PREFRONTAL LAYER 6 <EM >DRD1A-CRE+</EM> NEURONS SERVE AS A NEUROMODULATORY HUB OF CORTICAL NETWORK AROUSAL AND REWARD LEARNING IN MICE

Cortical layer 6 (L6) neurons play an important role in driving cortical arousal via the higher-order corticothalamic system. Many of these neurons are remnants of the subplate, which is involved in thalamocortical circuit development and implicated in neurodevelopmental disorders like autism and schizophrenia. In the somatosensory cortex, a L6 neuronal subpopulation expressing the dopamine D1 receptor (L6-Drd1a) selectively projects to higher-order thalamic nuclei and is highly sensitive to the wake-promoting neuropeptide orexin, playing an important role in gating cortical arousal and anxiety behavior. However, in the prefrontal cortex (PFC), the anatomical connectivity and neuromodulatory response of L6-Drd1a neurons, alongside their functional role in reward learning, remain elusive. Thus, we (i) identified PFC L6-Drd1a projection targets using anterograde adeno-associated viral targeting, (ii) characterized their dopaminergic and serotonergic modulation using whole-cell patch clamp and multielectrode array recordings, and (iii) investigated their in vivo activity compared to L5-Rbp4 neurons using freely-moving fiber photometry recordings during learning. We found that (i) PFC L6-Drd1a neurons project to higher-order thalamic nuclei, midbrain nuclei, and ventral hippocampus, (ii) dopamine D1 receptor agonist SKF-81297 and serotonin 5-HT2A receptor agonist psilocin selectively excite L6-Drd1a neurons to induce regional patterns of PFC network activity, and (iii) L6-Drd1a neurons respond to reward while L5-Rbp4 neurons respond to movement initiation during operant conditioning and probabilistic reversal learning. These results elucidate a novel role for PFC L6-Drd1a neurons as a neuromodulatory hub of cortical network arousal that facilitate reward learning in a laminar, cell type-specific manner, with relevant therapeutic implications for learning-related disorders.