Comprehensive characterization of Somatostatin-expressing neurons in the mouse Ventral Tegmental Area

The ventral tegmental area (VTA) in the mammalian brain contains dopaminergic neurons that give rise to the mesolimbic and mesocortical pathways, crucial for motivation, learning, and associated pathologies. Most research on the VTA has been focused on characterizing dopamine (DA) neurons, resulting in detailed knowledge of their heterogeneity, connectivity, and function. However, the VTA also harbours substantial populations of the γ-aminobutyric acidergic (GABA) and glutamatergic (Glu) neurons, which remain understudied. Considering the crucial tuning and synchronizing role of cortical GABA neurons, careful subtyping and description of the VTA GABA neurons are necessary for a better understanding of the reward circuitry organization. The somatostatin (Sst) gene expression marks one of the major inhibitory neuron populations in the cortical areas, yet the properties of this population in the VTA remain unknown. This project aimed to expand knowledge of VTA Sst+ neurons and employed a multimodal approach to study their characteristics from cellular to behavioural levels. Study I investigated the number, location, morphology, electrophysiological properties, and molecular characteristics of Sst neurons in the VTA of mice. By combining patch-clamp recordings, single-cell sequencing, and 3D morphology reconstruction, three distinct subtypes of Sst neurons were identified: afterdepolarizing (ADP), high-frequency firing (HFF), and Delayed neurons. These subtypes exhibited specific electrophysiological properties and differential expression of GABA, Glu, and DA-related genes. Functionally, ADP neurons were shown to participate in local VTA circuitry by inhibiting neighbouring DA cells through GABA release. Study II further described the VTA Sst+ neuron population from a behavioural perspective and discovered their brain-wide connections. Viral tracing revealed VTA Sst+ projections to five forebrain regions, including the ventral pallidum, lateral hypothalamus, central amygdala, bed nucleus of the stria terminalis, and paraventricular thalamic nucleus. Selective deletion of VTA Sst+ neurons resulted in altered responses in tests for stress, morphine sensitization, fear conditioning, and home-cage activity. Moreover, sex differences were observed in several behavioural alterations, suggesting potential divergent network functions of VTA Sst+ neurons between males and females. In summary, this work for the first time describes the previously overlooked somatostatin-expressing neuron population in the mouse VTA and advances our understanding of reward-circuitry organization.