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

The subjective effects that define psychedelics like LSD, psilocybin, and DOI are linked to activation of the serotonin 2A receptor (5-HT2AR), but what differentiates psychedelic from nonpsychedelic 5-HT2AR agonists is unclear. A new ex vivo platform was developed to measure drug-mediated activation of the Gq/11 pathway in mouse brain tissue by tracking inositol monophosphate (IP1) levels. In the frontal cortex of mice, DOI produced time-bound, dose-dependent IP1 increases that correlated with head twitch responses. LSD elevated IP1, while lisuride did not, consistent with their respective psychedelic and nonpsychedelic natures. MDMA also increased IP1, attributed to serotonin release, unlike the serotonin precursor 5-HTP or fluoxetine. This method provides mechanistic insights into psychedelic action and Gq/11-coupled receptors.

Study at a glance

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.