Developmental changes in NMDA receptor-mediated visual activity in the rat superior colliculus, and the effect of dark rearing

Abstract

N-methyl-D-aspartate (NMDA) receptor-mediated activity is considered important for experience-dependent plasticity in the developing visual system. We investigated the influence of age and experience on the role of NMDA receptors in the visual transmission in the superficial grey layer of the superior colliculus (SGS) of the superior colliculus, where, in the adult, NMDA receptors mediate a substantial part of the visual response. In normally reared (postnatal day 14, P14, to adult) rats, visual responses were challenged with NMDA receptor-selective iontophoretic applications of the antagonist D-2-amino-5-phosphonovalerate (AP5). After eye opening (at P14), there was a significant increase in the number of neurones whose visual responses were reduced during AP5 ejection, which peaked at P22 (85%; n = 21), and then declined to adult levels (66%; n = 47) at P25. The mean reduction of the response (from control levels) by AP5 was similar at all ages (approximately 40%). Dark rearing had striking effects on the role of NMDA receptors in visual transmission, especially when comparisons were made between age-matched subjects greater than P25. In these subjects, AP5 ejection reduced the visual responses of all neurones studied. In addition, AP5 ejection caused a significantly larger reduction of visual responses in dark-reared rats (mean reduction 62 +/- 4; n = 29) compared with age-matched controls (mean reduction 44 +/- 8; n = 23). The D,L-alpha-amino-3-hydroxy-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-dependent effects. We conclude that NMDA receptors, but not AMPA receptors, assume greater importance for visual transmission in the SGS of dark-reared rats.