Dihydralazine, which is used in the treatment of
hypertension, causes a long-lasting hypotensive action by a direct
vasodilator effect on arteriolar smooth muscle. The present study was carried out to investigate the effect of a daily single injection of
dihydralazine (20 mg/kg, s.c.) for 14 days on the
tyrosine hydroxylase (TH)
protein quantity in some catecholaminergic rat brainstem areas such as the dorsomedial medulla (DMM), the ventrolateral medulla (VLM) and the locus coeruleus (LC). This study demonstrates that the
dihydralazine produced (1) an 85% increase in TH
protein quantity exclusively in the rostral part of DMM, (2) a 58% increase of TH
protein content exclusively in the rostral part of the LC, and (3) a 37% increase of the TH
protein quantity in VLM catecholaminergic area. To determine whether the increase in TH
protein quantity could be related to a change in
norepinephrine (NE) content, the rate constant of disappearance (k) of NE was measured in the catecholaminergic regions of the same rats treated with
dihydralazine. Our results show that
dihydralazine causes an increase of the TH
protein, in addition to an elevation of NE content, within the subpopulations of catecholaminergic structures. These data suggest a selective response of the TH regulation to
dihydralazine within the rostral DMM area which receives barosensory inputs.