Histaminergic neurons of the ventral hippocampus and the baso-lateral amygdala of the rat: functional interaction on memory and learning mechanisms

The possibility of a functional interaction between the amygdala and the ventral hippocampus on learning of a conditioned avoidance response when both brain structures are chemically stimulated with histamine was studied in rats. Adult male rats were stereotaxically implanted under ether anaesthesia with guide cannulae into the baso-lateral amygdala and the ventral hippocampus. Seventy-two hours after the implant, rats were microinjected with: 1 μl saline solution into both structures (SAL+SAL group); 9 or 90 nmol doses of histamine into both structures (HA+HA groups); 9 or 90 nmol histamine into the hippocampus and saline into the amygdala (HA+SAL groups); saline into the hippocampus and 9 or 90 nmol histamine into the amygdala (SAL+HA groups). Five minutes following the injection, rats were subjected to a learning task in eight consecutive trials, consisting of avoiding an electric shock applied to the feet of the animal when an ultrasonic tone of 40 kHz is on for 30 s. Results show that histamine applied in any dose into both amygdala and hippocampus was able to significantly increase the escape latency and impair the efficacy of learning. Chemical stimulation with histamine of only hippocampus or amygdala affected selectively the escape latency. Results suggest that there is a functional interaction between histamine-sensitive neurons of the amygdala and hippocampus during processing of a learning task.