Neurosecretion

Summary Ultrastructural specializations characteristic of sites of release of neurosecretory material from axons were examined in several species of blattarian insects. Discharge of such material may take place within or outside of neurohemal organs and is not restricted to fiber terminals. Structurally distinctive areas serving this function occur intermittently and may be more or less transient. Many of these specialized zones face the extracellular stroma that forms sheaths and partitions of neurohemal organs (corpora cardiaca, perisympathetic organs), others contact various cellular elements (nerve fibers with or without neurosecretory granules, glial cells, non-neural endocrine cells).Irrespective of the milieu, these sites of release are characterized by small electron lucent vesicles clustered near the internal surface of the plasma membrane, and by variously shaped accumulations of electron dense material on either side of this membrane. These ultrastructural features are strikingly similar to those of the presynaptic component of conventional interneuronal junctions. However, the functional implications of this morphological resemblance seem to be limited. In neurosecretory systems, physiological phenomena comparable to chemical transmission are out of the question in the absence of postsynaptic cells.In peptidergic neurons of the insect species used in the present study, as in those of various mammals examined by other investigators, the small vesicles observed seem to be the result of fragmentation of neurosecretory granules prior to the discharge of their contents. The presence of variable intermediate stages speaks against a cholingergic role of these synapticlike vesicles at least some of which seem to contain neurosecretory material instead of a neurotransmitter. Furthermore, the variously shaped intra- and extracellular dense material in synaptoid areas seems to represent a neurosecretory product in transit and is therefore not equivalent to dense material customarily found within or on either side of the regular synaptic cleft.Sites of release not directly affiliated with the stroma often share a common narrow gap between adjoining neurosecretory fibers and face each other in mirror image fashion. It is here where the distinction from regular synapses is sometimes more difficult to make because the structural elements of one side of the paired complex may mimic postsynaptic dense material. A further source of difficulty in the interpretation of special contact areas of this sort is the existence of unusual junctions between two classes of neurosecretory neurons (B and A fibers) in which pre- and postsynaptic details are discernible. These, and synaptoid junctions with non-neural endocrine effector cells, seem to serve for the dispatch of local neurosecretory signals that resemble, but are nevertheless apart from, conventional neurohumoral communication. The special neurosesecretory products involved here do not qualify as neurohormones.Synaptoid neurosecretory contact areas with pre- and postsynaptic features should be classified as a group distinct from another group in which the postsynaptic component is absent.Supported by grants AM-3984, NB-00840, and NB-05219 from the U.S.P.H.S.I am greatly indebted to Mrs. Sarah Wurzelmann for her excellent technical assistance.