Learning and memory involves the activity dependent modification of synaptic strength. Thus a clear understanding of how the brain stores information ultimately requires a cellular and molecular dissection. The most compelling model for these cellular changes is long-term potentiation (LTP), in which brief repetitive stimulation results in a long lasting enhancement in synaptic transmission. Two critical lines of research are essential in terms of understanding LTP as a substrate for learning. First we need to identify the molecular machines that underlie the changes in synaptic strength. Work from my lab and others have provided evidence that the synaptic modification underlying LTP resides primarily on the postsynaptic side of the synapse, at least for the first hour. Furthermore, evidence exists that the insertion of AMPA receptors into the synapse underlies the increase in synaptic strength. Our lab has identified some key proteins involved in trafficking AMPA receptors. Based on the stargazer mutant mouse, we discovered that the mutated protein, stargazin, as well as other related proteins, act as auxiliary AMPA receptor subunits, important for surface delivery, synaptic targeting and receptor gating of AMPA receptors. However, the synaptic targeting of AMPA receptors also appears to involve synaptic scaffolding proteins, in particular PSD-95, but conflicting data exists.
Specific Aim 1 will use novel approaches to determine the role of scaffolding proteins in the anchoring of AMPA receptors to the synapse. AMPA receptors are tetrameric channels, assembled from four subunits;GluR1-4. Most receptors in pyramidal cells are either GluR1/2 or GluR2/3. It is proposed that the synaptic insertion of GluR1/2 receptors requires activity, whereas the GluR2/3 receptors constitutively cycle into and out of the synapse. It is also proposed that long-tem depression (LTD) requires the presence of GluR2 containing receptors. Here, again, there are discrepancies, depending on the experimental approach (i.e., over expression or gene targeted knock outs).
Specific Aim 2 will address these discrepancies by a combination of approaches. The second key issue to understanding how LTP is used as a mnemonic devise is to determine the ways that neuronal circuits and ionic conductances control the """"""""gating"""""""" of LTP. In addition to local inhibitory circuits, a powerful slow calcium activated potassium current (IAHP) regulates dendritic excitability and, in turn, the ability of synapses to generate LTP. Research from this grant showed previously that this current is the target of numerous modulatory neurotransmitters, such as norepinephrine and acetylcholine, which turn this current off. As a consequence the inhibitory check on LTP is released. This mechanism could well explain the salutary role of attention, which involves activation of the noradrenergic system, in learning and memory. Remarkably the molecular identity of this channel remains a mystery. Given the central role this channel plays in controlling synaptic plasticity we plan to identify this channel and determine the mechanism by which calcium and norepinephrine regulate its activity. The experiments outlined will provide fundamental insight into the cellular underpinnings of learning and memory and by extension to the numerous conditions that result in cognitive impairment.
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