Different patterns of circadian oscillation in the suprachiasmatic nucleus of hamster, mouse, and rat.

Although spontaneous neural firing in the mammalian suprachiasmatic nucleus is accepted to peak once during mid-subjective day, dual activity peaks have been reported in horizontal brain slices taken from hamsters. These two peaks were interpreted as new evidence for the theory of dual circadian oscillators and raised the expectation that such activity would be found in other circadian model systems. We examined hamster, mouse, and rat slices in both coronal and horizontal planes and found a second peak of activity only in hamster horizontal preparations. This raises interesting questions about the relative circadian physiology of these important experimental animals.

Temperature-sensitive properties of rat suprachiasmatic nucleus neurons.

The hypothalamic suprachiasmatic nucleus (SCN) contains a heterogeneous population of neurons, some of which are temperature sensitive in their firing rate activity. Neuronal thermosensitivity may provide cues that synchronize the circadian clock. In addition, through synaptic inhibition on nearby cells, thermosensitive neurons may provide temperature compensation to other SCN neurons, enabling postsynaptic neurons to maintain a constant firing rate despite changes in temperature. To identify mechanisms of neuronal thermosensitivity, whole cell patch recordings monitored resting and transient potentials of SCN neurons in rat hypothalamic tissue slices during changes in temperature. Firing rate temperature sensitivity is not due to thermally dependent changes in the resting membrane potential, action potential threshold, or amplitude of the fast afterhyperpolarizing potential (AHP). The primary mechanism of neuronal thermosensitivity resides in the depolarizing prepotential, which is the slow depolarization that occurs prior to the membrane potential reaching threshold. In thermosensitive neurons, warming increases the prepotential's rate of depolarization, such that threshold is reached sooner. This shortens the interspike interval and increases the firing rate. In some SCN neurons, the slow component of the AHP provides an additional mechanism for thermosensitivity. In these neurons, warming causes the slow AHP to begin at a more depolarized level, and this, in turn, shortens the interspike interval to increase firing rate.

Oscillation and light induction of timeless mRNA in the mammalian circadian clock.

Circadian rhythms in Drosophila melanogaster depend on a molecular feedback loop generated by oscillating products of the period (per) and timeless (tim) genes. In mammals, three per homologs are cyclically expressed in the suprachiasmatic nucleus (SCN), site of the circadian clock, and two of these, mPer1 and mPer2, are induced in response to light. Although this light response distinguishes the mammalian clock from its Drosophila counterpart, overall regulation, including homologous transcriptional activators, appears to be similar. Thus, the basic mechanisms used to generate circadian timing have been conserved. However, contrary to expectations, the recently isolated mammalian tim homolog was reported not to cycle. In this study, we examined mRNA levels of the same tim homolog using a different probe. We observed a significant (approximately threefold) diurnal variation in mTim expression within mouse SCN using two independent methods. Peak levels were evident at the day-to-night transition in light-entrained animals, and the oscillation persisted on the second day in constant conditions. Furthermore, light pulses known to induce phase delays caused significant elevation in mTim mRNA. In contrast, phase-advancing light pulses did not affect mTim levels. The mTim expression profile and the response to nocturnal light are similar to mPer2 and are delayed compared with mPer1. We conclude that temporal ordering of mTim and mPer2 parallels that of their fly homologs. We predict that mTIM may be the preferred functional partner for mPER2 and that expression of mTim and mPer2 may, in fact, be driven by mPER1.

A persistent circhoral ultradian rhythm is identified in human core temperature.

There have been inconclusive reports of intermittent rhythmic fluctuations in human core temperature, with the fluctuations having a period of about an hour. However, there has been no definitive demonstration of the phenomenon. This is likely due to the intermittency and seeming instability of the events. They have been assumed to be secondary rather than autonomous phenomena, putatively arising from the oscillation between rapid eye movement (REM) and non-REM (NREM) sleep. In this study, we report identification of a clear, persistent circhoral ultradian rhythm in core temperature with a period for this study sample of 64 +/- 8 minutes. It appeared simultaneously with an intact circadian core temperature rhythm, persisted despite complex perturbations in core temperature brought about by the sequelae of 40 h of sleep deprivation, and could not be attributed to sleep stage alternation or other endogenous or exogenous factors. Analysis of power spectra using the maximum entropy spectral analysis (MESA) method, which can uncover hidden rhythmicities, demonstrated that the apparent intermittency of the rhythm is due to periodic interference of this rhythm by other rhythmic events. The persistence of this oscillation suggests that, in this system as in the endocrine system, circhoral regulation is an integral component of thermoregulatory control. Identifying the source and functional role of this novel rhythm warrants further work.

Synaptic inhibition: its role in suprachiasmatic nucleus neuronal thermosensitivity and temperature compensation in the rat.

1. Whole-cell patch clamp recordings of neurones in the suprachiasmatic nucleus (SCN) from rat brain slices were analysed for changes in spontaneous synaptic activity during changes in temperature. While recent studies have identified temperature-sensitive responses in some SCN neurones, it is not known whether or how thermal information can be communicated through SCN neural networks, particularly since biological clocks such as the SCN are assumed to be temperature compensated. 2. Synaptic activity was predominantly inhibitory and mediated through GABAA receptor activation. Spontaneous inhibitory postsynaptic potentials (IPSPs) and currents (IPSCs) were usually blocked with perifusion of 10-50 microM bicuculline methiodide (BMI). BMI was used to test hypotheses that inhibitory synapses are capable of either enhancing or suppressing the thermosensitivity of SCN neurones. 3. Temperature had opposite effects on the amplitude of IPSPs and IPSCs. Warming decreased IPSP amplitude but increased IPSC amplitude. This suggests that thermally induced changes in IPSP amplitude are primarily influenced by resistance changes in the postsynaptic membrane. The thermal effect on IPSP amplitude contributed to an enhancement of thermosensitivity in some neurones. 4. In many SCN neurones, temperature affected the frequency of IPSPs and IPSCs. An increase in IPSP frequency with warming and a decrease in frequency during cooling made several SCN neurones temperature insensitive, allowing these neurones to maintain a relatively constant firing rate during changes in temperature. This temperature-adjusted change in synaptic frequency provides a mechanism of temperature compensation in the rat SCN.

Neuronal thermosensitivity and survival of rat hypothalamic slices in recording chambers.

Several studies have examined the activity of neurons in hypothalamic tissue slices. The present experiments studied relationships between neuronal activity (firing rate and thermosensitivity) and tissue survival as a function of time and slice thickness. Rat hypothalamic tissue slices were sectioned at different thicknesses (350, 450, and 600 microm) and maintained in an oxygenated interface chamber which was perfused with artificial cerebrospinal fluid (ACSF). Electron and light microscopy were used to examine tissue morphology at different depths from the slice surfaces, and extracellular recordings were used to measure each cell's spontaneous activity and response to changes in temperature. Tissue damage was most evident at tissue layers nearest the gas-exposed surface. At 9 h in the chamber, 350 microm thick slices showed subtle changes in morphology with little difference between the gas-exposed and ACSF-exposed surfaces. In the 450 and 600 microm thick slices, tissue degeneration became more evident with increased damage at the gas-exposed surface. This damage extended fully into the tissue of the 600 microm section. There were no differences in firing rate or thermosensitivity between 350 and 450 microm slices; but in 600 microm slices, there were fewer spontaneously active neurons, although these neurons had a higher mean thermosensitivity. Based on the incidence of spontaneous activity and morphological integrity, the results suggest that electrophysiological experiments using 350 microm slices are preferable to experiments using thicker slices.

Metabolic and thermal adaptations from endurance training in hot or cold water.

Metabolic and thermal adaptations resulting from endurance training in hot vs. cold water were compared. It was hypothesized that training in hot water would have greater effects on muscle glycogen use and blood lactate accumulation during exercise than training in cold water. Eighteen men exercised at 60% of maximal oxygen uptake while immersed in hot (n = 9) or cold water (n = 9) for 1 h, 5 days/wk, for 8 wk. Training in hot water (35 degrees C) potentiated body temperature increases during exercise, and training in cold water (20 degrees C) blunted body temperature increases during exercise. Before and after training, cardiorespiratory and thermoregulatory responses and muscle glycogen and blood lactate changes were assessed during a 1-h exercise trial in hot water and, on a separate day using the same intensity, in cold water. Oxygen uptake was similar for all trials, averaging 2.0 +/- 0.1 l/min. It was observed that 1) training reduced glycogen use and lactate accumulation during exercise, with no difference between cold and hot water training groups in the magnitude of this effect; 2) lactate accumulation during exercise was the same in hot water as in cold water; and 3) skin temperature decreased more rapidly during cold-water exercise after than before training, with no difference between cold and hot water training groups in the magnitude of this effect. Thus, exercise-induced body temperature increases are not an important stimulus for glycogen-sparing effects and blunted lactate accumulation associated with endurance training.