APOMORPHINE-SUSCEPTIBLE AND APOMORPHINE-UNSUSCEPTIBLE WISTAR RATS DIFFER IN NOVELTY-INDUCED CHANGES IN HIPPOCAMPAL DYNORPHIN-B EXPRESSION AND 2-WAY ACTIVE-AVOIDANCE - A NEW KEY IN THE SEARCH FOR THE ROLE OF THE HIPPOCAMPAL ACCUMBENS AXIS

The present study examines two characteristic traits of the hippocampu s in apomorphine-susceptible (APO-SUS) and apomorphine-unsusceptible ( APO-UNSUS) Wistar rat lines. Since hippocampal mossy fibers contain am ong others dynorphin B as transmitter, a radioimmunoassay was used to analyze the hippocampal dynorphin B expression in response to novelty in these lines. Dynorphin B expression at the end of the baseline cond ition was greater in APO-SUS rats than in APO-UNSUS rats, while exposu re to novelty decreased and increased the dynorphin B expression in AP O-SUS and APO-UNSUS rats, respectively. These interline differences in dynorphin B expression could be due to (a) an interline difference in the size of the mossy fiber terminal fields, (b) an interline differe nce in the regulation of the firing rate of mossy fibers by corticoste roids, and/or (c) an interline difference in the release of corticoste roids in response to novelty. Since the size of the mossy fiber infra/ intrapyramidal terminal field is inversely related to two-way active a voidance performance, APO-SUS and APO-UNSUS rats (n = 9 per line) were given this task: APO-UNSUS rats performed much better than APO-SUS ra ts. It is concluded that the neurochemical and behavioural function of the hippocampus significantly differs between lines. Given the alread y known interline differences in the function of the nucleus accumbens , the present results provide a new avenue in search for the functiona l relationship between the hippocampus and the nucleus accumbens.