Modulation Of Ltp Induction

BOX 5, cont'd (C) BDNF also modulates LTP induction in response to theta-frequency-type stimulation. Two stimulating electrodes were positioned on either side of a single recording electrode to stimulate two different groups of afferents converging in the same dendritic field in CA1. Stimulation was applied to Schaffer collaterals alternately at low frequency (one per minute). After a period of baseline recording, LTP was induced with a theta-burst stimulation applied at time 0 only to one pathway (S2, filled squares). Simultaneous recording of an independent pathway (S1, open circles) showed no change in its synaptic strength after the theta burst was delivered to S1. BDNF (closed squares) selectively facilitates the induction of LTP in the tetanized pathway without affecting the synaptic efficacy of the untetanized pathway. EPSPs were recorded in the CA1 area of BDNF-treated slices. Synaptic efficacy (initial slope of field EPSPs) is expressed as a percentage of baseline value recorded during the 20 minutes before the tetanus. Representative traces of field EPSPs from S1 and S2 pathways were taken 10 minutes before and 40 minutes after the theta-burst stimulation. Adapted from Gottschalk, Pozzo-Miller, and Figurov (35).

FIGURE 9 Mechanisms of induction, maintenance, and expression. This diagram highlights the importance of considering that each different phase of LTP may have separate and parallel induction, maintenance, and expression mechanisms.

FIGURE 9 Mechanisms of induction, maintenance, and expression. This diagram highlights the importance of considering that each different phase of LTP may have separate and parallel induction, maintenance, and expression mechanisms.

Finally, it is important for the reader to synthesize the concepts of induction, maintenance, and expression with the concept of phases. Simply stated, three phases of LTP (I-, E-, and L-LTP) times three distinct underlying mechanisms for each phase (induction, maintenance, and expression) gives nine separate categories into which any particular molecular mechanism contributing to LTP may fit (see Figure 9). Added to this is the complexity that one phase could be largely presynaptic and another largely postsynaptic. Interesting implications begin to arise from thinking about LTP this way. How is it that the different mechanisms for the different phases interact with each other? Is the maintenance mechanism for one phase the induction mechanism for the next, or do the mechanisms for the phases operate independently? If the maintenance and expression mechanisms for the phases are independent, how does the magnitude of LTP stay constant as the shorter-lasting phase decays? How does the mechanism for L-LTP know where to stop, so that the magnitude of L-LTP is the same as the magnitude that E-LTP had attained? In reference 21, Joey English, Eric Roberson, and I discussed some hypothetical answers to these questions. While in this earlier publication we discussed them in the context of LTP specifically, it also is important to keep in mind that in many ways the same considerations apply to memory itself. If memory is encoded as some complex set of molecular changes, how is it that fidelity of memory is maintained as short-term memory transitions into long-term memory, for example. Although we won't arrive at an answer to these many questions, I believe that it is instructive to begin to formulate a hypothetical framework for their discussion. We will return to these hypothetical issues in the last chapter of the book.

In some cases for these questions, we can begin to formulate specific answers in molecular terms. Those cases where a molecular mechanism can be hypothesized constitute the issues that are addressed in the next three chapters. In Chapter 6, I will discuss mechanisms contributing to the induction of E-LTP, although, of course, many of those same molecular events are prerequisites for L-LTP as well. In Chapter 7, I will discuss mechanisms contributing to the maintenance and expression of E-LTP. In Chapter 8, I will discuss mechanisms that appear to be unique to the induction and expression of L-LTP.

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