Eicosanoids in non-febrile thermoregulation

Paracetamol-Induced Hypothermia in Rodents: A Review on Pharmacodynamics

Paracetamol can induce hypothermia in humans and rodents. The study’s aim is to review the mechanisms of paracetamol-induced hypothermia in rodents or the results issued from in vitro studies on the same species’ tissues (in doses that do not produce hepatic impairment) using the latest developments published in scientific journals over the last 15 years. Available human studies are also analysed. An extensive search in PubMed databases exploring the hypothermic response to paracetamol was conducted. 4669 articles about paracetamol’s effects on body temperature in mice or rats were found. After applying additional filters, 20 articles were selected for review, with 9 of them presented in tabular forms. The analysis of these articles found that the hypothermic effect of paracetamol is due to the inhibition of a cyclooxygenase-1 variant, is potentiated by endothelin receptor antagonists, and can be mediated through GABAA receptors and possibly through transient receptor potential cation channel subfamily A member 1 via N-acetyl-p-benzoquinone imine in the central nervous system. Human studies confirm the in vivo and in vitro experiments in rodents regarding the presence of a hypothermic effect after high, non-toxic doses of paracetamol. Further research is required to understand the mechanisms behind paracetamol’s hypothermic effect in humans.

A compendium of G-protein-coupled receptors and cyclic nucleotide regulation of adipose tissue metabolism and energy expenditure

With the ever-increasing burden of obesity and Type 2 diabetes, it is generally acknowledged that there remains a need for developing new therapeutics. One potential mechanism to combat obesity is to raise energy expenditure via increasing the amount of uncoupled respiration from the mitochondria-rich brown and beige adipocytes. With the recent appreciation of thermogenic adipocytes in humans, much effort is being made to elucidate the signaling pathways that regulate the browning of adipose tissue. In this review, we focus on the ligand-receptor signaling pathways that influence the cyclic nucleotides, cAMP and cGMP, in adipocytes. We chose to focus on G-protein-coupled receptor (GPCR), guanylyl cyclase and phosphodiesterase regulation of adipocytes because they are the targets of a large proportion of all currently available therapeutics. Furthermore, there is a large overlap in their signaling pathways, as signaling events that raise cAMP or cGMP generally increase adipocyte lipolysis and cause changes that are commonly referred to as browning: increasing mitochondrial biogenesis, uncoupling protein 1 (UCP1) expression and respiration.

Effect of Acetaminophen Ingestion on Thermoregulation of Normothermic, Non-febrile Humans

In non-febrile mouse models, high dose acetaminophen administration causes profound hypothermia. However, this potentially hazardous side-effect has not been confirmed in non-febrile humans. Thus, we sought to ascertain whether an acute therapeutic dose (20 mg⋅kg lean body mass) of acetaminophen would reduce non-febrile human core temperature in a sub-neutral environment. Ten apparently healthy (normal core temperature, no musculoskeletal injury, no evidence of acute illness) Caucasian males participated in a preliminary study (Study 1) to determine plasma acetaminophen concentration following oral ingestion of 20 mg⋅kg lean body mass acetaminophen. Plasma samples (every 20 min up to 2-hours post ingestion) were analyzed via enzyme linked immunosorbent assay. Thirteen (eight recruited from Study 1) apparently healthy Caucasian males participated in Study 2, and were passively exposed to 20°C, 40% r.h. for 120 min on two occasions in a randomized, repeated measures, crossover design. In a double blind manner, participants ingested acetaminophen (20 mg⋅kg lean body mass) or a placebo (dextrose) immediately prior to entering the environmental chamber. Rectal temperature, skin temperature, heart rate, and thermal sensation were monitored continuously and recorded every 10 min. In Study 1, the peak concentration of acetaminophen (14 ± 4 μg/ml) in plasma arose between 80 and 100 min following oral ingestion. In Study 2, acetaminophen ingestion reduced the core temperature of all participants, whereas there was no significant change in core temperature over time in the placebo trial. Mean core temperature was significantly lower in the acetaminophen trial compared with that of a placebo (p < 0.05). The peak reduction in core temperature in the acetaminophen trial was reached at 120 min in six of the thirteen participants, and ranged from 0.1 to 0.39°C (average peak reduction from baseline = 0.19 ± 0.09°C). There was no significant difference in skin temperature, heart rate, or thermal sensation between the acetaminophen and placebo trials (p > 0.05). The results indicate oral acetaminophen reduces core temperature of humans exposed to an environment beneath the thermal neutral zone. These results suggest that acetaminophen may inhibit the thermogenic mechanisms required to regulate core temperature during exposure to sub-neutral environments.

Is prostaglandin E2 (PGE2) involved in the thermogenic response to environmental cooling in healthy humans?

Prostaglandin E2 (PGE2) is an eicosanoid derived from cyclooxygenase, an enzyme responsible for the cyclisation and oxygenation of arachidonic acid. In response to bacterial infection, PGE2 binds to EP3 receptors on a population of GABAergic neurons in the pre-optic area. Activation of the EP3 receptor decreases the intracellular cyclic adenosine monophosphate (cAMP) concentrations of these neurons, and the resulting dis-inhibition activates spinal motor outputs responsible for shivering thermogenesis, tachycardia, and brown adipose tissue activation. These involuntary responses increase core body temperature to varying degrees depending on the magnitude of infection; an immune response which is crucial for the survival of the host. However, evidence in animal and human models, primarily through the use of cyclooxygenase inhibitors (which block the production of PGE2), suggests that PGE2 may also be an important molecule for the defence of core temperature against body cooling and cold stress (in the absence of fever). In this paper, evidence within human and animal models is discussed which supports the hypothesis that the eicosanoid PGE2 has a role in maintaining human core temperature during environmental cooling. Given that over-the-counter PGE2 inhibiting drugs [i.e. Non-Steroidal Anti Inflammatory Drugs (NSAIDS)] are frequently used worldwide, it is possible that the use of such medication during environmental cooling could impair one's ability to thermoregulate. Support for such findings could have major implications in the pathology of hypothermia, thus, we suggest that future researchers investigate this specific hypothesis in vivo, using healthy human models. Suggestions for the implementation of such experiments are provided in the present work.

Effect of manipulation of incubation temperature on fatty acid profiles and antioxidant enzyme activities in meat-type chicken embryos

Eggs (n = 1,800) obtained from Ross broiler breeders at 32 and 48 wk of age were incubated at either a constant temperature of 37.6°C throughout (T1), or the temperature was reduced for 6 h to 36.6°C each day during embryonic age (EA) 10 to 18 (T2). Yolk sac, liver, and brain fatty acid profiles and oxidant and antioxidant status of liver and brain were measured at EA 14, 19, and day of hatch (DOH). Fatty acid profiles of yolk sac, liver, and brain were influenced by age of breeder with significant breeder hen age × incubation temperature interactions. At EA 14, higher levels of 20:4n-6 and 22:6n-3 had been transferred from the yolk sac to T2 embryos from younger than older breeders, whereas for T1 and T2 embryos, yolk sac 20:4n-6 and 22.6n-3 values were similar for older breeders. Accumulation of 20:4n-6 and 22:6n-3 fatty acids in the liver of T1 and T2 embryos from younger breeders was similar; however, T2 embryos from older breeders had higher liver levels of 20:4n-6 and 22:6n-3 than T1 embryos. At EA 19, liver nitric oxide levels were higher for T2 embryos from younger breeders than those from breeders incubated at T1. Brain catalase levels of T2 embryos from younger breeders were higher than those from older breeders at DOH. Thus, changes in fatty acid profiles and catalase and nitric oxide production of brain and liver tissues resulting from 1°C lower incubation temperature from EA 10 to 18 reflect adaptive changes.

Dietary fatty acid composition influences tissue lipid profiles and regulation of body temperature in Japanese quail

Many avian species reduce their body temperature (T(b)) to conserve energy during periods of inactivity, and we recently characterized how ambient temperature (T(a)) and nutritional stress interact with one another to influence physiologically controlled hypothermic responses in Japanese quail (Coturnix japonica). In the present study, we examined how the fatty acid (FA) composition of the diet influences the FA composition of phospholipids in major organs and how these affect controlled hypothermic responses and metabolic rates in fasted birds. For 5 weeks prior to fasting, quail were fed a standard diet and gavaged each morning with 0.7 ml of water (control), or a vegetable oil comprising saturated fatty acids (SFA; coconut oil), or unsaturated fatty acids (UFA; canola oil). Birds were then fasted for 4 days at a T(a) of 15°C. We found that, while fasting, both photophase and scotophase T(b) decreased significantly more in the SFA treatment group than in the control group; apparently the former down-regulated their T(b) set point. This deeper hypothermic response was correlated with changes in the phospholipid composition of the skeletal muscle and liver, which contained significantly more oleic acid (18:1) and less arachidonic acid (20:4), respectively. Our data imply that these two FAs may be associated with thermoregulation.

Two pathways for cyclooxygenase-2 protein degradation in vivo

COX-2, formally known as prostaglandin endoperoxide H synthase-2 (PGHS-2), catalyzes the committed step in prostaglandin biosynthesis. COX-2 is induced during inflammation and is overexpressed in colon cancer. In vitro, an 18-amino acid segment, residues 595-612, immediately upstream of the C-terminal endoplasmic reticulum targeting sequence is required for N-glycosylation of Asn(594), which permits COX-2 protein to enter the endoplasmic reticulum-associated protein degradation system. To determine the importance of this COX-2 degradation pathway in vivo, we engineered a del595-612 PGHS-2 (Delta 18 COX-2) knock-in mouse lacking this 18-amino acid segment. Delta 18 COX-2 knock-in mice do not exhibit the renal or reproductive abnormalities of COX-2 null mice. Delta 18 COX-2 mice do have elevated urinary prostaglandin E(2) metabolite levels and display a more pronounced and prolonged bacterial endotoxin-induced febrile response than wild type (WT) mice. Normal brain tissue, cultured resident peritoneal macrophages, and cultured skin fibroblasts from Delta 18 COX-2 mice overexpress Delta 18 COX-2 relative to WT COX-2 expression in control mice. These results indicate that COX-2 can be degraded via the endoplasmic reticulum-associated protein degradation pathway in vivo. Treatment of cultured cells from WT or Delta 18 COX-2 mice with flurbiprofen, which blocks substrate-dependent degradation, attenuates COX-2 degradation, and treatment of normal mice with ibuprofen increases the levels of COX-2 in brain tissue. Thus, substrate turnover-dependent COX-2 degradation appears to contribute to COX-2 degradation in vivo. Curiously, WT and Delta 18 COX-2 protein levels are similar in kidneys and spleens from WT and Delta 18 COX-2 mice. There must be compensatory mechanisms to maintain constant COX-2 levels in these tissues.

Opioid, cannabinoid CB1 and NOP receptors do not mediate APAP-induced hypothermia in rats

Acetaminophen (APAP) produces antinociception and hypothermia. Because the antinociceptive effect in rats is partially dependent on opioid and cannabinoid CB1 receptor activation, we determined if activation of these receptors also contributes to the hypothermic effect of APAP. Rats injected with APAP (100, 250, 375 or 500 mg/kg, i.p.) displayed dose-related hypothermia. For combined administration, the hypothermic effect of APAP (400 mg/kg, i.p.) was not altered by pretreatment with: naltrexone (10 mg/kg, s.c.), a non-selective opioid antagonist; naltrindole (1 mg/kg, s.c.), a delta opioid antagonist; nor-binaltorphimine (10 mg/kg, i.p.), a kappa opioid antagonist; SR 141716A (3 mg/kg, i.m.), a cannabinoid CB1 receptor antagonist; or JTC-801(1 mg/kg, i.p.), a nociceptin/orphanin FQ peptide (NOP) receptor antagonist. The demonstration that APAP produces hypothermia independent of opioid, cannabinoid CB1 or NOP receptor activation is contrary to its antinociceptive effect, which requires opioid and cannabinoid CB1 receptor activation.

Acetaminophen: antipyretic or hypothermic in mice? In either case, PGHS-1b (COX-3) is irrelevant

Acetaminophen (AC) reduces the core temperatures (T(c)) of febrile and non-febrile mice alike. Evidence has been adduced that the selectively AC-sensitive PGHS isoform, PGHS-1b (COX-3), mediates these effects. PGHS-1b, however, has no catalytic potency in mice. To resolve this contradiction, AC was injected intravenously (i.v.) into conscious PGHS-1 gene-sufficient (wild-type (WT)) and -deficient (PGHS-1(-/-)) mice 60 min before or after pyrogen-free saline (PFS) or E. coli LPS (10 microg/kg) i.v. T(c) was monitored continuously; brain and plasma PGE(2) levels were determined hourly. AC at <160 mg/kg did not affect T(c) when given before PFS or LPS; at 160 mg/kg, it caused a approximately 2.5 degrees C T(c) fall in 60 min. LPS given after AC (all doses) induced a approximately 1 degrees C fever, not different from that in AC-untreated mice. But this rise was insufficient to overcome the hypothermia of the 160 mg/kg-treated mice; their T(c) culminated 1 degrees C below baseline. LPS given before AC similarly elevated T(c) approximately 1 degrees C. This rise was reduced to baseline in 30 min by 80 mg AC/kg; T(c) rebounded to its febrile level over the next 30 min. At 160 mg/kg, AC reduced T(c) to 4 degrees C below baseline in 60 min, where it remained until the end of the experiment. WT and PGHS-1(-/-) mice responded similarly to all the treatments. The basal brain and plasma PGE(2) levels of PFS mice and the elevated plasma levels of LPS mice were unchanged by AC at 160 mg/kg; but the latter's brain levels were reduced at 1h, then recovered. Thus, AC could exert an anti-PGHS-2 effect when this enzyme is upregulated in the brain of febrile mice. The hypothermia it induces in non-febrile mice, therefore, is due to another mechanism. PGHS-1b is not involved in either case.