Precise connections in many mammalian nervous systems require a great deal of remodeling during development. In the visual system, many excess synapses are originally formed in the lateral geniculate nucleus and striate cortex. Only the correct set of axon terminals are retained during normal development, while imprecise ones withdraw. The mechanism by which only correct axons are retained requires neural activity, and may be regulated by specific receptors at synapses.
The transmission of neural signals at these synapses is carried out in part by the glutamate-activated NMDA receptor. It is hypothesized that NMDA receptor activation plays a crucial role in enhancing only those connections in the immature system which will form a retinotopically correct map in the LGN and cortex. NMDA receptor activation requires depolarization of the neuron membrane. Possibly, only neurons transmitting information from nearby areas in the retina summate to produce NMDA receptor- mediated currents. The result is an influx of Ca++ ions that has been shown to cause trophic effects within the cell that could enhance the synaptic connection. Thus, NMDA receptors may act to detect coincident neural activity in immature animals, thereby allowing only visuo-topically related axon terminals to undergo enhancement of synaptic transmission and structure. As development proceeds, NMDA receptor function decreases, possibly reducing these intracellular effects.
Blocking NMDA receptor activation experimentally does alter the normal set of connections in the visual system. Yet, is there a direct cause- and-effect relation between NMDA receptor activity and anatomical changes? Many cellular events probably result from NMDA-mediated currents. Intracellular changes in phosphorylation states and protein levels could eventually alter a synapse at the anatomical level. Study of the changing NMDA receptor subunit types making up the receptor within visual system structures could reveal, in part, the means by which plasticity is down-regulated. The experimental regulation of these subunits in vivo could reveal important information concerning their specific function if plasticity and development were to be altered as a result. A summary of previous studies, and proposals for further research concerning the role of the NMDA receptor and its various types in developing visual pathways are presented in this manuscript.
| Identifer | oai:union.ndltd.org:vcu.edu/oai:scholarscompass.vcu.edu:etd-6331 |
| Date | 01 January 1995 |
| Creators | Perens, Gregory S. |
| Publisher | VCU Scholars Compass |
| Source Sets | Virginia Commonwealth University |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Theses and Dissertations |
| Rights | © The Author |
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