Regulation of beta- and gamma-actin mRNA by thyroid hormone in the developing rat brain.

The relative sensitivity of the two isotypes of actin mRNA (beta and gamma) to thyroid hormone (T3) was examined by comparing the steady state levels of these mRNAs with their rates of transcription in cerebra from normal and hypothyroid rats of different ages, covering the period of synaptogenesis (days 1-20 postnatal). Hypothyroidism was associated with a reduction in the steady state level of beta-actin mRNA during the entire period, but levels of gamma-actin mRNA were reduced only during the early phase of synaptogenesis (first week). The rats of transcription of both beta- and gamma-actin mRNAs were stimulated by T3 during the early phase but that of beta-actin mRNAs was relatively more sensitive to T3 than gamma-actin mRNA. The lack of correlation between the rates of transcription of beta- and gamma-actin mRNA at different ages with their corresponding steady-state levels suggests additional post-transcriptional regulation.