Sources of Synaptic Input to Neurons in the Nucleus Accumbens Shell That Project to the Ventral Pallidum

The shell of the nucleus accumbens (NAcSh) regulates motivation and reward via its dense projection to the ventral pallidum (VP). This ventral striatopallidal system has also been shown to regulate the activity of midbrain dopamine neurons and the release of dopamine in the NAcSh. The present study applied monosynaptic rabies tracing in the rat to quantify the brain-wide sources of synaptic input to neurons in the medial NAcSh that project to the ventromedial VP. The ventral subiculum of the hippocampus (vSub) was the largest source of input cells to the NAcSh-VP projection neurons. Anterograde tracing of vSub-NAcSh projection neurons demonstrated that their fibers terminated densely in the NAcSh largely avoiding other regions of the striatum. Another relatively strong source of input cells included the anterior part of the paraventricular nucleus of the thalamus (aPVT). The CA1, lateral septal nucleus, VP, paratenial thalamic nucleus, bed nucleus of the stria terminalis, lateral preoptic area and dorsomedial nucleus of the hypothalamus were moderately strong sources of input neurons. The prefrontal cortex, amygdala, and the basolateral nucleus of the amygdala were found to be relatively weak sources of input. A lack of sex differences for all the sources of input identified indicates that there is no apparent sexual dimorphism in the afferents to the striatopallidal system. In summary, the vSub and the aPVT are the major sources of cortical and thalamic monosynaptic inputs to the NAcSh-VP projection neurons where these inputs converge to regulate behavior and dopamine release in the NAcSh.