Histamine and histamine receptors: behavioral thermoregulation in the salamander Necturus maculosus

Naturally occurring hypothermia promotes survival in severe anaphylaxis

Although hypothermia has received substantial attention as an indicator of severity in anaphylaxis, it has been neglected from the perspective of whether it could act as a disease-modifying factor in this condition. Here, the impact of naturally occurring (spontaneous) hypothermia on anaphylaxis was evaluated in a murine model of ovalbumin (OVA)-induced allergy. Nonextreme changes in the ambient temperature (T_a) were used to modulate the magnitude of spontaneous hypothermia. At a T_a of 24°C, challenge with OVA intraperitoneally or intravenously resulted in a rapid, transient fall in body core temperature, which reached its nadir 4-6°C below baseline in 30 min. This hypothermic response was largely attenuated when the mice were kept at a T_a of 34°C. The T_a-dependent attenuation of hypothermia resulted in a survival rate of only 30%, as opposed to survival of 100% in the condition that favored the development of hypothermia. The protective effect of hypothermia did not involve changes in the rate of mast cell degranulation, as assessed by the concentration of mast cell protease-1 in bodily fluids. On the other hand, hypothermia improved oxygenation of the brain and kidneys, as indicated by higher NAD⁺/NADH ratios. Therefore, it is plausible to propose that naturally occurring hypothermia makes organs more resistant to the anaphylactic insult.

Thermoregulation as a disease tolerance defense strategy

Physiological responses that occur during infection are most often thought of in terms of effectors of microbial destruction through the execution of resistance mechanisms, due to a direct action of the microbe, or are maladaptive consequences of host-pathogen interplay. However, an examination of the cellular and organ-level consequences of one such response, thermoregulation that leads to fever or hypothermia, reveals that these actions cannot be readily explained within the traditional paradigms of microbial killing or maladaptive consequences of host-pathogen interactions. In this review, the concept of disease tolerance is applied to thermoregulation during infection, inflammation and trauma, and we discuss the physiological consequences of thermoregulation during disease including tissue susceptibility to damage, inflammation, behavior and toxin neutralization.

The role of histamine H1 receptors in the thermoregulatory effect of morphine in mice

Morphine is known to release histamine from mast cells and increase the turnover of neuronal histamine. It is also known that histamine receptors mediate some of the morphine effects. The contribution of histamine H1 and H2 receptors to the thermoregulatory effect of morphine in mice was investigated in the present experiments. Morphine produced a hypothermic effect, especially at the dose of 10 mg/kg. Although the histamine H1 receptor antagonist, dimethindene (0.1 mg/kg, i.p.), attenuated the hypothermic effect of morphine (10 mg/kg), a histamine H2 receptor antagonist, ranitidine (100 mg/kg, i.p.), had no effect. These results suggest that the hypothermic effect of morphine in mice is mediated, at least partly, through histamine H1 receptors.