Glutamate receptors and development of the visual cortex: effect of metabotropic agonists on cAMP

Glutamate receptors are more active in several respects in young animals than in adults. Here we examine the effect of metabotropic glutamate agonists on rat cortical cAMP during and after the critical period for visual cortex plasticity. Quisqualate produced a substantial increase in cAMP, which was larger during the critical period than in the adult. The increase was not affected by CNQX or APV, showing that it was not due to the action of quisqualate on ionotropic glutamate receptors. Both Type I mGluRs (mGluRs 1 and/or 5) and Type II mGluRs (mGluRs 2 and/or 3) probably contributed to the cAMP increase because (i) ACPD and L-CCG-I, which are more active on Type II mGluRs, were more effective than DHPG, which is more active on Type I mGluRs; and (ii) there was a significant difference in the effect of ACPD on the increase in cAMP, comparing mGluR1 knockout mice with control mice. Agonists which produce large stimulation of cAMP production (ACPD, L-CCG-I), as well as L-AP4, also produced small attenuations of forskolin-stimulated cAMP, but only at high concentrations. Thus, we conclude that it is the stimulation and/or potentiation of cAMP production that is significant, rather than the attenuation of forskolin-stimulated cAMP. Since this stimulation and/or potentiation is higher during the critical period than in the adult, and the cAMP second messenger system has been implicated in hippocampal plasticity, it may also play a role in visual cortex plasticity.