Reversible reduction in dendritic spines in CA1 of rat and ground squirrel subjected to hypothermia-normothermia in vivo: A three-dimensional electron microscope study.
ABSTRACT A study was made at electron microscope level of changes in the three-dimensional (3-D) morphology of dendritic spines and postsynaptic densities (PSDs) in CA1 of the hippocampus in ground squirrels, taken either at low temperature during hibernation (brain temperature 2-4 degrees C), or after warming and recovery to the normothermic state (34 degrees C). In addition, the morphology of PSDs and spines was measured in a non-hibernating mammal, rat, subjected to cooling at 2 degrees C at which time core rectal temperature was 15 degrees C, and then after warming to normothermic conditions. Significant differences were found in the proportion of thin and stubby spines, and shaft synapses in CA1 for rats and ground squirrels for normothermia compared with cooling or hibernation. Hypothermia induced a decrease in the proportion of thin spines, and an increase in stubby and shaft spines, but no change in the proportion of mushroom spines. The changes in redistribution of these three categories of spines in ground squirrel are more prominent than in rat. There were no significant differences in synapse density determined for ground squirrels or rats at normal compared with low temperature. Measurement of spine and PSD volume (for mushroom and thin spines) also showed no significant differences between the two functional states in either rats or ground squirrels, nor were there any differences in distances between neighboring synapses. Spinules on dendritic shafts were notable qualitatively during hibernation, but absent in normothermia. These data show that hypothermia results in morphological changes which are essentially similar in both a hibernating and a non-hibernating animal.
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- "These changes have been characterized in other species of ground squirrels and include: a large decrease in pyramidal cell soma size   , decreases in dendritic branching and spine density of CA1 and CA3 cells   , fewer mossy fiber terminals   and up to a 65% loss of synapses  . Upon arousal from hibernation, there is a rapid increase in cell soma size, dendritic branching and spine density within several hours     and this appears to parallel a recovery of function based on behavioral tests . However, none of these studies examined sex as a variable. "
ABSTRACT: Studies across and within species suggest that hippocampus size is sexually dimorphic in polygamous species, but not in monogamous species. Although hippocampal volume varies with sex, season and mating system, few studies have simultaneously tested for sex and seasonal differences. Here, we test for sex and seasonal differences in the hippocampal volume of wild Richardson's ground squirrels (Urocitellus richardsonii), a polygamous species that lives in matrilineal, kin-based social groups and has profound sex differences in behavior. Based on the behavior and ecology of this species, we predicted that males would have a significantly larger hippocampus than females and that the hippocampus would be largest in males during the breeding season. Analyses of both absolute and relative volumes of the hippocampus yielded a significant difference between the sexes and seasons as well as an interaction between the two such that non-breeding males have significantly larger hippocampal volumes than breeding males or females from either season. Dentate gyrus, CA1 and CA3 subfield volumes were generally larger in the non-breeding season and in males, but no significant interaction effects were detected. This sex and seasonal variation in hippocampal volume is likely the result of their social organization and male-only food caching behavior during the non-breeding season. The demonstration of a sex and seasonal variation in hippocampal volume suggests that Richardson's ground squirrel may be a useful model for understanding hippocampal plasticity within a natural context.Behavioural brain research 09/2012; 236(1):131-8. DOI:10.1016/j.bbr.2012.08.044 · 3.39 Impact Factor
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- "Most hibernators periodically interrupt the state of hibernation (torpor) by euthermic episodes or arousal, a process responsible for up to 90% of the energy consumed during hibernation. Previously, using Golgi and electron microscope studies in hippocampal tissue of hibernating ground squirrels we have shown marked structural alterations in the components of neural circuitry, with the reversible retraction of dendritic spines and synapses in CA1 and CA3  . "
ABSTRACT: Neurogenesis occurs in the adult mammalian hippocampus, a region of the brain important for learning and memory. Hibernation in Siberian ground squirrels provides a natural model to study mitosis as the rapid fall in body temperature in 24 h (from 35-36°C to +4-6°C) permits accumulation of mitotic cells at different stages of the cell cycle. Histological methods used to study adult neurogenesis are limited largely to fixed tissue, and the mitotic state elucidated depends on the specific phase of mitosis at the time of day. However, using an immunohistochemical study of doublecortin (DCX) and BrdU-labelled neurons, we demonstrate that the dentate gyrus of the ground squirrel hippocampus contains a population of immature cells which appear to possess mitotic activity. Our data suggest that doublecortin-labelled immature cells exist in a mitotic state and may represent a renewable pool for generation of new neurons within the dentate gyrus.Neural Plasticity 05/2011; 2011:867525. DOI:10.1155/2011/867525 · 3.60 Impact Factor
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- "Recent electron-microscopic data indicate that spine-pruned cortical neurons do lose their connection with afferent inputs (Knott et al., 2006). On the other hand, in hibernating animals there is a marked decrease in spine density during hibernation but there is an increase in shaft synapses (Popov et al., 2007; von der Ohe et al., 2006), and when the animals wake up from hibernation they regain the spines and appear to remember tasks learnt before hibernation, indicating that regardless of the persistence of spines, memories are retained (Clemens et al., 2009). In fact, if trained 24 h after arousal from hibernation, they remember better than controls (Weltzin et al., 2006). "
ABSTRACT: An emerging view of structure-function relations of synapses in central spiny neurons asserts that larger spines produce large synaptic currents and that these large spines are persistent ('memory') compared to small spines which are transient. Furthermore, 'learning' involves enlargement of small spine heads and their conversion to being large and stable. It is also assumed that the number of spines, hence the number of synapses, is reflected in the frequency of miniature excitatory postsynaptic currents (mEPSCs). Consequently, there is an assumption that the size and number of mEPSCs are closely correlated with, respectively, the physical size of synapses and number of spines. However, several recent observations do not conform to these generalizations, necessitating a reassessment of the model: spine dimension and synaptic responses are not always correlated. It is proposed that spines are formed and shaped by ongoing network activity, not necessarily by a 'learning' event, to the extent that, in the absence of such activity, new spines are not formed and existing ones disappear or convert into thin filopodia. In the absence of spines, neurons can still maintain synapses with afferent fibers, which can now terminate on its dendritic shaft. Shaft synapses are likely to produce larger synaptic currents than spine synapses. Following loss of their spines, neurons are less able to cope with the large synaptic inputs impinging on their dendritic shafts, and these inputs may lead to their eventual death. Thus, dendritic spines protect neurons from synaptic activity-induced rises in intracellular calcium concentrations.European Journal of Neuroscience 06/2010; 31(12):2178-84. DOI:10.1111/j.1460-9568.2010.07270.x · 3.67 Impact Factor