REGIONAL SPECIFICITY OF LONG-TERM REGULATION OF TYROSINE-HYDROXYLASE IN SOME CATECHOLAMINERGIC RAT BRAIN-STEM AREAS .2. EFFECT OF A CHRONICDIHYDRALAZINE TREATMENT

Dihydralazine, which is used in the treatment of hypertension, causes a long-lasting hypotensive action by a direct vasodilator effect on ar teriolar smooth muscle. The present study was carried out to investiga te 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 me dulla (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 D MM, (2) a 58% increase of TH protein content exclusively in the rostra l 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 p rotein quantity could be related to a change in norepinephrine (NE) co ntent, the rate constant of disappearance (k) of NE was measured in th e catecholaminergic regions of the same rats treated with dihydralazin e. Our results show that dihydralazine causes an increase of the TH pr otein, in addition to an elevation of NE content, within the subpopula tions of catecholaminergic structures. These data suggest a selective response of the TH regulation to dihydralazine within the rostral DMM area which receives barosensory inputs.