Opposite temporal changes after a single central administration of B-endorphin: tolerance and sensitization.

Rats were given a single unilateral microinjection of B-endorphin in the periaqueductal gray, followed by a second microinjection of the same dose of B-endorphin in the same site a week later. A decrease in the analgesic action (i.e., tolerance) but an increase in the hyperthermic action (i.e., sensitization) was observed over this interval. These results suggest that different receptors may mediate these actions of B-endorphin. In addition, these results indicate the need for caution in repeated-measurements studies using this opiate peptide, since the assumption that such temporal effects dissipate within 3-5 days, with resulting minimal carry-over effects from the preceding treatment appears to be unjustified.