A bizarre receptor mediating stimulatory effect of histamine on cyclic AMP formation in duck pineal gland

In agreement with our previous findings in chick pineal, histamine also appeared to be a potent stimulator of cAMP formation in intact duck pineal gland (about ten-fold increase above control at 100 microM), showing an EC50 value of 3-5 microM. The effect of histamine in the duck pineal was mimicked by several histaminergic drugs, with the following order of potency: histamine = 4-methylhistamine > 2-methylhistamine > R-alpha-methylhistamine > amthamine > 2-thiazolylethylamine = immepip > imetit > dimaprit. The effect of 100 microM histamine was unaffected by H1-, H2- and H3-receptor selective antagonists, mepyramine, ranitidine and thioperamide, respectively; yet, two other H2-antagonists, aminopotentidine and tiotidine, reduced the histamine effect by only 40%. Thus, these results, being in line with our previous chick data, give support to a suggestion that histamine may be considered a modulator of the pineal activity, and that the avian pineal may contain a novel or avian-specific histamine receptor modulating cAMP formation, whose pharmacology is different from that typical for H1-, H2-, or H3-type receptor.