Role of 5-HT2A, 5-HT2C, 5-HT1A and TAAR1 Receptors in the Head Twitch Response Induced by 5-Hydroxytryptophan and Psilocybin: Translational Implications.

There is increasing interest in the therapeutic potential of psilocybin. In rodents, the serotonin precursor, 5-hydroxytryptophan (5-HTP) and psilocybin induce a characteristic 5-HT2A receptor (5-HT2AR)-mediated head twitch response (HTR), which is correlated with the human psychedelic trip. We examined the role of other serotonergic receptors and the trace amine -associated receptor 1 (TAAR1) in modulating 5-HTP- and psilocybin-induced HTR. Male C57BL/6J mice (11 weeks, ~30 g) were administered 5-HTP, 50-250 mg/kg i.p., 200 mg/kg i.p. after pretreatment with 5-HT/TAAR1 receptor modulators, psilocybin 0.1-25.6 mg/kg i.p. or 4.4 mg/kg i.p., immediately preceded by 5-HT/TAAR1 receptor modulators. HTR was assessed in a custom-built magnetometer. 5-HTP and psilocybin induced a dose-dependent increase in the frequency of HTR over 20 min with attenuation by the 5-HT2AR antagonist, M100907, and the 5-HT1AR agonist, 8-OH-DPAT. The 5-HT2CR antagonist, RS-102221, enhanced HTR at lower doses but reduced it at higher doses. The TAAR1 antagonist, EPPTB, reduced 5-HTP- but not psilocybin-induced HTR. We have confirmed the key role of 5-HT2AR in HTR, an inhibitory effect of 5-HT1AR, a bimodal contribution of 5-HT2CR and a role of TAAR1 in modulating HTR induced by 5-HTP. Compounds that modulate psychedelic-induced HTR have important potential in the emerging therapeutic use of these compounds.

Striking Long Term Beneficial Effects of Single Dose Psilocybin and Psychedelic Mushroom Extract in the SAPAP3 Rodent Model of OCD-Like Excessive Self-Grooming.

Obsessive compulsive disorder (OCD) is a highly prevalent disorder that causes serious disability. Available treatments leave 40% or more of people with OCD significantly symptomatic. There is an urgent need for novel therapeutic approaches. Mice that carry a homozygous deletion of the SAPAP3 gene (SAPAP3 KO) manifest a phenotype of excessive self-grooming, tic-like head-body twitches and anxiety. These behaviors closely resemble pathological self-grooming behaviors observed in humans in conditions that overlap with OCD. Following a preliminary report that the tryptaminergic psychedelic, psilocybin, may reduce symptoms in patients with OCD, we undertook a randomized controlled trial of psilocybin in 50 SAPAP3 KO mice (28 male, 22 female). Mice that fulfilled inclusion criteria were randomly assigned to a single intraperitoneal injection of psilocybin (4.4 mg/kg), psychedelic mushroom extract (encompassing the same psilocybin dose) or vehicle control and were evaluated after 2, 4 and 21 days by a rater blind to treatment allocation for grooming characteristics, head-body twitches, anxiety and other behavioral features. Mice treated with vehicle (n=18) manifested a 118.71±95.96 % increase in total self-grooming (the primary outcome measure) over the 21-day observation period. In contrast, total self-grooming decreased by 14.60%±17.90% in mice treated with psilocybin (n=16) and by 19.20±20.05% in mice treated with psychedelic mushroom extract (n=16) (p=.001 for effect of time; p=.0001 for time × treatment interaction). 5 mice were dropped from the vehicle group because they developed skin lesions; 4 from the psilocybin group and none from the psychedelic mushroom extract group. Secondary outcome measures such as head-body twitches and anxiety all showed a significant improvement over 21 days. Notably, in mice that responded to psilocybin (n=12) and psychedelic mushroom extract (n=13), the beneficial effect of a single treatment persisted up to 7 weeks. Mice initially treated with vehicle and non-responsive, showed a clear and lasting therapeutic response when treated with a single dose of psilocybin or psychedelic mushroom extract and followed for a further 3 weeks. While equivalent to psilocybin in overall effect on self-grooming, psychedelic mushroom extract showed superior effects in alleviating head-body twitches and anxiety. These findings strongly justify clinical trials of psilocybin in the treatment of OCD and further studies aimed at elucidating mechanisms that underlie the long-term effects to alleviate excessive self-grooming observed in this study.