The Effects of Chronic Psilocybin Administration on Physiological, Metabolic and Behavioural Outcomes in Control and High-Fat Diet-Fed Male and Female C57BL/6J Mice

Abstract

Psilocybin, the prodrug, and psilocin, the psychoactive compound and serotonin receptor agonist found in Psilocybe mushrooms has been used by Mesoamerican communities for thousands of years for spiritual and medicinal purposes. In Canada, psilocybin is designated as a Schedule III substance prohibiting its use recreationally and in research. However, in 2018, psilocybin received “Breakthrough Therapy” status from the United States Food and Drug Administration, due to its impressive safety profile, and clinical evidence in the treatment of several psychiatric disorders, including substance-related and addictive disorders, when used in conjunction with psychotherapy. While obesity, which affects 1 in 4 Canadians, can be associated with a number of contributing factors; for those affected by overeating, obesity shares neurobiological and environmental parallels with other addictive disorders. For this reason, psilocybin is of interest as a potential treatment for obesity. To date, only three pre-clinical studies have examined psilocybin in diet-induced obesity models, and none have studied its chronic effects in female animals or effects on metabolic parameters, glucose tolerance and insulin sensitivity. Therefore, this thesis investigated the expression of serotonin (5-HT) receptors in 3T3-L1 adipocyte differentiation, in addition to the physiological, metabolic and behavioural effects of chronic psilocybin administration in female and male mice fed control diet (CD) or high-fat diet (HFD). In doing so, the expression of 5-HT receptors involved in regulating feeding behaviour and satiety (i.e., 5-HTR1B, 5-HTR2A-C, 5-HTR3 and 5-HTR7) were examined in 3T3-L1 adipocytes over 16 days of differentiation. Interestingly, only the expression of 5Htr2a significantly changed during differentiation, further signifying its vital role in regulating lipogenesis. Other than 5-HTR1B, which was expressed but did not vary during differentiation, all other 5-HT receptors showed minimal or no expression in differentiating and mature adipocytes indicating their rather dispensable role in these cells. In subsequent studies, eight-week-old male and female C57BL/6J mice were randomized to receive a 45 kcal% HFD or 10 kcal% CD for 17-weeks to induce divergent body masses, then they were further randomized to also receive weekly i.p. injections of 1 mg/kg psilocybin or vehicle (isotonic saline) for an additional 19-weeks. Towards the end of the injection period, mice were subjected to a test battery consisting of behavioural and metabolic tests: open field testing and elevated plus maze to monitor the movement of mice and to assess for anxiolytic behaviour; the Oxymax Comprehensive Lab Animal Monitoring System by Columbus Instruments® to assess respiratory gas exchange and ambulatory, rearing and total locomotion over a 24-h period; the Low-Profile Wireless Running Wheels by Med Associates® to assess voluntary running; glucose tolerance and insulin sensitivity testing to evaluate glucose response; and food intakes and body weights were measured on a weekly basis for the duration of the study, while organ weights were collected at study termination. Collectively, all studies showed no differences in body weight, food intake, respiratory gas exchange, organ weight, glucoregulation, anxiolytic behaviour or spontaneous movement following chronic psilocybin treatment in HFD-fed male and female mice when compared to the CD-fed mice. Although there were no differences due to drug treatment, differences due to diet were present in nearly all the measures evaluated. Contrary to my hypotheses, psilocybin was unable to correct and restore the physiological, metabolic and behavioural alterations caused by HFD feeding to CD-fed levels. Nonetheless, CD-fed male mice treated with psilocybin had improved insulin sensitivity compared to all other groups. This novel finding leads us to speculate a role for psilocybin in glucose regulation, which may be elucidated in future studies. Therefore, this thesis, through its proposed studies, answered a vital question – chronic psilocybin treatment does not affect satiety or feeding behaviour in mice.