Apomorphine-induced modulation of neural activities in the ventrolateral striatum of rats

The dopaminergic system in the ventrolateral portion of the striatum (Svl), part of the basal ganglia, regulates orofacial movements; bilateral co-stimulation of both dopamine D1 -like and D2 -like receptors elicits repetitive jaw movements in rats. However, how the activities of Svl neurons are modulated by the activation of dopaminergic receptors remains unknown. We systematically injected apomorphine, a non-selective dopamine receptor agonist that induced jaw movements under urethane anesthesia, and performed multi-channel unit recording from Svl neurons. The Svl neurons were classified into two subgroups: (1) the phasically active (PA) neurons represented by mainly the medium spiny neurons and the GABAergic interneurons in part, and (2) the tonically active (TA) neurons composed of mainly the cholinergic interneurons. Apomorphine modulated PA neuron firing frequency with wide variability; 33.3% of the PA neurons were facilitated, while 38.3% were suppressed. In the majority of TA neurons, the firing frequency was reduced by apomorphine (71.1%). The cross-correlations between PA and PA, PA and TA, and TA and TA neurons were analyzed, and pairs of PA neurons and pairs of PA and TA neurons, showed negligible apomorphine-induced effect on the number of synchronized spikes. In contrast, pairs between TA neurons showed a consistent decrease in the number of synchronized spikes. The apomorphine-induced suppression of TA neuron activities with decreased synchronized outputs is likely to reduce the amount of locally released acetylcholine, which may contribute to the induction of apomorphine-induced jaw movements in rats.