In the present study we investigated the regulation of tyrosine hydroxylase (TH) by angiotensin II (Ang II) in an attempt to provide cellular and molecular evidence that this hormone has increased neuromodulatory actions in the spontaneously hypertensive (SH) rat brain. Neuronal cells in primary culture from the hypothalamus-brain stem of both normotensive [Wistar-Kyoto (WKY)] and SH rats have been used. These cultures mimic in vivo situations. Ang II caused a time-dependent increase in TH activity in WKY rat brain neurons. A maximal increase of 2.5-fold was observed with 100 nM Ang II in an actinomycin- and cycloheximide-dependent process. In addition, Ang II caused a parallel increase in TH messenger RNA (mRNA) levels, with a maximal stimulation of 5-fold in 4 h by 100 nM Ang II in WKY rat brain neurons. The stimulation of TH mRNA was mediated by the AT1 receptor subtype, resulted from an increase in its transcription, and involved activation of phospholipase C and protein kinase C. Antisense oligonucleotide for c-fos attenuated Ang II stimulation of TH mRNA in a time- and dose-dependent fashion, indicating an involvement of c-fos as a putative third messenger in Ang II stimulation of TH. Ang II also caused stimulation of TH activity and its mRNA levels in neuronal cultures of SH rat brain by a mechanism similar to that observed for neuronal cultures of WKY rat brain, involving AT1 receptors, protein kinase C, and c-fos. However, the stimulation of TH activity and that of TH mRNA were approximately 30% and 80% higher, respectively, in the SH rat brain neurons than those in the WKY rat brain neurons. In vivo experiments have been carried out to validate the elevated response of TH gene expression to Ang II in SH rat brain neuronal cultures. Ang II stimulated both TH activity and TH mRNA levels in the hypothalami and brain stems of adult WKY and SH rats. The level of stimulation in the brain of the SH rat was significantly higher than that in the WKY rat. These observations are consistent with an increase in AT1, receptor gene expression and suggest that increased TH gene expression could be the cellular/molecular basis for the greater neuromodulatory action of Ang II in the SH rat brain.