Water temperature determines neurochemical and behavioural responses to forced swim stress: an in vivo microdialysis and biotelemetry study in rats

Forced swimming is a behavioural stress model increasingly used to investigate the neurocircuitry of stress responses. Although forced swim stress clearly is a psychological stressor (anxiety, panic), its physical aspects are often neglected. There are indications that behavioural and neurochemical responses to swim stress depend on the water temperature. Thus, we investigated the responsiveness of hippocampal serotonergic neurotransmission (important in the coordination of stress responses), and of behaviour and core body temperature to forced swimming at different water temperatures (19, 25 and 35 degrees C). In vivo microdialysis and biotelemetry in freely-behaving rats were used. Dialysates were analysed for serotonin (5-HT) and its metabolite 5-HIAA (5-hydroxyindoleacetic acid) by HPLC with electrochemical detection. Forced swimming in water at 25 and 19 degrees C decreased core body temperature by 8 and 12 degrees C, respectively. A rapid and pronounced increase in hippocampal 5-HT and 5-HIAA was found in rats that swam at 35 degrees C, whereas biphasic responses in 5-HT and 5-HIAA were observed at 25 and 19 degrees C. Also swim stress behaviour and post-stress home cage behaviour depended on the water temperature. Comparing the serotonergic and core body temperature changes revealed that a combination of two different 5-HT and 5-HIAA responses seems to shape the neurotransmitter response. Swimming-induced increases in hippocampal extracellular concentrations of 5-HT and 5-HIAA occurred at all water temperatures, but these increases were temporarily quenched, or concentrations were transistently decreased, when core body temperature fell below 31 degrees C in water at 25 or 19 degrees C. These data demonstrate that water temperature is a key factor determining the impact of forced swim stress on behaviour and neurochemistry, and underscore that changes in these parameters should be interpreted in the light of the autonomic responses induced by this stressor.     

Thermal homeostasis in pregnant rats during heat stress.

Rats exposed to inescapable heat stress maintained a controlled hyperthermia while increasing heat loss by cutaneous vasodilatation and by grooming behavior. In nonpregnant rats, the evaporation of saliva groomed onto the body surfaces increased exponentially as a function of ambient temperature above 36 degrees C. This was associated with a decrease in the body temperature threshold for salivary secretion from the submaxillary gland, which then began at approximately the same body temperature as cutaneous vasodilatation. In addition, the pregnant rats maintained a lower level of controlled hyperthermia during heat stress than did nonpregnant rats. This appeared to result from a decreased production of metabolic heat, reduced insulation on the ventral surface, and an increased motivation to keep cool during heat stress. These changes met the increased need for thermolysis during pregnancy and provided for thermal homeostasis both in the pregnant rat and in the unborn fetuses.     

Cold-induced impairment of delayed matching in rats

Exposure to moderate, nonhypothermic cold temperature has been reported to affect a variety of behavioral and neural functions. To elucidate the effects of mild cold stress on short-term (working) memory, Long-Evans rats were exposed to an ambient temperature of either 2 degrees or 23 degrees C while performing a delayed matching task. At the beginning of each trial, rats were required to respond on one of two levers cued by a light. Following a delay of 2, 8, or 16 s, a response on the lever previously cued produced food reinforcement. Relative to performance at 23 degrees C, exposure to 2 degrees C occasioned no change in matching accuracy at the 2-s delay, a modest decrement at the 8-s delay, and a larger decrement at the 16-s delay. The cold exposure did not decrease colonic temperature. In addition to accuracy decrements, matching response times were consistently shorter during cold exposures. Cold-induced impairments were absent during removal of the memory component from the task, indicating the observed cold effects on memory were not due to impaired attentional, sensory, or motor processes. These data suggest that mild cold stress may impair active maintenance of information in working memory but not processes related to reference memory.     

Long lasting increase in nociceptive threshold induced in mice by forced swimming: involvement of an endorphinergic mechanism

Mice submitted to forced swimming session(s) displayed a long lasting modification in their nociceptive threshold, assessed through their jump latency from a hot plate (55 degrees C). Thus two forced swimming sessions (6 min each, 8h apart), in water at 33 degrees C, increased by about 50% the jump latency when the hot plate test was performed 14 hours, 3 days or 6 days thereafter. The water temperature (16 degrees C vs 33 degrees C) had no critical influence in this respect. To be clearly effective (at 33 degrees C) the swimming session had to be performed twice (when performed only once it was irregularly effective); it apparently culminated for a 6 min duration, since its effectiveness was not significantly increased by extending the swimming time to 12 min or 18 min. Performing 2 forced swimming sessions (6 min each, 8h apart), 5 consecutive days, resulted in a suppression of the increase in jump latency in the hot plate test. The two forced swimming episodes-induced analgesia was prevented by the s.c. administration of diazepam (from 0.125 mg/kg) or morphine (from 5 mg/kg) or scopolamine (1 mg/kg) before each forced swimming episode. Morphine (7.5 mg/kg) was uneffective to prevent the induction of two forced swimming episodes-induced analgesia when it was administered immediately after each forced swimming session. Finally this analgesia was dose dependently reversed by naloxone (ID(50) = 0.14 mg/kg, s.c., 30 min before the hot plate test). It is hypothesized that the handling of mice immediately before the hot plate test induces the remembrance of the stress induced by previous forced swimming episodes, triggering a fear reaction which increases the nociceptive threshold.     

The effect of high ambient temperature on timing behavior in rats

The present experiments demonstrated a reliable within-session increase in rectal temperature (T(re)) at 25 degrees C during stable differential-reinforcement-of-low-rate (DRL) performance. The thermal response was found to be independent of the DRL value and reinforced DRL performance, but dependent on the state of the animal's deprivation. Exposure to a 35 degrees -C environment increased the post-session T(re) significantly above the 25 degrees -C post-session T(re) for seven of eight subjects. Response and reinforcement rate at 35 degrees C was found to be independent of DRL value, although it decreased as DRL value increased at 25 degrees C. A discriminative stimulus used to mark the end of the interval increased the reinforcement rate at 25 degrees C but provided no advantage at 35 degrees C. Measurement of the pattern of responding during DRL performance revealed increases in the proportion of long interresponse times at 35 degrees C. Reinforcement rate was found to decrease progressively at 35 degrees C, reaching a minimum within 40 to 50 min of the 90-min session. Visual observation of overt behaviors during DRL performance at 35 degrees C revealed a reduction in the frequency of overt behavior, characteristic of 25 degrees -C performance, and a time-dependent increase in the probability of an alternative set of behaviors.     

Hyperthermia impairs retrieval of an overtrained spatial task in the Morris water maze

Fifteen rats were trained to learn the location of a spatially fixed platform hidden in a Morris water maze (40 +/- 2 degrees C). Then retention of the spatial task was assessed immediately after raising core body temperature (Tc) to 42 or 40 degrees C or stabilizing at 37 degrees C (the normothermic control). The hyperthermic treatment order was counterbalanced according to a Latin-square design. Hyperthermia at 42 degrees C Tc significantly impaired spatial performance. Hyperthermic animals were cooled to normothermia (Tc = 37 degrees C) and spatial performance was tested again approximately 30 min later. Cooling resulted in a complete recovery of spatial performance. These results demonstrate that hyperthermia-induced amnesia can be obtained on an overtrained spatial-mapping strategy and cooling to normothermia initiates recovery of spatial performance.     

The role of central and peripheral vision in postural control during walking

Three hypotheses have been proposed for the roles of central and peripheral vision in the perception and control of self-motion: (1) peripheral dominance, (2) retinal invariance, and (3) differential sensitivity to radial flow. We investigated postural responses to optic flow patterns presented at different retinal eccentricities during walking in two experiments. Oscillating displays of radial flow (0 degree driver direction), lamellar flow (90 degrees), and intermediate flow (30 degrees, 45 degrees) patterns were presented at retinal eccentricities of 0 degree, 30 degrees, 45 degrees, 60 degrees, or 90 degrees to participants walking on a treadmill, while compensatory body sway was measured. In general, postural responses were directionally specific, of comparable amplitude, and strongly coupled to the display for all flow patterns at all retinal eccentricities. One intermediate flow pattern (45 degrees) yielded a bias in sway direction that was consistent with triangulation errors in locating the focus of expansion from visible flow vectors. The results demonstrate functionally specific postural responses of both central and peripheral vision, contrary to the peripheral dominance and differential sensitivity hypotheses, but consistent with retinal invariance. This finding emphasizes the importance of optic flow structure for postural control regardless of the retinal locus of stimulation.     

Thermopreferendum of the rat: inter- and intra-subject variabilities

The thermopreferendum of male hooded rats was studied in a temporal thermal gradient. The preferred ambient temperature was found to be approximately 19 degrees C. In addition to the thermopreferendum, three other physiological functions (resting body temperature, daily food intake, and daily ambulatory activity) were each measured in eight rats for 8 days. Overall variability of the thermopreferendum data was higher than that of body temperature and food intake data but lower than the variability of ambulatory activity data. Coefficients of inter- and intrasubject variabilities were computed for each of the four variables. Inter-subject variability was equivalent in all variables, but intrasubject variability was smaller for thermopreferendum and activity than for food intake and core temperature. The thermopreferendum of 19 degrees C is consistent with previously published results but inconsistent with the available data on the thermoneutral zone of the rat (27-28 degrees C). The cause of this inconsistency remains to be determined.     

Gustatory,salivary, and oral thermal responses to solutions of sodium chloride at four temperatures

Using eight highly trained Ss, sensitivity to near threshold levels of NaCl was significantly greater at solution temperatures of 22° and 37°C than at 0° or 55°C. Perceived intensity increased linearly with concentration (0.04%–0.64% NaCl) at all four solution temperatures, with the two lower considered slightly more intense than the two higher temperatures. Biomodal distributions were obtained for hedonic judgments at all temperatures, with three Ss showing greater liking and five Ss showing greater disliking of increasing concentrations. Parotid salivary flow was inversely related to the taste sensitivity, i.e., significantly lower flow rates were obtained for the intermediate than for the hot or cold solutions, independent of salt content. When solution temperature was O°C, the minimum temperature of the oral cavity was 9°–20°C; when solution temperature was 55°C, the maximum temperature of the oral cavity was46°–49°C.     

Facial skin temperature decreases in infants with joyful expression

Studies of infant emotion rely on the assessment of expressive behavior and physiological response because infants cannot tell their feelings. Little is known about the physiological response of infants when they are in a joyful emotion. In this study, we examined changes in facial skin temperature as a physiological response, when infants are laughing, an expressive behavior of joyful emotion. Using thermography, skin temperatures of the nose, forehead and cheek were evaluated at 2-3 months, 4-6 months and 8-10 months. A decrease in facial skin temperature occurred when they laughed. The decrease was most dramatic in the nose dropping as much as 2.0 degrees C in 2 min. The response was evident in infants older than 4 months. These results suggest that a joyful emotion could be associated with a drop in facial skin temperature, which had been considered only as a sign of an unpleasant emotion. This response is developmentally controlled.     

The mechanical properties and dislocation structure of a LaNi5 alloy

A LaNi5 alloy was deformed in compression at high temperatures. Below 850 degrees C, it is brittle but, above 900 degrees C, it showed plasticity. The thermodynamic parameters for plastic deformation have been determined. The dislocation structure was observed by transmission electron microscopy and the Burgers vectors determined without ambiguity. It is concluded that plastic deformation of this alloy at high temperatures is controlled by a Peierls mechanism.     

Selective blockade of 5-HT2A receptors attenuates the increased temperature response in brown adipose tissue to restraint stress in rats

Previous studies have demonstrated that 5-HT2A receptors may be involved in the central control of thermoregulation and of the cardiovascular system. Our aim was to test whether these receptors mediate thermogenic and tachycardiac responses induced by acute psychological stress. Three groups of adult male Hooded Wistar rats were instrumented with: (i) a thermistor in the interscapular area (for recording brown adipose tissue temperature) and an ultrasound Doppler probe (to record tail blood flow); (ii) temperature dataloggers to record core body temperature; (iii) ECG electrodes. On the day of the experiment, rats were subjected to a 30-min restraint stress preceded by s.c. injection of either vehicle or SR-46349B (a serotonin 2A receptor antagonist) at doses of 0.01, 0.1 and 1.0 mg/kg. The restraint stress caused a rise in brown adipose tissue temperature (from, mean +/- s.e.m., 36.6 +/- 0.2 to 38.0 +/- 0.2 degrees C), transient cutaneous vasoconstriction (tail blood flow decreased from 12 +/- 2 to 5 +/- 1 cm/s), increase in heart rate (from 303 +/- 15 to 453 +/- 15 bpm at the peak, then reduced to 393 +/- 12 bpm at the steady state), and defaecation (6 +/- 1 pellets per restraint session). The core body temperature was not affected by the restraint. Blockade of 5-HT2A receptors attenuated the increase in brown adipose tissue temperature and transient cutaneous vasoconstriction, but not tachycardia and defaecation elicited by restraint stress. These results indicate that psychological stress causes activation of 5-HT2A receptors in neural pathways that control thermogenesis in the brown adipose tissue and facilitate cutaneous vasoconstriction.     

Is fat perception a thermal effect?

It has been generally assumed that fat is detected by its flavour and by its lubrication of the oral mucosa. A recent study reported a correlation of -.99 between perceived temperature of a product and its fat content. This was significantly higher than correlations of sensory scores for fat flavour, mouthfeel, and afterfeel. This suggested a third detection mechanism; fat may be detected via its effect on the thermal conductivity of the food. In 3 studies, thermal sensitivity in humans was investigated to verify whether oral thermal receptors are sufficiently rapid and accurate to play a role in the perception of fats. The thermal sensitivity of the lips and oral mucosa of the anterior and middle one-third of the tongue were assessed using a Peltier device. Subjects detected 0.5 Hz fluctuations in temperature of 0.08'C on the lower lip, 0.26 degrees C and 1.36 degrees C at the tip and dorsum of the tongue, demonstrating that the lips are sufficiently sensitive to detect small differences in temperature. In two further experiments subjects ingested custards and mayonnaises and then spat out samples after 5, 10, or 20 sec. The temperature of the food and oral mucosa was measured before and after spitting and the rates of heating were calculated. Results suggest assessment of thermal conductivity of food may be used to assess fat content.     

Anisotropic tracking: evidence for automatic synergy formation in a bimanual task

Investigation of interlimb synergy has become synonymous with the study of coordination dynamics and is largely confined to periodic movement. Based on a computational approach this paper demonstrates a method of investigating the formation of a novel synergy in the context of stochastic, spatially asymmetric movements. Nine right-handed participants performed a two degrees of freedom (2D) "etch-a-sketch" tracking task where the right hand controlled the horizontal position of the response cursor on the display while the left hand controlled the vertical position. In a pre-practice 2D tracking task, measures of phase lag between the irregularly moving target and the response showed that participants controlled left and right hands independently, performance of the right hand being slightly superior to the left. Participants then undertook 4 h 16 min distributed practice of a one degree of freedom etch-a-sketch task where the target was constrained to move irregularly in only the 45 degrees direction on the display. To track such a target accurately participants had to make in-phase coupled stochastic movements of the hands. In a post-practice 2D task, measures of phase lag showed anisotropic improvement in performance, the amount of improvement depending on the direction of motion on the display. Improvement was greatest in the practised 45 degrees and least in the orthogonal 135 degrees direction. Best and worst performances were no longer in the directions associated with right and left hands independently, but in directions requiring coupled movements of the two hands. These data support the proposal that the nervous system can establish a model of novel coupling between the hands and thereby form a task-dependent bimanual synergy for controlling the stochastic coupled movements as an entity.     

Ontogeny of thermoregulatory mechanisms in the rat.

The ability of rats to select a warm environment was studied as a function of postnatal age (birth to 13 days). Animals younger than 5 days demonstrated no choice response (movement to a warm compartment, 36-37 degrees C); however, they did demonstrate movement within the start compartment (23 degrees C). Increasing the motor capabilities of the pups, by injection of L-3,4 dihydroxyphenylalanine (L-dopa, 50 mg/kg), elicited a choice response in 4-5 day-old animals. Younger animals demonstrated no choice of a warm environment even though they moved considerably. Also there was no difference between L-dopa-treated and control animals in the magnitude of temperature change in pups isolated from their mother for 1 hr. The evidence suggests development of behavioral thermoregulatory mechanisms prior to abilities for internal regulation.     

Temperature-induced changes in neuromuscular function: central and peripheral mechanisms

Three series of experimental tests were conducted on subjects under both elevated and depressed thermal conditions. Tripartite series consisted of whole-body immersion excepting the head, whole-body immersion excepting the head and response limb, and immersion of the discrete-response limb. Measures of physiological and behavioural responses were made at sequential .4 degrees C changes during whole-body immersions and approximately 5 degrees C changes of water temperature during the immersion of a limb only. Results suggested that velocity of nerve conduction decreased with thermal depression. Premotor, motor, simple, and choice reaction times varied differentially as a function of the hot and cold conditions. Implications of these differential effects on neuromuscular function are examined with respect to person-machine performance in artificially induced or naturally occurring extremes of ambient temperature.     

Magnitude estimation of warmth: intra- and intersubject variability

Fifty-two subjects, both male and female, 18 to 62 years old, were asked to hold a metal bar maintained at different temperatures (33 degrees to 42 degrees C), and to estimate the magnitude of the resulting thermal sensation. On the average, the estimates were linearly related to the stimulus temperature, although most individual subjects either under- or overestimated middle-range values. Intersubject variability, which was twice as large as intrasubject variability, could not be accounted for by gender, age, or body-temperature differences.     

Use of temperature variations to combat drivers' drowsiness

The purpose of the present study was to evaluate the efficiency of temperature variations as a measure against drivers' drowsiness. A system for regulation of the air temperature in the cab was installed in two trucks. Irregular temperature reductions of about 8 degrees C were introduced. The periods between the temperature reductions varied among 5, 6, 7, and 8 min. The periods for the temperature reductions varied between 2, 4, 6, and 8 min. The system was tested with 6 drivers on 54 trips. The system was applied for periods varying between 30 to 300 min. Changes in wakefulness were analysed using subjective ratings by the lorry drivers. The application of the system significantly improved subjectively rated wakefulness. The wakefulness was rated with a questionnaire completed after 10 min. use of a cooling system and at the end of its use. Analysis showed individual differences in the usefulness of the system: 3 of the 6 truck drivers reported positive reactions to the use of the temperature cooling system.     

Apparent higher brain weight in suicide victims: possible reasons

In 2000 we tested previously reported findings by Salib and Tadros that brain weight of fatal self-harm victims is higher than of those who died of natural causes. Our results were based on data from 15 suicides and 15 deaths of other causes. Data included matching variables of age, sex, time between death and postmortem examination, and temperature of the surrounding environment. The exploratory variables were brain weight and method of death. No significant difference was found between the brain weights of suicides and others. On the other hand, some differences were obtained for different suicide methods, which also differed in the temperature of the environment, this being lower for the group of suicides that occurred outdoors (around or below 0 degrees C). Once we excluded all the outdoor cases and controls, a significantly higher brain weight was obtained for suicide cases. These and previous results are intriguing and require explanation. Respirator brain syndrome as described by Moseley, Molinari, and Walker in 1976 may provide only a partial explanation. Another possible suggestion is that higher brain weight in suicide victims may be related to previously demonstrated increased amygdala blood flow and subsequent amygdala enlargement due to the increased processing of emotional information.     

High ambient temperature reduces rate of body-weight loss produced by wheel running

This study examined the effect of ambient temperature (AT) on the relationship between activity and weight loss. Compared with a neutral AT of 21 degrees C, high ATs of 27-29 degrees C produced a slower rate of weight loss in rats given 1.5-hr food access and 22.5-hr running-wheel access in a standard activity-based anorexia (ABA) procedure (Experiments 1 and 2). The high AT did not affect food intake or wheel running in Experiment 1, but did reduce running in Experiment 2. Switching from neutral to high AT had only a transient effect on weight loss when wheel access was maintained (Experiment 2) but resulted in less weight loss when wheel access was prevented (Experiment 3). Giving rats only 3 hr of wheel access each day at a neutral AT also produced substantial weight loss, but less if for the rest of each day they were maintained at a high AT (Experiment 4).