BINDING OF [H-3] TRIAMCINOLONE ACETONIDE TO GLUCOCORTICOID RECEPTORS IN BRAIN CYTOSOL FRACTIONS OF RATS WITH INTACT ADRENALS

Binding of [H-3]triamcinolone acetonide (TA) increased with prolongati on of incubation periods of up to 5 h after the onset of incubation at 2 degrees C, with a plateau thereafter persisting for at least up to 48 h in brain cytosol fractions of rats with intact adrenals. Elevatio n of incubation temperature to 30 degrees C resulted in a marked reduc tion of the binding at equilibrium which persisted for only 1 to 2 h w ith complete abolition thereafter. The addition of sodium molybdate wa s effective in doubling the maximal value at 30 degrees C without mark edly affecting the binding at 2 degrees C. [H-3]TA binding at equilibr ium determined at 2 degrees C was a reversible, saturable and structur e selective process with uneven distribution profiles in rat brain. Am ong a variety of steroid hormones tested, TA was the most potent displ acer with progressively less potent displacement by dexamethasone, deo xycorticosterone, progesterone, prednisolone, hydrocortisone and corti costerone. Among discrete brain regions examined, the highest density was detected in the cerebellum followed by the hippocampus, cerebral c ortex, midbrain, striatum, hypothalamus and medulla-pons in a rank ord er of decreasing density. In contrast, both the cerebellum and medulla -pons had significantly higher affinities for [H-3]TA than the cerebra l cortex. Moreover, the binding was markedly inhibited by Zn2+ ions at 10 mu M due to a decrease in the affinity. These results suggest that [H-3]TA labels a ligand recognition domain on the cytoplastic glucoco rticoid receptor complex with different affinities in rat brain.