Synaptic Vesicle Classes in Torpedo and Skate Electric Organ and Muscle

Abstract

Presynaptic nerve terminals contain spherical membrane-bound structures termed synaptic vesicles, which contain transmitter substances (Whittaker, 1984, Zimmermann, 1982). A calcium mediated mechanism fuses vesicles with the presynaptic membrane resulting in the release of transmitter into the synaptic cleft. The increase in internal calcium results from an inward calcium current through a voltage dependent calcium channel which is activated by the action potential. The vesicular shell either pulls back (Ceccarelli and Hurlbut, 1973) or is later internalized by a membrane recycling mechanism (Heuser and Reese, 1973) and subsequently is filled with transmitter for another cycle. Physiologically, transmitter is spontaneously released in the form of quanta which produce miniature end-plate potentials (MEPP); and, these quanta are synchronously released with a nerve action potential to generate the evoked end-plate potential (Fatt and Katz, 1952; del Castillo & Katz, 1954; Boyd and Martin, 1956). These observations form the basis of the quantum of transmitter release. There is evidence from freeze fracture studies with potentiated release that the fusion of 1 vesicle generates 1 quantum (Heuser, Reese, Jan, Jan, and Evans, 1978; Katz and Miledi, 1979) However, several problems have arisen with the hypothesis that one vesicle generates one MEPP. One has been the identification of the sub-MEPP class of quanta which appears in normal preparations as 1/10 the size of the larger class and both classes have the same time characteristics (Erxleben & Kriebel, 1988a; Kriebel and Gross, 1974; Kriebel et al., 1976, 1982). In addition, MEPP amplitude histograms show integral peaks which suggest an alternate hypothesis whereby each MEPP contains 7 to 10 sub-units that are the size of sub-MEPPs (Kriebel and Gross, 1974; Kriebel et al. 1976, 1982; Carlson & Kriebel, 1985; Erxleben & Kriebel, 1988b). Anatomically, this means that each MEPP may represent either the simultaneous release of 10 vesicles or that sub- units are contained within one vesicle. Potentiated release with an action potential shows that quantal content is in excess of the number of vesicles contacting the presynaptic membrane (Katz and Miledi, 1979) which implies that it may be impossible to correlate vesicles and quantal content of an end-plate potential. The sub-MEPPs have been observed in the skate (Kriebel, Gross & Pappas, 1986 and in Torpedo (Muller & Dunant, 1986; Kriebel, Fox, and K6tting this issue) and there is some indication of a subunit composition of MEPPs. The classes of MEPP quanta have not been correlated with different classes of vesicles (Kriebel & Florey, 1983; Kriebel, Hanna, Muniak, 1986; Kriebel & Pappas, 1987). However in the Torpedo electric organ a smaller class of vesicles is thought to represent the refilled vesicles (Zimmermann and Whittaker, 1974). More recent studies indicate that this population may represent those vesicles actively involved in transmission and recycling (Stadler & Kiene, 1987) with the larger diameter vesicles functioning more in a storage capacity. Stadler and Kiene (1987) have demonstrated three classes of vesicles based on ACh content with a ratio of 0:1:5-10. The two classes containing ACh may correspond to the MEPP and sub-MEPP class, but only one vesicle class was found in the skate (Kriebel, Gross, and Pappas, 1987).