How the brain distinguishes between pain and itch

Pain and itch are both unpleasant sensations, but they trigger different responses -- pain often prompts withdrawal, while itching leads to scratching. Until now, scientists have struggled to understand how the brain processes these sensations separately, as they share overlapping neural pathways from the spinal cord to the brain.

Both stimuli are transmitted from the spinal cord to the thalamus and brainstem, eventually reaching the ACC. The ACC is a key brain region involved in various functions, ranging from basic sensory processing to higher-order cognition. However, a comprehensive understanding of how a limited number of neurons within the ACC execute such diverse functions has been lacking. This study provides fundamental insights into how ACC neurons selectively process pain and itch information.

By analyzing neuronal response patterns in the ACC to pain and itch stimuli, the research team identified two distinct neuronal populations:

1. Non-selective neurons, which respond to both pain and itch stimuli indiscriminately.

2. Stimulus-specific neurons, which were selectively activated by either pain or itch stimuli.

Furthermore, using the dual-eGRASP technique -- an advanced synaptic analysis method developed by Kaang's research team (Science, 2018) -- the researchers discovered that stimulus-specific neurons in the ACC receive distinct synaptic inputs from the mediodorsal thalamus (MD). This finding indicates that pain and itch are processed by independent neuronal populations within the ACC, which receive differentiated synaptic inputs, providing fundamental insights into the neural mechanisms of pain and itch processing.

To further confirm the role of these neurons, the team used chemogenetic techniques to selectively deactivate either pain-specific or itch-specific neurons. The results showed suppressing pain neurons reduced pain perception without affecting itch, and vice versa. This discovery suggests that these neurons play a direct role in shaping how we experience pain and itch.

This study presents a groundbreaking discovery that the role of ACC neurons in processing pain or itch is predetermined. Importantly, the study demonstrates that pain- and itch-specific neurons in the ACC are synaptically paired with corresponding stimulus-specific neurons in the MD, establishing independent neural circuits for pain and itch processing. These findings challenge the conventional assumption that pain and itch signals follow overlapping pathways and instead highlight distinct neural mechanisms for each sensation.

Given that the ACC is known to mediate the affective aspects of pain and itch, this study suggests that separate neuronal populations are responsible for encoding the subjective experience of pain and itch. Building upon these findings, the research team aims to further investigate the brain's complex sensory processing mechanisms.

Corresponding author KAANG Bong-Kiun stated, "The ACC is an important brain area not only for memory storage but also for processing higher-order emotions such as pain and conflict. Through this study, we have taken a step further in understanding emotional memory at the synaptic level."

Co-corresponding author and first author KO Hyoung-Gon commented, "I am particularly interested in how these pain- and itch-selective neural circuits change under pathological conditions. Moving forward, we plan to expand our research to explore the interactions between these circuits."