Tachykinins enhance the depression of spinal nociceptive neurons caused by cutaneously applied vibration in the cat

The present investigation was prompted by previous studies in our laboratory which have indicated that tachykinins enhance depressant effects of purines and that the purine adenosine mediates a vibration-induced depression of nociceptive dorsal horn neurons. Extracellular recordings were made from single nociceptive neurons in the lower lumbar segments of anaesthetized cats. Vibration (80 Hz; 2.5-3.5 s every 20-25 s) was applied to the hindlimb using a feedback-controlled mechanical stimulator. The tachykinins physalaemin, substance P and neurokinin A were administered by iontophoresis. Physalaemin was tested on vibration-induced responses of 29 neurons; each neuron was excited by this tachykinin. To control for possible changes in the response to vibration caused by the excitation per se, statistical comparisons were made of the vibration-induced responses during excitation by tachykinins and during excitation by glutamate. In 16 cases the magnitude of the vibration-induced depression was significantly greater during the excitation caused by physalaemin. With the remaining neurons the response to vibration failed to differ during the excitation by physalaemin compared with the excitation by glutamate. In four of the 16 cases subthreshold applications of vibration caused depression after administration of physalaemin. The P1-purinergic (adenosine) antagonist, caffeine, was administered in three cases where vibration caused depression only with application of physalaemin. In each of these cases the depression was reversibly blocked by caffeine (10 or 40 mg kg-1 i.v.). The magnitude of vibration-induced depression was significantly increased during excitation by neurokinin A (5/14 neurons) or by substance P (1/9 neurons). From the results of the present study we suggest that tachykinins enhance the vibration-induced depression. This enhancement may be due to enhanced depression by adenosine.(ABSTRACT TRUNCATED AT 250 WORDS)