Question

3. You are investigating a class of drugs that might alter long-term potentiation (LTP) within neurons. For each of the following, describe how the compound might influence Early and Late LTP.

a) A drug that blocks NMDA receptors.

b) A drug that blocks intracellular calcium signaling.

c) A drug that stimulates PKM-zeta.

d) A drug that causes the Phosphorylation of AMPA receptors.

Answer 1

a) NMDA receptors are also associated with synaptic plasticity. The idea that both synaptic and extrasynaptic NMDA receptors can affect long-term potentiation (LTP) and long-term depression (LTD) differently has also been explored. Experimental data suggest that extrasynaptic NMDA receptors inhibit LTP while producing LTD. Inhibition of LTP can be prevented with the introduction of a NMDA antagonist. A theta burst stimulation that usually induces LTP with synaptic NMDARs, when applied selectively to extrasynaptic NMDARs produces a LTD. Experimentation also indicates that extrasynaptic activity is not required for the formation of LTP. In addition, both synaptic and extrasynaptic are involved in expressing a full LTD.

b)Synaptic plasticity is thought to be crucial for information processing in the brain and to underlie learning and memory. Widely studied models for synaptic plasticity are long-term potentiation (LTP) and long-term depression (LTD). LTP is a cellular model underlying learning and memory, which has been described in all excitatory pathways in the hippocampus and in different other brain regions .

LTP is usually divided into three temporal phases. The first stage is initial LTP or referred as short-term potentiation (STP) and is characterized as being protein-kinase and protein-synthesis independent. The next phase is early LTP (E-LTP) and its expression is mediated by activation of various protein kinases and the insertion of glutamate receptors into the postsynaptic membrane . The third phase is late LTP (L-LTP) and lasts from a few hours to several days and is correlated to long-term memory. The critical biochemical feature for L-LTP is a requirement for new gene expression and protein synthesis . An essential event necessary for the induction of all types of LTP appears to be the influx of calcium into the postsynaptic spine. Indeed, LTP induction can occur when postsynaptic hippocampal neurons are loaded with calcium . Conversely, LTP can be blocked with calcium chelators preventing the postsynaptic rise in calcium . Extracellular calcium influx is not, however, the only event controlling LTP. Depletion of ER calcium stores can block LTP, suggesting that calcium release from intracellular stores is also critical for LTP induction.

In the majority of synapses that support LTP, the postsynaptic increase in calcium is mediated by the N-methyl-D-aspartate receptor (NMDAR) . The requirement for NMDAR activity in LTP was not only demonstrated in the hippocampus, but also in other brain regions, such as the amygdala and frontal cortex . In addition animal studies confirmed the importance of NMDAR for learning and memory, by showing that NMDAR inhibition blocked spatial and associative learning . LTP induction, however, can also occur in the CA1 region as a result of L-type VGCC activation . Furthermore, several studies have shown that intracellular calcium stores may also play a role in the increase of postsynaptic calcium levels upon synaptic activity. Indeed, depletion of ER calcium stores with thapsigargin has been shown to inhibit the induction of LTP in brain hippocampal slices and inhibition of the ER calcium channels RyRs or InsP3Rs appeared to affect specific LTP forms .

c) When the criteria of necessity, occlusion, erasure, and persistence are examined in detail, the cumulative evidence strongly supports the persistent action of PKMζ as a core molecular mechanism of late-LTP and long-term memory maintenance. CaMKII appears to have two roles: an enzymatic role that is essential for the induction of LTP, and a structural role involving interaction with the NMDAR that maintains synaptic transmission regardless of the state of potentiation. Further work will be required to evaluate whether this structural role of CaMKII is also: 1) one of several transient, post-translational mechanisms upregulated in early-LTP, 2) an expression mechanism of late-LTP and long-term memory, downstream of maintenance by PKMζ, or 3) as Lisman proposed, a maintenance mechanism of a form of synaptic plasticity, independent of PKMζ. A fundamental difference between the molecules is that CaMKII structural inhibitors generally disrupt AMPAR-mediated synaptic transmission, including basal transmission, whereas PKMζ inhibitors specifically disrupt only potentiated synaptic transmission during late-LTP. Thus, one scenario that remains to be fully investigated is that CaMKII maintains the synaptic plasticity involving the initial “AMPAfication” of NMDAR-only, “silent” synapses that occurs during development, and PKMζ maintains further potentiation of only a few of these NMDAR/AMPAR-containing synapses to sparsely encode and store information acquired during learning and experience. With John gone, we hope someone picks up his mantle to explore these and other possibilities

d)AMPA receptors (AMPAR) are both glutamate receptors and cation channels that are integral to plasticity and synaptic transmission at many postsynaptic membranes. One of the most widely and thoroughly investigated forms of plasticity in the nervous system is known as long-term potentiation, or LTP. There are two necessary components of LTP: presynaptic glutamate release and postsynaptic depolarization. Therefore, LTP can be induced experimentally in a paired electrophysiological recording when a presynaptic cell is stimulated to release glutamate on a postsynaptic cell that is depolarized. The typical LTP induction protocol involves a “tetanus” stimulation, which is a 100 Hz stimulation for 1 second. When one applies this protocol to a pair of cells, one will see a sustained increase of the amplitude of the EPSP following tetanus. This response is interesting since it is thought to be the physiological correlate for learning and memory in the cell. In fact, it was recently shown that, following a single paired-avoidance paradigm in mice, LTP could be recorded in some hippocampal synapses in vivo.

The molecular basis for LTP has been extensively studied, and AMPARs have been shown to play an integral role in the process. Both GluR1 and GluR2 play an important role in synaptic plasticity. It is now known that the underlying physiological correlate for the increase in EPSP size is a postsynaptic upregulation of AMPARs at the membrane, which is accomplished through the interactions of AMPARs with many cellular proteins.