Electrophysiological properties of nigrothalamic neurons after 6-hydroxydopamine lesions in the rat.

Extracellular recordings were made from electrophysiologically identified nigrothalamic cells in the substantia nigra pars reticulata of anaesthetized rats. The firing rate, firing pattern and responses to striatal stimulation were investigated in normal animals and in animals in which dopamine concentration in the ipsilateral striatum was reduced by more than 90%. At relatively short times after the lesion (less than 10 days) the mean firing rate of the spontaneously active cells in the population was significantly reduced and there was an increase in the occurrence of bursting activity. There was also a significant increase in the number of silent cells, located by antidromic stimulation from the thalamus. In spite of this reduction in mean firing rate the responses of neurons to stimulation of either the ipsilateral striatum or ventromedial thalamus was much larger in cells from lesioned animals. At longer times after the lesion (more than six months) the average firing rate of the neurons had returned to normal but there was still a prevalence of bursting activity and a consequent reduction in mean inter-spike intervals. There was little evidence of the previous hyper-responsiveness to thalamic stimulation but the responsiveness to striatal stimulation was still significantly elevated.