The development of N-methyl-D-aspartate receptors in cat visual cortex.

During a critical period of early postnatal development, the visual cortex of kittens is susceptible to experience-dependent modifications of neuronal response properties. Recently, the activation of N-methyl-D-aspartate (NMDA) receptors has been identified as an indispensable prerequisite for the induction of such modifications. We therefore investigated developmental changes in the density and distribution of NMDA receptors and questioned whether these showed a relation to the time course of the critical period. We determined the proportion of [3H]glutamate binding sites that were displaced by the NMDA receptor antagonist 2-amino-5-phosphonovalerate (APV) on 10-microns-thick cryostat sections of the primary visual cortex. The overall density of APV-sensitive [3H]glutamate binding sites increased dramatically between the second and the fourth week and stayed at this level throughout the critical period. Towards the end of the critical period, these binding sites decreased and finally reached adult values that were slightly above those of 2-week-old kittens. APV-sensitive binding sites were present in all cortical layers of the age groups investigated. While the general pattern of developmental changes was similar in all layers, slight differences existed in the time course. These observations are compatible with the notion that NMDA receptor activation is required for the expression of use-dependent change of response properties in the kitten visual cortex. Furthermore, they suggest as a possible reason for the decline of malleability towards the end of the critical period the reduction of NMDA receptors.