Serotonergic influences on brain stem thermoregulatory mechanisms in the cat

Control of Shivering During Targeted Temperature Management

Targeted temperature management (TTM) is used frequently in patients with a variety of diseases, especially those who have experienced brain injury and/or cardiac arrest. Shivering is one of the main adverse effects of TTM that can often limit its implementation and efficacy. Shivering is the body's natural response to hypothermia and its deleterious effects can negate the benefits of TTM. The purpose of this article is to provide an overview of TTM strategies and shivering management.

Prophylactic effects of intrathecal Meperidine and intravenous Ondansetron on shivering in patients undergoing lower extremity orthopedic surgery under spinal anesthesia

Objective:

Intraoperative hypothermia is a common problem with anesthesia. Spinal anesthesia, the same as general anesthesia, affects the process of temperature regulation. The aim of this study was to compare the prophylactic effect of intravenous (IV) ondansetron with intrathecal (IT) meperidine on prevention of shivering during spinal anesthesia in patients underwent orthopedic surgery of the lower limb.

Methods:

In this study, 120 patients with American Society of Anesthesiologists physical status I to II, between the ages 16 and 65 were randomized into three groups. Group O and Group M were given IV ondansetron 8 mg and IT meperidine 0.2 mg/kg, before spinal anesthesia, respectively. Group C received IV saline 0.9%. The core and ambient temperatures, the incidence and intensity of shivering, blood pressure, heart rate, and maximum level of sensory block were recorded.

Findings:

Shivering was observed in 15%, 2.5%, and 37.5% of patients in Groups O, M, and C, respectively. There was a significant difference between Group O and M compared to Group C (P = 0.023 for Group O vs. Group C, P < 0.001 for Group M vs. Group C, P = 0.049 for Group M vs. Group O). Shivering incidence and intensity in Group M was significantly lower than Group O (P = 0.049 and P = 0.047, respectively). Twenty-two patients required additional IV meperidine among which 15 patients were from Group C (37.5%), six patients from Group O (15%) and one patient from Group M (2.5%).

Conclusion:

We concluded that IT meperidine and IV ondansetron comparably can decrease intensity and incidence of shivering compared to control group as well as decreasing the requirement to additional doses of meperidine for shivering the control without any hemodynamic side effect.

Pharmacologic options for reducing the shivering response to therapeutic hypothermia

Recent literature has demonstrated significant improvements in neurologic outcomes in patients who have received induced hypothermia in the setting of out-of-hospital cardiac arrest. Through multiple metabolic mechanisms, the induction of hypothermia slows the progression and devastation of transient cerebral hypoxia. Despite these benefits, the desired reduction in core temperature is often a challenging venture as the body attempts to maintain homeostasis through the induction of thermoregulatory processes aimed at elevating body temperature. Shivering is an involuntary muscular activity that enhances heat production in an attempt to restore homeostasis. For successful induction and maintenance of induced hypothermia, shivering, as well as other thermoregulatory responses, must be overcome. Several pharmacologic options are available, either used alone or in combination, that safely and effectively prevent or treat shivering after the induction of hypothermia. We conducted a PubMed search (1966-March 2009) to identify all human investigations published in English that discussed pharmacologic mechanisms for the control of shivering. Among these options, clonidine, dexmedetomidine, and meperidine have demonstrated the greatest and most clinically relevant impact on depression of the shivering threshold. More research in this area is needed, however, and the role of the clinical pharmacist in the development and implementation of this therapy needs to be defined.

Exercise and heat stress: cerebral challenges and consequences

This review deals with new aspects of exercise in the heat as a challenge that not only influences the locomotive and cardiovascular systems, but also affects the brain. Activation of the brain during such exercise is manifested in the lowering of the cerebral glucose to oxygen uptake ratio, the elevated ratings of perceived exertion and increased release of hypothalamic hormones. While the slowing of the electroencephalographic (EEG), the decreased endurance and hampered ability to activate the skeletal muscles maximally during sustained isometric and repeated isokinetic contractions appear to relate to central fatigue arising as the core/brain increases, the central fatigue during exercise with hyperthermia thus can be considered as the ultimate safety break against catastrophic hyperthermia. This would force the subject to stop exercising or decrease the internal heat production. It appears that the dopaminergic system is important, but several other factors may interact and feedback from the skeletal muscles and internal temperature sensors are probably also involved. The complexity of brain fatigue response is discussed based on our own investigations and in the light of recent literature.

Cerebral perturbations provoked by prolonged exercise

This review addresses cerebral metabolic and neurohumoral alterations during prolonged exercise in humans with special focus on associations with fatigue. Global energy turnover in the brain is unaltered by the transition from rest to moderately intense exercise, apparently because exercise-induced activation of some brain regions including cortical motor areas is compensated for by reduced activity in other regions of the brain. However, strenuous exercise is associated with cerebral metabolic and neurohumoral alterations that may relate to central fatigue. Fatigue should be acknowledged as a complex phenomenon influenced by both peripheral and central factors. However, failure to drive the motorneurons adequately as a consequence of neurophysiological alterations seems to play a dominant role under some circumstances. During exercise with hyperthermia excessive accumulation of heat in the brain due to impeded heat removal by the cerebral circulation may elevate the brain temperature to >40 degrees C and impair the ability to sustain maximal motor activation. Also, when prolonged exercise results in hypoglycaemia, perceived exertion increases at the same time as the cerebral glucose uptake becomes low, and centrally mediated fatigue appears to arise as the cerebral energy turnover becomes restricted by the availability of substrates for the brain. Changes in serotonergic activity, inhibitory feed-back from the exercising muscles, elevated ammonia levels, and alterations in regional dopaminergic activity may also contribute to the impaired voluntary activation of the motorneurons after prolonged and strenuous exercise. Furthermore, central fatigue may involve depletion of cerebral glycogen stores, as signified by the observation that following exhaustive exercise the cerebral glucose uptake increases out of proportion to that of oxygen. In summary, prolonged exercise may induce homeostatic disturbances within the central nervous system (CNS) that subsequently attenuates motor activation. Therefore, strenuous exercise is a challenge not only to the cardiorespiratory and locomotive systems but also to the brain.

Neurohumoral responses during prolonged exercise in humans

This study examined neurohumoral alterations during prolonged exercise with and without hyperthermia. The cerebral oxygen-to-carbohydrate uptake ratio (O2/CHO = arteriovenous oxygen difference divided by arteriovenous glucose difference plus one-half lactate), the cerebral balances of dopamine, and the metabolic precursor of serotonin, tryptophan, were evaluated in eight endurance-trained subjects during exercise randomized to be with or without hyperthermia. The core temperature stabilized at 37.9 +/- 0.1 degrees C (mean +/- SE) in the control trial, whereas it increased to 39.7 +/- 0.2 degrees C in the hyperthermic trial, with a concomitant increase in perceived exertion (P < 0.05). At rest, the brain had a small release of tryptophan (arteriovenous difference of -1.2 +/- 0.3 micromol/l), whereas a net balance was obtained during the two exercise trials. Both the arterial and jugular venous dopamine levels became elevated during the hyperthermic trial, but the net release from the brain was unchanged. During exercise, the O2/CHO was similar across trials, but, during recovery from the hyperthermic trial, the ratio decreased to 3.8 +/- 0.3 (P < 0.05), whereas it returned to the baseline level of approximately 6 within 5 min after the control trial. The lowering of O2/CHO was established by an increased arteriovenous glucose difference (1.1 +/- 0.1 mmol/l during recovery from hyperthermia vs. 0.7 +/- 0.1 mmol/l in control; P < 0.05). The present findings indicate that the brain has an increased need for carbohydrates during recovery from strenuous exercise, whereas enhanced perception of effort as observed during exercise with hyperthermia was not related to alterations in the cerebral balances of dopamine or tryptophan.

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.

Higher environmental temperature-induced increase in body temperature: involvement of serotonin in GABA mediated interaction of opioidergic system

Exposure (2 h) of adult male albino rats to higher environmental temperature (HET, 40 degrees C) significantly increased body temperature (BT). Administration of (a) 5-HTP (5 mg/kg, i.p.) or morphine (1 mg/kg, i.p.) or physostigmine (0.2 mg/kg, i.p.) alone significantly increased and (b) methysergide (1 mg/kg, i.p.) or naloxone (1 mg/kg, i.p.) or atropine (5 mg/kg, i.p.) reduced the BT of both normal and HET exposed rats. Further, it was observed that morphine prevented the methysergide-induced hypothermia and 5-HTP potentiated the morphine-induced hyperthermia in both normal and HET exposed conditions. Biochemical study also indicates that serotonin metabolism was increased but GABA utilization was reduced following exposure to HET.5-HTP or bicuculline-induced hyperthermia in control and HET exposed rat was potentiated with the coadministration of bicuculline and 5-HTP. The cotreatment of bicuculline with methysergide prevented the methysergide-induced attenuation of BT of heat exposed rat, rather BT was significantly enhanced indicating that inhibition of GABA system under heat exposed condition may activate the serotonergic activity. Further (a) enhancement of (i) morphine-induced hyperthermia with physostigmine (ii) physostigmine- or morphine+physostigmine-induced increase of BT with 5-HTP and (b) reduction of (i) morphine- or morphine + 5-HTP-induced hyperthermia with atropine and (ii) atropine-induced hypothermia with 5-HTP in both normal and HET exposed conditions suggest that HET exposure activates the cholinergic system through the activation of opioidergic and serotonergic system and hence increased the BT. Thus, it may be concluded that there is an involvement of serotonergic regulation in the opioidergic-cholinergic interaction via GABA system in HET-induced increase in BT.

Facilitation of masseter EMG and masseteric (jaw-closure) reflex by serotonin in behaving cats

The trigeminal motor nucleus (MoV) contains the somata of the motoneurons that control jaw position and jaw movements. This nucleus is of neurochemical interest because it receives a dense serotonergic input. We examined the effects of application of serotonin or fluoxetine, a serotonin reuptake blocker, into this nucleus on the spontaneous or reflex (jaw-closure) electrical activity of the masseter muscle in behaving cats. Serotonin produced a clearcut enhancement of both spontaneous and reflex activities. This action was attenuated by previous systemic injection of the serotonin receptor antagonist methysergide. The effect was mimicked to a certain extent by fluoxetine. These data provide evidence that the serotonergic input to MoV exerts a general facilitatory influence on masseter motoneurons activity.

Temperature-related release of serotonin from unrestrained rats' pre-optic area perfused with ethanol

The circumscribed effect of ethyl alcohol on the local efflux of serotonin (5-HT) within the thermosensitive region of the anterior hypothalamic, pre-optic area (AH/POA) of the unrestrained rat was examined in relation to core temperature. A single guide cannula for push-pull perfusion was implanted stereotaxically in the AH/POA within coronal planes AP 7.0-8.2. Following 3-4 push-pull perfusions with control artificial CSF of a site identified as reactive to 5-HT, ethanol in a concentration of 2.75 (471 mM) or 5.5 (942 mM) percent was perfused at the same locus over a 5-10 min interval. Successive samples of perfusate were assayed for their content of 5-HT by high performance liquid chromatography using electrochemical detection (HPLC-EC). Within a circumscribed region of the AH/POA of the rat maintained at an ambient temperature of 22 degrees C, ethanol induced either an immediate or delayed hypothermia of short latency or a transient decline followed by an immediate increase in core temperature. In each case, the shift in temperature depended on the anatomical site of push-pull perfusion. Overall, the fall in core temperature was accompanied by an inhibition in the efflux of 5-HT. However, the consequent rise in the rat's core temperature was associated with an enhanced release of 5-HT in the samples of perfusate collected from the AH/POA. These results suggest that serotonergic synapses within the AH/POA are apparently involved in the thermolytic effects of ethanol as well as in the thermogenesis following the interval of heat loss.

Hyperthermia induced by morphine administration to the VTA of the rat brain: an effect dissociable from morphine-induced reward and hyperactivity

Morphine action at opiate receptors in the ventral tegmental area (VTA) of the rat brain has been implicated in the production of increased locomotor activity and in morphine's rewarding properties. In the present experiments, bilateral administration of morphine (18 micrograms tapped into the tips of 28 gauge cannulae) into the VTA resulted in an increase in body temperature in rats. This effect was both reversed and blocked by a systemic injection of the opiate receptor blocker, naloxone, suggesting that it was due to morphine action at opiate receptors. The neuroleptic, pimozide, injected systemically four hours prior to morphine administration completely blocked the increased locomotor activity but had no effect on the hyperthermia. These data demonstrate that the hyperthermia was not brought about by the increased physical activity. Furthermore, these results suggest that while morphine-induced reward and increased locomotor activity may be mediated by an interaction of morphine and the ascending mesolimbic dopamine system, the hyperthermia is not. In an additional experiment, the effect of systemic injections of the central neurotransmitter receptor antagonists, scopolamine, phenoxybenzamine, and methergoline, on the hyperthermia induced by morphine in the VTA was investigated. Only the serotonin antagonist, methergoline, attenuated the hyperthermia.

Capsaicin impairs preoptic serotonin-sensitive structures mediating hypothermia in rats

In an attempt to elucidate the possible mechanisms involved in the development of the thermoregulatory disturbances induced by systemic capsaicin treatment, the effects of 5-HT injected into the preoptic region or into the cisterna magna on the body temperature and on tail skin vasodilation were studied in control and capsaicin-treated rats. Intracisternal 5-HT elicited a comparable decrease in body temperature in both groups of animals. In contrast, intrapreoptic injection of different doses of 5-HT-induced tail skin vasodilation and hypothermia in the controls, but not in the capsaicin-treated rats. It is suggested that changes in the sensitivity of preoptic warm-responsive structures to 5-HT may contribute essentially to the specific thermoregulatory impairment brought about by systemic capsaicin treatment.

Changes in the hypothalamic mechanisms involved in the control of body temperature induced by the early thermal environment

The experiments reported in this study were designed to investigate the influence of the early thermal environment on the functional properties of certain putative thermoregulatory neurotransmitters within the hypothalamus of the Sprague-Dawley rat. The effects on body temperature of serotonin, dopamine, carbachol, PGE2 and endotoxin when microinjected into the anterior hypothalamic/preoptic region of the brain have been examined in warm-reared, control and warm-acclimated animals. Serotonin and PGE2 are shown to have different effects on body temperature in warm-reared as compared to warm-acclimated and control animals. The thermoregulatory effects of intrahypothalamic dopamine are shown to be changed by the normal acclimation process, while carbachol and endotoxin have similar effects on the body temperature of all 3 groups of animals. These data suggest that the thermal environment may significantly affect the roles which specific neurotransmitters play in the control of body temperature.

A role for an indoleamine other than 5-hydroxytryptamine in the hypothalamic thermoregulatory pathways of the rat

1. Intrahypothalamic injection of either 5-hydroxytryptamine (5-HT) (20 mug) or tryptamine (1 mug) caused hypothermia and hyperthermia respectively in lightly restrained rats maintained at an ambient temperature of 20 +/- 1 degrees C.2. Both the 5-HT- and the tryptamine-sensitive sites were located within the same region of the preoptic area.3. When rats were tested at different ambient temperatures (4, 20 and 29 degrees C), intrahypothalamic injection of 5-HT caused a marked fall in core temperature (-1.3 degrees C) in rats maintained at 4 degrees C, but smaller responses were obtained at 20 and 29 degrees C (-0.9 and -0.5 degrees C respectively). Tryptamine caused a significant hyperthermia in rats kept at 20 degrees C, but had no significant effect in rats maintained at either 4 or 29 degrees C.4. The hypothermic effect of 5-HT was selectively antagonized by systemic pre-treatment with cyproheptadine (2.5 mg/kg), but not by methergoline (0.625 mg/kg) and methysergide (0.2 mg/kg). In contrast, the hyperthermic effect of tryptamine was blocked by methergoline and methysergide, but not by cyproheptadine.5. Cyproheptadine (2.5 mg/kg) reduced the ability of rats to cope with a heat load but had no effect on the response to cold. In contrast, methergoline (0.625 mg/kg) and methysergide (0.2 mg/kg) reduced the ability to cope with cold but the rats' ability to cope with a heat load remained intact.6. These results suggest the existence of two indoleamine pathways within the preoptic anterior hypothalamus involved in the control of body temperature: a serotonergic pathway mediating heat loss and a non-serotonergic pathway mediating heat gain. The non-serotonergic system may exert its effects by modulating the activity of a central serotonergic system.

Changes in body temperature after administration of adrenergic and serotonergic agents and related drugs including antidepressants

This survey, the third in a series, presents extensive tabulations of literature, primarily since 1965, on thermoregulatory effects of adrenergic and serotonergic agonists and their antagonists including ergot alkaloids, amphetamines, tryptamines, monoamine oxidase inhibitors, tricyclic and other antidepressants, a variety of other agents which alter presynaptic aminergic mechanisms including reserpine, 6-hydroxydopamine, p-chlorophenylalanine, alpha-methyltyrosines, cocaine, guanethidine and bretylium. 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 drug.

Thermoregulatory noradrenergic and serotonergic pathways to hypothalamic units

1. In guinea-pigs hypothalamic single units were extracellularly tested for their response to thermal stimulation of the skin and to electrical stimulation of two different pontine areas, the nucleus raphé magnus and the dorsomedial reticular formation. Furthermore, thermoregulatory control actions were measured during the stimulations.2. Electrical stimulation of those reticular formation areas containing noradrenaline cells caused an increase of oxygen uptake, electrical muscle activity and body temperature, while stimulation of the nucleus raphé magnus, known to contain serotonin cells, brought about inhibition or had no effect.3. The recorded units could be subdivided into three groups. Cell type a. Neurones on the boundary of preoptic and anterior hypothalamic regions which increased their firing rate when the skin was cooled and decreased it when the nucleus raphé magnus was stimulated. Cell type b. Neurones in the anterior hypothalamus which did not respond to brain-stem stimulation. Cell type c. More posterior neurones which increased their firing rate when the skin was warmed or when the nucleus raphé magnus was stimulated and decreased their firing rate when the reticular formation was stimulated.4. Cell type a seems to represent interneurones which are connected to the ascending serotonergic thermoregulatory pathway. As for cell type c, it is inferred that it could represent interneurones which control the threshold for shivering and non-shivering thermogenesis.

Opposing temperature responses to intrahypothalamic injections of 5-hydroxytryptamine in the pigeon exposed to cold

5-Hydroxytryptamine injected into posterior and anterior parts of the pigeon hypothalamus evoked a short lasting hyperthermia or hypothermia, respectively. Variable responses obtained within the same brain region suggest the existence of different 5-HT systems, even in rather limited hypothalamic areas.

Precise location within the preoptic area where noradrenaline produces hypothermia

Noradrenaline (5 microgram in 0.5 microliter) was microinjected into 87 different sites within the preoptic anterior hypothermic area of cat brain to determine the anatomical location most sensitive to the hypothermic action of the amine. At 26 of these sites noradrenaline produced a hypothermia greater than 0.5 degrees C. Such falls in body temperature were invariably accompanied by vasodilation and sometimes by a marked increase in respiratory rate. During the hypothermia most animals appeared sedated. Histological analysis of the sites where hypothermia was produced indicated that the site of maximum sensitivity occurred between the optic chiasm and the anterior commissure at A 15.0, L 2.5, H--2.5. The results are discussed in terms of the physiological control of body temperature.