EFFECT OF 6-HYDROXYDOPAMINE INJECTION INTO THE ARCUATE HYPOTHALAMIC NUCLEUS ON THE OSMOTIC RELEASE OF VASOPRESSIN IN CONSCIOUS RATS

The aim of the present study was to evaluate a role in vasopressin sec retion of the catecholaminergic neurons, including the tuberohypophysi al dopaminergic neurons situated in the arcuate hypothalamic nucleus. A neurotoxin, 6-hydroxydopamine (6 g/l), was injected locally into the arcuate nucleus and its effects on catecholamine levels of the hypoth alamic tissue and the neurointermediate lobe, and on the plasma vasopr essin concentrations before and during iv infusion (0.1 ml kg(-1) min( -1)) of isotonic (0.15 mol/l) or hyper-tonic saline (2.5 mol/l), were examined in conscious rats. The infusion of hyper-tonic saline produce d increases of plasma vasopressin 15 and 30 min later, accompanied by elevations of plasma osmolality, sodium, chloride and arterial pressur e. The vasopressin response was potentiated markedly by the 6-hydroxyd opamine injection performed 8 days before, which hardly affected the r esponses of the other variables. Histological examination indicated th at the injection sites of 6-hydroxydopamine in those rats had been loc ated in the area ranging from rostral to medial arcuate nucleus. The i v infusion of isotonic saline did not change plasma vasopressin, osmol ality, sodium, chloride or arterial pressure, regardless of the presen ce or absence of pretreatment with 6-hydroxydopamine. It was confirmed that when 6-hydroxydopamine was injected into the arcuate nucleus reg ion 8 days before, noradrenaline and adrenaline concentrations of the hypoathalamic tissue containing the injection site were decreased rema rkably, although we could not detect any significant alteration in the dopamine concentration of the hypothalamic tissue or the neurointerme diate lobe. On the basis of these results, we concluded that catechola minergic neurons in the arcuate nucleus may act to inhibit osmotic vas opressin secretion.