Vagal CCK and 5-HT(3) receptors are unlikely to mediate LPS or IL-1beta-induced fever

5-HT Receptors and Temperature Homeostasis

The present review summarizes the data concerning the influence of serotonin (5-HT) receptors on body temperature in warm-blooded animals and on processes associated with its maintenance. This review includes the most important part of investigations from the first studies to the latest ones. The established results on the pharmacological activation of 5-HT1A, 5-HT3, 5-HT7 and 5-HT2 receptor types are discussed. Such activation of the first 3 type of receptors causes a decrease in body temperature, whereas the 5-HT2 activation causes its increase. Physiological mechanisms leading to changes in body temperature as a result of 5-HT receptors' activation are discussed. In case of 5-HT1A receptor, they include an inhibition of shivering and non-shivering thermogenesis, as well simultaneous increase of peripheral blood flow, i.e., the processes of heat production and heat loss. The physiological processes mediated by 5-HT2 receptor are opposite to those of the 5-HT1A receptor. Mechanisms of 5-HT3 and 5-HT7 receptor participation in these processes are yet to be studied in more detail. Some facts indicating that in natural conditions, without pharmacological impact, these 5-HT receptors are important links in the system of temperature homeostasis, are also discussed.

TLR4 Signaling Selectively and Directly Promotes CGRP Release from Vagal Afferents in the Mouse

There has been a long-standing debate regarding the role of peripheral afferents in mediating rapid-onset anorexia among other responses elicited by peripheral inflammatory insults. Thus, the current study assessed the sufficiency of peripheral afferents expressing toll-like receptor 4 (TLR4) to the initiation of the anorexia caused by peripheral bacterial lipopolysaccharide (LPS). We generated a Tlr4 null (Tlr4^LoxTB) mouse in which Tlr4 expression is globally disrupted by a loxP-flanked transcription blocking (TB) cassette. This novel mouse model allowed us to restore the endogenous TLR4 expression in specific cell types. Using Zp3-Cre and Na_v1.8-Cre mice, we produced mice that express TLR4 in all cells (Tlr4^LoxTB X Zp3-Cre) and in peripheral afferents (Tlr4^LoxTB X Na_v1.8-Cre), respectively. We validated the Tlr4^LoxTB mice, which were phenotypically identical to previously reported global TLR4 knock-out mice. Contrary to our expectations, the administration of LPS did not cause rapid-onset anorexia in mice with Na_v1.8-restricted TLR4. The later result prompted us to identify Tlr4-expressing vagal afferents using in situ hybridization (ISH). In vivo, we found that Tlr4 mRNA was primarily enriched in vagal Na_v1.8 afferents located in the jugular ganglion that co-expressed calcitonin gene-related peptide (CGRP). In vitro, the application of LPS to cultured Na_v1.8-restricted TLR4 afferents was sufficient to stimulate the release of CGRP. In summary, we demonstrated using a new mouse model that vagally-expressed TLR4 is selectively involved in stimulating the release of CGRP but not in causing anorexia.

CCK2 receptor nullification attenuates lipopolysaccharide-induced sickness behavior

Systemic infection produces a highly regulated set of responses such as fever, anorexia, adipsia, inactivity, and cachexia, collectively referred to as sickness behavior. Although the expression of sickness behavior requires immune-brain communication, the mechanisms by which peripheral cytokines signal the brain are unclear. Several mechanisms have been proposed for neuroimmune communication, including the interaction of cytokines with peripheral nerves. A critical role has been ascribed to the vagus nerve in mediating sickness behavior after intraperitoneally delivered immune activation, and converging evidence suggests that this communication may involve neurochemical intermediaries afferent and/or efferent to this nerve. Mice lacking functional CCK2/gastrin receptors (CCK2KO) and wild-type (WT) controls were administered LPS (50, 500, or 2,500 μg/kg; serotype 0111:B4; ip). Results indicate a role for CCK2 receptor activation in the initiation and maintenance of LPS-induced sickness behavior. Compared with WT controls, CCK2KO mice were significantly less affected by LPS on measures of body temperature, activity, body weight, and food intake, with the magnitude of effects increasing with increasing LPS dose. Although activation of CCK2 receptors at the level of the vagus nerve cannot be excluded, a possible role for these receptors in nonvagal routes of immune-brain communication is suggested.

Absence of the cholecystokinin-A receptor deteriorates homeostasis of body temperature in response to changes in ambient temperature

The circadian rhythm of the body core temperature (Tc) and the effects of changes in ambient temperatures on the homeostasis of Tc in Otsuka Long Evans Tokushima Fatty (OLETF) rats, which are naturally occurring cholecystokinin (CCK)-A receptor (CCK-AR) gene knockout (−/−) rats, were examined. In addition, the peripheral responses to warming or cooling of the preoptic and anterior hypothalamic region (PO/AH) were determined. The circadian rhythm of Tc in OLETF rats was similar to that in Long-Evans Tokushima (LETO) rats; this rhythm was characterized by a higher Tc during the dark period and a lower Tc during the light period. When the ambient temperature was changed within the limits of 0°C to 30°C, the changes in Tc of LETO rats were associated with the changes in ambient temperature, whereas those in OLETF rats were dissociated from the temperature changes. The OLETF rats showed a large hysteresis. The peripheral responses to warming or cooling of PO/AH, including shivering of the neck muscle and changes in skin temperature of the tail and footpad, were similar in OLETF and LETO rats. To confirm the role of CCK-AR in the regulation of body temperature, the values of Tc in the CCK-AR(−/−) mice were compared with those in CCK-B receptor (CCK-BR) (−/−), CCK-AR(−/−)BR(−/−), and wild-type mice. In the mice, the circadian rhythms of Tc were the same, regardless of the genotype. Mice without CCK-AR showed larger hysteresis than mice with CCK-AR. From these results, we conclude that the lack of CCK-AR causes homeostasis of Tc in rats and mice to deteriorate.

Cholecystokinin inhibits food intake independent of interleukin-1 beta expression in the brain

Increasing evidence has suggested that cholecystokinin (CCK) is involved in immune-to-brain communication. The afferent vagus nerve is an important component for transmitting peripheral immune signals to the brain, such as those determining interleukin (IL)-1beta expression in the brain and anorexia. In the present study, we investigated whether the anorexic effect of CCK, which also activates the afferent vagus nerve, is mediated via IL-1beta expression in the brain. CCK-8 dose-dependently (8-320 microg/kg, i.p.) inhibited food intake in mice. However, IL-1beta transcripts in the hypothalamus, the hippocampus and the brainstem were not significantly increased after the administration of CCK-8, even at the larger dose of 320 microg/kg. These findings suggest that the CCK-induced inhibition of food intake may be independent of IL-1beta production in the brain, and indicate the diverse role of CCK in the regulation of the neuro-immune interaction.

Compromised neuroimmune status in rats with experimental colitis

In colitis, chronic and recurrent inflammation is associated with a breakdown in host defence mechanisms that leads to local and systemic infection. Whether this is due to a compromised neuroimmune response has not been studied. Our aim was to determine if colitis altered the host neuroimmune response as reflected in either body temperature rhythm or the febrile responses to lipopolysaccharide (LPS). Body temperature was monitored by telemetry from conscious, unrestrained male rats treated with trinitrobenzene sulphonic acid or saline. Twenty-six days after initial induction, colitis was reactivated. Animals were given LPS (50 microg kg-1 Escherichia coli LPS) during colitis and after reactivation. At the peak of colitis, treated rats showed a disruption of circadian body temperature rhythm, manifested as day-time fever followed by night-time hypothermia. In response to LPS, controls displayed a characteristic fever, whereas treated animals had a significantly reduced fever and low plasma levels of interleukin-6 (IL-6) and tumour necrosis factor-alpha (TNF-alpha). During reactivation of colitis, treated animals did not mount a fever or exhibit increased plasma levels of IL-6 and TNF-alpha after LPS. We conclude that experimental colitis is associated with a compromised neuroimmune status.

Role for the cholecystokinin-A receptor in fever: a study of a mutant rat strain and a pharmacological analysis

The involvement of the cholecystokinin (CCK)-A receptor in fever was studied. The polyphasic febrile responses to lipopolysaccharide (LPS; 10 microg kg-1, I.V.) were compared between wild-type Long-Evans (LE) rats and the CCK-A-receptor-deficient Otsuka LE Tokushima Fatty (OLETF) rats. The response of the wild-type rats was biphasic, which is typical for LE rats. Phases 1 and 2 of the response of the OLETF rats were similar to those of the LE rats, but the OLETF rats also developed a robust phase 3. This late enhancement of the febrile response could reflect either the absence of the A receptor per se or a secondary trait of the mutant strain. To distinguish between these possibilities, we conducted a pharmacological analysis. We studied whether the normally low phase 3 of LE rats can be enhanced by a CCK-A-receptor antagonist, sodium lorglumide (4.3 microg kg-1 min-1, 120 min, I.V.), and whether the normally high phase 3 of Wistar rats can be attenuated by a CCK-A receptor agonist, sulphated CCK-8 (up to 0.17 microg kg-1 min-1, 120 min, I.V.). The dose of sodium lorglumide used was sufficient to increase food intake (to block satiety), but it did not affect the fever response. In both febrile and afebrile rats, CCK-8 induced dose-dependent skin vasodilatation and decreased body temperature, but it failed to produce any effects specific for phase 3. We conclude that the exaggeration of phase 3 in OLETF rats reflects a secondary trait of this strain and not the lack of the CCK-A receptor per se. None of the three known phases of the febrile response of rats to LPS requires the CCK-A receptor.

Cholecystokinin and thermoregulation--a minireview

Thermoregulatory effects of cholecystokinin (CCK) peptides are reviewed with special emphasis on two types of responses, that is hypothermia or hyperthermia. In rodents exposed to cold a dose-dependent hypothermia has been observed on peripheral injection of CCK probably acting on CCKA receptors. Central microinjection of CCK in rats induced a thermogenic response that could be attenuated by CCKB receptor antagonists, but some authors observed a hypothermia. It is suggested that neuronal CCK may have a specific role in the development of hyperthermia, and endogenous CCK-ergic mechanisms could contribute to the mediation of fever. Possible connections between thermoregulatory and other autonomic functional changes induced by CCK are discussed.