Thyroidal iodine and thyroglobulin in hibernating and active Richardson's ground squirrels Spermophilus richardsoni

Thyroidal iodine, protein and thyroglobulin (TG) were investigated in a hibernator, Spermophilus richardsoni, sampled in the field and from laboratory-held winter colonies. In field animals, thyroidal iodine and protein contents were similar at onset of hibernation and at terminal arousal. Iodine content was increased in laboratory-held animals, a function of alimentary supply, but no differences were observed between hibernators and non-hibernators. Density sucrose gradients showed that approx. 20% of the TG was present as the 12S precursor sub-unit. No variations in the iodine content, nor the iodoamino acid composition of the TG occurred as a function of hibernation.     

Uridine uptake and RNA synthesis in the brain of torpid and awakened ground squirrels.

1. Uptake of [3H]uridine into the nucleotide precursor pool after intraventricular injection occurs with the same intensity in the brain of torpid and normothermic awakened ground squirrels. This indicates that the membrane uridine transporters and uridine kinases operate in the hibernator's brain in a hypothermia-tolerant way. 2. Utilization of the [3H]uridine pool for synthesis of the rapidly labelled RNA in the brain of torpid ground squirrels falls more than eight times against RNA labelling in the brain of the active animals between bouts of hibernation. 3. Two hours from the beginning of the artificially provoked awakening, RNA uridine incorporation in the brain of ground squirrels has risen 6.5 times. 4. Drastic changes in [3H]uridine RNA labelling under the stable uridine uptake exclude the precursors and energy supply as the main factors determining changes in intensity of the brain RNA synthesis in the different stages of hibernation.     

Ubiquitylation of proteins in livers of hibernating golden-mantled ground squirrels, Spermophilus lateralis

Rodent hibernators experience low core body temperature (as low as −2 °C) and reduced metabolic rates during hibernation. Concordant with energetic constraints, protein synthesis is negligible during torpor. To maintain pools of key regulatory proteins, proteolysis must be depressed as well. Ubiquitin-dependent proteolysis consists of two major steps: (1) ubiquitylation or tagging of a protein substrate by ubiquitin and (2) the protein substrate’s subsequent degradation by the 26S proteasome. Earlier, we demonstrated that the low temperatures typical of torpor virtually arrest proteolytic processing. Here, we demonstrate that in vitro ubiquitylation still continues at greater than 30% of maximal rates at temperatures as low as 0 °C. Continued ubiquitylation in the presence of severely depressed proteolysis may explain the previously observed 2- to 3-fold increase of ubiquitin conjugates during torpor. We determined if there is a qualitative change in the type of ubiquitylation e.g., monoubiquitylation vs polyubiquitylation that occurs during torpor. We found no bias for monoubiquitylation in any state of the torpor cycle. We further determined that substrate limitation of free ubiquitin is not limiting ubiquitylation during torpor. We conclude that while the cold temperatures of torpor may limit proteolysis in accordance with metabolic demands, continued ubiquitylation may result in increased ubiquitin conjugate concentrations that must be processed upon arousal.     

Seasonal variation in daily activity patterns of free-ranging European ground squirrels (Spermophilus citellus)

Daily aboveground activity of European ground squirrels (Spermophilus citellus) in their natural habitat was recorded with a visual scanning procedure during the active seasons of 1992 and 1993. Activity patterns were analyzed with respect to time of year and to the animal's reproductive state. Aboveground activity started on average 3.9 h (SD 0.6 h, n = 37 days) after civil twilight at dawn and ended on average 3.2 h (SD 0.9 h, n = 37 days) before civil twilight at dusk. Between onset and offset of activity, 54% was spent aboveground, of which 73% was spent foraging. Activity patterns were influenced by photoperiod, rainfall, and by reproductive state. During mating, reproductively active males started activity earlier than females and reproductively inactive males. For females, time spent foraging was high during lactation. The midpoint of daily activity was at 12:16 h (SD 0.37 h, n = 37 days). Activity patterns of European ground squirrels thus appear robustly positioned in the middle of the photoperiod.     

Relationships between the thermal environment and activity of Piute ground squirrels (Spermophilus mollis)

(1) We examined relationships between ground squirrel activity and operative temperature (T_e) in two habitats in southwestern Idaho, USA to determine how activity patterns are tied to environmental conditions and how habitat can mediate the thermal environment. (2) Midday environmental conditions were too hot throughout much of the active season for Piute ground squirrels to remain active in a grassland habitat. (3) Ground squirrels were most active in grassland at a T_e of approximately 25°C and showed a strong bimodal activity pattern during the latter half of their active season (after 15 April). (4) Suitable aboveground microclimates existed throughout most of the season in sagebrush (Artemisia tridentata) habitats and ground squirrels remained active throughout the day. (5) The availability of cooler microhabitats may make sagebrush a better habitat than grassland for Piute ground squirrels, especially during the latter portion of the active season and in hotter than normal years.     

Ubiquitylation of proteins in livers of hibernating golden-mantled ground squirrels,Spermophilus lateralis

Rodent hibernators experience low core body temperature (as low as −2 °C) and reduced metabolic rates during hibernation. Concordant with energetic constraints, protein synthesis is negligible during torpor. To maintain pools of key regulatory proteins, proteolysis must be depressed as well. Ubiquitin-dependent proteolysis consists of two major steps: (1) ubiquitylation or tagging of a protein substrate by ubiquitin and (2) the protein substrate’s subsequent degradation by the 26S proteasome. Earlier, we demonstrated that the low temperatures typical of torpor virtually arrest proteolytic processing. Here, we demonstrate that in vitro ubiquitylation still continues at greater than 30% of maximal rates at temperatures as low as 0 °C. Continued ubiquitylation in the presence of severely depressed proteolysis may explain the previously observed 2- to 3-fold increase of ubiquitin conjugates during torpor. We determined if there is a qualitative change in the type of ubiquitylation e.g., monoubiquitylation vs polyubiquitylation that occurs during torpor. We found no bias for monoubiquitylation in any state of the torpor cycle. We further determined that substrate limitation of free ubiquitin is not limiting ubiquitylation during torpor. We conclude that while the cold temperatures of torpor may limit proteolysis in accordance with metabolic demands, continued ubiquitylation may result in increased ubiquitin conjugate concentrations that must be processed upon arousal.     

Capture, care, and captive breeding of 13-lined ground squirrels, Spermophilus tridecemlineatus

Researchers use the 13-lined ground squirrel for studies of hibernation biochemistry and physiology, as well as for modeling a variety of potential biomedical applications of hibernation physiology. It is currently necessary to capture research specimens from the wild; this presents a host of unknown variables, not least of which is the stress of captivity. Moreover, many investigators are unfamiliar with the husbandry of this species. The authors describe practical methods for their capture, year-round care (including hibernation), captive mating, and rearing of the young. These practices will allow the researcher to better standardize his or her population of research animals, optimizing the use of this interesting model organism.     

Effects of hibernation on multicatalytic proteinase complex in thirteen-lined ground squirrels, Spermophilus tridecemlineatus

Multicatalytic proteinase complex (MCP) was studied in skeletal muscle of the hibernating ground squirrel, Spermophilus tridecemlineatus. MCP was partially purified using a S-400 gel filtration column and Centricon concentrating devices and assayed fluorometrically using three AMC-labeled substrates. K_m and V_max values were determined for each substrate with no significant differences between the enzyme from euthermic versus hibernating animals when assayed at 23 C. However, properties of MCP from euthermic and hibernating ground squirrels were differentially affected by low assay temperature (8–10 C) and also differed from the mouse enzyme, the data indicating that ground squirrel MCP is better suited for low temperature function. MCP preferentially degrades oxidatively-damaged proteins and quantification of protein carbonyl content showed that the level of oxidatively-damaged protein in skeletal muscle decreased by > 75% during hibernation suggesting a continuing role for the MCP in the torpid state. (Mol Cell Biochem 271: 205–213, 2005)     

Hibernation effects on memory in European ground squirrels (Spermophilus citellus).

Effects of hibernation on memory were tested in European ground squirrels (Spermophilus citellus). The animals were trained in summer to successfully accomplish two tasks: a spatial memory task in a maze and an operant task on a feeding machine. One group hibernated normally, and the other was prevented from hibernation by maintaining ambient temperature at 22 degrees C. In spring, the same tasks were repeated for both groups and their individual performances compared to the initial training phase. The experimental groups differed significantly in both tasks. The nonhibernating animals had higher levels of retention and needed significantly fewer trials to relearn the tasks than the group that had hibernated. In addition to testing the retention of conditioned tasks, social memory was also studied. The ground squirrels were given a social recognition test in spring with one familiar and one unfamiliar conspecific. In contrast to the conditioned tasks, social memory did not seem to be affected by hibernation. The results indicate negative effects of hibernation on the retention of conditioned tasks, which could produce important constraints on animals. A potential explanation for this memory loss might be changes in neuronal activity, which occur during hibernation.     

Sciurid phylogeny and the paraphyly of Holarctic ground squirrels (Spermophilus)

The squirrel family, Sciuridae, is one of the largest and most widely dispersed families of mammals. In spite of the wide distribution and conspicuousness of this group, phylogenetic relationships remain poorly understood. We used DNA sequence data from the mitochondrial cytochrome b gene of 114 species in 21 genera to infer phylogenetic relationships among sciurids based on maximum parsimony and Bayesian phylogenetic methods. Although we evaluated more complex alternative models of nucleotide substitution to reconstruct Bayesian phylogenies, none provided a better fit to the data than the GTR+G+I model. We used the reconstructed phylogenies to evaluate the current taxonomy of the Sciuridae. At essentially all levels of relationships, we found the phylogeny of squirrels to be in substantial conflict with the current taxonomy. At the highest level, the flying squirrels do not represent a basal divergence, and the current division of Sciuridae into two subfamilies is therefore not phylogenetically informative. At the tribal level, the Neotropical pygmy squirrel, Sciurillus, represents a basal divergence and is not closely related to the other members of the tribe Sciurini. At the genus level, the sciurine genus Sciurus is paraphyletic with respect to the dwarf squirrels (Microsciurus), and the Holarctic ground squirrels (Spermophilus) are paraphyletic with respect to antelope squirrels (Ammospermophilus), prairie dogs (Cynomys), and marmots (Marmota). Finally, several species of chipmunks and Holarctic ground squirrels do not appear monophyletic, indicating a need for reevaluation of alpha taxonomy.     

Pineal sensitivity to nighttime swimming stress changes during the active season in Richardson's ground squirrels (Spermophilus richardsonii)

Melatonin synthesis in the pineal gland, which is primarily regulated by the environmental lighting regime, can also be influenced by other factors that elicit modifications in sympathetic tone. The objectives of this study were to determine if forced swimming alters the normal pattern of melatonin production in the pineal gland of the Richardson's ground squirrel (Spermophilus richardsonii). In early June, the squirrels were forced to swim for 10 min during the photophase or during the scotophase. In mid-July squirrels swam only during the scotophase. Animals were sacrificed 15, 30, or 60 min after the onset of swimming. Activities of pineal N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT) were assessed by radioenzyme assay, and pineal melatonin content was measured by radioimmunoassay. Daytime swimming elicited no major changes in enzyme activity or pineal melatonin. In June, swimming at night prevented the normal rises in NAT activity and pineal melatonin seen in nonswimming controls. In contrast, the pineals of squirrels that were tested 6 weeks later in mid-July did not appear to be as sensitive to nighttime swimming, as there were only minor differences in both NAT activity and melatonin content compared to controls. These results demonstrate that forced nighttime swimming, unlike several other aversive stimuli, can evoke changes in the normal pattern of pineal melatonin production in this species. Furthermore, the pineal's response to such stimuli may not be stable over the course of the active season.