Effects of temperature on the interaction of morphine with opioid receptors.

The electrically stimulated guinea pig ileum preparation was used to determine the effects of temperature on the affinity of morphine for opioid receptors. The potency of morphine (expressed as the concentration which produces 50% inhibition--IC50) was significantly decreased at 30 degrees C (IC50 41.0 x 10(-8) mol litre-1) and increased at 40 degrees C (IC50 5.1 x 10(-8) mol litre-1) when compared with its potency at 37 degrees C (IC50 8.8 x 10(-8) mol litre-1). Experiments carried out in the presence of naloxone (a competitive opioid antagonist) indicated that the affinity of opioid receptors for this antagonist was not affected by temperature. Further studies using B-funaltrexamine (a mu-specific, non-reversible opioid antagonist) revealed an increase in morphine receptor affinity when temperature was increased from 30 to 37 degrees C. The data demonstrated that the potency of morphine increased with temperature; the affinity of naloxone for opioid receptors was unaltered by temperature; and the affinity of morphine for mu-receptors reached an optimal value within the range 30-37 degrees C.