Localization of tyrosine hydroxylase mRNA in the axons of the hypothalamo-neurohypophysial system.

With in situ hybridization we examined the localization of mRNA coding for tyrosine hydroxylase (TH) in the rat hypothalamo-neurohypophysial system (HNS) under conditions of acute osmotic stress. Fifteen min after salt loading, hybridization signal of TH mRNA could be located in the magnocellular hypothalamic nuclei and in the median eminence (ME). In untreated animals, TH mRNA was detected only in the ME. In osmotically challenged animals that had been pretreated with colchicine, signals for TH mRNA remained confined to the ME, while pretreatment of salt loaded rats with a polymerase II transcription inhibitor resulted in labelling of the magnocellular perikarya but a decrease of the hybridization signal in the ME. Our results suggest that also TH mRNA is among the RNAs which are axonally transported in the HNS. TH mRNA can probably be stored in axons of the hypothalamo-neurohypophysial tract, to be transported retrogradely and translated upon certain stimuli.