nNOS-expressing neurons in the ventral tegmental area and substantia nigra pars compacta

Abstract

GABA neurons in the ventral tegmental area (VTA) and substantia nigra pars compact (SNc) play key roles in reward and aversion through their local inhibitory control of dopamine neuron activity and through long-range projections to several target regions including the nucleus accumbens. It is not clear if some of these GABA neurons are dedicated local interneurons or if they all collateralize and send projections externally as well as making local synaptic connections. Testing between these possibilities has been challenging in the absence of interneuron-specific molecular markers. We hypothesised that one potential candidate might be neuronal nitric oxide synthase (nNOS), a common interneuronal marker in other brain regions. To test this, we used a combination of immunolabelling (including antibodies for nNOS that we validated in tissue from nNOS-deficient mice) and cell-type-specific virus-based anterograde tracing in mice. We found that nNOS-expressing neurons, in the parabrachial pigmented (PBP) part of the VTA and the SNc were GABAergic and did not make detectable projections, suggesting they may be interneurons. In contrast, nNOS-expressing neurons in the Rostral Linear Nucleus (RLi) were mostly glutamatergic and projected to a number of regions, including the lateral hypothalamus, the ventral pallidum, and the median raphe nucleus. Taken together, these findings indicate that nNOS is expressed by neurochemically- and anatomically-distinct neuronal sub-groups in a sub-region-specific manner in the VTA and SNc.

Significance Statement GABA neurons in the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) play important roles in reward and aversion through their local control of dopamine neuron activity and long-range projections to regions such as the nucleus accumbens. It is not clear if some of these neurons are dedicated interneurons, or if they all project externally and synapse locally. We find that nNOS is expressed by some GABAergic neurons that do not make detectable projections, suggesting that they may be interneurons. In addition, nNOS is expressed by a subgroup of glutamatergic neurons that project to regions including the ventral pallidum and median raphe nucleus. Our study paves the way for future investigation of the function of these molecularly-defined populations.