Quantitative and morphological studies on the plasticity of mixed synapses as exemplified by clear, spherical vesicles.

Large mixed axosomatic synapses (synaptic membrane with chemically "active zones" and electric "gap-junction" parts) within the region of the oculomotor nucleus of the trout were stimulated for a prolonged period of time (14 days) under physiological conditions. Concerning the great number of clear vesicles that are found in the presynaptic part of this type of synapse it can be said that they can be subdivided morphometrically into two classes with respect to the special membrane complexes of the mixed synapse (active zone, gap junction). This paper is designed to find out to what an extent the proven adaptability of the "mixed synapse" system does reflect the quantitative changes in the population of these organelles. For this purpose the number (numerical density of vesicles NVv) as well as the size (vesicular volume Vv) of the vesicles were determined by stereologic methods from segments of vesicles that were visible in the electron microscopic picture. Stimulation of the contact caused an accumulation of vesicles in the presynaptic area of the active zone as well as in the region of the terminal axonal cylinder (myelin-free presynaptic part of the axon--"marginal zone"). At the gap junctions the number of vesicles remained unchanged, but their size diminished significantly. On the one hand, the findings obtained are discussed as an expression of potential compensatory mechanisms involved in vesicle supply or formation (axonal transport, recycling) for chemical transmission, and, on the other, as indicating possible interactions between vesicles and gap junctions for electric transmission. They suggest that the behaviour of the vesicle population of mixed synapses is governed by laws of its own.