Functional regional distribution of histamine receptors in the rat hippocampus: modulation of learning of an active avoidance response

The possible functional regional distribution of the histamine sensitive sites controlling the behavioral performance of learning of an active avoidance response in the hippocampal structure was studied. Adult male rats were stereotaxically implanted with microinjection cannula into three different regions of the hippocampus: the ventral, the dorsal posterior and the dorsal anterior hippocampus. Different groups of rats were microinjected into these subregions with either saline, 9, 45 or 90 nmol of histamine. Five min later all groups were subjected to learning of a conditioned response to an ultrasonic tone warning an electric shock to the feet of the animals. Three sessions of 8 trials were necessary in order to control animals learn the task. Results show that histamine microinjections into the ventral hippocampus were able to significantly inhibit the natural decrease of the latency to escape during trials, impairing the efficiency of learning. Histamine administration into the dorsal posterior hippocampus was slightly effective in prolonging latency and impairing learning, while injections into the dorsal anterior region of the hippocampus had no effect. Results give a further support to the idea that histamine sensitive neurons in the ventral hippocampus are modulating the memory and learning mechanisms in the rat. In addition, evidence has been found suggesting a regional distribution of histamine sensitive neurons controlling learning of avoidance responses along the septo-temporal axis of the rat hippocampus.