Observations on the ultrastructure of synapses in some pulmonate molluscs

1432-0878

Springer Online Journal Archives 1860-2000

Biology

Medicine

Summary Submicroscopic structure of synapses of Vaginula solea, Helix pomatia, and Cryptomphallus aspersa was studied. Synaptic junctions in these nervous tissues are always of the axo-axonal type. Perikarya are located at the periphery of the ganglion and never show synaptic contacts. These appear beyond the axon hillock, at the neuropile situated in the core of the ganglion. The postsynaptic membranes are mainly located on finger-like axonal branches. The presynaptic endings may be either “en passant” or of the terminal type, the latter being much more abundant than the former. The general structure of these synapses does not differ in their submicroscopic features from those described in vertebrate synapses, but they may contain, apart from mitochondria, different types of vesicular material: 1) Clear synaptic vesicles (CSV) (450–600 Å in methacrylate, 600–800 Å in epon) similar in structure to classical synaptic vesicles, 2) dense synaptic vesicles (DSV) (600 to 750 Å in methacrylate, 800–1,100 Å in epon) containing a very dense granule. DSV highly resemble vesicles observed in vertebrate adrenergic nerve endings, and 3) “neurosecretory” synaptic vesicles (NSV) (800–1,300 Å in methacrylate, 1,200–1,400 Å in epon) similar to elementary neurosecretory granules observed in “neurosecretory” neurons of both Vertebrates and Invertebrates. Other contents of synapses are: a dense granular ribosome-like material which appears mixed with CSV; clear empty vesicles of 800–900 Å and multivesicular bodies that are observed accompanying NSV. According to the distribution of this vesicular material, synaptic endings are classified as: Type I: contain only CSV; Type II: contain DSV and clear vesicles similar to CSV (proper CSV or empty forms of DSV) and Type III: contain NSV, clear vesicles of 800–900 Å, clear vesicles resembling CSV and multivesicular bodies (Table 2). These findings are discussed on the bases of available biochemical and physiological data. Microphysiological and biochemical work has demonstrated in these ganglia the existence of cholinergic and non-cholinergic synaptic transmitters. As CSV are analogous to classical synaptic vesicles present in cholinergic synapses, endings of the type I are postulated as possibly cholinergic. Non-cholinergic synaptic input is hypothetically related to DSV, therefore to synapses of the type II and the possible nature of the involved transmitters is considered. The “neurosecretory” character of NSV is discussed and its possible participation in nervous activity is postulated. Main facts and hypothesis are also summarized in Table 1, 2 and 3.

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