Previous Studies from This Laboratory Have Brought Forth Evidence Suggesting That Im- Balances between Carotid and Vertebral

701 PREVIOUS STUDIES FROM THIS LABORATORY HAVE BROUGHT FORTH EVIDENCE SUGGESTING THAT IMBALANCES BETWEEN CAROTID AND VERTEBRAL blood flow and temperature underlie the increase in preoptichypothalamic temperature during rapid eye movement (REM) sleep in several mammalian species.1 Particularly relevant to the mechanism of such a temperature increase are the effects on preoptic-hypothalamic temperature of short-lasting (≤ 100 seconds), bilateral occlusion of the common carotid artery during wakefulness, non-rapid eye movement (NREM) sleep, and REM sleep in the cat2 and rabbit.3 During wakefulness and NREM sleep, this procedure decreases the ear pinna temperature and increases the preoptic-hypothalamic temperature. In contrast, during REM sleep, the same procedure does not significantly affect the spontaneous decrease in ear pinna temperature and increase in preoptic-hypothalamic temperature. This result supports the inference that the bilateral occlusion of the common carotid artery in wakefulness and NREM sleep would mimic the hemodynamic condition occurring spontaneously in REM sleep. Consistent with this view, bilateral occlusion of the common carotid artery just after the end of REM sleep stops the spontaneous increase in ear pinna temperature and decrease in preoptic-hypothalamic temperature. This result indirectly points out that the blood flow in the common carotid artery spontaneously increases after the end of REM sleep. Only the thermal data support the inference that a physiologically significant drop in common carotid blood flow characterizes REM sleep. In this respect, the systemic circulatory changes observed during REM sleep in cats,4 rabbits,5 and rats6 lack statistical significance. The aim of the present study was to appraise directly whether state-dependent changes in the systemic cardiovascular variables would underlie a physiologically significant spontaneous decrease in common carotid blood flow during REM sleep in rabbits.