Norepinephrine regulates the proestrous and estradiol- induced LH surge by binding to aradrenergic receptors. The density of α1-receptors may be regulated by estradiol, photoperiod, and noradrenergic neuronal activity. We wished to determine whether α1-receptors exhibit a diurnal rhythm in ovariectomized and/or estradiol-treated female rats, whether estradiol regulates arreceptors in those areas of brain involved with LH secretion and/or sexual behavior, and whether the concentrations of α-receptors vary inversely relative to previously reported norepinephrine turnover patterns. Young female rats, maintained on a 14:10 light-dark cycle were ovariectomized. One week later, half of them were outfitted sc with Silastic capsules containing estradiol. Groups of animals were decapitated 2 days later at 0300, 1000, 1300, 1500, 1800, and 2300 h. Brains were removed, frozen, and sectioned at 20 Î¼m. Sections were incubated with [3H]prazosin in Tris-HCl buffer, washed, dried, and exposed to LKB Ultrofilm. The densities of aireceptors were quantitated using a computerized image analysis system. In ovariectomized rats, the density of α1-receptors exhibited a diurnal rhythm in the suprachiasmatic nucleus (SCN), medial preoptic nucleus (MPN), and pineal gland. In SCN and MPN, receptor concentrations were lowest during the middle of the day and rose to peak levels at 1800 h. In the pineal gland, the density of α1-receptors was lowest at middark phase, rose to peak levels before lights on, and remained elevated during the day. Estradiol suppressed the density of α1 binding sites in the SCN, MPN, median eminence, ventromedial nucleus, and the pineal gland but had no effect on the lateral septum. Estrogen treatment altered the rhythm of receptor densities in MPN, median eminence, and the pineal gland. In MPN, a transient suppression in the density of receptors occurred at 1500 h, a time associated with increased norepinephrine turnover and the onset of the LH surge. Therefore, we conclude that arreceptor densities undergo a diurnal rhythm in brain regions associated with entrainment to the photoperiod. Estrogen alters the rhythm of α1-receptors in areas involved with the regulation of LH secretion and decreases the density in other estrogen-responsive regions.
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