Hippocampal histamine receptors: possible role on the mechanisms of memory in the rat, II.

In this paper it was studied the role of histamine and histamine receptors in the hippocampus of rats on an active avoidance response induced by an ultrasonic tone. The animals had to learn to walk through a swinging door into a safe compartment only after the conditioning ultrasonic tone was on in order to avoid an electric shock to their feet. Trained animals were implanted in the ventral hippocampus with microinjection cannulae and injected twice with 1 microliter of saline solution containing pyrilamine (PYR, H1-HA antagonist), ranitidine (RAN, H2-HA antagonist) or histamine. The histamine antagonists were applied in a dose of 65.5 nmol each while histamine was administered in a dose of 45 nmol. The two variables measured were the time in sec the rats take to present the conditioned avoidance response and the accumulated percentage of conditioned avoidance response (CAR). Results showed that histamine administration significantly increased the latency time to escape and decreased the % CAR. These effects were not blocked by the administration of RAN. However, administration of PYR completely counteracted the HA effects. Present findings confirm our previous findings about the inhibitory effect of histamine on the hippocampal retrieval mechanisms and give further support to the hypothesis that HA acts on the memory processes in the hippocampal formation, by activation of H1-histamine receptors.