Snapshot of 5-HT 2A receptor activation in the mouse brain via IP 1 detection.

Psychedelics like LSD, psilocybin, and DOI produce their distinct effects by activating the serotonin 2A receptor (5-HT2AR). A new ex vivo method measures drug-induced activation of this receptor in mouse brain tissue by tracking changes in inositol monophosphate (IP1), a downstream signaling molecule. The method was specific to 5-HT2AR, as IP1 increases were absent in knockout mice. Head-twitch response counts, a behavioral correlate of psychedelic effects, correlated with IP1 levels in the frontal cortex. LSD increased IP1, while lisuride, a non-psychedelic 5-HT2AR agonist, did not. MDMA also raised IP1, likely by releasing serotonin, unlike 5-HTP or fluoxetine. This approach offers mechanistic insights into psychedelic and serotonergic drug action.

Study at a glance

Abstract

The distinct subjective effects that define psychedelics such as LSD, psilocybin or DOI as drug class are causally linked to activation of the serotonin 2A receptor (5-HT 2A R). However, some aspects of 5-HT 2A R pharmacology remain elusive, such as what molecular drivers differentiate psychedelic from non-psychedelic 5-HT 2A R agonists. We developed an ex vivo platform to obtain snapshots of drug-mediated 5-HT 2A R engagement of the canonical G q/11 pathway in native tissue. This non-radioactive methodology captures the pharmacokinetic and pharmacodynamic events leading up to changes in inositol monophosphate (IP 1 ) in the mouse brain. The specificity of this method was assessed by comparing IP 1 levels in homogenates from the frontal cortex in DOI-treated wild-type and 5-HT 2A R-KO animals compared to other brain regions, namely striatum and cerebellum. Furthermore, we encountered that head-twitch response (HTR) counts and IP 1 in the frontal cortex were correlated. We observed that IP 1 levels in frontal cortex homogenates from mice treated with LSD and lisuride vary in magnitude, consistent with LSD's 5-HT 2A R agonism and psychedelic nature, and lisuride's lack thereof. MDMA evoked an increase of IP 1 signal in the frontal cortex that were not matched by the serotonin precursor 5-HTP or the serotonin reuptake inhibitor fluoxetine. We attribute differences in the readout primarily to the indirect stimulation of 5-HT 2A R by MDMA via serotonin release from its presynaptic terminals. This methodology enables capturing a snapshot of IP 1 turnover in the mouse brain that can provide mechanistic insights in the study of psychedelics and other serotonergic agents pharmacodynamics.