Beta-funaltrexamine disrupts the day-night rhythm of nociception in mice

Determinations were made of the effects of beta-funaltrexamine (beta-FNA), an irreversible mu-opioid receptor antagonist, on the day-night rhythm of nociception in male mice. Peripheral administration of beta-FNA (20 and 40 mg/kg) disrupted the day-night rhythm of foot-licking response to aversive thermal (50 degrees C) stimulation. The peak nocturnal response latency was attenuated and the marked increases and decreases in response latency present at the light-dark and dark-light transitions, respectively, were suppressed. These results suggest that mu opioids are associated with the generation and expression of the day-night rhythm of this particular measure of nociception in mice.