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Tolerance to hyperthermia produced by morphine in rat.
Psychopharmacology
|January 1, 1987
Summary
Rats develop significant tolerance to the hyperthermic effects of morphine, challenging the idea that tolerance only affects depressant opiate actions. This study demonstrates tolerance to both hypothermic and hyperthermic responses.
Area of Science:
- Pharmacology
- Neuroscience
- Physiology
Background:
- The prevailing view suggests rats develop tolerance to morphine's hypothermic but not hyperthermic effects.
- Opioid-induced hyperthermia is a complex thermoregulatory response.
- Understanding tolerance to different opioid effects is crucial for clinical applications.
Purpose of the Study:
- To investigate the development of tolerance to the hyperthermic effects of morphine in rats.
- To challenge the established notion regarding differential tolerance to morphine's thermic effects.
Main Methods:
- Rats were rendered tolerant and dependent on morphine via daily intraperitoneal injections.
- Rectal temperatures were measured after various morphine test doses in different environmental conditions.
- Dose-response curves and time/effect curves were analyzed, including serum morphine levels.
Main Results:
- Confirmed tolerance to morphine's hypothermic effect.
- Demonstrated significant, dose-proportional decreases in morphine-induced hyperthermia in tolerant rats.
- Observed reduced hyperthermia despite elevated serum morphine levels in treated animals.
Conclusions:
- Substantial tolerance develops to the hyperthermic effects of opiates in rats.
- Previous difficulties in observing this tolerance may stem from opposing acute thermic effects in naive animals.
- Findings challenge the view that tolerance is limited to depressant opiate effects.