Participation of histamine in the step-through active avoidance response and its inhibition by H1-blockers

The effects of intravenous and intracerebroventricular administrations of certain H1-blockers on the active avoidance response in rats were studied. Among the classic H1-blockers used in this study: pyrilamine, diphenydramine, promethazine and chlorpheniramine, promethazine was the most effective and chlorpheniramine the least in inhibiting the active avoidance response; namely, a variation of prolongation in the response latency of the avoidance response. Meanwhile, ketotifen most potently inhibited the active avoidance response when the drugs were administered intracerebroventricularly. Mequitazine, astemizole and oxatomide were weak depressants when administered by either route. Azelastine was less effective than the classic H1-blockers by intravenous injection, while by intracerebroventricular injection, the inhibition was almost identical to those induced by the classic H1-blockers. Intracerebroventricular injection of histamine was antagonized the prolonged latency in the avoidance response induced by pyrilamine or diphenhydramine. A similar effect was also produced by 2-methylhistamine, but 4-methylhistamine had no effect. Intracerebroventricular injection of acetylcholine was restored the retarded avoidance response induced by pyrilamine, but a dose 20 times greater than that of histamine was required. From these findings, it can be concluded that inhibition of the active avoidance response induced by H1-blockers may be exerted through interaction with H1-receptors in the brain.