Apomorphine-susceptible and apomorphine-unsusceptible Wistar rats differ in novelty-induced changes in hippocampal dynorphin B expression and two-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 hippocampus in apomorphine-susceptible (APO-SUS) and apomorphine-unsusceptible (APO-UNSUS) Wistar rat lines. Since hippocampal mossy fibers contain among 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 condition was greater in APO-SUS rats than in APO-UNSUS rats, while exposure to novelty decreased and increased the dynorphin B expression in APO-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 difference in the regulation of the firing rate of mossy fibers by corticosteroids, and/or (c) an interline difference in the release of corticosteroids in response to novelty. Since the size of the mossy fiber infra/intrapyramidal terminal field is inversely related to two-way active avoidance performance, APO-SUS and APO-UNSUS rats (n = 9 per line) were given this task: APO-UNSUS rats performed much better than APO-SUS rats. It is concluded that the neurochemical and behavioural function of the hippocampus significantly differs between lines. Given the already known interline differences in the function of the nucleus accumbens, the present results provide a new avenue in search for the functional relationship between the hippocampus and the nucleus accumbens.