Cell type and conduction velocity of rat primary sensory neurons with substance P-like immunoreactivity.

A double labelling immunohistochemical study of rat L4 dorsal root ganglia was made with an anti-substance P antibody and with an antibody, RT97, which specifically labels the light cell population. Substance P-like immunoreactivity was found in 30% of the small dark neurons and 10% of the light neurons. Of the neurons with substance P-like immunoreactivity, 30% were light neurons and 70% were small dark neurons. A study of electrophysiologically characterized neurons in lumbar dorsal root ganglia with dye injection revealed substance P-like immunoreactivity in neurons with conduction velocities ranging from 0.5 to 9.5 m/s. It was seen in 50% of C-fibre neurons, in 20% of A delta-fibre neurons, and in no A alpha/beta neurons. Overall, substance-P-like immunoreactivity was found in 10% of A-fibre neurons sampled.