Cross-tolerance between morphine and swim analgesia in mice selectively bred for high and low stress-induced analgesia.
Mice selectively bred for high (HA) and for low analgesia (LA) induced by 3-min swimming at 20 degrees C and unselected controls (C) were injected three times daily for 3 days with 20 mg/kg morphine HCl. The analgesic effect of 10 mg/kg morphine in nontolerant mice differed between the lines in the rank order of HA > C > LA and significantly decreased after repeated treatment with morphine, as revealed by the hotplate test (56 degrees C). The tolerance to morphine analgesia was more pronounced in HA than in C mice but did not develop at all in LA mice. Similarly, the magnitude of swim-induced analgesia in morphine tolerant mice decreased to a greater degree in the HA than the C line but did not change in LA mice. Naloxone HCl (1 and 10 mg/kg) attenuated swim analgesia more in nontolerant HA than C mice but had no effect in morphine-tolerant HA and C and in all LA mice. The differential degree of morphine tolerance and cross-tolerance with swim analgesia suggests that the strategy of selective breeding toward divergent magnitudes of stress-induced analgesia has differentiated opioid involvement in endogenous pain inhibition in the selected lines.
Sadowski
B
B
Department of Behavioral Physiology, Institute of Genetics and Animal Breeding, Polish Academy of Sciences, Mrokow.
Panocka
I
I
D000698:Analgesia; D000818:Animals; D004361:Drug Tolerance; D008297:Male; D051379:Mice; D008815:Mice, Inbred Strains; D009020:Morphine / Q000494:pharmacology*; D009270:Naloxone / Q000494:pharmacology; D010147:Pain Measurement / Q000187:drug effects; D011817:Rabbits; D013045:Species Specificity; D013315:Stress, Psychological / Q000235:genetics / Q000523:psychology*; D013550:Swimming
D009270:Naloxone; D009020:Morphine