Dopamine D1 and D2 receptor agonists induce opposite changes in the firing rate of ventral pallidal neurons.

Selective dopamine D1 and D2 agonists were used to determine the contributions of each receptor subtype in the modulation of firing rate of ventral pallidum/substantia innominata (VP/SI) neurons. Administration of cumulative doses of the D2 agonist, quinpirole, decreased activity in 59% of the VP/SI cells tested. The decrease in firing rate was dose-dependent between 0.002-0.2 mg/kg i.v. and was blocked by the D2 antagonist, sulpiride (12.5 mg/kg i.v.). In addition, the magnitude and the distribution of responses of VP/SI neurons was not changed following administration of quinpirole as a single versus a divided cumulative dose of 0.1 mg/kg. In contrast, administration of the D1 agonist, SKF38393, excited 69% of the neurons sampled. Similar maximal responses were observed following administration of either a single or a divided cumulative dose of 3.2 mg/kg of SKF38393. The D1 receptor antagonist, SCH23390 (0.1-0.4 mg/kg i.v.) often attenuated the SKF38393-induced increases. The results illustrate that, (1) VP/SI neurons are sensitive to systemically administered dopamine agonists, (2) D1 or D2 receptor activation is sufficient to change the activity of these neurons and (3) these selective agonists mediate opposite effects on VP/SI neuronal activity. These differential responses contrast with effects observed for other dopaminoceptive brain regions, and distinguish VP/SI neurons from morphologically related neurons of the dorsal globus pallidus.