The effects of histamine and some related compounds on conditioned avoidance response in rats

When histamine (Hi) and other agonists were applied intraventricularly, Hi caused a dose-dependent inhibition of the avoidance response in rats; its ED50 was 3.60 micrograms. 1-methylHi, 1-methylimidazole acetic acid and imidazole acetic acid which are major metabolites of Hi produced no inhibitory effect even at 50 micrograms. H1-agonists (2-methylHi and 2-thiazolylethylamine) also depressed the avoidance response; their dose-response lines run parallel to that of Hi. The depressant effects of H2-agonists (4-methylHi and dimaprit) were relatively weak; their dose-response lines were not parallel to that of Hi. When antagonists were pretreated intravenously, Hi action was clearly antagonized by diphehydramine and pyrilamine, but not by cimetidine or ranitidine. Intraventricular injection of Hi mixed with cimetidine or ranitidine did not change the effect induced by Hi alone. The avoidance response was not affected by noradrenaline, dopamine or 5-hydroxytryptamine. Although acetylcholine (ACh) suppressed the avoidance response dose-dependently, its effect was much weaker than that of Hi. Pretreatment with cholinergic blocking drugs (atropine and scopolamine) antagonized ACh action but not Hi action. From these results, it is assumed that the inhibitory effect of Hi on the avoidance response is preferentially linked to the H1-receptor. After intraventricular application of 3H-Hi, the highest radioactivity was determined in the hypothalamus.