Abstract
An in vitro approach was used to determine the opioid receptor subtype mediating beta-endorphin inhibition of endogenous norepinephrine release from the A2 nucleus in the caudal dorsomedial medulla of rats. The voltage-sensitive Na+ channel blocker tetrodotoxin was used to investigate the role of Na+-dependent action potentials in beta-endorphin inhibition of K+-evoked norepinephrine release. Human beta-endorphin(1-31) inhibited K+-evoked norepinephrine release in a concentration-dependent fashion. Activation of delta- and kappa-opioid receptors had no effect on endogenous norepinephrine release. The inhibitory effect of beta-endorphin was blocked in a concentration-dependent manner by the mu-opioid receptor antagonist CTOP (Cys2, Tyr3, Orn5, Pen7 amide). Tetrodotoxin (TTX) inhibited norepinephrine release evoked by 25 mM K+ in a concentration-dependent manner and blocked the inhibitory effects of beta-endorphin. These results indicate that beta-endorphin acts on mu-opioid receptors to inhibit K+-evoked norepinephrine release from A2 neurons and suggest that the receptors involved are not located on noradrenergic nerve terminals.
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