Abstract
Serotonin (5-HT) is a neuromodulator secreted from serotonergic neurons located in the pons and upper brain stem in a behavioral context-dependent manner. The serotonergic axon terminals innervate almost the whole brain, causing modulatory actions on various brain functions including vision. Our previous study demonstrated the visual responses of neurons in the primary visual cortex (V1) of anesthetized monkeys were modulated by the activation of 5-HT receptors depending on the response magnitude, in which 5-HT2A receptor-selective agonists enhanced weak visual responses but not strong responses. This observation suggests that the activation of serotonergic receptors modulates neuronal visual information processing to improve the behavioral detectability of a stimulus. However, it remains unknown if 5-HT improves visual detectability at the behavioral level. To investigate this point, visual detectability was measured as contrast sensitivity (CS) in freely moving rats using a two-alternative forced-choice visual detection task (2AFC-VDT) combined with the staircase method. The grating contrast was decreased or increased step by step after a correct choice (hit) or incorrect choice (miss), respectively. CS was evaluated as an inverse value of the visual contrast threshold. The effect of the intraperitoneal administration of fluoxetine (FLX, 5 mg/kg), a selective serotonin reuptake inhibitor, on CS was tested. The CS of rats was significantly higher in FLX than control conditions, and the drug effect showed specificity for the spatial frequency (SF) of a grating stimulus, in which CS improvement was observed at optimal SF but not non-optimal high SF. Thus, we conclude that endogenously-secreted serotonin in the brain improves visual detectability, which may be mediated by vision-related neurons acquiring SF information of the visual stimulus.
Highlights
Information processing in the brain dynamically changes depending on the arousal state and behavioral context [1,2]
The contrast values of the stimulus for all 60 min sessions were averaged over all trial numbers, and the averaged data were plotted as average transition curves (Fig 2C and 2D)
5-HT modulation in behavioral visual detectability level for 5 min were measured after the administration of saline or FLX (Fig 4), but we found no difference between the two conditions
Summary
Information processing in the brain dynamically changes depending on the arousal state and behavioral context [1,2]. The dynamics are mediated by neuromodulators, including 5-hydroxytryptamine (5-HT, or serotonin) depending on behavioral contexts. Serotonin is secreted from afferent terminals originating from neurons in the dorsal and median raphe nuclei, which act on subcortical and cortical areas [3,4] to regulate various brain functions [5,6,7]. Serotonergic neurons regularly fire at low frequencies JP16H01869 to S.S, and Grant-in-Aid for JSPS Fellows JP17J08499 to A.Y.S
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