Multimodal interrogation of ventral pallidum projections reveals projection-specific signatures and effects on cocaine reward.

The ventral pallidum (VP) is a central hub in the reward circuitry with diverse projections that have different behavioral roles attributed mostly to the connectivity with the downstream target. However, different VP projections may represent, as in the striatum, separate neuronal populations that differ in more than just connectivity. In this study we performed in mice of both sexes a multimodal dissection of four major projections of the VP - to the lateral hypothalamus (VP→LH), ventral tegmental area (VP→VTA), lateral habenula (VP→LHb) and mediodorsal thalamus (VP→MDT) - with physiological, anatomical, genetic and behavioral tools. We also tested for physiological differences between VP neurons receiving input from nucleus accumbens (NAc) medium spiny neurons (MSNs) that express either the D1 (D1-MSNs) or the D2 (D2-MSNs) dopamine receptor. We show that each VP projection 1) when inhibited during a cocaine conditioned place preference (CPP) test affects performance differently; 2) receives a different pattern of inputs using rabies retrograde labeling; 3) shows differentially-expressed genes using RNA sequencing; 4) has projection-specific characteristics in excitability and synaptic input characteristics using whole-cell patch clamp. VP→LH and VP→VTA projections have different effects on CPP and show low overlap in circuit tracing experiments, as VP→VTA neurons receive more striatal input while VP→LH neurons receive more olfactory input. Additionally, VP→VTA neurons are less excitable while VP→LH neurons are more excitable than the average VP neuron, a difference driven mainly by D2-MSN-responding neurons. Thus, VP→VTA and VP→LH neurons may represent largely distinct populations of VP neurons.Significance statement Many modern studies in neuroscience assign distinct behavioral roles to specific circuits. These behavioral roles are often attributed to the mere connectivity between two brain regions, although different projections may also differ in other aspects and may originate, like in the striatum, from largely separate neuronal populations.The ventral pallidum (VP) is a major structure of the reward system that sends projections to many different targets. In this work we provide for the first time a comprehensive and multimodal characterization of four major outputs of the VP, with evidence for various differences between the projections. We also suggest that two of these projections, to the ventral tegmental area and the lateral hypothalamus originate in largely separate neuronal populations.