Behavioral alterations and loss of caudate modulation in the centrum medianum-parafascicular complex of the cat following electrolytic lesions of the substantia nigra

Extracellular activity in the centrum medianum-parafascicular complex was examined following electrolytic lesions of the substantia nigra in the cat. Three major changes were observed in the activity of these polysensory medial thalamic neurons: (1) normally quiescent in the intact animal, about 70% of the neurons now dsiplayed spontaneous activity; (2) many of these spontaneously active neurons no longer responded to caudate stimulation and the responses to sensory stimulation were not changed; and (3) while caudate stimulation alone did inhibit some neurons, such stimulation in a conditioning test procedure often failed to inhibit the test response to limb stimulation which ‘broke through’ the inhibitory period. All of these changes were correlated with the extent of damage to the ipsilateral substantia nigra. Additionally, a noticeable change was observed in the behavior of the lesioned animals; intense circling, contralateral to the side of the lesion, persisted for several days following surgery. Other symptoms were arrest reactions, abnormal rhythmic pawing and sluggishness of movement. The results indicate that the substantia nigra is an important element in funneling caudatofugal activity to the centromedian-parafascicular complex where it interacts with, and modulates incoming sensory activity. The loss of this modulation and the removal of a tonic background inhibition, indicated by the increased proportion of spontaneously active neurons, may be important factors in producing the observed behavioral abnormalities.