Neurons that encode sweetness identified: Mouse study

Summary: SatB2PBN-expressing neurons in the parabrachial nucleus play a key role in processing and encoding sweet tastes. The SatB2PBN neurons relay sweet taste signals from the gustatory thalamus to the cortex in mouse models.

Source: National Institutes of Natural Sciences

While the peripheral taste system has been extensively investigated, relatively little is known about the contribution of CNS gustatory neurons in the sensation of taste. In this new study, researchers have identified neurons in the brainstem that are responsible for encoding sweet tastes.

In mice, the parabrachial nucleus of the pons in the brainstem is a major hub that receives sensory information about hunger, satiety, and taste information and relays it to the cortex via the gustatory thalamus. One clue to the molecular properties of gustatory neurons in the parabrachial nucleus may lie in the neuronal expression of SatB2; the role of neurons in the parabrachial nucleus that possess this transcription factor has so far remained a mystery. Ken-ichiro Nakajima and his research team at the National Institute for Physiological Sciences have found that SatB2-expressing neurons in the parabrachial nucleus of mice encode sweet tastes and those that projected to the gustatory thalamus induced appetitive lick behaviors in mice. They recently published their findings in Cell Reports.

“We’ve known about the presence of taste-responsive neurons in the parabrachial nucleus for over 40 years,” Nakajima says. “Only recently have we had the appropriate molecular markers and imaging methods to properly characterize these neurons–we used cell ablation, in vivo calcium imaging, and optogenetics to define the role of SatB2-expressing neurons is in the sensation of taste.”

Selective ablation of SatB2-expressing neurons led to the loss of normal sweet taste sensing, which was measured by licking behavior in mice, but had little impact on the sensitivities to umami, bitter, sour, and salty tastes. This indicates that SatB2-expressing neurons have selective roles in sweet taste transduction.

This shows the network from the gustatory thalamus to the cortex in mice

SatB2-expressing neurons localized in the parabrachial nucleus (PBN) of the mouse brainstem encode positive valance and selectively transmit sweet taste signals to the gustatory thalamus (VPMpc). The image is credited to Ken-ichiro Nakajima / National Institute for Physiological Sciences.

Furthermore, the researchers clarified the functional role of SatB2-expressing neurons. Artificial activation by optogenetics caused dramatic changes in licking behavior; mice intensively licked tasteless water as if it were the sweet-tasting solution. These findings indicated that SatB2-expressing neurons convey sweet taste-specific signals.

“Our findings indicate that different taste qualities are processed by different types of neurons, at least in the brainstem,” says lead author Ou Fu. “The next important step will be to identify a whole set of gustatory neurons, including SatB2 neurons, in the mouse parabrachial nucleus. This will allow us to understand how their assemblage forms complex flavors.”

This new work could be pivotal in characterizing taste processing at molecular and cellular levels.

About this neuroscience research article

Source:
National Institutes of Natural Sciences
Media Contacts:
Ken-ichiro Nakajima – National Institutes of Natural Sciences
Image Source:
The image is credited to Ken-ichiro Nakajima / National Institute for Physiological Sciences.

Original Research: Open access
“SatB2-Expressing Neurons in the Parabrachial Nucleus Encode Sweet Taste”. Ou Fu, Yuu Iwai, Kunio Kondoh, Takumi Misaka, Yasuhiko Minokoshi, Ken-ichiro Nakajima.
Cell Reports doi:10.1016/j.celrep.2019.04.040

Abstract

SatB2-Expressing Neurons in the Parabrachial Nucleus Encode Sweet Taste

Highlights
• Loss of SatB2PBN neurons lead to selective impairment of sweet taste sensing
• SatB2PBN neurons respond to sweet taste but not to other tastes
• Optogenetic activation of SatB2PBN neurons enhances water lick behavior
• The gustatory thalamus-projecting SatB2PBN neurons encode positive valance

Summary
The gustatory system plays an important role in sensing appetitive and aversive tastes for evaluating food quality. In mice, taste signals are relayed by multiple brain regions, including the parabrachial nucleus (PBN) of the pons, before reaching the gustatory cortex via the gustatory thalamus. Recent studies show that taste information at the periphery is encoded in a labeled-line manner, such that each taste modality has its own receptors and neuronal pathway. In contrast, the molecular identity of gustatory neurons in the CNS remains unknown. Here, we show that SatB2-expressing neurons in the PBN play a pivotal role in sweet taste transduction. With cell ablation, in vivo calcium imaging, and optogenetics, we reveal that SatB2PBN neurons encode positive valance and selectively transmit sweet taste signals to the gustatory thalamus.

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