NM DA Receptors: Neural Map Designers and Refiners?

Sensory maps are created as a result of the precise architecture of neurons and their projections within the brain. The formation of these maps during development requires the appropriate positioning of neuronal axons and dendrites. Glutamate receptors of the N-methyl-Daspartate (NMDA) class have been shown to play a clear role in neuronal plasticity that underlies learning and memory, their role in sensory map formation is less clear. One of the best examples of sensory map formation is seen in the visual system, eye-specific segregation occurs such that inputs from the two eyes form eyespecific zones within the same brain region. Adjacent points in visual space are represented by adjacent points within each eye-specific region, generating what is referred to as a retinotopic map. Recent findings from Cline and colleagues [1] indicate that, in amphibians, eye-specific segregation depends on NMDA receptor activity. On the surface, these findings appear to contradict previous research in mammalian systems, where NMDA receptors do not appear to affect eye-specific segregation [2,3]. NMDA receptor blockade does, however, affect neuronal structure — for example, the size and shape of neuronal axons and dendrites — in mammalian systems, and so it may affect other features of map formation, such as retinotopy. We therefore felt a closer examination of the effects of NMDA receptors on neuronal morphology in a number of systems could provide some interesting insights into sensory map formation. A table summarising the effects of NMDA receptor blockade on axon and dendritic morphology can be found at the web site http://www.bms.ed.ac.uk/ research/idg/gendev/PKind/P.Kind’s%20Research.htm.