MODULATION BY ANGIOTENSIN-III OF NOCICEPTION-RELATED AND ARTERIAL PRESSURE-RELATED NEURONAL RESPONSIVENESS IN THE NUCLEUS-RETICULARIS GIGANTOCELLULARIS OF THE RAT

We evaluated possible modulation by angiotensin III (AIII) of the inte ractive effect of noxious stimuli and elevation in systemic arterial p ressure on the responsiveness of neurons in the nucleus reticularis gi gantocellularis (NRGC) of the medulla oblongata. Combined extracellula r single-neuron recording and microiontophoresis were carried out on m ale, adult Sprague-Dawley rats anesthetized with pentobarbital sodium. The responsiveness of NRGC neurons to nociception (tail clamp) and/or transient hypertension elicited by phenylephrine (5 mug/kg, i.v.), in the absence or presence of AIII, was used as the experimental index. Microiontophoretic application of the heptapeptide suppressed the resp onses of spontaneously active NRGC neurons to individually delivered n ociception or hypertension. Interestingly, the preferential reduction in responsiveness to tail clamp upon simultaneous elevation in arteria l pressure was reversed to one that favored nociception in the presenc e of AIII. These actions of the heptapeptide appeared to be receptor-s pecific, since they were discernibly blocked by its selective antagoni st, Ile7-angiotensin III. Our results reveal that neuropeptides such a s AIII may differentially modulate neuronal responsiveness according t o the prevailing physiologic input(s) to the central nervous system of the animal.