Study on the expression of tyrosine hydroxylase (TH) mRNA in the brains of a stress-induced depression-model rats

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The abnormality of brain noradrenergic function is one of the hypotheses of depressive illness which sometimes manifests or aggravates itself following stressful situation. Therefore, to see how the noradrenaline (NA) synthesis is influenced in the brain by chronic stress, using in situ hybridization technique we examined the expression of tyrosine hydroxylase (TH) mRNA in the locus coeruleus (LC) of "depression-model rats" with low activity following exposure to long-term (14 days) forced walking stress (FWS). We also examined it in rats stressed for 30 minutes, 3 hours, 1 day, 2 days (short-term), 6 days and 12 days (long-term). The THmRNA expression markedly increased in rats exposed to FWS for 1-12 days, but neither 30 minutes nor 3 hours. It increased significantly in the depression-model rats too, though its level was lower than that of rats stressed for 12 days. No difference of expression was found between control rats and "spontaneous recovery rats" whose activity was restored after the long-term stress. These results suggest that FWS is a mild and unadaptable stress in terms of delayd but prolonged increase in THmRNA expression and that the increased function of NA synthesis continues in the FWS-induced depression-model rats because of the maintenance of high level of THmRNA expression.