Serotonin Regulates the Sensitivity of Brain Cells Involved in Hearing

You may have heard of serotonin, a chemical found throughout the brain that regulates a host of mental states such as mood, appetite and alertness. When we have enough of it, we have an overall sense of wellbeing and happiness. When we're running low on it, we can experience depression.

Scientists discovered that serotonin alters the sensitivity of an auditory neuron (large red bulb) by interacting with one of its axons (yellow box), a threadlike projection that carries electrical signals on to other neurons. The scientists used an ultrathin glass pipette tip (asterisk) to record electrical activity in the neuron.

Nace Golding, a neuroscientist at the University of Texas at Austin, and his team were surprised to find that serotonin also regulates the sensitivity of auditory neurons, brain cells involved in hearing. The more sensitive a neuron, the more likely it is to fire after receiving signals from other neurons. When serotonin levels are low, auditory neurons are less sensitive, and vice versa.

"Serotonin affects the region of the cell where electrical impulses are generated, and in doing so it acts like a thermostat, changing the neuron's sensitivity to the activity of its inputs, so it can be more or less selective about what signals it lets through," says Golding.

The findings were published online in the journal Nature Neuroscience on April 25.

The discovery is intriguing, but it raises broader questions. Does serotonin have the same effect on other types of neurons? Does serotonin affect the sense of hearing in people with depression (who have lower serotonin levels)?

Golding notes that the type of serotonin receptor they studied is the most common subtype found in the brain and is the target of many antidepressant drugs. Which raises yet another intriguing question: Does serotonin affect the sense of hearing in people taking antidepressants?

The researchers made the discovery in living brain tissue from gerbils. One technique allowed them to send electrical signals to individual neurons and record their electrical outputs. In another technique, they added chemical "photoswitches" to the brain tissue, which allowed them to modify their electrical properties by simply changing the color of light shining on them.

Kwang Woo Ko, a former graduate student working in Golding's lab and lead author of the current study, conducted the bulk of the research. He is currently at Washington University School of Medicine in St. Louis.

The study's other co-authors are Matthew Rasband at Baylor University; and Richard Kramer and Victor Meseguer at the University of California, Berkeley.

Funding for this research was provided by the National Institute on Deafness and Other Communication Disorders, the National Institute of Neurological Disorders and Stroke, and the National Eye Institute.