Neuroendocrine Correlates of Circadian Rhythmicity in Crustaceans

The secretion of neurohormones from the crustacean X-organ –sinus gland system is controlled by environmental influences,light being the most conspicuous. Two sets of photoreceptorsappear to mediate the influence of light on neurosecretion basedon intracellular recordings from X-organ neurons and estimationsof hormone release. Extra-retinal photoreceptors can initiateneurohormonal release from the eyestalk. Neurosecretory activity is also influenced by putative neurotransmitters.GABA is found in high concentrations in the medulla temninalisof the eyestalk and is released by stimulation, in a calcium-dependentmanner. Diurnal variations occur in the amounts of eyestalk neurohormones,either those present in the eyestalk or released by electricalstimulation of the isolated sinus gland. Rhythm phases varyfrom one hormone to another. Neurohormones secreted in the eyestalkare also found in other regions of the central nervous system.Rhythms of neurosecretion are present both in the secretionin the isolated eyestalk and in eyestalkless animals, thus indicatingthat rhythmicity is a distributed property of the neurosecretorysystem.