Skeletal muscle thermogenesis: its role in the hyperthermia of conscious rats given morphine or beta-endorphin.

The role of skeletal muscle thermogenesis (increases in skeletal muscle tone) in the hyperthermic responses of conscious, unrestrained rats given acute or repeated i.p. or i.c.v. injections of morphine sulfate (MS) or beta-endorphin was investigated. Initial blood gas experiments showed that rats given acute i.p. injections of MS caused PO2 and pH to decrease by 60 min postadministration in a dose-related fashion whereas PCO2 increased; with repeated MS administration the respiratory acidosis seen with acute injections was reduced. Acute i.p. injections of MS (1, 10 or 20 mg/kg) caused catalepsy scores, plasma lactate levels and electromyographic (EMG) amplitude to be elevated in a dose-related fashion along with a rise in rectal temperatures (TRS). Surface (tail) temperatures also rose after the acute MS injections but only after the increase in TR. Significant increases in EMG amplitude after acute injections of MS occurred even before TRS increased and, with subsequent naloxone HCl administration (10 mg/kg i.p.), a rapid and marked fall in EMG amplitude occurred before TRS fell back to saline control levels. Acute i.c.v. injections of 1.1 nmol of either MS or beta-endorphin also caused EMG amplitudes to rise significantly before TRS began to increase. Tail temperatures again increased passively after i.c.v. injection of either drug. Subsequent naloxone injections (10 mg/kg i.p.) to these groups also caused EMG amplitudes to decrease before TRS decreased back to control TRS. Repeated i.p. injections of MS (10 mg/kg i.p. daily for 5 days) caused TRS to be higher than those seen after the initial injection but catalepsy scores, plasma lactates and EMG amplitudes were below those respective levels seen upon acute MS administration. Similar chronic findings occurred in other groups of rats given 1.1 nmol of either MS or beta-endorphin i.c.v., when they had been previously given repeated i.p. injections of MS (5 mg/kg i.p. twice daily for 2 days). The results indicate that acute peripheral or central injections of MS or beta-endorphin to conscious rats cause skeletal muscle to be activated, resulting in nonlocomotor, catatonic behavior. This skeletal muscle activation occurs before the rise in TR and is thought to be an important and possibly the primary cause of the resultant hyperthermia seen in rats after acute central or peripheral administration of MS or beta-endorphin. Repeated injections of morphine cause TRS to escalate higher, compared to that seen with acute MS administration, yet catalepsy scores, plasma lactate levels and EMG amplitude changes did not increase likewise.(ABSTRACT TRUNCATED AT 400 WORDS)