Synaptic Spinules Attendant on Post-tetanic Potentiation in Cat Sympathetic Ganglion.

Abstract

A synaptic spinule formed on the postsynaptic membrane in the cat superior cervical ganglion (SCG) in vivo was electron microscopically studied. When the preganglionic nerve fiber was tetanically stimulated at 10Hz for more than 10 seconds, a post-tetanic potentiation (PTP) was electrophysiologically detected and maintained up to 30min. Concurrently with PTP, a small process, termed as synaptic spinule, was generated as a small projection standing out from the postsynaptic dendritic membrane with a straightly extended contour. The spinule produced a break at about a middle point of the postsynaptic density (PSD) and divided it into two parts forming a perforated synapse. Then, the spinule increased in size, invaginated into the cytoplasm of the nerve terminal, and subdivided the presynaptic terminal bag into two compartments. Each of the compartments retained a morphology of an active synaptic site with a cluster of synaptic vesicles associated with a presynaptic active zone standing opposite to the one part of the subdivided PSD. The longer the tetanic stimulation in period, the more the synapses with the spinule increased in number. In addition to this, a few spinules (multiple type of spinule) were often formed after the stimulation of 1min and more than two compartments were generated in a single nerve ending. The present observations have suggested that the synaptic spinule is a morphological representation of the “synaptic plasticity” which is electrophysiologically displayed as the PTP in cat SCG in vivo.