Hyperthermia in the rat from handling stress blocked by naltrexone injected into the preoptic-anterior hypothalamus

Independent effects of rapid eye movement sleep deprivation and exposure to environmental heat stress on aerobic performance and thermoregulatory responses in exercising rats

Evidence indicates that aerobic performance is degraded either by environmental heat stress or sleep deprivation. However, whether these conditions interact to produce more significant performance impairment deserves further investigation. Therefore, this study investigated the effects of experimental sleep deprivation (24 h or 96 h) on aerobic performance and thermoregulatory responses in rats exercised on a treadmill at different environmental conditions. Adult male Wistar rats were subjected to rapid eye movement sleep deprivation (RSD) using the modified multiple platform method and were then subjected to an incremental-speed exercise until they were fatigued. Treadmill running was performed in a temperate (24°C) or warm (31°C) environment, and the colonic temperature (an index of core body temperature; T_CORE) and the tail-skin temperature (T_SKIN; an index of cutaneous heat loss) were recorded. 24-h and 96-h RSD produced small magnitude reductions in aerobic performance (Cohen's d = 0.47-0.58) and minor changes in thermoregulation. Relative to control rats, sleep-deprived rats showed a higher T_CORE at the exercise initiation and a higher threshold for activating cutaneous heat loss, but unchanged T_CORE and T_SKIN at fatigue. Exercise at 31°C induced large reductions in performance (d = 0.82-1.29) and marked changes in thermoregulation, as evidenced by higher T_CORE and T_SKIN at fatigue, compared to exercise at 24°C. Interestingly, none of the effects induced by RSD were exacerbated by environmental heat stress and vice-versa, indicating that both conditions did not interact. We conclude that RSD and heat stress modulate aerobic performance and thermoregulatory responses by acting independently.

Stress-induced hyperthermia and hypothermia

Stress affects core body temperature (T_c). Many kinds of stress induce transient, monophasic hyperthermia, which diminishes gradually if the stressor is terminated. Stronger stressors produce a longer-lasting effect. Repeated/chronic stress induces anticipatory hyperthermia, reduces diurnal changes in T_c, or slightly increases T_c throughout the day. Animals that are exposed to chronic stress or a cold environment exhibit an enhanced hyperthermic response to a novel stress. These changes persist for several days after cessation of stress exposure. In contrast, long-lasting inescapable stress sometimes induces hypothermia. In healthy humans, psychologic stress induces slight increases in T_c, which are within the normal range of T_c or just above it. Some individuals, however, develop extremely high T_c (up to 41°C) when they are exposed to emotional events or show persistent low-grade high T_c (37-38°C) during or after chronic stress situations. In addition to the nature of the stressor itself, such stress-induced thermal responses are modulated by sex, age, ambient temperature, cage mates, past stressful experiences and cold exposure, and coping. Stress-induced hyperthermia is driven by mechanisms distinct from infectious fever, which requires inflammatory mediators. However, both stress and infection activate the dorsomedial hypothalamus-rostral medullary raphe region-sympathetic nerve axis to increase T_c.

Psychogenic fever: how psychological stress affects body temperature in the clinical population

Psychogenic fever is a stress-related, psychosomatic disease especially seen in young women. Some patients develop extremely high core body temperature (Tc) (up to 41°C) when they are exposed to emotional events, whereas others show persistent low-grade high Tc (37-38°C) during situations of chronic stress. The mechanism for psychogenic fever is not yet fully understood. However, clinical case reports demonstrate that psychogenic fever is not attenuated by antipyretic drugs, but by psychotropic drugs that display anxiolytic and sedative properties, or by resolving patients' difficulties via natural means or psychotherapy. Animal studies have demonstrated that psychological stress increases Tc via mechanisms distinct from infectious fever (which requires proinflammatory mediators) and that the sympathetic nervous system, particularly β3-adrenoceptor-mediated non-shivering thermogenesis in brown adipose tissue, plays an important role in the development of psychological stress-induced hyperthermia. Acute psychological stress induces a transient, monophasic increase in Tc. In contrast, repeated stress induces anticipatory hyperthermia, reduces diurnal changes in Tc, or slightly increases Tc throughout the day. Chronically stressed animals also display an enhanced hyperthermic response to a novel stress, while past fearful experiences induce conditioned hyperthermia to the fear context. The high Tc that psychogenic fever patients develop may be a complex of these diverse kinds of hyperthermic responses.

The role of motivation and reward neural systems in vocal communication in songbirds

Many vertebrates are highly motivated to communicate, suggesting that the consequences of communication may be rewarding. Past studies show that dopamine and opioids in the medial preoptic nucleus (mPOA) and ventral tegmental area (VTA) play distinct roles in motivation and reward. In songbirds, multiple lines of recent evidence indicate that the roles of dopamine and opioid activity in mPOA and VTA in male birdsong differ depending upon whether song is used to attract females (sexually-motivated) or is produced spontaneously (undirected). Evidence is reviewed supporting the hypotheses that (1) mPOA and VTA interact to influence the context in which a male sings, (2) distinct patterns of dopamine activity underlie the motivation to produce sexually-motivated and undirected song, (3) sexually-motivated communication is externally reinforced by opioids released as part of social interactions, and (4) undirected communication is facilitated and rewarded by immediate opioid release linked to the act of singing.

Control of hormonal stress reactivity by the endogenous opioid system

Regulations of hormonal stress responses entail the initiation, amplitude and termination of the reaction, as well as its integration with other stress response systems. This study investigates the role of endogenous opioids in the regulation and integration of behavioral, thermal and hormonal stress responses, as these neuromodulators and their receptors are expressed in limbic structures responsible for stress responses. For this purpose, we subjected mice with selective deletion of beta-endorphin, enkephalin or dynorphin to the zero-maze test, a mildly stressful situation, and registered behaviors and stress hormone levels. Behavioral stress reactivity was assessed using zero-maze, light-dark and startle-reactivity paradigms. Animals lacking enkephalin displayed increased anxiety-related behavioral responses in each three, dynorphin knockouts in two models, whereas the responses of beta-endorphin knockouts indicated lower anxiety level in the zero-maze test. All knockout strains showed marked changes in hormonal stress reactivity. Increase in ACTH level after zero-maze test situation, unlike in wild type animals, failed to reach the level of significance in Penk1(-/-) and Pdyn(-/-) mice. Corticosterone plasma levels rapidly increased in all strains, with a lower peak response in knockouts. In wild-type and beta-endorphin-deficient mice, corticosterone levels returned to baseline within 60min after stress exposure. In contrast, mice lacking dynorphin and enkephalin showed longer-lasting elevated corticosterone levels, indicating a delayed termination of the stress reaction. Importantly, the behavioral and hormonal responses correlated in wild-type but not in knockout mice. Hyperthermia elicited by stress was reduced in animals lacking dynorphin and absent in Penk1(-/-) mice, despite of the heightened behavioral anxiety level of these strains. These results demonstrate an important role on the endogenous opioid system in the integration of behavioral and hormonal stress responses.

The stress-induced hyperthermia paradigm as a physiological animal model for anxiety: A review of pharmacological and genetic studies in the mouse

This paper reviews the function, brain mechanisms and pharmacology of stress-induced hyperthermia (SIH) in a broad context. Hyperthermia itself is induced by all stressful stimuli and can be found across numerous species, including humans. As a model for anxiety, the process of insertion of a rectal probe increases temperature ranging from about 0.5-1.5 degrees C in 10-15min is called SIH. This temperature increase can be blocked by anxiolytic drugs. The methodological as well as pharmacological aspects of the group- (G-SIH) and singly housed (SIH) version of the paradigm are described in detail. Also, an overview is presented about studies using the SIH procedure in genetically modified mice together with the potential interference with immunological induction of a febrile response. The paper also presents data that highlight some of the limitations of the SIH procedure for use of drugs like nicotine, which contain particular characteristics such as short in vivo half-life, and/or disturbance of thermoregulation. The advantages and disadvantages of the SIH procedure as a physiological model of anxiety are discussed.

Cytokine and hormone profiles in mice subjected to handling combined with rectal temperature measurement stress and handling only stress

Stress is known to either up or down regulate immunity. In this study, mice were subjected to handling combined with rectal temperature measurement (RTM) stress or handling only stress. We investigated whether there were any significant differences in the effect of handling combined with RTM and handling only on NK cell activity, serum cytokine (IL-1beta, IL-6, and TNF-alpha) and ACTH and beta-endorphin levels, and splenic cytokine (IL-1beta, IL-6, TNF-alpha, IFN-alpha, and IFN-beta) levels. Circulating cytokines and hormones and splenic cytokine mRNA levels were measured in individual mice. NK cell activity was significantly increased in both stress groups when compared to the control group. Handling combined with RTM produced significantly increased serum levels of IL-1beta, IL-6, and beta-endorphin. Serum IL-1beta, ACTH, and beta-endorphin were elevated significantly in the handling only group. Splenic TNFalpha mRNA in both of the stress groups and IL-6 mRNA in handling only group decreased significantly. Our observations are supported by existing literature demonstrating that various stressors have differential effects on immune functions and the neuroendocrine hormones and cytokines, which regulate them.

Fetal exposure to nicotine does not alter the core temperature response of 7- to 8-week-old rats to intracerebroventricular administration of PGE(1)

Prenatal exposure to nicotine attenuates stress-induced hyperthermia in adult male and female rats upon exposure to a novel environment. Given that prostaglandins play an important role in mediating stress-induced hyperthermia, our current experiments were carried out to determine if prenatal exposure to nicotine alters the thermogenic response of adult rats to an E-series prostaglandin. Forty-eight chronically instrumented adults rats (24 males and 24 females) received an intracerebroventricular injection of prostaglandin E(1) (PGE(1); 0.2 microg in 10 microl of artificial cerebrospinal fluid [aCSF]) or vehicle (10 microl aCSF) at 7-8 weeks of postnatal life (i.e. adulthood as defined by the ability to reproduce) following prenatal exposure to nicotine (6 mg of nicotine tartrate per kilogram of maternal body weight per day) or vehicle via a maternally implanted osmotic mini-pump from Day 6 or 7 of gestation to term. In female rats, intracerebroventricular injection of PGE(1) following prenatal exposure to vehicle produced a monophasic fever with a magnitude of approximately 1.5 degrees C and a duration of approximately 66 min. In male rats, however, intracerebroventricular injection of PGE(1) following prenatal exposure to vehicle produced a monophasic fever with a magnitude of only approximately 0.9 degrees C and a duration of approximately 42 min. Prenatal exposure to nicotine did not significantly alter the febrile responses of male or female rats to intracerebroventricular injection of PGE(1) as compared to that observed following prenatal exposure to vehicle. Thus, prenatal exposure to nicotine does not significantly alter the thermogenic response of adult rats to central administration of the pyrogen PGE(1). It is unlikely, therefore, that an altered thermoregulatory effector response to E-series prostaglandins is responsible for mediating the attenuated stress-induced hyperthermia in adult male and female rats upon exposure to a novel environment following prenatal exposure to nicotine.

Naltrexone attenuates plasma nitric oxide release following acute heat stress

Previous studies have shown that naltrexone attenuates morbidity and mortality in heat stress by inhibiting endogenous opioids. In this study, we hypothesized that naltrexone can decrease heat stress by attenuating nitric oxide release. Male Sprague-Dawley rats were pretreated with naltrexone or normal saline, and exposed to 45 degrees C for 25 min; controls were exposed to 25 degrees C. Colonic temperatures were recorded and plasma samples from an in-dwelling i.v. cannula were analyzed for nitrate/nitrite levels. Following heat stress, peak colonic temperature was significantly diminished (P < 0.05) in naltrexone-treated rats compared to saline-treated rats. Plasma nitrate/nitrite levels were significantly lower (P < 0.05) in naltrexone-treated rats compared to saline-treated rats. These findings suggest that naltrexone is able to attenuate the rise in plasma nitric oxide levels commonly observed after heat stress.

Prenatal exposure to nicotine attenuates stress-induced hyperthermia in 7- to 8-week-old rats upon exposure to a novel environment

Given that approximately 25% of women in the United States continue to smoke cigarettes during pregnancy, it is important to know if exposure to nicotine during development alters the physiological response of the adult to the "stressors" of everyday life. Our current experiments were carried out to determine if prenatal exposure to nicotine alters "stress-induced hyperthermia" in adult rats upon exposure to a novel environment such as a simulated open field. Forty-eight rats (23 males and 25 females) were exposed to a simulated open field or left in their home cage at 7 to 8 weeks of postnatal life (i.e., adulthood as defined by the ability to reproduce) following prenatal exposure to vehicle or nicotine (6 mg of nicotine tartrate per kilogram of maternal body weight per day) via a maternally implanted osmotic minipump from Day 6 or 7 of gestation. The simulated open field consisted of a 30(W)x60(L)x24(H)-in. white acrylic finish box illuminated by two hanging fluorescent lights and core temperature was measured by telemetry. Exposure to a simulated open field following prenatal exposure to vehicle elicited an increase in core temperature in male and female rats with a magnitude of approximately 1.2 degrees C and a duration of greater than 170 min. Prenatal exposure to nicotine significantly attenuated the thermogenic response of both genders; this was not only evident in the latency, magnitude and duration of the core temperature response but also in the core temperature index determined from the 3-h period following exposure to a simulated open field. Thus, our data provide evidence that prenatal exposure to nicotine attenuates stress-induced hyperthermia in male and female 7- to 8-week-old rats upon exposure to a "stressor" of everyday life (i.e., a novel environment).

Mechanisms and mediators of psychological stress-induced rise in core temperature

OBJECTIVE

Despite numerous case reports on "psychogenic fever," it remains uncertain how psychological stress raises core temperature and whether the rise in core temperature is a real fever or a hyperthermia. This article reviews studies on the psychological stress-induced rise in core temperature (PSRCT) in animals with the aim to facilitate studies on the mechanisms of so-called psychogenic fever in humans.

METHODS

To address this question, we reviewed the mechanisms and mediators of the PSRCT and classic conditioning of the fever response in animals.

RESULTS

The PSRCT is not due to the increased locomotor activity during stress, and the magnitude of the PSRCT is the same in warm and cold environments, indicating that it is a centrally regulated rise in temperature due to an elevated thermoregulatory "set point." The PSRCT caused by conventional psychological stress models, such as open-field stress, is attenuated by cyclooxygenase inhibitors, which block prostaglandin synthesis. On the other hand, the PSRCT elicited by an "anticipatory anxiety stress" is not inhibited by cyclooxygenase inhibitors but by benzodiazepines and serotonin Type 1A receptor agonists. The febrile response can be conditioned to neutral stimuli after paired presentation with unconditioned stimuli such as injection of lipopolysaccharide, a typical pyrogen.

CONCLUSIONS

Most findings indicate that the PSRCT is a fever, a rise in the thermoregulatory set point. The PSRCT may occur through prostaglandin E2-dependent mechanisms and prostaglandin E2-independent, 5-HT-mediated mechanisms. The febrile response can be conditioned. Thus, these mechanisms might be involved in psychogenic fever in humans.

Stress fever magnitude in laboratory-maintained California ground squirrels varies with season

A previous study demonstrated that California ground squirrels (Spermophilus beecheyi) living in the natural environment had, independent of season, a significantly higher mean diurnal body temperature (T(b)) (39.6 degrees C) than either summer (37.5 degrees C) or winter (36.5 degrees C) laboratory maintained animals. Based upon the previous study it has been suggested that California ground squirrels living in the natural environment may have an elevated set-point for body temperature in a manner analogous to a stress fever response. The present study was conducted to determine if season and/or duration of laboratory open-field exposure influenced the magnitude of laboratory open-field stress fever. If stress fever was involved to some extent in the higher body temperature observed in animals from the natural environment, laboratory maintained animals should exhibit a lower magnitude stress fever during the summer months and a higher magnitude stress fever during the winter months. It was hypothesized that laboratory maintained animals would exhibit the same set-point for stress fever T(b) independent of season, and that the duration of open-field exposure would not influence the magnitude of stress fever. Adult California ground squirrels were acclimated to an ambient temperature of 20+/-1.0 degrees C under either LD 14:10 (summer) or LD 10:14 (winter) photoperiod conditions and individuals from both photoperiod conditions were exposed for periods of 2, 4, and 6 h to an open-field arena. An analysis of the data with a two-factor ANOVA demonstrated that season (photoperiod) significantly influenced the magnitude of the stress fever response (1.1+/-0.1 degrees C for summer animals; 2.1+/-0.2 degrees C for winter animals) while there was no significant influence of open-field exposure duration on stress fever magnitude. These results demonstrate that although the set-point for body temperature in unstressed laboratory maintained California ground squirrels varies with season, the set-point for body temperature in open-field stressed animals does not vary with season. These data lend support to the hypothesis that something like stress fever may play some role in the higher body temperature observed in California ground squirrels living in the natural environment.

Psychoneuroendocrine immunology: perception of stress can alter body temperature and natural killer cell activity

Psychoimmunology has been credited with using the mind as a way to alter immunity. The problem with this concept is that many of the current psychoimmunology techniques in use are aimed at alleviating stress effects on the immune system rather than at direct augmentation of immunity by the brain. Studies in animals provide a model that permits us to approach the difficulties associated with gaining an understanding of the CNS-immune system connection. A particular advantage of using animals over humans is that psychological and social contributions play a less prominent role for animals than for human subjects, since the animals are all inbred and reared under identical controlled conditions. If the insightful information provided by animal studies is correct, then psychotherapy for the treatment of diseases might be made more effective if some aspect of this knowledge is included in the design of the treatment. We emphasize conditioning as a regimen and an acceptable way to train the brain to remember an output pathway to raise immunity. We propose that a specific drug or perception (mild stress, represented by rotation, total body heating or handling) could substitute and kindle the same output pathway without the need for conditioning. If this view is correct, then instead of using conditioning, it may be possible to use an antigen to activate desired immune cells, and substitute a drug or an external environmental sensory stimulus (perception) to energize the output pathway to these cells. Alternatively, monitoring alterations of body temperature in response to a drug or perception might allow us to follow how effectively the brain is performing in altering immunity. Studies with animals suggest that there are alternative ways to use the mind to raise natural or acquired immunity in man.

Role of AVP in mediating the altered core temperature response to a simulated open field in pregnant rats

Near the term of pregnancy, rats have an attenuated core temperature response on exposure to a novel environment (e.g., a simulated open field) compared with that observed early in pregnancy or in nonpregnant rats. The present experiments were carried out on 26 nonpregnant and 26 pregnant rats to test the hypothesis that arginine vasopressin, functioning as an endogenous antipyretic substance in the central nervous system, mediates this attenuated core temperature response. Exposure to a simulated open field after intracerebroventricular (ICV) vehicle produced a significant increase in core temperature in both nonpregnant and pregnant animals, the magnitude and duration of which were greater in the nonpregnant rats. In nonpregnant rats, exposure to a simulated open field after ICV vasopressin V(1)-receptor antagonist altered the pattern of the core temperature response but not the core temperature index compared with that observed on exposure to a simulated open field after ICV vehicle. In pregnant animals, ICV vasopressin V(1)-receptor antagonist did not alter the core temperature response to a simulated open field compared with that observed after ICV vehicle. Thus our data do not support the hypothesis that a pregnancy-related activation of arginine vasopressin attenuates the core temperature response to a simulated open field in rats near the term of pregnancy.

Gender influences the core temperature response to a simulated open field in adult guinea pigs

The induction of psychological stress is often accompanied by a transient increase in core temperature, commonly referred to as stress induced hyperthermia. Although stress-induced hyperthermia occurs when rats, mice, and pigs are exposed to a novel stimulus (e.g., a simulated open field, restraint, etc.), whether or not it occurs in guinea pigs has not been investigated. The present experiments were therefore carried out to investigate the thermoregulatory responses of both male (n = 7) and female (n = 7) adult guinea pigs when they were exposed to a simulated open field. Unexpectedly, neither the male nor female guinea pigs developed stress-induced hyperthermia. To the contrary, female but not male guinea pigs significantly decreased their core temperature during an open field experiment. The mechanism of the gender-specific thermoregulatory response of the adult guinea pig to psychological stress is presently unknown.

Coordination of autonomic and behavioral thermoregulatory responses during exposure to a novel stimulus in rats

The induction of psychological stress in rats is accompanied by an elevation of core temperature. Our experiments were carried out to determine whether the latency, duration, magnitude, or effector mechanisms of the core temperature response to psychological stress would be altered when rats were allowed to use behavioral as well as autonomic thermoregulation. Core temperature, oxygen consumption, and ambient temperature were measured in adult rats before and after handling and a sham intraperitoneal injection. Seven rats were studied in a thermocline (gradient of 7 to 42 degrees C) and eight rats were studied in a metabolic chamber (25 degrees C). The rats studied in the thermocline selected a warm ambient temperature following the sham intraperitoneal injection and exhibited an increase in core temperature of shorter latency, greater magnitude, and greater duration than those studied in the metabolic chamber. The rats studied in the metabolic chamber exhibited an oxygen consumption response of greater magnitude and duration than the animals studied in the thermocline. Thus the characteristics in addition to the effector mechanisms of the core temperature response to psychological stress are altered when rats are allowed to use behavioral as well as autonomic thermoregulatory effectors.

Pregnancy alters body-core temperature response to a simulated open field in rats

Exposure of a rat to a novel environment (e.g., a simulated open field) induces a transient increase in body-core temperature, which is often called stress-induced hyperthermia. Although pregnancy is known to influence thermoregulatory control, its effect on stress-induced hyperthermia is unknown. Therefore, 24 Sprague-Dawley rats (8 nonpregnant and 16 pregnant) were studied to test the hypothesis that pregnancy would alter the development of stress-induced hyperthermia after exposure to a simulated open field. Body-core temperature index increased significantly after exposure to a simulated open field in nonpregnant and gestation day-10 rats but not in gestation day-15 and day-20 rats. Thus our data provide evidence that pregnancy influences the body-core temperature response of rats exposed to a simulated open field in a gestation-dependent fashion. The functional consequences as well as the mechanisms involved remain to be determined.

Body temperature, motor activity, and feeding behavior of mice treated with beta-chlornaltrexamine

The effects of an irreversible long term opioid antagonism on circadian rhythms in body temperature (Tb), locomotor activity (Act) and feeding under normal conditions and following lipopolysaccharide administration (LPS; 2.5 mg/kg) have been investigated in unrestrained mice housed at their thermoneutral zone (30 degrees C). beta-chlornaltrexamine (beta-CNA; 5 mg/kg) given intraperitoneally decreased Tb on the day of injection, depressed Act, and reduced food and water intake for several days. The drug destroyed circadian rhythm in Tb for 4 consecutive days after administration due to prevention of the night time increases in temperature, whereas mean day time Tb of mice treated with beta-CNA remained similar to controls. Between days 5-8 the day-time Tb of beta-CNA-injected mice decreased, and the mice started displaying regular daily variations albeit with smaller amplitude and at lower level than controls. The depressive effect of beta-CNA on circadian variation in activity was more prolonged than its effect on Tb suggesting that these two variables are independently regulated. beta-CNA prevented the febrile response of the mice to LPS and enhanced the hypophagic effect of LPS. We conclude that normal circadian rhythms in Tb and Act, as well as certain symptoms of sickness behavior, have an opioid component.

Fever: causes and consequences

The present review distinguishes pathogenic, neurogenic, and psychogenic fever, but focuses largely on pathogenic fever, the hallmark of infectious disease. The data presented show that a complex cascade of events underlies pathogenic fever, which in broad outline - and with frank disregard of contradictory data - can be described as follows. An invading microorganism releases endotoxin that stimulates macrophages to synthesize a variety of pyrogenic compounds called cytokines. Carried in blood, these cytokines reach the perivascular spaces of the organum vasculosum laminae terminalis (OVLT) and other regions near the brain where they promote the synthesis and release of prostaglandin (PGE2). This prostaglandin then penetrates the blood-brain barrier to evoke the autonomic and behavioral responses characteristic of fever. But then once expressed, fever does not continue unchecked; endogenous antipyretics likely act on the septum to limit the rise in body temperature. The present review also examines fever-resistance in neonates, the blunting of fever in the aged, and the behaviorally induced rise in body temperature following infection in ectotherms. And finally it takes up the question of whether fever enhances immune responsiveness, and through such enhancement contributes to host survival.

Indomethacin and sodium carbonate effects on conditioned fever and NK cell activity

The augmentation of natural killer (NK) cell activity and elevation of body temperature (fever) can both be conditioned using camphor odor as the conditioned stimulus (CS) and poly I:C as the unconditioned stimulus (US). While both responses can be conditioned in parallel fashion as shown previously, our results indicate the conditioned learning of these responses may not follow along a common path. We found that injection of a 1% solution of sodium carbonate was able to consistently block the CS/US learning of the NK cell response but did not block conditioning of the fever response. In contrary fashion, mice treated with indomethacin (which inhibits prostaglandin-induced fever) dissolved in the sodium carbonate solution did not learn in consistent fashion the fever response. However, indomethacin-treated animals were able to recall the NK cell response. These results support the view that although the same mediator, IFN-beta, is responsible for the conditioned learning of the NK cell and fever responses both the learning and recall of the responses are initiated along separate pathways.

Stress hyperthermia: physiological arguments that it is a fever

The theory that stress (or emotional) rise in central temperature (Tc) in rats is a fever with an upward shift of the set-point temperature was tested with three experiments: 1) Measurement of tail skin temperature and Tc during the emotional Tc rise; 2) Investigation of the effect of ambient temperature on the emotional Tc rise; and 3) The assessment of emotional Tc rise during daytime and nighttime. Skin vasomotor responses helped the increase of Tc toward a higher level and contributed to the regulation of central temperature at this new higher level. The cold environment did not diminish the emotional rise of central temperature as it would be expected in the case of a hyperthermia. However, at night emotional fever reached a higher level than during the daytime, suggesting that prostaglandin rise in Tc is distinct from emotional or stress-induced hyperthermia. In conclusion, the experiments reported here confirm the hypothesis that the rise of Tc induced by handling or disturbance of the rats is regulated, and is due to a shift of the set-point as occurs in fever.

Electroacupuncture in rats: evidence for naloxone and naltrexone potentiation of analgesia

Low frequency electroacupuncture (EA) analgesia has been thought to be mediated by endogenous opioids. Among other lines of evidence, it has been reported that EA stimulation delivered at 2 and 2-15 Hz in rats could be blocked or partially antagonized by naloxone (NAL) and naltrexone (NTX). In contrast, experiments in one of our laboratories (D.J.M.) showed that NAL did not inhibit 2 Hz, and even potentiated 125 Hz EA analgesia. In an attempt to resolve these discrepancies, we conducted joint experiments in the U.S.A. and in China using the methods which previously yielded NAL reversibility of EA analgesia. In no experiment did opiate antagonists block or reduce EA analgesia. On the contrary, we found that, in most experiments, NAL and NTX potentiated 2 and 2-15 Hz EA analgesia respectively. The potentiation occurred independently of laboratory methods, geographic location of the experiment, strain (Chinese or American), tail temperature, sex, and weight of rats. This potentiation suggests the existence of an opioid anti-analgesic system or that NAL and NTX acquired analgesic properties following EA. These results indicate that EA analgesia in rats is a variable phenomenon even when laboratory methods are rigorously replicated. The EA stimulation may activate multiple conflicting neural circuits which interact and ultimately modulate the analgesic outcome.

Naltrexone blocks the expression of the conditioned elevation of natural killer cell activity in BALB/c mice

An elevation of natural killer (NK) cell activity was conditioned by the association of a camphor odor conditioning stimulus (CS) with an injection of 20 micrograms polyinosinic:poly-cytidylic acid (poly I:C), the unconditioned stimulus (US). Poly I:C elicits the production and secretion of interferon (IFN), which induces an increase in NK cell activity. Reexposure to the CS occurred on Days 3 and 5 after the association trial on Day 0. Immediately following the CS exposure on Day 5, 1 microgram poly I:C was administered to all animals. This procedure resulted in an increased NK cell activity in the conditioned (CND), but not the nonconditioned (NC), mice. In this study we have shown that the expression of the conditioned response was blocked by an injection of naltrexone (NTX) at 10 mg/kg ip when given immediately prior to the two test CS odor exposures. Peripheral treatment (ip) with a quaternary form of naltrexone (QNTX), which is a less potent opiate antagonist, at the same dose and at the same time relative to the CS odor reexposure did not block the conditioned response. The formation of the conditioned association did not appear to be disrupted by NTX at the 10 mg/kg dose when given immediately prior to the trial odor exposure on Day 0. No modulation of NK cell activity was observed in any of the control groups treated with naltrexone or the quaternary analog. Because of the inability of the QNTX to block the conditioned response, we hypothesize that the opiate receptors involved in the conditioned response and blocked by NTX were within the central nervous system (CNS). Whether this response is peripherally or centrally mediated, we have shown that opiate receptors represent part of the mechanism which mediates the conditioned augmentation of NK cell activity.

Effects of the opiate antagonist naloxone upon hypothalamically elicited affective defense behavior in the cat

The opiate antagonist naloxone hydrochloride was employed in order to determine whether endogenous opioids play a role in the control of affective defense behavior elicited from the medial hypothalamus in the cat. The effects of naloxone upon quiet biting attack behavior elicited from the lateral hypothalamus were also assessed. A comparison of the differences in response latencies or thresholds before and after naloxone (i.p.) administration was made. Naloxone (1, 4 and 10 mg/kg) was found to significantly facilitate affective defense behavior in a dose- and time-dependent manner. The duration of facilitation ranged from 30 min after a 1 mg/kg injection to 180 min after a 10 mg/kg injection. The data also suggest that the effects of naloxone upon affective defense behavior are opposite to those seen with quiet biting attack. In two animals, quiet biting attack behavior was suppressed for 30 min following a 10 mg/kg injection of naloxone. Naloxone was also administered to cats in which hypothalamic stimulation elicited predatory responses coupled with components of affective defense behavior. In these cases, naloxone was ineffective in altering latencies for this 'mixed' response. These findings suggest that the opiate peptide system selectively inhibits affective defense behavior elicited from the medial hypothalamus of the cat.

Potentiation of lethality and increase in body temperature by combined use of d-methamphetamine and morphine in mice

Lethality and change in body temperature in mice were examined after subcutaneous injection of d-methamphetamine and morphine alone or in combination. The LD50 values for methamphetamine and morphine were calculated to be 95 and 670 mg/kg body wt., respectively. When a non-lethal dose of morphine (300 mg/kg) was administered with various doses of methamphetamine, the LD50 for methamphetamine was reduced to 5 mg/kg, indicating a marked potentiation of toxicity by combined use of both drugs. Injection of 5 mg/kg of methamphetamine produced slight hyperthermia, while 300 mg/kg of morphine decreased the body temperature of mice. However, when both drugs were used concomitantly, a marked increase in body temperature was observed. Hyperthermia was also observed when the dose of morphine was reduced to 50 mg/kg. It is postulated that hyperthermia is probably one of the contributory factors in the potentiated toxicity by combined use of morphine and methamphetamine.

Naloxone, naltrexone and body temperature

The temperature effects of naloxone and naltrexone (1-30 mg/kg) were examined in well habituated male rats. These drugs had a similar time course and potency, producing a dose-dependent hypothermia followed several hours later by a hyperthermia. A subsequent study found that not only did 30 mg/kg of naloxone or naltrexone produce an equivalent hypothermia but this hypothermia was just as pronounced during the dark as in the light part of the cycle.

Conditional shifts in thermic responses to sequentially paired drugs and the "conditional hyperactivity" hypothesis

Previous experiments have demonstrated that upward shifts in a rat's thermic response to certain drugs may be observed when these drugs have been paired on several occasions with agents that induce hypothermia. A "conditional hyperactivity" hypothesis suggests that these upward shifts may simply reflect drug elicited increases in body movements which translate into higher temperatures. The present experiment explored this hypothesis. Atropine sulfate (10 mg/kg) was paired with sodium pentobarbital (40 mg/kg) on multiple occasions and several tests were conducted with both drugs. This treatment yielded a conditional hyperthermic response to atropine, but the drug was not found to elicit an increase in gross motor movements. Of greatest interest was the finding that the atropine, when injected 30 min prior to a hypnotic dose of pentobarbital (80 mg/kg), attenuated the hypothermia normally induced by this barbiturate while leaving the duration of hypnosis unaffected. This upward thermic shift cannot be accounted for by the "conditional hyperactivity" hypothesis because the animals were immobile while under pentobarbital's influence. These findings suggest that autonomic events, as yet unspecified, may underlie certain conditional temperature increases.

Body temperature effects of opioids in rats: intracerebroventricular administration

A variety of opioids were injected intracerebroventricularly into unrestrained rats, and rectal temperatures were monitored over several hours. Complete dose-response data for morphine, heroin, etorphine, d- and l-ethylketazocine, d- and l-pentazocine, and d- and l-N-allylnormetazocine revealed a predominant response of hyperthermia. Only etorphine in high doses caused a pronounced decrease in body temperature. These results differed considerably from those obtained previously with subcutaneously administered opioids but can be explained in terms of a dual-receptor theory of temperature control.