Role of metabotropic glutamate receptors in the cat's visual cortex during development.

We have studied the effect of metabotropic glutamate receptors on the second messenger cAMP, and how it varies with age in light- and dark-reared cats; the overall level of the metabotropic glutamate receptors mGluR1, 2/3, and 5 during development; the laminar distribution of these receptors; and how the physiological effect of the metabotropic glutamate receptor agonist ACPD varies with layer. The increase in cAMP produced by ACPD correlates well with the critical period for ocular dominance plasticity in both light- and dark-reared animals. Basal levels of cAMP also correlate well, but overall levels of mGluRs do not. Thus, the second messenger is likely to be the critical factor in plasticity, rather than the mGluRs. Both group I mGluRs (1 and 5) and group II mGluRs (2 and 3) contribute to the increase in cAMP. However, mGluR5 is affected by rearing in the dark, while mGluR1 is not. Moreover, the laminar distribution of mGluRs 2/3 and 5 changes with age, while mGluR1 does not. The laminar distribution is correlated with the functional effect of ACPD, which varies with layer. In upper layers, ACPD has a depressive effect on both visual response and spontaneous activity, while in lower layers it has a depressive effect on visual response and a facilitatory effect on spontaneous activity. These variations in functional effect with layer need to be taken into account before the role of metabotropic glutamate receptors in development will be fully understood.