Effect of histaminergic drugs on integumental melanophores of adult Bufo melanostictus.

Histamine and 2-methyl histamine caused dose-dependent aggregation of melanophores in toad B. melanostictus. The effects were effectively antagonised by mepyramine, a specific H1 histamine receptor antagonist, and metiamide a specific H2 receptor antagonist. On the other, hand 4-methyl histamine, a specific H2 receptor agonist dispersed the melanophores. The results suggest that adult Bufo melanophores have H1 histamine receptors which mediate melanophore aggregation, however, dispersion of melanophores may be controlled by undifferentiated histamine receptors of H2 type.

Role of endogenous opioids and histamine in morphine induced emesis.

The role of opioid and histaminergic system in morphine induced emesis was investigated in dogs. Morphine (25 micrograms, icv) consistently evoked emesis with an average latency of 195 +/- 29 sec which was fully accounted for by an action on the chemoreceptor trigger zone (CTZ) as its ablation rendered animals refractory to vomiting. Intraventricular pretreatment with opioid antagonist naloxone, histamine H1 antagonist mepyramine and H2 antagonists metiamide and cimetidine afforded protection to icv morphine emesis. The CSF histamine concentration was significantly raised 5 min after icv morphine administration. The results suggest that both endogenous opioid and histamine are involved in morphine emesis. Naloxone in high doses (1600 micrograms, icv) elicited emesis which was not blocked by CTZ ablation confirming our earlier report.

Evidence for histamine and 5-hydroxytryptamine like effects of MK-212 on guinea pig ileum, taenia coli and rat fundus strip.

MK-212 (1 x 10(-7)M -- 1 x 10(-5)M) produced dose-dependent contractions of guinea pig ileum, taenia coil and rat fundus strip. The responses to MK-212 in all three preparations were blocked competitively by cyproheptadine (1 x 10(-8)M) a 5-HT receptor antagonist. Mepyramine (1 x 10(-8)M)-H1 receptor antagonist also inhibited competitively the responses of guinea pig ileum and taenia coli to MK-212. However, it failed to block significantly the responses of rat fundus strip to MK-212. Metiamide (1 x 10(-6)M), propranolol (1 x 10(-6)M) or atropine (1 x 10(-6)M) did not produce any significant effects on MK-212 induced contractile responses of guinea pig ileum, taenia coli and rat fundus strip. Our findings suggest that MK-212 produces both 5-HT as well as histamine like effects on the guinea-pig ileum, taenia coli and rat fundus strip.

Evidence for both H and H2-histamine receptors in rabbit atria.

Spontaneously beating isolated atria of rabbits responded to histamine (0.5-16 micrograms/ml) with positive chrono- and inotropism. However, the inotropic response was greater than chronotropic one. The concentration-response curve of histamine for chronotropic effect was markedly shifted to the right in the presence of 0.5 micrograms/ml metiamide (H2-receptor antagonist), which per se augmented the control contractile amplitude in all the experiments. The rightward shift of chronotropic concentration response curve with mepyramine (H1-antagonist) was, however, moderate. On the contrary, the inotropic concentration response curve of histamine was shifted to much greater extent to right with mepyramine (0.62 micrograms/ml) than with metiamide, thus suggesting a greater share of H1 than H2-receptors in the mediation of positive inotropic effect of histamine. The chronotropic effect appears to be mediated predominently by H2-receptors. Unlike metiamide, mepyramine did not alter the spontaneous frequency or amplitude of contraction. The present study, thus lends support for dual histamine receptors in rabbit atria.

Histamine receptors in guinea-pig isolated urinary bladder.

Spasmogenic action of histamine, 2-(2-aminoethyl) pyridine (H1 receptor agonist) and 4 methyl-histamine (H2 receptor agonist), have been studied in guinea pig isolated urinary bladder in the presence of mepyramine (H1 antagonist) and metiamide (H2 antagonist) to identify the presence of H1 and H2 receptors. The study suggested the presence of H1 as well as H2 receptors in this preparation.