Evaluation of the rdw rats as an animal model for hereditary dwarfism and the causing factor for the dwarfism

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The rdw rat (gene symbol ; rdw) represents hereditary dwarfism inherited as an autosomal recessive trait. The rdw pituitary expressed 30-100 times less growth hormone (GH) and prolactin (PRL) mRNA than normal control. The pituitary-specific transcription factor Pit-1, which is involved not only in the gene expression of GH and PRL but in somatotroph, lactotroph and thyro-troph, was examined. No difference in mRNA expression for Pit-1 was found between rdw rats and controls. We focussed the cause of rdw dwarfism on the thyroid function. The rdw rats had reduced thyroid hormone levels and raised thyroid stimulating hormone (TSH) levels in blood compared with normal controls. The raised serum TSH in rdw rats showed equivalent biological activity in FRTL-5 cells to radioimmunoassay. To clarify the feature of rdw dwarfism, we focused the function of the thyroid gland in rdw rats. We carried out thyroid hormone supplement therapy in rdw rats. The rdw rats were fed with the rat chow added with 0.025% desiccated thyroid powder from weaning (5 weeks of age) to sacrifice time. A significant increase in body weight was noted in the rdw rats fed with the thyroid powder supplemented chow. A significant increase in body weight was also observed in the rdw rats transplanted with the thyroid gland. Equivalent expression of GH and PRL mRNA to the normal control rats was noted in the thyroid transplanted rdw at sacrifice time. The male rdw rats transplanted with the thyroid gland acquired repro-ductive function and had about 50% homozygous rdw/rdw offsprings in mating with hetrozygous rdw/+ females. These results indicate Primary hypothyroidism in the rdw rat.