LOCALIZATION OF SYNAPSIN-I IN NORMAL FIBERS AND REGENERATING AXONAL SPROUTS OF THE RAT SCIATIC-NERVE

The localization of synapsin I, a synaptic vesicle-associated protein, was investigated immunocytochemically in normal nerve fibers and rege nerating axonal sprouts following crush-injuries to the rat sciatic ne rve. In normal myelinated axons, weak synapsin I immunoreactivity was found in the axoplasmic/smooth endoplasmic domains, but not in the cyt oskeletal domains comprising neurofilaments and microtubules. In non-m yelinated axons without dense cytoskeletal structures, moderate immuno reactivity was distributed diffusely throughout the axoplasm. In the c rush-injured nerves, intense synapsin I immunoreactivity was demonstra ted by Light microscopy in early regenerating sprouts emerging from no des of Ranvier These nodal sprouts subsequently elongated as regenerat ing axons through the space between the basal lamina and the myelin sh eath (or Schwann cell plasma membrane). Intense synapsin I immunoreact ivity was also found in the growth cones of such long regenerating axo ns. Electron microscopy revealed that synapsin I immunoreactivity was associated mainly with vesicular organelles in the nodal sprouts and g rowth cones of regenerating axons. Long regenerating axons exhibited n o synapsin I immunoreactivity in the shaft, which contained an abundan ce of neurofilaments. However, vesicle accumulations remaining in the periphery of the shaft still exhibited intense synapsin I immunoreacti vity. Thus, it can be concluded that synapsin I is localized at especi ally high density in the domains comprising vesicular organelles, whic h are characteristic of early nodal sprouts, as well as in growth cone s of regenerating axons. These findings, together with the proposed fu nctions of synapsin I investigated in other studies, suggest that syna psin I may play important roles in vesicular dynamics including the tr anslocation of vesicles to the plasma membrane in sprouts and growth c ones of regenerating axons.