Regional blood flow changes in response to thermal stimulation of the brain and spinal cord in the Pekin duck

Summary In conscious Pekin ducks, carotid and sciatic blood flows, respiratory rate, core and skin temperatures were measured during selective thermal stimulations of the spinal cord and rostral brain stem in thermoneutral (20 °C) and warm (32 °C) ambient conditions.At thermoneutral ambient temperature selective heating of the spinal cord by 2–3 °C (to 43–44 °C) increased the carotid blood flow by 138% and the sciatic blood flow by 46%. Increase in blood flows was correlated with increased breathing rate and beak and web skin temperatures.Selective cooling of the spinal cord at warm ambient temperatures and panting reduced the blood flow in both arteries and decreased the breathing rate.Heating or cooling of the brain stem showed generally very weak but otherwise similar responses as thermal stimulation of the spinal cord. In one duck out of six there was a marked effect on regional blood flow during brain stimulation.The results show that thermal stimulation of the spinal cord exerts a marked influence on regional blood flow important in thermoregulation, whereas the lower brain stem shows only a weak thermosensitivity, and stimulation caused only small cardiovascular changes of no major consequence in thermoregulation.     

Thermoregulatory responses to temperature manipulation of the spinal cord in the marmot

Yellow-bellied marmots (M. flaviventris) acclimated to T_a = 20 °C were implanted with U-shaped polyethylene thermodes in the peridural space of the spinal cord. Decreasing the temperature of the cervical, thoracic, or lumbar areas of the cord increased heart rate, electromyographic activity, and oxygen consumption in the animals. These responses differed qualitatively from those elicited by heating the same cord areas, indicating specificity of the response to the temperature change.Increases in heat production were proportional to the amount of cooling of the cord. The thoracic area was found to be more thermosensitive than the lumbar area. No behavioral or physical thermoregulation was apparent when the spinal cord temperature was changed in these animals.In addition to the conclusion that regulation of spinal cord temperature may be important in the euthermic marmot, it was postulated that the temperature receptors located in the thoracic cord of the marmot may be important in maintaining shivering thermogenesis during arousal from hibernation.     

Force variability depends on core and muscle temperature

The aim of this study was to investigate if voluntary activation and force variability during maximal voluntary contraction (MVC) depends more on muscle (local) or body (core) temperature. Ten volunteers performed a 2-min MVC of the knee extensors under the control (CON) conditions (ambient temperature (21 °C), relative humidity (30%), and air velocity (∼0.1 m/s)) as well as after heating (HT) and cooling (CL) of the lower body. During water manipulation procedure lower body was immersed up to the waist in a water bath at ∼44 °C for 45 min for HT experiment, and ∼15 °C for 30 min for CL experiment. Peak torque, torque variability, muscle voluntary activation and half-relaxation time were assessed during the exercise. HT increased muscle (2.8±0.2 °C) and rectal (1.9±0.1 °C) temperatures while CL lowered muscle (2.2±0.2 °C) temperature, but did not affect rectal temperature. During 2-min MVC, peak torque decreased (P<0.05; SP>90%) and to a lower level in HT compared to CON and CL experiments (52.6±2.3% versus 69.0±2.3% and 65.6±1.9% MVC, respectively, P<0.05; SP>90%). Torque variability increased significantly during exercise and was significantly larger in HT and lower in CL compared to CON experiment. Voluntary activation of exercising muscle was more depressed in HT (i.e. greater central fatigue) and the smallest effect was found in CL compared to CON. In conclusion increased core and muscle temperature impairs voluntary activation and increases force variability of the exercising muscles while a local muscle cooling decrease force variability but has a small effect on central fatigue.     

Convergence of impulses on anterior hypothalamic neurons during local temperature stimulation of the skin

Unit responses in the medial preoptic region of the anterior hypothalamus and septum to local temperature stimulation of various parts of the skin were studied in chronic experiments on rabbits. The temperature of an area of skin on the back (zone I) and head (zone II) was altered by means of thermodes: heated to 38–40°C and cooled to 22–26°C. Of 111 neurons tested 21 responded to a change of skin temperature (mainly to cooling). Temperature-sensitive hypothalamic neurons were shown to react to temperature stimulation of both skin zones stimulated. The types of the responses recorded are described.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 13, No. 4, pp. 365–370, July–August, 1981.     

Effects of microwave irradiation and temperature on spontaneous ventral root discharges in prog spinal cord

Aperiodic firing independent of extraneous stimuli at rates varying between 3–8 and 65–100 Hz (spontaneous firing activity, or SFA) was recorded at ventral root filaments of isolated, sagitally hemisected frog spinal cord. Lowest activity level was observed at temperatures of 7–11°C and an increased rate at either higher or lower temperatures. Some consistent short-lasting changes in SFA were noted straight away during the course of thermal changes: heating and cooling the preparation increased and reduced discharge rate, respectively. Characteristic activity rate for a given temperature level would set in 1–3 min after this level had stabilized. Microwave radiation of the spinal cord (6.45 GHz; specific absorption rates: 0.1, 0.4, and 2.0 W/g; duration 5 min) brought about no significant alteration in SFA at a steady temperature level. Microwave heating of the preparation and raised temperature both produced the same effects in all trials. Results would indicate that a thermal mechanism underlies the microwave effects on SFA at the ventral roots of frog spinal cord.Institute of Medical Radiology, Academy of Medical Sciences of the USSR, Obninsk. Translated from Neirofiziologiya, Vol. 20, No. 6, pp. 723–728, November-December, 1988.     

Effect of nifedipine on core cooling in rats during tail cold water immersion

Male rats (450 g, n=11/group) were heated at an ambient temperature of 42°C until a rectal temperature of 42.8°C was attained. Rats, then received either saline (30°C)+tail ice water immersion (F+I) or saline (30°C)+tail ice water immersion+Nifedipine, a peripheral vasodilator, (F+I+N) to determine cooling rate effectiveness and survivability. The time to reach a rectal temperature of 42.8°C averaged 172 min in both groups resulting in similar heating rates (0.029°C/min). The cooling rates in group F+I and F+I+N were not significantly different from each other. We conclude that since Nifedipine did not improve cooling rates when combined with fluid+tail ice water immersion, its use as a cooling adjunct does not seem warranted.     

Habituation of the metabolic and ventilatory responses to cold-water immersion in humans

An experiment was undertaken to answer long-standing questions concerning the nature of metabolic habituation in repeatedly cooled humans. It was hypothesised that repeated skin and deep-body cooling would produce such a habituation that would be specific to the magnitude of the cooling experienced, and that skin cooling alone would dampen the cold-shock but not the metabolic response to cold-water immersion. Twenty-one male participants were divided into three groups, each of which completed two experimental immersions in 12 °C water, lasting until either rectal temperature fell to 35 °C or 90 min had elapsed. Between these two immersions, the control group avoided cold exposures, whilst two experimental groups completed five additional immersions (12 °C). One experimental group repeatedly immersed for 45 min in average, resulting in deep-body (1.18 °C) and skin temperature reductions. The immersions in the second experimental group were designed to result only in skin temperature reductions, and lasted only 5 min. Only the deep-body cooling group displayed a significantly blunted metabolic response during the second experimental immersion until rectal temperature decreased by 1.18 °C, but no habituation was observed when they were cooled further. The skin cooling group showed a significant habituation in the ventilatory response during the initial 5 min of the second experimental immersion, but no alteration in the metabolic response. It is concluded that repeated falls of skin and deep-body temperature can habituate the metabolic response, which shows tissue temperature specificity. However, skin temperature cooling only will lower the cold-shock response, but appears not to elicit an alteration in the metabolic response.     

Circadian and circannual variations in growth and cold resistance of warm acclimated rats and their relationship with the geomagnetic activity

In winter, summer, spring and fall, groups of 200 g Sprague Dawley male rats were kept for 14 days at constant Ta 28°C±°C and LD 12:12(07:00–19:00 h). They were fed a laboratory chow (P) or a semi-purified diet (H). They were weighed at 0, 7 and 14 days. Cold resistance was determined by the fall in rectal temperature during 4 hours of restrained exposure to +1°C either in the morning (09:00 – 13:00 h) or in the afternoon (14:00 – 18:00 h). Rats fed the H diet could better tolerate cold exposure in the morning than in the afternoon, whereas the chow-fed rats were more vulnerable and became severely hypothermic both in morning and afternoon. The greater morning resistance provided by the H diet was evident in summer and winter but not in spring or fall. With both diets, cold resistance as well as growth decreased during spring and fall. Frozen storage of the diets and the water for use during other seasons showed that the nutrients per se were not a factor in the observed seasonal cycle. Although humidity in the 28°C room varied between a minimum of 26.5% in April to a maximum of 44.3% in August, it was not a statistically significant factor in the growth cycle. Arguments are presented to rule out relative humidity as a significant factor in the seasonal variation in the degree of cold resistance. A significant correlation was found for growth and cold resistance with geomagnetic activity.NRCC # 17310     

Generation and processing of peripheral temperature signals in mammals

Temperature transduction in peripheral cold receptors and processing of peripheral temperature signals in the spinal cord were studied in cats and rats. The temperature dependence of the generator potential is attributed to different temperature coefficients of an electrogenic Na-efflux and the passive Na-influx. Cold receptor activity and particularly its bursting pattern is considerably modulated by the local Ca-concentration, but the effect of elevated Ca-concentration is abolished by the ATPase blocker ouabain. — The peripheral temperature signals from the scrotal skin of rats are transformed in dorsal horn neurones (DHN) into temperature reactions, which occur only above (warm reaction) or below (cold reaction) a certain temperature threshold and are limited to an operational range of 1–4°C. Convergency of different temperature inputs were observed in one and the same DHN. Supraspinal control of temperature reactive DHN appears to be complex but predominantly excitatory.Presented at the Eighth International Congress of Biometeorology, 9–14 September 1979, Shefayim, Israel.     

Sows’ responses to increased heat load – A review

There is a comprehensive body of literature on how increased air temperature affects the physiology, production and behaviour of sows, while very few studies consider the thermal effects of air humidity and air velocity.This review summarises studies that have investigated effects of air temperature by reviewing published literature in which sows were exposed to at least two different levels of air temperature ranging from 15 °C to 39 °C. Increased rectal temperature was investigated in the majority of the studies (26) and on average, the rectal temperature increased by 0.099 °C per °C increased air temperature above 25 °C. The increase was smaller at lower air temperatures, and it was suggested that rectal temperature is practically unaffected by air temperatures in the range of 15 °C–21 °C. This review elucidates how air temperature also affects performance indicators such as respiration rate, vaginal temperature, skin temperature, feed intake, milk yield, body weight loss during lactation, mortality, litter daily weight gain during lactation and sow behaviour.One study reported how respiration rate, rectal temperature, vaginal temperature and skin temperature were affected by both air temperature and air humidity, and the results suggest that the relative significance of air temperature and humidity may be similar for sows and finishing pigs (e.g. an increase of 40% relative humidity at an air temperature of 30 °C has a similar effect as a 1.9 °C increase in temperature).Studies on mitigation methods against the effects of high temperature and humidity such as snout cooling, drip cooling and floor cooling were reviewed to extract knowledge related to the effects of air velocity, temperatures of surrounding surfaces and the opportunity for sows to moisten their skin.     

Cold tolerance in rats following administration of streptozotocin,glucose, insulin and glucose plus insulin

Fall in rectal temperature (T_re) and survival time was determined on exposure to–20°C in adult normoglycemic and diabetic (streptozotocin treated) rats and 1 h following glucose feeding or insulin administration or both and on exposure to–10°C in young rats with and without glucose feeding. The susceptibility to frostbite was determined by exposure of the limbs to freezing mixture of–19°C or–23°C. The rate of fall of T_re was less and the survival time more in glucose and insulin plus glucose treated animals. On the other hand, the rate of fall of T_re was more and the survival time less, in dia betic and insulin-treated animals. The rectal temperature at which the animal died was the same in the control and the treated animals. The susceptibility to frost bite was more in insulin treated and diabetic animals and less in glucose-fed animals. Exposure to cold during the second h after glucose or glucose plus insulin injection did not alter the blood glucose from that obtained at room temperature. In insulin-treated animals the rate of rise of blood glucose during the second h was much higher at low temperature than at room temperature. The rise in blood glucose in diabetic animals was much higher than in normoglycemic animals exposed to cold.     

The effects of two types of clothing on seasonal cold tolerance

This study investigated the effects of two types of clothing, leaving legs covered or uncovered, on seasonal cold tolerance in women. Experiments were carried out to compare cold tolerance at an ambient temperature (T _a) of 10° C in December between two groups of subjects, who wore either knee-length skirts (skirt group) or full-length trousers (trouser group) for 3 months from September to November. The main results are summarized as follows: rectal temperatures continued to fall for 40 min in the trouser group when the subjects were covered by a blanket, while it became stable in 30 min in the skirt group; rectal temperatures showed greater increases in the skirt group when the blanket was removed after 40 min exposure to T _a of 10° C; metabolic heat production was kept significantly lower in the skirt group when uncovered or covered by a blanket at T _a of 10° C; metabolic heat production was negatively correlated with mean skin temperature and was always higher in the trouser group when measured at the same mean skin temperature; in the uncovered condition diastolic blood pressure increased significantly in the trouser group but not in the skirt group. These results would suggest that the subjects who wore skirts for 3 months from September to November had improved their ability to tolerate the cold.     

Effects of hyperthermia and hypothermia on spontaneous contractions of the adult rabbit testicular capsule

Spontaneous contractions of the isolated testicular capsule of the adult rabbit have been found to be markedly sensitive to heat and cold stress. Testicular capsular contractions may provide a propulsive pumping action for transporting nonmotile sperm out of the testis and into the epididymis where they can then attain motility. An optimal temperature for the amplitude of spontaneous contractions of the rabbit testicular capsule occurred at 32 – 34°C. An increase in thein vitro organ bath temperature from 37 to 40°C caused a marked decrease in the amplitude of spontaneous contractions. A complete and irreversible cessation of spontaneous contractions occurred at 48°C for at least 30 min after cooling to 37°C. A decrease in temperature from 37 to 26°C resulted in a marked decrease in frequency and amplitude progressing to a complete but reversible cessation of spontaneous contractions at 16°C. Marked changes in the frequency and amplitude of spontaneous contractions of the isolated testicular capsule began to be observed when the tissue was exposed to organ bath temperatures of 3°C above and below the normal intra-testicular temperature. These data suggest that exposure of men to fever or excessively hot baths as well as swimming in excessively cold water or extreme cold weather exposure may have inhibitory effects on testicular capsular spontaneous contractions which may interfere with sperm transport.     

Thermoregulatory responses to preoptic-anterior hypothalamic heating and cooling in the bat,Eptesicus fuscus

Summary The central nervous control of temperature regulation in the bat, Eptesicus fuscus, was evaluated by heating the preoptic-anterior hypothalamus (PO/AH) of active, unanaesthetized bats. Because bats are metabolically very variable, change in body temperature was used as the criterion of change in heat balance in response to change in brain temperature and change in wing temperature as an indicator of vasomotor changes.Heating the preoptic-anterior hypothalamic area (PO/AH) of the bat Eptesicus fuscus caused an average increase in wing temperature due to vasodilation of 1.0° C and an average increase in body temperature of 0.4° C. Conversely, cooling the PO/AH led to an average decline in wing temperature due to vasoconstriction of 0.9° C and an average decline in body temperature of 0.4° C.Bats were heat-stressed to augment the responsiveness of the PO/AH. Heat-stress alone causes a rise in body temperature and wing temperature. Release from heat stress causes a fall in body temperature and a fall in wing temperature. When the PO/AH is heated following a period of high heat-stress, the body temperature continues to fall but wing temperature reverses its direction of change and rises. When bats are given a low heat-stress and simultaneous heating of the PO/AH, wing temperature rises in response to PO/AH temperature and the body temperature stabilizes. When the PO/AH is cooled in bats under high heat-stress, body temperature stabilizes and wing temperature falls. When bats are cold-stressed, body temperature and wing temperature fall regardless of heating of the PO/AH.These responses are related to the life habits of the bat.It is concluded that the PO/AH of the bat Eptesicus fuscus may be less thermally sensitive than the PO/AH in other vertebrates studied, and that other central nervous structures have acquired an increased thermoregulatory function.We thank Mrs. Ruth Chalmers for her excellent histological preparstions.This work was supported, in part, by National science Foundation grant GB 6303 and GB 13797.     

Responses to whole body and finger cooling before and after an Antarctic expedition

Eight subjects, who were indoor workers and not habitually exposed to cold, spent 53 days in Antarctica. They did mainly geological field work often requiring the use of bare hands. The effects of the expedition on responses to a whole body cold exposure test, a finger blood flow test and a cold pressor test were studied. After the expedition, during whole-body cooling the time for the onset of shivering was delayed by 36 min (P<0.001) and forearm and thigh temperatures were 1.5°C higher (P<0.05) at the end of exposure. During local cooling of the finger with 10°C perfusion, finger vascular resistance was 14.9 (SEM 6.6) mmHg · ml^–1 · min · 100 ml (P<0.05) lower and finger temperature 3.9 (SEM 0.8) °C higher (P< 0.01). However, the decrease in rectal temperature during wholebody cooling was unaltered and the response to a cold pressor test was unchanged. The data would indicate that partial acclimatization to cold had been developed. Changes in forearm temperature were correlated with the duration of cold exposure of the hands (P < 0.05) and finger vascular resistance and finger temperature were correlated with responses to cooling before the expedition (P<0.001 and P<0.01, respectively). Because the ambient temperature was not clearly lower in Antarctica in comparison to Finland, the reason for the changes developed seems to be the increased exposure to the outdoor climate in Antarctica.     

Central nervous versus total body thermosensitivity of the duck

Ducks were chronically implanted with thermodes in the POAH region, the lower brainstem or the vertebral canal. At thermoneutral conditions, lowering the temperature of the spinal cord (T_vc) or the lower brainstem (T_mb) stimulated metabolic heat production (M) with a subsequent rise of core temperature (T_c). Lowering the temperature of the POAH region (T_hy) induced a fall of T_c due to paradoxical activation of heat defence and, thus, induced slight to moderate general hypothermia depending on the cooling intensity. When T_hy was normalized, the hypothermia temporarily stimulated metabolic heat production until T_c was normalized. Cold sensitivity of the entire body, as revealed by the metabolic response to the hypothermia induced by preceding POAH cooling, and cold sensitivity of the spinal cord and the lower brainstem, as revealed by the metabolic response to local cooling, were quantified by calculating the quotient M/T from the maximum metabolic response and the experimentally induced drop of T_c, T_mb and T_vc. With lower brainstem cooling M/T_mbdid not exceed –0.4 W/(kg · C). With spinal cord cooling, M/T_vc did not exceed –0.6 W/(kg · C). The mean value of M/T_c after hypothermia induced by POAH cooling was –4.02 W/(kg · C). The results indicate that the cold sensitivity residing in the CNS of ducks represents only a small fraction of the entire cold sensitivity of the body.Presented at the Eighth International Congress of Biometeorology, 9–14 September 1979, Shefayim, Israel.     

Combined effects of temperature and salinity on critical thermal minima of pacific white shrimp Litopenaeus vannamei (Crustacea: Penaeidae)

Critical thermal minima (CTMin) were determined for the Pacific white shrimp Litopenaeus vannamei juveniles from four different acclimation temperatures (15, 20, 25, and 30 °C) and salinities (10‰, 20‰, 30‰, and 40‰). The lowest and highest CTMin of shrimp ranged between 7.2 °C at 15 °C/30‰ and 11.44 °C at 30 °C/20‰ at the cooling rate of 1 °C h⁻¹. Acclimation temperature and salinity, as well as the interaction of both parameters, had significant effects on the CTMin values of L. vannamei (P<0.01). Yet, the results showed a much more profound effect of temperature on low thermal tolerance of juveniles. Only 40‰ salinity had an influence on the CTMin values (P<0.01). As the acclimation temperature was lowered from 30 to 15 °C thermal tolerance of the shrimp significantly increased by 3.25–4.14 °C. The acclimation response ratio (ARR) of the Pacific white shrimp exposed to different combinations of salinity and temperature ranged between 0.25 and 0.27. When this species is farmed in sub-tropical regions, its pond water temperature in the over-wintering facilities (regardless of the water salinity level) must never fall below 12 °C throughout the cold season to prevent mortalities.     

The effects of warming by active and passive means on the subsequent responses to cold water immersion

Two experiments were undertaken to investigate the effects of warming the body upon the responses during a subsequent cold water immersion (CWI). In both experiments the subjects, wearing swimming costumes, undertook two 45-min CWIs in water at 15° C. In experiment 1, 12 subjects exercised on a cycle ergometer until their rectal temperatures (T _re) rose by an average of 0.73°C. They were then immediately immersed in the cold water. Before their other CWI they rested seated on a cycle ergometer (control condition). In experiment 2, 16 different subjects were immersed in a hot bath (40° C) until their T _re rose by an average of 0.9° C; they were then immediately immersed in the cold water. Before their other CWI they were immersed in thermoneutral water (35° C; control condition). Heart rate in both experiments and respiratory frequency in experiment 1 were significantly (P < 0.05) higher during the first 30 s of CWI following active warming. In experiment 1, the rate of fall of T _re during the final 15 min of CWI was significantly (P < 0.01) faster when CWI followed active warming (2.46° C · h^–1) compared with the control condition (1.68°C · h^–1). However, this rate was observed when absolute T _re was still above that seen in the control CWIs. It is possible, therefore, that if longer CWIs had been undertaken, the two temperature curves may have converged and thereafter fallen at similar rates; this was the case with the aural temperature (T _au) seen in experiment 1 and the T _au and T _re in experiment 2. It is concluded that pre-warming is neither beneficial nor detrimental to survival prospects during a subsequent CWI.     

Thermoregulation mediated by conditioned heart-rate changes in pigeons

Summary Pigeons were subjected to an apparent heat (44 °C) or cold (30 °C) load by means of a thermode located next to their thermosensitive spinal cord. They were then able to obtain brief thermonormality (40 °C) spells if they changed their heart-rate by a preset amount above (or below) their baseline heartrate. In the final experiments the animals increased (or decreased) their heart-rate by about 25% within 12 training sessions, thereby achieving a partial thermoregulation: they were able to maintain their spinal temperature at a normal level for about 25% of the time. Extinction sessions, in which the heart-rate changes did not have any thermoregulatory effect, and control sessions, in which thermonormality periods were given independently of the heart activity, yielded baseline heart-rates. It is concluded that the cardiac activity related to thermoregulation may be subject to modification through learning and thus may not differ fundamentally in this respect from some homeothermic behavioural responses.This work was supported by the Interdisciplinary Research Fund of the Ruhr-Universität and the Deutsche Forschungsgemeinschaft through its Sonderforschungsbereich 114. We thank Dr. I. Schmidt (Bad Nauheim) for much advice and discussion and Dr. J. Emmerton, Miss A. Lohmann and Mrs. D. Hagenkötter for assistance of various kinds.