Effects of intracerebroventricular L-DOPA on caudate unit firing

Single units were recorded bilaterally from the caudate nuclei of cats before and after 50 microliter injections of either Merlis solution or L-DOPA (200 microgram) dissolved in Merlis into the left lateral cerebral ventricle. After injection of L-DOPA, but not Merlis solution, there was a period of unit silence in both caudates with an onset of about 10 min and a duration of 30-50 min. This period of caudate silence was coincident with a significant reduction of systolic blood pressure and heart rate. Upon resumption of unit firing in caudate, it was found that the interspike intervals on the contralateral side were markedly increased from 30 min-2 hr postinjection while there was no change on the side of injection. The similarity of this response to that following lesions which interrupt the caudate to thalamus pathway unilaterally was discussed. It is suggested that L-DOPA injection may produce this lesion-like effect by altering the firing of caudate output neurons.