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 als o appeared to be a potent stimulator of cAMP formation in intact duck pineal gland (about ten-fold increase above control at 100 mu M), show ing an EC(50) Value of 3-5 mu M. The effect of histamine in the duck p ineal 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 mu M histamine was unaffected b y H-1-, H-2- and H-3-receptor selective antagonists, mepyramine, ranit idine and thioperamide, respectively; yet, two other H-2-antagonists, aminopotentidine and tiotidine, reduced the histamine effect by only 4 0%. Thus, these results, being in line with our previous chick data, g ive support to a suggestion that histamine may be considered a modulat or of the pineal activity, and that the avian pineal may contain a nov el or avian-specific histamine receptor modulating cAMP formation, who se pharmacology is different from that typical for H-1-, H-2-, or H-3- type receptor.