Noradrenaline contributes to modulation of the carotid sinus baroreflex in the nucleus tractus solitarii area in the rabbit.

The authors examined the contribution of the noradrenergic system to central cardiovascular regulation in the dorso-medial region of the medulla oblongata, including the nucleus tractus solitarii (NTS) area, in anaesthetized rabbits. First, to determine the modulation of the carotid sinus baroreflex system by alpha-blocker, which blocks the action of noradrenaline, the open loop relationship between isolated carotid sinus pressure (CSP) and renal nerve activity (RNA) was examined before and after injection of alpha-blocker (phentolamine, 10 micrograms, bilateral). alpha-Blocker injected to the NTS area caused a decrease in RNA at low CSP level (40-50 mmHg), and RNA response remained higher at high CSP levels (130 mmHg), in comparison with control. Baroreflex sensitivity was attenuated significantly from 10.9 +/- 2.2 to 5.0 +/- 0.5% (mmHg-1). The data show that the NTS area is one of the important sites in central modulation of the carotid sinus baroreflex function. It is suggested that the noradrenergic system in the NTS area contributed to facilitating carotid sinus baroreflex function. Second, using the dialysis technique, a dialysis probe was implanted in the dorso-medial region of the medulla oblongata, including the NTS area, and perfused with Ringer's solution. Dialysates were collected from this region and noradrenaline (NA) output was determined by high-performance liquid chromatography/electrochemical methods. Drug-induced hypotension caused a 30% decrease in arterial pressure and an increase in dialysate NA levels from 6.8 +/- 0.2 to 9.0 +/- 0.4 pg sample-1. The results show that noradrenaline in the NTS area is involved in the regulation of arterial pressure through the central modulation of arterial baroreflex function.