Temperature effects on evoked potentials of hippocampal slices from euthermic chipmunks, hamsters and rats

Neuronal Activity in the Hibernating Brain

Hibernation is a natural phenomenon in many species which helps them to survive under extreme ambient conditions, such as cold temperatures and reduced availability of food in the winter months. It is characterized by a dramatic and regulated drop of body temperature, which in some cases can be near 0°C. Additionally, neural control of hibernation is maintained over all phases of a hibernation bout, including entrance into, during and arousal from torpor, despite a marked decrease in overall neural activity in torpor. In the present review, we provide an overview on what we know about neuronal activity in the hibernating brain focusing on cold-induced adaptations. We discuss pioneer and more recent in vitro and in vivo electrophysiological data and molecular analyses of activity markers which strikingly contributed to our understanding of the brain's sensitivity to dramatic changes in temperature across the hibernation cycle. Neuronal activity is markedly reduced with decreasing body temperature, and many neurons may fire infrequently in torpor at low brain temperatures. Still, there is convincing evidence that specific regions maintain their ability to generate action potentials in deep torpor, at least in response to adequate stimuli. Those regions include the peripheral system and primary central regions. However, further experiments on neuronal activity are needed to more precisely determine temperature effects on neuronal activity in specific cell types and specific brain nuclei.

Neuronal plasticity in hibernation and the proposed role of the microtubule-associated protein tau as a "master switch" regulating synaptic gain in neuronal networks

The present paper provides an overview of adaptive changes in brain structure and learning abilities during hibernation as a behavioral strategy used by several mammalian species to minimize energy expenditure under current or anticipated inhospitable environmental conditions. One cellular mechanism that contributes to the regulated suppression of metabolism and thermogenesis during hibernation is reversible phosphorylation of enzymes and proteins, which limits rates of flux through metabolic pathways. Reversible phosphorylation during hibernation also affects synaptic membrane proteins, a process known to be involved in synaptic plasticity. This mechanism of reversible protein phosphorylation also affects the microtubule-associated protein tau, thereby generating a condition that in the adult human brain is associated with aggregation of tau protein to paired helical filaments (PHFs), as observed in Alzheimer's disease. Here, we put forward the concept that phosphorylation of tau is a neuroprotective mechanism to escape NMDA-mediated hyperexcitability of neurons that would otherwise occur during slow gradual cooling of the brain. Phosphorylation of tau and its subsequent targeting to subsynaptic sites might, thus, work as a kind of "master switch," regulating NMDA receptor-mediated synaptic gain in a wide array of neuronal networks, thereby enabling entry into torpor. If this condition lasts too long, however, it may eventually turn into a pathological trigger, driving a cascade of events leading to neurodegeneration, as in Alzheimer's disease or other "tauopathies".

Thermal dependence of neural activity in the hamster hippocampal slice preparation

1. Neural activity was recorded in an in vitro hamster hippocampal slice preparation while the temperature of the Ringer's solution bathing in the slice was controlled at selected levels. 2. The amplitude of the population spike (action potentials from a group of pyramidal cells) was measured as bath temperature was lowered from 35 degrees C to temperatures where a response could not be evoked. 3. Plots of population spike amplitude versus temperature have bell-shaped curves. The population spikes increased in amplitude as temperature was lowered from 35 degrees C, reached a peak amplitude between 25 and 20 degrees C, and then decreased until a response could not be evoked when temperature was further lowered. 4. These in vitro results obtained in the slice preparation are related to in vivo hippocampal studies. Results are interpreted as consistent with the proposal reviewed here that neural activity in the hippocampus plays a role at specific stages of entrance into and arousal from hibernation.

Diurnal modulation of long-term potentiation in the hamster hippocampal slice

Long-term potentiation (LTP) was examined in hippocampal slices from Syrian hamsters entrained to a LD 14:10 cycle. Population spike (PS) amplitudes from CA1 pyramidal cells were measured before (control) and after tetanizing the Schaffer/collateral commissural pathway. Slices from animals sacrificed during the day, between zeitgeber time (ZT) 0430 and 0530, were incubated, and then tetanized between ZT 1340 and 1930, where ZT=0 denotes lights on. Slices from animals sacrificed during the night, between ZT 1830 and 1930, were incubated, and tetanized between ZT 0030 and 0410. LTP, a sustained increase in PS amplitude following tetanus, was evoked in both groups. PS amplitude increased by 102.7+/-20.3% in animals sacrificed during the day and by 48.0+/-7.5% in animals sacrificed during the night (p<0.05). Thus hamster slices prepared during the day show more robust LTP (a doubling of PS amplitude), a difference persisting in slices incubated for several hours.

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.

Rapid temperature changes induce adenosine-mediated depression of synaptic transmission in hippocampal slices from rats (non-hibernators) but not in slices from golden hamsters (hibernators)

Disturbances in neuronal communication induced by rapid temperature changes are a risk in the context of accidental hypothermia and would be fatal for hibernators during arousal from hibernation. Therefore, we investigated the effects of rapid temperature changes on synaptically induced CA1 population spikes in hippocampal slices from golden hamsters (hibernators) and rats (non-hibernators). Temperature was changed ramp-like by 0.3 degrees C/min, which corresponds to the rise of body temperature in golden hamsters during arousal from hibernation. During cooling from 35 to 10-15 degrees C, the population spike amplitude increased, reached maximal values at 25-30 degrees C and 20-25 degrees C in hamster and rat slices, respectively, and then decreased with further cooling. During rewarming, hamster slices displayed the same temperature dependence as during cooling. In contrast, in rat slices dynamic effects of the temperature change occurred. These were most obvious in a strong depression of the spike amplitude during rewarming as compared to cooling. Above 26-29 degrees C, the depression was superimposed by an excitatory effect. The depression was largely attenuated by theophylline (100-200 microM) and thus seems to be based on an increase of the concentration of endogenous adenosine, which in turn may result from an imbalance in energy metabolism during warming. The lack of warming-related depression in hamster slices can be explained by a lower sensitivity for adenosine as compared to rat slices. In addition, a better resistance of metabolic balance against rapid temperature changes may prevent large elevations of endogenous adenosine in the hamster hippocampus. For hibernators, the avoidance of temperature change-induced disturbances of neuronal communication may be a prerequisite for safe arousal from hibernation.

Synaptic transmission and paired-pulse behaviour of CA1 pyramidal cells in hippocampal slices from a hibernator at low temperature: importance of ionic environment

To investigate the effects of ionic changes possibly associated with hibernation, hippocampal slices prepared from golden hamsters were studied in artificial cerebrospinal fluid (ACSF) of variable composition (K+ 3-5 mM, Ca2+ 2-4 mM, Mg2+ 2-4 mM, pH 7.0-7.7) at temperatures of 15-20 degrees C, just above the temperature below which synaptic transmission is blocked. Population action potentials (population spikes, PSs) of CA1 pyramidal cells were evoked by stimulation of the Schaffer collaterals/commissural fibers with paired pulses (interpulse interval 50 ms, interval between pairs 30 s). The responses evoked at given temperatures were investigated as a function of extracellular ion concentrations. In ACSF containing 3 mM K+, 2 mM Ca2+ and 2 mM Mg2+, PSs could be evoked at temperatures of > approximately 16 degrees C whereas at lower temperatures synaptic transmission was blocked. The threshold temperature was slightly higher for the first (PS1) than for the second PS (PS2) evoked by paired-pulse stimulation. The slices displayed paired-pulse facilitation (PPF) at all temperatures. Elevation of [K+]o from 3 to 5 mM depressed the amplitudes of both PS1 and PS2, with a stronger effect on PS2. PPF was reduced and, at near-threshold temperatures, turned into paired-pulse depression (PPD). Elevation of [Ca2+]o from 2 to 4 mM increased the amplitude of PS1. The amplitude of PS2, in contrast, was reduced at near-threshold temperatures. PPF turned into PPD. Elevation of [Mg2+]o from 2 to 4 mM reduced the amplitudes of both PS1 and PS2, with a stronger effect on PS1. Accordingly, PPF was increased. Acidification by 0.3 pH units strongly depressed the amplitudes of PS1 as well as PS2 and increased PPF. Alkalization by 0.4 pH units had only weak effects in the opposite direction. Changes in the ionic composition comparable to those investigated in the present study presumably occur in the brain interstitium of hamsters during entrance into hibernation. According to our results, such changes depress synaptic transmission at low temperatures in the hamster hippocampus in vitro. This modulation may be important for the regulation of neuronal activity during entrance into hibernation.

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.

Thermal dependence of serotonergic modulation of neural activity in the hamster

1. The modulatory effect of serotonin on CA1 pyramidal cells in the hamster (Mesocricetus auratus) hippocampus was examined over a range of temperatures. 2. Following repetitive Schaffer collateral/commissural stimulation, changes in the amplitude of population spikes (the synchronous firing of CA1 pyramidal cells) were recorded in the hamster, a hibernator. Amplitudes were measured after 10 microM serotonin was added to and then withdrawn from the perfusing medium with the temperature of the bath fixed at different temperatures. 3. Between 35 degrees C and 15 degrees C a depression in population spike amplitude of at least 10% was seen in 36 of 43 trials, with an average depression of 68%. No significant temperature dependence of the depressive effect was seen. 4. Following the removal of serotonin from the perfusate, the spike amplitude was enhanced over the same range of temperatures, averaging 33% higher than control values. The enhancement was most pronounced at 35 degrees C and 15 degrees C and smallest at 25 degrees C. 5. Thus, over the entire temperature range of 35 degrees C to 15 degrees C, serotonin exerted a dual modulatory effect on the spike amplitude, a depression followed by an enhancement. Serotonin's modulatory effects on pyramidal cell excitation persist over temperatures encountered as the hamster enters hibernation.

Action potential duration increases as body temperature decreases during hibernation

The effect of temperature on the duration of both population spikes and action potentials of single neurons has been investigated in a variety of in vitro preparations. A few studies have examined the influence of temperature on spike potentials from spinal motoneurons of intact, anesthetized mammals. In all cases, the duration of the action potential or population spike increased as temperature decreased. A similar increase in the duration of action potentials accompanied hibernation in the ground squirrel (Spermophilus lateralis). Oscilloscope traces of 2 brainstem reticular formations, and 8 posterior thalamic single units were photographed at a body temperature (Tb) of 34-36 degrees C during euthermia prior to entrance into hibernation and at Tb's ranging from 10 to 27 degrees C during hibernation. There was a significant increase in the duration of the second component of the diphasic action potential at the lower Tb (P less than 0.01). This temperature effect was reversible, i.e. action potential durations returned to preentrance euthermic values following arousal from hibernation (Tb = 34-36 degrees C). This study is the first to use behaving animals to demonstrate that changes in biophysical characteristics of central nervous system neurons occur at low Tb. These changes in membrane characteristics probably result in alterations in neuronal functioning and information processing during hibernation.

Hypothermia impairs performance in the Morris water maze

Fifteen rats were trained to learn the location of a spatially fixed platform hidden in a Morris water maze (14-16 degrees C). Asymptotic performance was achieved over six training days (10 trials/day). Then retention of the task was assessed immediately after lowering core body temperature (Tc) to 28 degrees or 30 degrees C or stabilizing at 37 degrees C (the normothermic control). The hypothermia treatment order was counterbalanced according to a Latin-square design. Hypothermia significantly impaired all measures of spatial performance. Hypothermic animals were then rewarmed in a 40 degrees C water bath to 37 degrees C Tc and spatial performance tested again. Artificial rewarming resulted in complete recovery of all measures of spatial performance. These results demonstrate that hypothermia impairs performance in the Morris water maze, possibly by an amnesic mechanism, and that returning Tc to normothermic levels initiates recovery of performance.