Do H3-receptors participate in the effects of histamine on electrically-evoked contractions of rat vas deferens?

1. The specific effects of histamine were studied on rat vas deferens contractions induced by electrical field stimulation (0.1 Hz, 1 pulse, 1 msec duration, supramaximal voltage). 2. Histamine (5 x 10(-7)-10(-4) M) inhibited the electrically-evoked contractions (EEC). This effect was unchanged after the H1-receptor antagonist diphenhydramine (5 x 10(-6) M) and was decreased after the H2-receptor antagonist cimetidine (10(-5) M). 3. Betahistine, described as H1-agonist (in guinea pig ileum) and H3-autoreceptor antagonist (in rat cerebral cortex) slightly decreased the EEC, but competitively inhibited the histamine effect. 4. The results obtained suggest the existence of a specific third type presynaptic H-receptors in rat vas deferens, which are pharmacologically similar to the presynaptic H3-autoreceptors in rat cerebral cortex.