Regulation of tyrosine hydroxylase and phenylethanolamine N-methyltransferase mRNA levels in the sympathoadrenal system by the pituitary-adrenocortical axis

The pituitary-adrenocortical axis plays a complex role in the regulation of the levels of enzymes of the catecholamine biosynthetic pathway. In this report we have explored molecular mechanisms of these regulations, by examining the effects of hypophysectomy (HPX) and dexamethasone (DEX) on tyrosine hydroxylase (TH) and phenylethanolamine N-methyltransferase (PNMT) mRNA levels in the adrenal medulla (AM) and superior cervical ganglia (SCG). Three weeks after hypophysectomy weights (−48%), total RNA (−49%), and DNA (−22%) contents in AM were significantly reduced, when compared to sham-operated animals (SO). In SCG decreases in weight (−23%) and in the ratio of RNA/DNA (−25%) were also found. TH mRNA contents paralleled decreases in total RNA levels and no significant change in the relative abundance of TH mRNA was found. When HPX rats were injected for 5 days with DEX (1 mg/kg, i.p.), TH mRNA levels in the SCG (+51%) and in the AM (+74%) were significantly increased when compared to saline-treated HPX animals. DEX given to SO rats increased TH mRNA in SCG (+49%); a 27% increase in TH mRNA in the AM was also observed. The relative abundance of PNMT mRNA in the AM was reduced after hypophysectomy (−64%). This decrease was completely reversed by DEX. In contrast, DEX did not affect PNMT mRNA levels in the AM of SO rats. PNMT mRNA was not detected in SCG of saline- or DEX-treated rats. In conclusion, our findings suggest that the pituitary-adrenocortical axis is involved in the regulation of the steady-state levels of TH and PNMT mRNAs. This regulation involves: (1) induction of TH mRNA contents in AM and SCG by increased plasma glucocorticoid levels; and (2) maintenance of the steady-state levels of PNMT mRNA in AM by glucocorticoid-dependent mechanisms.