Question

What are “silent synapses”? Describe the molecular mechanism that
induces silent synapses to exhibit LTP(long term potentiation). 500words

Answer 1

Silent Synapses: It is excitatory synapses. It is a glutamatergic synapse type whose postsynaptic membrane contains NMDA-type glutamate receptors but no AMPA-type glutamate receptors. These are so because of the fact that normal AMPA receptor-mediated signaling is absent; this renders synapse inactive under typical conditions. These synapses are classically thought to be immature glutamatergic synapses.

As we know some of the synapses do have a functional release site contain surface expressed NMDARs however, they don’t have surface-expressed AMPARs at their postsynaptic membrane. This is further known that the former (NMDARs) have much higher affinity for glutamate than AMPARs.   There is further evidence to support that the amino acid (glutamate) could ‘spill over’ from neighboring active synapses onto the postsynaptic receptors at presynaptically inactive synapses when its concentration is very low and activates NMDARs although AMPARs may be also postsynaptic membrane. In this context, LTP is further demonstrated by an elevated discharge prospect at these presynaptically silent synapses. The circumstances under which the original silent synapse experiments were performed were suggested to increase this leak over as they were done at room temperature. This is the main circumstance compared to others that decreases the high affinity glutamate transport and therefore suggested to increase spill over. Following this, further studies suggested an alternative mechanism for silent synapses in which glutamate is released at a slow rate and incompletely from immature presynaptic terminals at invariably low enough concentration to only activate the postsynaptic NMDARs at the opposite postsynaptic membrane even though AMPARs are also present. In this regard, LTP at silent synapses because of rapid change in the presynaptic release machinery to a mature state. In the latter there is high concentration of glutamate that is quickly released so that both NMDARs and AMPARs are activated.