The effect of carbachol on the excitatory synaptic transmission was studied in rat neostriatal neurons using intracellular and whole-cell voltage clamp-recording methods. Depolarizing excitatory postsynaptic potentials (EPSPs) were evoked by cortical stimulation. Superfusion of carbachol (0.01-3 microM) reversibly decreases the EPSP amplitude in a concentration-dependent manner and with an estimated IC50 of 0.3 microM. While, neither the N-methyl-D-aspartate (NMDA, 100 microM)- nor (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA, 100 microM)-induced response was affected by carbachol (0.1 microM). In addition, the inhibitory effect induced by carbachol at a low concentration of 0.1 microM was attenuated by 4-diphenylacetoxy-N,N-methyl-piperidine (4-DAMP), a selective M3 muscarinic receptor antagonist. However, other muscarinic subtype (M1 or M2) antagonists could also block the inhibitory effect by carbachol 0.1 microM. The rank order of antagonist potency was: 4-DAMP (M3 antagonist) > methoctramine (M2 antagonist) > pirenzepine (M1 antagonist). Based on these findings, we conclude that carbachol at a low concentration (< or = 0.1 microM) reduced the excitatory response of neostriatal neurons following cortical stimulation via presynaptic M3 muscarinic receptors located on the terminals of corticostriatal neurons.