Effects of seizure and subseizure posttrial hippocampal stimulation on appetitive operant behavior in mice

Abstract

Ninety-three BALB/c male mice were implanted bilaterally in the dorsal hippocampus. For each animal, the individual afterdischarge threshold intensity was determined for a 4-sec bilateral hippocampal stimulation train. Several days later, all animals underwent an initial 15 min session in a continuous reinforcement appetitive task (Skinner box). In Expt 1, all animals except a control group were stimulated 30 sec after the learning session either at half of the individual afterdischarge threshold value for 80 sec, or at the after-discharge threshold value for 4 or 80 sec. A 30-min second session took place 24 hr later. In the control group, an improvement of performance, the reminiscence effect, was found. In the subseizure stimulated group, the observed reminiscence effect was higher than that of the control group. Conversely, with the two durations of seizure stimulation, the reminiscence effect was erased. In Expt 2, the same learning paradigm was used, but the animals were stimulated at their individual afterdischarge threshold value at different times after the end of the first session (from 30 sec to 180 min). During the second session, 24 hr later, the impairment of the reminiscence effect for the different groups decreased as the learning-stimulation delay was increased. Thus, the effects of posttrial hippocampal stimulation on learning were studied. The duration of the long temporal amnesia gradient of seizure stimulation and of the shorter facilitatory gradient previously described at a half threshold value were discussed. The gradient duration in the first case could be related to the spread of afterdischarges from the hippocampus to other parts of the brain.