Stimulus specificity of peripherally evoked substance P release from the rabbit dorsal horn in situ

Characteristics of in situ substance P release from the lumbar dorsal horn were investigated in decerebrated rabbits. Noxious mechanical stimuli produced by pinching the skin of a hind leg ipsilateral to the perfusion site remarkably and significantly increased the release of immunoreactive substance P, which was identified as substance P itself, using separation with high-performance liquid chromatography and radioimmunoassay. The noxious pinch did not affect the release of immunoreactive substance P, when applied to the contralateral hind leg. Both the basal and pinch-evoked release of immunoreactive substance P were largest in the dorsolateral part of the dorsal horn. The pinch-evoked release of immunoreactive substance P was abolished when the dorsal horn was perfused with a Ca2+-free medium containing 7 mM Mg2+ or with a medium with 10 microM tetrodotoxin added. The evoked release of immunoreactive substance P was also abolished following pretreatment of a stimulated region with the local anesthetic dibucaine, a procedure which inhibited the pinch-evoked aversive behavior in freely-moving rabbits. Among a variety of natural stimuli applied to the hind leg, noxious pinch and a subcutaneous injection of formaldehyde solution significantly evoked the release of immunoreactive substance P from the dorsal horn. The most intense heat or scalding stimulation increased the immunoreactive substance P release in two out of five experiments. However, other natural stimuli such as ice-cold, warm, noxious heat and innocuous mechanical stimuli produced no apparent changes in the release of immunoreactive substance P. These results suggest that among the noxious stimuli, only mechanical and inflammatory but not thermal stimuli lead to a release of substance P from the primary afferent terminals in the dorsal horn. The present findings suggest that, at least in rabbits, substance P-containing primary afferents have high-threshold mechanoreceptors. Substance P may participate in the transmission of information related to noxious mechanical and inflammatory stimulation from the periphery to the dorsal horn.