András Garami

The Transient Receptor Potential Vanilloid-1 Channel in Thermoregulation: A Thermosensor It Is Not

The development of antagonists of the transient receptor potential vanilloid-1 (TRPV1) channel as pain therapeutics has revealed that these compounds cause hyperthermia in humans. This undesirable on-target side effect has triggered a surge of interest in the role of TRPV1 in thermoregulation and revived the hypothesis that TRPV1 channels serve as thermosensors. We review literature data on the distribution of TRPV1 channels in the body and on thermoregulatory responses to TRPV1 agonists and antagonists. We propose that two principal populations of TRPV1-expressing cells have connections with efferent thermoeffector pathways: 1) first-order sensory (polymodal), glutamatergic dorsal-root (and possibly nodose) ganglia neurons that innervate the abdominal viscera and 2) higher-order sensory, glutamatergic neurons presumably located in the median preoptic hypothalamic nucleus. We further hypothesize that all thermoregulatory responses to TRPV1 agonists and antagonists and thermoregulatory manifestations of TRPV1 desensitization stem from primary actions on these two neuronal populations. Agonists act primarily centrally on population 2; antagonists act primarily peripherally on population 1. We analyze what roles TRPV1 might play in thermoregulation and conclude that this channel does not serve as a thermosensor, at least not under physiological conditions. In the hypothalamus, TRPV1 channels are inactive at common brain temperatures. In the abdomen, TRPV1 channels are tonically activated, but not by temperature. However, tonic activation of visceral TRPV1 by nonthermal factors suppresses autonomic cold-defense effectors and, consequently, body temperature. Blockade of this activation by TRPV1 antagonists disinhibits thermoeffectors and causes hyperthermia. Strategies for creating hyperthermia-free TRPV1 antagonists are outlined. The potential physiological and pathological significance of TRPV1-mediated thermoregulatory effects is discussed.

Pharmacological blockade of the cold receptor TRPM8 attenuates autonomic and behavioral cold defenses and decreases deep body temperature

We studied N-(2-aminoethyl)-N-(4-(benzyloxy)-3-methoxybenzyl)thiophene-2-carboxamide hydrochloride (M8-B), a selective and potent antagonist of the transient receptor potential melastatin-8 (TRPM8) channel. In vitro, M8-B blocked cold-induced and TRPM8-agonist-induced activation of rat, human, and murine TRPM8 channels, including those on primary sensory neurons. In vivo, M8-B decreased deep body temperature (Tb) in Trpm8+/+ mice and rats, but not in Trpm8−/− mice, thus suggesting an on-target action. Intravenous administration of M8-B was more effective in decreasing Tb in rats than intrathecal or intracerebroventricular administration, indicating a peripheral action. M8-B attenuated cold-induced c-Fos expression in the lateral parabrachial nucleus, thus indicating a site of action within the cutaneous cooling neural pathway to thermoeffectors, presumably on sensory neurons. A low intravenous dose of M8-B did not affect Tb at either a constantly high or a constantly low ambient temperature (Ta), but the same dose readily decreased Tb if rats were kept at a high Ta during the M8-B infusion and transferred to a low Ta immediately thereafter. These data suggest that both a successful delivery of M8-B to the skin (high cutaneous perfusion) and the activation of cutaneous TRPM8 channels (by cold) are required for the hypothermic action of M8-B. At tail-skin temperatures <23°C, the magnitude of the M8-B-induced decrease in Tb was inversely related to skin temperature, thus suggesting that M8-B blocks thermal (cold) activation of TRPM8. M8-B affected all thermoeffectors studied (thermopreferendum, tail-skin vasoconstriction, and brown fat thermogenesis), thus suggesting that TRPM8 is a universal cold receptor in the thermoregulation system.

Contributions of Different Modes of TRPV1 Activation to TRPV1 Antagonist-Induced Hyperthermia

Transient receptor potential vanilloid-1 (TRPV1) antagonists are widely viewed as next-generation pain therapeutics. However, these compounds cause hyperthermia, a serious side effect. TRPV1 antagonists differentially block three modes of TRPV1 activation: by heat, protons, and chemical ligands (e.g., capsaicin). We asked what combination of potencies in these three modes of TRPV1 activation corresponds to the lowest potency of a TRPV1 antagonist to cause hyperthermia. We studied hyperthermic responses of rats, mice, and guinea pigs to eight TRPV1 antagonists with different pharmacological profiles and used mathematical modeling to find a relative contribution of the blockade of each activation mode to the development of hyperthermia. We found that the hyperthermic effect has the highest sensitivity to the extent of TRPV1 blockade in the proton mode (0.43 to 0.65) with no to moderate sensitivity in the capsaicin mode (−0.01 to 0.34) and no sensitivity in the heat mode (0.00 to 0.01). We conclude that hyperthermia-free TRPV1 antagonists do not block TRPV1 activation by protons, even if they are potent blockers of the heat mode, and that decreasing the potency to block the capsaicin mode may further decrease the potency to cause hyperthermia.

Thermoregulatory Phenotype of the Trpv1 Knockout Mouse: Thermoeffector Dysbalance with Hyperkinesis

This study aimed at determining the thermoregulatory phenotype of mice lacking transient receptor potential vanilloid-1 (TRPV1) channels. We used Trpv1 knockout (KO) mice and their genetically unaltered littermates to study diurnal variations in deep body temperature (Tb) and thermoeffector activities under basal conditions, as well as thermoregulatory responses to severe heat and cold. Only subtle alterations were found in the basal Tb of Trpv1 KO mice or in their Tb responses to thermal challenges. The main thermoregulatory abnormality of Trpv1 KO mice was a different pattern of thermoeffectors used to regulate Tb. On the autonomic side, Trpv1 KO mice were hypometabolic (had a lower oxygen consumption) and hypervasoconstricted (had a lower tail skin temperature). In agreement with the enhanced skin vasoconstriction, Trpv1 KO mice had a higher thermoneutral zone. On the behavioral side, Trpv1 KO mice preferred a lower ambient temperature and expressed a higher locomotor activity. Experiments with pharmacological TRPV1 agonists (resiniferatoxin and anandamide) and a TRPV1 antagonist (AMG0347) confirmed that TRPV1 channels located outside the brain tonically inhibit locomotor activity. With age (observed for up to 14 months), the body mass of Trpv1 KO mice exceeded that of controls, sometimes approaching 60 g. In summary, Trpv1 KO mice possess a distinct thermoregulatory phenotype, which is coupled with a predisposition to age-associated overweight and includes hypometabolism, enhanced skin vasoconstriction, decreased thermopreferendum, and hyperkinesis. The latter may be one of the primary deficiencies in Trpv1 KO mice. We propose that TRPV1-mediated signals from the periphery tonically suppress the general locomotor activity.

Transient Receptor Potential Channel Ankyrin-1 Is Not a Cold Sensor for Autonomic Thermoregulation in Rodents

The rodent transient receptor potential ankyrin-1 (TRPA1) channel has been hypothesized to serve as a temperature sensor for thermoregulation in the cold. We tested this hypothesis by using deletion of the Trpa1 gene in mice and pharmacological blockade of the TRPA1 channel in rats. In both Trpa1−/− and Trpa1+/+ mice, severe cold exposure (8°C) resulted in decreases of skin and deep body temperatures to ∼8°C and 13°C, respectively, both temperatures being below the reported 17°C threshold temperature for TRPA1 activation. Under these conditions, Trpa1−/− mice had the same dynamics of body temperature as Trpa1+/+ mice and showed no weakness in the tail skin vasoconstriction response or thermogenic response to cold. In rats, the effects of pharmacological blockade were studied by using two chemically unrelated TRPA1 antagonists: the highly potent and selective compound A967079, which had been characterized earlier, and the relatively new compound 43 ((4R)-1,2,3,4-tetrahydro-4-[3-(3-methoxypropoxy)phenyl]-2-thioxo-5H-indeno[1,2-d]pyrimidin-5-one), which we further characterized in the present study and found to be highly potent (IC50 against cold of ∼8 nm) and selective. Intragastric administration of either antagonist at 30 mg/kg before severe (3°C) cold exposure did not affect the thermoregulatory responses (deep body and tail skin temperatures) of rats, even though plasma concentrations of both antagonists well exceeded their IC50 value at the end of the experiment. In the same experimental setup, blocking the melastatin-8 (TRPM8) channel with AMG2850 (30 mg/kg) attenuated cold-defense mechanisms and led to hypothermia. We conclude that TRPA1 channels do not drive autonomic thermoregulatory responses to cold in rodents.

Age-dependence of alpha-MSH-induced anorexia

Long-term regulation of energy balance involves two major trends: first age-related obesity develops in the middle-aged, later it is followed by anorexia of aging (sarcopenia and/or cachexia). A dynamic balance between orexigenic and anorexigenic neuropeptides is essential for the regulation of energy homeostasis. Special imbalances of neuropeptide effects may be assumed corresponding to different age-periods. Anorexia induced by acute alpha-MSH (alpha-melanocyte stimulating hormone; endogenous melanocortin agonist) injections was analyzed in male Wistar rats aged 6-9 weeks (juvenile), 3-4 months (young adult), 6 or 12 months (two middle-aged groups), 18 months (aging) and 24-26 months (old). Alpha-MSH injected through a preimplanted intracerebroventricular (ICV) cannula (compared with saline injection) dose-dependently suppressed spontaneous food intake and also re-feeding following 24-h fasting, but the rate of suppression varied between age-groups. An ICV injection of 5 microg alpha-MSH attenuated the 2-h re-feeding by 21.9+/-3.2% in juvenile rats, strongly (68.7+/-2.5%) suppressed it in young adults, the suppression became progressively weaker in the two middle-aged groups (55.7+/-4.9%, vs. 26.4+/-4.9%, respectively), but it turned extreme in aging (94.7+/-4.2%) and old (74.3+/-4.5%) rats. Body composition also changed with age: unlike the tibialis anterior muscle, the epididymal and retroperitoneal fat pads increased until middle-age and remained large even in old animals, while the measured indicator of muscle mass decreased in the oldest group. The food intake suppressing and body weight decreasing effects of a 7-day-long ICV infusion of 1 microg/h alpha-MSH were weakest in the 12-month-old and most pronounced in the 24 month-old rats. In conclusion, responsiveness to the anorexic effect of alpha-MSH varies with age, with a nadir of the curve in the middle-aged, and a peak in the aging and old animals. This age-related nadir of melanocortin-responsiveness may promote obesity in middle-aged rats, while the tendency for anorexia and incipient sarcopenia of old (still obese) rats may result from age-related melanocortin-hypersensitivity rather than from adiposity.

Energetics of fasting heterothermia in TRPV1-KO and wild type mice.

To learn the possible role of TRPV1 in the changes of temperature regulation induced by short-term energy lack, TRPV1-KO and wild type mice were exposed to complete fasting for 2 or 3 days while their core temperature and locomotor activity were recorded using a biotelemetry method. In both types of mice, fasting led to progressive daytime hypothermia with night-time core temperature being maintained at normothermia (collectively called heterothermia). During fasting rises of locomotor activity were observed parallel to night-time normothermia with occasional increases of both parameters recorded every 2 to 3 hours (ultradian rhythms). The daytime fall of core temperature was significantly greater in wild type than in TRPV1-KO mice, in the former an advance of the temperature/activity rhythm having been observed in spite of the presence of a 12/12 hour light/darkness schedule. Re-feeding applied at the beginning of the light-period led to rapid reappearance of normothermia in both types of mice without a large increase in locomotor activity. It is concluded that the TRPV1-gene may have a role in the development of adaptive daytime hypothermia (and hence saving some energy) in mice during complete fasting but still allowing normothermia maintained at night, a strategy probably serving survival under natural conditions in small size rodents such as the mouse. The possible role of muscle thermogenesis either with or without gross bodily movement during fasting or on re-feeding, respectively, may be based on different mechanisms yet to be clarified.

Aging reverses the role of the transient receptor potential vanilloid-1 channel in systemic inflammation from anti-inflammatory to proinflammatory

Studies in young rodents have shown that the transient receptor potential vanilloid-1 (TRPV1) channel plays a suppressive role in the systemic inflammatory response syndrome (SIRS) by inhibiting production of tumor necrosis factor (TNF)α and possibly by other mechanisms. We asked whether the anti-inflammatory role of TRPV1 changes with age. First, we studied the effect of AMG517, a selective and potent TRPV1 antagonist, on aseptic, lipopolysaccharide (LPS)-induced SIRS in young (12 wk) mice. In agreement with previous studies, AMG517 increased LPS-induced mortality in the young. We then studied the effects of TRPV1 antagonism (AMG517 or genetic deletion of TRPV1) on SIRS in middle-aged (43–44 wk) mice. Both types of TRPV1 antagonism delayed and decreased LPS-induced mortality, indicating a reversal of the anti-inflammatory role of TRPV1 with aging. In addition, deletion of TRPV1 decreased the serum TNFα response to LPS, suggesting that the suppressive control of TRPV1 on TNFα production is also reversed with aging. In contrast to aseptic SIRS, polymicrobial sepsis (induced by cecal ligation and puncture) caused accelerated mortality in aged TRPV1-deficient mice as compared with wild-type littermates. The recovery of TRPV1-deficient mice from hypothermia associated with the cecal ligation and puncture procedure was delayed. Hence, the reversal of the anti-inflammatory role of TRPV1 found in the aged and their decreased systemic inflammatory response are coupled with suppressed defense against microbial infection. These results caution that TRPV1 antagonists, widely viewed as new-generation painkillers, may decrease the resistance of older patients to infection and sepsis.

Central alpha-MSH infusion in rats: Disparate anorexic vs. Metabolic changes with aging

UNLABELLED Changes of the anorexigenic and hypermetabolic components of the overall catabolic effect of alpha-MSH were studied in rats as a function of age. In male Wistar rats a 7 day-long intracerebroventricular infusion of alpha-MSH suppressed food intake and caused a fall in body weight in 2 and 3-4 month-old (young) groups, but it was most effective in the 24 month-old group and had hardly any effect in the 12 month-old (middle-aged) animals. In contrast, metabolic rate as well as biotelemetric measurements of core temperature and heart rate revealed the most pronounced hypermetabolic effects of such infusions at age 12 months. The hypermetabolic effect was still high in the oldest group, but low in the younger groups. IN CONCLUSION Changes of the anorexigenic and hypermetabolic effects in the course of aging are not concordant. The overall catabolic activity of alpha-MSH is smallest in the middle-aged and highest in the oldest group.

Low fermentable oligosaccharides, disaccharides, monosaccharides and polyols (FODMAP) diet improves symptoms in adults suffering from irritable bowel syndrome (IBS) compared to standard IBS diet: A meta-analysis of clinical studies

Background Irritable bowel syndrome (IBS) and functional digestive tract disorders, e.g. functional bloating, carbohydrate maldigestion and intolerances, are very common disorders frequently causing significant symptoms that challenge health care systems. A low Fermentable Oligosaccharides, Disaccharides, Monosaccharides and Polyols (FODMAP) diet is one of the possible therapeutic approaches for decreasing abdominal symptoms and improving quality of life. Objectives We aimed to meta-analyze data on the therapeutic effect of a low-FODMAP diet on symptoms of IBS and quality of life and compare its effectiveness to a regular, standard IBS diet with high FODMAP content, using a common scoring system, the IBS Symptom Severity Score (IBS-SSS). Methods A systematic literature search was conducted in PubMed, EMBASE and the Cochrane Library as well as in the references in a recent meta-analysis. Adult patients diagnosed with IBS according to the Rome II, Rome III, Rome IV or NICE criteria were included in the analysis. Statistical methods Mean differences with 95% confidence intervals were calculated from studies that contained means, standard deviation (SD) or mean differences and SD of differences and p-values. A random effect model was used because of the heterogeneity (Q test (χ2) and I2 indicator). A p-value of less than 0.05 was chosen to indicate a significant difference. Results The literature search yielded 902 publications, but only 10 were eligible for our meta-analysis. Both regular and low-FODMAP diets proved to be effective in IBS, but post-diet IBS-SSS values were significantly lower (p = 0.002) in the low-FODMAP group. The low-FODMAP diet showed a correlation with the improvement of general symptoms (by IBS-SSS) in patients with IBS. Conclusions This meta-analysis provides high-grade evidence of an improved general symptom score among patients with irritable bowel syndrome who have maintained a low-FODMAP diet compared to those on a traditional IBS diet, therefore showing its superiority to regular IBS dietary therapy. These data suggest that a low-FODMAP diet with dietitian control can be a candidate for first-line therapeutic modality in IBS. Because of a lack of data, well-planned randomized controlled studies are needed to ascertain the correlation between improvement of separate key IBS symptoms and the effect of a low-FODMAP diet.