Effects of hypothalamic lesions on body temperature in the chicken

Oral Administration of L-Citrulline Changes Brain Free Amino Acid and Monoamine Metabolism in Heat-Exposed Broiler Chickens

High ambient temperatures (HT) in summer are becoming more severe due to global warming, leading to severe adverse effects on poultry production. Recently, we have reported that oral administration of L-citrulline (L-Cit) can minimize hyperthermia in chickens under HT. However, whether oral L-Cit can enter the brain, the center for thermoregulation, has not been studied. We investigated the effects of oral administration of L-Cit on free amino acids and monoamines in the diencephalon region of the brain of heat-exposed broilers. Broilers were treated with L-Cit (40 mmol/20 ml/bird), then moved to a chamber at HT (30 ± 1°C) or to a thermoneutral temperature (CT: 22 ± 1°C) chamber for 2 h. Control groups were given methyl cellulose solution and placed in the CT or HT chambers. After 2 h of exposure to HT, there were increased brain concentrations of Cit in comparison with concentrations in broilers exposed to CT, whereas brain ornithine (Orn) concentrations were decreased, and arginine (Arg) concentrations were not changed. Interestingly, oral administration of L-Cit increased brain concentration of Cit, Arg, and Orn under both CT and HT. Tryptophan and its metabolite, serotonin (5-HT) concentrations were lower in the brain under HT than under CT. HT did not change brain concentrations of tyrosine, but dopamine (DA, a metabolite of tyrosine) concentrations decreased, and methoxyhydroxyphenylglycol (MHPG, a metabolite of DA) concentrations increased in comparison with CT. Oral administration of L-Cit decreased brain concentrations of both tryptophan and tyrosine under CT and HT without changing 5-HT; however, DA levels declined under HT. Moreover, MHPG concentrations increased. In conclusion, these results suggest that metabolism of amino acids and metabolism of DA can be enhanced in the brain by oral administration of L-Cit. Metabolic changes in the brain in response to oral administration of L-Cit may influence the thermoregulatory center in the brain, leading to a reduction in body temperature and conferring thermotolerance in heat-exposed broiler chickens.

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.

Effects of hypothalamic lesions on temperature regulation in pigeons

The effect of hypothalamic lesions on temperature regulation was studied in pigeons by recording deep and skin temperatures, shivering, panting and oxygen consumption. Thresholds of shivering and panting were assessed before and after lesions. Survival was 100% when lesions were done in awake animals in a cool environment. Lesions anterior to the anterior commissure which included the preoptic area resulted in an increased threshold of panting or a lack of panting response at body temperatures up to 44.5 degrees C. Shivering response was unchanged in these animals. Lesions posterior to the anterior commissure were followed by a lack of shivering response with no change in panting threshold. This points to a segregation of the hypothalamic structures controlling heat loss and heat gain in the pigeon.

Classification of the neurons in fowl hypothalamic nuclei based on their dendritic patterns

1. Dendroarchitectonic analysis of the hypothalamic nuclei in the fowl was investigated by the Golgi-Cox, Nissl and silver impregnation techniques. 2. All hypothalamic nuclei appeared to consist of leptodendritic neurons constituting a variety within the isodendritic family. They showed triangular, spindle and round shapes and had a few long, relatively straight, fine dendrites bearing very fine dendritic spines. 3. Neurons in the nucleus preopticus medialis et lateralis showed a bipolar spindle-shape and had stout, relatively long primary dendrites. 4. Those in the nucleus hypothalamicus rostralis medialis et lateralis were characterised by bipolar, spindle-shaped and multipolar leptodendritic neurons and by short fine primary dendrites bifurcating to fine longer secondary segments. 5. Neurons in the nucleus hypothalamicus caudalis medialis et lateralis and nucleus hypothalamicus inferior were of the multipolar leptodendritic type and had thick, long, unbranching dendrites emerging directly from the cell body. 6. The fowl hypothalamus is dendroarchitectonically divided into preoptic, rostral and caudal hypothalamic regions; the borderlines separating them are the commissura rostralis and the region between the nucleus hypothalamicus rostralis medialis and nucleus hypothalamicus caudalis medialis, respectively.

Effects of altering rostral brain stem temperature on temperature regulation in the Adelie penguin, Pygoscelis adeliae

In 4 Adelie penguins, thermodes were implanted in the rostral brain stem. Two animals were additionally equipped with spinal canal thermodes. At thermoneutral (+8 to +16 degrees C) and cold (-18 to -22 degrees C) ambient conditions, the effects of hypothalamic 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.- Heating the rostral brain stem induced heat defence responses: Heat production was reduced in the cold and skin vasodilatation was evoked at thermoneutral ambient conditions. As a rule, core temperature fell during rostral brain stem heating.- Cooling the rostral brain stem did not induce clear-cut cold defence responses. On the contrary, strong cooling at thermo-neutral ambient conditions induced vasodilation in the skin. In the cold, even slight degrees of rostral brain stem cooling decreased metabolic heat production. As a rule, core temperature fell when the rostral brain stem was cooled.- It is concluded from the results that thermosensitive structures in the stimulated section of the rostral brain stem of the Adelie penguin contribute to the central temperature signal input in the range of normal to elevated core temperatures. These hypothalamic warm signals appear to be at least as effective as spinal warm signals in controlling skin blood flow and metabolic heat production. The inhibition of ongoing thermoregulatory effector activity by rostral brain stem cooling suggests positive temperature coefficients of the integrative and/or efferent neurons in the hypothalamic temperature regulation center of the Adelie penguin.

Effects of noradrenaline infused into the chick hypothalamus on thermoregulation below thermoneutrality

1. Hypothermia induced by infusion of noradrenaline into the hypothalamus of 2-3 week old chicks, within their thermoneutral range, was considerably potentiated by lowering ambient temperature. 2. Noradrenaline-induced hypothermia was associated with reduced carbon dioxide elimination and reduced blood lactate concentrations whereas leg temperature, electromyographic activity, plasma NEFA and plasma glucose concentrations were increased. 3. Mechanisms postulated to explain the phenomenon are inhibitory and facilitatory effects of noradrenaline on some, but not all, heat production and heat loss mechanisms. Increased electromyographic activity after intrahypothalamic noradrenaline is assumed to be due to lack of effect of noradrenaline on spinal thermosensitive centres; increased plasma NEFA concentration may be due to inhibition of NEFA utilization.

The role of hypothalamic temperature in the control of panting in the chicken exposed to heat

1. In unanaesthetized chickens, the temperatures of the hypothalamus, colon and skin have been recorded in relation to the onset and cessation of thermally induced panting.2. During control conditions, hypothalamic temperature showed fluctuations associated with arousal and movement. It was lower than colonic temperature by about 0.9 degrees C but this difference generally decreased during exposure to heat.3. When the birds were exposed abruptly to 40 degrees C, or to ambient temperatures increasing gradually from 20 to 52 degrees C, there was a delay of 15-65 min and marked increases in both peripheral and central body temperatures before panting commenced.4. Infra-red irradiation of the thorax and abdomen caused vasodilatation in the comb and toe before detectable increases in the deep body temperatures. Increasing the colonic temperature by up to 2 degrees C did not cause panting until hypothalamic temperature was also raised. This inhibitory effect of normal hypothalamic temperature was enhanced by low ambient temperature.5. Infra-red irradiation of the head increased hypothalamic temperature by up to 3.5 degrees C and caused vasodilatation in the toe without changes in colonic temperature. Panting, however, did not occur so long as colonic temperature was within the normal range. The inhibitory effect of normal colonic temperature was again enhanced by low ambient temperature.6. In anaesthetized chickens, selective heating of the head and body caused panting only after increases in both the hypothalamic and colonic temperatures.7. Repeated exposure of birds to 40 degrees C did not reduce the time delay before panting started.8. It is concluded that panting in the fowl requires an increase in some extracranial deep body temperature as well as in that of the hypothalamus.

Effects of catecholamines infused into the brain of young chickens

1. (-)-Noradrenaline, (-)-alpha-methylnoradrenaline and (-)-isoprenaline were infused into various brain regions of 12-21 day chicks. When infused into the hypothalamic area, but not the cerebral hemisphere or posterior mesencephalon, these amines produced behavioural sleep, lowered temperature and blood pressure and reduced oxygen consumption; electrocortical sleep activity usually ensued but this was not marked and frequently dissociation between electrocortical activity and behaviour occurred. After monoamine oxidase inhibition, which prolonged the action of noradrenaline, dopamine had similar effects.2. The effects of the catecholamines were prevented or substantially reduced by pretreatment with phenoxybenzamine given intravenously or into the hypothalamus but not by intravenous injection of propranolol. However, intrahypothalamic infusion of propranolol prevented the temperature, but not the behavioural effects of noradrenaline. The implications of this are discussed.3. That the effects were similar but more intense, apart from electrocortical changes, and of longer duration than those seen after intravenous injection of catecholamines suggests that in young chicks these amines penetrate from the blood into the brain and elicit their effects through a localized region, presumably the hypothalamus.