Effect of Oestrus Cyclicity on the Number of Brain Opioid Mu Receptors in the Rat

Summary

The present experiments have been performed in order to analyse whether the binding characteristics of brain opioid receptors of the mu type vary during the different phases of the oestrous cycle in the female rat. To this pupose different groups of females with a regular 4-day oestrous cycle were killed by decapitation in different phases of their oestrous cycle, i.e. at 10.00 and 16.00 h of the first and second day of dioestrus, at 10.00, 12.00, 14.00, 16.00, 18.00 and 20.00 of the day of pro-oestrus, and at 10.00, 12.00, 14.00, 16.00 and 18.00 of the day of oestrus. The total brains, after discarding the cerebellum, were homogenized and crude membrane preparations were obtained. On these preparations the maximal binding capacity (Bmax, index of the number of receptors) and the constant of affinity (Ka) for dihydromorhine, a typical ligand of mu opioid receptors were evaluated. Serum concentrations of luteinizing hormone (LH), follicle-stimulating hormone (FSH) and prolactin were measured by specific radioimmunoassays in order to exactly ascertain the different phases of the oestrous cycle. The results obtained show that the number of mu opioid receptors in the whole brain presents significant changes during the different phases of the oestrous cycle. In particular, an increase in the concentration of these receptors was observed at 12.00 h of the day of pro-oestrus and at 18.00 h of the day of oestrus; these fluctations of the number of mu receptors were not accompanied by any change of their affinity for the ligand.

Theses results suggest that the number of brain mu opioid receptors depends upon the phase of the oestrous cycle in the female rat. Further experiments have to be designed in order to clarify whether these oestrus-linked fluctuations are a primary phenomenon or whether they depend upon the changes of serum concentrations of gonadal steroids occuring in the different phases of the oestrous cycle.

Key-Words

Oestrous - Mu Receptors - Rat