Changes in hypothalamic temperature of rats after daily exposure to heat at a fixed time

Rats were subjected to an ambient temperature (T_a) of 33°C for ca. 5 h during the last half of the dark phase for 5, 14 or 28 consecutive days (heat-exposed rats, HE), while control rats were kept at a constant T_a of 24°C. After the heat exposure schedule, the levels of hypothalamic temperature (T_hy) as an index of body core temperature in the HE were significantly lower than those of the controls for 2–4 h in the last half of the dark phase. The low levels of T_hy persisted during the specific period for 1, 3 and 6 days after the end of the 5-, 14- and 28-day heat exposure schedules, respectively. These results confirm that, in rats subjected to daily heat exposure for ca. 5 h at a fixed time per day, their T_hy falls during the period when the rats were previously exposed to heat, and suggest that the duration of the specific T_hy change observed after completing the heat exposure schedule depends on the length of the heat exposure schedule.     

Circadian temperature and wake rhythms of rats exposed to prolonged continuous illumination

The purpose of this study was to simultaneously measure temperature and sleep in the rat under continuous illumination in an attempt to reveal properties of the underlying circadian oscillators. At first, the circadian rhythms of temperature and wake free-ran in parallel. Within weeks or months, circadian arrhythmicity developed in most animals. Both circadian rhythms eventually damped out, even at fairly low light intensities. The circadian rhythm of wake was weaker and disintegrated sooner than the circadian rhythm of temperature. Although the data did not rule out control by separate circadian oscillators, one for temperature and one for wake, a single oscillator model was sufficient to explain this phenomenon. Ultradian variations with a period of about 2–5 hr were superimposed upon the circadian rhythms. When the circadian rhythms damped out, the ultradian variations remained. The ultradian bursts of wake preceded the ultradian bursts of temperature, suggesting a causal relationship. On the other hand, the circadian rhythm of temperature could not be dependent on the circadian rhythm of wakefulness, because the temperature rhythm could persist while the wake rhythm was absent.     

Analysis of feeding patterns in normal and vagotomized rabbits.

Spontaneous meal sizes, intermeal intervals, and 24 hr feeding rhythms were monitored in normal and 60 day recovered vagotomized rabbits fed solid laboratory chow. Mean sizes of meals and intermeal intervals, and the circadian distribution of food intake did not differ between the two groups, but vagotomy was associated with increased frequencies of both smaller and larger than average meals. Positive meal to postmeal interval correlations were evident in intact but not vagotomized animals, whereas vagotomized animals displayed a meal to premeal interval correlation in the light phase that was not present in normal rabbits.     

Duration and Placement of Sleep in a "Disentrained" Environment

Sleep/wake cycles of 9 young adults were electrographically recorded during 60 hrs of enforced bedrest. During this period subjects were required to lie quietly, with no time cues and minimal exogenous stimulation. Sleep and wakefulness patterns were clearly modified under these conditions. There was an alternation of waking periods with an average length of 2.7 hrs and sleep episodes with a mean duration of 2.99 hrs. Eighty percent of both sleep and waking periods were less than 4 hrs duration. The circadian pattern of sleep period duration persisted in disentrainment, but temporal organization of sleep episodes was substantially disrupted; sleep episodes occurred throughout the 24-hr day. The results suggest the presence of two distinct components of the human sleep system–one, sleep duration, is controlled by an endogenous circadian oscillatory system; another, sleep placement, is controlled primarily by behavioral controls, in the form of social and occupational pressures, and self-imposed behavioral alternatives to sleep.     

Circadian variation of blood clotting time and circulating vitamin K in the athletic horse

In equine sport medicine, blood clotting and fibrinolysis variations are well investigated, given the practical implications of several pathophysiological conditions affecting the athlete horse such as exercise-induced pulmonary haemorrhage (EIPH) and other bleeding disorders whose etiology and pathogenesis mechanisms are not yet clearly understood. The purpose of the present investigation was to gain evidence of a daily rhythm of several blood clotting indices such as prothrombin time (PT), activated partial thromboplastin time (APTT), thrombin time (TT), plasma fibrinogen concentration and serum vitamin K concentration in the athletic horses. Blood samples from five thoroughbred mares were collected at 4-h intervals for 48 h (starting at 08:00 h on day 1 and finishing at 4:00 on day 2 via an intravenous catheter inserted into the jugular vein. Prothrombin time, activated partial thromboplastin time, thrombin time and plasma fibrinogen concentration were assessed by means of a Seac Clot 2 coagulometer (SEAC, Italy), while serum vitamin K concentration was measured by HPLC. Data analysis was conducted by one-way repeated analysis of variance (ANOVA) and by the single cosinor method. ANOVA showed a significant influence of time on all parameters investigated, in all horses, on either day. Cosinor analysis defined the periodic parameters and their acrophases (expressed in hours) during the 2 days of monitoring. PT showed a nocturnal acrophase, whereas serum vitamin K concentration acrophase occurred during the evening. The results of this study reflect the physiological peculiarities of the horse that is subjected to a number of exogenous (environmental, nutritional, physical) and endogenous stimuli capable of entraining the circadian rhythm specifically and thus producing time-dependent variations not always comparable with those observed in humans or laboratory animals.     

Control of the rat's circadian self-stimulation rhythm by light-dark cycles.

Rats with hypothalamic and septal electrodes were maintained in continuous test environments where bar-press responses produced brief reinforcing electrical stimulations. Long-term trends in response emission were measured under continuous exposure to light, dark and 12 hr light-dark alternations. In addition, transient behavioral adjustment to sudden 180 degrees phase shifts in the light-dark schedule was studied. The ambient light condition was found to control the period and phase of the circadian rhythm of brain self-stimulation behavior, as quantified by Fourier analysis. The circadian period was greatest under constant light (up to 24.90 hr under dim illumination), and approximated 24.00 hr under constant dark. Successful nocturnal entrainment to 12 hr light-dark alternations was obtained, with the peak of the 24 hr Fourier fundamental occurring in the middle-to-late dark segments. Three to 11 days were required for re-entrainment to 180 degrees light-dark phase shifts, during which the behavioral oscillation period increased to values comparable to periods under constant light. The rate of re-entrainment appeared to be proportional to illumination intensity during light segments.     

Disruption of diurnal feeding and weight gain cycles in weanling rats by ventromedial and dorsomedial hypothalamic lesions

At the age of 28 days, male Sprague-Dawley rats received bilateral electrolytic lesions in the ventromedial (VMN) and dorsomedial (DMN) hypothalamic nuclei, repectively. Sham-operated rats served as controls. Food intake and body weight gains were measured for 13 days at the beginning of the light period (0800–2000 hr) and prior to the start of the dark period (2000-0800 hr). Both types of hypothalamic lesions caused a disruption of the naturally occurring diurnal feeding and weight gaining cycles. In accordance with previous data, the VMN rats remained normophagic and made normal weight gains while DMN animals were hypophagic and gained weight subnormally; linear growth was reduced in both types of lesioned animals, but only the VMN rats became obese. The data suggest the existence in the medial hypothalamus of an area that is involved in feeding and weight gaining cycles. From the standpoint of overall-caloric intake, this area consists of two types of neuronal assemblies that differ fuctionally as profoundly as they are anatomically separate.     

The adrenergic modulation of firings of respiratory rhythm-generating neurons in medulla-spinal cord preparation from newborn rat

We analysed the modulation of respiratory neurons by adrenaline or noradrenaline (NA) in a newborn rat brainstem-spinal cord preparation. Adrenaline or NA caused a dose-dependent depression of the respiratory rhythm and induced C4 spinal tonic discharges. The inhibitory effect of adrenaline (ED₅₀=0.5 μM) on the respiratory rhythm was stronger than NA (ED₅₀=5 μM). The adrenaline respiratory rhythm depression was partially blocked by the α₁-antagonist prazosin or by the α₂-antagonist yohimbine. The C4 tonic discharge elicited by adrenaline was blocked by the α₁-antagonist prazosin. The direct effects of adrenaline on pre-inspiratory (Pre-I) neurons were examined in a synaptic blockade solution (low Ca), and fifty-six percent of Pre-I neurons were found to continue firing. In low-Ca solution, Pre-I neurons were excited (n=29 of 39) or depressed (n=5 of 39) by adrenaline, and excited by α₁-agonist phenylephrine or depressed by α₂-agonist clonidine. These results suggest that the respiratory rhythm depression under intact network conditions is mediated by some other inhibitory system. The inhibitory effect of adrenaline on the respiratory rhythm was partially blocked by the GABA_A-antagonists bicuculline or picrotoxin, but not by the GABA_B-antagonist phaclofen. The present results suggest that: (1) respiratory rhythm generation is more sensitive to adrenaline than NA through α-adrenergic action of adrenaline; (2) the activity of Pre-I neurons could be directly regulated by excitation via α₁-receptors and inhibition via α₂-receptors; and (3) the depression of the respiratory rhythm by adrenaline is partly mediated by GABA_Aergic neurons. Received: 8 April 1997 / Accepted: 6 October 1997     

Genetic variation in locomotor activity rhythm among populations ofLeptopilina heterotoma (Hymenoptera: Eucoilidae), a larval parasitoid ofDrosophila species

The locomotor activity rhythm ofLeptopilina heterotoma, a parasitoid insect ofDrosophila larvae, was investigated under laboratory conditions. Under LD 1212, the locomotor activity of females shows a clear rhythm which persists under continuous darkness (circadian rhythm). However, comparative study of five populations indicates that both the rate of activity and the profile of the rhythm vary according to the origin of females. The Mediterranean populations (Tunisia and Antibes) show two peaks of activity, at the beginning and at the end of the photophase, whereas more northern populations (Lyon and the Netherlands) are mostly active during the afternoon. Females originating from the area of Lyon have a very low level of activity. Reciprocal crosses (F₁ hybrids and backcrosses) between the French and the Tunisian strains demonstrated the genetic basis of these variations and the biparental inheritance of the trait. This genetic variability is interpreted as a consequence of selective pressures and suggests a local adaptation of natural populations in host foraging behavior. The selective factors which could act on the daily organization of parasitoid behaviors are discussed.     

Phase shifts of circadian rhythms in activity entrained to food access

Rats anticipate daily 2 hr meals with a sharp increase in activity several hours prior to food availability. The present experiment examined the response to phase shifts of food access in rats with lesions of the suprachiasmatic nuclei (SCN). Following entrainment of activity to 2 hr of food per day, food access was phase delayed or phase advanced by 4, 6, or 8 hr. All rats responded to phase delays of 4 or 6 hr with an increase in the duration of anticipatory activity so that transients appeared mostly in activity onset. Following 8 hr phase delays, clear delaying transients in both activity onset and end were observed. Only a few rats showed advancing transients in activity after phase advances of food access. In response to 6 hr and 8 hr phase advances, 3 different responses occurred: (a) activity re-entrained to food access by the 2nd or 3rd day without clear intervening transients, (b) activity phase shifted by means of distinct delaying transients and (c) delaying transients occurred in one component of activity while a second component of activity appeared at the new phase position by the second or third day. These results provide further evidence that anticipation of food access is mediated by a circadian mechanism which is functionally independent of the SCN and illustrate some similarities as well as considerable differences between circadian rhythms entrained by feeding and those entrained by light-dark cycles.