The distribution of substance P-containing neurons in the cat Edinger-Westphal nucleus: relationship to efferent projection systems.

The light microscopic localization of substance P-like immunoreactivity (SPLI) was examined in the cat Edinger--Westphal complex using the peroxidase--antiperoxidase method. A high density of cell bodies and processes staining for SPLI were found in the caudal part of the Edinger--Westphal complex (EWc) capping the somatic divisions of the oculomotor nucleus. This distribution continued rostrally into the anteromedian nucleus (AM). Cells labeled with SPLI were also found arranged in a thin layer dorsally capping the oculomotor nucleus, and scattered cells were found in the periaqueductal gray region at the same level. This distribution of SPLI-positive cells was then compared with the distribution of cells in EWc and AM that are retrogradely labeled by horseradish peroxidase or Nuclear Yellow injections into spinal cord, cerebellum, or ciliary ganglion. Injections of horseradish peroxidase into both cervical and lumbar cord labeled a large number of cells throughout the length of EWc and the more rostral AM. A similar pattern of labeling was seen following injections of Nuclear Yellow into the deep cerebellar nuclei. In contrast, cells innervating the ciliary ganglion were found predominantly outside of the Edinger--Westphal complex in AM, the rostral periaqueductal region, and the tegmentum ventral to the oculomotor complex. The distribution of cells projecting to spinal cord or cerebellum and the pattern of SPLI staining was found to closely overlap, evidence that substance P may be contained in cells that give rise to the central projections of the Edinger--Westphal complex.