Delta but not mu-opioid receptors in the spinal cord are involved in antinociception induced by beta-endorphin given intracerebroventricularly in mice.

The present studies were designed to determine what type of opioid receptor, mu or delta, in the spinal cord was involved in beta-endorphin-induced antinociception. The tail-flick response was used as an antinociceptive test. Intrathecal (i.t.) injection of ICI-174,864 [(Allyl)2-Tyr-Aib-Aib-Phe-Leu-OH] (10 micrograms) or ICI-154,129 [(N,N-Bisallyl)-Tyr-Gly-Gly-psi-(CH2S)-Phe-Leu-OH] (20 micrograms), delta-opioid receptor antagonists, but not beta-funaltrexamine (0.025 microgram), a mu-opioid receptor antagonist, antagonized inhibition of the tail-flick response induced by beta-endorphin given i.c.v. However, i.t. injection of the same dose of ICI-174,864, ICI-154,129 or beta-funaltrexamine did not affect inhibition of the tail-flick response induced by morphine given i.c.v. Mice were pretreated i.c.v. with either beta-endorphin (2 micrograms), morphine (2 micrograms) or saline (5 microliters) for 2, 3 or 4 hr (which were times that the tail-flick response was no longer inhibited) and were injected i.t. with various doses of D-Ala2-NMePhe4-Gly-ol-enkephalin (a selective mu-opioid receptor agonist), D-Ala2-D-Leu5-enkephalin (a mu- and delta-opioid receptor agonist) or D-Pen2-D-Pen5-enkephalin (a delta-opioid receptor agonists). The tail-flick response was performed 10 min after i.t. injection. A single i.c.v. pretreatment with beta-endorphin for 2 and 3 hr, but not 4 hr, reduced markedly the inhibition of the tail-flick response induced by D-Pen2-D-Pen5-enkephalin and D-Ala2-DLeu5-enkephalin, but not D-Ala2-NMePhe4-Gly-ol-enkephalin, injected i.t.(ABSTRACT TRUNCATED AT 250 WORDS)