5-HT_2AR agonists

Psychedelics have a wide range of binding profiles with the 5-HT receptor family (Ray, Reference Ray2010) (Figure 1). The 5-HT_2AR is of particular interest in the field of psychedelic research as it is largely associated with the psychotropic effects of psychedelics (Nichols, Reference Nichols2004; Preller et al., Reference Preller, Burt, Ji, Schleifer, Adkinson, Stampfli, Seifritz, Repovs, Krystal, Murray, Vollenweider and Anticevic2018; Madsen et al., Reference Madsen, Fisher, Burmester, Dyssegaard, Stenbaek, Kristiansen, Johansen, Lehel, Linnet, Svarer, Erritzoe, Ozenne and Knudsen2019; Shao et al., Reference Shao, Liao, Davoudian, Savalia, Jiang, Wojtasiewicz, Tan, Nothnagel, Liu, Woodburn, Bilash, Kim, Che and Kwan2025). This receptor is widely distributed throughout the cortex, whereas moderate to low levels of expression can be found in the limbic system, including the amygdala and the hippocampus, which plays a crucial role in emotional processing, memory formation, and behavioural regulation (Saulin et al., Reference Saulin, Savli and Lanzenberger2012).

Psychedelics can induce different downstream effects after activation of the 5-HT_2AR. Psychedelics display biased agonism, also known as functional selectivity, referring to the ability of different ligands to selectively stabilise distinct receptor conformations, leading to the activation of specific signalling pathways over others (Inserra et al., Reference Inserra, De Gregorio and Gobbi2021).

In the context of 5-HT receptors, psychedelics can induce alternative 5-HT receptor conformations, leading to altered ligand affinity recruitment of intracellular effector proteins, including β-arrestins, which mediate receptor desensitisation, internalisation, and signalling pathways independent of G proteins. For example, LSD induces a receptor conformational change that will preferentially recruit β-arrestin. Interestingly, psychedelic-related effects induced by LSD appear to be mediated by β-arrestin (Rodriguiz et al., Reference Rodriguiz, Nadkarni, Means, Pogorelov, Chiu, Roth and Wetsel2021) unlike the full 5-HT_2AR agonist 2,5-dimethoxy-4-iodoamphetamine [(R)-DOI] (Schmid et al., Reference Schmid, Raehal and Bohn2008).

A recent meta-analysis found that there are no significant differences in selectivity between N,N-DMT, LSD, and psilocin, relative to the 5-HT_1AR, but they reported that LSD induced a significantly higher formation of inositol phosphate (Shinozuka et al., Reference Shinozuka, Jerotic, Mediano, Zhao, Preller, Carhart-Harris and Kringelbach2024).

The 5-HT_2CR has been shown to be involved in the head-twitch response (HTR), a commonly used measure for a hallucinogenic-like experience in rodents. It remains unclear exactly how this receptor contributes to this behavioural response as some studies have found that antagonists increase – and agonists attenuate – the HTR (Canal et al., Reference Canal, Booth and Morgan2013; Erkizia-Santamaria et al., Reference Erkizia-Santamaria, Alles-Pascual, Horrillo, Meana and Ortega2022), whereas some studies have found that its activation can induce this behaviour (Custodio et al., Reference Custodio, Ortiz, Lee, Sayson, Kim, Lee, Kim, Cheong and Kim2023).

Psychedelics mediate their effects through 5-HT_2A receptors, to distinct signalling and functional outcomes that occur depending on the localisation of receptor activation (Olson, Reference Olson2021). Extracellular 5-HT_2A activation by psychedelics like psilocin initiates Gq/11-PLC signalling, which exceeds 70% efficacy threshold to induce hallucinations and HTR, while sub-threshold agonists (e.g., lisuride) lack psychedelic effects (de Vos et al., Reference De Vos, Mason and Kuypers2021; Wallach et al., Reference Wallach, Cao, Calkins, Heim, Lanham, Bonniwell, Hennessey, Bock, Anderson, Sherwood, Morris, De Klein, Klein, Cuccurazzu, Gamrat, Fannana, Zauhar, Halberstadt and Mccorvy2023). β-Arrestin2 recruitment via cell-surface receptors does not correlate with psychedelic potential or with the psychedelic experience and may instead promote receptor downregulation (Schmid and Bohn, Reference Schmid and Bohn2010).

Lipophilic psychedelics (e.g., DMT) activate intracellular 5-HT2A pools, triggering neuroplasticity via transglutaminase 2 (TGM2)-mediated serotonylation of Rac1, which enhances dendritic arborisation and synaptogenesis (Ly et al., Reference Ly, Greb, Cameron, Wong, Barragan, Wilson, Burbach, Soltanzadeh Zarandi, Sood, Paddy, Duim, Dennis, Mcallister, Ori-Mckenney, Gray and Olson2018; de la Fuente Revenga et al., Reference de la Fuente Revenga, Zhu, Guevara, Naler, Saunders, Zhou, Toneatti, Sierra, Wolstenholme, Beardsley and Huntley2021). This intracellular signalling may contribute to the sustained therapeutic effects observed in clinical trials for the treatment of depression, independent of hallucinogenic activity (Vargas et al., Reference Vargas, Dunlap, Dong, Carter, Tombari, Jami, Cameron, Patel, Hennessey, Saeger, Mccorvy, Gray, Tian and Olson2023). Emerging strategies aim to decouple neuroplasticity from psychedelic effects by targeting intracellular receptors, using specific β-arrestin modulators or designing Gq-sub-threshold agonists, offering a pathway for non-hallucinogenic therapeutics (Dunlap et al., Reference Dunlap, Azinfar, C., Cameron, Viswanathan, Tombari, Myers-Turnbull, Taylor, Grodzki, Lein, Kokel and Olson2020; Wallach et al., Reference Wallach, Cao, Calkins, Heim, Lanham, Bonniwell, Hennessey, Bock, Anderson, Sherwood, Morris, De Klein, Klein, Cuccurazzu, Gamrat, Fannana, Zauhar, Halberstadt and Mccorvy2023).

β2-arrestin biased signalling modulates MAPK pathways such as ERK1/2, JNK, and p38, influencing immune cell activity through scaffold-mediated regulation. β-arrestins form complexes with ERK1/2, sequestering it in the cytosol and preventing nuclear translocation, altering transcription, and dampening pro-inflammatory cytokine production (Sharma and Parameswaran, Reference Sharma and Parameswaran2015). In macrophages, β-arrestin2 inhibits TLR2/ERK1/2 signalling, reducing TNF-α expression (Fan, Reference Fan2014). β-arrestin1 promotes CD4⁺ T-cell survival by upregulating Bcl2, while β-arrestin2 suppresses NK cell cytotoxicity by interacting with inhibitory receptors (Crepieux et al., Reference Crepieux, Poupon, Langonne-Gallay, Reiter, Delgado, Schaefer, Bourquard, Serrano and Kiel2017). β-arrestin2 also negatively regulates NF-κB by stabilising IκBα, limiting inflammatory cytokine release in sepsis models (Fan, Reference Fan2014). β-arrestin appears to act in a modulatory fashion in immune regulation, balancing pro- and anti-inflammatory responses through modulation of MAPK activity and crosstalk with PRR signalling (Sharma and Parameswaran, Reference Sharma and Parameswaran2015).

Certain immune cells, including monocytes, macrophages, dendritic cells, and T cells, express high levels of 5-HT_2A receptors making them functionally sensitive to co-ligation by psychedelics that target 5-HT receptors and activate β-arrestin signalling through crosstalk with PRR pathways such as TLRs, and can influence key signalling cascades like NF-κB and MAPK. β-arrestin-biased agonists at 5-HT_2ARs can further fine-tune these effects, potentially reducing pro-inflammatory responses (Nau et al., Reference Nau, Yu, Martin and Nichols2013; Szabo, Reference Szabo2015; Flanagan and Nichols, Reference Flanagan and Nichols2018).

5-HT_2AR and regulation of brain glia

Brain glia, and specifically microglia, have been shown to express various forms of 5-HT receptors including the 5-HT_2AR (Krabbe et al., Reference Krabbe, Matyash, Pannasch, Mamer, Boddeke and Kettenmann2012; Glebov et al., Reference Glebov, Lochner, Jabs, Lau, Merkel, Schloss, Steinhauser and Walter2015) and 5-HT_2BR influencing microglial development (Kolodziejczak et al., Reference Kolodziejczak, Bechade, Gervasi, Irinopoulou, Banas, Cordier, Rebsam, Roumier and Maroteaux2015; Turkin et al., Reference Turkin, Tuchina and Klempin2021). Although the exact functions of these remain unclear, studies to date suggest that activation of these 5-HT_2AR promotes the release of microglia-derived vesicles known as exosomes containing a variety of proteins and ribonucleic acids (RNA) (Glebov et al., Reference Glebov, Lochner, Jabs, Lau, Merkel, Schloss, Steinhauser and Walter2015). 5-HT_2AR influences the dynamic extensions of microglia which play an important role in surveillance and maintenance functions within the CNS (Krabbe et al., Reference Krabbe, Matyash, Pannasch, Mamer, Boddeke and Kettenmann2012). Microglia may play a role in the neuropharmacological and therapeutic effects of various drugs, including dissociative NMDA receptor psychedelics like ketamine and 5-HT_2AR psychedelics (VanderZwaag et al., Reference Vanderzwaag, Halvorson, Dolhan, Simoncicova, Ben-Azu and Tremblay2023).

Astrocytes express multiple forms of 5-HT receptors, including the 5-HT_2AR (Hagberg et al., Reference Hagberg, Blomstrand, Nilsson, Tamir and Hansson1998; Hirst et al., Reference Hirst, Cheung, Rattray, Price and Wilkin1998; Cohen et al., Reference Cohen, Bouchelet, Olivier, Villemure, Ball, Stanimirovic and Hamel1999; Maxishima et al., Reference Maxishima, Shiga, Shutoh, Hamada, Maeshima and Okado2001; Kong et al., Reference Kong, Peng, Chen, Yu and Hertz2002; Verkhratsky et al., Reference Verkhratsky, Parpura, Scuderi and Li2021). Although more research is required, 5-HT_2AR seems to play a role in 5-HT-driven astrocytic calcium signalling with a possible role in synaptic plasticity (Jalonen et al., Reference Jalonen, Margraf, Wielt, Charniga, Linne and Kimelberg1997; Hagberg et al., Reference Hagberg, Blomstrand, Nilsson, Tamir and Hansson1998; Gonzalez-Arias et al., Reference Gonzalez-Arias, Sanchez-Ruiz, Esparza, Sanchez-Puelles, Arancibia, Ramirez-Franco, Gobbo, Kirchhoff and Perea2023). An immunocytochemical study found increased 5-HT_2AR expression in astrocytes within the prefrontal cortex of Alzheimer’s disease patients and the caudate nucleus of Huntington’s disease patients (Wu et al., 1999). Astrocytes also express the serotonin transporter (SERT) (Fitzgerald et al., Reference Fitzgerald, Kaplinsky and Kimelberg1990; Bel et al., Reference Bel, Figueras, Vilaro, Sunol and Artigas1997), enabling them to regulate the extracellular availability of 5-HT (Edmondson et al., Reference Edmondson, Binda and Mattevi2007).

Oligodendrocytes form the myelin sheath around neuronal axons and support neuronal plasticity (Jang et al., Reference Jang, Gould, Xu, Kim and Kim2019). Both in vitro and in vivo studies demonstrate that these cells express the 5-HT_2AR, with elevated serotonin levels altering myelination via this receptor (Simpson et al., Reference Simpson, Weaver, De Villers-Sidani, Lu, Cai, Pang, Rodriguez-Porcel, Paul, Merzenich and Lin2011; Fan et al., Reference Fan, Bhatt, Tien, Zheng, Simpson, Lin, Cai, Kumar and Pang2015). Psilocybin elicits an increase in cellular activation indicated by expression of the immediate early gene c-Fos in neurons and in oligodendrocytes (Funk et al., Reference Funk, Araujo, Slassi, Lanthier, Atkinson, Feng, Lau, Le and Higgins2024). This increase was found in 10–20% of neurons and 25% of oligodendrocytes in the medial prefrontal cortex (mPFC), basolateral amygdala, and the dorsal raphe nucleus of male rats.

Effect of 5-HT on the immune system

In the periphery, most 5-HT production occurs in enterochromaffin cells located in the gut. Gut-derived 5-HT modulates gastrointestinal functions and immune cells in or near the gut epithelium. It can also enter the bloodstream, where platelets absorb it via SERT (Cloutier et al., Reference Cloutier, Allaeys, Marcoux, Machlus, Mailhot, Zufferey, Levesque, Becker, Tessandier, Melki, Zhi, Poirier, Rondina, Italiano, Flamand, Mckenzie, Cote, Nieswandt, Khan, Flick, Newman, Lacroix, Fortin and Boilard2018).

5-HT is known to play a versatile role within the immune system influencing macrophage and monocyte activity, dendritic cell maturation, and natural killer (NK) cell cytotoxicity (Herr et al., Reference Herr, Bode and Duerschmied2017; Roumier et al., Reference Roumier, Béchade, Maroteaux and Pilowsky2019). By binding to various 5-HT receptors, 5-HT can either promote or suppress immune responses via the modulation of cytokine production and release. 5-HT receptors are abundantly expressed by immune cells [see Hodo et al. (Reference Hodo, de Aquino, Shimamoto and Shanker2020) for a thorough review on expression patterns and functions of each receptor]. Figure 2 summarises the expression pattern for 5-HT receptors in peripheral immune cells and associated functions.

Figure 1. Heat map visualisation of serotonergic receptor binding profiles. Yellow cells with crosses indicate known binding activity of ligands (rows) to specific 5-HT receptors (columns), while dark purple cells indicate no reported binding. Data compiled from: Nichols (Reference Nichols2004); Kitson (Reference Kitson2007); Keiser et al. (Reference Keiser, Setola, Irwin, Laggner, Abbas, Hufeisen, Jensen, Kuijer, Matos, Tran, Whaley, Glennon, Hert, Thomas, Edwards, Shoichet and Roth2009); Besnard et al. (Reference Besnard, Ruda, Setola, Abecassis, Rodriguiz, Huang, Norval, Sassano, Shin, Webster, Simeons, Stojanovski, Prat, Seidah, Constam, Bickerton, Read, Wetsel, Gilbert, Roth and Hopkins2012); Rickli et al. (Reference Rickli, Luethi, Reinisch, Buchy, Hoener and Liechti2015); Wsol (2023); Hatzipantelis and Olson (Reference Hatzipantelis and Olson2024); Ippolito et al. (Reference Ippolito, Vasudevan, Hurley, Gilmour, Westhorpe, Churchill and Sharp2024). 2C-(x) refers to the family of 2,5-dimethoxy-phenethylamine analogues. Note that binding affinity varies based on pharmacological method, cell type, and experimental conditions. The psychoactive drug screening programme (PDSP) has been a primary source for standardised binding data (Ki values) for many of these compounds, as reviewed in Alexander et al. (Reference Alexander, Anderson, Baxter, Claydon, Rucker and Robinson2024); Hatzipantelis and Olson (Reference Hatzipantelis and Olson2024).

Previous in vitro studies demonstrate that exogenous 5-HT regulates the release of pro- and anti-inflammatory cytokines and chemokines (e.g., TNF-α) through 5-HT₃R, 5-HT₄R, and 5-HT₇R activation in lipopolysaccharide (LPS)-stimulated peripheral blood mononuclear cell (PBMC) cultures and whole blood (Kubera et al., Reference Kubera, Kenis, Bosmans, Scharpe and Maes2000; Cloez-Tayarani et al., Reference Cloez-Tayarani, Petit-Bertron, Venters and Cavaillon2003; Durk et al., Reference Durk, Panther, Muller, Sorichter, Ferrari, Pizzirani, Di Virgilio, Myrtek, Norgauer and Idzko2005; Kubera et al., Reference Kubera, Maes, Kenis, Kim and Lason2005). Exogenous 5-HT also promoted anti-inflammatory mechanisms in splenocytes, lymph nodes, and PBMC cultures (Toh and Miossec, Reference Toh and Miossec2007; Chabbi-Achengli et al., Reference Chabbi-Achengli, Coman, Collet, Callebert, Corcelli, Lin, Rignault, Dy, De Vernejoul and Cote2016; Sacramento et al., Reference Sacramento, Monteiro, Dias, Kasahara, Ferreira, Hygino, Wing, Andrade, Rueda, Sales, Vasconcelos and Bento2018). In contrast, exogenous 5-HT impaired the ability of mouse-derived dendritic cells (DCs) to induce Type 1 regulatory (T-reg1) cells and reduced expression of the anti-inflammatory cytokine IL-10 (Liao et al., Reference Liao, Tao, Wang, Wu, Yao, Yang, Huang, Liu, Yang and Yang2023).

In vitro evidence also suggests that eosinophils can be modulated by 5-HT. Indeed, treatment with 5-HT caused human eosinophils to roll onto vascular cell adhesion molecule (Vcam)-1, a process important for immune cell migration (Kang et al., Reference Kang, Ha, Bahaie, Hosseinkhani, Ge, Blumenthal, Rao and Sriramarao2013). This effect was found to be driven by 5-HT_2AR activation and was coupled with an increase in intracellular calcium (Ca²⁺) levels and with distinct changes in the cytoskeleton and cell shape.

These observations suggest that 5-HT has different cell-specific effects on the immune system, and that these immunomodulatory effects are dependent on various 5-HT receptors.

Immunomodulatory effects have also been observed with commonly prescribed selective 5-HT reuptake inhibitors (SSRIs) and 5-HT and noradrenaline reuptake inhibitors (SNRIs) in whole blood (Diamond et al., Reference Diamond, Kelly and Connor2006), PBMC (Taler et al., Reference Taler, Gil-Ad, Lomnitski, Korov, Baharav, Bar, Zolokov and Weizman2007), and microglial cultures (Horikawa et al., Reference Horikawa, Kato, Mizoguchi, Monji, Seki, Ohkuri, Gotoh, Yonaha, Ueda, Hashioka and Kanba2010; Tynan et al., Reference Tynan, Weidenhofer, Hinwood, Cairns, Day and Walker2012; Liu et al., Reference Liu, Zou, Wang, Zhu, Lai, Zhou, Chen, Zhang and Zhu2014), and in in vivo immune stimulation paradigms (Pellegrino and Bayer, Reference Pellegrino and Bayer2000; Dong et al., Reference Dong, Zhang, Yao, Ren, Yang, Ma, Han, Saito and Hashimoto2016; Tomaz et al., Reference Tomaz, Chaves Filho, Cordeiro, Juca, Soares, Barroso, Cristino, Jiang, Teixeira, De Lucena and Macedo2020).

Other mechanisms by which SSRIs might modulate the immune system include signalling pathways such as extracellular signal-regulated-protein kinase (Erk) and p38 MAPK cascades (Russo-Neustadt et al., Reference Russo-Neustadt, Alejandre, Garcia, Ivy and Chen2004; Mercier et al., Reference Mercier, Lennon, Renouf, Dessouroux, Ramauge, Courtin and Pierre2004; Chilmonczyk et al., Reference Chilmonczyk, Bojarski, Pilc and Sylte2017), cAMP production (Zhou et al., Reference Zhou, Ma, Yeung, Wong, Tsim, So, Lam and Chung2016), membrane-associated lipid rafts (Singh et al., Reference Singh, Wray, Schappi and Rasenick2018), glucocorticoid receptors (Pariante et al., Reference Pariante, Makoff, Lovestone, Feroli, Heyden, Miller and Kerwin2001; Antonioli et al., Reference Antonioli, Rybka and Carvalho2012; Gobin et al., Reference Gobin, Van Steendam, Denys and Deforce2014), the brain derived neurotrophic factor (BDNF) (Wang et al., Reference Wang, Wang, Wu, Huang and Yang2022), and the Sigma-1 receptor (Nguyen et al., Reference Nguyen, Lucke-Wold, Mookerjee, Cavendish, Robson, Scandinaro and Matsumoto2015; Hashimoto, Reference Hashimoto2015; Rosen et al., Reference Rosen, Seki, Fernandez-Castaneda, Beiter, Eccles, Woodfolk and Gaultier2019; Salaciak and Pytka, Reference Salaciak and Pytka2022), as well as directly modulating the vagal nerve (Ondicova et al., Reference Ondicova, Tillinger, Pecenak and Mravec2019). This suggests that pathways independent of the serotonergic system may be involved in the immunomodulatory effects of SSRIs.

Finally, classical psychedelics moderately influence platelet function and immune responses via serotonin 5-HT2A receptor pathways on platelets and other immune cells but lack significant clotting risks (Szabo, Reference Szabo2015). In contrast, MDMA can markedly disrupt hemostasis, triggering coagulopathies like thrombocytopenia and disseminated intravascular coagulation (DIC) through serotonin syndrome and rhabdomyolysis, highlighting critical safety concerns in therapeutic use (Szabo, Reference Szabo2015; Doyle et al., Reference Doyle, Meyer, Breen and Hunt2020).

While preclinical models provide valuable insights into 5-HT’s immunomodulatory potential, there are critical interspecies differences in receptor expression patterns and pharmacological responses that are crucial to keep in mind. For example, the 5-HT₆R is abundantly expressed in human and rat striatal regions, however it shows negligible expression in mouse brain tissue (Hirst et al., Reference Hirst, Abrahamsen, Blaney, Calver, Aloj, Price and Medhurst2003; Kirkpatrick, Reference Kirkpatrick2004). Receptor structural variations render many compounds targeting 5-HT₆R ineffective in murine models (Hirst et al., Reference Hirst, Abrahamsen, Blaney, Calver, Aloj, Price and Medhurst2003; Kirkpatrick, Reference Kirkpatrick2004). These variations are relevant for studying serotonergic psychedelics, as demonstrated by Haberzettl et al. (Reference Haberzettl, Bert, Fink and Fox2013) in their systematic analysis of serotonin syndrome models. Their work revealed that monoamine oxidase (MAO)-A knockout mice exhibit greater sensitivity to 5-HT-enhancing drugs compared to wild-type strains, whereas human MAO polymorphisms show more nuanced clinical manifestations (Haberzettl et al., Reference Haberzettl, Bert, Fink and Fox2013; Chiew and Isbister, Reference Chiew and Isbister2024). This difference originates not only from receptor diversity but also from differences in systemic 5-HT homeostasis and storage mechanisms (Mossner and Lesch, Reference Mossner and Lesch1998). When considering neuroimmune interactions, physiological disparities complicate translation, particularly given that psychedelics like LSD demonstrate species-specific binding kinetics at the 5-HT_2AR critical for both psychoactive and immunomodulatory effects (Szabo, Reference Szabo2015; Canal, Reference Canal2018). These findings underscore the necessity of validating preclinical observations in human cell systems (Figure 2).

Figure 2. Sankey diagram depicting 5-HT receptor expression in peripheral immune cells and their associated cellular functions. Function-specific references are provided for each cell type. Second messengers and their targets are identified for specific receptors in certain immune cells: 5-HT _2A R in Eos is associated to increased intracellular Ca²⁺ and to the activation of ROCK, MAPK, PI3K, PKC, and Calmodulin (Boehme et al., Reference Boehme, Lio, Sikora, Pandit, Lavrador, Rao and Sriramarao2004; Kang et al., Reference Kang, Ha, Bahaie, Hosseinkhani, Ge, Blumenthal, Rao and Sriramarao2013), 5-HT _1A R in both T- and B-cells has been associated to NF-κB translocation (Abdouh et al., Reference Abdouh, Albert, Drobetsky, Filep and Kouassi2004), 5-HT ₇ R in T-cells has been found associated to the activation of ERK1/2 and translocation of NF-κB (León-Ponte et al., Reference León-Ponte, Ahern and O’connell2007), 5-HT _2C R in macrophages has been associated to increased intracellular Ca²⁺ (Mikulski et al., Reference Mikulski, Zaslona, Cakarova, Hartmann, Wilhelm, Tecott, Lohmeyer and Kummer2010), and 5-HT ₄ R and 5-HT ₇ R in dendritic cells have been associated to increases in cAMP levels (Muller et al., Reference Muller, Durk, Blumenthal, Grimm, Cicko, Panther, Sorichter, Herouy, Di Virgilio, Ferrari, Norgauer and Idzko2009).

Effect of the immune system on serotonergic transmission

Systemic inflammation has consistently been demonstrated to influence serotonergic signalling in the brain. Systemic LPS injection had a wide range of effects on the brain including increased reuptake of 5-HT from the synaptic cleft via SERT in the mPFC in mice (van Heesch et al., Reference Van Heesch, Prins, Konsman, Korte-Bouws, Westphal, Rybka, Olivier, Kraneveld and Korte2014), reduced 5-HT concentrations in the hippocampus in mice (Zhao et al., Reference Zhao, Cao, Liu, Li, Xu, Liu, Zhang, Yang, Yi, Xu, Fan and Ma2019) and in the anteroventral preoptic region in rats (Mota et al., Reference Mota, Rodrigues-Santos, Fernandez, Carolino, Antunes-Rodrigues, Anselmo-Franci and Branco2017), increased 5-HT_2AR mRNA levels in the PFC and hippocampus in mice (Couch et al., Reference Couch, Xie, lundberg, Sharp and Anthony2015), increased the functional response to (R)-DOI (Couch et al., Reference Couch, Xie, lundberg, Sharp and Anthony2015), and increased 5-HT turnover in the PFC and hippocampus in mice (Swiergiel and Dunn, Reference Swiergiel and Dunn2006).

In rats treated with the immune stimulus and toll-like receptor (TLR)-3 agonist polyinosinic:polycytidylic acid [Poly(I:C)], levels of messenger RNA (mRNA) coding for SERT increased in the cortex, cerebellum, medial preoptic area, and paraventricular hypothalamic nucleus, which in turn was associated with a decrease in extracellular 5-HT concentrations as measured by microdialysis (Katafuchi et al., Reference Katafuchi, Kondo, Take and Yoshimura2005).

In zebrafish, intra-cerebroventricular (ICV) microinjection of IL-4 suppressed 5-HT production, whereas ICV microinjection of 5-HT suppressed neurogenesis in periventricular neurons via a neuron-glia interaction involving the induction of BDNF (Bhattarai et al., Reference Bhattarai, Cosacak, Mashkaryan, Demir, Popova, Govindarajan, Brandt, zhang, Chang, Ampatzis and Kizil2020).

Systemic LPS injection also leads to induction of the kynurenine pathway in the hippocampus by increasing the expression of the indoleamine-2,3-dioxygenase (IDO) enzyme involved in tryptophan metabolism, leading to an increase in the production of kynurenine from tryptophan (Zhao et al., Reference Zhao, Cao, Liu, Li, Xu, Liu, Zhang, Yang, Yi, Xu, Fan and Ma2019; Marx et al., Reference Marx, Mcguinness, Rocks, Ruusunen, Cleminson, Walker, Gomes-Da-Costa, Lane, Sanches, Diaz, Tseng, Lin, Berk, Clarke, O’neil, Jacka, Stubbs, Carvalho, Quevedo, Soares and Fernandes2021). As tryptophan is an essential amino acid required for the biosynthesis of 5-HT, it has been proposed that IDO induction may limit the availability of tryptophan for 5-HT biosynthesis.

Effects of 5-HT_2AR psychedelics on peripheral blood mononuclear cells

In LPS or Poly(I:C)-stimulated primary monocyte-derived dendritic cells (moDC), N,N-DMT, and the methoxylated derivative 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT) inhibit the production of pro-inflammatory cytokines IL-1β, IL-6, and TNF-α and the chemokine IL-8, while increasing the secretion of the anti-inflammatory cytokine IL-10 (Szabo et al., Reference Szabo, Kovacs, Frecska and Rajnavolgyi2014). This treatment also reduced the amount of induced T helper (Th)-1 and Th17 effector T-cells versus vehicle in a SIG-1R-dependent manner. Considering that both compounds are endogenous ligands of SIG-1R, the authors propose that activation of the SIG-1R in peripheral immune cells with psychedelics like N,N-DMT may trigger a unique cluster of differentiation (CD)4⁺ T-cell response upon viral or bacterial stimulation.

In CD4⁺ and CD8⁺ T-cell cultures stimulated with concanavalin A (ConA), a lectin mitogen, (R)-DOI was found to provoke a stimulatory response with an increase in the production of cytokines interferon (IFN)-γ and IL-2 (Inoue et al., Reference Inoue, Okazaki, Kitazono, Mizushima, Omata and Ozaki2011). This effect was reversed by the 5-HT_2AR antagonist sarpogrelate hydrochloride. This study found higher doses of (R)-DOI to enhance IL-2 and IFN-γ production, indicating a link between 5-HT_2AR activity and IFN-γ production. The use of ConA, known to induce a distinct anti-cancer T-cell response, might explain the observed difference compared to other studies (Wiersma, Reference Wiersma and Chouaib2020).

In primary cultures of CD4⁺ and CD8⁺ T-cells stimulated with anti-human CD3, a widely used T-cell stimulant, levels of TNF-α were unaffected by LSD, N,N-DMT, psilocin, or mescaline (Rudin et al., Reference Rudin, Areesanan, Liechti and Grundemann2023). The authors suggest that an initial anti-inflammatory cortisol release upon administration of psychedelics may be responsible for the effects measured in previous investigations (Dos Santos et al., Reference Dos Santos, Valle, Bouso, Nomdedéu, Rodríguez-Espinosa, McIlhenny, Barker, Barbanoj and Riba2011) which examined blood and serum samples following administration of psychedelics. Furthermore, cell cultures in this study were treated for 24 to 72 hours which may miss the initial window of immunomodulatory action. Notably, the peak anti-proliferative effects of ayahuasca – a psychoactive decoction containing N,N-DMT and monoamine oxidase inhibitors – on CD3+, CD4+, and CD8+ cell populations begin to diminish two hours after administration (Dos Santos et al., Reference Dos Santos, Valle, Bouso, Nomdedéu, Rodríguez-Espinosa, McIlhenny, Barker, Barbanoj and Riba2011).

In cultured RAW 264.7 macrophages, Nkadimeng et al. (Reference Nkadimeng, Nabatanzi, Steinmann and Eloff2020) demonstrated that psilocybin-containing mushroom extracts significantly reduced the expression levels of IL-1β following LPS stimulation. Similarly, Oppong-Damoah et al. (Reference Oppong-Damoah, Curry, Blough, Rice and Murnane2019) observed that macrophages stimulated with ethanol produce NO, an effect that was reduced by (R)-DOI.

Ghasemi Gojani et al. (Reference Ghasemi Gojani, Wang, Li, Kovalchuk and Kovalchuk2024) found that a concentration of 15 μM psilocybin was sufficient to inhibit NLRP3 inflammasome activation and downregulate inflammatory-regulating transcription factors in LPS-stimulated human monocytes. Both 15 and 10 μM doses resulted in the downregulation of NF-κB, Tyrosine Kinase 2 (TYK2), Signal Transducer and Activator of Transcription (STAT)-1, and STAT3 activation – key regulators of IL-1β, IL-6, TNF-α, and cyclooxygenase (COX)-2. This was accompanied by a reduction of COX-2, Pro-TNFα, IL-1β, IL-6, and Pro-IL-1β levels, and suppression of IL-1β release. Interestingly, the 5 μM dose stimulated an increase in the levels of these transcription factors. Higher doses of psilocybin did not alter total NF-κB levels but prevented its nuclear translocation, thereby inhibiting transcription of the regulated genes. In the cases of IL-6 and COX-2, protein levels – but not mRNA levels – were reduced, suggesting that psilocybin may modulate post-translational processing or modifications. This study also found that the LPS-stimulation decreased protein levels of 5-HT_2A and 5-HT_2B receptors while psilocybin attenuated this downregulation in LPS-stimulated cells, upregulating 5-HT_2A receptors above baseline at the highest dose. In unstimulated cells, psilocybin reduced 5-HT_2A receptors in a dose-dependent fashion. This research emphasises the immune-modifying capabilities of psilocybin, while also revealing distinct outcomes when applied to inflamed versus non-inflamed biological systems.

Psilocybin inducing opposing immunoregulatory effects at low compared to higher doses has also been observed in resting macrophages, with lower doses inducing higher levels of TNF-α (Laabi et al., Reference Laabi, LeMmon, Vogel, Chacon and Jimenez2024). Whereas in LPS-stimulated and classically activated macrophages, post-treatment with psilocin, but not psilocybin, produced anti-inflammatory-like effects, reducing levels of TNFα, and increasing levels of IL-10 (Laabi et al., Reference Laabi, LeMmon, Vogel, Chacon and Jimenez2024). Peyote extract, containing the phenethylamine psychedelic mescaline, activated nitric oxide (NO) production by murine macrophages (Franco-Molina et al., Reference Franco-Molina, Gomez-Flores, Tamez-Guerra, Tamez-Guerra, Castillo-Leon and Rodriguez-Padilla2003). This study further reported that peyote extract stimulated murine thymic lymphocyte proliferation and induced an increase in mRNA levels of IL-1, IL-6, and IL-8 in human leukocytes.

Tourino et al. (Reference Tourino, De Oliveira, Belle, Knebel, Albuquerque, Dorr, Okada, Migliorini, Soares and Campa2013) aimed to assess the effects of N,N-DMT and tryptamine (TRY) on IDO activity and the subsequent production of kynurenine. They found that both compounds acted as classical non-competitive inhibitors of IDO. N,N-DMT and TRY also increased cytotoxic activity in co-culture assays of A172 glioblastoma cells with PBMCs, suggesting that IDO inhibition by these compounds contributes to a more effective tumour-reactive response by PBMCs. IDO inhibitors have previously been found to produce antidepressant-like properties in animal models of sickness behaviour (O’Farrell and Harkin, Reference O’Farrell and Harkin2017; O’Connor et al., Reference O’Connor, Lawson, André, Moreau, Lestage, Castanon, Kelley and Dantzer2009).

These findings highlight the potential therapeutic application of psychedelics like N,N-DMT in enhancing immune responses against tumours, warranting further investigation into their use in cancer immunotherapy.

Effects of 5-HT_2AR psychedelics on microglia

In LPS-stimulated microglial cultures derived from mice, N,N-DMT and psilocybin reduce expression levels of TLR4 (Kozłowska et al., Reference Kozłowska, Klimczak, Wiatr and Figiel2021). This suggests their potential to dampen the inflammatory cascade. Additionally, treatment with N,N-DMT and psilocybin altered microglial morphology, with treated cells exhibiting a more rounded and compact shape compared to controls. Furthermore, both drugs decreased the expression of co-stimulatory T-cell molecule CD80 and NF-κβ protein, suggestive of a decreased ability of presenting antigens to T-cells. Interestingly, treatment with psilocybin, but not N,N-DMT, reduced phagocytosis of healthy neurons by LPS-stimulated microglia, suggesting increased neuroprotection. Additionally, psilocybin upregulated a regulator of microglial phagocytosis and synaptic pruning, Triggering Receptor Expressed on Myeloid cells 2 (TREM2). Mutations in TREM2 have been associated to various neurodegenerative diseases including Alzheimer’s disease and Parkinson’s disease, and its pathway is implicated in anti-inflammatory and neuroprotective functions. If psilocybin upregulates a functional form of TREM2, this could produce potent anti-inflammatory effects, decreasing expression of pro-inflammatory proteins, thus ameliorating disease outcomes.

Proteome analysis of cerebral organoids treated with 5-MeO-DMT found significant alterations in genes associated with long term potentiation, dendritic spine formation, cellular protrusion formation, microtubule and cytoskeletal organisation, and mild activation of T lymphocyte differentiation (Dakic et al., Reference Dakic, Minardi Nascimento, Costa Sartore, Maciel, De Araujo, Ribeiro, Martins-de-Souza and Rehen2017). Following 5-MeO-DMT treatment, they also observed downregulation in protein expression of pathways associated to nuclear factor of activated T-cells (NFAT) and NF-κB signalling via TLR- and Gq-coupled receptors.

In TNF-α and IFN-γ stimulated C6-glioma cells, (R)-DOI produced a dose-dependent inhibition of cytokine-induced NO levels (Miller and Gonzalez, Reference Miller and Gonzalez1998). This effect was not seen when (R)-DOI was added more than 2 hours after the immune stimulation was added. This suggests that (R)-DOI suppression of NO occurs at a transcription regulation level.

Another study looking at NO release in LPS and IFN-γ stimulated BV-2 murine microglia showed that pre-treatment with psilocin reduced NO release. This effect was dependent on the 5-HT_2AR, as inhibiting it with the antagonists cyproheptadine and risperidone prevents the reduction in NO release seen by psilocin (Wiens et al., Reference Wiens, Brooks, Riar, Greuel, Lindhout and Klegeris2024). Additionally, this study demonstrated that psilocin could reduce levels of ROS in human microglia-like cells primed with LPS and stimulated with the bacterial peptide N-formyl-Met-Leu-Phe (fMLP) (Wiens et al., Reference Wiens, Brooks, Riar, Greuel, Lindhout and Klegeris2024). Increases in ROS have been seen in clinical studies and preclinical models of neuropsychiatric disorders like major depressive disorder (MDD) and are also believed to play a role in the pathogenesis of neurodegenerative diseases (Simpson and Oliver, Reference Simpson and Oliver2020; Rossetti et al., Reference Rossetti, Paladini, Riva and Molteni2020). The capability of psychedelics to reduce the production of ROS might be one of the mechanisms with which they modulate the inflammatory response.

Although the research reviewed so far in cell cultures or organoids indicate immunomodulatory properties of some 5-HT_2AR psychedelics, there is no clear consensus on whether psychedelics are pro- or anti-inflammatory. Additional research is warranted to definitively establish whether psychedelics can directly affect PBMC proliferation and function, microglial activation, and the production and release of cytokines in vivo and the mechanisms underlying such effects.

Immune organs and psychedelics

In LPS-stimulated mouse splenic cell cultures, low concentrations of LSD enhanced both baseline and IL-2-augmented NK cell function, but higher doses suppressed the NK response (House et al., Reference House, Thomas and Bhargava1994). These high LSD doses also led to suppression of B-cell function, macrophage function, reduced numbers of cytotoxic lymphocytes, and a reduction in Th1 cell IL-2 production.

In rat primary aortic smooth muscle cells treated with TNF-α, pre-treatment and co-treatment with (R)-DOI for 24 hours inhibited the TNF-α-induced inflammatory responses in a 5-HT_2AR-dependent fashion (Yu et al., Reference Yu, Becnel, Zerfaoui, Rohatgi, Boulares and Nichols2008). (R)-DOI potently suppressed the expression of key inflammatory mediators, notably the cytokine IL-6, the intracellular adhesion molecules ICAM-1 and VCAM-1, important components of atherosclerotic plaque formation, compared to TNF-α stimulated cells alone. Notably, (R)-DOI by itself did not elicit any response. Furthermore, the authors showed similar inhibitory effects on TNF-α induced inflammation with other 5-HT_2AR agonists, including a phenethylamine [(4-bromo-3,6-dimethoxybenzocyclobuten-1-yl) methylamine (2C-BCB)], and two indolealkylamines, lysergic acid 2,4-dimethylazetidide (LA-SS-Az) and LSD, suggesting a broader class effect.

Psilocybin was found to reduce expression levels of TNFα, IFN-γ, IL-6 and IL-8, in a human 3D epi-Intestinal tissue model treated with TFN-α/IFN-γ, providing additional evidence for the anti-inflammatory properties of psychedelics (Robinson et al., Reference Robinson, Li, Wang, Rahman, Gerasymchuk, Hudson, Kovalchuk and Kovalchuk2023).

The studies mentioned so far have used a range of concentrations of psychedelics. The clinically relevant psychedelics psilocybin, N,N-DMT, 5-MeO-DMT, and LSD were all used in supraphysiological concentrations (House et al., Reference House, Thomas and Bhargava1994; Kozłowska et al., Reference Kozłowska, Klimczak, Wiatr and Figiel2021; Szabo et al., Reference Szabo, Kovacs, Frecska and Rajnavolgyi2014; Rudin et al., Reference Rudin, Areesanan, Liechti and Grundemann2023), meaning that the doses used may have exceeded circulating concentrations as observed in clinical trials of psychedelics for the treatment of MDD (Ross et al., Reference Ross, Bossis, Guss, Agin-Liebes, Malone, Cohen, Mennenga, Belser, Kalliontzi, Babb, Su, Corby and Schmidt2016; Carhart-Harris et al., Reference Carhart-Harris, Roseman, Bolstridge, Demetriou, Pannekoek, Wall, Tanner, Kaelen, Mcgonigle, Murphy, Leech, Curran and Nutt2017; Carhart-Harris et al., Reference Carhart-Harris, Bolstridge, Day, Rucker, Watts, Erritzoe, Kaelen, Giribaldi, Bloomfield, Pilling, Rickard, Forbes, Feilding, Taylor, Curran and Nutt2018; Doss et al., Reference Doss, Považan, Rosenberg, Sepeda, Davis, Finan, Smith, Pekar, Barker, Griffiths and Barrett2021; Carhart-Harris et al., Reference Carhart-Harris, Giribaldi, Watts, Baker-Jones, Murphy-Beiner, Murphy, Martell, Blemings, Erritzoe and Nutt2021; Goodwin et al., Reference Goodwin, Aaronson, Alvarez, Arden, Baker, Bennett, Bird, Blom, Brennan, Brusch, Burke, Campbell-Coker, Carhart-Harris, Cattell, Daniel, Debattista, Dunlop, Eisen, Feifel, Forbes, Haumann, Hellerstein, Hoppe, Husain, Jelen, Kamphuis, Kawasaki, Kelly, Key, Kishon, Knatz Peck, Knight, Koolen, Lean, Licht, Maples-Keller, Mars, Marwood, Mcelhiney, Miller, Mirow, Mistry, Mletzko-Crowe, Modlin, Nielsen, Nielson, Offerhaus, O’keane, Palenicek, Printz, Rademaker, Van and Malievskaia2022, Reference Goodwin, Aaronson, Alvarez, Atli, Bennett, Croal, Debattista, Dunlop, Feifel, Hellerstein, Husain, Kelly, Lennard-Jones, Licht, Marwood, Mistry, Páleníček, Redjep, Repantis, Schoevers, Septimus, Simmons, Soares, Somers, Stansfield, Stuart, Tadley, Thiara, Tsai, Wahba, Williams, Winzer, Young, Young, Zisook and Malievskaia2023a, Reference Goodwin, Croal, Feifel, Kelly, Marwood, Mistry, O’keane, Peck, Simmons, Sisa, Stansfield, Tsai, Williams and Malievskaia2023c;Raison et al., Reference Raison, Sanacora, Woolley, Heinzerling, Dunlop, Brown, Kakar, Hassman, Trivedi, Robison, Gukasyan, Nayak, Hu, O’donnell, Kelmendi, Sloshower, Penn, Bradley, Kelly, Mletzko, Nicholas, Hutson, Tarpley, Utzinger, Lenoch, Warchol, Gapasin, Davis, Nelson-Douthit, Wilson, Brown, Linton, Ross and Griffiths2023).

Receptors mediating the immunomodulatory effects of 5-HT_2AR psychedelics

Although the involvement of the 5-HT_2AR in the immunomodulatory effects of psychedelics has not yet been fully elucidated, the studies explored so far have reported anti-inflammatory effects which appear to be 5-HT_2AR-dependent (Yu et al., Reference Yu, Becnel, Zerfaoui, Rohatgi, Boulares and Nichols2008; Nau et al., Reference Nau, Yu, Martin and Nichols2013). The 5-HT_2AR is likely the major binding site which is most closely associated with the anti-inflammatory effects observed in preclinical studies. However, 5-HT_2AR psychedelics have been reported to bind other receptors.

Animal models

Preclinical models are not perfect representations of human diseases; however, they offer many advantages to elucidate the mechanisms underlying the effect of psychedelics on the CNS and the immune system. These advantages include a controlled environment to isolate specific factors, the accessibility of cells and tissues at different life stages to track progression, genetic manipulation, and the means to test experimental drugs on bi-directional immune-nervous system interactions. There have been several studies investigating the effect of psilocybin, (R)-DOI, LSD, ayahuasca, and novel psychedelics, on immune responses and mediators in vivo, in healthy animals as well as in disease models.

In an exploratory study, Bove and Mokler (Reference Bove and Mokler2022) administered psilocybin to healthy female rats and measured pro- and anti-inflammatory markers in peripheral serum. Psilocybin induced the release of pro-inflammatory factors IL-1β, TNF-α, IL-13, MCP-1, and C-X-C motif chemokine ligand 10 (CXCL10), and anti-inflammatory factors IFN-γ, IL-10, granulocyte colony-stimulating factor (G-CSF), into the serum, consistent with a generalised activation of the immune system. This difference was persistent seven days after psilocybin injection, however, as the authors note, high variability in the results limit the conclusions that can be drawn from this study. In support of this, Custodio et al. (Reference Custodio, Ortiz, Lee, Sayson, Kim, Lee, Kim, Cheong and Kim2023) reported neurotoxic properties of (R)-DOI and other 5-HT_2CR agonists in healthy mice, with increased expression of ionised calcium binding adaptor molecule 1 (Iba1), IL-6, and TNF-α, however, these effects were reported at very high doses (30 mg/kg).

LSD, administered to healthy mice, was found to reduce hippocampal levels of kynurenine, altering the kynurenine/tryptophan and the kynurenine/5-HT ratios significantly, without altering levels of hippocampal 5-HT (Inserra et al., Reference Inserra, Giorgini, Lacroix, Bertazzo, Choo, Markopolous, Grant, Abolghasemi, De Gregorio, Flamand, Rogers, Comai, Silvestri, Gobbi and Di Marzo2023). This highlights psychedelics’ potential therapeutic role in disorders with a dysregulated kynurenine pathway, which is discussed in detail later. The same group also observed modulation of endocannabinoid-related metabolites in the hippocampus following LSD administration. These effects were correlated with increased sociability (Inserra et al., Reference Inserra, Giorgini, Lacroix, Bertazzo, Choo, Markopolous, Grant, Abolghasemi, De Gregorio, Flamand, Rogers, Comai, Silvestri, Gobbi and Di Marzo2023).

In the context of disease models, 5-HT_2AR psychedelics have consistently been reported to suppress the immune response.

Mice injected with sheep erythrocytes – activating both T and B lymphocytes (McAllister et al., Reference Mcallister, Apgar, Leung, Rickert and Jellusova2017) – and subsequently treated with (R)-DOI exhibited a marked suppression of the immune response (Davydova et al., Reference Davydova, Cheido, Gevorgyan and Idova2010). This was evidenced by a significant decrease in CD8⁺ T-cells, both in peripheral blood and the spleen. Notably, administration of ketanserin, a 5-HT_2AR antagonist, produced the opposite effect. These findings strongly support the involvement of 5-HT_2AR in mechanisms leading to immunosuppression.

Nichols and colleagues have been able to demonstrate anti-inflammatory properties for psychedelics specifically via 5-HT₂ receptor sub-type activation using various animal models. For example, a rapid immune response can be induced after systemic administration of exogenous TNF-α, with an increase in circulating IL-6 and VCAM-1. Administration of (R)-DOI efficiently blocks these effects of TNF-α, with reduced IL-6 and VCAM-1 expression in the aortic arch and small intestine (Nau et al., Reference Nau, Yu, Martin and Nichols2013). The mechanism underlying this potent anti-inflammatory response appears to be the activation of 5-HT_2AR.

Ovalbumin (OVA) induced acute allergic asthma in mice provides a model to study immune-modulatory properties of experimental compounds. Using this model, (R)-DOI was found to attenuate various asthma parameters in response to a non-selective muscarinic receptor agonist methacholine, including elevated airway hyperresponsiveness and pulmonary inflammation (Flanagan et al., Reference Flanagan, Sebastian, Battaglia, Foster, Cormier and Nichols2019a). Interestingly, the authors measured a reduction in expression of certain pro-inflammatory cytokines such as IL-15 and IL-9, but an increase in expression of other pro-inflammatory cytokines IL-13 and IL-33 which are known contributors to a persistent chronic asthma state. In a later study using the OVA model, (R)-DOI was found to prevent OVA-induced increases in mRNA levels for pro-inflammatory cytokines IL-5, IL-6, TNF-α and IL-1β (Flanagan et al., Reference Flanagan, Billac, Landry, Sebastian, Cormier and Nichols2021). The authors identified 2,5-dimethoxyphenethylamine (2C-H) from 21 different 5-HT_2A agonists as the key pharmacophore mediating effective anti-inflammatory properties. This may be a useful starting point for future drug development focusing on anti-inflammatory action.

More recently, Flanagan et al. (Reference Flanagan, Foster, Galbato, Lum, Louie, Song, Halberstadt, Billac and Nichols2024) aimed to identify key structural components of 5-HT_2AR agonists mediating their anti-inflammatory effects focusing on their ability to suppress Arginase-1 (Arg1) expression in peripheral tissues. Arg1 catalyses the conversion of L-arginine into L-ornithine and urea. Upregulation of Arg1 has been shown to contribute to inflammation and airway obstruction (North et al., Reference North, Khanna, Marsden, Grasemann and Scott2009), whereas in the absence of Arg1 activity, L-arginine is instead converted into NO via NOS activity, contributing to relaxation of bronchial smooth muscle and inhibition of inflammation (Cloots et al., Reference Cloots, Sankaranarayanan, De Theije, Poynter, Terwindt, Van Dijk, Hakvoort, Lamers and Kohler2013). Despite having similar in vitro activity on 5-HT_2AR and similar behavioural potency, the novel agonist (R)-2,5-dimethoxy-4-trifluoromethylamphetamine [(R)-DOTFM] did not exhibit anti-inflammatory properties like (R)-DOI. Only (R)-DOI led to significant reductions in levels of Arg1, IL-6, and CXCL10. The authors argue that the different effects of (R)-DOTFM and (R)-DOI may originate from differences in receptor stabilisation and conformation which could lead to separate downstream effectors and pathways being recruited. These experiments are the first to study and identify differences in functional selectivity of 5-HT_2AR agonists in peripheral tissues, indicating molecular and cellular sensitivities underlying anti-inflammatory properties of serotonergic 5-HT_2AR psychedelics. This may inform future studies to identify novel anti-inflammatory compounds devoid of subjective psychedelic effects.

There are additional animal models that have provided insights into the anti-inflammatory effects of psychedelics. In a mouse model of cardiovascular disease [Apolipoprotein E (ApoE)^-/- on a high fat diet], continuous systemic infusion of low-dose (R)-DOI resulted in significant reductions in mRNA expression of pro-inflammatory markers, including IL-6, TNF-α and CXCL10 (Flanagan et al., Reference Flanagan, Sebastian, Battaglia, Foster, Maillet and Nichols2019b). (R)-DOI treatment normalised glucose homeostasis and reduced circulating cholesterol. Although this study is not directly comparable, research in human vascular smooth cell cultures found that serotonin increased IL-6 production through 5-HT_2AR, suggesting that psychedelics and serotonin may activate different downstream cellular pathways (Ito et al., Reference Ito, Ikeda, Shimpo, Yamamoto and Shimada2000).

Psilocybin has been shown to have anti-inflammatory properties in different models of peripheral inflammation. In mice systemically injected with LPS, psilocybin treatment, pre- or post-administration of LPS, was found to reduce expression of IL-6 and TNF-α cytokines in homogenised brain tissue and in peripheral blood (Zanikov et al., Reference Zanikov, Gerasymchuk, Ghasemi Gojani, Robinson, Asghari, Groves, Haselhorst, Nandakumar, Stahl, Cameron, Li, Rodriguez-Juarez, Snelling, Hudson, Fiselier, Kovalchuk and Kovalchuk2023). Inflammation can also be induced via the gut-brain-axis by orally administering dextran sulphate sodium (DSS) to mice or rats to induce acute colitis. This has been shown to lead to immune responses in the gastrointestinal tract as well as in the CNS (Dempsey et al., Reference Dempsey, Abautret-Daly, Docherty, Medina and Harkin2019). In a recent study using this model, post-treatment with psilocybin and eugenol – a positive allosteric modulator of the gamma-aminobutyric acid (GABA)-A receptor – reduced expression of pro-inflammatory cytokines and markers IL-1β, IL-6, and COX-2 in the brain (Zanikov et al., Reference Zanikov, Gerasymchuk, Robinson, Gojani, Asghari, Groves, Cameron, Rodriguez-Juarez, Snelling, Hudson, Fiselier, Kovalchuk and Kovalchuk2024). These results demonstrate a clear immunomodulatory property of psilocybin in the context of induced systemic inflammation.

Streptozotocin (STZ), a glucosamine–nitrosourea compound leading to an insulin-resistant brain state, has been previously used to induce an Alzheimer’s disease model in the rat as intraperitoneal or intracerebroventricular injection of STZ leads to production of amyloid-beta (Kadhim et al., Reference Kadhim, Al-Mumen, Nahi and Hamidi2022). Using this model, Afshar et al. (Reference Afshar, Shahidi, Rohani, Soleimani Asl and Komaki2019) reported that both a selective antagonist of 5-HT_1AR (NAD-299) and a selective agonist of 5-HT_2AR (TCB-2) reduced oxidative stress in the hippocampus and provided neuroprotection. In support of this, a separate study using an Aβ-induced mouse model of AD found that N,N-DMT alleviated astrocytic activation and astrogliosis in the hippocampus and dentate gyrus measured via GFAP immunostaining (Borbély et al., Reference Borbély, Varga, Szögi, Schuster, Bozsó, Penke and Fülöp2022).

N,N-DMT administration also produced anti-inflammatory and pro-neurotrophic effects in a rat ischaemic brain injury model. Pretreatment with N,N-DMT led to lower expression of both mRNA and protein levels of a key activator of the apoptotic cascade (Apoptotic Protease Activating Factor 1 – APAF-1), and higher BNDF expression in this model, as well as a reduction in TNF-α, IL-1β, IL-6 and an increase in IL-10 levels (Nardai et al., Reference Nardai, László, Szabó, Alpár, Hanics, Zahola, Merkely, Frecska and Nagy2020).

Szabo et al. (Reference Szabo, Varga, Dvoracsko, Farkas, Kormoczi, Berkecz, Kecskes, Menyhart, Frank, Hantosi, Cozzi, Frecska, Tomboly, Krizbai, Bari and Farkas2021) also demonstrated a neuroprotective effect in the ischaemic rat brain following N,N-DMT administration, with a reduction in the number of apoptotic cells and improved astrocytic survival. However, they demonstrated this effect to be mediated by Sig-1R activation. In a more recent study, pre-treatment with psilocybin reduced brain infarction and neurological deficits following induced stroke in rats, whereas post-treatment also led to downregulation of Iba1 (Yu et al., Reference Yu, Wu, Wang, Bae, Chianelli, Bambakidis and Wang2024). These effects were attenuated by the BDNF inhibitor and TrkB antagonist ANA12.

A recent study used caecal ligation and puncture to model sepsis in rats and assess how ayahuasca might produce anti-inflammatory and neuroprotective effects (de Camargo et al., Reference de Camargo, Joaquim, Machado, de Souza Ramos, da Rosa, de Novais Junior, Mathias, Maximiano, Strickert, Nord, Gava, Scarpari, Martins, Lins, Chaves, da Silva, de Oliveira, da Silva, Fernandes, Tiscoski, Piacentini, Santos, Inserra, Bobinski, Rezin, Yonamine, Petronilho and de Bitencourt2024). Ayahuasca pre-treatment increased levels of IL-4 and BDNF in the cortex, while enhancing neutrophil activation and decreasing nitric oxide signalling. At a behavioural level, ayahuasca reduced anxiety-like measures in behavioural tests, suggesting that it can prevent sepsis-induced neuroinflammatory and oxidative stress, and reduce anxiety-like behaviour.

Overall, these results suggest that modulation of serotonin receptors, and possibly (SIG-1R), offer protection against neuroinflammation induced by various disease salient stimuli, highlighting the potential role of the serotonergic system and the Sig-1R in inflammation associated neurodegenerative processes.

Studies have also found that psychedelics modulate the expression of immunological factors reported to be increased in stress associated behavioural paradigms. Kelley et al. (Reference Kelley, Venable, Destouni, Billac, Ebenezer, Stadler, Nichols, Barker and Francis2022) measured an increased expression of IL-1β and its receptor IL1r1 in a rat model for post-traumatic stress disorder in the prefrontal cortex. They found that IL1r1 was significantly reduced after treatment with N,N-DMT, or a combination of N,N-DMT and harmaline, as well as TLR4, TLR6, and TLR7 (Kelley et al., Reference Kelley, Venable, Destouni, Billac, Ebenezer, Stadler, Nichols, Barker and Francis2022). In response to repeated social aggression, stressed mice exhibit elevated cytokine gene expression and increased TNFα levels in plasma and CSF (Krupp et al., Reference Krupp, Yaeger, Ledesma, Withanage, Gale, Howe, Allen, Sathyanesan, Newton and Summers2024). A single administration of (R)-DOI after stress induction reduced plasma and limbic brain levels of TNFα and promoted escape, a dynamic coping strategy, indicating anxiolytic effects.

This emerging evidence suggests potent immunomodulatory properties of classical psychedelics, particularly (R)-DOI and psilocybin. In vivo studies in both mice and rats have demonstrated that 5-HT_2AR agonists can influence responses in immune-challenge models via a suppression of factors associated with the inflammatory response system (Nau et al., Reference Nau, Yu, Martin and Nichols2013; Flanagan et al., Reference Flanagan, Sebastian, Battaglia, Foster, Cormier and Nichols2019a, b, Reference Flanagan, Billac, Landry, Sebastian, Cormier and Nichols2021; Nardai et al., Reference Nardai, László, Szabó, Alpár, Hanics, Zahola, Merkely, Frecska and Nagy2020; Zanikov et al., Reference Zanikov, Gerasymchuk, Ghasemi Gojani, Robinson, Asghari, Groves, Haselhorst, Nandakumar, Stahl, Cameron, Li, Rodriguez-Juarez, Snelling, Hudson, Fiselier, Kovalchuk and Kovalchuk2023), a decrease in CD8⁺ T-cells proportion (Davydova et al., Reference Davydova, Cheido, Gevorgyan and Idova2010), and a reduction in oxidative stress (Afshar et al., Reference Afshar, Shahidi, Rohani, Soleimani Asl and Komaki2019; Szabo et al., Reference Szabo, Varga, Dvoracsko, Farkas, Kormoczi, Berkecz, Kecskes, Menyhart, Frank, Hantosi, Cozzi, Frecska, Tomboly, Krizbai, Bari and Farkas2021). Interestingly, as referred to earlier, psilocybin led to a general activation of the immune system with a release of pro- and anti-inflammatory cytokines in healthy unstimulated rats, suggesting possibly different responses based on activation state (Bove and Mokler, Reference Bove and Mokler2022). Similarly, (R)-DOI treatment has been associated with reduced response induced in the OVA model, such as a significant reduction in mucus production and pulmonary inflammation (Flanagan et al., Reference Flanagan, Sebastian, Battaglia, Foster, Cormier and Nichols2019a), and normalised glucose homeostasis in the ApoE^-/- model, in mice (Flanagan et al., Reference Flanagan, Sebastian, Battaglia, Foster, Maillet and Nichols2019b).

According to these preclinical in vivo studies, psychedelics appear to have a dual effect on the immune system. Under normal conditions, without external triggers, psychedelics tend to stimulate immune system activity. However, when the immune system is already activated by external factors, such as with LPS administration, psychedelics seem to inhibit the immune response to these triggers. This suggests that psychedelics may have a regulatory effect on immune function, potentially stimulating or suppressing immune activity depending on the body’s current state.

Human studies

Following administration of a high dose of psilocybin in healthy volunteers, Hasler et al. (Reference Hasler, Grimberg, Benz, Huber and Vollenweider2004) observed a significant rise in thyroid–stimulating hormone (TSH), prolactin, adrenocorticotropic hormone (ACTH), and cortisol. Interestingly, this increased hormonal release wasn’t associated with heightened anxiety. The authors suggest this observation aligns with known effects of 5-HT₂ receptor stimulation, which can trigger the HPA axis, leading to elevated ACTH and cortisol levels. Given that cortisol is known for its anti-inflammatory properties (Rhen and Cidlowski, Reference Rhen and Cidlowski2005), it might be expected that elevated cortisol levels could influence inflammatory markers. Conversely, Burmester et al. (Reference Burmester, Madsen, Szabo, Aripaka, Stenbaek, Frokjaer, Elfving, Mikkelsen, Knudsen and Fisher2023) reported no change in circulating inflammatory markers [C-reactive protein (CRP) and TNF-α] after a single psilocybin dose in 16 healthy participants. Similarly, four weeks after a psilocybin dose, a transient increase in peripheral cytokine production was reported. However, this increase was not consistent across different patient populations which included healthy participants, patients with depression, anxiety, and cancers of various types (DiRenzo et al., Reference DiRenzo, Barrett, Perin, Darrah, Christopher-Stine and Griffiths2024).

Studies on N,N-DMT, a key component of ayahuasca, indicate its potential ability to acutely influence neuroendocrine and immune markers. After N,N-DMT administration using freeze-dried ayahuasca capsules, Dos Santos et al. (Reference Dos Santos, Valle, Bouso, Nomdedéu, Rodríguez-Espinosa, McIlhenny, Barker, Barbanoj and Riba2011) observed decreased CD4⁺ and CD3⁺ cell populations in healthy individuals and those with treatment-resistant depression (TRD), increased proportion of NK cells, and increased prolactin and cortisol levels. Notably, this modulatory effect was transient, with peak effects occurring two hours after intake and returning to baseline within 24 hours. A larger study by Galvao et al. (Reference Galvao, De Almeida, Silva, Freire, Palhano-Fontes, Onias, Arcoverde, Maia-De-Oliveira, De Araujo, Lobao-Soares and Galvao-Coelho2018) demonstrated a rise in salivary cortisol levels in both healthy controls and patients with TRD during the ayahuasca session compared to placebo, returning to baseline within 48 hours. Interestingly, subsequent research revealed a significant correlation between greater reductions in CRP and lower depressive symptoms at 48 hours post-ayahuasca in both patient and control groups (Galvao-Coelho et al., Reference Galvao-Coelho, De Menezes Galvao, De Almeida, Palhano-Fontes, Campos Braga, Lobao Soares, Maia-De-Oliveira, Perkins, Sarris and De Araujo2020). These findings suggest a potential link between ayahuasca’s antidepressant action and its effect on neuroendocrine and immune systems.

Preliminary evidence also indicates that 5-MeO-DMT has immunomodulatory properties. In experienced psychedelic users, a significant decrease in salivary IL-6 and a rise in cortisol was measured after 5-MeO-DMT inhalation (Uthaug et al., Reference Uthaug, Lancelotta, Szabo, Davis, Riba and Ramaekers2020). However, further investigation is required as this study lacked a suitable placebo, had a small sample size, and used variable dosing. 5-MeO-DMT is similarly active at 5-HT_2A and 5-HT_1A receptors and binds to 5-HT_1AR as a biased agonist, similar to the partial agonist and anxiolytic buspirone (Warren et al., Reference Warren, Lankri, Cunningham, Serrano, Parise, Kruegel, Duggan, Zilberg, Capper, Havel, Russo, Sames and Wacker2024).

Table 1. Summary of in vitro studies having measured immunomodulatory properties of 5-HT_2AR psychedelics

Table 2. Summary of in vivo studies, both in rodents and humans, having measured immunomodulatory properties of 5-HT2AR psychedelics