Summary: Researchers have identified a brain pathway near the midbrain where taste and pain signals converge, according to a new study.
Source: University of Oklahoma
University of Oklahoma neuroscientists have identified a neural pathway where taste and pain signals meet. The study, initially designed to examine how taste and food temperature interact, unexpectedly revealed shared circuitry for taste and nociceptive (pain) signals. The research team received a five-year, $1.6 million grant from the National Institutes of Health to pursue this line of inquiry.
“Our original goal was to understand how taste interacts with thermal sensation to inform how taste influences food preferences, health, and well-being,” said Christian Lemon, principal investigator on the grant and associate professor in the OU Department of Biology, OU College of Arts and Sciences. “Taste is closely linked to emotion and behavior, so mapping how the brain processes different tastes has broad significance. We were surprised to find that, near the midbrain, temperature signals converged with taste — and that taste and pain signals also used the same circuitry.”
Lemon and OU research associate Jinrong Li applied molecular biology, electrophysiological, and optogenetic techniques to examine how taste and somatosensory (thermal and chemesthetic) pathways converge with pain-related inputs. The researchers found that neurons carrying aversive taste information — such as bitter tastes — also respond to nociceptive stimuli. This overlap suggests a protective neural function and raises the possibility that taste input can influence how pain signals are processed, although further study is needed to define the behavioral consequences.
The sense of taste functions as a complex detector of nutrients and potential toxins, guiding ingestion and avoidance behaviors. Bitter tastes typically signal potentially harmful substances and prompt avoidance, while somatosensory cues such as noxious heat and chemical irritants warn of immediate physical harm. By identifying the parabrachial region as a site of overlap between gustatory and nociceptive processing, the study connects two sensory systems that together support protective behaviors.
In anesthetized mice of both sexes, the team recorded spike activity from taste-responsive neurons in the parabrachial nucleus (PbN) while delivering oral thermal and chemesthetic stimuli, including agonists that activate nociceptive transient receptor potential (TRP) ion channels. They also electrically stimulated an oral somatosensory region of the spinal trigeminal subnucleus caudalis (Vc), which projects to the PbN, to test trigeminal inputs. Neurons categorized by classic taste response profiles — including sodium-, electrolyte-, appetitive-, and bitter-responsive cells — were tested for responses to trigeminal stimulation and nociceptive stimuli.
Most taste-active PbN neurons responded to electrical pulses in the Vc, indicating trigeminal projections reach gustatory neurons in the PbN. A subset of neurons that responded to bitter tastants (quinine, cycloheximide) and aversive concentrations of sodium also fired in response to nociceptive TRP channel agonists such as capsaicin, mustard oil, and noxious heat. These multisensory neurons were concentrated in the lateral PbN. Importantly, optogenetic inhibition of the trigeminal input reduced nociceptive responses in bitter-responsive PbN neurons, supporting the idea that convergent trigeminal input contributes to their activity.
With the circuit identified, OU researchers plan to manipulate these pathways to test how they influence behaviors linked to taste and pain. Understanding this cross-modal circuitry has implications for human conditions related to eating behavior and sensory processing, including obesity, diabetes, and disorders where aversive taste or altered pain perception may play a role.
Source:
University of Oklahoma
Media Contact:
Jana Smith – University of Oklahoma
Image Source:
Image credit: University of Oklahoma.
Original Research:
“Mouse Parabrachial Neurons Signal a Relationship between Bitter Taste and Nociceptive Stimuli”
Jinrong Li, Christian H. Lemon
Journal of Neuroscience 27 February 2019, 39 (9) 1631-1648; DOI: 10.1523/JNEUROSCI.2000-18.2018
Abstract (summary)
Both taste and somatosensation serve protective roles: bitter taste helps avoid ingesting toxins, while pain signals warn against harmful stimuli. Although traditionally studied separately, evidence suggested overlap between gustatory and trigeminal somatosensory pathways. Using electrophysiology and optogenetics in mice, researchers recorded from taste-active neurons in the PbN during oral delivery of thermal and chemesthetic stimuli, including TRP channel agonists. They also examined responses to electrical stimulation of the trigeminal subnucleus caudalis (Vc), which projects to the PbN. Neurons fell into classic taste categories, and many responded to Vc stimulation, indicating trigeminal projections to PbN gustatory cells. A subset of bitter-responsive neurons also fired to nociceptive stimuli such as capsaicin, mustard oil, and noxious heat. These neurons were mainly in the lateral PbN, and optogenetic suppression of trigeminal input reduced their nociceptive responsiveness. The findings reveal that gustatory neurons in the PbN participate in cross-system signaling related to protection.
SIGNIFICANCE STATEMENT
This study demonstrates overlap between taste and somatosensory processing in the parabrachial area. Parabrachial gustatory neurons receive trigeminal input and can respond to oral nociceptive stimuli; activation by such stimuli is associated with responsiveness to aversive bitter tastants. Silencing trigeminal projections suppresses nociceptive activity in bitter-responsive PbN neurons, indicating a predictable convergence of taste and somatosensory signals that supports protective coding.