To beat the heat, hypothalamus neurons in mice ramp up their firing

A strong, long-lasting sensory stimulus—be it visual, auditory, olfactory or tactile—triggers plasticity in the neurons that respond to it. But as a scientist long interested in temperature, Jan Siemens wondered: Does the same principle apply to prolonged heat?

In mammals, the body changes when temperatures soar—blood vessels dilate, heat-generating brown adipose tissue shuts off, the heart rate lowers, locomotion slows—but it wasn’t clear if the brain played a role in these changes, or even changed itself, says Siemens, professor of pharmacology at the University of Heidelberg.

Siemens and his team started a search for heat-induced neuronal plasticity in the ventromedial preoptic area of the hypothalamus (VMPO) in mice. They chose the region because of its involvement in regulating body temperature and generating fever; neurons there receive temperature information downstream from cells innervating the skin, whereas others are themselves warm-sensitive. They identified cells to target by measuring the expression of c-FOS, a gene that is activated by neuronal activity, after housing the mice at 36 degrees Celsius for up to eight hours.

At first, however, their investigative trail went cold. In brain slices, those warm-responding cells showed only slight and inconsistent changes in synaptic plasticity. “That was actually quite humbling and disappointing,” Siemens says.

But then they made a “serendipitous observation,” he says: A subgroup of neurons expressing the leptin receptor became almost constantly active after four weeks of heat acclimation. The firing was so synchronized and regular that Wojciech Ambroziak, a postdoctoral scholar in the lab at the time, described it as “soldiers marching in a line,” Siemens recalls.

 

 

The neurons also became warm-sensitive, meaning they fired more often in response to even higher temperatures, the team reported in a paper published this week in Nature Neuroscience. These changes contribute to heat tolerance and are caused by an increased sodium current, additional experiments from the paper show.

The work “brings substantial information about how the brain produces acclimation to warm environments,” says Natalia Machado, assistant professor of neurology at Beth Israel Deaconess Medical Center, who was not involved in the study.

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nce the housing for the heat-acclimated mice was set back to its usual 23 degrees Celsius, the increased firing lowered to baseline levels within a week but rebounded after only two days of the mice being exposed again to 36-degree conditions. This type of “memory” also occurs in peripheral organs, the researchers write in their paper, so they wondered if the neuronal changes played a role in increasing the heat tolerance of the animal.

Mice that had four weeks to acclimate to higher temperatures could maintain a healthy body temperature for 24 hours during a 39-degree “heat challenge,” whereas unacclimated animals reached an unsafe body temperature within 6 hours, on average. In the acclimated animals, greater heat tolerance was linked to increased firing in the leptin receptor VMPO neurons. When the researchers silenced those neurons using chemogenetics during the heat challenge, acclimated animals could no longer maintain a healthy body temperature and quickly developed hyperthermia, but inhibiting the cells did not worsen the performance of unacclimated animals.

This experiment does not definitively show that the neurons’ only job is orchestrating heat tolerance, Machado says, because the leptin receptor neurons could also be used to detect and respond to acute temperature changes. “If [the mice] are not performing well in the heat challenge, is it because they are not able to heat dissipate, they are just not losing heat because the acute neurons are also gone?” she says. “Or is it because you are shutting down the neurons that are necessary for acclimation?”

A follow-up experiment could answer this by applying an activity-based marker such as c-FOS to heat-responsive VMPO neurons before or after heat acclimation, and then blocking only the neurons that become active after acclimation, Machado adds.

Inducing the increased activity in the leptin receptor neurons via optogenetics made mice heat-tolerant in only three days, and without being exposed to heat. “They can do the same thing during a four-week heat acclimation or doing a three-day stimulation—that was sufficient,” says Heike Muenzberg-Gruening, professor of neuroscience and metabolism at the Pennington Biomedical Research Center at Louisiana State University, who wasn’t involved in the work. “I think that is kind of remarkable.”

Leptin levels in the blood decreased as the mice lost weight over the course of their heat acclimation, which could mean that leptin prevents the VMPO neurons from firing under normal conditions, Siemens hypothesizes. But increasing or decreasing leptin levels in acclimated mice—either via injection or food deprivation, respectively—had only a minor effect on the increased firing pattern. The neurons express other receptors in addition to the leptin receptor, so the activity changes are driven by “probably many different things coming together, and leptin being one of them, but not a major one,” Siemens says.

The resting membrane potential is 10 millivolts more depolarized, on average, in acclimated VMPO neurons than in unacclimated neurons, the team found. Increased sodium current from the voltage-gated channel Nav1.3 contributes to this difference, pharmacology and electrophysiology experiments show, but other channels are also likely involved, Siemens says.

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ogether, one interpretation of these results is that after heat acclimation, “these neurons gain much more power, more impact, more impetus to control heat tolerance than before,” Siemens says. Over the course of heat acclimation, their activity primes the organs to start developing tolerance mechanisms, whereas their activity during acute challenges instructs the body to dissipate heat even further, he adds.

The assay the team used­—maintaining body temperature during a heat challenge—demonstrated the “ultimate” level of heat tolerance, Siemens says: survival. “But now the question is really, what are the downstream organ systems that are engaged by these pathways?” His lab has begun to explore the relationship between activity in these neurons and changes in specific organs and behaviors, he says.

Most research in the field of thermoregulation has focused on how groups of neurons respond to acute temperature changes, says Ramón Piñol, staff scientist at the U.S. National Institute of Diabetes and Digestive and Kidney Diseases, who wasn’t involved in the study. Chronic temperature change has not received much attention. “The integration of ambient temperature over time, I think that is underappreciated and not studied a lot in the brain,” Piñol says. “For body temperature regulation, we have mostly focused on acute things: ‘We now put the mouse from cold to warm and see what happens. We now put the mouse from warm to cold and see what happens.’”

As a result, researchers have identified several types of neurons—those that respond to warm or cold cues from neurons in the skin, those that are warm- or cold-sensitive themselves and those that control fever—but have not yet determined how those populations overlap and interact with one another. Or, in the case of this study, how a subgroup’s properties can morph in response to a changing environment.

The neurons that gain warm sensitivity after heat acclimation come from subgroups that respond to cold in vivo and neurons that don’t respond to temperature changes in brain slices, the work shows. “They are basically developing this ability that they didn’t have before,” Machado says. “As far as I know, this is the first observation, and it will be very interesting to know if other cell types also can do similar things.” For example, she says, would neurons that aren’t typically cold-sensitive also change their properties during long-term exposure to cold? And do neurons in other brain regions also change in this way?

“It’s beautiful,” Piñol says. “I like seeing that neurons can be recruited to perform a certain role depending on the environment of the animal. It just shows that we can adapt.”