Substance P-like immunoreactivity in the trigeminal ganglion. A fluorescence, light and electron microscope study.

The trigeminal ganglion of rat and guinea pig was studied for the presence of immunoreactive substance-P using fluorescence, light and electronmicroscopy. In untreated animals substance P containing cells, with a diameter of 15 to 50 micron, were distributed throughout the ganglion and comprised 10-30% of all ganglion cells. Colchicine, injected intraventricularly to inhibit intra-axonal transport, had no effect on the number of substance P cells; but when the drug was injected directly into the posterior root of the ganglion, the proportion of these cells increased to as much as 50%. In the electron microscope, immunoreactive substance-P was confined to ganglion cells classified as B type according to the arrangement of subcellular organelles, and to unmyelinated nerve fibers. Subcellularly the immunoreactivity appeared in cytoplasmic vesicles, as well as dispersed in the nerve fibers and the perikarya of neurons. The great number of substance P immunoreactive ganglion cells suggests that they do not comprise a well defined subpopulation of the B-cells. However, the immunoreactivity was restricted to a distinct ultrastructural type of neurons with unmyelinated nerve fibers, suggesting that they also may share some distinct functions.