Comparison of associative and non-associative conditioning procedures in the induction of LTD in CA1 of the hippocampus.

Recent reports have indicated that weak activity in a test input, negatively correlated (out-of-phase) with tetanization of a separate, converging input, produces an NMDA-independent, associative long-term depression (LTD) of the test input synapses, in hippocampal field CA1 (Stanton and Sejnowski, 1989; Stanton et al., 1991). Associative LTD has also been observed in the dentate gyrus, in vivo, but only following "priming" of the test path with 5 Hz stimulation prior to associative conditioning (Christie and Abraham, 1992b). We have used these stimulus protocols, in vitro, in order to compare the induction of non-associative and associative LTD in field CA1 of the adult rat hippocampus. Stimulation in normal solution evoked a small non-associative LTD, but no associative LTD. Addition of picrotoxin to the medium facilitated the induction of NMDA-dependent non-associative LTD, but not associative LTD. Previously potentiated pathways were not different from naive pathways in expression of LTD of either kind. Finally, 'priming' stimulation (5 Hz) of the test pathway produced a weak, selective enhancement of associative LTD that was, however, not significantly greater than non-associative LTD. These results indicate that, for our experimental conditions, negatively correlated co-activity during afferent tetanization does not induce a substantial associative LTD in area CA1.