Regulation of neurosecretory activity in the freshwater pulmonate Lymnaea stagnalis (L.) with particular reference to the role of the eyes: a quantitative electron microscopical study.

The process of neurosecretion in the Caudo-Dorsal Cells (CDC) of the freshwater snail Lymnaea stagnalis, which produce an ovulation hormone, shows a diurnal rhythmicity. Synthesis, transport and release of the neurosecretory material (NSM) is high during the evening and the early night and low during the rest of the day, while storage of NSM mainly occurs during the daytime. In the present study the role of the eyes in the regulation of the CDC-rhythm was investigated. During a 24-hr period, at time intervals of 6 hrs, cerebral ganglia, which contain CDC, of blinded and control snails (5 per group) were fixed and the CDC were studied with quantitative electron microscopical methods. The CDC of the controls showed a distinct diurnal thythmicity. Blinding, on the other hand, clearly affected this rhythmicity. The results indicate that after blinding the circadian CDC-rhythms of individual snails are no longer synchronous with each other ("interanimal desynchronization"). It is suggested that the rhythm of CDC neurosecretory activity is synchronized by the natural light/dark cycle via the eyes. The information from the eyes probably reaches the CDC via a nervous pathway. True snyapses and three types of synapse-like structures were found on the CDC. Their role in the regulation of CDC-activity is discussed. The effect of blinding is specific for the CDC; blinding does not influence the diurnal rhythmicity of another type of cerebral neurosecretory cells, the Light Green Cells (LGC). The CDC within a cluster act synchronously. This synchrony does not depend upon the presence of the eyes. Some structures which may be involved in establishing this synchrony, such as subsurface cisterns, desmosome-like structures and "specific release sites", are described.