Medial Prefrontal Cortex Innervation of the Basal Forebrain Regulates Behavioral Activation with Prelimbic and Infralimbic Projections Contributing to Distinct Affective Functions in Rats

Abstract

The medial prefrontal cortex (mPFC) regulates affective and motivational behaviors through its projections to subcortical targets, yet the functional contribution of its innervation of the basal forebrain (BF) remains poorly understood. Here, we combined viral tracing, neural activity mapping, molecular characterization, and pathway-specific optogenetic manipulations in adult rats (27 males, 28 females) to investigate the role of prelimbic (PrL) and infralimbic (IL) projections to BF. Using retrograde tracing combined with stress-induced c-Fos mapping, we showed that BF-projecting neurons in both PrL and IL cortices are robustly recruited by acute and chronic stress. Optogenetic activation of these neurons in the BF increased activity, reduced conditioned freezing, and led to real-time place avoidance, indicating a role in behavioral activation and fear suppression. Subregion-specific manipulations of mPFC revealed a functional dissociation. Selective activation of the IL-BF pathway increased general activity, suppressed freezing during extinction, and produced place preference. In contrast, activation of the PrL-BF projections produced place avoidance without altering activity levels or conditioned fear responses. Anatomical analysis revealed that BF-projecting mPFC neurons represent a heterogenous population that may express CaMKIIa as well as different calcium-binding proteins, while some of their axonal terminals in the BF contain VGluT2 or are located in close apposition to PSD95-positive postsynaptic puncta. The majority of the postsynaptic targets were hDlx-expressing inhibitory neurons in the ventral pallidum. Together, these findings identify the BF as a key target in prefrontal control of affective functions, through which PrL and IL projections differentially regulate motivational state, valence processing, and fear memory. Significance Statement The medial prefrontal cortex (mPFC) exerts top-down control over affective behaviors through subcortical projections, with distinct subregions often mediating opposing or complementary functions. We identify the basal forebrain (BF) as a critical mPFC target through which prelimbic (PrL) and infralimbic (IL) glutamatergic projections exert dissociable effects. IL inputs to the BF, which mainly target inhibitory neurons in the ventral pallidum, promote behavioral activation, suppress fear expression, and produce place preference, reflecting positive valence. In contrast, the PrL to BF pathway, which exhibits comparatively sparse innervation, conveys aversive signals and drives behavioral avoidance. These findings reveal a novel circuit mechanism by which medial prefrontal subregions differentially regulate motivational state, valence processing, and fear memory.