Oscillatory firing of dopamine neurons: differences between cells in the substantia nigra and ventral tegmental area

Neuronal oscillations have been suggested to play an important role in information processing in the brain. Using spectral analysis, we have recently shown that the repetitive burst-like firing in many dopamine (DA) neurons in the ventral tegmental area (VTA) can be described as a slow oscillation (SO) in firing rate. In this study, we examined whether DA neurons in the adjacent substantia nigra (SN) also display a SO. DA neurons were recorded extracellularly using the cells/track technique in chloral hydrate-anesthetized rats. Spectral analysis showed that firing patterns of SN DA neurons exhibited a SO similar to that observed in VTA DA neurons. The amplitude of the SO, however, was much reduced in the SN compared with that in the VTA and so was the number of DA neurons qualified as high-SO cells. In high-SO DA neurons, the amplitude of the SO was strongly correlated with the degree of bursting, and this correlation was observed in both the VTA and SN. In low-SO cells, however, the SO was more significantly correlated with the variability of firing than with firing rate and bursting. Since the generation of the SO depends on afferent inputs to DA neurons, a better understanding of its difference between the SN and VTA may provide important insights into the neural networks that control DA neurons in the two areas.