Effect of histamine and its blockers on plasma beta-endorphin-like immunoreactivity in rats

The effect of histamine and related compounds on the plasma beta-endorphin-like immunoreactivity (beta-En-Li) levels in rats were studied. Histamine (2.5 mg/kg), mepyramine (10 mg/kg) or famotidine (5.0 mg/kg) was injected i.p., and the animals were serially decapitated. The plasma beta-En-Li levels were measured by radioimmunoassay. Effects of histamine, mepyramine and famotidine on beta-En-LI release from the anterior pituitary were also investigated by means of an in vitro experiment. The beta-En-LI content in the hypothalamus and pituitary gland did not change significantly after histamine, mepyramine and famotidine injection. The plasma beta-En-LI levels increased significantly in a dose-related manner with a zenith at 20 min after histamine injection and decreased significantly after mepyramine injection, but did not change significantly after famotidine injection. Effects of histamine on plasma beta-En-LI levels were prevented with the pretreatment of mepyramine. The beta-En-LI release from the anterior pituitary was enhanced with the addition of histamine to the medium, and histamine's effects were blocked with an addition of mepyramine to the medium. These findings suggest that histamine acts as to stimulate beta-En-LI release from the anterior pituitary, and that histamine's effects may be mediated via H1-receptor.