Melatonin-induced phase and dose responses in a diurnal mammal, Funambulus pennantii

Sirtuins and the circadian clock interplay in cardioprotection: focus on sirtuin 1

Chronic disruption of circadian rhythms which include intricate molecular transcription-translation feedback loops of evolutionarily conserved clock genes has serious health consequences and negatively affects cardiovascular physiology. Sirtuins (SIRTs) are nuclear, cytoplasmic and mitochondrial histone deacetylases that influence the circadian clock with clock-controlled oscillatory protein, NAMPT, and its metabolite NAD+. Sirtuins are linked to the multi-organ protective role of melatonin, particularly in acute kidney injury and in cardiovascular diseases, where melatonin, via upregulation of SIRT1 expression, inhibits the apoptotic pathway. This review focuses on SIRT1, an NAD+-dependent class III histone deacetylase which counterbalances the intrinsic histone acetyltransferase activity of one of the clock genes, CLOCK. SIRT1 is involved in the development of cardiomyocytes, regulation of voltage-gated cardiac sodium ion channels via deacetylation, prevention of atherosclerotic plaque formation in the cardiovascular system, protection against oxidative damage and anti-thrombotic actions. Overall, SIRT1 has a see-saw effect on cardioprotection, with low levels being cardioprotective and higher levels leading to cardiac hypertrophy.

Wheel-running activity rhythms and masking responses in the diurnal palm squirrel, Funambulus pennantii

Several studies have reported activity patterns of various diurnal species from the order Rodentia, in which most of the species are nocturnal. Most of these studies have been performed under controlled laboratory conditions. These studies found that most of these species change their activity patterns when held under laboratory conditions, have a diverse masking response to light, and their activity pattern is influenced by the presence of a running wheel. Squirrels are reported to be strictly diurnal both in the field as well as in laboratory settings, and, therefore, form an interesting species to study to better understand the switch to diurnality. The aim of the current study is to characterize the masking response and temporal organization of wheel-running activity rhythms in the palm squirrel, Funambulus pennantii, under semi-natural (NLD) and controlled laboratory conditions using different lighting schedules. Squirrels were housed individually in a resting cage with running wheel under NLD (n = 10) and squared 12:12 h of light-dark cycle (LD) (n = 20). After stable entrainment under the LD condition, squirrels were divided into two groups. One group was housed under constant darkness (DD) (n = 10) and another group under constant light (LL) (n = 10). Following the stable free-running rhythm under DD and LL, the LD condition was reinforced. The kinetics of the endogenous pacemaker was studied following a 6 h phase advance or delay of LD cycle. Further, palm squirrels were subjected to a 3.5: 3.5 h LD cycle to evaluate the masking response to light and dark. Squirrels demonstrated stable, clear, robust, and strict diurnal activity rhythm during NLD and LD. In DD and LL, F. pennantii free-ran from the phase of the previous LD cycle, and the free-running period was longer in LL than in DD. The percentage of activity during the light phase was significantly higher in NLD and LD (above 96%) compared to activity during the subjective day in the DD and LL conditions (above 91%). The alpha/rho ratio was significantly higher in the LL compared to other lighting schedules. Further, all ten squirrels re-entrained to both 6 h advance and delay shifts within 11 days. In the ultradian cycle, significant positive masking of light was evident in nine of ten squirrels. These results suggest that the: (i) circadian system of F. pennantii is stable and functional under various lighting conditions; (ii) basic temporal organization in activity pattern remained unaltered even in the presence of a running wheel; (iii) diurnality is the inherent trait of F. pennantii, and (iv) behavioral activity rhythms are governed by both the circadian clock and external masking. Thus, palm squirrels can be used as a suitable diurnal model in circadian biology to study the underlying mechanisms of diurnality and effects of different light schedules, wavelengths, and non-photic cues on physiological and behavioral parameters.