The retinas of many species show regional specialisations that are evident in the differences in the processing of visual input from different parts of the visual field. Regional specialisation is thought to reflect an adaptation to the natural visual environment, optical constraints, and lifestyle of the species. Yet, little is known about regional differences in synaptic circuitry. Here, we were interested in the topographical distribution of connexin-36 (Cx36), the major constituent of electrical synapses in the retina. We compared the retinas of mice, rats, and cats to include species with different patterns of regional specialisations in the analysis. First, we used the density of Prox1-immunoreactive amacrine cells as a marker of any regional specialisation, with higher cell density signifying more central regions. Double-labelling experiments showed that Prox1 is expressed in AII amacrine cells in all three species. Interestingly, large Cx36 plaques were attached to about 8–10% of Prox1-positive amacrine cell somata, suggesting the strong electrical coupling of pairs or small clusters of cell bodies. When analysing the regional changes in the volumetric density of Cx36-immunoreactive plaques, we found a tight correlation with the density of Prox1-expressing amacrine cells in the ON, but not in the OFF sublamina in all three species. The results suggest that the relative contribution of electrical synapses to the ON- and OFF-pathways of the retina changes with retinal location, which may contribute to functional ON/OFF asymmetries across the visual field.

Non-primate mammals have dichromatic colour vision with two types of cone photoreceptors (S and ML) but the neural pathway carrying S/ML ("blue-yellow") chromatic opponent responses has never been described in these animals. In search of such a pathway, we have set out to characterise the cone inputs to the receptive fields (RFs) of neurons in the lateral geniculate nucleus (LGN) of domestic cats. Five anaesthetised, paralysed and artificially ventilated adult cats were used. Circular patches of drifting sinusoidal gratings were presented at the RF centre of each single unit recorded while the stimulus driven action potential responses were recorded from the LGN using metal microelectrodes. Colour and contrast of the gratings were calibrated to allow specific modulation of S- or ML-cones. The majority (n=30) of the 41 characterized neurons was strongly driven by ML-cones but no evidence of S-cone input was found in these cells. The rest of the cells (n=11, found in the deep, C layers of the LGN) showed input from both cone types in a colour opponent manner. Moreover, the summation fields of S- and ML-cone inputs were equal in size. These results show that the receptive field structure of S-cone driven cells corresponds to that of primate blue-ON cells. We suggest that these cells are part of a "blue-yellow" colour opponent pathway and form the basis of colour vision in the cat.