To investigate diurnal variations in opiate receptor binding, the amount of specifically bound [3H]naloxone was measured at 4-h intervals across a 24-h period in the forebrains of rats that had been housed under a controlled light--dark cycle (lights on from 07.00 to 19.00 h) for 3 weeks. A significant rhythm with a peak at 22.00 h was found, the amplitude was 46--78%. In the absence of time cues, this circadian rhythm persisted with a peak at 02.00--06.00 h and an amplitude of 88%. Scatchard analysis indicated that the differences in binding throughout the day were due not to changes in affinity, but to changes in the number of binding sites.
