Question: In the rodent hippocampus, long term potentiation (LTP) can be produced in the mossy fiber pathway that innervates the CA-3 cells. LTP in the mossy fiber pathway is both input specific and state dependent, but LTP in this pathway is not associative.
Briefly summarize what we mean when we say that LTP is associative.
The associative property essentially means that a synapse can still develop LTP even if the presynaptic cell is not strongly releasing glutamate neurotransmitter. The way it can do this is by just being active, even if it is only releasing a little bit of Glu, as long as it does this at the same time that the postsynaptic cell is experiencing a strong depolarization (like say, from an action potential it received from another synapse).
What might be a reason that LTP in the mossy fiber pathway is not associative? Explain completely.
Something that comes to mind is that perhaps the depolarizing current from synapse 1 (that strongly release Glu and induced an action potential in the postsynaptic cell) fails to reach synapse 2 (that weakly released Glu). Although both synapses are input specific (both are active and have Glu) and state dependence is present (the postsynaptic cell has experienced a strong depolarization), synapse 2 can still not experience LTP if it doesn't experience that strong depolarizing current that came from synapse 1.
The molecular reason for this (bear with me) is because NMDA channels are responsible for LTP in that they strengthen synapses by conducting Ca2+ ions into the postsynaptic cell, but it can only do this once a Mg2+ that blocks the channel is knocked off from a strong intracellular depolarization through electrostatic repulsion. Therefore, if the strong depolarizing current fails to reach synapse 2 from synapse 1, the NMDA channels at synapse 2 will still be blocked and synapse 2 cannot potentiate.
