No evidence for direct physical interaction of 5-HT 2A -mGluR2 receptors in vitro or in vivo

Activation of mGluR2, the primary presynaptic autoreceptor for glutamate in the brain, attenuates the behavioral and electrophysiological effects of psychedelics. The mechanisms behind this are debated, with two competing hypotheses: direct actions via mGluR2/5-HT2A heterodimers, or presynaptic inhibition of glutamate release. In mice expressing tagged receptors, mGluR2 agonist pretreatment reduced the head twitch response induced by the psychedelic DOI. Multiple orthogonal in vivo and in vitro approaches found no evidence for receptor colocalization or oligomerization under basal or agonist-exposed conditions, nor for mGluR2-mediated modulation of 5-HT2A ligand binding. The findings support models where mGluR2 signaling modulates 5-HT2A receptor activity in layer V pyramidal neurons rather than requiring mGluR2/5-HT2A multimers.

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Abstract

Abstract Activation of mGluR2 (metabotropic glutamate receptor 2), the primary presynaptic autoreceptor for glutamate in the brain, is well established to attenuate the psychedelics-mediated behavioral and electrophysiological effects. However, the mechanisms responsible for these actions are controversial. The two competing mechanistic hypotheses have been proposed to explain this phenomenon are: (1) direct actions mediated by mGluR2/5-HT 2A heterodimers, and (2) inhibition of 5-HT 2A -mediated excitation of pyramidal neurons via presynaptic inhibition of glutamate release by mGluR2 receptors. Consistent with prior reports, we show that mGluR2 agonist pretreatment attenuates the head twitch response induced by the psychedelic drug 1-(2,5-Dimethoxy-4-iodophenyl)-2-aminopropane (DOI) in mice engineered to express mGluR2-mCherry and 5-HT 2A -eGFP-CT tagged receptors. We next employed multiple orthogonal in vivo and in vitro approaches to explore the potential for direct physical interactions between mGluR2 and 5-HT 2A receptors. Across all approaches, we found no evidence for receptor colocalization or oligomerization under basal or 5-HT 2A agonist-exposed conditions in vitro or in vivo . Radioligand binding and kinetic analyses revealed no evidence for mGluR2-mediated modulation of 5-HT 2A ligand binding in vitro or in vivo . Collectively, our findings support models in which mGluR2 signaling modulates the activity of Gα q -coupled 5-HT 2A receptors in layer V pyramidal neurons, rather than models positing the requirement of mGluR2/5-HT 2A multimers.