Continuous light exposure and sympathectomy suppress circadian rhythm of blood pressure in rats

Background: Although the 24-hour rhythm in blood pressure is well known, it is not clear how environmental light controls circadian cardiovascular and behavioral rhythms. Methods and Results: The prolonged exposure of Wistar rats to continuous light for 17 weeks, beginning at 5 weeks old, induced a complete suppression of their blood pressure, heart rate, spontaneous locomotor activity, and body temperature circadian rhythms. Daily subcutaneous melatonin injections at the theoretical onset of darkness for 21 days could not restore light-suppressed blood pressure circadian rhythm, whereas it partially synchronized heart rate and body temperature rhythms and it fully restored spontaneous locomotor activity rhythms, as measured by radiotelemetry. The transfer of these rats from constant light to a standrad 12:12-hour light/dark photoperiod fully restored circadian rhythmicity within 2 to 5 days, although their 24-hour diastolic blood pressure remained elevated. Synchronized rats were then subjected to superior cervical ganglionectomy (SCGx) and 6-hydroxydopamine sympathectomy (SYMPx). SCGx plus SYMPx completely abolished the circadian rhythm in blood pressure and significantly reduced those in heart rate, spontaneous locomotor activity, and body temperature. Conclusions: We conclude that in Wistar rats exposed to continuous light, the light-induced increase in sympathetic outflow can suppress blood pressure circadian rhythm, and sustained cardiac wall stress can alter diastolic function at rest. Preserved inotropy in these conditions must result from an adaptative hypertrophic response of myocytes.