The results of a study in rodents carried out by scientists at Nagoya University suggest that a group of neurons, called EP3 neurons, in the preoptic area (POA) of the brain play a key role in regulating body temperature in mammals. The discovery could pave the way for the development of technology that artificially adjusts body temperature, as a way of helping to treat conditions such as heat stroke, or hypothermia, and potentially obesity.
“On top of that, this technology could lead to new strategies for survival of people in hotter global environments, which are becoming a serious worldwide problem,” said research lead Kazuhiro Nakamura, PhD. The scientists reported on their study in Science Advances, in a paper titled, “Prostaglandin EP3 receptor–expressing preoptic neurons bidirectionally control body temperature via tonic GABAergic signaling.”
Thermoregulation is a physiological function that is fundamental to homeostasis in mammals, the authors noted. “Body core temperature is maintained within a control range by autonomous regulation of the balance between heat production within the body and heat loss to the environment.” In humans and many other mammals, body temperature is regulated at around 37°C (98.6°F), which optimizes all regulatory functions. But when body temperature noticeably deviates from the normal range, these functions can be impaired, potentially leading to heat stroke, hypothermia, and, in the worst case, death. These conditions might possibly be treated if body temperature could be artificially adjusted to remain within the normal range.
The brain’s temperature regulation center resides in the preoptic area, a part of the hypothalamus that controls the body’s vital functions. For example, when the preoptic area receives signals from a mediator called prostaglandin E (PGE2) that is produced in response to infections, this area releases a command to raise body temperature to fight against viruses, bacteria, and other disease-causing organisms. “The POA receives and integrates thermosensory (cool- and warm-sensory) neural signals from skin thermoreceptors and a pyrogenic humoral signal mediated by prostaglandin E2 (PGE2), which is produced in response to infections,” the team noted.
However, it is still unclear exactly which neurons in the preoptic area release commands to increase or decrease body temperature. To identify such neurons, Nakamura, together with Yoshiko Nakamura, PhD, and colleagues at Nagoya University, in collaboration with Hiroyuki Hioki, PhD, at Juntendo University, designed a study in rodents. They focused on how EP3 neurons in the preoptic area that express the EP3 subtype of PGE2 receptor may be involved in regulating body temperature. “… we investigated the physiological role of POAEP3R neurons in the central circuit mechanisms of thermoregulation and fever,” the team explained.
Nakamura and colleagues started by looking at how the activity of EP3 neurons in the preoptic area varied in response to changes in ambient temperature. “We first examined activation of POAEP3R neurons in response to ambient thermal challenges and then histochemically and physiologically determined the neurotransmitter phenotype of POAEP3R neurons,” they noted. A comfortable environmental temperature for rats is around 28°C. For two hours, the researchers exposed the rats to cold (4°C), room (24°C), and hot (36°C) temperatures. Results showed that exposure to 36°C activated EP3 neurons, while exposure to 4°C and 24°C did not. “These observations indicate that the POAEP3R neuronal group includes a substantial subpopulation of warming-activated neurons but not cooling-activated neurons,” the investigators suggested.
The group then observed nerve fibers of EP3 neurons in the preoptic area to identify where the signals from EP3 neurons are transmitted. Their observations revealed that nerve fibers are distributed to various brain regions, particularly to the dorsomedial hypothalamus (DMH), which activates the sympathetic nervous system. Their analysis also showed that the substance that EP3 neurons use for signal transmission to DMH is gamma-aminobutyric acid (GABA), a major inhibitor of neuronal excitation. “Although many POAEP3R neuronal cell bodies express a glutamatergic messenger RNA marker, their axons in the DMH predominantly release γ-aminobutyric acid (GABA), and their GABAergic terminals are increased by chronic heat exposure,” they stated. The combined results, the team further wrote, “… demonstrate that POAEP3R neuron–derived axons predominantly form GABAergic synapses onto DMH neurons.”
To further investigate the role of EP3 neurons in temperature regulation, researchers artificially manipulated their activity using a chemogenetic approach. They found that activating the neurons led to a decrease in body temperature, whereas suppressing their activity led to their increase. “Chemogenetic stimulation of POAEP3R neurons at room temperature reduces body temperature by enhancing heat dissipation, whereas inhibition of them elicits hyperthermia involving brown fat thermogenesis, mimicking fever,” they noted.
The combined study results demonstrated that EP3 neurons in the preoptic area play a key role in regulating body temperature by releasing GABA to send inhibitory signals to DMH neurons to control sympathetic responses. “The present study demonstrates that POAEP3R neurons, a target of PGE2 for its pyrogenic action, play a pivotal role in the preoptic efferent control of central sympathetic outflow for basal thermoregulation,” the team wrote. “Our study shows strong evidence that POAEP3R neurons provide tonic GABAergic inhibitory signaling to sympathoexcitatory efferent pathways as a fundamental determinant of body temperature for thermal homeostasis and fever.”
Lead author Nakamura further suggested, “Probably, EP3 neurons in the preoptic area can precisely regulate the signal strength to fine-tune body temperature. For example, in a hot environment, signals are augmented to suppress sympathetic outputs, resulting in increased blood flow in the skin to facilitate the radiation of the body’s heat to prevent heat stroke. However, in a cold environment, signals are reduced to activate sympathetic outputs, which promote heat production in brown adipose tissue and other organs to prevent hypothermia. Furthermore, at the time of infection, PGE2 acts on EP3 neurons to suppress their activity, resulting in activation of sympathetic outputs to develop fever.”
This study’s findings could pave the way for the development of a technology that artificially adjusts body temperature, which might then be applied to a wide range of medical fields. Interestingly, this technology may also be helpful in the treatment of obesity, by keeping body temperature slightly higher than normal to promote fat burning.
“On top of that, this technology could lead to new strategies for survival of people in hotter global environments, which are becoming a serious worldwide problem,” said Nakamura.
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