Temporal variations in the effect of morphine and nalorphine on total brain acetylcholine content in the rat

The effect of increasing doses of morphine and nalorphine on total rat brain acetylcholine (ACh) content was determined 30 min after the administration of the drugs at 07h30 and 19h30. The results indicate that the effects of morphine and nalorphine on brain ACh levels varied depending upon the time of administration. For instance, 10 mg/kg of morphine did not significantly change total brain ACh when injected in the morning whereas a 45% increase in ACh level was observed after administration of the same dose at night. Depending on the time of administration, nalorphine increased or decreased ACh content of rat brain. The results are discussed and related to the circadian rhythms that were demonstrated in brain levels of ACh, in opioid receptors and in the secretion of opioid peptides.