SUMMARY

Studies of long-term potentiation (LTP) induction and persistence in awake animals have revealed intriguing regional differences. LTP and heterosynaptic LTD in the dentate gyrus both inevitably appear to be decremental in nature, with the time constant of decay related to the strength of the induction stimulus. On the other hand, nondecremental LTP can be induced in area CA1 with a very mild stimulus that, when given in the dentate gyrus, evokes little or no LTP at all. CA1 synapses can also exhibit decremental LTP. LTP in neocortex has so far been observed only after multiple, spaced tetanization episodes, but this LTP can then be quite long-lasting. These data indicate intriguing differences in the pattern of LTP persistence across brain regions that may reflect different contributions to the memory or information storage process in behaving animals.

The dentate gyrus and CA1 appear to differ in a number of ways at molecular, cellular, and network levels that could account for the apparent differences in LTP persistence. These include the ability to produce voltage-dependent calcium channel (VDCC)-dependent LTP, the contribution by catecholamines to the persistence of LTP, the pattern of constitutive gene expression, and the pattern of tetanic stimulation commonly used to study LTP in these two regions. Few of these features appear to strongly differentiate the two hippocampal regions, however, making it conceivable that the dentate gyrus has the inherent capacity for nondecremental LTP, if only the right induction conditions could be met. Indeed, apparently nondecremental LTP has been reported for perforant path synapses in studies that have employed electroconvulsive shocks.