Opposite Effects of δ and μ Opioid Receptor Agonists on the In Vitro Release of Substance P‐Like Material from the Rat Spinal Cord

Superfusion of slices from the dorsal half of the lumbar enlargement of rat spinal cord with Krebs‐Henseleit medium supplemented with 30 μ M bacitracin allowed the collection of substance P‐like immunoreactive material (SPLI), which was released at a rate of ∼ 10 pg/4 min. Tissue depolarization by an excess of K + (30–60 m M ) or veratridine (50 μ M ) induced a marked increase in SPLI outflow, provided that Ca 2+ was present in the superfusing fluid. K + ‐or veratridine‐induced SPLI overflow could be modulated in opposite directions by μ and δ opioid receptor agonists. Thus, the two preferential μ agonists Tyr‐ d ‐Ala‐Gly‐MePhe‐Gly‐ol (DAGO; 10 μ M ) and Tyr‐ d ‐Ala‐Gly‐MePhe‐Met(O) 5 ‐OH (FK‐33824; 0.1 μ M ) enhanced SPLI overflow from depolarized tissues, whereas the selective δ agonists Tyr‐ d ‐Thr‐Gly‐Phe‐Leu‐Thr (deltakephalin; 3 μ M ) and [2‐ d ‐penicillamine, 5‐ d ‐penicillamine]enkephalin (50 μ M ) reduced it. The effect of DAGO was antagonized by a low concentration (1 μ M ) of naloxone but not by the selective δ antagonist ICI‐154129 (50 μ M ). In contrast, the latter drug prevented the inhibitory influence of δ agonists on K + ‐induced SPLI release. Complementary experiments with morphine (10 μ M ) and [2‐ d ‐alanine, 5‐ d ‐leucine]enkephalinamide (3 μ M ), in combination with 1 μ M naloxone or 50 μ M ICI‐154129 for the selective blockade of μ or δ receptors, respectively, confirmed that the stimulation of μ receptors increased, whereas the stimulation of δ receptors reduced, SPLI overflow. The results suggest that, at the spinal level, the antinociceptive action of δ but not μ agonists might involve a presynaptic inhibition of substance P‐containing primary afferent fibers.