The zinc iodide-osmium (ZIO) reaction in the central nervous system: localization and relations to functional activity.

The fine structural characteristics of ZIO reaction was studied in the cerebral and cerebellar cortex and olfactory bulb of the rat and in synaptosomes prepared from rat cerebellar cortex. It was concluded that: 1. Organelles of different nerve cell types exhibit different ZIO reactions provided that the impregnation was carried out under standardized conditions. 2. 6...10 times more synaptic vesicles were stained by ZIO in the inhibitory terminals than in the excitatory ones. 3. ZIO positivity was found in all types of synaptosomes prepared from cerebral cortex. Following electrical or chemical (KCl) depolarization there was a decrease in the number of ZIO positive synaptic vesicles, which decrease was directly proportional to the parameters of stimulation. 4. By x-ray microanalysis Os, Zn and Ca were consistently detected in the ZIO precipitates. Iodine, however, could not always be found. After stimulation the presence of Ca was observed even in those synaptosomes in which the ZIO reaction product was absent. 5. On the basis of the staining characteristics the reaction, under standard conditions, can reflect certain functional states of the nerve terminals.