Psychedelic neuroplasticity of cortical neurons lacking 5-HT2A receptors

Classical psychedelic drugs show promise as a treatment for major depressive disorder and related psychiatric disorders. This therapeutic efficacy stems from long-lasting psychedelic-induced neuroplasticity onto prefrontal cortical neurons and is thought to require the postsynaptic expression of serotonin 2A receptors (5-HT_2AR). However, other cortical regions such as the granular retrosplenial cortex (RSG) - important for memory, spatial orientation, fear extinction, and imagining oneself in the future, but impaired in Alzheimer's disease - lack 5-HT_2AR and are thus considered unlikely to benefit from psychedelic therapy. Here, we show that RSG pyramidal cells lacking postsynaptic 5-HT_2A receptors still undergo long-lasting psychedelic-induced synaptic enhancement. A newly engineered CRISPR-Cas-based conditional knockout mouse line reveals that this form of psychedelic-induced retrosplenial plasticity requires presynaptic 5-HT_2A receptors expressed on anterior thalamic axonal inputs to RSG. These results highlight a broader psychedelic therapeutic utility than currently appreciated, suggesting potential for augmenting RSG circuit function in Alzheimer's disease, post-traumatic stress disorder, and other neuropsychiatric conditions, despite the lack of postsynaptic 5-HT_2A receptors.