Nmda Receptorindependent

Although the vast majority of studies of LTP and its molecular mechanisms have investigated NMDA receptor-dependent processes, as mentioned previously there also are several types of NMDA receptor-independent LTP. We will not discuss the mechanisms for these types of LTP very much in this book for two reasons. First, there have not been many studies investigating the molecular basis of NMDA receptor-independent LTP, at least relative to its NMDA receptor-dependent counterpart. Second, in those cases where mechanisms for this type of LTP have been investigated, there has been considerable controversy. I believe that it is wise to wait for the field to advance somewhat before attempting to integrate molecular mechanisms for NMDA receptor-independent LTP into the better-established models for NMDA receptor-dependent LTP. Nevertheless, it is clearly worthwhile to describe briefly a few different types of NMDA receptor-independent LTP as background material and to highlight them as important areas for future investigation.

NMDA receptor-independent LTP can be induced at the Schaffer-collateral synapses in area CA1, the same synapses we have been discussing thus far. This allows for somewhat of a compare-and-contrast of two different types of LTP at the same synapse. A protocol that elicits NMDA receptor-independent LTP in area CA1 is the use of four 0.5-second, 200-Hz stimuli separated by 5 seconds (14). LTP induced with this stimulation protocol is insensitive to NMDA receptor-selective antagonists such as APV (see Figure 13). It is interesting that simply doubling the rate of tetanic stimulation from 100 to 200 Hz appears to shift activity-dependent mechanisms for synaptic potentiation into NMDA receptor independence. At the simplest level of thinking, this indicates that there is some unique type of temporal integration going on at the higher-frequency stimulation that allows for superseding the necessity for NMDA receptor activation. What might the 200-Hz stimulation be uniquely stimulating? One appealing hypothesis arises from the observation that 200-Hz LTP is blocked by blockers of voltage-sensitive calcium channels. Thus, the current working model is that 200-Hz stimulation elicits sufficiently large and sufficiently prolonged membrane depolarization, resulting in the opening of voltage-dependent calcium channels, to trigger elevation of postsy-naptic calcium sufficient to trigger synaptic potentiation. One observation consistent with this hypothesis is that injection of post-synaptic calcium chelators blocks 200-Hz stimulation-induced LTP.

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