Thermal dependence of neural activity in the hamster hippocampal slice preparation

Is bigger always better? A critical appraisal of the use of volumetric analysis in the study of the hippocampus

A well-developed spatial memory is important for many animals, but appears especially important for scatter-hoarding species. Consequently, the scatter-hoarding system provides an excellent paradigm in which to study the integrative aspects of memory use within an ecological and evolutionary framework. One of the main tenets of this paradigm is that selection for enhanced spatial memory for cache locations should specialize the brain areas involved in memory. One such brain area is the hippocampus (Hp). Many studies have examined this adaptive specialization hypothesis, typically relating spatial memory to Hp volume. However, it is unclear how the volume of the Hp is related to its function for spatial memory. Thus, the goal of this article is to evaluate volume as a main measurement of the degree of morphological and physiological adaptation of the Hp as it relates to memory. We will briefly review the evidence for the specialization of memory in food-hoarding animals and discuss the philosophy behind volume as the main currency. We will then examine the problems associated with this approach, attempting to understand the advantages and limitations of using volume and discuss alternatives that might yield more specific hypotheses. Overall, there is strong evidence that the Hp is involved in the specialization of spatial memory in scatter-hoarding animals. However, volume may be only a coarse proxy for more relevant and subtle changes in the structure of the brain underlying changes in behaviour. To better understand the nature of this brain/memory relationship, we suggest focusing on more specific and relevant features of the Hp, such as the number or size of neurons, variation in connectivity depending on dendritic and axonal arborization and the number of synapses. These should generate more specific hypotheses derived from a solid theoretical background and should provide a better understanding of both neural mechanisms of memory and their evolution.

The ecological relevance of sleep: the trade-off between sleep, memory and energy conservation

All animals in which sleep has been studied express signs of sleep-like behaviour, suggesting that sleep must have some fundamental functions that are sustained by natural selection. Those functions, however, are still not clear. Here, we examine the ecological relevance of sleep from the perspective of behavioural trade-offs that might affect fitness. Specifically, we highlight the advantage of using food-caching animals as a system in which a conflict might occur between engaging in sleep for memory/learning and hypothermia/torpor to conserve energy. We briefly review the evidence for the importance of sleep for memory, the importance of memory for food-caching animals and the conflicts that might occur between sleep and energy conservation in these animals. We suggest that the food-caching paradigm represents a naturalistic and experimentally practical system that provides the opportunity for a new direction in sleep research that will expand our understanding of sleep, especially within the context of ecological and evolutionary processes.

Hippocampal synaptophysin immunoreactivity is reduced during natural hypothermia in ground squirrels

Natural hypothermia during hibernation results in physiological and behavioral deficits. These changes may be traced at the level of hippocampal signal transduction. We investigated synaptophysin immunoreactivity (SYN-ir) in the hippocampus after short and long periods of hypothermia and short and long periods of euthermy in hibernating ground squirrels. SYN-ir in the stratum lucidum of the hippocampus was transiently reduced during natural hypothermia. Natural hypothermia thus reduces synaptic efficacy. This may play a role in the reduced neuronal connectivity of CA3 pyramidal cell dendrites observed in hibernating ground squirrels.

Temperature-dependent separation of a delayed-onset long-lasting enhancement mediated by coactivation of NMDA and metabotropic glutamate receptors following a transient exposure of extracellular high Ca(2+)

Transient increases in the concentration of extracellular Ca(2+) play an essential role in various physiological and/or pathological implications. Here the effect of low temperatures on synaptic transmission modulated by a brief increase in extracellular high Ca(2+) was studied in CA1 area of hippocampal slices from hamsters. A high Ca(2+) pulse (4.5 mM) induced a long-lasting enhancement at 25, 20, or 17 degrees C, but not at 15 degrees C. While the temperature was lowered to 17 degrees C, the overall expression of synaptic responses following a high Ca(2+) pulse was separated into two sequential enhanced components: an initial component (approximately 30-40 min in duration), followed by a delayed-onset component that was sustained throughout the remainder of the experiment. Application of 100 microM D-2-amino-5-phosphonovalerate, a selective antagonist of N-methyl-D-asparate receptors, or (RS)-alpha-Methyl-4-carboxyphenylglycine, a selective antagonist of metabotropic glutamate (mGlu) receptors, during a high Ca(2+) pulse at 17 degrees C blocked the development of the delayed-onset enhanced component without affecting the initial enhanced component significantly. In contrast, the application of D-2-amino-5-phosphonovalerate or (RS)-alpha-Methyl-4-carboxyphenylglycine immediately after a high Ca(2+) pulse at 17 degrees C had no discernible effects on the development of both components. These results indicate that low temperatures (e.g. 17 degrees C) unmasked two enhanced components that cannot be seen as separate components in the overall potentiation, while long-lasting enhancement was generated at higher temperatures (e.g. 25 degrees C). The development of the delayed-onset enhanced component primarily depended on coactivation of N-methyl-D-asparate and mGlu receptors during a high Ca(2+) challenge at 17 degrees C. The findings here may provide new understanding of the use of low temperatures and promise significant insight into a novel therapeutic intervention in the CNS while the glutamatergic signaling pathway is abnormally activated by certain ambient insults, such as transient increases in the concentration of extracellular Ca(2+).

Spontaneous activity of preoptic neurons in slice preparations of the hypothalamus of European hamsters (Cricetus cricetus) and Wistar rats under different states of hypothermia

Hypothermia was induced in European hamsters (hibernators) and Wistar rats (nonhibernators), and changes in the firing rate and spike duration of extracellularly recorded action potentials were investigated in hypothalamic slices in vitro. At slice temperatures close to normal body temperature (37 +/- 3 degreesC), 32 and 57% of spontaneously active neurons in the medial preoptic area were classified as warm-sensitive in rats and hamsters, respectively. With decreasing slice temperature, the number of active neurons decreased progressively without a significant difference between rats and hamsters. At a slice temperature of 10 degreesC, 57% of all hypothalamic neurons in rats and 42% in hamsters were still spontaneously active. The average temperature at which activity ceased completely when the temperature was decreased further (the cut-off temperature) was 7.9 +/- 0.3 degreesC (n = 14) in rats but was significantly lower at 4.9 +/- 0.4 degreesC (n = 8) in hamsters (P < 0.001). Firing rates and temperature coefficients did not differ in their temperature dependence between rats and hamsters. Action potential duration increased with decreasing slice temperature in both species, but the increase in duration was significantly greater in rats.

Serotonergic modulation of hippocampal pyramidal cells in euthermic, cold-acclimated, and hibernating hamsters

Serotonergic fibers project to the hippocampus, a brain area previously shown to have distinctive changes in electroencephalograph (EEG) activity during entrance into and arousal from hibernation. The EEG activity is generated by pyramidal cells in both hibernating and nonhibernating species. Using the brain slice preparation, we characterized serotonergic responses of these CA1 pyramidal cells in euthermic, cold-acclimated, and hibernating Syrian hamsters. Stimulation of Shaffer-collateral/commissural fibers evoked fast synaptic excitation of CA1 pyramidal cells, a response monitored by recording population spikes (the synchronous generation of action potentials). Neuromodulation by serotonin (5-HT) decreased population spike amplitude by 54% in cold-acclimated animals, 80% in hibernating hamsters, and 63% in euthermic animals. The depression was significantly greater in slices from hibernators than from cold-acclimated animals. In slices from euthermic animals, changes in extracellular K+ concentration between 2.5 and 5.0 mM did not significantly alter serotonergic responses. The 5-HT1A agonist 8-hydroxy-2(di-n-propylamino)tetralin mimicked serotonergic inhibition in euthermic hamsters. Results show that 5-HT is a robust neuromodulator not only in euthermic animals but also in cold-acclimated and hibernating hamsters.

Thermal effects on long-term potentiation in the hamster hippocampus

Extracellular CA1 pyramidal cell activity was measured at different temperatures in hippocampal slices from the Syrian hamster (Mesocricetus auratus), a hibernator. Control records taken before and after tetanic stimulation of Schaffer collateral/commissural pathways were compared to determine if long-term potentiation (LTP) was established. LTP (an enhancement of the population spike amplitude or population synaptic response following tetanus) was elicited in slices at temperatures above 22 degrees C, but not in slices at temperatures of 20 degrees C. When LTP was established at temperatures above 24 degrees C, however, lowering the temperature to 20 degrees C did not abolish the LTP. Furthermore, when a tetanus was delivered at 20 degrees C and the bath temperature was then raised above 22 degrees C, LTP was established. These results for step changes in temperature suggest that the sequence of cellular mechanisms leading to LTP is activated, but then arrested in slices maintained at a constant temperature of 20 degrees C. Assuming this type of activity in the slice parallels in vivo hippocampal activity, it follows that the ability to elicit LTP in CA1 hippocampal pyramidal cells is lost when the core temperature of an animal entering hibernation falls to 20 degrees C.