Neurosecretory phenomena in invertebrates

Abstract

Neurosecretion is a general phenomenon in invertebrates as well as in vertebrate phyla. In comparison with other general phylogenetic aspects, it can be assumed that we may learn fundamental structures and functions of neurosecretion especially from the neurosecretory cells of invertebrates. This may be demonstrated in the different developmental steps of neurohemal organs. Morphological studies, initially the basis of research work on cells both at the level of light and electron microscopy. But it is impossible to grasp the multiplicity of the terms “neurosecretion” and “neurosecretory cell” only with these morphological methods. In relation to results in vertebrates we can distinguish two types of secretory neurons: non-peptidergic neurons and the “classical” neurosecretory cell-peptidergic neurons. For the understanding of the phenomenon of neurosecretion and its functions we must include in addition to morphological features biochemical and physiological facts. This is discussed with special emphasis on new results of our investigations. The question of the relation of the “ordinary neuron” to the neurosecretory cell, and the problem of the origin of neurosecretory cells, are discussed with the reference to the multiple secretory activity in the central nervous system of annelids. In the ventral nerve cord of some limicoline oligochaetes, in addition to ordinary nervous cells, neurosecretory cells and glia cells, nonnervous secretory cells are also present. Another problem of general interest is concerned with comparison between the results of cytological studies of neurosecretion and biochemical and physiological investigations. This is discussed with reference to the neurosecretion of the brain of insects, especially of the pars intercerebralis. It seems evident that morphological methods do not prevail the different kinds of neurosecretory material produced by neurosecretory cells, as demonstrated by biochemical and physiological work. Finally, the functional differentiation of the neurosecretory system of the ventral chain of an insect is illustrated by the neurohormonal control of water balance and excretion in larvae of Corethra. In summary, through investigations on the phenomenon of neurosecretion in invertebrates may contribute to a fundamental knowledge of the neuroendocrinology of the whole animal kingdom.