Hypothalamic modulation of the midbrain dopaminergic system

Ventral tegmental area (VTA) dopaminergic (DAergic) and GABAergic neurons are critically involved in mechanisms of reward, reinforcement and emotional arousal. The hypothalamus regulates the homeostatic drive to eat and sends a massive output to the VTA, including projections from neurons containing orexins, the novel neuropeptides, which potently modulate arousal and feeding. Single-unit extracellular and whole-cell patch-clamp recordings, accompanied by the filling of the neuron with biocytin in order to perform post hoc immunostaining, were used to examine the effects of orexins and other hypothalamic neuropeptides on cells in the substantia nigra (SN) and the VTA in vitro. Orexins uniformly excited GABAergic neurons in the SN and the VTA, this effect was blocked by the prior application of a selective protein kinase A inhibitor. A distinct subgroup of GABAergic neurons in the VTA with a slow firing rate (0.8 Hz) was found. In DAergic VTA neurons, orexins caused an increase in firing frequency, burst firing or no change in firing. DAergic neurons in the SN were not affected by orexins. Neurons showing oscillatory firing in response to orexins had smaller afterhyperpolarizations (AHP) than the other groups of dopamine neurons. Single-cell RT-PCR experiments revealed that the calcium binding protein calbindin that is usually present in cells with the smaller AHPs, was only expressed in neurons, which also expressed orexin receptors. All VTA neurons from a recently described group, which express both TH and GAD, expressed orexin receptors and did not express calbindin. In the VTA, in contrast to dorsal raphe, the expression of both orexin receptors was not related to the presence or absence of transient receptor potential canonical channel (TRPC) subunits. Orexins did not affect field potentials in ventral and dorsal striatum. Another stimulator of food intake, neuropeptide Y (NPY) inhibited half of the DAergic and GABAergic neurons in the VTA, whereas the anorectic neuropeptide corticotropin-releasing factor (CRF), which exerts anxiety and arousal, excited a subgroup of DAergic neurons and all tested GABAergic neurons as well. Melanin-concentrating hormone (MCH), agouti-related protein (AGRP), ghrelin, leptin and cocaine and amphetamine-related transcript (CART) did not affect membrane potential or firing rate of the VTA neurons. Substance P (SP) increased the firing rate of the majority of DAergic and all tested GABAergic neurons in VTA. Histamine, a strong wake-promoter, did not affect the firing frequency of DAergic neurons but increased the firing of GABAergic neurons in SN and VTA. This effect was blocked by prior …