The induction of corticostriatal long-term depression (LTD) in striatal spiny projection neurons (SPNs) requires coactivation of group I metabotropic glutamate receptors (mGluRs) and L-type Ca²⁺ channels. This combination leads to the postsynaptic production of endocannabinoids that act presynaptically to reduce glutamate release. Although the necessity of coactivation is agreed upon, why it is necessary in physiologically meaningful settings is not. The studies described here attempt to answer this question by using two-photon laser scanning microscopy and patch-clamp electrophysiology to interrogate the dendritic synapses of SPNs in ex vivo brain slices from transgenic mice. These experiments revealed that postsynaptic action potentials induce robust ryanodine receptor (RYR)-dependent Ca²⁺-induced-Ca²⁺ release (CICR) in SPN dendritic spines. Depolarization-induced opening of voltage-gated Ca²⁺ channels was necessary for CICR. CICR was more robust in indirect pathway SPNs than in direct pathway SPNs, particularly in distal dendrites. Although it did not increase intracellular Ca²⁺ concentration alone, group I mGluR activation enhanced CICR and slowed Ca²⁺ clearance, extending the activity-evoked intraspine transient. The mGluR modulation of CICR was sensitive to antagonism of inositol trisphosphate receptors, RYRs, src kinase, and Ca_v1.3 L-type Ca²⁺ channels. Uncaging glutamate at individual spines effectively activated mGluRs and facilitated CICR induced by back-propagating action potentials. Disrupting CICR by antagonizing RYRs prevented the induction of corticostriatal LTD with spike-timing protocols. In contrast, mGluRs had no effect on the induction of long-term potentiation. Taken together, these results make clearer how coactivation of mGluRs and L-type Ca²⁺ channels promotes the induction of activity-dependent LTD in SPNs.