Tachykinns mediate changes in spinal reflexes after activation of unmyelinated muscle afferents in the rat

The effect of intrathecally applied tachykinin antagonist D-NicLys¹, 3-Pal³, D-Cl² Phe⁵, Ama⁶, Trp^7,8, Nle¹¹-substance P, spantide II, on the long-term increase of spinal cord excitability after activation of unmyelinated muscle afferents was studied in decerebrate, spinalized, unanaesthetized rats. A conditioning stimulus train (1 Hz, 20 s) that activated unmyelinated fibres in the gastrocnemius muscle nerve facilitated the flexor reflex for about 1 h, which was strongly blocked by pretreatment with spantide II (3 µg). The present results indicate that the facilitation of the flexor reflex by conditioning stimulation of a muscle nerve is mediated by tachykinins and possibly other neuropeptides which may be released from the central terminals of these unmyelinated afferents.