Microinjection of prostaglandin E2 and muscimol into the preoptic area in conscious rats: comparison of effects on plasma adrenocorticotrophic hormone (ACTH), body temperature, locomotor activity, and cardiovascular function

A hypothalamomedullary network for physiological responses to environmental stresses

Various environmental stressors, such as extreme temperatures (hot and cold), pathogens, predators and insufficient food, can threaten life. Remarkable progress has recently been made in understanding the central circuit mechanisms of physiological responses to such stressors. A hypothalamomedullary neural pathway from the dorsomedial hypothalamus (DMH) to the rostral medullary raphe region (rMR) regulates sympathetic outflows to effector organs for homeostasis. Thermal and infection stress inputs to the preoptic area dynamically alter the DMH → rMR transmission to elicit thermoregulatory, febrile and cardiovascular responses. Psychological stress signalling from a ventromedial prefrontal cortical area to the DMH drives sympathetic and behavioural responses for stress coping, representing a psychosomatic connection from the corticolimbic emotion circuit to the autonomic and somatic motor systems. Under starvation stress, medullary reticular neurons activated by hunger signalling from the hypothalamus suppress thermogenic drive from the rMR for energy saving and prime mastication to promote food intake. This Perspective presents a combined neural network for environmental stress responses, providing insights into the central circuit mechanism for the integrative regulation of systemic organs.

Prostaglandin E2 depresses GABA release onto parvocellular neuroendocrine neurones in the paraventricular nucleus of the hypothalamus via presynaptic receptors

Inflammation-induced activation of the hypothalamic-pituitary-adrenal (HPA) axis and the ensuing release of anti-inflammatory glucocorticoids are critical for the fine-tuning of the inflammatory response. This immune-induced neuroendocrine response is in large part mediated by prostaglandin E₂ (PGE₂ ), the central actions of which ultimately translate into the excitation of parvocellular neuroendocrine cells (PNCs) in the hypothalamic paraventricular nucleus. However, the neuronal mechanisms by which PGE₂ excites PNCs remain incompletely understood. In the present study, we report that PGE₂ potently depresses GABAergic inhibitory synaptic transmission onto PNCs. Using whole-cell patch clamp recordings obtained from PNCs in ex vivo hypothalamic slices from rats, we found that bath application of PGE₂ (0.01-100 μmol L^-1 ) concentration-dependently decreased the amplitude of evoked inhibitory postsynaptic currents (eIPSCs) with maximum effects at 10 μmol L^-1 . The PGE₂ -mediated depression of eIPSCs had a rapid onset and was long-lasting, and also was accompanied by an increase in paired pulse ratio. In addition, PGE₂ decreased the frequency but not the amplitude of both spontaneous IPSCs and miniature IPSCs. These results collectively indicate that PGE₂ acts at a presynaptic locus to decrease the probability of GABA release. Using pharmacological approaches, we also demonstrated that the EP3 subtype of the PGE₂ receptor mediated the actions of PGE₂ on GABA synapses. Taken together, our results show that PGE₂ , via actions of presynaptic EP3 receptors, potently depresses GABA release onto PNCs, providing a plausible mechanism for the disinhibition of HPA axis output during inflammation.

Central Mechanisms for Thermoregulation

Maintenance of a homeostatic body core temperature is a critical brain function accomplished by a central neural network. This orchestrates a complex behavioral and autonomic repertoire in response to environmental temperature challenges or declining energy homeostasis and in support of immune responses and many behavioral states. This review summarizes the anatomical, neurotransmitter, and functional relationships within the central neural network that controls the principal thermoeffectors: cutaneous vasoconstriction regulating heat loss and shivering and brown adipose tissue for heat production. The core thermoregulatory network regulating these thermoeffectors consists of parallel but distinct central efferent pathways that share a common peripheral thermal sensory input. Delineating the neural circuit mechanism underlying central thermoregulation provides a useful platform for exploring its functional organization, elucidating the molecular underpinnings of its neuronal interactions, and discovering novel therapeutic approaches to modulating body temperature and energy homeostasis.

Neuroleptic malignant syndrome and serotonin syndrome

The clinical manifestation of drug-induced abnormalities in thermoregulation occurs across a variety of drug mechanisms. The aim of this chapter is to review two of the most common drug-induced hyperthermic states, serotonin syndrome and neuroleptic malignant syndrome. Clinical features, pathophysiology, and treatment strategies will be discussed, in addition to differentiating between these two syndromes and differentiating them from other hyperthermic or febrile syndromes. Our goal is to both review the current literature and to provide a practical guide to identification and treatment of these potentially life-threatening illnesses. The diagnostic and treatment recommendations made by us, and by other authors, are likely to change with a better understanding of the pathophysiology of these syndromes.

Afferent pathways for autonomic and shivering thermoeffectors

Body core temperature of mammals is regulated by the central nervous system, in which the preoptic area (POA) of the hypothalamus plays a pivotal role. The POA receives peripheral and central thermosensory neural information and provides command signals to effector organs to elicit involuntary thermoregulatory responses, including shivering thermogenesis, nonshivering brown adipose tissue thermogenesis, and cutaneous vasoconstriction. Cool-sensory and warm-sensory signals from cutaneous thermoreceptors, monitoring environmental temperature, are separately transmitted through the spinal-parabrachial-POA neural pathways, distinct from the spinothalamocortical pathway for perception of skin temperature. These cutaneous thermosensory inputs to the POA likely impinge on warm-sensitive POA neurons, which monitor body core (brain) temperature, to alter thermoregulatory command outflows from the POA. The cutaneous thermosensory afferents elicit rapid thermoregulatory responses to environmental thermal challenges before they impact body core temperature. Peripheral humoral signals also act on neurons in the POA to transmit afferent information of systemic infection and energy storage to induce fever and to regulate energy balance, respectively. This chapter describes the thermoregulatory afferent mechanisms that convey cutaneous thermosensory signals to the POA and that integrate the neural and humoral afferent inputs to the POA to provide descending command signals to thermoregulatory effectors.

Role of the lateral preoptic area in cardiovascular and neuroendocrine responses to acute restraint stress in rats

The lateral preoptic area (LPO) is connected with limbic structures involved in physiological and behavioral responses to stress. Accordingly, exposure to stressors stimuli activates neurons within the LPO. In spite of these evidence, an involvement of the LPO on cardiovascular and neuroendocrine adjustments during aversive threats has not yet been investigated. Therefore, in the present study we tested the hypothesis that the LPO is involved in the control of cardiovascular and neuroendocrine responses to acute restraint stress in rats. Bilateral microinjection of the nonselective synaptic blocker CoCl₂ (0.1nmol/100nl) into the LPO did not affect basal values of either arterial pressure, heart rate, tail skin temperature, or plasma corticosterone concentration. However, LPO treatment with CoCl₂ enhanced the tachycardiac response and the increase in plasma corticosterone concentration caused by restraint stress. Conversely, LPO synaptic blockade decreased restraint-evoked pressor response. Sympathetic-mediated cutaneous vasoconstriction during restraint stress was not affected by LPO pharmacological treatment. These findings indicate an inhibitory influence of LPO on tachycardiac and plasma corticosterone responses evoked during aversive threats. Additionally, data suggest that LPO plays a facilitatory influence on stress-evoked pressor response.

A hypothalamic circuit that controls body temperature

The homeostatic control of body temperature is essential for survival in mammals and is known to be regulated in part by temperature-sensitive neurons in the hypothalamus. However, the specific neural pathways and corresponding neural populations have not been fully elucidated. To identify these pathways, we used cFos staining to identify neurons that are activated by a thermal challenge and found induced expression in subsets of neurons within the ventral part of the lateral preoptic nucleus (vLPO) and the dorsal part of the dorsomedial hypothalamus (DMD). Activation of GABAergic neurons in the vLPO using optogenetics reduced body temperature, along with a decrease in physical activity. Optogenetic inhibition of these neurons resulted in fever-level hyperthermia. These GABAergic neurons project from the vLPO to the DMD and optogenetic stimulation of the nerve terminals in the DMD also reduced body temperature and activity. Electrophysiological recording revealed that the vLPO GABAergic neurons suppressed neural activity in DMD neurons, and fiber photometry of calcium transients revealed that DMD neurons were activated by cold. Accordingly, activation of DMD neurons using designer receptors exclusively activated by designer drugs (DREADDs) or optogenetics increased body temperature with a strong increase in energy expenditure and activity. Finally, optogenetic inhibition of DMD neurons triggered hypothermia, similar to stimulation of the GABAergic neurons in the vLPO. Thus, vLPO GABAergic neurons suppressed the thermogenic effect of DMD neurons. In aggregate, our data identify vLPO→DMD neural pathways that reduce core temperature in response to a thermal challenge, and we show that outputs from the DMD can induce activity-induced thermogenesis.

Antipsychotic inductors of brain hypothermia and torpor-like states: perspectives of application

Hypothermia and hypometabolism (hypometabothermia) normally observed during natural hibernation and torpor, allow animals to protect their body and brain against the damaging effects of adverse environment. A similar state of hypothermia can be achieved under artificial conditions through physical cooling or pharmacological effects directed at suppression of metabolism and the processes of thermoregulation. In these conditions called torpor-like states, the mammalian ability to recover from stroke, heart attack, and traumatic injuries greatly increases. Therefore, the development of therapeutic methods for different pathologies is a matter of great concern. With the discovery of the antipsychotic drug chlorpromazine in the 1950s of the last century, the first attempts to create a pharmacologically induced state of hibernation for therapeutic purposes were made. That was the beginning of numerous studies in animals and the broad use of therapeutic hypothermia in medicine. Over the last years, many new agents have been discovered which were capable of lowering the body temperature and inhibiting the metabolism. The psychotropic agents occupy a significant place among them, which, in our opinion, is not sufficiently recognized in the contemporary literature. In this review, we summarized the latest achievements related to the ability of modern antipsychotics to target specific receptors in the brain, responsible for the initiation of hypometabothermia.

Central neural control of thermoregulation and brown adipose tissue

Central neural circuits orchestrate the homeostatic repertoire that maintains body temperature during environmental temperature challenges and alters body temperature during the inflammatory response. This review summarizes the experimental underpinnings of our current model of the CNS pathways controlling the principal thermoeffectors for body temperature regulation: cutaneous vasoconstriction controlling heat loss, and shivering and brown adipose tissue for thermogenesis. The activation of these effectors is regulated by parallel but distinct, effector-specific, core efferent pathways within the CNS that share a common peripheral thermal sensory input. Via the lateral parabrachial nucleus, skin thermal afferent input reaches the hypothalamic preoptic area to inhibit warm-sensitive, inhibitory output neurons which control heat production by inhibiting thermogenesis-promoting neurons in the dorsomedial hypothalamus that project to thermogenesis-controlling premotor neurons in the rostral ventromedial medulla, including the raphe pallidus, that descend to provide the excitation of spinal circuits necessary to drive thermogenic thermal effectors. A distinct population of warm-sensitive preoptic neurons controls heat loss through an inhibitory input to raphe pallidus sympathetic premotor neurons controlling cutaneous vasoconstriction. The model proposed for central thermoregulatory control provides a useful platform for further understanding of the functional organization of central thermoregulation and elucidating the hypothalamic circuitry and neurotransmitters involved in body temperature regulation.

Central nervous system regulation of brown adipose tissue

Thermogenesis, the production of heat energy, in brown adipose tissue is a significant component of the homeostatic repertoire to maintain body temperature during the challenge of low environmental temperature in many species from mouse to man and plays a key role in elevating body temperature during the febrile response to infection. The sympathetic neural outflow determining brown adipose tissue (BAT) thermogenesis is regulated by neural networks in the CNS which increase BAT sympathetic nerve activity in response to cutaneous and deep body thermoreceptor signals. Many behavioral states, including wakefulness, immunologic responses, and stress, are characterized by elevations in core body temperature to which central command-driven BAT activation makes a significant contribution. Since energy consumption during BAT thermogenesis involves oxidation of lipid and glucose fuel molecules, the CNS network driving cold-defensive and behavioral state-related BAT activation is strongly influenced by signals reflecting the short- and long-term availability of the fuel molecules essential for BAT metabolism and, in turn, the regulation of BAT thermogenesis in response to metabolic signals can contribute to energy balance, regulation of body adipose stores and glucose utilization. This review summarizes our understanding of the functional organization and neurochemical influences within the CNS networks that modulate the level of BAT sympathetic nerve activity to produce the thermoregulatory and metabolic alterations in BAT thermogenesis and BAT energy expenditure that contribute to overall energy homeostasis and the autonomic support of behavior.

Fever, immunity, and molecular adaptations

The heat shock response (HSR) is an ancient and highly conserved process that is essential for coping with environmental stresses, including extremes of temperature. Fever is a more recently evolved response, during which organisms temporarily subject themselves to thermal stress in the face of infections. We review the phylogenetically conserved mechanisms that regulate fever and discuss the effects that febrile-range temperatures have on multiple biological processes involved in host defense and cell death and survival, including the HSR and its implications for patients with severe sepsis, trauma, and other acute systemic inflammatory states. Heat shock factor-1, a heat-induced transcriptional enhancer is not only the central regulator of the HSR but also regulates expression of pivotal cytokines and early response genes. Febrile-range temperatures exert additional immunomodulatory effects by activating mitogen-activated protein kinase cascades and accelerating apoptosis in some cell types. This results in accelerated pathogen clearance, but increased collateral tissue injury, thus the net effect of exposure to febrile range temperature depends in part on the site and nature of the pathologic process and the specific treatment provided.

Distribution of Fos-immunoreactive cells in rat forebrain and midbrain following social defeat stress and diazepam treatment

The anxiolytic diazepam selectively inhibits psychological stress-induced autonomic and behavioral responses without causing noticeable suppression of other central performances. This pharmacological property of diazepam led us to the idea that neurons that exhibit diazepam-sensitive, psychological stress-induced activation are potentially those recruited for stress responses. To obtain neuroanatomical clues for the central stress circuitries, we examined the effects of diazepam on psychological stress-induced neuronal activation in broad brain regions. Rats were exposed to a social defeat stress, which caused an abrupt increase in body temperature by up to 2°C. Pretreatment with diazepam (4mg/kg, i.p.) attenuated the stress-induced hyperthermia, confirming an inhibitory physiological effect of diazepam on the autonomic stress response. Subsequently, the distribution of cells expressing Fos, a marker of neuronal activation, was examined in 113 forebrain and midbrain regions of these rats after the stress exposure and diazepam treatment. The stress following vehicle treatment markedly increased Fos-immunoreactive (IR) cells in most regions of the cerebral cortex, limbic system, thalamus, hypothalamus and midbrain, which included parts of the autonomic, neuroendocrine, emotional and arousal systems. The diazepam treatment significantly reduced the stress-induced Fos expression in many brain regions including the prefrontal, sensory and motor cortices, septum, medial amygdaloid nucleus, medial and lateral preoptic areas, parvicellular paraventricular hypothalamic nucleus, dorsomedial hypothalamus, perifornical nucleus, tuberomammillary nucleus, association, midline and intralaminar thalami, and median and dorsal raphe nuclei. In contrast, diazepam increased Fos-IR cells in the central amygdaloid nucleus, medial habenular nucleus, ventromedial hypothalamic nucleus and magnocellular lateral hypothalamus. These results provide important information for elucidating the neural circuitries that mediate the autonomic and behavioral responses to psychosocial stressors.

Autonomic regulation of brown adipose tissue thermogenesis in health and disease: potential clinical applications for altering BAT thermogenesis

From mouse to man, brown adipose tissue (BAT) is a significant source of thermogenesis contributing to the maintenance of the body temperature homeostasis during the challenge of low environmental temperature. In rodents, BAT thermogenesis also contributes to the febrile increase in core temperature during the immune response. BAT sympathetic nerve activity controlling BAT thermogenesis is regulated by CNS neural networks which respond reflexively to thermal afferent signals from cutaneous and body core thermoreceptors, as well as to alterations in the discharge of central neurons with intrinsic thermosensitivity. Superimposed on the core thermoregulatory circuit for the activation of BAT thermogenesis, is the permissive, modulatory influence of central neural networks controlling metabolic aspects of energy homeostasis. The recent confirmation of the presence of BAT in human and its function as an energy consuming organ have stimulated interest in the potential for the pharmacological activation of BAT to reduce adiposity in the obese. In contrast, the inhibition of BAT thermogenesis could facilitate the induction of therapeutic hypothermia for fever reduction or to improve outcomes in stroke or cardiac ischemia by reducing infarct size through a lowering of metabolic oxygen demand. This review summarizes the central circuits for the autonomic control of BAT thermogenesis and highlights the potential clinical relevance of the pharmacological inhibition or activation of BAT thermogenesis.

Cyclooxygenase-2-related signaling in the hypothalamus plays differential roles in response to various acute stresses

We previously suggested that cyclooxygenase (COX)-2 plays a role as a common mediator of stresses in the brain. In the present study, we evaluated the possible involvement of COX-2-related signaling in the activation of the hypothalamic-pituitary-adrenal (HPA) axis under three different stress conditions, namely infectious (lipopolysaccharide, LPS), hypoglycemic (2-deoxy-d-glucose, 2DG) and restraint (1h) stresses in rats. Both an unselective COX inhibitor (indomethacin) and a selective COX-2 inhibitor (NS-398) significantly attenuated the increase of serum corticosterone levels after LPS and restraint stresses, but not after 2DG injection. COX-2 and microsomal prostaglandin E synthase (mPGES)-1 mRNA levels in the hypothalamus were significantly increased after LPS injection in intact rats. In adrenalectomized (ADX) rats, the expression of both genes was significantly increased after 2DG and restraint stresses, which was blocked by treatment with corticosterone. Interleukin-1β (IL-1β) mRNA levels in the hypothalamus in intact rats were increased only by LPS injection, though those in ADX rats were increased by all three stress stimuli. These results suggest that the relationship between COX-2-related signaling and activation of the HPA axis is stress-specific, and that COX-2-related signaling preferably mediates infectious and restraint stresses. Furthermore, the expression of COX-2 and mPGES-1 mRNA under the infectious stress condition was not negatively regulated by endogenous glucocorticoids, likely due to an increase in IL-1β levels.

Estrous cycle variations in GABA(A) receptor phosphorylation enable rapid modulation by anabolic androgenic steroids in the medial preoptic area

Anabolic androgenic steroids (AAS), synthetic testosterone derivatives that are used for ergogenic purposes, alter neurotransmission and behaviors mediated by GABA(A) receptors. Some of these effects may reflect direct and rapid action of these synthetic steroids at the receptor. The ability of other natural allosteric steroid modulators to alter GABA(A) receptor-mediated currents is dependent upon the phosphorylation state of the receptor complex. Here we show that phosphorylation of the GABA(A) receptor complex immunoprecipitated by β(2)/β(3) subunit-specific antibodies from the medial preoptic area (mPOA) of the mouse varies across the estrous cycle; with levels being significantly lower in estrus. Acute exposure to the AAS, 17α-methyltestosterone (17α-MeT), had no effect on the amplitude or kinetics of inhibitory postsynaptic currents in the mPOA of estrous mice when phosphorylation was low, but increased the amplitude of these currents from mice in diestrus, when it was high. Inclusion of the protein kinase C (PKC) inhibitor, calphostin, in the recording pipette eliminated the ability of 17α-MeT to enhance currents from diestrous animals, suggesting that PKC-receptor phosphorylation is critical for the allosteric modulation elicited by AAS during this phase. In addition, a single injection of 17α-MeT was found to impair an mPOA-mediated behavior (nest building) in diestrus, but not in estrus. PKC is known to target specific serine residues in the β(3) subunit of the GABA(A) receptor. Although phosphorylation of these β(3) serine residues showed a similar profile across the cycle, as did phosphoserine in mPOA lysates immunoprecipitated with β2/β3 antibody (lower in estrus than in diestrus or proestrus), the differences were not significant. These data suggest that the phosphorylation state of the receptor complex regulates both the ability of AAS to modulate receptor function in the mPOA and the expression of a simple mPOA-dependent behavior through a PKC-dependent mechanism that involves the β(3) subunit and other sites within the GABA(A) receptor complex.

Central control of brown adipose tissue thermogenesis

Thermogenesis, the production of heat energy, is an essential component of the homeostatic repertoire to maintain body temperature during the challenge of low environmental temperature and plays a key role in elevating body temperature during the febrile response to infection. Mitochondrial oxidation in brown adipose tissue (BAT) is a significant source of neurally regulated metabolic heat production in many species from mouse to man. BAT thermogenesis is regulated by neural networks in the central nervous system which responds to feedforward afferent signals from cutaneous and core body thermoreceptors and to feedback signals from brain thermosensitive neurons to activate BAT sympathetic nerve activity. This review summarizes the research leading to a model of the feedforward reflex pathway through which environmental cold stimulates BAT thermogenesis and includes the influence on this thermoregulatory network of the pyrogenic mediator, prostaglandin E(2), to increase body temperature during fever. The cold thermal afferent circuit from cutaneous thermal receptors, through second-order thermosensory neurons in the dorsal horn of the spinal cord ascends to activate neurons in the lateral parabrachial nucleus which drive GABAergic interneurons in the preoptic area (POA) to inhibit warm-sensitive, inhibitory output neurons of the POA. The resulting disinhibition of BAT thermogenesis-promoting neurons in the dorsomedial hypothalamus activates BAT sympathetic premotor neurons in the rostral ventromedial medulla, including the rostral raphe pallidus, which provide excitatory, and possibly disinhibitory, inputs to spinal sympathetic circuits to drive BAT thermogenesis. Other recently recognized central sites influencing BAT thermogenesis and energy expenditure are also described.

Central circuitries for body temperature regulation and fever

Body temperature regulation is a fundamental homeostatic function that is governed by the central nervous system in homeothermic animals, including humans. The central thermoregulatory system also functions for host defense from invading pathogens by elevating body core temperature, a response known as fever. Thermoregulation and fever involve a variety of involuntary effector responses, and this review summarizes the current understandings of the central circuitry mechanisms that underlie nonshivering thermogenesis in brown adipose tissue, shivering thermogenesis in skeletal muscles, thermoregulatory cardiac regulation, heat-loss regulation through cutaneous vasomotion, and ACTH release. To defend thermal homeostasis from environmental thermal challenges, feedforward thermosensory information on environmental temperature sensed by skin thermoreceptors ascends through the spinal cord and lateral parabrachial nucleus to the preoptic area (POA). The POA also receives feedback signals from local thermosensitive neurons, as well as pyrogenic signals of prostaglandin E(2) produced in response to infection. These afferent signals are integrated and affect the activity of GABAergic inhibitory projection neurons descending from the POA to the dorsomedial hypothalamus (DMH) or to the rostral medullary raphe region (rMR). Attenuation of the descending inhibition by cooling or pyrogenic signals leads to disinhibition of thermogenic neurons in the DMH and sympathetic and somatic premotor neurons in the rMR, which then drive spinal motor output mechanisms to elicit thermogenesis, tachycardia, and cutaneous vasoconstriction. Warming signals enhance the descending inhibition from the POA to inhibit the motor outputs, resulting in cutaneous vasodilation and inhibited thermogenesis. This central thermoregulatory mechanism also functions for metabolic regulation and stress-induced hyperthermia.

Neural controls of prostaglandin 2 pyrogenic, tachycardic, and anorexic actions are anatomically distributed

Fever and anorexia are induced by immune system challenges. Because these responses are adaptive when short lasting but deleterious when prolonged, an understanding of the mediating neural circuitry is important. Prostaglandins (PGE) are a critical signaling element for these immune responses. Despite the widespread distribution of PGE receptors throughout the brain, research focuses on the hypothalamic preoptic area as the mediating site of PGE action. Paraventricular nucleus of the hypothalamus (PVH), parabrachial nucleus (PBN), and nucleus tractus solitarius (NTS) neurons also express PGE receptors and are activated during systemic pathogen infection. A role for these neurons in PGE-induced fever, tachycardia, and anorexia is unexplored and is the subject of this report. A range of PGE₂ doses was microinjected into third or fourth ventricles (v), or directly into the dorsal PVH, lateral PBN, and medial NTS, and core and brown adipose tissue temperature, heart rate, locomotor activity, and food intake were measured in awake, behaving rats. PGE₂ delivery to multiple brain sites (third or fourth v, PVH, or PBN) induced a short- latency (< 10 min) fever and tachycardia. By contrast, an anorexic effect was observed only in response to third v and PVH stimulation. NTS PGE₂ stimulation was without effect; locomotor activity was not affected for any of the sites. The data are consistent with a view of PGE₂-induced effects as mediated by anatomically distributed sites rather than a single center. The data also underscore a potential anatomical dissociation of the neural pathways mediating pyrogenic and anorexic effects of PGE₂.

The role of orexin-1 receptors in physiologic responses evoked by microinjection of PgE2 or muscimol into the medial preoptic area

The medial preoptic area (mPOA) of the hypothalamus has long been thought to play an important role in both fever production and thermoregulation. Microinjections of prostaglandin E2 (PgE2) or the GABA(A) agonist muscimol into the mPOA cause similar increases in body temperature, heart rate, and blood pressure. Microinjections of these compounds however evoke different behavioral responses with muscimol increasing and PgE2 having no effect on locomotion. The purpose of this study was to determine the role of orexin-1 receptors in mediating these dissimilar responses. Systemic injections of the orexin-1 receptor antagonist SB-334867 reduced temperature and cardiovascular responses produced by microinjections of muscimol, but had no effect on either response produced by PgE2. SB-334867 did not significantly decrease locomotion evoked by microinjections of muscimol into the mPOA. These data suggest that there are two central nervous system circuits involved in increasing body temperature, heart rate and blood pressure: one circuit activated by muscimol, involving orexin neurons, and a separate orexin-independent circuit activated by PgE2.

Stress-free microinjections in conscious rats

Microinjections are a major tool in modern neuroscience. Microinjection techniques in conscious animals typically involve four steps: (1) animal adapts to experimental setup; (2) injection system is filled and the microinjector is carefully inserted; (3) a drug solution is injected; (4) 1-2 min later the microinjector is carefully removed. Steps 2 and 4 are difficult to perform in rodents without disturbing the animal. This disruption can cause stress and accompanying tachycardia and hyperthermia - unwanted artifacts in physiological research. To reduce these effects, we altered the traditional approach. Our procedure of microinjection consisted of the following steps: (1) we filled the injection setup and fixed the microinjector in its guide cannula; (2) allowed an animal to adapt to the setup; (3) performed an experiment including microinjection(s); (4) removed the microinjector after the experiment was complete. The key change we incorporated was a 1m long piece of tubing with a small internal diameter; it allowed us to inject nanoliter volumes through the injector which had been placed into the guide cannula in advance. This way we avoided the usual manipulations related to microinjection, and minimized extraneous disturbances to the rat. In this report we describe the details of this technique in conscious rats and provide examples of the effects and the reproducibility of a 100 nL drug injection on cardiovascular function.

2010 Carl Ludwig Distinguished Lectureship of the APS Neural Control and Autonomic Regulation Section: Central neural pathways for thermoregulatory cold defense

Central neural circuits orchestrate the homeostatic repertoire to maintain body temperature during environmental temperature challenges and to alter body temperature during the inflammatory response. This review summarizes the research leading to a model representing our current understanding of the neural pathways through which cutaneous thermal receptors alter thermoregulatory effectors: the cutaneous circulation for control of heat loss, and brown adipose tissue, skeletal muscle, and the heart for thermogenesis. The activation of these effectors is regulated by parallel but distinct, effector-specific core efferent pathways within the central nervous system (CNS) that share a common peripheral thermal sensory input. The thermal afferent circuit from cutaneous thermal receptors includes neurons in the spinal dorsal horn projecting to lateral parabrachial nucleus neurons that project to the medial aspect of the preoptic area. Within the preoptic area, warm-sensitive, inhibitory output neurons control heat production by reducing the discharge of thermogenesis-promoting neurons in the dorsomedial hypothalamus. The rostral ventromedial medulla, including the raphe pallidus, receives projections form the dorsomedial hypothalamus and contains spinally projecting premotor neurons that provide the excitatory drive to spinal circuits controlling the activity of thermogenic effectors. A distinct population of warm-sensitive preoptic neurons controls heat loss through an inhibitory input to raphe pallidus sympathetic premotor neurons controlling cutaneous vasoconstriction. The model proposed for central thermoregulatory control provides a platform for further understanding of the functional organization of central thermoregulation.

Central neural pathways for thermoregulation

Central neural circuits orchestrate a homeostatic repertoire to maintain body temperature during environmental temperature challenges and to alter body temperature during the inflammatory response. This review summarizes the functional organization of the neural pathways through which cutaneous thermal receptors alter thermoregulatory effectors: the cutaneous circulation for heat loss, the brown adipose tissue, skeletal muscle and heart for thermogenesis and species-dependent mechanisms (sweating, panting and saliva spreading) for evaporative heat loss. These effectors are regulated by parallel but distinct, effector-specific neural pathways that share a common peripheral thermal sensory input. The thermal afferent circuits include cutaneous thermal receptors, spinal dorsal horn neurons and lateral parabrachial nucleus neurons projecting to the preoptic area to influence warm-sensitive, inhibitory output neurons which control thermogenesis-promoting neurons in the dorsomedial hypothalamus that project to premotor neurons in the rostral ventromedial medulla, including the raphe pallidus, that descend to provide the excitation necessary to drive thermogenic thermal effectors. A distinct population of warm-sensitive preoptic neurons controls heat loss through an inhibitory input to raphe pallidus neurons controlling cutaneous vasoconstriction.

Increase in plasma ACTH induced by urethane is not a consequence of hyperosmolality

Although anesthetic doses of urethane increase plasma levels of ACTH, the exact mechanism through which this occurs is unclear. We theorized that these increases might be a consequence of an increased systemic osmolality owing to the large doses of urethane usually employed. To evaluate this possibility, we measured plasma osmolality and ACTH in a total of six rats after graded infusions of urethane (N=3 rats) or equimolar amounts of mannitol (N=3 rats). Rats received infusions at 15 min intervals up to a cumulative dose equivalent to an anesthetic dose for urethane (1.4 g/kg). Blood samples (0.35 ml) were withdrawn at baseline and 10 min after each infusion. Urethane and mannitol produced significant and equivalent increases in plasma osmolality. However, only urethane evoked increases in plasma ACTH which were maximal (252+/-55 pg/ml from a baseline of 27+/-7 pg/ml) after a cumulative dose of 1 g/kg. Thus, increases in plasma ACTH seen after anesthetic doses of urethane are unlikely to be a consequence of its effect on plasma osmolality.

Dorsomedial hypothalamus mediates autonomic, neuroendocrine, and locomotor responses evoked from the medial preoptic area

Previous studies suggest that sympathetic responses evoked from the preoptic area in anesthetized rats require activation of neurons in the dorsomedial hypothalamus. Disinhibition of neurons in the dorsomedial hypothalamus in conscious rats produces physiological and behavioral changes resembling those evoked by microinjection of muscimol, a GABA(A) receptor agonist and neuronal inhibitor, into the medial preoptic area. We tested the hypothesis that all of these effects evoked from the medial preoptic area are mediated through neurons in the dorsomedial hypothalamus by assessing the effect of bilateral microinjection of muscimol into the DMH on these changes. After injection of vehicle into the dorsomedial hypothalamus, injection of muscimol into the medial preoptic area elicited marked increases in heart rate, arterial pressure, body temperature, plasma ACTH, and locomotor activity and also increased c-Fos expression in the hypothalamic paraventricular nucleus, a region known to control the release of ACTH from the adenohypophysis. Prior bilateral microinjection of muscimol into the dorsomedial hypothalamus produced a modest depression of baseline heart rate and body temperature but completely abolished all changes evoked from the medial preoptic area. Microinjection of muscimol just anterior to the dorsomedial hypothalamus had no effect on autonomic and neuroendocrine changes evoked from the medial preoptic area. Thus, activity of neurons in the dorsomedial hypothalamus mediates a diverse array of physiological and behavioral responses elicited from the medial preoptic area, suggesting that the latter region represents an important source of inhibitory tone to key neurons in the dorsomedial hypothalamus.

Neural regulation of endocrine and autonomic stress responses

The survival and well-being of all species requires appropriate physiological responses to environmental and homeostatic challenges. The re- establishment and maintenance of homeostasis entails the coordinated activation and control of neuroendocrine and autonomic stress systems. These collective stress responses are mediated by largely overlapping circuits in the limbic forebrain, the hypothalamus and the brainstem, so that the respective contributions of the neuroendocrine and autonomic systems are tuned in accordance with stressor modality and intensity. Limbic regions that are responsible for regulating stress responses intersect with circuits that are responsible for memory and reward, providing a means to tailor the stress response with respect to prior experience and anticipated outcomes.

Roles of two preoptic cell groups in tonic and febrile control of rat tail sympathetic fibers

In response to cold and in fever, heat dissipation from the skin is reduced by sympathetic vasoconstriction. The preoptic region has been implicated in regulating basal, thermal, and febrile vasoconstriction of cutaneous vessels such as the rat's tail, but the neurons responsible for these functions have not been well localized. We recorded activity from single sympathetic nerve fibers supplying tail vessels in urethane-anesthetized rats, while microinjections of GABA (300 mM, 15–30 nl) were used to inhibit neurons in different parts of the preoptic region. Tail fiber activity increased promptly after GABA injections in two distinct regions: a rostromedial preoptic region (RMPO) centered around the organum vasculosum of the lamina terminalis, and a second region centered ∼1 mm caudolaterally (CLPO). Responses to GABA within each region were similar. The febrile mediator, PGE2 (0.2 or 1 ng in 15 nl) was then microinjected into GABA-sensitive preoptic sites. Injections of PGE2 into the RMPO induced a rapid increase in tail fiber activity followed by a rise in core temperature; injections into the rostromedial part of CLPO gave delayed tail fiber responses; injections into the central and caudal parts of CLPO were without effect. These results indicate that neurons in two distinct preoptic regions provide tonic inhibitory drive to the tail vasoconstrictor supply, but febrile vasoconstriction is mediated by PGE2 selectively inhibiting neurons in the rostromedial region.

Cardiovascular and thermal responses evoked from the periaqueductal grey require neuronal activity in the hypothalamus

Stimulation of neurons in the lateral/dorsolateral periaqueductal grey (l/dlPAG) produces increases in heart rate (HR) and mean arterial pressure (MAP) that are, according to traditional views, mediated through projections to medullary autonomic centres and independent of forebrain mechanisms. Recent studies in rats suggest that neurons in the l/dlPAG are downstream effectors responsible for responses evoked from the dorsomedial hypothalamus (DMH) from which similar cardiovascular changes and increase in core body temperature (T(co)) can be elicited. We hypothesized that, instead, autonomic effects evoked from the l/dlPAG depend on neuronal activity in the DMH. Thus, we examined the effect of microinjection of the neuronal inhibitor muscimol into the DMH on increases in HR, MAP and T(co) produced by microinjection of N-methyl-D-aspartate (NMDA) into the l/dlPAG in conscious rats. Microinjection of muscimol alone modestly decreased baseline HR and MAP but failed to alter T(co). Microinjection of NMDA into the l/dlPAG caused marked increases in all three variables, and these were virtually abolished by prior injection of muscimol into the DMH. Similar microinjection of glutamate receptor antagonists into the DMH also suppressed increases in HR and abolished increases in T(co) evoked from the PAG. In contrast, microinjection of muscimol into the hypothalamic paraventricular nucleus failed to reduce changes evoked from the PAG and actually enhanced the increase in T(co). Thus, our data suggest that increases in HR, MAP and T(co) evoked from the l/dlPAG require neuronal activity in the DMH, challenging traditional views of the place of the PAG in central autonomic neural circuitry.

Blockade of prostaglandin E2-induced thermogenesis by unilateral microinjection of GABAA receptor antagonist into the preoptic area

Previous studies have demonstrated that pretreatment of rats with a GABA(A) receptor antagonist microinjected bilaterally into the preoptic area (POA) blocked cold- or lipopolysaccharide-induced thermogenesis. Here, the involvement of GABA(A) receptors in prostaglandin (PG)E2-induced fever was examined. Thermogenic, tachycardic, vasoconstrictive, and hyperthermic responses were elicited by the unilateral microinjection of 0.57-1.1 pmol PGE2 into the region adjacent to the organum vasculosum of the lamina terminalis in urethane-chloralose-anesthetized rats. All these responses were blocked 10 min after pretreatment of the rats with a GABA(A) receptor antagonist, bicuculline methiodide or gabazine (50-500 pmol), microinjected unilaterally into the POA; and recovery occurred at approximately 70 min. Though the antagonist treatment alone had no effect on the O2 consumption rate or colonic temperature, it did elicit a bradycardic response. Pretreatment with the vehicle, saline, had no effect on the PGE2-induced responses. However, the blocking action of bicuculline/gabazine was efficacious when the agent was administered unilaterally, but not necessarily bilaterally, into the POA either contralateral or ipsilateral to the PGE2 injection site. These results suggest that the PGE2-induced responses are not simply mediated by the GABAergic transmission from the PGE2-sensitive site to the thermoefferent structure in the POA, although a tonic inhibitory input to POA neurons has a permissive role for the full expression of PGE2-induced fever.

Angiotensin and NMDA receptors in the median preoptic nucleus mediate hemodynamic response patterns to stress

The brain renin-angiotensin system plays an important role in the regulation of arterial pressure in response to stress, in part due to activation of AT1 receptors in the hypothalamic median preoptic nucleus (MnPO) by endogenous angiotensin II (ANG II). N-methyl-d-aspartate (NMDA) receptors are also involved in the angiotensinergic signaling pathway through the MnPO. We investigated whether AT1 and NMDA receptors in the MnPO are responsible for variable hemodynamic response patterns to stress. Cocaine or startle with cold water evoked a pressor response in Sprague-Dawley rats due, in some rats [vascular responders (VR)], to a large increase in systemic vascular resistance (SVR) and, in other rats [mixed responders (MR)], to small increases in SVR and cardiac output (CO). Microinjection of the GABAA agonist muscimol into the MnPO to block synaptic transmission attenuated the cocaine- or stress-induced increase in SVR and the decrease in CO seen in VR without altering either response in MR. Likewise, administration of either an AT1 receptor antagonist, losartan, or an NMDA receptor antagonist, MK-801, attenuated the increase in SVR and the decrease in CO in VR in response to either cocaine (losartan and MK-801) or startle with cold water (losartan) without altering either response in MR. We propose that the MnPO is responsible for greater SVR responses in VR and that AT1 and NMDA receptors play an important role in greater SVR responses in VR. These data provide additional support for the critical role of the MnPO in cardiovascular responses to stress.

Central control of thermogenesis in mammals

Thermogenesis, the production of heat energy, is an essential component of the homeostatic repertoire to maintain body temperature in mammals and birds during the challenge of low environmental temperature and plays a key role in elevating body temperature during the febrile response to infection. The primary sources of neurally regulated metabolic heat production are mitochondrial oxidation in brown adipose tissue, increases in heart rate and shivering in skeletal muscle. Thermogenesis is regulated in each of these tissues by parallel networks in the central nervous system, which respond to feedforward afferent signals from cutaneous and core body thermoreceptors and to feedback signals from brain thermosensitive neurons to activate the appropriate sympathetic and somatic efferents. This review summarizes the research leading to a model of the feedforward reflex pathway through which environmental cold stimulates thermogenesis and discusses the influence on this thermoregulatory network of the pyrogenic mediator, prostaglandin E(2), to increase body temperature. The cold thermal afferent circuit from cutaneous thermal receptors ascends via second-order thermosensory neurons in the dorsal horn of the spinal cord to activate neurons in the lateral parabrachial nucleus, which drive GABAergic interneurons in the preoptic area to inhibit warm-sensitive, inhibitory output neurons of the preoptic area. The resulting disinhibition of thermogenesis-promoting neurons in the dorsomedial hypothalamus and possibly of sympathetic and somatic premotor neurons in the rostral ventromedial medulla, including the raphe pallidus, activates excitatory inputs to spinal sympathetic and somatic motor circuits to drive thermogenesis.

MyD88 signaling in brain endothelial cells is essential for the neuronal activity and glucocorticoid release during systemic inflammation

Activation of neuronal circuits involved in the control of autonomic responses is critical for the host survival to immune threats. The brain vascular system plays a key role in such immune-CNS communication, but the signaling pathway and exact type of cells within the blood-brain barrier (BBB) mediating these functions have yet to be uncovered. To elucidate this issue we used myeloid differentiation factor 88 (MyD88)-deficient mice, because these animals do not show any responses to the cytokine interleukin-1beta (IL-1beta). We created chimeric mice with competent MyD88 signaling in either the BBB endothelium or perivascular microglia of bone marrow origin and challenged them with IL-1beta. Systemic treatment with the cytokine caused a robust transcriptional activation of genes involved in the prostaglandin E(2) (PGE(2)) production by vascular cells of the brain. Upregulation of these genes is dependent on a functional MyD88 signaling in the endothelium, because MyD88-deficient mice that received bone marrow stem cells from wild-type animals (for example, functional perivascular microglia) exhibited no response to systemic IL-1beta administration. MyD88 competent endothelial cells also mediate neuronal activation and plasma release of glucocorticoids, whereas chimeric mice with MyD88-competent perivascular microglia did not show a significant increase of these functions. Moreover, competent endothelial cells for the gene encoding Toll-like receptor 4 (TLR4) are essential for the release of plasma corticosterone in response to low and high doses of lipopolysaccharide. Therefore, BBB endothelial cells and not perivascular microglia are the main target of circulating inflammatory mediators to activate the brain circuits and key autonomic functions during systemic immune challenges.

Prostaglandins are necessary and sufficient to induce contextual fear learning impairments after interleukin-1 beta injections into the dorsal hippocampus

The intra-hippocampal administration of interleukin-1beta (IL-1beta) as well as the induction of elevated but physiological levels of IL-1beta within the hippocampus interferes with the formation of long-term memory. There is evidence suggesting that the induction of prostaglandin (PG) formation by IL-1beta is involved in impairments in working and spatial memory following IL-1beta. The present experiments extend these findings by showing that PGs are responsible for memory deficits in contextual fear conditioning that occur following IL-1beta injection into the dorsal hippocampus of Sprague-Dawley rats. Cyclooxygenase (COX) inhibition blocked the disruption in contextual fear conditioning produced by IL-1beta and COX inhibition alone also disrupted contextual memory, suggesting an inverted U-shaped relationship between PG levels and memory. In addition to demonstrating the necessity of PGs in IL-1beta-mediated memory deficits, we also show that PGs injected directly into the dorsal hippocampus are sufficient to impair context memory and significantly reduce post-conditioning levels of BDNF within the hippocampus, suggesting a possible mechanism for the memory-impairing effects of PGs.

Effect of hypernatraemia and the neurohypophysial peptide, arginine vasotocin (AVT) on behavioural thermoregulation in the agamid lizard, Ctenophorus ornatus

Hypernatraemia induced by chronic injections of sodium chloride provokes thermal depression in the agamid lizard, Ctenophorus (formerly Amphibolurus) ornatus, with a fall of two degrees Celsius in the mean body temperature selected behaviourally in a photo-thermal gradient. The placement of an electrolytic lesion in the base of the hypothalamus, designed to eliminate secretion of the neuropeptide arginine vasotocin (AVT), did not affect the lizards' thermoregulatory behaviour and their Preferred Body Temperature (PBT) was not significantly different from that of unoperated controls. Saline loading, however, did not induce thermal depression in these tract-operated individuals and their PBT was significantly higher than that of salt-loaded intact individuals. When AVT was injected into operated, salt-loaded, animals, however, thermal depression was observed, supporting the hypothesis that thermal depression brought about by hypernatraemia is mediated through the action of AVT. AVT similarly significantly depressed the PBT of injected intact individuals by 3.2 degrees C when compared with hydrated controls. Immunostaining for AVT confirmed that the lesions placed in the region of the median eminence virtually eliminated AVT located in the neurohypophysial tract, and the pars nervosa. This is the first report of an effect of this peptide on behavioural thermoregulation in a lizard.