Segmental distribution of peptide‐like immunoreactivity in cell bodies of the thoracolumbar sympathetic nuclei of the cat

Abstract

The distribution of leucine-enkephalin, methionine-enkephalin, neurotensin, somatostatin, substance P, oxytocin, vasopressin, and neurophysin II in cell bodies of sympathetic autonomic nuclei of the thoracolumbar (T-L) spinal cord was studied immunohistochemically in cats after intrathecal administration of colchicine. Neurons containing only enkephalin-, neurotensin-, somatostatin-, and substance P-like immunoreactivity (ENK, NT, SS, SP, respectively) were found in the intermediolateral nucleus pars principalis (IMLp) and pars funicularis (IMLf), the nucleus intercalatus (IC), and the central autonomic area (CA). The size, shape, location, and numbers of the peptide-positive neurons in the IMLp, IMLf, and IC suggested that they were sympathetic preganglionic neurons (SPN). This was confirmed by a combined retrograde tracing/immunohistochemical study showing that most of these neurons at the levels of the T-L cord known to provide preganglionic fibers to the stellate ganglion were SPN. On the other hand, the functional identification of the neurons in the CA is uncertain as neurons were not observed which were both retrogradely labelled and contained ENK, NT, SS, or SP. Immunoreactive neurons in each area were counted in ten sections from each segment from C8 to L4. In the IMLp, the SPN with ENK were greatest in number (up to 25) in segments T4-T7 and L2-L3. The maximum number of SPN containing NT was found in segments T4-T7 (45 neurons). Of the four peptides, neurons containing SS were found in the greatest number (up to 48 in segments T2-T6); neurons containing SP were found in the smallest number (15 or fewer per segment). Few SPN containing each of the four peptides were found in the IC; CA neurons with ENK and NT were also few in number. A comparison of the numbers of immunoreactive neurons in the IML with earlier estimates for the total numbers of SPN in the IML at each level showed that the proportions of IML neurons containing each of the four peptides were fairly consistent throughout the T-L cord, with some exceptions. These results suggest that the innervation of visceral organs is not obviously peptide-specific, although some organs may be innervated by a greater proportion of SPN containing one of these peptides. Finally, the presence of ENK, NT, SS, and SP in SPN suggests that these four peptides act as neurotransmitters in preganglionic pathways to sympathetic ganglia.