The effects of stress on tyrosine hydroxylase gene transcription rate in the locus coeruleus

I developed and validated an intron-specific in situ hybridization histochemical technique (ISHH) using oligodeoxynucleotide probes specific for intron 2 area of rat tyrosine hydroxylase (TH), the first and rate-limiting enzyme in the catecholamine biosynthesis, to measure TH transcription rate in the adrenal medulla. The results from this method were highly correlated with those obtained using both RNase protection and nuclear run-on assays.;This method was then applied to the examination of the effects of stress on the TH transcription rate in the locus coeruleus (LC), which mediates the brain's response to stressful stimuli. Previous studies have shown that there is an increase in the synthesis, release, and catabolism of norepinephrine in LC neurons in response to stress. It is also well documented that the stress-induced depletion and subsequent elevated synthesis of norepinephrine is accompanied by an increase in the enzymatic activity, protein, and mRNA levels of TH. Furthermore, studies from examining TH induction in the adrenal gland and LC neurons have suggested that induction of TH mRNA may be partially mediated transcriptionally. However, the impact of stress or pharmacological activation on TH transcription rate has never been directly examined in individual LC neurons.;From the study using the intron-specific ISHH, I observed that there was a marked increase in the TH primary transcript levels in LC neurons after pharmacological stimulation and electric footshock, and this increase in TH transcription rate was accompanied by an increase in TH mRNA levels in these neurons. Furthermore, all these treatments increased the percentage of cells expressing increased TH primary transcript levels in LC neurons significantly. I also examined changes in TH transcription rate and mRNA levels in the LC at different times after 15 min footshock. There were different temporal patterns of increases in TH primary transcript and mRNA levels in LC neurons following footshock.;The present study suggests that the increase in TH mRNA levels in LC neurons in response to stress is dependent, at least in part, on the increase in TH transcription rate in these cells and this increase occurs very fast.