Specificity of the Adrenal Steroid Receptor System in Rat Hippocampus*

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Abstract
The binding properties of the adrenal steroid receptor system in rat hippocampus were analyzed in vitro in cytosol binding experiments and in vivo with cell nuclear uptake studies. All experimental animals were adrenalectomized 3 days previously for removal of endogenous adrenal steroids. Scatchard analysis of [3H]corticosterone and [3H]dexamethasone in vitro binding in cytosol gave linear plots. Binding parameters for [3H]corticosterone-labeled sites were a maximal binding capacity (Bmax) of 424 fmol/mg protein and a Kd of 3.4 nM and for [3H] dexamethasone-labeled sites, a Bmax of 507 fmol/mg protein and a Kd of 5.0 nM. Scatchard analysis of [3H]aldosterone binding showed two populations of binding sites: one with high affinity (Kd= 2.6 nM) and low capacity (Bma = 80 fmol/mg protein) and the other with low affinity (Kd = 29.8 nM) and higher capacity (Bmax = 347 fmol/mg protein). Inclusion in the incubation medium of 0.6-fold excess unlabeled corticosterone or of 100-fold excess of the “pure” glucocorticoid RU 26988 gave a linear [3H] aldosterone Scatchard plot with only the high affinity sites detectable. Relative binding affinity (RBA) of various steroids for [3H]corticosterone-labeled sites was: corticosterone = deoxycorticosterone (DOC)> progesterone= dexamethasone > aldosterone. The order of potency was the same for [3H]dexamethasone- labeled sites. For [3H]aldosterone-labeled sites the order of RBA was: corticosterone = DOC > progesterone > dexamethasone > aldosterone. With a 100-fold excess of the pure glucocorticoid RU 26988 in the incubation medium, the RBA for [3H]aldosterone-labeled sites changed to: corticosterone = DOC> progesterone = aldosterone > dexamethasone. In vivo uptake of [3H]corticosterone in hippocampal cell nuclei was blocked by prior administration of corticosterone, aldosterone, and DOC, while dexamethasone and progesterone (in doses of 30 μg/lOO g BW, sc) did not interfere. It is concluded that 1) there is a mineralocorticoid receptor system, distinct from the glucocorticoid receptor system, present in rat hippocampus; 2) the data of this study are in line with the notion of heterogeneity within the glucocorticoid receptor system; 3) corticosterone and DOC have the highest affinity in vitro to all populations of adrenal steroid receptor sites; 4) the receptor system, involved in cell nuclear retention of corticosterone in hippocampal neurons, has a high affinity towards the mineralocorticoids aldosterone and DOC as well.