Substance P-containing pathways to avian sympathetic preganglionic neurons: evidence for major spinal-spinal circuitry.

The origin of substance P-like immunoreactivity (SPLI) in the neuropil of sympathetic preganglionic neurons (SPNs) was investigated in pigeon (Columba livia). Previous investigators had suggested that a major SPLI-containing projection to SPNs arose from caudal brainstem (Johansson, O., T. Hokfelt, B. Pernow, S. L. Jeffcoate, N. White, H. W. M. Steinbusch, A. A. J. Verhofstad, P. C. Emson, and E. Spindel (1981) Neuroscience 6: 1857-1881; Gilbert, R. F. T., P.C. Emson, S. P. Hunt, G. W. Bennett, C. A. Marsden, B. E. B. Sandberg, H. W. M. Steinbusch, and A. A. J. Verhofstad (1982) Neuroscience 7: 69-87; Helke, C. J. J. J. Neil, V. J. Massari, and A. D. Loewy (1982) Brain Res. 243: 147-152) and, furthermore, that the bulbospinal fibers and terminals in the SPN neuropil which contained SPLI also contained serotonin-like immunoreactivity (5-HTLI) (Johansson, O., T. Hokfelt, B. Pernow, S. L. Jeffcoate, N. White, H. W. M. Steinbusch, A. A. J. Verhofstad, P. C. Emson, and E. Spindel (1981) Neuroscience 6: 1857-1881; Gilbert, R. F. T., P.C. Emson, S. P. Hunt, G. W. Bennett, C.A. Marsden, B. E. B. Sandberg, H. W. M. Steinbusch, and A. A. J. Verhofstad (1982) Neuroscience 7: 69-87). In the present study, various spinal lesions were made and the SPLI and 5-HTLI content of thoracic spinal cord was examined using immunohistochemistry. Interruption of descending bulbospinal fibers by cervical hemisection had no demonstrable effect on SPLI in the SPN neuropil, while 5-HTLI was almost totally depleted in the half of the spinal gray (including the SPN cell column) ipsilateral to the hemisection. Following a complete thoracic transection, SPLI was only depleted 2 to 3 mm rostral and caudal to the lesion, while normal SPLI staining was present in the remainder of the spinal cord. 5-HTLI was totally depleted caudal to a thoracic transection. Dorsal rhizotomy of three to six sequential spinal segments significantly depleted SPLI in the dorsal horn but had no effect on SPLI in the region of SPNs. Major depletion of SPLI within the SPN cell column was only seen when portions of thoracic spinal cord were isolated by complete transections or unilateral hemisections. Finally, evidence was found for intraspinal SPLI-containing fiber systems. These results demonstrate that the majority of SPLI in the SPN cell column in the pigeon is probably of intraspinal origin. The data also confirm that 5-HTLI is contained in axons and terminals arising from cell bodies of supraspinal origin.