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

¹ Department of Biology, Tulane University, New Orleans, Louisiana 70118

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.

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