Triiodothyronine receptors in adult rat brain: topographical distribution and effect of hypothyroidism.

The binding properties (affinity constant Ka and number of binding sites) of the nuclear receptors for triiodothyronine (T3) were studied in selected brain areas of the adult rat: cerebral cortex, cerebellum, olfactory bulb, caudate nucleus, hippocampus, hypothalamus and pituitary. Normal rats were compared to hypothyroid animals. The Ka of the nuclear receptors in the different parts of the brain had a similar order of magnitude (10(9) M-1) except for the hypothalamus where the value was lower (6 X 10(7) M-1). In hypothyroid animals, two main changes were observed in the properties of nuclear receptors: the Ka in the cerebral cortex was reduced by half, whereas in the pituitary the value was three times higher than in controls. In contrast to the decreased affinity, hypothyroidism enhanced the maximal binding capacity, especially in the cerebral cortex, the olfactory bulb, the caudate nucleus and the hippocampus. Of particular interest were the effects of hypothyroidism on the pituitary receptors which showed a markedly increased affinity for T3 and a reduced number of binding sites; this observation could be related to the control feedback mechanism of thyroid-stimulating hormone secretion by thyroid hormones.