Brain stem sites mediating specific and non-specific temperature effects on thermoregulation in the pekin duck

Understanding the role of temperature in seasonal timing: Effects on behavioural, physiological and molecular phenotypes

Organisms adapt to daily and seasonal environmental changes to maximise their metabolic and reproductive fitness. For seasonally breeding animals, photoperiod is considered the most robust cue to drive these changes. It, however, does not explain the interannual variations in different seasonal phenotypes. Several studies have repeatedly shown the influence of ambient temperature on the timing of different seasonal physiologies including the timing of migration, reproduction and its associated behaviours, etc. In the present review, we have discussed the effects of changes in ambient temperature on different seasonal events in endotherms with a focus on migratory birds as they have evolved to draw benefits from distinct but largely predictable seasonal patterns of natural resources. We have further discussed the physiological and molecular mechanisms by which temperature affects seasonal timings. The primary brain area involved in detecting temperature changes is the hypothalamic preoptic area. This area receives thermal inputs via sensory neurons in the peripheral ganglia that measure changes in thermoregulatory tissues such as the skin and spinal cord. For the input signals, several thermal sensory TRP (transient receptor potential ion channels) channels have been identified across different classes of vertebrates. These channels are activated at specific thermal ranges. Once perceived, this information should activate an effector function. However, the link between temperature sensation and the effector pathways is not properly understood yet. Here, we have summarised the available information that may help us understand how temperature information is translated into seasonal timing.

A thermoregulatory role of the medullary raphe in birds

The brainstem region medullary raphe modulates non-shivering and shivering thermogenesis and cutaneous vasomotion in rodents. Whether the same scenario occurs in the other endothermic group, i.e. birds, is still unknown. Therefore, we hypothesised that the medullary raphe modulates heat gain and loss thermoeffectors in birds. We investigated the effect of glutamatergic and GABAergic inhibitions in this specific region on body temperature (Tb), oxygen consumption (thermogenesis), ventilation (O2 supply in cold, thermal tachypnea in heat) and heat loss index (cutaneous vasomotion) in one-week-old chicken exposed to neutral (31°C), cold (26°C) and heat (36°C) conditions. Intra-medullary raphe antagonism of NMDA glutamate (AP5; 0.5, 5 mM) and GABAA (bicuculline; 0.05, 0.5 mM) receptors reduced Tb of chicks at 31°C and 26oC, due mainly to an O2 consumption decrease. AP5 transiently increased breathing frequency during cold exposure. At 31°C, heat loss index was higher in the bicuculline and AP5 groups (higher doses) than vehicle at the beginning of the Tb reduction. No treatment affected any variable tested at 36oC. The results suggest that glutamatergic and GABAergic excitatory influences on the medullary raphe of chicks modulate thermogenesis and glutamatergic stimulation prevents tachypnea, without having any role in warmth-defence responses. A double excitation influence on the medullary raphe may provide a protective neural mechanism for supporting thermogenesis during early life, when energy expenditure to support growth and homeothermy is high. This novel demonstration of a thermoregulatory role for the raphe in birds suggests a convergent brainstem neurochemical regulation of body temperature in endotherms.

Role of glucagon in the control of heat production in ducklings

Glucagon is known to be a central modulator of neural activity and a peripheral thermogenic effect. The purpose of this study was to better understand the role of glucagon in the control of heat production, shivering and particularly as a mediator of nonshivering thermogenesis (NST) in ducklings. In order to study the mechanism of NST, an intracerebroventricular (i.c.v.) injection of glucagon (10(-7) M) in to thermoneutral (TN), chronically glucagon treated (GT) and cold acclimatized (CA) ducklings exposed to acute cold (4 degrees C) or a thermoneutrality (25 degrees C), was performed. At 25 degrees C ambient temperature (Ta), the metabolic rate (MR) remained unchanged after glucagon injection. At 4 degrees C Ta i.c.v. glucagon injection, no significant change in MR was observed in GT and CA ducklings during 160 min of cold exposure, whereas there was 63% decrease in MR in (TN) ducklings (5.02 +/- 0.1 2 vs 7.91 +/- 0.1 4 W/kg(-1) p < 0.05). Shivering activity was completely suppressed in TN and GT ducklings after glucagon administration. The NST was estimated to be 3.26 W/kg. This findings suggest that glucagon administered into the brain has no thermogenic effect but could be involved in the central control of somatic motricity, and here we demonstrated for the first time, of our knowledge, that central glucagon have a role in the development of nonshivering thermogenesis during prolonged cold via an inhibition of shivering in birds.

Physiology of temperature regulation: comparative aspects

Few environmental factors have a larger influence on animal energetics than temperature, a fact that makes thermoregulation a very important process for survival. In general, endothermic species, i.e., mammals and birds, maintain a constant body temperature (Tb) in fluctuating environmental temperatures using autonomic and behavioural mechanisms. Most of the knowledge on thermoregulatory physiology has emerged from studies using mammalian species, particularly rats. However, studies with all vertebrate groups are essential for a more complete understanding of the mechanisms involved in the regulation of Tb. Ectothermic vertebrates-fish, amphibians and reptiles-thermoregulate essentially by behavioural mechanisms. With few exceptions, both endotherms and ectotherms develop fever (a regulated increase in Tb) in response to exogenous pyrogens, and regulated hypothermia (anapyrexia) in response to hypoxia. This review focuses on the mechanisms, particularly neuromediators and regions in the central nervous system, involved in thermoregulation in vertebrates, in conditions of euthermia, fever and anapyrexia.

Inhibition of shivering thermogenesis by centrally applied glucagon in muscovy ducklings

Glucagon has marked thermogenic and lipolytic effects in birds but could also be involved in the central modulation of neural activity on the basis of the recently discovered glucagon receptors in several areas of the brain in ducklings. The aim of this work was to investigate the possible role of these receptors in the modulation of thermogenic processes. Glucagon was infused into the lateral ventricle of the brain in ducklings after an acute cold exposure (4 degrees C, 2 h) or at thermoneutrality (25 degrees C). Electromyographic (EMG) data were simultaneously recorded with electrodes implanted in the gastrocnemius muscle. Glucagon (10(-4) M) was infused at a rate of 8 microliters/min. When acutely exposed to cold, ducklings increased their metabolic rate by shivering thermogenesis. A significant decrease in shivering activity was elicited after 5 min of glucagon infusion. After 16 +/- 2 min of glucagon infusion, shivering was completely inhibited, corresponding to a total dose of 36 +/- 4 micrograms/kg. The suppression of shivering was accompanied by a diminution of metabolic rate (5.3 +/- 0.3 vs. 8.5 +/- 0.2 W/kg, P < 0.05). The values of metabolic rate obtained at 4 degrees C after glucagon infusion were not significantly different from those measured at 25 degrees C before glucagon infusion (6.4 +/- 0.3 W/kg, P > 0.05). The infusion of the same dose of glucagon did not induce any change in EMG activity and resting metabolic rate at 25 degrees C. These findings suggest that glucagon infused into the brain has no thermogenic effect but could be involved in the central control of somatic motricity. Although the origin and the mechanisms of action of the endogenous peptide still remain unknown, glucagon might have a role in the development of non shivering thermogenesis during prolonged cold exposure via an inhibition of shivering in birds.

The effect of brain transection on the response to forced submergence in ducks

The effect of brain transection at two levels on cardiovascular responses to forced submergence has been investigated in ducks. Compared with intact ducks, neither decerebration nor brain stem transection at the rostral mesencephalic (RM) level had any effect on development of diving bradycardia, or heart rate at the end of two-min dives. Arterial blood pressure was maintained in brain transected ducks as well as in intact ducks. Furthermore, end-dive arterial blood gases and pH were also similar in intact and brain transected ducks confirming that the oxygen sparing cardiovascular adjustments, involving a massive increase in total peripheral resistance, were unimpaired by brain transection. In this respect, ducks with RM transections tolerated four-min dives. However, the increase in post-dive VE seen in intact and decerebrated ducks was prevented by RM transection. We conclude that control of the circulatory response to diving resides in the lower brainstem, is reflexogenic in nature, and does not depend on the cognitive perception of 'fearful' stimuli.

Convergence of thermal signals on the reticulospinal neurons in the midbrain, pons and medulla oblongata

A total of 670 reticulospinal (RfS) and non-RfS neurons in the mesencephalic, pontile and medullary reticular formation (mRf, pRf and mdRf) were studied for the responsiveness to changes in temperatures of local brain sites, preoptic and anterior hypothalamus (PO/AH) and skin in the urethane anesthetized rat. Local thermoresponsiveness was found in 49.6% of 139 mRf neurons, 61.9% of 160 pRf neurons and 75.4% of 126 mdRf neurons. While the ventromedial region of pRf and mdRf contained predominantly warm-responsive neurons (54.8% and 62.5%), cold-responsive neurons were much more frequently found in the mRf (33.8%) and the dorsolateral region of pRf (41.9%) and mdRf (50.0%). Responsiveness to hypothalamic temperature and/or skin temperature was observed in about 40-74% of Rf neurons. Higher incidence of responsiveness to remote temperatures was found among locally thermoresponsive neurons than among locally thermounresponsive neurons in all three areas. Particularly, there was a high degree of convergence of 'cold' signals from local and remote sites on the RfS neurons in the mRf and the dorsolateral pRf and mdRf. Microinjections of procaine and glutamate into these regions decreased and increased the cold-induced increase in EMG activity and shivering without any correlated changes in cardiovascular and respiratory parameters and pilomotor activity. The results suggest that RfS and non-RfS neurons in the mRf and the dorsolateral pRf and mdRf are involved in the control of thermoregulatory muscle tone and shivering.

Thermally-induced activities of the mesencephalic reticulospinal and rubrospinal neurons in the rat

Unit activities of 226 midbrain reticulospinal (mRfS) and non-mRfS neurons and 238 rubrospinal (RbS) and non-RbS neurons were investigated during changes in temperatures of midbrain (Tmb), preoptic and anterior hypothalamus (Thyp) and skin (Ts) in the urethane-anesthetized rat. Responsiveness to Tmb, Thyp and Ts were found in 43.5%, 41.6% and 51.5% of neurons of midbrain reticular formation (mRf), and in 35.2%, 32.7% and 17.6% of neurons of red nucleus (Rb). Higher incidence of responsiveness to remote temperatures was found among Tmb responsive neurons than Tmb unresponsive neurons in both mRf and Rb. The mRf contains significantly greater numbers of neurons having such multiple thermal responsiveness and also of neurons which were activated by falls in temperatures (cold-responsive neurons) than the Rb. These characteristics were more conspicuously seen among mRfS neurons, showing a high degree of convergence of cold signals from different sites of body. On the other hand, RbS neurons did not differ from non-RbS neurons regarding thermal characteristics and showed no particular combinations of responsiveness to temperatures of different sites. Microinjection of procaine and glutamate into the mRf just dorsolateral to the Rb, but not into the Rb, decreased and increased cold-induced increase in EMG activity and shivering without changes in cardiovascular and respiratory parameters and pilomotor activity. The results suggest that mRfS neurons are involved in the control of thermoregulatory muscle tone and shivering.

Appropriate and inappropriate hypothalamic cold thermosensitivity in Willow ptarmigan

Hypothalamic thermosensitivity has been investigated in conscious Willow Ptarmigan (Lagopus lagopus lagopus) provided with chronically implanted hypothalamic perfusion thermodes. The birds were exposed to either cold (Ta - 10 degrees C) or warm (Ta + 25 degrees C) ambient conditions while hypothalamic temperature (Thy) was clamped for periods of 20 min at different set levels between 28 degrees C and 43 degrees C. The responses of the animals to hypothalamic thermal stimulation were classified by comparing them with those normally found in mammals. At Ta - 10 degrees C hypothalamic heating inhibited ongoing shivering, causing a fall in body-core temperature (Tc) (appropriate mammalian-like response). Strong levels of hypothalamic cooling (Thy less than 34.0 degrees C) also caused a fall in Tc due to inhibition of shivering (inappropriate mammalian-like response). However, weaker levels of hypothalamic cooling (Thy 34-36 degrees C), facilitated ongoing shivering, resulting in small increases in Tc (appropriate mammalian-like response). At Ta + 25 degrees C hypothalamic heating facilitated ongoing panting while weak (Thy 38 degrees C) levels of hypothalamic cooling inhibited ongoing panting (both mammalian-like responses). The observation of a weak mammalian-like cold hypothalamic thermosensitivity in Willow Ptarmigan indicates that these birds possess some specific cold thermosensors in the hypothalamic region. This finding suggests that hypothalamic temperature dependence in birds and mammals is fundamentally similar.

Comparison between hypothalamic thermoresponsive neurons from duck and rat slices

Neuronal thermoresponsiveness in the preoptic and anterior hypothalamic (PO/AH) region of a bird and a mammal were compared in vitro by recording the activity of 48 units from ducks and 37 units from rats in tissue slices subjected to temperature changes. Warm-responsive units were found in similar proportions in duck and rat PO/AH slices. The average degrees of thermoresponsiveness did not differ between the two species. Neurons exhibiting thresholds of warm responsiveness had higher threshold temperatures (2P less than 0.01) in duck (38.8 +/- 0.2 degrees C) than in rat (37.4 +/- 0.4 degrees C) slices (means +/- standard errors). Firing rates at threshold temperatures and thermoresponsiveness below and above thresholds did not differ between ducks and rats. During synaptic blockade in a Ca2+-free/high-Mg2+ medium, warm-responsiveness was retained in 9 out of 13 units in duck slices and in 8 out of 13 units in rat slices. In two instances in ducks and in one case in rats positive temperature coefficients were converted into negative temperature coefficients. Among two cold-responsive units tested in duck slices one retained its cold-responsiveness. It is concluded that in vitro evaluation of PO/AH neuronal thermoresponsiveness in a bird and a mammal has not revealed differences at the single unit level which might explain the diverging contributions of the avian and mammalian hypothalamus to deep body temperature perception.

A comparison between total body thermosensitivity and local thermosensitivity in mammals and birds

We have investigated how total body thermosensitivity in various mammalian and avian species (mouse, rat, golden hamster, guinea pig, rabbit, dog, goat, pigeon, duck, goose) is related to their respective local thermosensitivities in the hypothalamus, spinal cord and skin. Local and total thermosensitivities were determined by measuring the relationship between the response of one thermoregulatory effector, metabolic heat production, and the appropriate temperature. Local cooling was performed with chronically implanted, water perfused thermodes, and local thermosensitivities were estimated by relating the maximum activation of metabolic heat production to the induced decreases in local temperature. Total body cooling was achieved by means of chronically implanted intravascular heat exchangers or with thermodes inserted into the lower intestinal tract, and total body thermosensitivity was assessed by relating the rise in metabolic heat production to the induced fall in core temperature. These analyses plus previous estimations derived from the literature show total body thermosensitivity in the different species to range from -4.0 to -12.0 W X kg-1 . C-1. We also measured rabbit spinal cord thermosensitivity and guinea pig hypothalamic and spinal cord thermosensitivity; values for local thermosensitivity in other species were derived from the literature. In all species, local thermosensitivities determined as cold sensitivities in the described way were smaller than the corresponding total body core sensitivities. We conclude that thermosensitive structures outside of the investigated thermosensitive areas contribute a major input to the controller of body temperature, particularly in avian species in which hypothalamic thermosensitivity is lacking.(ABSTRACT TRUNCATED AT 250 WORDS)

Properties of high Q10 units in the conscious duck's hypothalamus responsive to changes of core temperature

1. Five Pekin ducks were chronically implanted with a device allowing thermal stimulation of the hypothalamus and simultaneous recording of hypothalamic single unit activity on the conscious animals in repeated experiments. In addition, the core temperature of the animals could be lowered by means of a thermode tube which was placed in the colon and was perfused by a cold solution. Hypothalamic temperature was measured in the centre of the hypothalamic thermode array; core temperature was measured in the axillary pit.2. Each unit was tested in two periods of hypothalamic ramp cooling, one was performed at normal core temperature, and the other at a lowered core temperature during sustained intestinal cooling.3. Among forty-six neurones exhibiting a local Q(10) > 2 of their discharge rate, intestinal cooling was found to activate 26% (fall feed-back units), to inhibit 44% (rise feed-back units), and not to affect 30% (non-reactive units). The local Q(10) values of the fall feed-back units were, on average, significantly higher than those of the rise feed-back units.4. By multiple linear regression analysis the thermal coefficients (impulses/sec. degrees C) relating unit discharge to hypothalamic (local) and to core (remote) temperature changes were evaluated. The fall feed-back units exhibited average local temperature coefficients of 0.79+/-0.11 and remote coefficients of -2.75+/-0.56 (means+/-s.e. of mean); the corresponding coefficients of the rise feed-back units were determined as 0.44+/-0.08 and +2.33+/-0.41.5. The results of this study support the hypothesis that the activity of hypothalamic neurones conveying extrahypothalamic cold signals is depressed more by hypothalamic cooling than that of the neurons conveying extrahypothalamic warm signals. This would explain the paradoxical effects of hypothalamic cooling on thermoregulatory effector activity in birds.

Effects of CNS temperature on generation and transmission of temperature signals in homeotherms. A common concept for mammalian and avian thermoregulation

Neurophysiological studies on avian hypothalamic thermosensitivity have presented evidence for a higher Q10 of cold than of warm signal transmission in the CNS of birds. An identical temperature dependence of central cold and warm signal transmission in mammals is suggested by considerations on the phylogeny of temperature regulation. By taking into account the experimental evidence for the existence of thermosensory afferents in the CNS of mammals and birds, being differently developed in the various sections of the neural axis and exerting quantitatively different influences on the various thermoregulatory effectors, a common concept of homeothermic thermoregulation is proposed resting on the same basic assumptions for mammals and birds. The great diversity of negative as well as positive feedback effects of CNS temperature displacements on homeothermic thermoregulation, which is particularly expressed in avian autonomic and behavioral thermoregulation and, further, certain pathophysiological conditions of disturbed thermoregulation could be accounted for by assuming quantitatively different contributions of the central thermosensory inputs of thermoregulatory effector control, but maintaining the Q10 values of hypothalamic warn and cold signal transmission constant. The proposed model, while basically additive in its mathematical design, meets a number of properties described by multiplicative models of thermoregulation. In additionally generalizes these models of predicting that changes of hypothalamic temperature modify the sensitivities with which any thermoregulatory effector responds to any thermosensory input.