Fever

"Many species use a temporary decrease in body temperature and metabolic rate (torpor) as a strategy to survive food scarcity in a cool environment. Torpor is caused by preoptic neurons that express a variety of peptides and receptors1–7 , but no single genetic marker has been found for this population. Here we report that expression of the prostaglandin EP3 receptor (EP3R) marks a unique population of median preoptic nucleus (MnPO) neurons that are required for both torpor and lipopolysaccharideinduced fever8 . The MnPO-EP3R neurons produce persistent fever responses when inhibited and prolonged hypothermic responses when activated either chemogenetically or optogenetically, even for brief periods of time. The mechanism for these prolonged responses appears to involve increases in intracellular levels of cAMP and calcium that may persist for many minutes up to hours beyond the termination of a stimulus. These properties endow the population of MnPO-EP3R neurons with the ability to act as a two-way switch for the hypothermic and hyperthermic responses that are required for survival."

Fever is a common phenomenon during infection or inflammatory conditions. This stereotypic rise in body temperature (Tb) in response to inflammatory stimuli is a result of autonomic responses triggered by prostaglandin E2 action on EP3 receptors expressed by neurons in the median preoptic nucleus (MnPO^EP3R neurons). To investigate the identity of MnPO^EP3R neurons, we first used in situ hybridization to show coexpression of EP3R and the VGluT2 transporter in MnPO neurons. Retrograde tracing showed extensive direct projections from MnPO^VGluT2 but few from MnPO^Vgat neurons to a key site for fever production, the raphe pallidus. Ablation of MnPO^VGluT2 but not MnPO^Vgat neurons abolished fever responses but not changes in Tb induced by behavioral stress or thermal challenges. Finally, we crossed EP3R conditional knock-out mice with either VGluT2-IRES-cre or Vgat-IRES-cre mice and used both male and female mice to confirm that the neurons that express EP3R and mediate fever are glutamatergic, not GABAergic. This finding will require rethinking current concepts concerning the central thermoregulatory pathways based on the MnPO^EP3R neurons being GABAergic.