Properties and Distribution of Glucocorticoid-Binding Sites in Cytosol of the Spinal Cord

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Abstract
We have examined the spinal cord for the presence of glucocorticoid-binding sites. For this purpose, cytosol from the spinal cord of adrenalectomized rats was incubated with (3H)-dexamethasone. Maximal binding was obtained after 20 h of incubation at 0 °C in the presence of 20 mM molybdate, whereas at 20 °C the maximum was at 2 h. Using a range of (·Η)-dexamethasone concentrations (0.2–30 nM), low capacity (161 ± 23 fmol/mg protein) and high affinity (Kd 3.2 ± 0.3 nM) sites were measured. Binding sites decreased by 25% and Kd increased 2.5-fold after incubation with a pure glucocorticoid (RU 26988). Relative binding affinities of several competitors of 10 nM(3H)-dexamethasone were: triamcinolone acetonide 108, dexamethasone 100, RU 26988 54, corticosterone 18, progesterone 17, aldosterone 7, estradiol and testosterone 3H)-dexamethasone was decreased by omitting a SH-protective agent from the buffer or by addition of SH-blocking reagents such as N-ethylmaleimide and p-chloromercuribenzoate. Using rats of different ages, it was found that binding sites were much lower in spinal cord from 2- to 8-day-old rats than in rats of 13–20 days and adults. Regional distribution studies using cytosol from spinal cords dissected between vertebrae C1–C2, C3–C7, T1–T8, T9–L3 and L4–L6 revealed that binding sites were higher in regions containing the cervical (C3-C7) and lumbar (T9-L3) enlargements, with respect to L4–L6. These studies suggested that the spinal cord contains cytosolic-binding sites resembling glucocorticoid receptors, and that the reported action of these steroids on spinal cord function could be mediated by these sites.