Nucleus tractus solitarii lesions alter the metabolic and hyperthermic response to central prostaglandin E1 in the rat

The biological actions of prostanoids in adipose tissue in physiological and pathophysiological conditions

Adipose tissue has been established as an endocrine organ that plays an important role in maintaining metabolic homeostasis. Adipose tissue releases several bioactive molecules called adipokines. Inflammation, dysregulation of adipokine synthesis, and secretion are observed in obesity and related diseases and cause adipose tissue dysfunction. Prostanoids, belonging to the eicosanoid family of lipid mediators, can be synthesized in adipose tissue and play a critical role in adipose tissue biology. In this review, we summarized the current knowledge regarding the interaction of prostanoids with adipokines, the expression of prostanoid receptors, and prostanoid synthase enzymes in adipose tissues in health and disease. Furthermore, the involvement of prostanoids in the physiological function or dysfunction of adipose tissue including inflammation, lipolysis, adipogenesis, thermogenesis, browning of adipocytes, and vascular tone regulation was also discussed by examining studies using pharmacological approaches or genetically modified animals for prostanoid receptors/synthase enzymes. Overall, the present review provides a perspective on the evidence from literature regarding the biological effects of prostanoids in adipose tissue. Among prostanoids, prostaglandin E2 (PGE₂) is prominent in regards to its substantial role in both adipose tissue physiology and pathophysiology. Targeting prostanoids may serve as a potential therapeutic strategy for preventing or treating obesity and related diseases.

Injections of Galanin-Like Peptide directly into the nucleus of the tractus solitarius (NTS) reduces food intake and body weight but increases metabolic rate and plasma leptin

Galanin-Like Peptide (GALP) is a hypothalamic neuromediator of metabolism and reproduction. GALP is known to stimulate reproduction and alter food intake and body weight in multiple species. The regulation of body weight involves control of both energy intake and energy expenditure. Since GALP is known to alter food intake - possibly via the autonomic nervous system - we first hypothesized that GALP would increase metabolic rate. First, male Sprague-Dawley rats were implanted with intracerebroventricular (ICV) cannulae and abdominal radiotelemetry temperature transmitters. Following ICV injection with either 5nmol GALP or vehicle, the oxygen consumption of each rat was monitored for 8h. Food intake, core temperature, and general motor activity were monitored for 24h. GALP significantly increased oxygen consumption, an indirect estimator of metabolic rate, without having any significant effect on motor activity. Compared to controls, GALP increased core body temperature during the photophase and reduced food intake over the 24h period following injection. ICV GALP also increased plasma levels of luteinizing hormone (LH). A second group of male Sprague-Dawley rats were implanted with abdominal transmitters and given injections of GALP directly into the nucleus of the tractus solitarius (NTS). These injections resulted in a significant reduction in food intake, and a significant increase in both oxygen consumption and core body temperature compared to vehicle injections. Direct injections of GALP into the NTS compared to vehicle also resulted in a significant increase in plasma leptin levels, but not LH levels. GALP appears to increase energy expenditure in addition to decreasing energy input by actions within the NTS and thus may play an important role in the hypothalamic regulation of body weight.

Involvement of endogenous central hydrogen sulfide (H2S) in hypoxia-induced hypothermia in spontaneously hypertensive rats

Spontaneously hypertensive rats (SHR) display autonomic imbalance and abnormal body temperature (Tb) adjustments. Hydrogen sulfide (H2S) modulates hypoxia-induced hypothermia, but its role in SHR thermoregulation is unknown. We tested the hypothesis that SHR display peculiar thermoregulatory response to hypoxia and that endogenous H2S overproduced in the caudal nucleus of the solitary tract (NTS) of SHR modulates this response. SHR and Wistar rats were microinjected into the fourth ventricle with aminooxyacetate (AOA, H2S-synthezing enzyme inhibitor) or sodium sulfide (Na2S, H2S donor) and exposed to normoxia (21% inspired O2) or hypoxia (10% inspired O2, 30 min). Tb was continuously measured, and H2S production rate was assessed in caudal NTS homogenates. In both groups, AOA, Na2S, or saline (i.e., control; 1 μL) did not affect euthermia. Hypoxia caused similar decreases in Tb in both groups. AOA presented a longer latency to potentiate hypoxic hypothermia in SHR. Caudal NTS H2S production rate was higher in SHR. We suggest that increased bioavailability of H2S in the caudal NTS of SHR enables the adequate modulation of excitability of peripheral chemoreceptor-activated NTS neurons that ultimately induce suppression of brown adipose tissue thermogenesis, thus accounting for the normal hypoxic hypothermia.

Central mediators involved in the febrile response: effects of antipyretic drugs

Fever is a complex signal of inflammatory and infectious diseases. It is generally initiated when peripherally produced endogenous pyrogens reach areas that surround the hypothalamus. These peripheral endogenous pyrogens are cytokines that are produced by leukocytes and other cells, the most known of which are interleukin-1β, tumor necrosis factor-α, and interleukin-6. Because of the capacity of these molecules to induce their own synthesis and the synthesis of other cytokines, they can also be synthesized in the central nervous system. However, these pyrogens are not the final mediators of the febrile response. These cytokines can induce the synthesis of cyclooxygenase-2, which produces prostaglandins. These prostanoids alter hypothalamic temperature control, leading to an increase in heat production, the conservation of heat, and ultimately fever. The effect of antipyretics is based on blocking prostaglandin synthesis. In this review, we discuss recent data on the importance of prostaglandins in the febrile response, and we show that some endogenous mediators can still induce the febrile response even when known antipyretics reduce the levels of prostaglandins in the central nervous system. These studies suggest that centrally produced mediators other than prostaglandins participate in the genesis of fever. Among the most studied central mediators of fever are corticotropin-releasing factor, endothelins, chemokines, endogenous opioids, and substance P, which are discussed herein. Additionally, recent evidence suggests that these different pathways of fever induction may be activated during different pathological conditions.

Central mediators involved in the febrile response: effects of antipyretic drugs

Fever is a complex signal of inflammatory and infectious diseases. It is generally initiated when peripherally produced endogenous pyrogens reach areas that surround the hypothalamus. These peripheral endogenous pyrogens are cytokines that are produced by leukocytes and other cells, the most known of which are interleukin-1β, tumor necrosis factor-α, and interleukin-6. Because of the capacity of these molecules to induce their own synthesis and the synthesis of other cytokines, they can also be synthesized in the central nervous system. However, these pyrogens are not the final mediators of the febrile response. These cytokines can induce the synthesis of cyclooxygenase-2, which produces prostaglandins. These prostanoids alter hypothalamic temperature control, leading to an increase in heat production, the conservation of heat, and ultimately fever. The effect of antipyretics is based on blocking prostaglandin synthesis. In this review, we discuss recent data on the importance of prostaglandins in the febrile response, and we show that some endogenous mediators can still induce the febrile response even when known antipyretics reduce the levels of prostaglandins in the central nervous system. These studies suggest that centrally produced mediators other than prostaglandins participate in the genesis of fever. Among the most studied central mediators of fever are corticotropin-releasing factor, endothelins, chemokines, endogenous opioids, and substance P, which are discussed herein. Additionally, recent evidence suggests that these different pathways of fever induction may be activated during different pathological conditions.

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.

Inhibition of brown adipose tissue thermogenesis by neurons in the ventrolateral medulla and in the nucleus tractus solitarius

Neurons in the ventrolateral medulla (VLM) and in the nucleus tractus solitarius (NTS) play important roles in the regulation of cardiovascular and other autonomic functions. In the present study, we demonstrate an inhibition of brown adipose tissue (BAT) thermogenesis evoked by activation of neurons in the VLM, as well as by neurons in the intermediate NTS, of chloralose/urethane-anesthetized, artificially ventilated rats. Activation of neurons in either rostral VLM or caudal VLM with N-methyl-d-aspartate (12 nmol) reversed the cold-evoked increase in BAT sympathetic nerve activity (SNA), BAT temperature, and end-expired CO(2). Disinhibition of neurons in either VLM or NTS with the GABA(A) receptor antagonist, bicuculline (30 pmol), reversed the increases in BAT SNA, BAT temperature, and end-expired CO(2) that were elicited 1) by cold defense; 2) during the febrile model of nanoinjection of prostaglandin E(2) into the medial preoptic area; 3) by activation of neurons in the dorsomedial hypothalamus or in the rostral raphe pallidus (rRPa); or 4) by the micro-opioid receptor agonist fentanyl. Combined, but not separate, inhibitions of neurons in the VLM and in the NTS, with the GABA(A) receptor agonist, muscimol (120 pmol/site), produced increases in BAT SNA, BAT temperature, and expired CO(2), which were reversed by nanoinjection of glycine (30 nmol) into the rRPa. These findings suggest that VLM and NTS contain neurons whose activation inhibits BAT thermogenesis, that these neurons receive GABAergic inputs that are active under these experimental conditions, and that neurons in both sites contribute to the tonic inhibition of sympathetic premotor neuronal activity in the rRPa that maintains a low level of BAT thermogenesis in normothermic conditions.

Reversible inactivation of the dorsal vagal complex blocks lipopolysaccharide-induced social withdrawal and c-Fos expression in central autonomic nuclei

Peripheral administration of lipopolysaccharide (LPS), a potent activator of the immune system, induces symptoms of behavioral depression, such as social withdrawal, concommitant with increases in c-Fos expression in central autonomic network nuclei. Previous studies implicated vagal visceral sensory nerves in transduction of immune-related signals relevant to for the induction of social withdrawal, a symptom of behavioral depression. Vagal sensory nerves terminate in the dorsal vagal complex (DVC) of the brainstem, a region that functions to integrate visceral signals and may also play a role in modulating arousal and affect. The objective of the current study was to determine whether the DVC contributes to immunosensory pathways driving symptoms of social withdrawal associated with LPS-induced behavioral depression, using a reversible lesion technique to temporarily inactivate the DVC. To assess the effects of DVC inactivation on LPS-induced social withdrawal and the subsequent changes in brain activation, we used behavioral assessment of social withdrawal, and analyzed c-Fos expression, a marker of neuronal activation, in the central nucleus of the amygdala (CEA), bed nucleus of the stria terminalis (BST), hypothalamic paraventricular nucleus (PVN), and ventromendial preoptic area (VMPO). Two hours following intraperitoneal LPS injection, there was a significant increase in c-Fos immunoreactivity in forebrain regions in animals treated with LPS. DVC inactivation completely blocked LPS-induced social withdrawal and dramatically reduced LPS-induced Fos expression in all four forebrain regions assessed. Collectively, these findings support the idea that the DVC acts as an immune-behavior interface between the peripheral stimuli and brain areas involved in modulating social behavior.

The vagus nerve in thermoregulation and energy metabolism

The vagus nerve may indirectly influence thermoregulation by modulation of energy balance: its afferent fibers convey signals that represent information on feeding state, resulting in either depression or stimulation of metabolic processes. A regulated metabolic depression can be detected in the background of fasting-induced hypometabolism and hypothermia. In its development (besides humoral signals) vagally transmitted neural signals of gastrointestinal and hepatoportal origin are important. These signals are related to hunger, to decrease of mechanical/chemical stimuli from the gut, to decline of blood glucose; they alter discharge rates of vagal afferents and activity of the nucleus of the solitary tract, eliciting inhibition of metabolic rate and enhancement of food intake. In this hunger-related metabolic inhibition the nucleus of the solitary tract is in interaction with hypothalamic nuclei, that contribute to neuropeptide changes characterized by high neuropeptide Y activity (with energy-conserving type of regulation) and depressed cholecystokinin and corticotropin releasing hormone activities (with depressed energy-expenditure). In postalimentary states the hypermetabolism and hyperthermia are due to opposite changes in metabolic regulation. Satiety-related stimulatory signals of abdominal origin, transmitted via hepatic vagal afferents to the nucleus of the solitary tract, contribute to enhancement of sympathetic activity and stress-responsiveness, leading to hypermetabolism and hyperthermia. Depressed neuropeptide Y release and enhanced cholecystokinin and corticotropin releasing hormone activities participate in the central regulatory changes, and in the high energy-expenditure. The biological role of these vagal functions is not directly the regulation of body temperature, rather the regulation of energy balance and energy content in the body.

Effect of nucleus tractus solitarius lesions on fever produced by interleukin-1beta

Partial elimination of vagal sensory afferents by subdiaphragmatic vagal section has variously been reported to eliminate, to reduce, or to have no effect on fever produced by peripheral lipopolysaccharide and interleukin-1beta (IL-1beta). However, to adequately test the idea that vagal afferents convey immune information to the brain, all vagal input to the central nervous system must be eliminated. This was accomplished by bilateral electrolytic lesions of the nucleus tractus solitarius (NTS). Reflex bradycardia evoked by intravenous phenylbiguanide was eliminated in NTS-lesioned rats, verifying the lesion's effectiveness. IL-1beta (2 microg/kg) was given to conscious, unrestrained rats via an indwelling intraperitoneal catheter and produced rapid fever (approximately 1 degree C) with an onset latency of 15 min and peak response at 30 min, with a second, smaller peak at 130 min. NTS lesions attenuated the first fever peak, with a lesser, non-significant effect on the second peak. The thermogenic capacity of NTS-lesioned rats was evaluated using 3 different strategies: (1) thermogenesis evoked by CNS injections of prostaglandin E2, (2) 3 h exposure to a 4 degrees C environment, and (3) heat production of intrascapular brown fat produced by intravenous infusion of the beta3-adrenergic agonist BRL 37344. NTS-lesioned rats were equivalent, or even superior to control animals in their thermogenic response to these non-immune-related stimuli. Therefore, the impaired febrile response of NTS-lesioned rats to IL-1beta cannot be attributed to reduced thermogenic capacity. Finally, these results suggest that fever elicited by intraperitoneal IL-1beta is, at least in part, dependent on the integrity of NTS neurons, but also that mechanisms independent of vagal afferent projections to the NTS must also play a role in immune-to-brain signaling.

Developmental changes in respiratory, febrile, and cardiovascular responses to PGE(2) in newborn lambs

PGE(2) has centrally mediated respiratory, febrile, and cardiovascular effects that markedly differ between fetal and adult life. We hypothesized that the transition from fetal to adult responses to PGE(2) occurs in the newborn period. Thus effects of an intracarotid infusion of PGE(2) (3 microg/min for 60 min) were determined in unanesthetized newborn lambs at 5, 10, and 15 days after birth. At 5 days, PGE(2) reduced central CO(2) sensitivity, reduced lung ventilation due to a decrease in breathing frequency, and induced hypercapnia. By 15 days, these effects of PGE(2) had waned significantly. In contrast, phasic (expiratory) thyroarytenoid muscle electromyogram activity, number of short apneas, and incidence of Biot periodic breathing were similarly increased at all three ages. PGE(2) induced a sustained fever at 10 and 15 days. Heart rate and mean arterial blood pressure were unchanged in contrast to marked increases observed by others in adults. Results showed that the transition from fetal to adult respiratory and febrile responses to PGE(2) occurs in early postnatal life, whereas adult cardiovascular responses develop later in life in sheep.

Brown adipose tissue. More than an effector of thermogenesis?

Brown adipose tissue (BAT) produces heat by oxidation of fatty acids. This takes place when the tissue is stimulated by norepinephrine; the molecular background for the ability of BAT to produce heat is the tissue-specific mitochondrial protein UCP1. In the classic view of BAT with respect to fever, BAT is an effector organ, producing heat especially during the onset phase of the fever. There is good evidence that BAT thermogenesis is stimulated via a lipopolysaccharide (LPS), interleukin (IL)-1 beta, IL-6, prostaglandin E cascade. Under physiologic conditions of constantly stimulated activity, BAT is expected to be recruited, but in fevers this is only evident in thyroxine fever. However, BAT may be more than merely an effector. There are indications of a correlation between the amount of BAT and the intensity of fevers, and brown adipocytes can indeed produce IL-1 alpha and IL-6. Furthermore, brown adipocytes are directly sensitive to LPS; this LPS sensitivity is augmented in brown adipocytes from IL-1 beta-deficient mice. Thus, BAT may also have a controlling role in thermoregulation. The existence of transgenic mice with ablations of proteins central in fever and in BAT thermogenesis opens up possibilities for identification and elucidation of this putative new role for brown adipose tissue as an endocrine organ involved in the control of fever.

Subdiaphragmatic vagotomy does not prevent fever following intracerebroventricular prostaglandin E2: further evidence for the importance of vagal afferents in immune-to-brain communication

Brain-mediated sickness responses can be blocked by subdiaphragmatic vagotomy, suggesting that vagal afferents signal peripheral inflammation or infection. This study tested whether subdiaphragmatic vagotomy disrupts sickness responses by interrupting effector pathways. If this explanation is correct, intracerebroventricular prostaglandin E2-induced fever should be blocked by this procedure. Fever was unaffected by subdiaphragmatic vagotomy, thus these data provide support for the conclusion that vagal afferents signal the brain during immune activation.

Bacterial endotoxin-induced gene expression in the choroid plexus and paraventricular and supraoptic hypothalamic nuclei of the sheep

The febrile and neuroendocrine responses to circulating endotoxin are effected, at least in part, by a central action of prostaglandins with interleukins serving as intermediaries. Data from rodents suggest that prostaglandin and interleukin (IL-1 beta) synthesis in response to endotoxin challenge may occur within the circumventricular organs of the brain, especially the choroid plexus; the present study investigated this possibility using the sheep as an experimental model. A pyretic dose of bacterial endotoxin (40 micrograms lipopolysaccharide) was given intravenously to sheep (n = 5) and the effect on gene expression in the choroid plexus after a 40 min interval was compared with that observed in vehicle-treated animals (n = 5) using in situ hybridisation histochemistry. Evidence of activational and synthetic events following endotoxin administration was provided by significant increases in c-fos (P < 0.05) and IL-1 beta (P < 0.01) mRNA expression. Constitutive cyclooxygenase (cox-1 mRNA) and inducible cyclooxygenase (cox-2 mRNA) synthesis were unchanged. The investigation also sought to provide evidence for endotoxin effects on neuroendocrine activity in this species by examining changes in hypothalamic gene expression. The results showed that c-fos mRNA increased in the paraventricular (P < 0.01) and supraoptic (P < 0.05) nuclei and that CRH mRNA was upregulated in the paraventricular nucleus (P < 0.001). However, in agreement with previous work, there was no change in vasopressin gene expression although oxytocin mRNA was enhanced throughout the paraventricular nucleus (P < 0.05). These findings suggest the following: (1) possible involvement of the choroid plexus in the response of sheep to immunological challenge: (2) endotoxin-induced changes in gene expression in the ovine hypothalamus similar in those caused by other stressors: and (3) possible changes in oxytocin synthesis concomitant with fever in the sheep.

Ontogenesis of prostaglandin E2 binding sites in the brainstem of the sheep

Perinatal changes in plasma prostaglandin E2 (PGE2) act at the level of the fetal brainstem to modulate respiratory activity. In-vitro receptor autoradiography was used to determine whether PGE2 binding sites (putative receptors) are located in brainstem regions responsible for respiratory control. Frozen brainstem sections were obtained from fetal sheep at 140 d of gestation, lambs at 5 h and 10 d after birth, and adults (n = 3 at each age). In the perinatal brainstem, high density binding was localized near the following nuclei: hypoglossal, tractus solitarius, parabrachial and spinal trigeminal-oral. Moderate levels of binding were localized near the nuclei of the spinal trigeminal tract, ambiguus, inferior olivary, cuneate, gracilis, facial, medial vestibular, superior vestibular, and abducens. Binding density did not change with age within the perinatal period. However, binding in three high density regions (hypoglossal, spinal trigeminal and parabrachial nuclei) decreased significantly in adulthood, whereas the binding in the nucleus of the solitary tract remained high. We hypothesize that the PGE2 binding sites that we identified near respiratory regions of the brainstem may mediate the central respiratory effects of PGE2.

Hyperthermia-associated changes in Fos protein in the median preoptic and other hypothalamic nuclei of the pig following intravenous administration of prostaglandin E2

Changes in body temperature and hypothalamic Fos production were investigated in pigs injected intravenously with prostaglandin E2 (PGE2, 20 micrograms/kg). PGE2 produced a sustained (> 150 min) hyperthermia (0.7-1 degree C) and significant increases in Fos labelling in the supraoptic and paraventricular nuclei. In addition, PGE2 induced Fos production in the median preoptic nucleus, a region previously demonstrated to be involved in thermoregulation in this species.