The influence of vasopressin on tonic activity of cardiovascular neurones in the ventrolateral medulla of the hypertensive rat

In normotensive Wistar rats, vasopressin may act as an excitatory neurotransmitter at synapses of paraventricular neurones on rostral ventrolateral medullary vasomotor neurones. We studied the influence of this neurotransmitter in spontaneously hypertensive rats to determine if it contributed to the increases in sympathetic nerve traffic in these rats. A five-barrel micropipette assembly was used for extracellular recording of neuronal activity and for microiontophoresis of drugs into the vicinity of identified medullary vasomotor neurones. Excitatory effects of iontophoretically applied vasopression were blocked by simultaneous iontophoretic application of V1a antagonist. Similar application of the vasopressin receptor antagonist did not block an excitatory effect of iontophoretically applied glutamate. Excitatory effects produced by activating paraventricular neurones were also blocked by the V1a antagonist. However, the vasopressin antagonist did not alter the ongoing activity of medullary vasomotor neurones. Therefore, in these anaesthetised hypertensive rats, we concluded that vasopressin neurones do not exert a significant tonic drive to rostral ventrolateral medullary–spinal vasomotor neurones.