Preganglionic nerve stimulation has been shown to induce delayed or long term changes in the neuron. Different models were used to study the trans-synaptic regulation of the tyrosine hydroxylase (TH): the rat adrenal medulla (AM) and the superior cervical ganglia (SCG). Northern blot analysis, western blot and enzymatic assays demonstrated that the TH mRNA paralleled both an increase in the protein amount and its enzymatic activity. Results of in vitro transcription on nuclei isolated from AM or from SCG after treatment with reserpine suggest that this increase in TH expression is due to an effect on the transcriptional activity of the TH gene. Other gene, cfos, is also induced by reserpine indicating that the TH transcription in these neurons may be mediated by "third messengers". Several putative regulatory elements, in particular an octamer sequence AP1 has been localized in the promoter of TH gene. Gel shift assays with nuclear extracts from untreated and phorbol ester treated cells strongly suggest that a protein complex binds to this AP1 like sequence. Comparative analysis of gel shift assays with AM nuclear extracts exhibit a similar pattern suggesting that this AP1 site could be involved in the trans-synaptic regulation of TH.