INTERHIPPOCAMPAL SYNTHESIS OF LATERALIZED PLACE NAVIGATION ENGRAMS

Rats were trained in a water maze in a dark room with the extramaze cu es restricted to only dimly back-lit shapes. We used lidocaine to reve rsibly lesion the dorsal hippocampus and this controlled-cue room in o rder to examine interhippocampal synthesis of lateralized place engram s. Experiment 1 showed that lidocaine injected into both hippocampi ef fectively abolished place navigation for up to 25 min but not at 45 mi n. In experiment 2, each day under lidocaine blockade of one hippocamp us, pretrained rats were trained in the water maze to locate the targe t according to two cues (e.g., AB). Two hours later, the contralateral hippocampus was inactivated and the rats were trained to the same loc ation with two other cues (CD). On day 5, intact brain retrieval was t ested in one of three conditions: ACQ (e.g., AB), one of the pairs of cues used in acquisition training; SYNTH (e.g., AC), one cue from each of the pairs used in acquisition; CONT (e.g., AE), one cue that was u sed in acquisition training and a novel cue. The results show that the hippocampi learned the two tasks independently and similarly [latency (L) at the asymptote = 7 s]. Retrieval performance was at the asympto te for ACQ (AB) and SYNTH (AC) (L = 6 and 7, respectively) but was dis rupted for CONT (L = 12). In experiment 3 as in experiment 2, the rats were trained, under unilateral blockade, to a new place for 4 days. O n day 5, retrieval with the trained hippocampus blocked was worse (L = 11) than with the untrained side blocked (L = 5). We conclude that in terhippocampal synthesis of lateralized place engrams is automatic and highly efficient. (C) 1995 Wiley-Liss, Inc.