Corticosterone and tri-iodothyronine transport into brain of obese (ob/ob) mice

Unidirectional influx of [3H]-corticosterone and [125I] 3,5,3'-tri-iodothyronine (T3) into brain, and of [3H]-corticosterone into livers, of 8-10 week old lean and obese (ob/ob) mice was measured with a bolus injection, tissue-sampling technique. Uptake of corticosterone by brain and liver of ob/ob mice was comparable to that observed in lean mice, and unaffected by corticosterone concentration in the bolus (17-32 nM or 25 microM) or by addition of lean or ob/ob mouse blood serum to the bolus injectate. Uptake of T3 by ob/ob mouse brain was lower than observed in lean mice when the bolus contained physiological concentrations of T3 (2.6-19 nM). An increase in T3 concentration in the injectate to 5 microM depressed T3 uptake by brain of lean mice, but not of ob/ob mice. Addition of blood serum from ob/ob mice to the bolus further lowered influx of T3 into the brain of lean and ob/ob mice. Even though unidirectional influx of T3 into the brain of ob/ob mice was impaired, steady-state T3 concentrations in ob/ob mice brain were similar to lean values. The enhanced sensitivity of ob/ob mice to the obesity-producing effects of glucocorticoids is not caused by an increased influx of corticosterone into brain or liver.