title: Distribution and diurnal variations of the mu opioid receptor expression in the arcuate nucleus of the male rat.
abstract: Using an in situ hybridization method with radioactively labeled riboprobes, the distribution of mu-opioid-receptor-expressing cells was investigated in the arcuate nucleus (ARC) of the male rat. We simultaneously tested the hypothesis that expression of mu-opioid receptor mRNA displayed diurnal variations in neurons of the ARC, and an image analysis system was used to measure cellular levels of mu-opioid receptor expression. Rats were sacrificed in groups of 5 at 06.00, 12.00 and 20.00 h. In the ARC, mu-opioid receptor mRNA was expressed in scattered neurons located in ventromedial, ventrodorsal and ventrolateral portions. The level of expression was very low in sections taken from animals sacrificed at 06.00 and 12.00 h. The level of mu-opioid receptor mRNA expression was significantly (p < 0.001) increased in neurons of animals 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 presence of low levels of mu-opioid-receptor-expressing cells in the ARC. We observed that the receptor synthesis increases in the evening. These results 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.
authors.affiliation: INSERM U422, Neuroendocrinologie et Physiopathologie Neuronale, Lille, France. mitchell@biserte.lille.inserm.fr
authors.forename: V
authors.initials: V
authors.lastname: Mitchell
authors.forename: V
authors.initials: V
authors.lastname: Prévot
authors.forename: J C
authors.initials: JC
authors.lastname: Beauvillain
mesh_terms: D000818:Animals; D001111:Arcuate Nucleus of Hypothalamus / Q000737:chemistry / Q000378:metabolism*; D002940:Circadian Rhythm; D015870:Gene Expression; D008297:Male; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D017450:Receptors, Opioid, mu / Q000032:analysis / Q000235:genetics*
chemical_list: D012333:RNA, Messenger; D017450:Receptors, Opioid, mu