Functional properties of dopamine neurons and co-expression of vasoactive intestinal polypeptide in the dorsal raphe nucleus and ventro-lateral periaqueductal grey

The dorsal raphe nucleus (DRN) and ventrolateral periaqueductal grey (vlPAG) regions contain populations of dopamine neurons, often considered to be a dorsal caudal extension of the A10 group [mostly found in the ventral tegmental area (VTA)]. Recent studies suggest they are involved in promoting wakefulness and mediate some of the antinociceptive and rewarding properties of opiates. However, little is known about their electrophysiological properties. To address this, we used Pitx3-GFP and tyrosine hydroxylase (TH)-GFP mice to carry out targeted whole-cell recordings from this population in acute brain slices. We found that DRN/vlPAG dopamine neurons have characteristics similar to most VTA dopamine neurons, but distinct from dorsal raphe serotonin neurons. They fire broad action potentials at a relatively slow, regular rate, exhibit a hyperpolarization-activated inward current and delayed repolarization, and show spike-frequency adaptation in response to prolonged depolarization. In addition, they receive fast excitatory and inhibitory synaptic inputs. Moreover, we found co-expression of vasoactive intestinal polypeptide in small, periaqueductal dopamine neurons, but generally not in larger, more ventral dopamine neurons.