Seasonal variation in the apparent number of binding sites for 3H-opioid agonists and antagonists

Throughout the year, a radioreceptor assay was used to investigate the number of binding sites for several opioid ligands in C57BL/6J mouse brain homogenates. The mean number of sites observed for ³H-morphine and ³H-dihydromorphine was about one third less than the mean for ³H-D-ala²-met⁵-enkephalinamide, ³H-naloxone or ³H-naltrexone. Some seasonal variation was observed in all the opioids studied; the seasonal fluctuation observed in the apparent number of naloxone binding sites was the greatest. These seasonal alterations may play a role in the annual pattern of several physiological processes in which endorphin systems are directly or indirectly involved.