BIO254:Silent: Difference between revisions

Silent synapses are abundant in development and are found in several brain regions, including the hippocampus, the cerebral cortex, and the spinal cord. The "silence" of these synapses is the result of Mg++ blockade of NMDA receptors, which are voltage-dependent. Interestingly, glutamate released at silent synapses binds only to NMDA receptors, without binding to AMPA receptors. For years, this specificity has puzzles neurobiologists, but one explanation is that NMDA and AMPA receptors have significantly different affinities for binding the released glutamate neurotransmitter. The concentration of glutamate may be sufficient enough to activate NMDA receptors (high-affinity), but not the low-affinity AMPA receptors. A second possibility states that both AMPA and NMDA receptors exist on the postsynaptic terminal, but only the NMDA receptors are fully functional. Or, some specific excitatory synapses only have NMDA receptors; growing evidence tends to support this latter model. Immunocytochemical experiments perhaps provide the most compelling evidence for this explanation: staining done by Gomperts et al. (2000) shows that select excitatory synapses only possess NMDA receptors.

The abundance of NMDA-receptor-only synapses peaks soon after post-natal development and decreases in adults. Hence, silent synapses appear not to be a separate class of excitatory synapses that are deficient in AMPA receptors, but are seen developmentally at an early stage of glutamatergic synapse maturation.