CHARACTERISTICS OF SUBSTANCE P-EVOKED RELEASE OF AMINO-ACIDS FROM NEONATAL RAT SPINAL-CORD

The characteristics of release of amino acids evoked by substance P from the neonatal rat spinal cord were examined. A hemisected spinal cord was continuously perfused and the release of amino acids into the perfusate was measured using high-performance liquid chromatography with a precolumn derivatization technique. Substance P (10 mu M) evoked a significant increase in the release of aspartate, glutamate, GABA, glycine and taurine. The substance P-evoked release of these five amino acids was not reduced by Ca2+-free medium, but was blocked by [D-Pro(4),D-Trp(7,9,10)] substance P-4-11 (10 mu M). Perfusion of the spinal cord with low-Na+ medium (22 mM) induced a marked increase in the high-K+ (90 mM)-evoked release of GABA, glutamate and glycine. In contrast, the substance P-evoked release of the five amino acids was significantly decreased by the low-Na+ medium. Similarly, perfusion of the spinal cord with low-Cl- medium (8 mM) increased the high-K+-evoked release of GABA and glycine, but decreased the substance P-evoked release of GABA, glycine and taurine. The substance P-evoked release of the five amino acids was dose-dependently blocked by d-tubocurarine (3-10 mu M), whereas it was not blocked by tetrodotoxin (0.2 mu M) or amiloride (30 mu M). Compound 48/80 (10 mu g/ml), a histamine-releasing agent on mast cells, evoked release of the amino acids from the spinal cord with characteristics similar to those of substance P-evoked amino acid release. These results suggest that, in the neonatal rat spinal cord, substance P evokes the release of neuroactive amino acids by inducing sodium and/or chloride influx that is blocked by d-tubocurarine and consequently by activating reversed uptake through transporters of the amino acids.