The role of glutamate and vasopressin in the excitation of RVL neurones by paraventricular neurones.

Neurones in the paraventricular nucleus of the hypothalamus project to rostral ventrolateral medullary spinally projecting vasomotor neurones. We studied the excitatory action and the role of glutamate and vasopressin in this pathway in anaesthetised 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. Iontophoresis of L-glutamate or vasopressin into the vicinity of a vasomotor neurone increased activity, effects which were blocked by simultaneous iontophoretic application of a glutamate receptor antagonist, or a vasopressin V(1a) antagonist respectively. Paraventricular neurones were activated either by microinjecting D,L-homocysteic acid or by disinhibition by microinjecting bicuculline. The excitatory effects on vasomotor neurones, of paraventricular nucleus stimulation at some sites were prevented by simultaneous microiontophoretic application of kynurenic acid or at other sites by application of V(1a) antagonist. Neither antagonist altered the ongoing activity of the vasomotor neurones. Therefore, glutamate or vasopressin may act as excitatory neurotransmitters at synapses of paraventricular neurones on rostral ventrolateral medullary vasomotor neurones.