Transient enhancement of inhibition following visual cortical lesions in the mouse superior colliculus

A non-canonical retina-ipRGCs-SCN-PVT visual pathway for mediating contagious itch behavior

Contagious itch behavior informs conspecifics of adverse environment and is crucial for the survival of social animals. Gastrin-releasing peptide (GRP) and its receptor (GRPR) in the suprachiasmatic nucleus (SCN) of the hypothalamus mediates contagious itch behavior in mice. Here, we show that intrinsically photosensitive retina ganglion cells (ipRGCs) convey visual itch information, independently of melanopsin, from the retina to GRP neurons via PACAP-PAC1R signaling. Moreover, GRPR neurons relay itch information to the paraventricular nucleus of the thalamus (PVT). Surprisingly, neither the visual cortex nor superior colliculus is involved in contagious itch. In vivo calcium imaging and extracellular recordings reveal contagious itch-specific neural dynamics of GRPR neurons. Thus, we propose that the retina-ipRGC-SCN-PVT pathway constitutes a previously unknown visual pathway that probably evolved for motion vision that encodes salient environmental cues and enables animals to imitate behaviors of conspecifics as an anticipatory mechanism to cope with adverse conditions.

It has been shown that GRP-GRPR neuropeptide signaling in the SCN is important for contagious itch behavior in mice. Gao et al. find that SCN-projecting ipRGCs are sufficient to relay itch information from the retina to the SCN by releasing neuropeptide PACAP to activate the GRP-GRPR pathway.

GABAergic mechanisms for shaping transient visual responses in the mouse superior colliculus

An object that suddenly appears in the visual field should be quickly detected and responded to because it could be beneficial or harmful. The superficial layer of the superior colliculus (sSC) is a brain structure capable of such functions, as sSC neurons exhibit sharp transient spike discharges with short latency in response to the appearance of a visual stimulus. However, how transient activity is generated in the sSC is poorly understood. Here, we show that inhibitory inputs actively shape transient activity in the sSC. Juxtacellular recordings from anesthetized mice demonstrate that almost all types of sSC neurons, which were identified by post hoc histochemistry, show transient spike discharges, i.e., ON activity, immediately after visual stimulus onset. ON activity was followed by a pause before the visual stimulus was turned off. To determine whether the pause reflected the absence of excitatory drive or inhibitory conductance, we injected depolarizing currents juxtasomally, which enabled us to observe inhibition as decreased discharges. The pause was observed even under this condition, suggesting that inhibitory input caused the pause. We further found that local application of a mixture of GABAA and GABAB receptor antagonists additively diminished the pause. These results indicate that GABAergic inputs produce transient ON responses by attenuating excitatory activity through the cooperative activation of GABAA and GABAB receptors, allowing sSC neurons to act as a saliency detector.