The superficial gray layer of the superior colliculus contains a map that represents the visual field, whereas the underlying intermediate gray layer contains a vector map of the saccades that shift the direction of gaze. These two maps are aligned so that a particular region of the visual field is represented directly above the neurons that orient the highest acuity area of the retina toward that region. Although it has been proposed that the transmission of information from the visuosensory to the motor map plays an important role in the generation of visually guided saccades, experiments have failed to demonstrate any functional linkage between the two layers. We examined synaptic transmission between these layers in vitro by stimulating the superficial layer while using whole- The superior colliculus receives sensory information about the location of objects and then processes this information to initiate motor command signals for the saccadic head and eye movements that orient gaze toward objects of interest (1). The proximity of these functions within the same structure makes the superior colliculus a powerful model for studying the fundamental problem of how the brain integrates sensory and motor systems to produce behavior.

Neurons in the superficial gray layer (SGS) of the superior colliculus receive visual input and excite intermediate layer (SGI) neurons that play a critical role in initiating rapid orienting movements of the eyes, called saccades. In the present study, two types of experiments demonstrate that a population of SGI neurons gives rise to a reciprocal pathway that inhibits neurons in SGS. First, in GAD67-GFP knockin mice, GABAergic SGI neurons that expressed GFP fluorescence were injected with the tracer biocytin to reveal their axonal projections. Axons arising from GFP-positive neurons in SGI terminated densely in SGS. Next, SGI neurons in rats and mice were stimulated by using the photolysis of caged glutamate, and in vitro whole-cell patch-clamp recordings were used to measure the responses evoked in SGS cells. Large, synaptically mediated outward currents were evoked in SGS neurons. These currents were blocked by gabazine, confirming that they were GABAA receptor-mediated inhibitory postsynaptic currents. This inhibitory pathway from SGI transiently suppresses visual activity in SGS, which in turn could have multiple effects. These effects could include reduction of perceptual blurring during saccades as well as prevention of eye movements that might be spuriously triggered by the sweep of the visual field across the retina.patch-clamp ͉ photostimulation ͉ sensorimotor ͉ superior colliculus ͉ visuomotor N eurons in the superficial layer (SGS) of the superior colliculus receive input from the retina and the visual cortex (1) and provide a powerful excitatory input to premotor neurons in the intermediate layer (SGI) that play a critical role in initiating rapid orienting movements of the eyes, called saccades (2-4). The activity of many of these superficial layer neurons is suppressed during saccades (5). By reducing the activity in the pathway from the superficial to the premotor cells in the intermediate layer, this suppression may prevent unwanted saccades that might otherwise be triggered by the rapid movement of the visual field that occurs during a saccade. SGS cells also project to nuclei in the dorsal thalamus that relay visual information to the cortex (1), and the reduction of activity in these relay nuclei may contribute to the attenuation of visual perception that occurs during the course of saccades (6-9).Experiments designed to identify the neural mechanisms responsible for saccadic suppression suggest that neither retinal input nor proprioceptive input from the periphery is responsible. For example, superficial layer neurons can respond to visual stimuli moving at the velocity of saccades (5), which suggests that the retinal input is suppressed after reaching SGS. Moreover, the observation that the suppression of activity in SGS can occur during eye movements in the dark indicates that the mechanism is not dependent on changes in the level of retinal activity (5). Similarly, the proposal that the suppression is mediated by input from receptors in extraocular eye muscles or tendons that detect the ...

Stratum griseum superficiale (SGS) of the superior colliculus receives a dense cholinergic input from the parabigeminal nucleus. In this study, we examined in vitro the modulatory influence of acetylcholine (ACh) on the responses of SGS neurons that project to the visual thalamus in the rat. We used whole-cell patch-clamp recording to measure the responses of these projection neurons to electrical stimulation of their afferents in the stratum opticum (SO) before and during local pressure injections of ACh. These colliculothalamic projection neurons (CTNs) were identified during the in vitro experiments by prelabeling them from the thalamus with the retrograde axonal tracer wheat germ agglutinin-apo-HRP-gold. In a group of cells that included the prelabeled neurons, EPSCs evoked by SO stimulation were significantly reduced by the application of ACh, whereas IPSC amplitudes were significantly enhanced. Similar effects were observed when the nicotinic ACh receptor agonist lobeline was used. Application of the selective GABA(B) receptor antagonist 3-[[(3,4-dichlorophenyl)-methyl]amino]propyl](diethoxymethyl)phosphinic acid blocked ACh-induced reduction in the evoked response. In contrast, the ACh-induced reduction was insensitive to application of the GABA(A) receptor antagonist bicuculline. The ACh-induced reduction was also diminished by bath application of muscimol at the low concentrations that selectively activate GABA(C) receptors. Because GABA(C) receptors may be specifically expressed by GABAergic SGS interneurons (Schmidt et al., 2001), our results support the hypothesis that ACh reduces CTN activity by nicotinic receptor-mediated excitation of local GABAergic interneurons. These interneurons in turn use GABA(B) receptors to inhibit the CTNs.

Attenuation of visual activity in the superficial layers (SLs), stratum griseum superficiale and stratum opticum, of the superior colliculus during saccades may contribute to reducing perceptual blur during saccades and also may help prevent subsequent unwanted saccades. GABAergic neurons in the intermediate, premotor, layer (SGI), stratum griseum intermedium, send an inhibitory input to SL. This pathway provided the basis for a model proposing that the SGI premotor cells that project to brainstem gaze centers and discharge before saccades also activate neighboring GABAergic neurons that suppress saccade-induced visual activity in SL.The in vitro method allowed us to test this model. We made whole-cell patch-clamp recordings in collicular slices from either rats or GAD67-GFP knock-in mice, in which GABAergic neurons could be identified by their expression of green fluorescence protein (GFP). Antidromic electrical stimulation of SGI premotor cells was produced by applying pulse currents in which their axons congregate after exiting the superior colliculus. The stimulation evoked monosynaptic EPSCs in SGI GABAergic neurons that project to SL, as would be predicted if these neurons receive excitatory input from the premotor cells. Second, IPSCs were evoked in SL neurons, some of which project to the visual thalamus. These IPSCs were polysynaptically mediated by the GABAergic neurons that were excited by the antidromically activated SGI neurons. These results support the hypothesis that collaterals of premotor neuron axons excite GABAergic neurons that inhibit SL visuosensory cells.