Statistical properties of neuronal spike trains in the substantia nigra: Cell types and their interactions

Summary Spike trains from spontaneously active neurons in the substantia nigra of locally anesthetized, immobilized rats could be included in one of three broad categories, on the basis of the form of their autocorrelation histograms and spike waveforms. Initial brief trough (IBT) cells show a tendency not to fire for 10–50 msec after generation of an action potential. The duration of this period, which is seen as an initial trough in the auto-correlation histogram, is inversely related to firing rate for these neurons. Cells of this type fire repetitively at rates above 15–20/sec, and are found predominantly in the pars reticulata of the substantia nigra. Initial prolonged trough (IPT) cells have substantially longer initial troughs (60–250 msec) in their auto-correlation histograms, from which firing probability recovers more gradually. IPT cells have a trough length which increases with increasing firing rate for rates below 3–6/sec, the threshold for repetitive firing. They are found in the pars compacta and probably represent the dopaminergic neurons of that area. Bursty cells are much less numerous than cells of the other types, and are found in both parts of the substantia nigra, although about twice as commonly in pars reticulata. They fire in bursts of variable length, and may represent nigral interneurons. Cross-correlations between these different cell types suggest several potential synaptic interactions between neurons intrinsic to the substantia nigra. In addition, the possibility of shared excitatory or inhibitory input to different classes of neurons in the substantia nigra is discussed, as well as the relation of these statistical properties to neuropharmacological and anatomical observations on neurons of the substantia nigra.