INHIBITORY EFFECT OF HISTAMINE ON THE RELEASE OF MELANIN-DISPERSING HORMONE IN THE FIDDLER CRAB,UCA PUGILATOR

Abstract

Histamine (HA), a stimulator of H1 and H2 receptors, produced dose-dependent inhibition of the melanin dispersion which normally occurs when fiddler crabs, Uca pugilator, are transferred from a white to a black background. The HA precursor L-histidine, and 4-methyl histamine (4-MeHA), an H2 receptor agonist, also inhibited melanin dispersion. 2-Methyl histamine (2-MeHA), an H1 receptor agonist, enhanced melanin dispersion. The inhibitory effects of HA and 4-MeHA were abolished by the H2 receptor blocker metiamide but not by blockers of either H1 receptors or alpha1 adrenoceptors. Melanin-dispersing hormone (MDH) release is accomplished mainly by stimulation of alpha1 adrenoceptors with norepinephrine appearing to be the neurotransmitter involved. The H1 receptor blockers pyrilamine and SA-97 antagonized 2-MeHA. HA-induced inhibition of melanin dispersion was potentiated by the noradrenergic neuron blocker bretylium and the alpha2 adrenoceptor agonist B-HT 933. HA did not significantly affect melanin dispersion in crabs pretreated with 6-hydroxydopamine which destroys catecholaminergic neuroterminals. None of these drugs affected the melanophores directly. On the basis of these and previously obtained results it is suggested that H1 and H2 receptors are present on norepinephrine neurons involved in triggering MDH secretion, and administered HA inhibits MDH release by decreasing impulse-mediated noradrenergic neurotransmission through stimulation of H2 receptors.