Mice respond to morphine with characteristic mydriasis which is antagonized by naloxone. The present study presents data on the diurnal variation of these responses. The mydriatic response is at its highest level at 0800 and has a nadir at 2400. This effect is not influenced by ambient light conditions. The miotic response to naloxone in the morphine-dilated pupil is maximal between 1200 and 1600 and minimal at 0400. The curves of the responses to the two drugs, therefore, differ as do their acrophases. Pharmacokinetic factors are probably not responsible for the observed variations. It is speculated that these diurnal changes may reflect cyclic alteration in the affinity of the opiate receptors to agonists and antagonists, respectively, or in the number of available receptors.
- Circadian rhythm