SYMPATHETIC NERVOUS ACTIVITY AND ARTERIAL BLOOD-PRESSURE CONTROL IN CONSCIOUS RAT DURING REST AND BEHAVIORAL STRESS

The object of this experiment is to analyze the neural control of arterial blood pressure (BP) during rest and a sudden behavioral stress. Sprague-Dawley rats were classically conditioned by following a 15-s tone (CS+) with a 0.5-s tail shock. Bipolar renal nerve electrodes and a caudal artery catheter were implanted. Two days later BP and sympathetic nervous activity (SNA) were recorded in the behaviorally trained animals. The CS+ evoked a large initial increase in BP (peak, 14 +/- 5 mmHg, mean +/- SD; n = 12) that lasted 3.9 +/- 0.8 s. An abrupt (latency = 0.16 +/- 0.03 s), short (duration = 0.58 +/- 0.12 s), and intense (4.09 +/- 1.02 times average control) burst in sympathetic activity preceded this first component (C-1) of the BP conditional response. The size of C-1 was related to the magnitude of the SNA burst. SNA then fell below control; this quiet period preceded a fall in BP after the C-1 peak. Pressure rose again (Ct; peak = 6 +/- 3 mmHg, average increase = 3 +/- 3 mmHg) for the remainder of the CSS. SNA increased to 1.24 +/- 0.14 of control during this second component of the BP conditional response. Ganglionic blockade eliminated the BP and SNA conditional response (n = 3). The C-1 pressure increase appears to result from an ''open-loop'' process in which a brief barrage of nerve activity governs BP changes lasting several seconds. The quiet period probably results from a negative feedback (i.e., baroreflex) relationship between SNA and BP. Finally, the baroreflex may be reset to regulate BP at a higher level during the remainder of the conditional response, perhaps because of altered central drive.