Steroid receptors in the rat hippocampus: a note to the methodology of their binding assay

Mineralocorticoid (MR) and glucocorticoid (GR) receptors in the rat hippocampus are linked to several cognitive functions of the animal and seem to play an important role in the response to various stressors. Their assessment by binding experiments brings about problems associated with their intracellular compartmentalization, and in particular with the separation of the bound and free ligands. Adrenalectomy 24 h before sacrificing is commonly used to clear the circulating adrenal steroids, and to facilitate their dissociation from hippocampal MR and GR. We have successful attempted to use dialysis to these purposes and thus, to avoid a potential surgical stress. Without dialysis, only GR can be measured in the cytosol from intact rats, while the corresponding pellet contains MR as a component of the cell nuclei. The bound ligand fraction was separated by filtration on polyethyleneimine pretreated glass fiber filters as suggested earlier. The method has clear-cut preferences compared to any alternative used up to now. Discrimination between the two receptor types can be optimally achieved in a cross-displacement experiment in which two labeled ligands possessing various affinities to individual receptors (in our case: corticosterone and aldosterone, or their synthetic analogs) are displaced with the two corresponding nonlabelled ligands from their receptors. Computations can be carried out with LIGAND software which yield accurate values of binding parameters.