A recent study published in Neuron investigated the potential effects of serotonin, a neurotransmitter, on the early stages of visual processing—and its ability to selectively gate transmission from specific retinal ganglion cell (RGC) axons within the thalamus.
Let’s talk about serotonin and vision.
In vision, RGC channels relay visual information such as luminance, motion, or fine details. Neuromodulators (like serotonin) determine if this information is sent to downstream brain regions for additional processing.
Now the study.
Using two-photon calcium imaging, researchers evaluated awake-behaving mice to track the activity of individual retinal axons when relaying visual signals to the thalamus.
They simultaneously increased the release of serotonin in the thalamus of the mice using an optogenetic stimulation—a technique used to intensify the activity of a set of genetically-defined neurons using light—as they entered the thalamus.
Why use serotonin?
According to the investigators, the use of serotonin in the mice was based on previous research that found it suppresses neurotransmitter release from the retinothalamic synapse via a specific serotonin receptor.
What did they find?
Serotonin was found to suppress calcium signals in retinal axons and the release of glutamate (a neurotransmitter) in the thalamus—leading to a decrease in visual signal transmissions from the eye to the thalamus.
What else?
Levels of serotonin were also found to strongly suppress different retinal axon classes, with those more sensitive to changes in light levels (rather than visual details) being more affected.
How did this compare to states of high arousal?
Conversely, periods of high arousal that were not accompanied by an increase in serotonin levels tended to suppress retinal axons— which influenced the transmission of information regarding fine visual details rather than changes in light levels.
Take home.
The findings suggest that filtering visual information can be modulated by different internal states (i.e. hunger, sexual desire, fear).
Study authors plan to continue investigating the effect of serotonin in altering thalamic output to the cortex as well as the effects of other neuromodulators with a retinothalamic connection to uncover further discoveries of potential diagnostic biomarkers and therapeutic targets.