Insulin plays a role in mediating worms' perceptions and behaviors
As imaging tools and techniques have improved, scientists have been building a detailed map of neural connections in the human brain, with the ultimate hope of understanding how the mind works. But in order to decode perceptions and behaviors, it is also necessary to know the routes that information takes in the brain. A recent study by Sreekanth Chalasani, in collaboration with UC San Diego doctoral student Sarah Leinwand, found a striking example of flexibility in neural circuitry and its influence on behaviors in worms.
In the study, reported in Nature Neuroscience, Chalasani's team built on established research on the roundworm Caenorhabditis elegans that had identified sensory neurons with distinct roles such as sensing temperature, pheromones, salt and odors. Imaging worms that expressed genetically encoded indicators in their neurons, which caused the cells to light up when active, they found the worms' olfactory sensory neuron lit up after exposure to an attractive but high concentration of salt. This revealed that more than one type of neuron is involved in processing sensory cues; previously it had been thought these were sensed only by single neurons. Chalasani and Leinwand further showed that the olfactory neuron was crucial for the worm's movement toward salt within a certain concentration range and that a neuropeptide was being released by the salt-sensing neuron to shape the animal's behavior. Tracking olfactory neuron responses to high salt in worms missing specific genes, they found that worms lacking the gene for an insulin neuropeptide known as INS-6 did not respond to increases in salt. Restoring this peptide reinstated the animal's normal responses to high salt.
It was a big surprise that insulin was the main signaling molecule recruiting the olfactory neuron into a salt-sensing circuit. Neuropeptides had been thought to modulate neuronal function over many seconds to many minutes. But in this instance, it appears that the insulin is acting in less than a second to transfer information from the salt-sensing neuron to the neuron that normally responds to odor. Similar neuropeptide communication may create flexible neural circuits that mediate the behaviors of other animals and people, so Chalasani and Leinwand plan to investigate whether other fast neural circuit switches exist in worms, and if so, what signaling mechanisms they use.