Effect of morphine on rectal temperature after acute and chronic treatment in the rat

• 1.

  1. In naive male rats morphine sulfate administered intraperitoneally in doses less than 25 mg/kg produced hyperthermia, while higher doses produced hypothermia.

• 2.

  2. In morphine tolerant rats, morphine produced only hyperthermia at all doses with a markedly reduced time of onset.

• 3.

  3. When morphine was acutely administered intravenously, all doses tested (2,5,10 and 20 mg/kg) led to an initial hypothermia; hyperthermia subsequently occurred at doses 2,5 and 10 mg/kg, but not at 20 mg/kg.

• 4.

  4. Naltrexone (5 mg/kg) antagonized both the hyperthermic and hypothermic response to intraperitoneally administered morphine, and when the hypothermic response was antagonized, hyperthermia occurred.

• 5.

  5. Adrenalectomy retarded the hyperthermic effects of intraperitoneally administered morphine in both naive and tolerant subjects.

• 6.

  6. It is suggested that hypothermia is a primary action of morphine on central thermoreceptors, and that certain components of the hyperthermic response are induced indirectly, perhaps through hormones of the adrenal glands.