SYNAPSES ON AXONS OF SYMPATHETIC PREGANGLIONIC NEURONS IN RAT AND RABBIT THORACIC SPINAL-CORD

Axosomatic and axodendritic synapses occur on sympathetic preganglioni c neurons, but it is not yet known whether their axons receive synapti c input, which could be particularly effective at regulating sympathet ic outflow. Here, we examined retrogradely labelled sympathetic pregan glionic axons to see if they received synapses. Cholera toxin B subuni t (CTB) or CTB conjugated to horseradish peroxidase (CTB-HRP) was used to label neurons projecting to the rat or rabbit superior cervical ga nglion, the rat adrenal medulla, or the rabbit stellate ganglion. At t he light microscopic level, small groups of CTB-immunoreactive axons t ravelled through the ventral horn near its lateral boundary, with occa sional axons taking a more medial course. The axons passed through the ventrolateral funiculus to exit at the ventral roots. In parasagittal section, a few axons branched within the ventral horn, sending proces ses rostrally and caudally for short distances before they turned vent rally to exit the spinal cord. At the ultrastructural level, CTB-immun oreactive rat and rabbit sympathetic preganglionic axons were almost e xclusively unmyelinated. In contrast, labelling with CTB-HRP revealed both myelinated and unmyelinated axons in the ventral horn, the ventro lateral white matter, and the ventral roots. CTB-HRP also allowed the detection of the initial segment of a sympathetic preganglionic axon. Synapses, with vesicles clustered presynaptically and membrane special izations postsynaptically, were found on some unmyelinated CTB-immunor eactive axons. Occasional axons received several synapses. Synapses we re most common on CTB-containing axons just ventral to the intermediol ateral cell column. One synapse was found on an axon within 2 mu m of its origin from a proximal dendrite. Rare synapses were found several hundred micrometers ventral to the intermediolateral cell column. One branching axon had synapses just below the branch point on both the ma in axon and the axonal branch. These findings indicate an extensive sy naptic input to the axons of at least some sympathetic preganglionic n eurons. These axoaxonic synapses could have a profound effect on sympa thetic activity. (C) 1995 Wiley-Liss, Inc.