Body temperature daily rhythms in the striped mouse Rhabdomys pumilio: the effects of alpha and beta blockade

Obligatory homeothermy of mesic adapted African striped mice, Rhabdomys pumilio, is governed by seasonal basal metabolism and year-round "thermogenic readiness" of brown adipose tissue

Small mammals undergo thermoregulatory adjustments in response to changing environmental conditions. Whereas small heterothermic mammals can employ torpor to save energy in the cold, homeothermic species must increase heat production to defend normothermia through the recruitment of brown adipose tissue (BAT). Here, we studied thermoregulatory adaptation in an obligate homeotherm, the African striped mouse (Rhabdomys pumilio), captured from a subpopulation living in a mesic, temperate climate with marked seasonal differences. Basal metabolic rate (BMR), non-shivering thermogenesis (NST) and summit metabolic rate (MSUM) increased from summer to winter, with NST and MSUM already reaching maximal rates in autumn, suggesting seasonal preparation to the cold. Typical of rodents, cold-induced metabolic rates positively correlate with BAT mass. Analysis of cytochrome c oxidase (COX) activity and UCP1 content, however, demonstrate that thermogenic capacity declines with BAT mass. This resulted in seasonal differences in NST being driven by changes in BMR. The increase in BMR is supported by a comprehensive anatomical analysis of metabolically active organs, revealing increased mass proportions in the cold season. The thermoregulatory response of R. pumilio is associated with the maintenance of body weight throughout the year (48.3±1.4 g), contrasting large summer-winter mass reductions often observed in Holarctic rodents. Collectively, bioenergetic adaptation of this Afrotropical rodent involves seasonal organ adjustments influencing BMR, combined with a constant thermogenic capacity dictated by trade-offs in thermogenic properties of BAT. Arguably, this high degree of plasticity was a response to unpredictable cold spells throughout the year. Consequently, the reliance on such a resource intensive thermoregulatory strategy may expose more energetic vulnerability in changing environments of food scarcity and extreme weather conditions due to climate change, with major ramifications for survival of the species.

The effect of α- and β-adrenergic blockade on daily rhythms of body temperature, urine production, and urinary 6-sulfatoxymelatonin of social voles Microtus socialis

To examine the effect of adrenergic blockade on daily rhythms of rectal body temperature (T(b)), urine production rate, and melatonin (MEL; measured as urinary 6-sulfatoxymelatonin; 6-SMT), social voles Microtus socialis received a single intra-peritoneal injection of either prazosin (PRAZ, 1 mg/kg) or propranolol (PROP, 4.5 mg/kg); alpha- and beta-adrenergic blocking agents respectively, 1 h prior to scotophase onset (light/dark, 12L:12D; lights on 07:00 h). Both blockers caused significant decrease in T(b) values mainly during scotophase. Nocturnal urine production rates were higher for M. socialis treated with the drugs compared with controls. Overall, urine production rates were systematically higher in PROP-voles over the 24 h period when compared with PRAZ-voles; however these differences were not statistically significant. Interestingly, PROP caused significant elevation in urinary 6-SMT at the second half of the dark phase, whereas PRAZ had no effects. These data suggest that the mechanisms regulating MEL synthesis and thermoregulatory responses in M. socialis are different from those described in other rodents' species. Importantly, the data also suggest that the beta-blockade-induced elevation in MEL levels may be directly associated with increased urination in M. socialis.

Nocturnal patterns and up-regulated excretion of the melatonin metabolite 6-sulfatoxymelatonin in the diurnal rodent Psammomys obesus post-weaning under a short photoperiod

The ontogeny of daily rhythms in body temperature (T(b)) oxygen intake (VO(2)) and urinary excretion of the major melatonin metabolite, 6-sulfatoxymelatonin (6SMT) was studied in the day-active rodent, Psammomys obesus. Generally, T(b) and VO2 were high during the light phase in this diurnal species. However, after weaning, and only under the short photoperiod, P. obesus individuals display elevated T(b) and VO2 levels during the dark phase, as in nocturnally active species. In parallel, 6SMT and nocturnal activity of pineal arylalkylamine N-acetyltransferase (AANAT) were greatly enhanced. The cDNA encoding P. obesus pineal AANAT was cloned and found to share 90.2% homology with rat and 83.8% with human AANAT, and based on homology modeling, to structurally resemble the ovine enzyme. A robust diurnal rhythm in P. obesus pineal AANAT-mRNA was found, with maximal levels at night. AANAT-mRNA levels were not enhanced in the post-weaning phase, suggesting post-transcriptional up-regulation of pineal AANAT activity. The photoperiod-dependent post-weaning change into nocturnal behavior and up-regulation melatonin production (as evidenced from the increase in both 6SMT and AANAT activity) represent a hitherto unobserved pattern of transition of a diurnal mammal into independent life. Possibly, this pattern may be physiologically important to facilitate T(b) maintenance in the cold nights of winter in the desert.

Effects of adrenergic blockade on the daily rhythms of body temperature and oxygen consumption of the black-tailed tree rat (Thallomys nigricauda) maintained under different photoperiods

(1) Measurements of VO(2) and T(b) daily rhythms in T. nigricauda were conducted. (2) Three different effects on VO(2) and T(b) were assessed: alpha and beta blockade, time of the day, and photoperiod regime. (3) Results show that in T. nigricauda, the response of T(b) and VO(2) differs significantly under the different photoperiod regimes. Rats acclimated to 16L:8D increased T(b) values as a response to beta blockade, while alpha blockade caused a decrease in T(b). (4) VO(2) values of T. nigricauda acclimated to 11.5L:12.5D responded differently to alpha and beta blockade. It can be assumed that the thermoregulatory responses with beta blockade are through a decrease in heat production, rather than through heat dissipation. However, in the case of alpha blockade, it may be assumed that heat dissipation, through vasodilation, is the cause for the decrease in T(b) values.