Effect of substance P on mechanosensitive units of tissues around and in the lumbar facet joint.

The purpose of this study was to examine how substance P affects the mechanosensitive afferent units identified in the tissues around and in the lumbar facet joint of the rabbit. Substance P was applied to the receptive fields of the units by microinjection, and the afferent activity of the units was recorded from dorsal root filaments. Changes in afferent discharge rates and von Frey thresholds were measured sequentially after the application of substance P. Most of the units (83.3%) had an increase in the ongoing discharge rates after the application of substance P: 54.2% of the units had immediate excitation and 29.2% had delayed excitation. One-third of the units had a decreased von Frey threshold after the application of substance P. Substance P had an excitatory effect on 81.8% of the units with a threshold of > 5.0 g and a conduction velocity of < 30 m/s, which may serve as nociceptors, and on 84.6% of the units with a threshold of < 2.0 g, which may serve as proprioceptors. These results suggest that substance P has an excitatory effect on both nociceptive and proprioceptive units in the tissues around and in the lumbar facet joint.