DEVELOPMENTAL-CHANGES IN NMDA RECEPTOR-MEDIATED VISUAL ACTIVITY IN THE RAT SUPERIOR COLLICULUS, AND THE EFFECT OF DARK REARING

N-methyl-D-aspartate (NMDA) receptor-mediated activity is considered i mportant for experience-dependent plasticity in the developing visual system. We investigated the influence of age and experience on the rol e of NMDA receptors in the visual transmission in the superficial grey layer of the superior colliculus (SGS) of the superior colliculus, wh ere, in the adult, NMDA receptors mediate a substantial part of the vi sual response. In normally reared (postnatal day 14, P14, to adult) ra ts, visual responses were challenged with NMDA receptor-selective iont ophoretic applications of the antagonist D-2-amino-5-phosphonovalerate (AP5). After eye opening (at P14), there was a significant increase i n the number of neurones whose visual responses were reduced during AP 5 ejection, which peaked at P22 (85%; n = 21), and then declined to ad ult levels (66%; n = 47) at P25. The mean reduction of the response (f rom control levels) by AP5 was similar at all ages (approximately 40%) . Dark rearing had striking effects on the role of NMDA receptors in v isual transmission, especially when comparisons were made between age- matched subjects greater than P25. In these subjects, AP5 ejection red uced the visual responses of all neurones studied. In addition, AP5 ej ection caused a significantly larger reduction of visual responses in dark-reared rats (mean reduction 62 +/- 4; n = 29) compared with age-m atched controls (mean reduction 44 +/- 8; n = 23). The lpha-amino-3-hy droxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist 6- cyano-7-nitroquinoxaline-2,3-dione (CNQX) reduced the visual responses of every neurone studied and there were no age-or experience-dependen t effects. We conclude that NMDA receptors, but not AMPA receptors, as sume greater importance for visual transmission in the SGS of dark-rea red rats.