Regulation of NMDA receptor mRNA during visual map formation and after receptor blockade

The topographic refinement of the rat retinocollicular projection is dependent on normal NMDA receptor function. Here we examined the expression of NMDA and non-NMDA glutamate receptor mRNA in the rat superior colliculus (SC) during this postnatal refinement period. The temporal expression pattern of mRNA coding for the NMDA receptor subunit NR1 in the superficial SC followed the time course of collicular synaptogenesis. A pronounced increase of NR1 mRNA levels occurred during the late stages of retinocollicular map refinement. In cortex, the time course of the expression of NR1 mRNA in cortex was found to be similar to that observed in SC, with low levels during the first postnatal week, a maximum at P19, and a decrease thereafter. In SC, but not in cortex, there was a change in the ratio of the two NR1 transcripts during the second postnatal week that parallels a previously demonstrated developmental change in the mean open time of NMDA channels in collicular neurons. In contrast, the mRNA expression pattern of the non-NMDA receptor subunit GluR2 in the developing SC was not closely correlated with synaptic changes. Chronic treatment of the SC with the NMDA receptor antagonist 2-amino-5-phosphonovalerate (APV) for 12 or 19 days, which disrupts retinocollicular map formation, appears to block the developmental rise in NR1 mRNA levels. These findings support a specific role for the NMDA receptor subtype of glutamate receptors in the control of synaptogenesis and developmental plasticity in the SC.