Psychedelic compounds directly excite 5-HT_2ALayer 5 Pyramidal Neurons in the Prefrontal Cortex through a 5-HT_2AGq -mediated activation mechanism

Abstract

Psilocin, the active compound in Psilocybe sp. mushrooms, is a serotonergic psychedelic that has recently gained renewed interest due to its potential as a therapeutic tool. Despite promising clinical findings, the underlying signaling mechanisms and brain region-specific effects of psilocin and other psychedelic drugs remain unclear. Psilocin, like other psychedelic compounds, is an agonist at many serotonin and other biogenic amine receptors; however, activation of serotonin (5-Hydroxytryptamine, or 5-HT) 2A receptors (5-HT2ARs) is understood as the main molecular target for the psychoactive effects in animals and humans. 5-HT2ARs are abundantly expressed in the prefrontal cortex (PFC); however, the biochemical actions of psilocin on PFC neurons remain poorly understood. In this study, we used in vitro slice electrophysiology to examine how psilocin acutely alters the activity and electrophysiological properties of layer 5 pyramidal neurons in the mouse PFC. Focal application of psilocin (10uM) onto nonspecified Layer 5 Pyramidal neurons in the prelimbic PFC of C57BL/6J mice produced variable effects on firing (increase, decrease, or no change). 5-HT2AR layer 5 pyramidal neurons in the mouse prelimbic PFC were identified via labeling in a 5-HT2A-ERT2-Cre mouse crossed with an Ai9 tdTomato reporter. Focal application of psilocin increased firing in all identified 5-HT2AR neurons but did not result in any significant changes in synaptic transmission. Overall, the results demonstrate that psilocin evokes strong firing changes in the PFC that are 5-HT2AR and Gq dependent, thereby providing valuable insights into the effects of psilocin on a brain region implicated in mediating psychedelic drug actions.

A suite of engineered mice for interrogating psychedelic drug actions

“…These findings are important because of a recent report suggesting that many HTR2A agonists may exert their actions via intracellular rather than plasma membrane receptors (Vargas et al, 2023). As we recently reported (Schmitz et al, 2022), psychedelic drugs increase the firing of genetically-identified HTR2A neurons in a manner similar to that reported here for 5-HT. As 5-HT is extracellularly applied, our results are consistent with the hypothesis that the psychedelic actions of HTR2A agonists are mediated by direct actions of pyramidal neuron firing via surface HTR2A receptors.…”

“…Series resistance was continuously monitored with a hyperpolarizing 10 mV pulse; cells with axis resistance >20 MΩ were excluded from the data set. HTR2A expressing neurons were identified via tdTomato expression using fluorescent optics and brief (<2 sec) episodic illumination as previously described (Schmitz et al, 2022). During voltage clamp recording (V hold = −70mV), electrophysiological properties of cells were determined by pClamp 10 Clampex software using a 10 mV pulse delivered after breaking the cellular membrane.…”

Cellular rules underlying psychedelic control of prefrontal pyramidal neurons

“…Although psychedelics do enhance some forms of neuronal excitability, these data indicate that the dominant and net effect is dose-dependent suppression of intrinsic excitability, necessitating a revision of the current paradigm ( 19–24 ). Our acute results differ from some previous studies that concluded that psychedelics increase intrinsic excitability, likely due to prior studies examining RMP changes or using only near-threshold current injections, which fail to reveal the larger intrinsic suppression changes ( 37 , 38 , 40 , 42 ). Additionally, we utilized psychedelic drugs ( 47–50 , 72–76 ), in contrast to many former studies that used 5-HT2AR agonists with non- or unknown psychedelic effects such as serotonin or alpha-methyl-serotonin ( 77–84 ).…”

Psychedelic 5-HT2A receptor agonism: neuronal signatures and altered neurovascular coupling.

“…Apart from neuromodulation [95,[128][129][130][131][132], serotonin has potent vasoactive effects [44,45,133]. For example, serotonin binding triggers vasoconstriction through intracellular signaling pathways involving phospholipase C, calcium release, and protein kinase C phosphorylation [134].…”