Die Bedeutung der zentralnerv�sen Thermosensitivit�t f�r die Temperaturregulation der Taube

Spinal neuronal thermosensitivity in vivo and in vitro in relation to hypothalamic neuronal thermosensitivity

In the spinal cord, temperature signals are generated which serve as specific inputs in the central nervous control of body temperature. Because of the spatially distinct organization of afferent and efferent neuronal systems at the spinal level, the afferent pathway for temperature signal transmission could be identified in vivo in the ascending, anterior and lateral tracts with a relationship of about 75:25% between warm and cold sensitive neuraxons. Analysis of spinal neuronal thermosensitivity in vitro on spinal cord tissue slices has been concerned, so far, with the superficial laminae of the dorsal horn as the site of origin of ascending nerve fibers conveying mostly temperature and pain signals, and with lamina X as a site of origin of afferent as well as efferent neurons. A relationship of about 95:5% between warm and cold sensitive neurons was found at the segmental level, indicating that warm sensitivity is the prevailing, primary property of spinal neurons, whereas cold sensitivity seems to be mainly generated by synaptic interaction as a secondary modality. Dynamic responses to temperature changes were frequently displayed in vitro at the spinal segmental level in lamina I + II but not in lamina X, even by neurons whose static activity was little influenced by local temperature. Dynamic thermosensitivity was found less frequently in ascending tract neuraxons and was not observed in hypothalamic neurons receiving temperature signal inputs from the spinal cord, and thus, does not seem to be relevant for the thermosensory function of spinal cord neurons, unlike peripheral warm and cold receptors. A majority of spinal warm sensitive neurons displayed both static and dynamic warm sensitivity as an inherent property after synaptic blockade. In the further analysis of spinal cord thermosensitivity, the in vitro approach permits application of the same electrophysiological and neuropharmacological methods as were established for the analysis of hypothalamic thermosensitivity. In addition, the topography of the spinal cord will provide additional structural and possibly histochemical information to characterize the functions of neurons independently of their thermal properties.

Parabronchial oxygen extraction in ducks during selective cooling of the spinal cord

Pekin ducks Anas platyrhynchos were chronically equipped with thermodes in the vertebral canal. Metabolic heat production, parabronchial oxygen extraction, vertebral canal temperature and body temperature were measured simultaneously before and during spinal cooling, at ambient temperatures ranging from 6 to 25 degrees C. Lowering vertebral canal temperature from 41.4 +/- 0.2 degrees C to 35.9 +/- 0.6 degrees C gave a mean increase in metabolic heat production of 1.54 +/- 0.26 W kg-1. Even though the spinal cooling had a clear metabolic effect, there was no concomitant change in parabronchial oxygen extraction. It is concluded that the thermosensitive structures residing in the spinal cord are not involved in the regulation of parabronchial gas exchange. The increase in parabronchial oxygen extraction, which is reported during cold exposure in birds, may therefore be induced by thermal inputs from peripheral thermoreceptors.

Respiratory modulation of shivering intensity in the pigeon

Respiration and shivering were measured in unanaesthetized, cold-exposed pigeons using pneumotachography and electromyography, respectively. The instantaneous intensity of shivering in the pectoral muscle varied in phase with respiration. Power spectral analysis showed that the main frequency components of respiration and demodulated EMG coincided exactly. The intensity of shivering was highest during end-expiration and lowest at end-inspiration. This was confirmed by cross-correlation analysis of respiration and demodulated EMG. The absolute level of modulation remained constant (c. 10 microV peak-to-peak) despite changes in the general intensity of shivering. On the other hand, the relative depth of modulation was highest during incipient shivering. These facts indicate that only a part of the motor units recruited for shivering is susceptible to respiratory modulation and that this part is first recruited during incipient shivering. Inhalation of 5% CO2 did not affect the interaction between respiration and shivering although respiration frequency varied from 25 to 60 min-1. Thus, pulmonary chemoreceptors do not mediate this effect. It is suggested that the interaction between respiration and shivering occurs directly in the CNS. The question whether the interaction is adaptive for the animal or merely reflects a common evolutionary history of the underlying neural circuits is discussed.

EMG activity in pectoral and femoral muscles during spinal cord cooling in exercising pigeons

Adult domestic pigeons, with thermodes chronically implanted in the vertebral canal, were trained to walk on a treadmill. In the first series of experiments, EMG activity in a pectoral (M. pectoralis) and a femoral muscle (M. biceps femoris) was measured to determine if shivering could occur during exercise. When the spinal cord was cooled (36.2 +/- 0.5 degrees C) during exercise (0.6 km/h), pectoral muscle EMG activity increased by 80%, while femoral muscle EMG activity did not change significantly. EMG activity remained unchanged during exercise in control experiments where the spinal cord was not cooled. In the second series of experiments, the spinal cord was first cooled (36.1 +/- 0.5 degrees C) for 5 min in resting pigeons and then the treadmill was started. Spinal cord cooling during rest significantly increased pectoral muscle EMG activity but not that of the femoral muscle. Within 1 s after the onset of exercise, EMG activity in the pectoral muscle decreased by 74%. In both series of experiments, shivering was not induced in the femoral muscle. The level of pectoral muscle EMG activity stimulated by spinal cord cooling during exercise in the first series of experiments corresponded to the level to which EMG activity was reduced by exercise during spinal cord cooling in the second series of experiments (192% and 186% in relation to the post-cooling level, respectively). It is concluded that shivering can be induced in the pectoral muscle by spinal cord cooling during exercise in the pigeon.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of short exposure to cold on plasma thyroxine in Coturnix quail: role of the infundibular complex and its neural afferents

Exposure of control quail to low ambient temperature (4 degrees) for a short duration (15 min) led to a rapid increase in plasma thyroxine (T4) levels. A peak appeared 40 min after the cold began and was followed by a progressive and slow decline. T4 levels were elevated in birds maintained for up to 3 hr at 4 degrees. Restraint stress could be accompanied by a slight and late decrease in thyroxine concentration, indicating that glucocorticoids could partly inhibit thyroid function. Both cold and restraint stresses elicited sustained adrenocortical activation. No thyroid response to cold appeared after complete or partial neural deafferentation of the hypothalamus, indicating that cold signals were conveyed to the thyrotropic centers from peripheral and/or deep thermoreceptors through neural posterior-lateral afferents to the hypothalamus. No thalamic relay appeared to be necessary since normal thyroidal stimulation was observed after thalamic-hypothalamic disconnection. Such a response persisted in hemispherectomized quail.

Cardiovascular and blood gas changes during panting responses induced by ambient or spinal cord heating in the pigeon

We measured respiratory frequency (fR), oxygen consumption (VO2), cardiovascular responses and arterial and mixed venous PCO2, PO2, pH and CO2 in six pigeons during hyperthermia caused by ambient heating (increase in body temperature = 1 degrees C) and during heating of the spinal cord alone (increase in vertebral canal temperature = 3 degrees C). Spinal cord heating caused an increase in fR to 546 min-1 (+/- 13 min-1 SE) and increases in VO2, heart frequency (fH) and cardiac output (CO); PaCO2 and PVCO2 decreased 3.4 and 4.6 torr, respectively, while PaCO2 increased 6.8 torr. fR during ambient heating was 489 +/- 36 min-1; cardiovascular and blood gas changes were, generally, in the same direction as those during spinal cord heating but of lesser magnitude. In six other pigeons, we characterized fR, VO2, cardiovascular and blood gas changes during a 4 degrees C rise in body temperature caused by increased ambient temperature. Those data showed that with increasing hyperthermia fR increased rapidly, though not stepwise, to a maximum while VO2, fH, CO, PaO2, pHa and the arterial-venous CO2 difference all gradually increased; PaCO2 and PVCO2 gradually decreased. We conclude that, generally, whole body heating by increased ambient temperature and heating of the spinal cord alone produce the same responses and that these responses are dependent on the magnitude of the heat stimulation.

Cardiovascular and blood gas responses to shivering produced by external and central cooling in the pigeon

Cardiovascular and blood gas responses of pigeons to spinal cord cooling (35-36 degrees C) were measured at thermoneutral (28 degrees C) and low (5 degrees C) ambient temperatures. Spinal cord cooling at thermoneutral temperatures caused immediate shivering and increases in heat production (223%), heart rate (152%) and cardiac output (169%), but blood pressure and stroke volume did not change. PaCO2 and PvCO2 increased slightly during the cooling; PaO2 and CaO2 decreased slightly while PvO2 and CvO2 decreased considerably (10 Torr and 1.7 mmol . l-1, respectively), resulting in a greater a-v difference in O2 content. Ambient cooling produced responses comparable to spinal cord cooling. Simultaneous spinal cord and ambient cooling produced similar responses that were generally greater in magnitude than either kind of cooling alone. Consequently, heart rate, cardiac output and O2 extraction from the blood were all significantly, linearly related to heat production over the wide range studied. Comparisons are made between cardiovascular responses of birds to shivering and exercise in regards to the relative importance of increases in heart rate, stroke volume and blood pressure. It is suggested that exercise and shivering may effect cardiovascular responses through similar receptor mechanisms.

Increase in oxygen consumption induced by selective spinal cord cooling in the exercising pigeon

Six domestic pigeons with chronically implanted spinal thermodes were exercised on a treadmill at neutral ambient temperature. During the exercise the spinal cord was cooled to 34.7 +/- 0.4 degrees C (mean +/- S.E.M.). Oxygen consumption of the pigeons increased from 28.3 +/- 2.1 to 61.2 +/- 3.7 ml X min-1 X kg-1 due to exercise per se, and superimposed cooling of the spinal cord during exercise induced an additional increase in oxygen consumption to 84.9 +/- 4.5 ml X min-1 X kg-1. The result demonstrates that cooling of the spinal cord elicits shivering in exercising pigeons at thermoneutral conditions.

Behavioral self warming and cooling of spinal canal by rats

Rats with a chronic thermode implanted in their spinal canal could bar-press to warm or cool their spinal cord. With a "cold" lever, they cooled their spinal canal less in a cold environment than in a warm environment. With a "warm" lever they behaved in the same way, i.e., warmed their spinal canal more in a warm than in a cold environment. In a two-lever situation they pressed the cold and the warm levers alternatively in warm environment, but did not press either in cold environment. These results suggest that cold and warm spinal cord provided the rats with rewards of a different nature.

Evaporative losses of water by birds

1. Birds lose water in evaporation from the respiratory tract and, in many species, through the skin. Anatomical arrangements in the nasal passages to conservation of water and hear from the expired air in the absence of heat loads. However, most species still expend more water in evaporation than they produce in metabolism when either quiescent or vigorously active. Certain small birds, several of them associated with arid environments, represent exceptions to this and their more favorable situation appears in part to reflect as an ability to curtail cutaneous water loss. 2. Birds typically resort to panting in dealing with substantial heat loads developing in hot environments or accumulated over bouts of activity. In a number of species this form of evaporative cooling is supplemented by gular fluttering. 3. The ubiquitousness of active heat defense appears to reflect more the importance for birds of dealing with heat loads existing following flight or sustained running than any universal affinity for hot climates. Panting can be sustained for hours, despite progressive dehydration and, in some instances, hypocapnia and respiratory alkalosis. The prominent involvement of thermoreceptors in the spinal cord in its initiation is of considerable interest.

Thermal and electromyographic correlates of shivering thermogenesis in the pigeon

1. Electromyographic, thermal and metabolic measurements were made on pigeons subjected to stepwise ambient cooling. Subsequent signal analysis of the EMG-recordings from the pectoral muscle showed that the mean rectified value (Umrv) is the most reliable EMG-based indicator of metabolic heat production (M). 2. The muscle-abdomen temperature difference was linearly related to M, and correlated strongly with Umrv. 3. The long-term and short-term fluctuations of shivering intensity (Umrv) were highly synchronous in the two main muscles, m. pectoralis and m. supracoracoideus. 4. The EMG-signal approached a Gaussian process at high shivering intensities. Evaluation of the changes in EMG median frequency suggested a size-dependent recruitment of motor units for shivering. 5. It is pointed out that Umrv and the integrated value of EMG are interconvertible and that Umrv should be preferred, because it has more physiological relevance.

Total body thermosensitivity and its spinal and supraspinal fractions in the conscious goose

1. Effects of general body cooling on heat production: an intravascular heat exchanger was used to alter total body temperature. Heat production increased with decreasing body temperature at an average rate of -12W/kg x degree C. The rate of rise was independent of air temperature. The threshold body temperature below which heat production rose was lower at higher air temperature. 2. Effects of spinal cord cooling: heat production increased with decreasing spinal temperature at an average rate of -0.3 W/kg x degree C. The rate of rise was not clearly affected by air temperature. The spinal threshold temperature was lower at warm ambient conditions. The results suggest that spinal thermosensitivity in the goose represents only a minor fraction of total body thermosensitivity. 3. Effects of head cooling: heat exchangers closing the carotid arteries were used to alter the temperature of the blood supplied to the head. Cooling increased heat production. When the thermosensitivity of the area, which was affected by heat exchanger, was calculated from the relationship between changes of heat production and brain temperature, values between -0.74 and -1.65 W/kg x degree C were obtained. Measurements of brain, spinal cord and head skin temperatures suggest that the thermosensitive structures which mediated the responses, were predominantly situated in the brain.

Diurnal changes of thermoregulatory functions in pigeons. II. Spinal thermosensitivity

In pigeons there exists a pronounced day-night variation of deep body temperature with a maximum at day and a minimum at night. To investigate a possible influence of spinal cord thermosensitivity in generating the body temperature rhythm, we studied the relationship between the experimentally changed spinal temperature and either heat production or panting, respectively. The experiments were performed in LD 12:12h and at constant ambient temperatures. At an ambient temperature of 25 degrees C, the responses to spinal cooling are reduced during the dark phase. The spinal temperature theshold for activating heat production is about 2 degrees C lower during night, whereas the gain of the metabolic response to spinal cooling is nearly unchanged. The lower ambient temperature, the greater are the heat production responses during the dark phase in relation to those during the light phase. Warming the spinal cord to the same amount leads to different responses of respiration rate during day and night, too. The temperature threshold for thermal panting is lowered during the dark phase similarly to the lowered threshold for heat production responses. The described diurnal variations of the responses to spinal cord warming and cooling support the hypothesis of an involvement of spinal thermosensitivity in the generation of daily body temperature fluctuations.

Cyclic and non-cyclic variations of spinal cord temperature related with temperature regulation in pigeons

1. The temperature of the spinal cord (Tvc) was measured in unanaesthetized pigeons at different ambient temperatures (Ta). 2. In short-term experiments spontaneous or noncyclic variations of Tvc at constant (20 and 10 degrees C) and changing Ta were correlated with the amplitude of the electromyogram EMG) which indicates shivering, i.e., heat production. At constant Ta no unequivocal correlation between Tvc (and also Tskin) and EMG was found. During ambient cooling there was often an increase of Tvc which resulted in a positive correlation whereas there was a negative correlation to Tskin. 3. In long-term experiments (24h, LD 12:12Y cyclic variations of Tvc were measured at different Ta and ocrrelated with O2-consumption, i.e., heat production. As with body temperature Tvc was lowered during the dark phase of the diurnal cycle. In the light phase both Tvc and heat production increased with decreasing Ta which results in a positive correlation. In the dark phase there was a smaller increase in heat production but a decrease in Tvc, i.e., a negative correlation. 4. The results show that natural variations of Tvc are often positively ocrrelated with heat production. This is in contrast to experimental changes of Tvc where a clear negative correlation to heat production can be observed.

Initiation of muscle activity in spinalized pigeons during spinal cord cooling and warming

1. The effect of spinal cord temperature changes on muscle activity was investigated in unanaesthetized intact and chronically spinalized pigeons and in acutely spinalized pigeons which were artifically respirated and lightly anaesthetized with ether. 2. Spinal cord cooling regularly produced an increase in muscle activity and visible muscle tremor in intact and spinalized pigeons. This motor cold defence reaction was less intensive in spinalized animals, but was qualitatively identical in all groups with regard to spindle shaped firing patterns and grouped discharges. 3. Intravenous injection of 60-100 mg/kg L-Dopa enhanced the motor response to spinal cord cooling in acutely spinalized pigeons. It is suggested that L-Dopa may act on dopaminergic or noradrenergic neurones in the spinal cord. 4. The results demonstrate that the generation of the motor cold defence response to cooling of the spinal cord in pigeons is basically independent from supra-spinal nervous mechanisms. The decreased intensity of cold induced muscle activity in spinalized animals may be attributed to the loss of excitatory or disinhibitory descending inputs to the spinal cord. 5. Spinal cord warming above normal body temperature (41.5 degrees C) produced an increase in muscle activity and slow muscle movements. The pattern was qualitatively different from that of the cold induced tremor.

Temperature changes of the hypothalamus and body core in ducks feeding in cold water

Hypothalamic (Thy), spinal(Tsc) and colonic (Tc) temperatures were measured in Pekin ducks spontaneously dabbling for food in cold water (5 degrees C). In agreement with observations in the pigeon and the fowl Thy was found to be consistently lower by about 0.5 degrees C than the other core temperatures. The drop of Thy during dipping head and neck into the cold water was not substantially greater than that of Tsc, while both changed more than Tc. The measurements do not support the assumption that the hypothalamic region in the duck is exposed to substantially greater temperature fluctuations than other thermosensitive parts of the body core.

Interactions of behavioral and autonomic thermoregulation in heat stressed pigeons

The interactions of behavioral and autonomic thermoregulation in pigeons during ambient heat load were studied by simultaneous measurements of instrumental response rate for cold air reinforcement and respiratory rate. When providing sufficient reinforcement-magnitudes, deep body temperatures were stabilized, due to a linear increase of response rate with ambient loads from 40-60 degrees C, without involving an increase in respiratory heat dissipation. This was effected by maintaining the temporal mean of air temperature and consequently of all skin temperatures at a level independent from load temperature (Fig. 3). When the efficiency of instrumental thermoregulation was limited by reducing the reinforcement-magnitude, not only the instrumental response rate increased, but in addition body temperatures and subsequently respiratory rate rose with the thermal load. Thus a positive correlation between body temperatures and response rate and a simultaneous increase of autonomic heat defence activities characterize incomplete behavioral thermoregulation. The instrumental response rate rapidly followed changes of external load temperature without preceding changes of core temperatures or skin temperatures at well feathered areas (Fig. 6). These findings suggest that the input signal controlling instrumental thermoregulatory behavior is related to the rate of change of temperatures at exposed areas of the body shell, whereas the autonomic heat defence response follows the steady displacements of body temperatures. This points to an important difference between the input signals controlling behavioral and autonomic heat defence in the pigeon.

Effects of altering spinal cord temperature on temperature regulation in the Adelie Penguin, Pygoscelis Adeliae

In 6 Adelie penguins, thermodes were implanted in the cervical and upper thoracic spinal canal. At thermoneutral (+8 to +16 degrees C) and cold (-18 to -22 degrees C) ambient conditions, the effects of spinal canal heating and cooling on the surface temperature of one flipper (skin blood flow), oxygen consumption (metabolic heat production) and esophageal (core) temperature were observed in the conscious animals.- At thermoneutral ambient conditions, spinal cord cooling reduced and spinal heating increased skin blood flow. Only very strong spinal cooling induced small increases of oxygen consumption, while spinal heating had no effect at all. The relation between spinal canal temperature and metabolic heat production at thermoneutral ambient conditions could be described by a linear regression with a slope of -0.05 W. KG-1 . DEGREES C-1. -At cold ambient conditions, the skin vessels of the flippers were permanently constricted and an increase of metabolic heat production by 5-50% of the resting rate developed within 1-3 h of cold exposure. Spinal cord cooling augmented metabolic heat production. Spinal heating reduced heat production but did not release skin vasoconstriction even at high stimulus intensities. The relation between spinal canal temperature and metabolic heat production in the cold could be described by a linear regression with a slope of -0,52 W. kg-1 . degrees C-1. -It is concluded that temperature sensors with specific functions in temperature regulation are located in the spinal cord of the Adelie penguin. These sensors contribute to the central temperature signal input in the hypothermic and hyperthermic ranges of core temperature. The peripheral thermal conditions strongly influence the responsiveness of the various thermoregulatory effectors to the spinal thermal stimulus.

Dog behaviour as related to spinal cord temperature

3 dogs could behaviourally modify their own spinal cord temperature (Tspin. cord). In a hot environment, 2 dogs did not cool their spinal cord, 1 dog warmed it. The higher the environmental temperature, the higher the chosen Tspin. cord. These results seem to imply that this latter dog tended, in warm environment, to behaviourally reduce: Ts greater than Tspin. cord (Ts mean skin temperature). Data obtained previously support this explanation.

Effect of spinal deafferentation on temperature regulation and spinal thermosensitivity in pigeons

1. To study the effect of spinal deafferentation on temperature regulation and spinal thermosensitivity in acute experiments, the spinal cord of pigeons was transected at Th4 and the dorsal roots cut carefully on both sides from Th4 to C6 or C4 (13 or 15 segments); only afferent signals from the upper neck and the head could reach the CNS. Selective changes of the spinal cord temperature in the deafferented region were performed by a thermode in the vertebral canal. 2. At thermoneutral ambient conditions (Ta 23-30 degrees C) the deafferented pigeons were able to maintain a normal body temperature (about 41 degrees C). During ambient cooling (Ta 1-10 degrees C) the core temperature was regulated at a lower level of about 38 degrees C, strong shivering occurred, and heat production was increased. 3. If the decreased spinal cord temperature at low Ta was adjusted experimentally to normal values (about 41 degrees C) then shivering stopped and oxygen consumption decreased. This decrease in heat production was followed by a continuous fall in rectal temperature to values as low as 33-34 degrees C without any initiation of shivering or extra heat production. This means that shivering in the deafferented pigeons must be elicited by cold sensors in the spinal cord alone and that there are no important cold sensors in the non-deafferented region including the brain. 4. Selective spinal cooling of the deafferented region at thermoneutral Ta was followed by an immediate onset of shivering and an increase in heat production. Spinal heating resulted in an increase in wing temperature which served as an indication of vasodilatation, i.e., an activation of a heat loss mechanism. This is a confirmation of the assumption that the spinal temperature sensors are indeed located in the spinal cord and that the responses to experimental changes of spinal canal temperature are not mediated by extraspinal thermoreceptors. The results show clearly that the regulation of body temperature in pigeons at moderate thermal loads can be mediated by these spinal sensors alone. 5. Continued spinal cooling resulted in an increase in body temperature by about 2 degrees C and a subsequent regulation at this high level. This means that there must exist warm sensors in the non-deafferented cranial region.

Temperature-sensitive ascending neurons in the spinal cord of pigeons

Ascending neuronal activity in the lateral funiculus of the spinal cord of pigeons (spinalized at about C4, recordings at about C6) has been studied with regard to effects of temperature changes with a thermode in the vertebral canal between Th4 and C8. 2. Both warm-sensitive (35) and cold-sensitive (14) neurons were found. According to the change in impulse frequency during steplike thermal stimuli, different reaction types could be distinguished. Twenty-four warm-sensitive and seven cold-sensitive units showed a proportional frequency change without any dynamic reaction. Three other warm-sensitive neurons had an additional dynamic reaction (excitatory overshoot during warming, inhibition during cooling). Five warm-sensitive and three cold-sensitive units showed no static sensitivity but responded with outstanding dynamic frequency changes during rising or falling temperature. The activity of some neurons stopped suddenly above (4) or below (3) a critical temperature, which was always near the normal spinal temperature (about 41 degrees C). Altogether the reaction to rapid temperature changes was consistently greatest near the normal body temperature. 3. The mean static sensitivity of 17 warm-sensitive units was + 4.2 plus or minus 1.3 imp./sec. degrees C (mean value and s. d.) and that of three cold-sensitive ones minus 2.3 plus or minus 0.3 imp./sec. degrees C in the range 35 degrees minus 45 degrees C (vertebral canal temperature). The temperature coefficient (Q10) which was calculated for the same neurons showed great variations with mean values of about 5 for both warm- and cold-sensitive units.