Orexin neurons are indispensable for prostaglandin E 2 ‐induced fever and defence against environmental cooling in mice

We recently showed that orexin neurons in the hypothalamus are indispensable for stress‐induced thermogenesis. In this study we examined whether the orexin neurons are also important for other forms of thermogenic processes, including brain prostaglandin E 2 (PGE 2 ) injection that mimics inflammatory fever and environmental cold exposure. As was the case with stress‐induced thermogenesis, orexin neuron‐ablated (ORX‐AB) mice exhibited a blunted PGE 2 ‐induced fever and intolerance to cold (5°C) exposure. Injection of retrograde tracer into the medullary raphe nucleus, where sympathetic premotor neurons regulating thermogenesis by the brown adipose tissue are located, revealed direct and indirect projection from the orexin neurons, of which the latter seemed to be preserved in the ORX‐AB mice. These results suggest that orexin neurons are important in general thermogenic processes, and their importance is not restricted to stress‐induced thermogenesis. Abstract We recently showed using prepro‐orexin knockout (ORX‐KO) mice and orexin neuron‐ablated (ORX‐AB) mice that orexin neurons in the hypothalamus, but not orexin peptides per se , are indispensable for stress‐induced thermogenesis. To examine whether orexin neurons are more generally involved in central thermoregulatory mechanisms, we applied other forms of thermogenic perturbations, including brain prostaglandin E 2 (PGE 2 ) injections which mimic inflammatory fever and environmental cold exposure, to ORX‐KO mice, ORX‐AB mice and their wild‐type (WT) litter mates. ORX‐AB mice, but not ORX‐KO mice, exhibited a blunted PGE 2 ‐induced fever and intolerance to cold (5°C) exposure, and these findings were similar to the results previously obtained with stress‐induced thermogenesis. PGE 2 ‐induced shivering was also attenuated in ORX‐AB mice. Both mutants responded similarly to environmental heating (39°C). In WT and ORX‐KO mice, the administration of PGE 2 and cold exposure activated orexin neurons, as revealed by increased levels of expression of c‐fos. Injection of retrograde tracer into the medullary raphe nucleus revealed direct and indirect projection from the orexin neurons, of which the latter seemed to be preserved in the ORX‐AB mice. In addition, we found that glutamate receptor antagonists ( d ‐(–)‐2‐amino‐5‐phosphonopentanoic acid and 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione) but not orexin receptor antagonists (SB334867 and OX2 29) successfully inhibited PGE 2 ‐induced fever in WT mice. These result