Modulation of excitatory glutamatergic transmission at corticostriatal synapses by a metabotropic glutamate receptor (mGluR) was examined using a newly developed cell culture preparation in which small explants of cortical tissue are grown in co-culture with isolated striatal neurons. Electrical stimulation of cortical tissue evoked excitatory postsynaptic currents (eEPSCs) observed during tight-seal, whole-cell recordings from striatal neurons. Transmission was mediated by activation of AMPA/kainate-type glutamate receptors. The mGluR agonists, 1SR,3RS-ACPD and DCG-IV, reduced eEPSC amplitude. The effect of 1SR,3RS-ACPD increased in a concentration-dependent manner. Application of phorbol diacetate (PDAc) potentiated eEPSC amplitude and reduced the inhibitory effect of mGluR activation. Pretreatment with pertussis toxin (PTX) also reduced inhibition by 1SR,3RS-ACPD. Under conditions in which transmission was independent of the function of voltage-gated calcium channels, mGluR activation reduced the frequency of occurrence of miniature EPSCs (mEPSCs), but did not alter mEPSC amplitude. This effect of mGluR activation was reduced by PDAc treatment. mGluR activation modulates glutamatergic transmission via a presynaptic autoreceptor at corticostriatal synapses in this newly-developed corticostriatal co-culture preparation as in striatal slices. Modulation of transmission occurs whether or not transmission involves activation of voltage-gated calcium channels. Furthermore, many of the characteristics of mGluR modulation of eEPSCs are shared by mGluR modulation of mEPSCs. These findings indicate that mechanisms downstream from calcium entry may contribute to modulation of synaptic transmission by mGluR autoreceptors.