Kyushu University identifies C. elegans circuit for selective odor forgetting

Researchers from Kyushu University have identified the specific molecular and neural pathways that enable the nematode C. elegans to actively and selectively forget specific odors. Published in Current Biology, the study reveals how a distinct neural circuit directs the erasure of olfactory memories, offering a detailed map of the biological mechanisms underlying forgetting in a model organism with a fully characterized nervous system.

The study addresses a fundamental question in neuroscience: how does the brain decide which memories to erase? While significant progress has been made in understanding how memories are formed and maintained, the mechanisms of active forgetting have received comparatively less attention. Professor Takeshi Ishihara, who led the study from Kyushu University’s Faculty of Science, noted that forgetting is often viewed as a failure of memory, but it is actually essential for adaptation and survival.

To investigate this mechanism, the team utilized C. elegans, a nematode with only 302 neurons. Because every neuron in this organism has been identified, it is possible to trace specific memories and their subsequent erasure to particular cells. Although C. elegans are blind, they possess an exceptional sense of smell. Previous work by Ishihara’s team in 2013 had already identified the neurons responsible for storing odor memories and those that signal their erasure, but the specific molecules activating this pathway remained unknown.

The current study, led by first author and PhD candidate Yuuki Onishi, aimed to identify these responsible molecules. The researchers trained C. elegans to learn a specific smell, diacetyl, and then monitored the retention of this memory over time. After approximately four hours, a period during which the organism typically forgets a smell, diacetyl was administered again. Movement toward the diacetyl smell signified that the memory had been forgotten; if not, the memory had been retained.

Building on previous findings that AWA neurons store the memory of the odor and that AWC neurons signal its erasure, the team focused on identifying the ‘forgetting’ signal released by AWC neurons. They identified the neuropeptide NLP-47 and its receptor GNRR-1 as key components of this pathway. Further investigation revealed that AIZ neurons, located between AWC and AWA neurons, were also involved in the circuit. This discovery clarified the sequential flow of signals that leads to the active forgetting of specific olfactory memories.

The neural circuit operates through a precise sequence of molecular interactions. First, AWC neurons release NLP-47, which activates the GNRR-1 receptor on AIZ neurons. The AIZ neurons then release glutamate, another neurotransmitter, which acts on the GLR-4 receptor on the AWA neurons that house the memory of the scent. This cascade of signals leads to the active forgetting of the odor. When the researchers used optogenetics to suppress the activity of the AWC and AIZ neurons, they found that the nematodes became more resilient to forgetting, confirming the necessity of this specific circuit for memory erasure.

Follow-up experiments demonstrated that this active forgetting process is highly specific rather than indiscriminate. When nematodes were trained to remember two odors, diacetyl and isoamyl alcohol, simultaneously, the NLP-47/GNRR-1 pathway only erased the memory of the diacetyl smell, leaving the memory of isoamyl alcohol intact. This selective nature of the forgetting mechanism is a striking finding, as it shows that the brain can target a specific memory for deletion while preserving others. C. elegans in which the NLP-47/GNRR-1 pathway or glutamate signaling did not function were more resistant to forgetting the smells, further highlighting the role of this circuit in memory loss.

Onishi expressed surprise at finding such a concrete mechanism of active forgetting with remarkable specificity, noting that he had not initially expected such a targeted process. This research provides a detailed look at the molecular and cellular basis of selective forgetting, a process that is fundamental to how organisms adapt to their environments.

The findings of this study may have broader implications for understanding memory-related conditions in humans. Ishihara noted that there are many diseases involving memory loss, and understanding how forgetting works in the brain is a prerequisite for understanding what goes wrong in these conditions. Interestingly, receptors like GNRR-1 are found in other organisms, including humans. In rodents, these receptors are located in the hippocampus, the brain’s center for memory, although their specific role there is not yet known. The discovery of this circuit in C. elegans may lead to a better understanding of the mechanisms by which mammals forget specific memories.

Subject of Research: Molecular and neural mechanisms of selective olfactory forgetting in C. elegans

Article Title: No neuralyzer needed. Identifying the molecular pathways and the circuit that triggers forgetting in C. elegans

Article References: No neuralyzer needed. Identifying the molecular pathways and the circuit that triggers forgetting in C. elegans. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: C. elegans, neuroscience, memory erasure, NLP-47, GNRR-1, AWA neurons, AWC neurons, AIZ neurons, glutamate, GLR-4 receptor, olfactory learning, selective forgetting, neuropeptide, Kyushu University, Current Biology, optogenetics, diacetyl, isoamyl alcohol, neural circuit, memory retention