Sleep Is a Complex and Heterogeneous State That Accounts for a Significant Fraction of the Behavioral Pattern Exhibited during A

328 SLEEP IS A COMPLEX AND HETEROGENEOUS STATE THAT ACCOUNTS FOR A SIGNIFICANT FRACTION OF THE BEHAVIORAL PATTERN EXHIBITED DURING A 24-hour period.1 The 2 states of natural sleep, namely slowwave or synchronized sleep and rapid eye movement (REM) or desynchronized sleep depend on distinct neural mechanisms and affect the regulation of the cardiovascular system differently.2 Hemodynamic control during the sleep phases is remarkably different among species. In cats, the most detailed species studied, a slight decrease in blood pressure, cardiac output, and heart rate during synchronized sleep and desynchronized sleep phases has been previously described.3-8 The fall in blood pressure becomes more intense during desynchronized sleep, mainly due to a decrease in total vascular resistance (caused by renal and splanchnic vasodilatation) and an intense bradycardia (that contributes to decrease cardiac output). In rats, blood pressure remains unchanged and stable during synchronized sleep and increases during desynchronized sleep.9-11 In humans, Khatri and Freis12 described a small reduction in blood pressure and cardiac output during synchronized sleep in relation to the awake state. The decline in cardiac output was associated with a reduction in heart rate rather than in stroke volume, and total vascular resistance remains essentially unchanged. Conversely, blood pressure, cardiac output, total vascular resistance, heart rate, and respiratory rate have been found to be increased during the desynchronized sleep phase.12 These hemodynamic adjustments, which occur during synchronized and desynchronized sleep phases, could be a consequence of a baroreflex buffering the blood pressure levels. Indeed, arterial baroreflex is one of the most powerful and rapid mechanisms for blood pressure control, buffering the short-term pressure fluctuations that accompany the 24-hour period.13 Several techniques can be used to evaluate the relative role of baroreceptors on hemodynamic control.14 Sinoaortic denervation (SAD) is a well-established experimental technique,9-11,15-17 described in rats in our laboratory,18 that involves the differentiation and resection of aortic and carotid baroreceptors. During the sleep phases, we have also reported that rats that undergo SAD have a paradoxic blood pressure response, with increased blood pressure levels during synchronized sleep and marked hypotension during desynchronized sleep.9-11 However, the hemodynamic behavior during the sleep phases after baroreceptor deafferentation has not been completely explored in the rat. To our knowledge, no previous study in rats has analyzed the changes in cardiac output and vascular resistance during sleep. In the present study, our objective was to determine the relative role of arterial baroreflex on hemodynamic parameters during the synchronized and desynchronized sleep phases of natural sleep in rats, especially the changes observed in cardiac output and total and regional vascular resistance. Autonomic And VAsculAr responses in sleep