Involvement of 5-HT2A receptors in the effects of a classic psychedelic on threat- and stress-related behavior in mice.

We currently witness a revival of interest in clinical explorations of 3,4-methylenedioxymethamphetamine (MDMA) and psilocybin in the treatment of psychiatric disorders. MDMA-assisted psychotherapy for post-traumatic stress disorder (PTSD) led to acute reduction in fear and long-term treatment efficacy. Psilocybin-assisted psychotherapy caused a long-lasting reduction of depressive symptoms in treatment-resistant patients. The American Food and Drug Administration has attributed a “breakthrough therapy” designation for MDMA and psilocybin in the treatment of PTSD and depression respectively while the Australian government has made it possible to prescribe these drugs under specific circumstances. However, the biological mechanisms that may contribute to the observed clinical effects of these compounds are still under the investigation.

MDMA increases the synaptic concentration of serotonin and other monoamines, and its effects on perception, emotional excitation and resilience to negative emotions depend on activation of 5-HT2A receptors. Conversely, psilocybin directly activates 5-HT2A receptors, and this is believed to drive its profound mind-altering effects. Nevertheless, it is a subject of ongoing debate whether the psychotropic effects elicited by hallucinogens are crucial for their therapeutic effects in humans. While activation of 5-HT2A receptors may drive neuroplastic changes, required for learning and adaptation to environmental challenges, drug-mediated decreases in 5-HT2A receptor function and expression have also been described for drugs with a documented efficacy in treatment of PTSD and depression. Moreover, recent preclinical works raise the possibility that anxiolytic, antidepressant and stress-ameliorating properties of classic psychedelics may be independent from their activity towards 5-HT2A receptors.

This thesis aims to explore how 5-HT2A activation may contribute to the beneficial effects of hallucinogens in the context of threat- and stress-related behavior. Since MDMA and classic psychedelics activate many pharmacological targets, we used a synthetic psychedelic with a fairly specific affinity towards 5-HT2A and 5-HT2C receptors, 2,5-dimethoxy-4-iodoamphetamine (DOI). We studied its effects on the behavior of C57BL/6J mice.

The first experimental part of the thesis focuses on fear and anxiety-like behavior in wild-type and 5-HT2A receptor knock-out (KO) mice. Administration of DOI had no effect on spatial working memory, as it did not impair performance in a spontaneous alternation Y-maze task but reduced anxiety-like avoidance behavior in the elevated plus maze and elevated zero maze tasks. The potentially anxiolytic effects of DOI were not observed in 5-HT2A KO mice, and we saw no evidence of psychostimulant effects of DOI in mice receiving the drug before a placement in familiar environment. Moreover, we found that DOI did not block memory recall but diminished fear expression in a passive avoidance task, which was also associated with an increased exploration of potentially threatening area of the apparatus. To examine if activation of 5-HT2A receptors could facilitate exposure-based psychotherapy, we tested the capacity of DOI to facilitate or improve the extinction of fear in an auditory fear extinction paradigm, a rodent model of a human exposure therapy. Application of DOI before fear extinction session has reduced the expression of fear in mice, but it did not improve the efficacy of the extinction training. The effect of DOI on fear expression was abolished in 5-HT2A receptor KO mice, suggesting the dependence of acute fear-related effects on these receptors. We next measured the neuronal response to DOI. Administration of DOI induced a significant increase of c-Fos expression in specific amygdala nuclei. Moreover, local infusion of the 5-HT2A receptor antagonist M100907 into the amygdala reversed the effect of systemic administration of DOI on fear expression while local administration of DOI into the amygdala was sufficient to suppress fear expression. Our data demonstrate that activation of 5-HT2A receptors in the amygdala are necessary and sufficient for suppression of fear expression by a psychedelic but provide no evidence for improvement of fear extinction by the drug.

The second experimental part of the thesis focused on the relationship between 5-HT2A receptors and stress coping behavior. Depression is frequently precipitated by exposure to stressful life events and the presence of this disorder is characterized by difficulties in stress coping and increased sensitivity to stressful events. In preclinical studies, exposure to severe stress has been associated with an increased sensitivity and expression of 5-HT2A receptors and classic psychedelics are known to induce a rapid downregulation of these receptors. We examined if the beneficial effects of classic psychedelics could be mediated by modulation of 5-HT2A receptor levels. Using the mouse forced swim test model, we examined whether the administration of DOI would downregulate 5-HT2A receptor levels in the medial prefrontal cortex and revert stress-induced changes in behavior. Mice subjected to swim stress developed a passive stress coping strategy when tested in the forced swim test 6 days later. This change in behavior was not associated with the hypothesized increase in 5-HT2A receptor-dependent head twitch behaviors or consistent changes in 5-HT2A receptor levels in the cortex. When DOI was administered 1 day before the forced swim test, we found no beneficial effect on immobility, a behavioral indicator of a passive stress coping. Nevertheless, DOI evoked a dose-dependent decrease in 5-HT2A levels in the medial prefrontal cortex of mice previously exposed to swim stress. Our findings do not support the hypothesis that the downregulation of 5-HT2A receptors in the mPFC contribute to the antidepressant-like properties of classic psychedelics.

Taken together, these data contribute to growing knowledge on 5-HT2A receptors and their role in the effects of hallucinogenic compounds.