Inhibiting the rabbit caudal ventrolateral medulla prevents baroreceptor-initiated secretion of vasopressin.

The A 1 noradrenergic neurones are known to project from the caudal ventrolateral medulla to the vasopressin-secreting neuroendocrine cells in the hypothalamus. They therefore represent a possible central pathway from the medulla to the hypothalamus for baroreceptor-initiated secretion of vasopressin. We tested this hypothesis in the anaesthetized rabbit. Muscimol, a gamma-aminobutyric-acid-receptor agonist, was injected into the caudal ventrolateral medulla to inhibit the A 1 noradrenergic neurones. Secretion of vasopressin, measured by radioimmunoassay, was initiated either by arterial haemorrhage or by constriction of the inferior vena cava. After injection of vehicle into the caudal ventrolateral medulla, or after injection of muscimol into nearby control areas, both haemorrhage and constriction of the inferior vena cava produced the expected elevation in plasma vasopressin. After injection of muscimol into the caudal ventrolateral medulla, secretion of vasopressin in response to haemorrhage and to constriction of the inferior vena cava, was completely abolished. The A 1 noradrenergic neurones may be the sole pathway transmitting the reflex for baroreceptor-initiated secretion of vasopressin from the medulla to the hypothalamus.