COMPARISON OF ASSOCIATIVE AND NONASSOCIATIVE CONDITIONING PROCEDURES IN THE INDUCTION OF LTD IN CA1 OF THE HIPPOCAMPUS

Recent reports have indicated that weak activity in a test input, nega tively correlated (out-of-phase) with tetanization of a separate, conv erging input, produces an NMDA-independent, associative long-term depr ession (LTD) of the test input synapses, in hippocampal field CA1 (Sta nton and Sejnowski, 1989; Stanton et al., 1991). Associative LTD has a lso 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 stimulu s protocols, in vitro, in order to compare the induction of non-associ ative and associative LTD in field CA1 of the adult rat hippocampus. S timulation in normal solution evoked a small non-associative LTD, but no associative LTD. Addition of picrotoxin to the medium facilitated t he induction of NMDA-dependent non-associative LTD, but not associativ e LTD. Previously potentiated pathways were not different from naive p athways in expression of LTD of either kind. Finally, 'priming' stimul ation (5 Hz) of the test pathway produced a weak, selective enhancemen t of associative LTD that was, however, not significantly greater than non-associative LTD. These results indicate that, for our experimenta l conditions, negatively correlated co-activity during afferent tetani zation does not induce a substantial associative LTD in area CA1. (C) 1993 Wiley-Liss, Inc.