Antagonism by indomethacin of neurogenic hyperthermia produced by unilateral puncture of the anterior hypothalamic/preoptic region

1. In unanaesthetized rats, restrained at an ambient temperature of 24 degrees C, the anterior hypothalamic/preoptic (AH/PO) region was lesioned unilaterally by acute mechanical puncture.2. In control (no pre-treatment) rats, unilateral AH/PO puncture produced a neurogenic hyperthermia which began immediately, reached its peak magnitude (mean peak magnitude = +2.3 degrees C) within 60-90 min and persisted usually for 8-16 hr. At defervescence, core temperature fell to a level near that of the pre-lesioning base line.3. The prostaglandin synthesis inhibitor, indomethacin, administered I.P. at doses of 5 and 15 mg/kg 1 hr before puncture of the AH/PO region, attenuated the lesion-induced hyperthermia in a dose dependent fashion. The higher dose reduced peak magnitude by 80% and the 6 hr Fever Index by 88%. The vehicle used to dissolve the indomethacin (60% DMSO/40% saline) did not significantly attenuate the hyperthermia.4. In rats that were hyperthermic after AH/PO damage, indomethacin (10-15 mg/kg I.P.) caused core temperature to fall promptly to near the prelesion base line. Reversal occurred whether the indomethacin was injected while core temperature was still rising or late in the plateau phase of the hyperthermia.5. It is suggested that the neurogenic hyperthermia elicited by unilateral lesioning of the AH/PO region was mediated by prostaglandins released from injured tissue and possibly from extravasated blood. Evidence is cited indicating that the most likely sites of action of the released prostaglandins are the surviving portion of the AH/PO region on the punctured side and the intact contralateral AH/PO region.     

Opioid binding in the rostral hypothalamus is reduced following lesion of the ventral noradrenergic tract in female rats

Summary Experiments were undertaken to establish whether opioid receptors exert a direct presynaptic influence on noradrenergic (NA) terminals in the preoptic/ anterior hypothalamus (PO/AH) of the female rat. Thus, opioid binding studies were performed in rats with lesions of the ventral NA tract (VNAT; the main NA projection to the hypothalamus) to assess whether a loss of NA terminals may also result in a decrease in opioid binding in the PO/AH. In the first experiment, unilateral electrolytic lesions of the VNAT caused a significant reduction in both the NA content and specific [³H]-diprenorphine binding to membrane homogenates in the ipsilateral PO/AH. In the second experiment bilateral 6-hydroxydopamine (6-OHDA)-induced lesions of the VNAT caused a significant reduction in NA levels in the PO/AH as well as significant decreases in the density of [³H]-diprenorphine binding to tissue sections of the PO/AH when compared to control animals. These results strongly suggest that the NA input to the PO/AH is regulated by endogenous opioid peptides, and provide an anatomical substrate to explain opioid-NA interactions in the control of gonadotrophin releasing hormone (GnRH) and gonadotrophin secretion.     

Action of preoptic injections of beta-endorphin on temperature regulation in rabbits

Male New Zealand White rabbits, Oryctolagus cuniculus, were stereotaxically implanted with a guide tube above the preoptic/anterior hypothalamus area (PO/AH) for the injection of the opioid peptide, beta-endorphin (beta-E), naloxone, sodium salicylate, or physiological saline. PO/AH and ear temperature, oxygen consumption, and evaporative heat loss (EHL) were recorded in free-moving rabbits before and after injection of saline followed with beta-E, naloxone, or sodium salicylate at ambient temperatures (Ta) of 2-31 degrees C. A 5-micrograms injection of beta-E promoted a rapid reduction in ear temperature followed by a prolonged rise in PO/AH (body) temperature. Preinjection with an isovolumetric amount of the opiate antagonist, naloxone, inhibited the thermoregulatory effects of beta-E. The beta-E-induced rise in body temperature was directly correlated with Ta. beta-E had no effect on oxygen consumption at Ta's of 5 and 27 degrees C. When measured 30 min after injection, beta-E demonstrated a significant inhibition of EHL at Ta's of 27 and 31 but not 5 degrees C. The beta-E-induced rise in body temperature was not antagonized with preinjections of sodium salicylate in the PO/AH. These data indicate that beta-E promotes a regulated increase in body temperature. The mechanism of activation appears to be distinct from that of an infectious fever.     

Characterization of midbrain component of the trigger for arousal from hibernation.

The ability of the midbrain reticular formation (MRF) to trigger arousal from hibernation and to change body temperature (Tb) during euthermia was tested in golden-mantled ground squirrels (Citellus lateralis). During hibernation (ambient temperature 5-6 degrees C) microinjections of cholinomimetic compounds (acetylcholine and carbachol) triggered full arousal or produced transient increases in Tb in 15 out of 19 tests; MRF microinjections of norepinephrine (NE) and 5-hydroxytryptamine (5-HT), at the same concentrations that produced full arousal when microinjected into the preoptic/anterior hypothalamus (PO/AH) area in previous experiments, has no effect on Tb in 13 out of 16 tests. In experiments on euthermic ground squirrels tested at an ambient temperature of 25 degrees C, MRF microinjections of acetylcholine raised Tb, whereas the monoamines had no significant effects. These results show that cholinoceptive neurons in the MRF can trigger arousal from hibernation and increase Tb during euthermia. The failure of NE and 5-HT to produce any significant effects at the same doses that were reported to be effective in the PO/AH indicates that the functional organization of the MRF portion of the arousal mechanism is different from that reported for the PO/AH.     

The role of preoptic area and anterior hypothalamus and median raphe nucleus on thermoregulatory system in freely moving rats

To clarify the role of the preoptic area and anterior hypothalamus (PO/AH) on thermoregulatory system and the effects of serotonergic innervation from the median raphe nucleus (MRN) on body temperature (Tb), we perfused tetrodotoxin (TTX) solution into the PO/AH or MRN by using a microdialysis technique at different ambient temperatures (5, 23 and 35 degrees C) in freely moving rats. Tb was continuously monitored by using a telemetry system. In the MRN, perfusion of TTX solution induced significant hypothermia in the normal environment, a greater decrease in Tb during cold exposure and had no effect on Tb during heat exposure. In the PO/AH, perfusion of TTX solution induced significant hyperthermia in normal environment, a greater increase in Tb during heat exposure and had no effect on Tb during cold exposure. Our results indicate that the PO/AH regulates mainly heat loss or inhibits the loci regulating heat production. Furthermore, heat production appears to be regulated by other loci receiving serotonergic innervation from the MRN.     

Thermoregulation in the rabbit following intracranial injection of norepinephrine.

The release of norepinephrine (NE) from nerve terminals in the anterior hypothalamic/preoptic area (AH/POA) of the rabbit may serve to raise body temperature. To further examine the putative neurotransmitter role of NE, bilateral microinjections of 5 or 10 mug NE were made into or near the AH/POA of 44 conscious rabbits exposed to an ambient temperature of 15 degrees C. Microinjections into the AH/POS did not cause fever; they either had no influence on thermoregulation or rapidly induced ear vasocilation and increased ear temperature accompanied by slight falls in rectal temperature. The latter averaged 0.32 degrees C (range: 0.16-0.45 degrees C) in 18 rabbits in which the effects were prominent. In contrast, the injection of 100 or 250 mug NE into the lateral cerebral ventricles of conscious rabbits in the 15 degrees C environment caused mean fevers of 0.62 +/- 0.0, and 1.04 +/- 0.14 degrees C (+/- SE, n equals 6), respectively, within 70 min. The febrile response to intraventricular injection of NE may be due to an action of the drug at a site other than the AH/POA. Alternatively, the response may depend critically on the particular distribution of NE that results from its diffusion from the third ventricle into the AH/POA.     

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<0.01) in duck (38.8±0.2° C) than in rat (37.4±0.4° 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.     

The effect of noradrenaline, injected into the hypothalamus, on thermoregulation in the cat.

1. Noradrenaline (NA) was microinjected into the anterior hypothalamic/preoptic area(AH/POA) of unanaesthetized cats held at ambient temperatures of 10, 22 or 35 degrees C. Loci in which injection of NA caused body temperature changes were also found to be sensitive to the febrile action of PGE1. 2. At all ambient temperatures, NA caused a dose-dependent fall in body temperature. However the mechanisms by which these temperature changes were brought about varied at different ambient temperatures. In cats maintained at the higher ambient temperature, NA activated heat loss mechanisms whereas in the cats maintained in the 10degrees C environment, the major effect of NA injection was an inhibition of heat conservation and heat production mechanisms. 3. We conclude that NA acts in cats not only as an inhibitor of heat conservation and production, but also acts in an excitatory manner on an active heat loss pathway within the AH/POA.     

Hypothalamic 11,12-epoxyeicosatrienoic acid attenuates fever induced by central interleukin-1beta in the rat

Inhibitors of cytochrome P-450 augment fever in rats and mice, indicating that the metabolite of the enzyme is candidate of endogenous antipyretic. Cytochrome P-450 of arachidonic acid cascade leads to the formation of regioisomeric 5,6-, 8,9- 11,12- and 14,15-epoxyeicosatrienoic acid (EET). Various isomers of EET were administrated into the preoptic area and anterior hypothalamus (PO/AH) to test their influence on fever induced by interleukin-1β (IL-1β) administrated into the PO/AH in conscious rats. The IL-β-induced fever was attenuated in the 11,12-EET-pretreated rats, although 5,6-, 8,9- and 14,15-EET did not affect the fever. Intra-PO/AH injection of 11,12-EET did not alter normal body temperature. The results suggest that 11,12-EET acts in the hypothalamus as an endogenous antipyretic.     

Effects of dopamine-depleting brain lesions on experimental hyperphagia in rats

Dopamine (DA)-depleting brain lesions of various sizes were produced in rats either by intracerebroventricular injections of 6-hydroxydopamine (6-HDA) or by electrolytic lesions of the lateral hypothalamic (LH) area. Among 30 animals that became aphagic and adipsic for at least four days after large LH or 6-HDA-induced brain lesions, only three developed hyperphagia after electrolytic lesions of the ventromedial hypothalamus (VMH) or daily injections of long-acting protamine-zinc insulin (PZI). In 20 rats with smaller LH or 6-HDA-induced lesions, which had not shown marked initial behavioral dysfunctions, only three gained as much weight after VMH lesions as the control animals. Similarly, 6 of 10 rats with smaller LH lesions could not tolerate a 15-day series of PZI treatments, although 14 of 17 rats with smaller 6-HDA-induced lesions increased their food intake and gained weight during the PZI treatments as did control animals. These results indicate that hypothalamic hyperphagia can be blocked by DA-depleting brain lesions that neither produce an initial period of aphagia and adipsia nor involve hypothalamic tissue. They further indicate that even small LH lesions may prevent the development of hyperphagia elicited by PZI, whereas only very large 6-HDA-induced lesions consistently have this effect.     

Hypothalamic warm-sensitive neurons possess a tetrodotoxin-sensitive sodium channel with a high Q10

We have investigated the ionic current responses to temperature of dissociated cells from the preoptic and anterior hypothalamus (PO/AH) of rat, using the 'whole-cell' configuration. The majority of the recorded neurons showed a linear increase in a non-inactivating inward current during warming (30-40 degrees C), and the Q10 was about 2. However, about 24% of PO/AH neurons were markedly sensitive to warming and the increase in non-inactivating inward current to a rise in temperature in the hyperthermic range (35-40 degrees C) had a high Q10 (4.3-7.0). This increase in current in the hyperthermic range was reversibly blocked by tetrodotoxin (TTX). The inward current in neurons with a Q10 of 2 was not affected by TTX. The results show that some neurons in the PO/AH possess a non-inactivating sodium channel that is highly temperature-sensitive in the hyperthermic range. These neurons are presumably the 'primary' warm-sensitive neurons.     

Locomotor response to changes in food intake and ambient temperature in rats with hypothalamic lesions

The changes in wheel running activity in response to changes in the availability of food and to changes in ambient temperature were studied in rats made hypoactive by the placement of either ventrolateral or ventromedial electrolytic hypothalamic lesions. Even though wheel running activity was decreased by approximately 90% and 60% by ventrolateral and ventromedial lesions, respectively, the relative changes in this activity that occur when the feeding regimen is changed sequentially from restricted food intake to ad lib food intake to complete food deprivation, and when ambient temperature is decreased from 24.5°C to 19°C were not impaired. These data suggest that, although ventrolateral and ventromedial hypothalamic lesions decrease spontaneous locomotor activity, they do not impair the locomotor response to changes in environmental stimuli.     

Effect of high pressure on metabolic response to cold in rats

The ability of rats to regulate body temperature in a cool helium-oxygen (heliox) atmosphere at high pressure was studied. Thermoregulatory ability in seven awake rats was assessed in a pressure chamber at 1.5, 21 and 41 ATA by monitoring body temperature and oxygen consumption during a gradual reduction of ambient temperature. Body temperature was measured by a radiotelemetry transmitter implanted into the intraperitoneal cavity. The thermal conductance of the rats increased from 0.37 to 0.75 W kg-1 degrees C-1 as air was substituted by heliox and increased further to 1.47 and 1.81 W kg-1 degrees C-1 as the pressure was increased to 21 and 41 ATA, respectively. At high pressure the rats were able to maintain body temperature until the metabolic rate reached a value of 21.9 +/- 2.0 W kg-1, which corresponds to peak metabolic rate. This occurred at an ambient temperature of approximately 21 degrees C. Below this ambient temperature the rats did not manage to maintain body temperature. The results show that the thermal effector mechanisms of rats respond adequately to a slowly decreasing ambient temperature in hyperbaric heliox environments.     

Influence of brain catecholamines on the development of fatigue in exercising rats in the heat

The purpose of the present study was to identify the effects of an acute injection of a dual dopamine (DA)/noradrenaline (NA) reuptake inhibitor (bupropion) on exercise performance, thermoregulation and neurotransmitters in the preoptic area and anterior hypothalamus (PO/AH) of the rat during exercise in the heat. Body core temperature (T(core)), brain temperature (T(brain)) and tail skin temperature (T(tail)) were measured. A microdialysis probe was inserted in the PO/AH, and samples for measurement of extracellular DA, NA and serotonin (5-HT) levels were collected. Rats received either bupropion (17 mg kg(-1); hot-BUP) or saline (1 ml kg(-1); hot) 20 min before the start of exercise and ran at a speed of 26 m min(-1) until exhaustion in a warm environment (30 degrees C). Rats also ran until exhaustion in a cool environment (18 degrees C; cool). Running time to exhaustion was significantly influenced by the ambient temperature, and it was increased by bupropion in the heat (cool, 143.6 +/- 21 min; hot, 65.8 +/- 13 min; hot-BUP, 86.3 +/- 7.2 min). T(core) and T(brain) at exhaustion were significantly higher in the bupropion group compared to the cool and hot groups, respectively. T(tail) measured at exhaustion was not significantly different between the two hot conditions. Extracellular concentrations of DA and NA in the PO/AH increased during exercise, and was significantly higher in the bupropion than in cool and hot groups (P < 0.05). No differences were observed between groups for 5-HT levels. These results suggest that DA and NA in the PO/AH might be responsible for the increase in exercise performance and T(core) and T(brain) in the bupropion group in hyperthermia. Moreover, these results support previous findings in humans that acute bupropion ingestion increases T(core) during exercise in the heat, indicating the possibility of an important role for DA and NA in thermoregulation.     

Serotonin release in the preoptic area and anterior hypothalamus is not involved in thermoregulation during low-intensity exercise in a warm environment

The aim of the present study was to assess the response of serotonin (5-hydroxytryptamine; 5-HT) in the preoptic area and anterior hypothalamus (PO/AH) to thermoregulation during exercise in a warm environment. In addition, it was investigated whether a rapid increase in extracellular 5-HT levels in the PO/AH modifies the thermoregulatory response under the same exercise conditions. Rats were made to run for 120 min at 10 m min⁻¹ on a treadmill at the ambient temperature of 30 °C. Body core temperature (T_core) was monitored using a biotelemetry system, and tail skin temperature (T_tail) was simultaneously measured as an index of heat loss response. Microdialysis in combination with HPLC was used to measure concentrations of monoamines in the PO/AH. Both T_core and T_tail increased during the first 20 min of exercise and remained stable until the end of the exercise period. Low-intensity exercise did not induce any changes in 5-HT release in the PO/AH, although the levels of norepinephrine and dopamine were increased. Moreover, increased extracellular 5-HT by local perfusion of 1 μM citalopram (selective 5-HT reuptake inhibitor; SSRI) in the PO/AH had no effect on the thermoregulatory response during acute low-intensity exercise in a warm environment. These results suggest that enhanced release of only 5-HT in the PO/AH may not intervene thermoregulation during exercise in a warm environment.     

Effects of lesions in central thermosensitive areas on thermoregulatory responses in rat

Behavioral and autonomic responses elicited by ambient heat were tested before and after lesioning five thermosensitive areas previously localized in the rat brain. Lesions in the PO/AH area, where local warming elicits prone body extension, markedly reduced or abolished extension but had no effect on locomotion, grooming, or tail vasodilation. Lesions in the posterior hypothalamus, where local warming induces grooming, and lesions in the septum, medial midbrain, and dorsomedial medulla, where local warming evokes locomotion, had no significant effects on thermoregulatory responses except for reduced body extension following midbrain lesions, which may have interrupted the efferent path from PO/AH thermosensors. It was concluded that extension is predominantly dependent on PO/AH thermosensors and/or integrative interneurons, while locomotion, grooming, and vasodilation are controlled by input from peripheral and central sensors. The present and earlier data from the rat are less consistent with a relatively localized general integrating mechanism producing a unitary output signal to thermoregulatory effectors than with a spatially distributed network of specific paths from sensors to separate response mechanisms, where summation of inputs may occur.     

A reevaluation of the role of the lateral hypothalamus in behavioral temperature regulation

Rats exposed to the cold (-9 degrees C) were trained to depress a lever to obtain pulses of infrared heat. Half of the animals received bilateral electrolytic lesions of the lateral hypothalamus and half were sham operated. Lesioned and control animals performed similarly postoperatively and responded appropriately to changes in reward duration and intensity. Thus, lateral hypothalamic lesions do not impair behavioral thermoregulation. Despite normal behavioral heat intake, body temperature of the lesioned rats fell significantly during the sessions, which can be attributed to an impairment in metabolic heat production due to the lesions.     

Body temperature regulation in rats during exercise of various intensities at different ambient temperatures

We examined the relationship between body temperature, tail vasomotor response, and work intensity at different ambient temperatures in rats, using a treadmill and continuously measuring oxygen uptake during exercise. At an ambient temperature (Ta) of 24 degrees C, rectal temperature (Tre) at the beginning of tail vasodilation during exercise increased in proportion to work intensity. After tail vasodilation Tre remained steady, and at the end of 30 min exercise Tre level was proportional to work intensity. At Ta of 14 degrees C, Tre at the end of exercise was slightly higher than at 24 degrees C, and was higher at higher work intensities. At Ta of 4 degrees C, Tre rose slower during exercise than at higher Tas and even dropped at relatively low work intensities. Tail vasodilation did not occur in most cases. At Ta of 34 degrees C, Tre rose continuously during exercise. These data indicate that body temperature of rats during exercise rises in proportion to work intensity, but the extent of body temperature rises differs according to Ta.     

Impairment of thermoregulatory cooling behavior by single cortical spreading depression in the rat

Effects of single waves of unilateral cortical spreading depression (CSD) on skin-cooling operant behavior were studied in the rat with and without unilateral lesion in the preoptic and anterior hypothalamus (PO/AH). The skin-cooling behavior was severely suppressed for 6 min, with a resulting rise in rectal temperature (T_re), when CSD entered the frontal cortex contralateral to the PO/AH lesion. However, CSD ipsilateral to the PO/AH lesion slightly suppressed the skin-cooling behavior for 4 min with small rise in T_re. In the PO/AH intact rat, while 0.9% NaCl did not affect the skin-cooling behavior, unilateral CSD slightly suppressed it for 3 min almost in the same way as that of the ipsilateral CSD trial in the PO/AH lesioned rat. The results add further evidence to support the involvement of the frontal cortex in the central control of thermoregulation.     

Effect of ambient temperature on sleep-waking cycle in cats with electrolytic dorsolateral pontine tegmental lesions.

The effects of ambient temperature on the sleep-waking cycle were studied in intact cats and those with bilateral electrolytic lesions in the pontine tegmentum. At a room temperature of 23 degrees C, the percentage of time spent in paradoxical sleep was significantly lower in the lesioned cats than in intact animals. The mean duration of paradoxical sleep episodes was also decreased in the lesioned animals. The reduction in slow-wave sleep was not significant. At a slightly warmer ambient temperature of 30 degrees C, both the mean duration of paradoxical sleep episodes and the total duration of paradoxical sleep in the lesioned animals were increased toward normal values. Slow-wave sleep increased slightly but not significantly. At a higher ambient temperature of 35 degrees C, as well as at colder ambient temperatures of 15 and 7 degrees C, the durations of both paradoxical sleep and slow-wave sleep were significantly reduced. Under these thermal loads, the reduction in the duration of sleep was significantly greater in the lesioned cats than in the intact animals. The results suggest that: (i) pontine lesions alter the sleep cycle of cats and ambient temperature influences this alteration; (ii) the effects of thermal loads on the sleep cycle are more severe in the lesioned cats; and (iii) a moderately warm ambient temperature (30 degrees C) improves the sleep of pontine-lesioned cats.