How the Brain Perceives Cooling Sensation

Summary: A new study delves into how our brains perceive temperature, particularly through the cooling sensation experienced when eating something like a chilled mint cookie. The study focuses on TRPM8 receptors in the mouth, which are activated by cold temperatures and menthol, explaining why mint tastes more intense when cold.

They discovered that removing TRPM8 receptors in mice alters the brain’s response to temperature, blurring the distinction between cool and warm sensations and influencing temperature preferences. This research not only sheds light on the complex process of temperature perception but also sets the stage for future explorations into how temperature affects taste and eating behavior.

Key Facts:

1. TRPM8 receptors, activated by cold temperatures and menthol, play a critical role in the brain’s perception of cooling sensations.
2. Removing these receptors in mice causes the brain to confuse cool and warm temperatures, affecting temperature preference behavior.
3. The study’s findings contribute to understanding how temperature perception impacts taste and dietary choices, promising insights into sensory processing and potential health implications.

Source: University of Oklahoma

Christian Lemon, Ph.D., an associate professor in the School of Biological Sciences at the University of Oklahoma, often thinks about temperature sensation and the brain when eating a chilled mint cookie.

Now, research from his lab examining oral temperature perception has been published in The Journal of Neuroscience. 

In their research, Lemon’s team investigates how cold receptors in the mouth are activated by cooling temperatures, how those signals are transmitted to the brain and how those transmissions are generated into a cooling sensation.  

“These receptors respond to cooling temperatures but are also activated by menthol from mint plants. This feature is probably why the flavor of a mint cookie can appear enhanced when eaten cold,” he said.

“While sometimes called a cold and menthol receptor, it’s technically known as TRPM8. These receptors begin to activate when temperature falls a few steps below your core body temperature.” 

According to prior research, TRPM8 receptors are activated by temperatures below about 86 degrees Fahrenheit, 30 degrees Celsius, and are strongly stimulated by colder temperatures near 50 degrees Fahrenheit, 10 degrees Celsius.  

“Our study found that genetically removing TRPM8 receptors in a mouse model reduced the brain’s response to mild cooling in the mouth, while responses to significantly colder temperatures remained partly intact,” he said.

“Interestingly, this process also impacted how the brain responded to warm temperatures. We found that without input from TRPM8 receptors, the brain’s response to warmth moved down into the cool range, essentially making cooler temperatures appear as warmer by the brain’s response.” 

Lemon’s team theorized that the brain might be confusing, or “blurring,” cooling and warming sensations when TRPM8 was silenced. To explore this idea, they precisely controlled the temperature of liquids consumed to monitor oral temperature preference behavior. These results compared how temperature messages from TRPM8 receptors in the mouth tracked along nerve fibers into the brain and influenced how the brain may interpret those signals. 

“We found that the control group with intact TRPM8 receptors preferred to drink mild cool and colder fluids and avoided warmed fluids. Those without the TRPM8 receptor, however, avoided sampling both warm and mild cool fluids,” he said.

“This common reaction to cool and warm temperatures agreed with the blurring of these temperature ranges we observed in the brain responses of TRPM8 silenced mice. This receptor appears to be required for the brain to correctly recognize warm temperatures inside the mouth and to distinguish them from cooling.” 

Based on these findings and because temperature is such a big component of oral sensation, Lemon’s team plans to explore how temperature sensory signals from TRPM8 and other pathways affect taste and eating preferences. They believe this could help understand the role of temperature sensing in a unique health-related context. 

“Combining our research findings with those from other labs and other papers will start to tell us the basics of how temperature recognition works in the brain in different settings,” he said.

“There’s still a lot of mysteries in the brain that we don’t understand, but the basic principles being defined in studies like ours are the building blocks to future discoveries.” 

About this Neuroscience research news

Author: Josh DeLozier
Source: University of Oklahoma
Contact: Josh DeLozier – University of Oklahoma
Image: The image is credited to Neuroscience News

Original Research: Closed access.
“Separation of Oral Cooling and Warming Requires TRPM8” by Christian Lemon et al. Journal of Neuroscience

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Abstract

Separation of Oral Cooling and Warming Requires TRPM8

Cooling sensations arise inside the mouth during ingestive and homeostasis behaviors. Oral presence of cooling temperature engages the cold and menthol receptor TRPM8 (transient receptor potential melastatin 8) on trigeminal afferents. Yet, how TRPM8 influences brain and behavioral responses to oral temperature is undefined.

Here we used in vivo neurophysiology to record action potentials stimulated by cooling and warming of oral tissues from trigeminal nucleus caudalis neurons in female and male wild-type and TRPM8 gene deficient mice.

Using these lines, we also measured orobehavioral licking responses to cool and warm water in a novel, temperature-controlled fluid choice test. Capture of antidromic electrophysiological responses to thalamic stimulation identified that wild-type central trigeminal neurons showed diverse responses to oral cooling.

Some neurons displayed relatively strong excitation to cold <10°C (COLD neurons) while others responded to only a segment of mild cool temperatures below 30°C (COOL neurons). Notably, TRPM8 deficient mice retained COLD-type but lacked COOL cells.

This deficit impaired population responses to mild cooling temperatures below 30°C and allowed warmth-like (≥35°C) neural activity to pervade the normally innocuous cool temperature range, predicting TRPM8 deficient mice would show anomalously similar orobehavioral responses to warm and cool temperatures.

Accordingly, TRPM8 deficient mice avoided both warm (35°C) and mild cool (≤30°C) water and sought colder temperatures in fluid licking tests, whereas control mice avoided warm but were indifferent to mild cool and colder water.

Results imply TRPM8 input separates cool from warm temperature sensing and suggest other thermoreceptors also participate in oral cooling sensation.