5-HT2A receptors in the prelimbic cortex VIP-expressing interneurons: A mechanism for psychedelic-induced innate fear attenuation.

Fear-related psychiatric disorders such as phobias and anxiety are insufficiently treated by current therapies. Psychedelics targeting 5-HT A receptors show promise in modulating fear circuits, but their specific neurobiological substrates remain unclear. This study investigates the effects of 4-acetoxy-N,N-dimethyltryptamine (4-AcO-DMT), a psychedelic compound, on innate fear responses and elucidates its underlying cortical microcircuits, focusing on predator odour-evoked 22-kHz ultrasonic vocalizations (USVs) in rats. Using an optimized paradigm of 2,4,5-trimethylthiazoline (TMT)-induced 22-kHz USVs in rats, the effects of 4-AcO-DMT were evaluated, and the dose-response relationships were investigated. In behavioural experiments, we demonstrated that 4-AcO-DMT suppresses 22-kHz USVs by reducing fear-related response in rats. Brain-wide EGR1 expression and targeted brain region lesions were employed to identify the brain region through which 4-AcO-DMT acts. Specific neuronal ablation and chemogenetic manipulation were applied to determine the types of neurons modulated by 4-AcO-DMT, and receptor antagonism and localized receptor knockout strategies were used to identify the receptor subtypes involved. 4-AcO-DMT significantly suppressed TMT evoked 22-kHz USVs via 5-HT A receptor activation. This fear attenuation was mediated by recruitment of VIP interneurons in the prelimbic cortex (PrL), as chemogenetic activation of these cells abolished the behavioural effect. A cortical microcircuit mechanism-5-HT A receptor-driven vasoactive intestinal polypeptide (VIP) interneuron engagement-is critical to 4-AcO-DMT -induced suppression of innate fear responses. Our study reveals that 5-HT A receptor activation by 4-AcO-DMT suppresses innate fear through VIP interneuron-mediated modulation in the prelimbic cortex, advancing the neurobiological understanding of serotonergic psychedelics.