EFFECT OF SUBSTANCE-P ON MECHANOSENSITIVE UNITS OF TISSUES AROUND ANDIN THE LUMBAR FACET JOINT

The purpose of this study was to examine how substance P affects the m echanosensitive 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 acti vity of the units was recorded from dorsal root filaments. Changes in afferent discharge rates and von Frey thresholds were measured sequent ially after the application of substance P. Most of the units (83.3%) had an increase in the ongoing discharge rates after the application o f substance P: 54.2% of the units had immediate excitation and 29.2% h ad delayed excitation. One-third of the units had a decreased von Frey threshold after the application of substance P. Substance P had an ex citatory 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 o n 84.6% of the units with a threshold of <2.0 g, which may serve as pr oprioceptors. These results suggest that substance P has an excitatory effect on both nociceptive and proprioceptive units in the tissues ar ound and in the lumbar facet joint.