5-Hydroxytryptamine Mediates Release of Moltinhibiting Hormone Activity From Isolated Crab Eyestalk Ganglia

Neurosecretory cells in crustacean eyestalk ganglia produce a putative molt-inhibiting hormone (MIH) which directly suppresses production of the steroid molting hormone, ecdysone, by the peripheral Y-organs. Neurotransmitter mediation of MIH release from isolated eyestalk ganglia of the crab, Cancer antennarius, was explored using an MIH bioassay based upon in vitro inhibition of Y-organ ecdysteroid production by eyestalk ganglion-conditioned saline. The conditioned saline (0.01-1.0 eyestalk equivalent) inhibited Y -organ ecdysteroid production dose-dependently and reversibly, and the effect of the saline was specific as to conditioning tissue. Isolated ganglia released a significant portion of their MIH activity in 2-h incubations, but also retained a significant portion. 5-hydroxytryptamine (5-HT) enhanced MIH release at concentrations of 10-10 M to 10-6 M. Acetylcholine, dopamine, octopamine, norepinephrine, or gamma-aminobutyric acid (10-7 M-10-6 M) did not alter basal MIH release. The 5-HT precursor, 5-hydroxytryptophan (10-7 M), stimulated MIH release, while the tryptophan hydroxylase inhibitor, p-chlorophenylalanine (10-6 M), and the 5-HT receptor antagonist, cyproheptadine (10-7 M), inhibited MIH release to below basal levels. Apparently, 5-HT neurons provide excitatory input to MIH-containing neurosecretory cells.