Opioid footshock-induced analgesia in mice acutely falls by stress prolongation.
The application of 80 footshocks (S-80) to mice induces a decrease in nociceptive responses as measured by the tail-flick test, which is opioid mediated as well as prevented by naloxone (10 mg/kg, SC). When the stress is prolonged up to 240 shocks (S-240) (i.e., from 6 min 40 s to 20 min), no analgesia can be seen immediately after the stress. We have examined the two most obvious possibilities, but they do not seem to be responsible for this fact. When morphine (1-5 mg/kg IP) is injected in the S-240 situation, a potentiation of its analgesic effects is seen, so that a desensitization of mu opioid receptors is unlikely. On the other hand, although cortisol (3-30 mg/kg IP) inhibits the analgesic response to S-80, metyrapone (40 and 80 mg/kg IP) and cortexolone (3-18 mg/kg IP) do not cause S-240 to be analgesic. Thus, an increase of endogenous glucocorticoids released during the long-duration stress does not seem responsible for the lack of analgesia after S-240.
Departamento de Medicina, Facultad de Medicina, Universidad de Oviedo, Asturias, Spain.
L
L
Menendez
F
F
Andres-Trelles
A
A
Hidalgo
A
A
Baamonde
D000818:Animals; D001143:Arousal / Q000187:drug effects* / Q000502:physiology; D003350:Cortodoxone / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004597:Electroshock; D006854:Hydrocortisone / Q000494:pharmacology; D007274:Injections, Intraperitoneal; D007279:Injections, Subcutaneous; D008297:Male; D008797:Metyrapone / Q000494:pharmacology; D051379:Mice; D009020:Morphine / Q000494:pharmacology*; D009270:Naloxone / Q000494:pharmacology*; D017288:Pain Threshold / Q000187:drug effects* / Q000502:physiology; D011930:Reaction Time / Q000187:drug effects / Q000502:physiology; D011957:Receptors, Opioid / Q000187:drug effects* / Q000502:physiology
D011957:Receptors, Opioid; D009270:Naloxone; D009020:Morphine; D003350:Cortodoxone; D006854:Hydrocortisone; D008797:Metyrapone