Freeze–fracture studies on unmyelinated axolemma of rat cervical sympathetic trunk : correlation with saxitoxin binding

Abstract

The density and diameter distributions of intramembranous particles (IMPS) within unmyelinated axolemma from rat cervical sympathetic trunk were examined with freeze–fracture electron microscopy. The axolemma displays a highly asymmetrical partitioning of IMPS withca. 1200 IMPS μm–2on P-faces andca. 100 IMPS μm–2on E-faces. Particle sizes (diameters) are unimodally distributed on both fracture faces, with a range from 2.4 nm to 15.6 nm. Approximately 16% of the particles on P-faces and 28% of particles on E-faces are of a large (greater than 9.6 nm) diameter. On both fracture faces, the IMPS appear to be randomly distributed; no aggregations of particles were observed. The results indicate that there areca. 230 large IMPS μm–2of unmyelinated axolemma from rat cervical sympathetic trunk. The density of these IMPS is similar to the density of saxitoxin binding sites on unmyelinated axolemma from rat cervical sympathetic trunk (Pellegrinoet al. 1984 (Brain Res. 305, 357–360)), which suggests that many of the large diameter particles may be the morphological correlate of voltage-sensitive Na+channels.