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In the cat caudate nucleus the same nine types of synapses are found as in putamen and fundus striati. The three parts of the striatum in the strict sense do not differ in the morphological differentiation of synapse types but only in their quantitative distribution. One-third of all synapses in the caudate nucleus are axo-spinous type IV synapses with a curved and divided asymmetric contact. This strongly suggests that the caudate nucleus interneuronal apparatus is dominated by centre-median input, in contrast to the putamen which is controlled by the cortico-striatal input to its internuncial cells and by its strong intrastriatal feedback mechanism. Extensive destruction of the convexity of the cortex and the medial cortex in one hemisphere results in dark degeneration of a large proportion of two of the nine types of caudate synapses: a) the axo-dendritic type VII synapses exciting the large spiny caudate neurons and b) the axo-spinous type III synapses making contact with the small spiny neurons of the interneuronal cell apparatus.

The ultrastructure of the hippocampal mossy fibre layer was studied in ultrathin sections and freeze-fracture preparations of rabbits under deep Nembutal anaesthesia, after recovery from ether anaesthesia, and 40 min after a single injection of methoxypyridoxine, that is, during the second generalized seizure discharge. The giant mossy fibre boutons contain two types of vesicles: evenly distributed, small round clear vesicles (50 nm) and a few scattered large dense-core vesicles (100 nm). In rare instances fusion of dense-core vesicles with the presynaptic membrane was observed. No differences in the morphology of the mossy fibre synapses were found between anaesthetized and unanaesthetized animals. During epileptiform seizures, however, the size and shape of clear and dense-core vesicles varied greatly. The active synaptic zones were covered with large, core-containing omega profiles or bumps and indentations. Only dense-core vesicles seem to undergo exocytosis. A fusion of clear vesicles with presynaptic membrane was not observed. Various explanations for the fact that only dense-core vesicles seem to undergo exocytosis are discussed. The hypothesis is put forward that in the mossy fibre bouton two morphologically and functionally distinct populations of synaptic vesicles exist and that only one of them undergoes visible irreversible exocytosis, whereas the majority, that is, the small vesicles discharge their transmitter by reversible fusion. After MP injection features of membrane retrieval were also prominent. Frequently, at the borders of the active synaptic zones coated membrane convolutes of both pre- and postsynaptic membranes had invaded the terminals as well as the postsynaptic spine. Thus, in contrast to electrical stimulation, the self-sustained seizures allows energy-expensive processes such as extensive membrane internalization to take place during the interictal pauses.

Isolated chromaffin granules incubated in 10 millimolar calcium chloride aggregated, forming contact sites with a pentalaminar membrane structure. These circular attachment sites were free of membrane-associated particles, which accumulated at the periphery. Incubation in 20 millimolar ethylenediaminetetraacetic acid reversed these changes, which are regarded as initial events in the membrane fusion reaction.

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