Muscarinic cholinergic receptor-mediated modulation on striatal c-fos mRNA expression induced by levodopa in rat brain

To clarify the interactions between dopamine receptors and muscarinic cholinergic receptors by which neurotransmitters may affect genetic responses, we studied the effects of the muscarinic cholinergic agonist, carbachol, and the muscarinic cholinergic antagonist, trihexyphenidyl, on levodopa-induced c-fos messenger RNA (mRNA) expression in rat striatum. Animals were administered levodopa (levodopa with one-tenth dosage of carbidopa), carbachol or thrihexyphenidyl alone or administered in combination as levodopa (100 mg/kg) + carbachol, or levodopa + trihexyphenidyl given as a single bolus. Levodopa given alone increase the expression of c-fos mRNA. Although carbachol or trihexyphenidyl alone was ineffective in inducing c-fos mRNA, the combination of levodopa and carbachol (≥ 0.1 mg/kg) significantly suppressed the induction of c-fos mRNA as compared with levodopa given alone. The combined administration of levodopa and trihexyphenidyl showed a trend toward an additive effect on the induction of c-fos mRNA vs levodopa alone. These findings suggest that the muscarinic cholinergic system may modulate the levodopa-induced c-fos mRNA expression which then regulates the expression of other mRNAs.