The hypothermic effect of pentobarbital in the rat: sites and mechanisms of action

Susceptibility of ascending dopamine projections to 6-hydroxydopamine in rats: effect of hypothermia

The aims of this study were to determine (1) whether mesolimbic and nigrostriatal DA cell bodies degenerate to different extents after 6-hydroxydopamine (6-OHDA) is administered into their respective terminal fields and (2) whether hypothermia, associated with sodium pentobarbital anesthesia, protects DA neurons from the toxic effects of 6-OHDA. To address these questions, 6-OHDA or vehicle was infused into either the ventral or dorsal striatum or into the medial forebrain bundle, under conditions of brain normothermia or hypothermia. Two weeks post-surgery, tyrosine hydroxylase-positive cell bodies were counted in the ventral tegmental area (VTA) and substantia nigra. In addition, autoradiographic labeling of tyrosine hydroxylase protein and dopamine transporter was quantified in dopamine terminal fields and cell body areas. Overall, DA cell bodies in the VTA were substantially less susceptible than those in the substantia nigra to depletion of dopaminergic markers. Hypothermia provided two types of neuroprotection. The first occurred when 6-OHDA was administered into the dorsal striatum, and was associated with a 30-50% increase in residual dopaminergic markers in the lateral portion of the VTA. The second neuroprotective effect of hypothermia occurred when 6-OHDA was given into the medial forebrain bundle. This was associated with a 200-300% increase in residual dopaminergic markers in the mesolimbic and nigrostriatal terminal fields; no significant protection occurred in the cell body regions.Collectively, these findings show that (1) the dopaminergic somata in the substantia nigra are more susceptible than those in the VTA to 6-OHDA-induced denervation, and (2) hypothermia can provide anatomically selective neuroprotection within the substantia nigra-VTA cell population. The continued survival of mesolimbic dopamine cell bodies after a 6-OHDA lesion may have functional implications relating to drugs of abuse, as somatodendritic release of dopamine in the VTA has been shown to play a role in the effectiveness of cocaine reward.

Hypothermia After Vascular Surgery: Complications, Prevention, and Treatment

Virtually all anesthetics render patients poikilothermic and body temperature invariably decreases during surgery. Dur ing vascular occlusion and resulting ischemia to the brain, kidneys, and spinal cord, hypothermia is often beneficial by decreasing metabolic demands and protecting from injury. Residual hypothermia, however, in the postoperative period is rarely desirable and hypothermia-related complications are well-known. Even mild hypothermia can exacerbate the stress response by activation of the sympathetic nervous system, resulting in increased catecholamines, which can precipitate myocardial ischemia and cardiac morbidity. As little as 2°C of core hypothermia impairs coagulation and predisposes to postoperative bleeding, which is especially problematic in the presence of fresh vascular anastomoses. Hypothermia also slows emergence from general anesthesia by both pharmacokinetic and pharmacodynamic mecha nisms. In vascular surgery patients, body temperature should be carefully monitored and controlled with the same level of attention that is given to the other vital signs. By active cooling and warming at the appropriate perioperative timepoints, outcomes can be improved and morbidity re duced in patients undergoing vascular surgery. Copyright © 2000 by W.B. Saunders Company.

Field potential recordings in dentate gyrus of anesthetized rats: stability of baseline

Urethane is a standard anesthetic utilized for in vivo recordings in the hippocampus. In studies of long-term potentiation (LTP), the measure of interest is the response amplitude minutes to hours following train delivery. In the absence of experimental treatment, we have consistently observed upward drift in the amplitude of the population spike (PS) and EPSP slope of the dentate gyrus (DG) evoked field response in acute surgical preparations performed in the urethanized rat. The present study systematically monitored PS amplitude and EPSP slope in the DG every 30 minutes for 6 hours following optimal positioning of Teflon-coated bipolar stainless steel electrodes under urethane anesthesia. At maximal stimulus intensities, large time-dependent increases in PS amplitude (70-80%) were observed over the first 2-4 hours, an effect that was exaggerated at lower stimulus intensities. Increases in the EPSP slope were smaller in magnitude (20-30%) and stabilized within a shorter period of time (1-2 hours). Animals were warmed on a heating pad and body and brain temperature remained constant over the recording session. Reducing stimulating electrode size and recording with glass micropipettes did not alleviate the upward drift in response amplitude. Similar increases were also seen under pentobarbital anesthesia. To dissociate anesthetic from surgical effects, recordings were obtained from animals previously prepared with indwelling electrodes and injected with urethane. Although slight declines (10-15%) in EPSP slope occurred over time, no significant alterations in PS amplitude were seen in the chronic preparation at high stimulus intensities. Low stimulus intensities yielded a more variable response pattern and, in direct contrast to the acute preparation, time-dependent declines, not increases, were noted in both parameters. These data suggest that generalized surgical trauma contributes to the upward drift in response amplitude and indicate that long stabilization periods are required in acute surgical preparations for accurate field potential recordings.

Hypothermia following whole-body heating of mice: effect of heating time and temperature

We have observed an acute and prolonged lowering of body temperature (hypothermia) following whole body heating (WBH) of mice. This phenomenon of heat-induced hypothermia and the subsequent recovery of normal temperatures have been systematically investigated. The hypothermic period can be characterized by two parameters: Tnadir and a recovery time constant (tau). For treatment temperatures below 41 degrees C and treatment durations of 1 h or less, a mild hypothermia (Tnadir greater than 33 degrees C) and fast recovery (tau less than 1 h) occur. Tnadir and tau vary slightly with treatment temperature and are almost independent of treatment length. At treatment temperatures 41 degrees C and above for up to 1 h, we observed acute hypothermia (Tnadir as low as 28 degrees C) and slow recovery (tau = several hours). This region of prolonged hypothermia is characterized by a rapid change of Tnadir and tau with temperature, and a much less rapid change with treatment duration. The WBH temperature-time range causing prolonged hypothermia is very narrow, and if exceeded results in lethality. Critical lethal temperatures have been estimated for several treatment durations from the time constant data. Post-WBH hypothermia can be minimized by keeping the animals in a 37 degrees C environment. However, we find that neither survivability nor intestinal cell repopulation is enhanced by this procedure.

Prenatal ethanol exposure affects temperature responses of adult rats to pentobarbital and diazepam alone and in combination with ethanol

Long-term effects of prenatal alcohol exposure on body temperature responses to pentobarbital and diazepam and to either drug in combination with ethanol were studied in adult rats who were the offspring of dams fed a 5.0% w/v ethanol-containing liquid diet during the last 2 weeks of gestation. Adult offspring of pair-fed and chow-fed dams served as nutritional and normal controls, respectively. Pentobarbital (6.25-25.0 mg/kg) and diazepam (2.5-10.0 mg/kg) produced significantly greater dose-related hypothermic responses in females than males. Following either pentobarbital or diazepam administration female prenatally ethanol-exposed (E) rats responded with a greater fall in body temperature than the controls. Significantly greater hypothermia occurred in both male and female E rats than in controls when ethanol (1.5 g/kg) was administered together with pentobarbital or diazepam. However, the drug combinations did not produce additive effects on body temperature in any prenatal treatment group. Pentobarbital produced acute cross-tolerance to ethanol while diazepam potentiated ethanol's effect. These studies confirm and extend our previous findings of enhanced hypothermic responses to ethanol in adult rats exposed to ethanol in utero and indicate that maternal alcohol consumption produces long-term effects on the central thermoregulatory systems of offspring.

Effect of sodium pentobarbital on behavioral thermoregulation in rats and mice

In this study on behavioral thermoregulation, male Sprague-Dawley rats were given intraperitoneal (IP) injections of sodium pentobarbital in doses of 0, 1, 5, 10 or 15 mg/kg and male CBA/J mice were given doses of 0, 5, 10, 15 or 30 mg/kg. The animals were immediately placed in a temperature gradient which allowed them to select their preferred ambient temperature (Ta). The preferred Ta of rats increased following an injection of 10 mg/kg sodium pentobarbital, whereas, the barbiturate had no effect on the preferred Ta of mice. In another study, male rats and mice were given sodium pentobarbital in doses of 0, 5, 10 and 15 mg/kg and then placed into a temperature-controlled environmental chamber set at 30 degrees C for mice and 25 degrees C for rats (i.e., their approximate preferred Ta when dosed with sodium pentobarbital). Colonic temperatures were taken one hour after injection. Sodium pentobarbital induced dose dependent hypothermia in rats at 25 degrees C and hyperthermia in mice at 30 degrees C. These data suggest a direct or indirect block of heat gain/conserving effectors in rats treated with sodium pentobarbital which results in hypothermia and an appropriate compensatory selection of a warmer Ta.

Changes in body temperature after administration of acetylcholine, histamine, morphine, prostaglandins and related agents: II

This survey continues a second series of compilations of data regarding changes in body temperature induced by drugs and related agents. The information listed includes the species used, the route of administration and dose of drug, the environmental temperature at which experiments were performed, the number of tests, the direction and magnitude of change in body temperature and remarks on the presence of special conditions, such as age or brain lesions. Also indicated is the influence of other drugs, such as antagonists, on the response to the primary agent. Most of the papers were published since 1979, but data from many earlier papers are also tabulated.

Production of systemic hyperthermia in the rat

The design of clinical trials employing whole-body hyperthermia in cancer therapy has been hampered due to lack of a suitable animal model. We describe a technique for reproducibly and efficiently inducing whole-body hyperthermia in Sprague-Dawley rats, using halothane and oxygen anesthesia and immersion in a hot water bath. Core body temperatures of between 41.5 and 43 degrees C were induced and maintained for periods of up to 200 min and survival curves were determined. The time of exposure at a given temperature that resulted in death in 50% of the animals within 24 hr after heating (LD50/24 hr) was calculated by linear logistic regression analysis. LD50 24 hr values of 115, 61, 57, 25 and 16 min were obtained for temperatures of 41.75, 42.0, 42.25, 42.5 and 42.75 degrees C respectively. This heating technique is compared to several more toxic methods for inducing whole-body hyperthermia with respect to possible pharmacological and physiological differences.

Thermal tolerance to whole body hyperthermia

Thermal tolerance, that is, a reduced sensitivity to a succeeding heat treatment, has been noted in vitro and following local hyperthermia in normal tissues and malignant tumors. However, information is sparse concerning thermal tolerance following systemic hyperthermia, thereby limiting our ability to design optimally fractionated systemic hyperthermia treatment protocols. A technique for reproducibly inducing systemic hyperthermia in the rat is described, and the survival curve for rats exposed at 42.5 degrees C for periods of up to 75 minutes is presented. Using this system, increased survival of rats to systemic hyperthermia at 42.5 degrees C was demonstrated 30 hours after an initial sublethal conditioning exposure (41.8 degrees C for 1 hour). The LD50 (the time of exposure lethal to 50% of the rats) at 42.5 degrees C was increased by a factor of approximately 2 in the animals exposed to the sublethal conditioning. This increase in LD50 demonstrates the development of thermal tolerance to killing by whole body hyperthermia.

Effects of ethanol on thermoregulation

Clinical reports of accidental hypothermia in alcohol intoxicated individuals exposed to low ambient temperature ( Paton , 1983) have generally been borne out by experimental studies in healthy volunteers. Small doses of ethanol, given to human subjects at normal ambient temperature (Ta), have very little effect on body temperature but a combination of large dose, low Ta and vasodilatation provoked by strenuous exercise, causes a sharp fall in rectal temperature. In experimental animals, the use of relatively larger doses of alcohol and more extreme temperatures, both above and below the thermoneutral zone, has shown that the effect of ethanol is essentially poikilothermic, i.e. an impairment of adaptation to both heat and cold. This effect has been studied in greater detail, in relation to each of the basic thermoregulatory processes. Though small doses of alcohol may increase the metabolic rate under some circumstances, the most common effect at low Ta is inhibition of shivering and therefore reduction of thermogenesis. At the same time it tends to cause increased heat loss by cutaneous vasodilatation. This makes for a greater feeling of comfort in the cold exposed subjects but increases in rate of fall of core temperature. The combination of decreased thermogenesis and increased heat loss, despite falling body temperature, is suggestive of a lowering of the set-point of the thermoregulatory control mechanisms. Consistent with this is a slight increase in ventilatory heat loss after low doses of ethanol but larger doses cause respiratory depression, so that heat loss through the lungs is minor. However, at high Ta ethanol caused hyperthermia in experimental animals and shows enhanced lethality, so that impairment of thermoregulatory effector mechanisms seems to be at least as important as change in set-point. Studies of the effects of ethanol on electrophysiological activity of single neurons in the pre-optic area and anterior hypothalamus (POAH), biochemical activities of neuronal membranes, hypothalamic blood flow, conventional neurotransmitters, amino acid putative neurotransmitters, neuropeptides, prostaglandins and inorganic ions have all failed so far to yield a clear comprehensive picture of the mechanisms by which ethanol affects thermoregulation. In each case, contradictory evidence has been obtained concerning the consequences of ethanol administration, whether by oral, intraperitoneal, intravenous, intracerebroventricular, or direct local (POAH) route.(ABSTRACT TRUNCATED AT 400 WORDS)

Changes in body temperature after administration of antipyretics, LSD, delta 9-THC, CNS depressants and stimulants, hormones, inorganic ions, gases, 2,4-DNP and miscellaneous agents

This survey concludes a series of complications of data from the literature, primarily published since 1965, on thermoregulatory effects of antipyretics in afebrile as well as in febrile subjects, LSD and other hallucinogens, cannabinoids, general CNS depressants, CNS stimulants including xanthines, hormones, inorganic ions, gases and fumes, 2,4-dinitrophenol and miscellaneous agents including capsaicin, cardiac glycosides, chemotherapeutic agents, cinchona alkaloids, cyclic nucleotides, cycloheximide, 2-deoxy-D-glucose, dimethylsulfoxide, insecticides, local anesthetics, poly I:poly C, spermidine and spermine, sugars, toxins and transport inhibitors. The information listed includes the species used, route of administration and dose of drug, the environmental temperature at which the experiments were performed, the number of tests, the direction and magnitude of body temperature change and remarks on the presence of special conditions such as age or lesions, or on the influence of other drugs, such as antagonists, on the response to the primary agents.

Hypothermic and toxic actions of 2-butyne-1, 4-diol and other related diols in the rat

The effects of 2-butyne-1,4-diol (BYDL) and related congeners on the body temperature of the rat have been determined. BYDL, at doses above 0·4 mmol kg−1 intraperitoneally, produced a significant fall in body temperature which was dose dependent. 2-butene-1,4-diol (BEDL) had no hypothermic action, and butane-1,4-diol (BDL) produced only a moderate fall in temperature (1·9°) which correlated with the time course of the hypnotic effect of the drug. γ-Butyrolac-tone (GBL), γ-hydroxybutyric acid (GHB) and pentobarbitone at hypnotic doses had similar hypothermic actions which correlated with the period of hypnosis. The hypothermia produced by BYDL could not be prevented by pretreatment with scopolamine or by maintaining the rats at high environmental temperature (32°). BYDL was far more toxic than the other diols examined (LD50 = 0·609-0·635 mmol kg−1 compared to BEDL: 3·71-3·74 mmol kg−1 and BDL: 11·87-11·90 mmol kg−1). Pretreatment of rats with pyrazole, an inhibitor of liver alcohol dehydrogenase, prevented the toxic and hypothermic actions of BYDL. From earlier studies with pyrazole and BDL it was concluded that BYDL itself was not active but that it was possibly converted in vivo by liver alcohol dehydrogenase to a toxic metabolite.