Oxytocin Neurotransmission in the A1-area of the Brainstem Induces Hormonal Vasopressin Release in Rats

To investigate the role of the oxytocin innervation of the caudal ventrolateral medulla, immunocytochemical techniques were used to demonstrate the presence of oxytocin fibres and terminals in close apposition to noradrenergic neurons of the A1-area. Subsequently, in freely moving animals fitted with an indwelling jugular venous catheter and a bilaterally implanted chronic cannula in the A1-area, it was examined whether infusions of oxytocin in this area were able to influence hormonal vasopressin release. It appeared that nanomolar (50-500 nM) concentrations of oxytocin induce a fourfold rise in plasma vasopressin values. The specificity of this effect was established with control infusions of Ringer, vasopressin, and the addition of an antagonist to oxytocin. It was not possible to demonstrate a major role for oxytocin in the A1-area in the release of hormonal vasopressin occurring during haemorrhage. These data permit us to conclude that oxytocin acts on presumably noradrenergic neurons of the A1-area leading to the release of vasopressin into the peripheral circulation. The circumstances under which oxytocin is released in this area remain to be established.