Characterization of NTS-to-VTA projection neurons reveals higher-order synaptic organization and distinct responsiveness to cholecystokinin.

The midbrain ventral tegmental area (VTA) shapes goal-oriented behaviours, including food intake, via dense dopaminergic projections to many key forebrain areas. In addition, the VTA serves as an integrator of neural inputs from throughout the brain that modulate VTA output to produce broad, state-dependent, changes that reflect the balance of internal and external considerations. The brainstem nucleus of the solitary tract (NTS) is the primary site for integration of viscerosensory and taste information conveyed through the cranial nerves and is essential for the control of food intake. Importantly, the NTS has been shown to form direct connections with the VTA, but little is known about the anatomical and neurophysiological nature of these projecting neurons. Using VTA-targeted retrobead injections, we characterized a subpopulation of NTS neurons that form monosynaptic connections to the VTA distributed across all levels of the nucleus, with higher counts in the medial and caudal NTS. VTA-projecting NTS neurons were largely catecholaminergic and/or GLP-1 containing, with the majority also expressing VGlut2, consistent with an excitatory glutamatergic phenotype. Functionally, VTA-projecting NTS neurons had noticeably different neurophysiological properties compared with unlabelled NTS neurons, with the majority being polysynaptically (higher-order) coupled to the viscerosensory afferents. Moreover, in contrast to unlabelled neurons, recorded VTA-projecting NTS neurons were largely inhibited by the feeding peptide cholecystokinin (CCK). Together, these findings demonstrate the distinct phenotypic and functional properties of VTA-projecting NTS neurons that may play a role in regulating mesolimbic output to control appetitive feeding responses. KEY POINTS: Ventral tegmental area (VTA)-projecting nucleus of the solitary tract (NTS) neurons are distributed rostro-caudally throughout the nucleus and express tyrosine hydroxylase and/or glucagon-like peptide-1 (GLP-1). VTA-projecting NTS neurons are higher order, receiving indirect, polysynaptic vagal afferent input. Most VTA-projecting NTS neurons are inhibited by cholecystokinin.