Hippocampal lesions block behavioral effects of central but not of peripheral pre-test injection of arginine vasopressin in an appetitive learning task.

The effects of peripheral and central administration of arginine-vasopressin (AVP) were examined in intact or hippocampal-lesioned mice, when administered just before the retention session of an appetitive visual discrimination task. All the subjects underwent a partial learning of this task and were tested 24 days later when a partial forgetting occurred in controls. In Expt. 1, intact mice received AVP just before the test session either subcutaneously (s.c., 1 microgram) or intracerebroventricularly (i.c.v., 1 ng). Both routes of administration resulted in an enhancement of the retention performance. In Expt. 2, bilateral lesions of the dorsal hippocampus were carried out 2 days after the end of initial learning. The lesions did not modify retrieval performance on the 24th day. In additional groups, AVP was injected in pre-test situation in lesioned animals. An s.c. AVP injection induced the same improvement of retrieval as observed in intact mice. On the contrary, i.c.v. administration of AVP accentuated forgetting in lesioned animals. In other words, hippocampal lesions reversed the effects of i.c.v. pre-test administration of AVP. The results are analyzed in terms of different mechanisms for both routes of administration of AVP.