Separable gain control of ongoing and evoked activity in the visual cortex by serotonergic input

Controlling gain of cortical activity is essential to modulate weights
between internal ongoing communication and external sensory drive. Here,
we show that serotonergic input has separable suppressive effects on the
gain of ongoing and evoked visual activity. We combined optogenetic
stimulation of the dorsal raphe nucleus (DRN) with wide-field calcium
imaging, extracellular recordings, and iontophoresis of serotonin (5-HT)
receptor antagonists in the mouse primary visual cortex. 5-HT1A receptors
promote divisive suppression of spontaneous activity, while 5-HT2A
receptors act divisively on visual response gain and account for
normalization of population responses over a range of visual contrasts in
awake and anesthetized states. Thus, 5-HT input provides balanced but
distinct suppressive effects on ongoing and evoked activity components
across neuronal populations. Imbalanced 5-HT1A/2A activation, either
through receptorspecific drug intake, genetically predisposed irregular
5-HT receptor density, or change in sensory bombardment may enhance
internal broadcasts and reduce sensory drive and vice versa.