Changes in noradrenergic transmission alter the concentration of cytoplasmic progestin receptors in hypothalamus

Abstract

These experiments were designed to determine whether drug-induced changes in NA transmission affect lordosis behavior of female guinea pigs by altering steroid action within hypothalamic target cells. In the first experiment, we examined the effects of the dopamine-β-hydroxylase inhibitor, U-14,624, on cytoplasmic progestin receptors (measured using a one-point [ 3 H]R5020 binding assay) in hypothalamus (HYPO), preoptic area (POA), cerebral cortex (CORT) and midbrain (MB) of estradiol benzoate (EB)-primed females. At 12 h after U-14,624 administration, specific binding of [ 3 H]R5020 was 36% less in cytosol from HYPO (binding in POA, CORT, and MB was not affected by U-14,624) in drug-treated than in non-drug-treated controls. To determine whether this reduction in [ 3 H]R5020 binding was due to competition of U-14,624 with [ 3 H]R5020 for progestin receptors, we examined the effects of U-14,624 on [ 3 H]R5020 binding in vitro. U-14,624 had no effects on [ 3 H]R5020 binding under these conditions. By using a range of [ 3 H]R5020 concentrations to assay cytoplasmic progestin receptors, we found that the reduction in [ 3 H]R5020 binding on hypothalamic cytosol after U-14,624 treatment was due to a lower concentration of progestin receptors rather than to a lower progestin receptor affinity for [ 3 H]R5020. Several lines of evidence indicate that the lower (compared to non-drug-treated females) concentration of these receptors was attributable to a reduction in NA transmission. First, U-14,624 caused a significant reduction in regional brain NE content. Second, activation of α-adrenergic receptors with clonidine completely reversed the effects of U-14,624 on cytoplasmic progestin receptors, although clonidine had no effect on progestin receptors when administered alone. Third, blockade of α-adrenergic receptors by i.p. injection of phenoxybenzamine (Pb) resulted in a relative reduction of specific [ 3 H]R5020 binding in hypothalamic (but not POA, CORT or MB) cytosol of EB-primed females. There was a 3–4 h delay between the blockade of α-receptors by Pb (determined using a [ 3 H]WB4101 binding assay) and a significant effect on the concentration of progestin receptors in cytoplasm of HYPO. The effects of Pb on hypothalamic progestin receptors did not appear to be due to a peripheral action of the drug. When administered intraventricularly, Pb caused a relative reduction in specific [ 3 H]R5020 binding in hypothalamic (but not POA, CORT or MB) cytosol of EB-primed females. The lower concentration of progestin receptors in HYPO after drug treatment also did not appear to be attributable to the release of adrenal progesterone. The concentrations of progesterone in plasma at 0.5, 2, 4, and 12 h after Pb and at 12 h after U-14,624 administration were not different from control values. Furthermore, the concentration of cytoplasmic progestin receptors was lower only in HYPO after Pb and U-14,624 treatment, whereas injection of 100 μg progesterone caused a reduction in the concentration of these receptors in all brain areas examined. The lower concentration of cytoplasmic progestin receptors in HYPO after drug treatment might be attributable to a drug-induced interference with the EB-induced increase in the concentration of these receptors. This hypothesis is supported by the finding that Pb has no effect on hypothalamic progestin receptors in the absence of EB priming. Thus, changes in NA transmission might alter EB action in HYPO in addition to altering target cell sensitivity (through effects on progestin receptor concentration) to progestins. On the basis of these results we propose that the modulation of target tissue responsiveness to steroids is an important mechanism by which neurotransmitters affect steroid-dependent processes. The operation of such a mechanism would provide a rapid means by which environmental, behavioral and emotional events could modulate steroid-dependent behaviors and anterior pituitary function.