The activation of histamine-sensitive sites of the ventral hippocampus modulates the consolidation of a learned active avoidance response in rats

Previous evidence from our laboratory has shown that histamine receptors located into the ventral hippocampus modulate learning and memory processes. Stimulation of histamine hippocampal sensitive receptors during the acquisition phase of a conditioned avoidance response to an ultrasonic tone was able to increase latency to escape and impair memory in the rat. Histamine application into the same hippocampal region also impaired the evocation of the response. The purpose of the present work was to evaluate if histaminergic neuron circuits have participation on the consolidation processes of the conditioned avoiding response. Male adult rats were implanted into the ventral hippocampus with microinjection cannulae and subjected consecutively to 2 sessions of 8 trials to learn an avoidance response after an ultrasonic tone of 40 kHz was on, as it was previously described. Immediately after the training period was over, or 15 min after, different groups of rats were microinjected with saline, histamine or a combination of histamine H1- or H2-receptor antagonists. Twenty four hours later, animals were tested in a new session for the retention of the avoiding response. Results showed that histamine treatment interfered with the consolidation of the avoiding response, affecting latency and the memory efficiency. This interference was mediated by histamine H1- and H2-receptors, since pretreatment with pyrilamine or ranitidine blocked the inhibitory effect of histamine. Results support the concept that histaminergic neurotransmission modulates learning and memory by affecting selectively the three stages of learning.