Thyroid hormone receptors in neonatal, prepubertal, and adult rat testis

Thyroid hormone (TH) is involved in the differentiation and development of rat testis, whereas its role in adult testis function is still undefined. T he aim of our work has been to further analyze the presence in the testis o f rats of various ages of messenger RNA (mRNA) coding the different TH rece ptor (TR) subtypes using a sensitive assay. such as reverse transcriptase-p olymerase chain reaction (RT-PCR). To rule out the possibility of an "illeg itimate transcription," we have analyzed both T-3-binding capacity of adult rat testis and the presence in the same organ of TR proteins by immunohist ochemistry, using specific antibodies directed against the various TR isofo rms. Messenger RNA coding for TR alpha1 and alpha2 isoforms was clearly vis ible in gels prepared from RT-PCR samples obtained from the testis of rats of all ages, including adults, whereas mRNA for the TR beta1-beta2 was abse nt. The T-3 maximal binding capacity (C-max) by nuclear extracts of testicu lar homogenates gradually decreased from birth to adulthood, still remainin g significantly detectable in adult testis, and represented approximately 1 % of the C-max observed in the liver. The immunostaining technique revealed an intense nuclear staining along the basement membrane of testicular tubu les prepared from rats of all ages and incubated with an antipeptide antibo dy specific for TR alpha1 (alpha1-403). Staining with an antipeptide antibo dy specific for TR beta1 (beta -62) was never present. Our data show that m RNAs coding for the functional TR alpha1, and also for the stilt undefined alpha2, are present in the testis of rats of all ages. T-3- binding activit y and immunohistochemical studies confirmed that the message is translated into proteins. The transcriptional activity clearly decreased from birth to adulthood, but it still remained significantly present. The presence of a TR alpha1 message indicates that the adult rat testis may be directly respo nsive to T-3 and, therefore, suggests an action of TH on rat testis that is not only developmental, but also metabolic.