The posterior basal diencephalon of rats enhances expression of an activated state.

STUDY OBJECTIVES: Various ablation studies have implicated the posterior basal diencephalon in the promotion of wakefulness. Although many studies have examined the role of this structure in promotion of cortical arousal, few investigations have attempted to examine its importance in regulation of motor activation (behavioral arousal). In the current study, recordings of freely moving decerebrate rats with and without a posterior basal diencephalon were performed. These studies allowed determination of the behavioral states expressed by the preparations and whether removal of the posterior basal diencephalon completely eliminated expression of both activated state with and activated state without limb movements. DESIGN: Muscle activity was recorded from limb and neck muscles. Eye movements and heart rate were also monitored. The percentage of time spent in various behavioral states and the proportion of limb movements expressed in each of these states were determined. MEASUREMENTS AND RESULTS: Rats with an intact posterior basal diencephalon cycled between all behavioral states. However, they spent most of the recording time in an activated state. In contrast, removal of the posterior basal diencephalon produced rats that spent most of the recording period in a quiescent state. Limb movements were expressed mainly by animals with an intact posterior basal diencephalon, and only when these animals were in the activated state. CONCLUSIONS: The results of this study suggest that the posterior basal diencephalon is required for expression of an activated state and specifically provide evidence for a descending projection from this region required for expression of this state and associated motor activation.

Sleep and mammalian hibernation: homologous adaptations and homologous processes?

Evidence from electroencephalographic, thermoregulatory and cellular neurophysiological studies suggests that sleep and hibernation may be homologous adaptations for energy conservation. However, despite the similarities between non-rapid eye movement (NREM) sleep and hibernation, the restorative function normally associated with slow wave sleep appears not to occur during hibernation, perhaps because of the low body temperature (Tb). Cellular neurophysiological studies also suggest that a bout of hibernation is not exclusively NREM sleep but is punctuated by periods of wakefulness. The entrance to hibernation involves both an inhibition of cortical activity and activation of hypothalamic regions, whereas the arousal from hibernation is primarily a hypothalamic function. Multiple neurochemical systems are affected by the arousal state change that occurs in hibernation, and a serotonergic-opiatergic interaction, in particular, may be important in regulating these events. Among regulated physiological systems affected by arousal state changes, the episodic respiration evident in hibernation shows striking similarities to the apneas observed during sleep in both humans and other mammals. Although the slight down-regulation of Tb and metabolism that accompanies the transition from wakefulness to NREM sleep may have served as a preadaptation for the evolution of hibernation among the mammals, increasing consideration must be given to the possibility that hibernation represents an arousal state distinct from any known normothermic arousal state.

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.

Neuronal activity during sleep and complete bouts of hibernation.

Changes in arousal state in a euthermic mammal exert powerful influences on major neural regulatory systems. Changes in behavioral state occur at body temperature (Tb) greater than 25 degrees C during hibernation. However, no information exists regarding alterations in arousal states during deep torpor. In this study we used a combination of electroencephalographic, electromyographic, and posterior thalamic neuronal activity in ground squirrels (Spermophilus lateralis) to evaluate arousal states during deep hibernation. No state homologous to rapid-eye-movement sleep was observed below Tb = 21 degrees C during hibernation. However, the animals did continue to cycle through states homologous to electrophysiologically defined wakefulness (AW) and non-rapid-eye-movement (NREM) sleep at all temperatures examined (Tb = 14-36 degrees C). These results extend previous observations that hibernation is not a homogeneous state. Instead, deep torpor consists primarily of a state similar to NREM sleep, interrupted periodically by short intervals of a form of AW. These periodic alterations in state should be accompanied by changes in the properties of many regulatory systems and must be accounted for in any theory of the neural control of hibernation.

Phosphagen and intracellular pH changes during contraction of creatine-depleted rat muscle.

To evaluate the functional role of phosphocreatine (PCr) and creatine in muscle metabolism, these compounds were depleted by feeding rats the creatine analogue, beta-guanidinopropionate (beta-GPA, 2% of diet). Changes in phosphate metabolites and intracellular pH were monitored in gastrocnemius muscle in situ by phosphorus nuclear magnetic resonance (31P-NMR) at 162 MHz using the surface coil technique. After 3 mo of feeding, 25 mumol/g of phosphorylated beta-GPA (beta-GPAP) had accumulated, and PCr, creatine, and ATP levels were reduced to 6, 17, and 56%, respectively, compared with muscles of control animals. In resting muscle, there was no measurable exchange of phosphate between beta-GPAP and ATP by the NMR saturation transfer method. During muscle stimulation at 1 and 5 Hz, the maximum net rate of beta-GPAP hydrolysis was 10% that of PCr in control muscles, so that after 150 s inorganic phosphate had increased to less than 50% of the level attained in control muscles. At both rates, peak twitch force declined toward a steady state more rapidly in beta-GPA-loaded muscles, but after 100 s force was either not different (1 Hz) or significantly greater (5 Hz) in the beta-GPA-fed animals. Intracellular pH initially decreased more rapidly during stimulation and recovered more rapidly afterward in the beta-GPA-loaded muscles compared with controls. This difference could be explained by the difference in expected proton consumption due to net PCr hydrolysis. However, despite buffering by PCr hydrolysis, pH ultimately decreased more in control muscle (6.1 vs. 6.3 for 5 Hz), indicating greater acid accumulation compared with beta-GPA-loaded muscles. In the superficial, predominantly fast-twitch glycolytic section of muscles clamp-frozen after 5-Hz stimulation for 150 s, lactate accumulation was twofold greater in controls. The results indicate that PCr is not essential for steady-state energy production but that the phosphate from PCr hydrolysis may be important for maximum activation of glycogenolysis and/or glycolysis.