Characterization of estrogen receptors in the hamster brain

Although the hamster is frequently used as an experimental animal for studying reproductive neuroendocrinology and sex behavior, estrogen receptors (ER) in the central nervous system have not been fully characterized. Using Sephadex LH-20 gel filtration and DNA-cellulose affinity chromatography, estrogen binding macromolecules having the physicochemical properties of classical ER were identified in cytosolic and nuclear extracts of brain tissues. These receptors exhibited high affinity for estradiol (Kd = 10(-9) M), limited capacity (30-50 fmol/g tissue), and estrogen specificity; however, competition studies indicate that brain and uterine ER have different binding kinetics. The neuroanatomic distribution of ER was similar in males and females with highest levels in the limbic brain and consistently low levels in remaining forebrain and mid/hindbrain. No sex differences in receptor number or other binding parameters were evident. Sucrose gradient centrifugation showed that cytosolic ER sedimented in the 7-8S region of a 5-20% linear gradient (no salt), whereas nuclear ER had a sedimentation coefficient of 5S under high ionic strength. On DNA-cellulose affinity columns, these receptors had an elution maximum of 0.18 M NaCl. After a single injection of estradiol, nuclear ER increased and cytosolic ER were depleted. The lower estradiol binding affinity and receptor levels in hamster brain as compared to the rat are consistent with observed species differences in neural sensitivity to estrogen. We expect these data in hamsters, a markedly photosensitive species, to provide a basis for future studies examining the role of receptors in mediating the effects of day-length on steroid dependent feedback and behavioral responses.