Different mechanisms mediate beta-endorphin- and morphine-induced inhibition of the tail-flick response in rats.
The studies were done to determine whether inhibition of the tail-flick response induced by beta-endorphin and morphine microinjected into periaqueductal central gray (CG) was mediated by stimulating different types of opioid receptors and by activating different descending pain modulatory systems in rats. beta-Endorphin (0.3-20 micrograms) or morphine (0.3-20 micrograms) microinjected bilaterally into CG produced a dose-dependent inhibition of the tail-flick response. Naloxone (0.3-3 micrograms) injected into CG was more effective in antagonizing inhibition of the tail-flick response induced by CG administered morphine than by beta-endorphin. beta-Endorphin-(1-27) (3 micrograms) injected CG effectively antagonized CG beta-endorphin-induced inhibition of the tail-flick response but slightly potentiated CG morphine-induced inhibition. Intrathecal injection of naloxone (0.3-30 micrograms) dose-dependently reversed inhibition of the tail-flick response induced by beta-endorphin (2 micrograms) but not by morphine (4 micrograms) injected into CG. On the other hand, yohimbine (0.3-30 micrograms) injected intrathecally dose-dependently antagonized inhibition of the tail-flick response induced by morphine (4 micrograms) but not by beta-endorphin (2 micrograms) given into CG. It was concluded that beta-endorphin and morphine produce inhibitions of the tail-flick response by stimulating epsilon and mu opioid receptors, respectively, and the descending pain modulatory system activated by beta-endorphin from CG involves spinal opioid receptors but not alpha-2 adrenoceptors whereas the descending system activated by morphine from CG involves spinal alpha-2 adrenoceptors but not opioid receptors.
Tseng
L F
LF
Department of Pharmacology and Toxicology, Medical College of Wisconsin, Milwaukee.
Tang
R
R
D000698:Analgesia; D000818:Animals; D004525:Efferent Pathways / Q000187:drug effects; D008297:Male; D008845:Microinjections; D009020:Morphine / Q000494:pharmacology*; D009270:Naloxone / Q000494:pharmacology; D010487:Periaqueductal Gray / Q000187:drug effects*; D051381:Rats; D011919:Rats, Inbred Strains; D011957:Receptors, Opioid / Q000187:drug effects; D015016:Yohimbine / Q000494:pharmacology; D001615:beta-Endorphin / Q000494:pharmacology
D011957:Receptors, Opioid; C043460:epsilon receptor; D015016:Yohimbine; D009270:Naloxone; D001615:beta-Endorphin; D009020:Morphine