Activation of metabotropic glutamate receptors (mGluRs) results in multiple second messenger responses in rat hippocampal slices, including stimulation of phosphoinositide hydrolysis and potentiation of cyclic AMP responses induced by agonists of other receptors that are directly coupled to adenylate cyclase. Alpha 1 adrenergic receptors and H1-histaminergic receptors are similar to mGluRs in that agonists of these receptors also induce both phosphoinositide hydrolysis and potentiation of cyclic AMP responses to other agonists. In each of these cases, it is not clear whether activation of phosphoinositide hydrolysis and potentiation of cyclic AMP responses are mediated by the same or different receptor subtypes. In the present studies, the pharmacological profiles of mGluR-mediated potentiation of cyclic AMP responses and mGluR-mediated activation of phosphoinositide hydrolysis were compared to determine whether these responses are mediated by the same or distinct receptor subtypes. In addition, the authors determined the effect of mGluR activation on cyclic AMP responses in various regions of the rat brain and at different stages of postnatal development. It was found that the rank order of efficacies and potencies of mGluR agonists for potentiating cyclic AMP responses is distinct from the rank order of efficacies and potencies of the same compounds at stimulating phosphoinositide hydrolysis. Furthermore, L-serine-O-phosphate competitively blocked mGluR-mediated potentiation of cyclic AMP responses but had little or no effect on activation of phosphoinositide hydrolysis by the active isomer of trans-1-aminocyclopentane-1,3-dicarboxylic acid. These data are consistent with the hypothesis that these two responses are mediated by distinct mGluR subtypes.