Comparative effects of scopolamine and quinpirole on the striatal fos expression induced by stimulation of D(1) dopamine receptors in the rat.

Treatment of intact rats with the full D(1) dopamine agonist A-77636 induced Fos-like immunoreactivity in the medial and, to a lesser extent, the lateral portions of the striatum. Pretreatment with the muscarinic antagonist scopolamine hydrobromide (1.5-6 mg/kg) potentiated the response to A-77636 and eliminated the mediolateral staining gradient seen after A-77636 alone. Similar effects were not produced by scopolamine methylbromide, which fails to cross the blood-brain barrier, demonstrating that the actions of scopolamine were centrally mediated. The effects of scopolamine were further compared to those of the D(2)-like dopamine agonist quinpirole using a factorial design in which subjects were pretreated with either scopolamine, quinpirole, or a combination of the two drugs before receiving A-77636. Pretreatment with either scopolamine or quinpirole increased staining in the lateral striatum, but the combination of the two drugs was no more effective than was quinpirole alone. Pretreatment with quinpirole, but not scopolamine, resulted in a markedly "patchy" pattern of staining and actually suppressed staining in the region between patches in the medial striatum. These findings demonstrate that there are both differences and similarities between the effects of scopolamine and quinpirole on D(1) agonist-induced Fos expression and suggest that although inhibition of cholinergic neurons may be one of the mechanisms through which the effects of quinpirole are produced, other factors must also contribute.