Top-down modulation of the retinal code via histaminergic neurons of the hypothalamus.

The mammalian retina is considered an autonomous circuit, yet work dating back to Ramon y Cajal indicates that it receives inputs from the brain. How such inputs affect retinal processing has remained unknown. We confirmed brain-to-retina projections of histaminergic neurons from the mouse hypothalamus. Histamine application ex vivo altered the activity of various retinal ganglion cells (RGCs), including direction-selective RGCs that gained responses to high motion velocities. These results were reproduced in vivo with optic tract recordings where histaminergic retinopetal axons were activated chemogenetically. Such changes could improve vision of fast-moving objects (e.g., while running), which fits with the known increased activity of histaminergic neurons during arousal. An antihistamine drug reduced optomotor responses to high-speed moving stimuli in freely moving mice. In humans, the same antihistamine nonuniformly modulated visual sensitivity across the visual field, indicating an evolutionary conserved function of the histaminergic system. Our findings expose a previously unappreciated role for brain-to-retina projections in modulating retinal function.

Dopamine differentially affects retinal circuits to shape the retinal code.

Dopamine has long been reported to enhance antagonistic surrounds of retinal ganglion cells (RGCs). Yet, the retina contains many different RGC subtypes and the effects of dopamine can be subtype-specific. Using multielectrode array (MEA) recordings we investigated how dopamine shapes the receptive fields of RGCs in the mouse retina. We found that the non-selective dopamine receptor agonist apomorphine can either increase or decrease RGCs' surround strength, depending on their subtype. We then used two-photon targeted patch-clamp to target a specific RGC subtype, the transient-Off-αRGC. In line with our MEA recordings, apomorphine did not increase the antagonistic surround of transient-Off-αRGCs but enhanced their responses to Off stimuli in the centre receptive field. Both D1 - and D2 -like family receptor (D1 -R and D2 -R) blockers had the opposite effect and reduced centre-mediated responses, but differently affected transient-Off-αRGC's surround. While D2 -R blocker reduced surround antagonism, D1 -R blocker led to surround activation, revealing On responses to large stimuli. Using voltage-clamp recordings we separated excitatory inputs from Off cone bipolar cells and inhibitory inputs from the primary rod pathway. In control conditions, cone inputs displayed strong surround antagonism, while inputs from the primary rod pathway showed no surround. Yet, the surround activation in the D1 -R blockade originated from the primary rod pathway. Our findings demonstrate that dopamine differentially affects RGC subtypes via distinct pathways, suggesting that dopamine has a more complex role in shaping the retinal code than previously reported. KEY POINTS: Receptive fields of retinal ganglion cells (RGCs) have a centre-surround organisation, and previous work has shown that this organisation can be modulated by dopamine in a light-intensity-dependent manner. Dopamine is thought to enhance RGCs' antagonistic surround, but a detailed understanding of how different RGC subtypes are affected is missing. Using a multielectrode array recordings, clustering analysis and pharmacological manipulations, we found that dopamine can either enhance or weaken antagonistic surrounds, and also change response kinetics, of RGCs in a subtype-specific manner. We performed targeted patch-clamp recordings of one RGC subtype, the transient-Off-αRGC, and identified the underlying circuits by which dopamine shapes its receptive field. Our findings demonstrate that dopamine acts in a subtype-specific manner and can have complex effects, which has implications for other retinal computations that rely on receptive field structure.