Serotonin regulates the transmission of visual data from the eyes to the thalamus.

Humans are known to perceive their surroundings differently depending on their circumstances, feelings, and sensations. Thus, internal states such as fear, arousal, or hunger can influence how the brain processes and registers sensory information.

Recently, a study was conducted by scientists at Beth Israel Deaconess Medical Center, Boston Children’s Hospital, and Peking University to determine whether the neurotransmitter serotonin, which is known to control sleep, mood, sexual desire, and other internal states, has any impact on how visual information is processed. Their results, which were reported in the journal Neuron, imply that serotonergic neurons in the brainstem (i.e., the central trunk of the mammalian brain) control the flow of visual data from the eyes to the thalamus, an egg-shaped region of the brain.

According to Chinfei Chen, one of the study’s authors, “internal states are known to affect sensory perception and processing, but this was generally thought to occur in the cortex or thalamus.”. One of our earlier studies showed that, before the information reaches the brain, arousal can suppress specific visual information channels at an earlier stage of the visual pathway, at the junction between the mouse retina and the thalamus. This type of information “filtering” suggests a very effective method of processing only pertinent information.

“One of our prior research found that arousal can inhibit specific visual information channels at an earlier stage of the visual pathway—at the junction between the mouse retina and the thalamus, before the information ever reaches the brain. This type of information’ filtering’ proposes a very effective method of processing just relevant information.”

Chinfei Chen, one of the researchers who carried out the study, 

Additionally, previous research has shown that neuromodulatory systems in the brainstem and other regions below the cortex, including the serotonergic system, mediate internal states, including arousal. In essence, this is the mechanism that uses serotonin to control physiological states.

In particular, Chen, her partner Mark Andermann, and their associates looked at how serotonin affected the early processing of visual information and its transmission from the eyes to the thalamus. They did this by monitoring the activity of individual retinal axons in the awake mouse brains as they viewed visual images on a computer monitor using a method known as two-photon calcium imaging.

By optogenetically stimulating serotonergic neurons as they entered the thalamus while studying the mice, the researchers also increased the release of serotonin there. Optogenetic stimulation is a research method that uses light to increase the activity of a group of genetically defined neurons.

“We specifically focused on serotonin because our previous work on mouse brain slices showed that serotonin can suppress neurotransmitter release from the retinothalamic synapse through a specific serotonin receptor, the 5HT1b receptor,” Chen said.

These researchers’ experiments produced some very intriguing findings. Notably, they demonstrated that serotonin can block calcium signals in retinal axons and glutamate release in the thalamus. By doing this, the amount of visual signals that are transmitted from the eye to the thalamus is ultimately decreased.

Andermann, Chen, and their colleagues further examined their findings and discovered that some classes of retinal axons tended to be suppressed by serotonin more than others, with classes that responded to broad changes in light levels being more impacted than classes responding to fine visual details.

The selective modulation seen in the researchers’ experiments was also compared to that seen in naturally highly aroused animals or people. They discovered that rather than changes in light levels, periods of high arousal that were not accompanied by an increase in serotonin release tended to suppress the retinal axons transmitting information about fine details.

Overall, the findings of this study support the notion that various internal states can influence how various types of visual information are processed. Future research examining the effects of neuromodulatory systems on downstream brain regions and on perception in both animal models and humans may be influenced by this insight, which may result in novel and intriguing findings.

By taking into account additional elements of the connections between the cortex and the thalamus that process the incoming retinal information, Chen continued, “We now plan to continue investigating how serotonin alters thalamic output to the cortex.”. We’ll also investigate the effects of inhibitory transmission and other well-known neuromodulators of the retinothalamic connection.”.

More information: Jasmine D.S. Reggiani et al, Brainstem serotonin neurons selectively gate retinal information flow to thalamus, Neuron (2022). DOI: 10.1016/j.neuron.2022.12.006

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