Ultrastructural localization of substance P-like immunoreactivity in the intermediolateral column of spontaneously hypertensive rats and Wistar-Kyoto rats

The distribution of substance P in the intermediolateral column of the upper thoracic spinal cord of spontaneously hypertensive (SHR) rats and Wistar-Kyoto (WKY) rats was studied by combined retrograde tracing of choleragen subunit-B horseradish peroxidase (CB-HRP) and immuno-electronmicroscopy. In the T-1-T-3 segments of the spinal cord, SP-like immunoreactive products were localized in the cell bodies and dendrites of the sympathetic preganglionic neurons as well as in a few pre-axon terminals or axon terminals. In the neuropil of the intermediolateral column (ILN), different synaptic configurations were observed including synaptic contacts between SP-like positive dendrites and negative axon terminals, and between SP-like positive axon terminals and SP-like positive dendrites. Furthermore, a single SP-like positive dendrite was sometimes postsynaptic to several axon terminals, a feature typical of glomerular synapses. The present findings suggest that most of the SP-like immunoreactive elements in the ILN were of intraspinal origin derived mainly from the sympathetic preganglionic neurons in SHR and WKY rats. Since there was no ultrastructural difference in the distribution of SP between the neural elements in the ILN of SHR and WKY rats, the present findings also suggest that SP may not be directly involved in the hyperactivity of the sympathetic nervous system in hypertension.