DISTRIBUTION AND DIURNAL-VARIATIONS OF THE MU-OPIOID RECEPTOR EXPRESSION IN THE ARCUATE NUCLEUS OF THE MALE-RAT

Using an in situ hybridization method with radioactively-labeled ribop robes, the distribution of mu-opioid-receptor-expressing cells was inv estigated in the arcuate nucleus (ARC) of the male rat. We simultaneou sly tested the hypothesis that expression of mu-opioid receptor mRNA d isplayed diurnal variations in neurons of the ARC, and an image analys is system was used to measure cellular levels of mu-opioid receptor ex pression. Rats were sacrificed in groups of 5 at 06.00. 12.00 and 20.0 0 h. In the ARC, mu-opioid receptor mRNA was expressed in scattered ne urons located in ventromedial, ventrodorsal and ventrolateral portions . The level of expression was very low in sections taken from animals sacrificed al 06.00 and 12.00 h. The level of mu-opioid receptor mRNA expression was significantly (p < 0.001) increased in neurons of anima ls killed at 20.00 h in comparison with the other hours of the day, i. e. 06.00 and 12.00 h. However, the number and general distribution of mu-receptor-expressing neurons did not vary across these time points ( p > 0.05). In conclusion, this study presents evidence for the presenc e of low levels of mu-opioid-receptor-expressing cells in the ARC. We observed that the receptor synthesis increases in the evening. These r esults suggest that the mu-opioid receptor may play a significant role in modulating the processing of neuropeptides located in this nucleus . The daily rhythm in mu-receptor expression is brought together with the diurnal fluctuation in pro-opiomelanocortin mRNA and the circadian variation in beta-endorphin concentrations in the ARC.