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

Abstract

Histamine (HA), a stimulator of H, and H 2 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 H 2 receptor agonist, also inhibited melanin dispersion. 2-Methyl histamine (2-MeHA), an H, receptor agonist, enhanced melanin dispersion. The inhibitory effects of HA and 4-MeHA were abolished by the H 2 receptor blocker metiamide but not by blockers of either H, receptors or alpha, adrenoceptors. Melanin-dispersing hormone (MDH) release is accomplished mainly by stimulation of alpha, adrenoceptors with norepinephrine appearing to be the neurotransmitter involved. The H, 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 alpha 2 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 H, and H 2 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 H 2 receptors.

Histamine operates Cl–-gated channels in crayfish neurosecretory cells

“…In addition it has been demonstrated that HA mediates presynaptic inhibition in the escape reaction of the Procambarus clarkii crayfish (el Manira and Clarac, 1994), as well as presynaptic inhibition of primary olfactory afferents in lobster (Wachowiak and Cohen, 1999). It is known that HA injected into the circulation inhibits the black pigment dispersion that normally occurs in crabs transferred from a white to a black background in a dose-dependent manner (Hanumante and Fingerman, 1982). Recently we found that plasma levels of HA in the crayfish fluctuate during the day, reaching a maximum value at daybreak (Cebada et al, 2006).…”