REGULATION OF NMDA RECEPTOR MESSENGER-RNA DURING VISUAL MAP FORMATIONAND AFTER RECEPTOR BLOCKADE

The topographic refinement of the rat retinocollicular projection is d ependent on normal NMDA receptor function. Here we examined the expres sion 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 synaptogene sis. 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 th at 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, t here was a change in the ratio of the two NR1 transcripts during the s econd postnatal week that parallels a previously demonstrated developm ental change in the mean open time of NMDA channels in collicular neur ons. In contrast, the mRNA expression pattern of the non-NMDA receptor subunit GluR2 in the developing SC was not closely correlated with sy naptic changes. Chronic treatment of the SC with the NMDA receptor ant agonist 2-amino-5-phosphonovalerate (APV) for 12 or 19 days, which dis rupts retinocollicular map formation, appears to block the development al rise in NR1 mRNA levels. These findings support a specific role for the NMDA receptor subtype of glutamate receptors in the control of sy naptogenesis and developmental plasticity in the SC.