Microinjection of anti-vasopressin serum into limbic structures of the rat brain: effects on passive avoidance responding and on local catecholamine utilization

Rats which had received bilateral microinjections of 1:50 diluted anti-vasopressin serum into the dorsal or ventral hippocampus, immediately after the learning trial of a one-trial passive avoidance test, showed a reduction in avoidance latency scores during subsequent retention tests 24 and 48 h later. Postlearning microinjection of anti-vasopressin serum into either the dorsalateral septum or the caudate nucleus was without effect on the retention of passive avoidance behavior. Microinjection of anti-vasopressin serum 1 h before the 24-h retention session into either the dorsal hippocampus, the ventral hippocampus or the dorsolateral septum attenuated avoidance responding during both the 24-h and 48-h retention sessions, whereas preretention microinjection of the serum into the caudate nucleus was not effective. Intracerebroventricular administration of the anti-vasopressin serum in amounts similar to those used in the microinjection experiments did not affect retention scores when given either immediately after the learning trial or before the first retention session. One week after the behavioral experiments, a repeated microinjection of anti-vasopressin serum decreased the local α-methyl-p-tyrosine methylester (α-MPT)-induced disappearance of noradrenaline in the ventral hippocampus and the dorsal hippocampus respectively. Microinjection of the antiserum in the dorsolateral septum enhanced noradrenaline disappearance in this brain region. No effect was found on α-MPT-induced dopamine disappearance in the caudate nucleus following local microinjection of anti-vasopressin serum. These data show that endogenous vasopressin in both the dorsal and the ventral hippocampus is functionally involved in consolidation processes as well as in retrieval processes related to passive avoidance behavior, while that in the dorsolateral septum seems to be involved in retrieval processes only. They also show that noradrenergic mechanisms in these 3 brain regions respond to the local reduction of the amount of bioavailable vasopressin in a direction opposite to that observed after local microinjection of vasopressin, which suggests that vasopressin might act by modulating noradrenergic neurotransmission