Snapshot IP 1 Detection Following 5-HT 2 A Receptor Stimulation in the Mouse Brain

Abstract

The distinct subjective effects that define psychedelics such as lysergic acid diethylamide (LSD), psilocybin, or 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) as drug class are causally linked to the 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 nonpsychedelic 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 nonradioactive 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 in homogenates from the frontal cortex in DOI-treated wild-type and 5-HT 2A R knockout (5-HT 2A R-KO) animals compared to other brain regions, namely, striatum and cerebellum. The effect of DOI on mouse frontal cortex IP 1 was time-bound, dose-dependent, and was correlated to head twitch response counts. We observed that IP 1 levels in frontal cortex homogenates from mice treated with LSD and lisuride varying in magnitude, consistent with LSD’s 5-HT 2A R agonism and psychedelic nature and lisuride’s lack thereof. 3,4-Methylenedioxymethamphetamine (MDMA) evoked an increase in the IP 1 signal in the frontal cortex that was not matched by the serotonin precursor 5-hydroxytryptophan or the serotonin reuptake inhibitor fluoxetine. We attribute the differences in the readout primarily to the indirect stimulation of 5-HT 2A R by MDMA via the release of serotonin from its presynaptic terminals. This methodology enables one to capture a snapshot of IP 1 turnover in the mouse brain that can provide mechanistic insights into the study of psychedelics and G q/11 -coupled receptors.