NMDA receptor binding declines differentially in three spinal motor nuclei during postnatal development

The NMDA subtype of glutamate receptors mediates a variety of neuronal processes involved in the development of dendritic morphology. For example, NMDA receptor antagonism during the early postnatal period attenuates dendritic growth in spinal motoneurons. NMDA receptors are present in high levels in the spinal cord early in the postnatal period and decline during development, a period of extensive dendritic plasticity in the spinal cord. Previous studies have suggested that an adult pattern of distribution of NMDA receptors is established as early as postnatal day (P)21 (day of birth = P1). However, given that dendritic growth in spinal motoneurons is not complete by this age and that NMDA receptor activation is necessary for dendritic growth, we assessed NMDA receptor binding in specific spinal motor nuclei during normal development. NMDA receptors were labeled with [3H]MK-801 at P7, P14, P28, P49, and in adult male rats. Receptor binding in the spinal nucleus of the bulbocavernosus (SNB), dorsolateral nucleus (DLN) and retrodorsolateral nucleus (RDLN) was measured using in vitro quantitative autoradiography. NMDA receptor binding over the SNB, DLN and RDLN in intact males was initially high, and declined to adult levels. However, the time course of the decline differed across nuclei. The local decline in NMDA receptor binding observed in the SNB and DLN is coincident with the periods of dendritic growth in these nuclei, further supporting a role for NMDA receptors in the development of motoneuron dendritic morphology.