Synaptic changes in frog brain after stimulation with potassium chloride

In vitro preparations of frog brains, stimulated by application of CKl were fixed by freeze substitution and examined electron microscopically. Control preparations were bathed in a calcium-free physiological solution with Mg added or in salt solution cooled to 5-10-degrees C. The isolated brain remains viable in the physiological solution as indicated by the direct cortical responses which can be led off from the forebrain. Control preparations were characterized by a row of vesicles situated close to the presynaptic membrane and by the absence of a well-developed postsynaptic web. In KCl stimulated preparations there were, in addition to synapses resembling those in the controls, synapses exhibiting fusion of synaptic vesicles with the membrane of the axonal ending, synapses in which the vesicles had retreated from the presynaptic membrane but were attached to it by a narrow stalk and synapses exhibiting a pronounced postsynaptic web. The synaptic gap was of a less uniform width than in the control preparations. The KCl stimulated preparations were furthermore characterized by a paucity of extracellular space and often showed invaginations formed by the presynaptic membrane and the plasma membranes of the postsynaptic or adjacent glial structure.