Properties and distribution of binding sites for the mineralocorticoid receptor antagonist [3H]ZK 91587 in brain.

We have studied the binding of the synthetic antimineralocorticoid [3H]ZK 91587 to soluble receptors in brain of adrenalectomized rats. It was observed that [3H]ZK 91587 labeled a single receptor class with high affinity (Kd 1.3 nM) and low capacity (51.1 fmol/mg prot.) in cytosol of hippocampus (HIPPO). The ligand was efficiently displaced in vitro from the receptor by aldosterone (IC50 2.0 nM) and corticosterone (2.3), while dexamethasone showed less potency (IC50 5.1 nM) and the pure antiglucocorticoid RU 28362 competed weakly (161 nM). Furthermore, there was a widespread distribution of binding sites all over the brain for this compound, but with CA1 and CA3 regions of HIPPO, some amygdaloid nuclei and lateral septum containing most of the binding sites, as revealed by binding assays employing 16 different microdissected brain regions. Finally, the receptor labeled with [3H]ZK 91587 was readily displaced by administration of aldosterone in vivo in physiological amounts, from 5 whole brain regions examined, but preferentially from preoptic area, amygdala and HIPPO. It is concluded that [3H]ZK 91587 is a useful ligand for further studies on putative mineralocorticoid responsive cells in brain, due to its high affinity, stability and lack of cross reactivity with glucocorticoid receptors. Its brain distribution is similar to that previously obtained using [3H]aldosterone in the presence of RU 28362 to block ligand binding to the glucocorticoid receptor.