The pleiotropic actions of neuromodulators on pre- and postsynaptic targets present challenges to disentangling the mechanisms underlying regulation of synaptic transmission. Within the striatum, acetylcholine modulates glutamate release via activation of muscarinic receptors (mAchRs), although the consequences for postsynaptic signaling are unclear. Using 2-photon microscopy and glutamate uncaging to examine individual synapses in the rat striatum, we find that glutamatergic afferents exhibit a high degree of multivesicular release (MVR) in the absence of postsynaptic receptor saturation. We show that mAchR activation decreases both the probability of release and the concentration of glutamate in the synaptic cleft. The corresponding decrease in synaptic potency reduces the duration of synaptic potentials and limits temporal summation of afferent inputs. These findings reveal a mechanism by which a combination of basal MVR and low receptor saturation allow the presynaptic actions of a neuromodulator to control the engagement of postsynaptic nonlinearities and regulate synaptic integration.