A light and electron microscopic study of the rat superior cervical ganglion cells by intracellular HRP-labeling.

Horseradish peroxidase (HRP) was injected intracellularly into single neurons of the isolated rat superior cervical ganglia. Intracellular iontophoresis of HRP did not seem to damage the sympatheticneurons or to affect synaptic transmission. Under the light microscope, 9 of the 27 HRP-labeled sympathetic neurons exhibited varicosities in their dendrites, but not in their axons; the varicose dendrites came into close contacts with adjacent non-labeled neurons. With the electron microscope, the varicose dendrites of 3 separate neurons lightly stained with HRP, morphological features of synapses could be identified at the contact site: clusters of vesicles in the varicose dendrites, intercellular space of about 20 nm separating the apposed membranes, and an intermediate density on the postjunctional membrane. These findings suggest at the ultrastructural level the occurrence of dendrodendritic and dendro-somatic synapses in mammalian sympathetic ganglia.