Systemic administration of the muscarinic receptor antagonist, scopolamine, augments, whereas the muscarinic receptor agonist, oxotremorine, attenuates behaviors (locomotion and stereotypies) and preprodynorphin (PPD) and substance P (SP) gene expression in striatonigral neurons induced by the indirect dopamine receptor agonist, amphetamine (AMPH). In contrast, systemic scopolamine blocks, whereas oxotremorine augments, AMPH-stimulated preproenkephalin (PPE) gene expression in striatopallidal neurons. This study investigated the site of action of these effects by administering scopolamine and oxotremorine directly into the striatum and assessing the expression of neuropeptide mRNAs with quantitative in situ hybridization. Unilateral injection of scopolamine into the dorsal striatum augmented, and oxotremorine attenuated, AMPH (2.5 mg/kg, i.p.)-stimulated behaviors. Intrastriatal scopolamine at a concentration of 62 mM, but not 6.2 mM, increased basal levels of PPD and SP mRNAs in the dorsal striatum. In addition, both 6.2 and 62 mM scopolamine significantly augmented AMPH-stimulated PPD and SP mRNA levels. Intrastriatal infusion of 1.6 or 8.1 mM oxotremorine did not alter basal levels of striatal PPD and SP mRNAs. However, both concentrations of oxotremorine completely blocked AMPH-stimulated SP mRNA and oxotremorine at 8.1 mM blocked AMPH-stimulated PPD mRNA. In contrast, PPE induction by AMPH was blocked by 62, but not 6.2, mM scopolamine. Both concentrations of oxotremorine tended to augment basal and AMPH-stimulated PPE mRNA in the dorsal striatum but the trend was not significant. These data demonstrate an inhibition of striatonigral, and facilitation of striatopallidal, gene expression through activation of local striatal muscarinic receptors, which is consistent with the changes seen after systemic administration of muscarinic agents. Therefore, muscarinic cholinergic regulation of basal and stimulated expression of neuropeptide mRNA is processed within the striatum.