DIFFERENTIAL REGULATION OF TYROSINE-HYDROXYLASE PROTEIN AND ACTIVITY IN RABBIT SYMPATHETIC NEURONS AFTER LONG-TERM COLD-EXPOSURE - ALTERED RESPONSES IN AGING

The aim of this study was to investigate the response of sympathetic n eurones to prolonged neural stimulation, using cold exposure as a non- invasive experimental paradigm. We examined the effects of prolonged ( 8 days and 4 wk) cold exposure on tyrosine hydroxylase (TH) protein an d activity and neuropeptide Y (NPY) levels in sympathetic neurones of the superior cervical ganglion (SCG), together with NPY levels in the ear artery from young and aged rabbits. The main findings were as foll ows. In young rabbits, TH levels and TH activity were differentially r egulated in response to prolonged cold exposure. TH levels rose whilst TH activity tended to decline. Decentralization of SCG from young ani mals before cold exposure abolished the rise in TH levels. TH activity in SCG from young rabbits was reduced by decentralization whilst cold exposure resulted in an increase in TH activity. Thus, TH activity wa s induced in the SCG in the absence of pre-ganglionic input, demonstra ting a non-synaptic regulatory mechanism. In old rabbits, cold-induced changes were either delayed or failed to occur, indicating that the r esponses of sympathetic neurones to cold stress are impaired in old ag e.