Further evidence for norepinephrine as a neurotransmitter stimulating release of melanin-dispersing hormone in the fiddler crab, Uca pugilator: the changes in the melanophores of the crabs following reserpine, 6-hydroxydopamine and bretylium administration.

1. The effects on the melanophores of the fiddler crab, Uca pugilator, of the amine depletor reserpine, the catecholamine neurotoxin 6-hydroxydopamine and the noradrenergic neuron blocker bretylium were investigated. 2. The most striking effect of these drugs was that they strongly reduced the amount of melanin dispersion that normally results on transferring the crabs from a white to a black background. the drugs had no effect on pigment migration in melanophores of isolated legs. These results are in accord with the hypothesis that norepinephrine serves as a neurotransmitter stimulating release of a melanin-dispersing hormone in Uca pugilator. 3. Norepinephrine was found in the central nervous system of Uca pugilator.