Physiological Properties of Zebra Finch Ventral Tegmental Area and Substantia Nigra Pars Compacta Neurons

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Abstract
The neurotransmitter dopamine plays important roles in motor control, learning, and motivation in mammals and probably other animals as well. The strong dopaminergic projection to striatal regions and more moderate dopaminergic projections to other regions of the telencephalon predominantly arise from midbrain dopaminergic neurons in the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA). Homologous dopaminergic cell groups in songbirds project anatomically in a manner that may allow dopamine to influence song learning or song production. The electrophysiological properties of SNc and VTA neurons have not previously been studied in birds. Here we used whole cell recordings in brain slices in combination with tyrosine-hydroxylase immunolabeling as a marker of dopaminergic neurons to determine electrophysiological and pharmacological properties of dopaminergic and nondopaminergic neurons in the zebra finch SNc and VTA. Our results show that zebra finch dopaminergic neurons possess physiological properties very similar to those of mammalian dopaminergic neurons, including broad action potentials, calcium- and apamin-sensitive membrane-potential oscillations underlying pacemaker firing, powerful spike-frequency adaptation, and autoinhibition via D2 dopamine receptors. Moreover, the zebra finch SNc and VTA also contain nondopaminergic neurons with similarities (fast-firing, inhibition by the μ-opioid receptor agonist [d-Ala2, N-Me-Phe4, Gly-ol5]-enkephalin (DAMGO)) and differences (strong h-current that contributes to spontaneous firing) compared with GABAergic neurons in the mammalian SNc and VTA. Our results provide insight into the intrinsic membrane properties that regulate the activity of dopaminergic neurons in songbirds and add to strong evidence for anatomical, physiological, and functional similarities between the dopaminergic systems of mammals and birds.