Multiple single-unit recordings in the striatum of freely moving animals: effects of apomorphine and D-amphetamine in normal and unilateral 6-hydroxydopamine-lesioned rats.

Ensembles of striatal neurons were recorded in freely moving normal and unilateral 6-hydroxydopamine (6-OHDA)-lesioned rats using chronically implanted electrode arrays. Animals received bilateral striatal implants of two 16-microwire arrays 1 week before recordings. Identified striatal neurons were categorized as medium spiny-like and large aspiny-like based on a combination of their activity autocorrelations and firing rates. Baseline firing rates of medium spiny-like neurons in the 6-OHDA-lesioned striata were significantly faster than were firing rates of the same neurons in the intact hemispheres of 6-OHDA-lesioned rats or normal animals. However, firing rates of large aspiny-like neurons were faster in both hemispheres of the 6-OHDA-lesioned rats as compared to normal animals. Interestingly, firing rates of neurons in all groups decreased by fivefold or greater under urethane anesthesia, although the relative firing rates between hemispheres were unchanged. d-Amphetamine (5.0 mg/kg, s.c.) increased the firing rates of both types of striatal neurons by twofold or greater in normal rats and in the intact hemispheres of 6-OHDA-lesioned animals. By contrast, this treatment did not alter neuron firing in the 6-OHDA-lesioned striata. Apomorphine (0.05 mg/kg, s.c.) did not affect neuronal firing rates either in normal rat striatum or in the unlesioned hemispheres of 6-OHDA-lesioned animals. However, it did significantly increase the firing rate of the medium spiny-like neurons in 6-OHDA-lesioned striata. These results demonstrate that the dopaminergic innervation of the striatum differentially influences two electrophysiologically distinct sets of striatal neurons in freely moving rats.