RECEPTIVE-FIELD ORGANIZATION OF RETINAL GANGLION-CELLS IN THE SPASTIC MUTANT MOUSE

1. We examined the receptive field properties of retinal ganglion cells in the isolated, superfused retinae of spastic mutant mice (B6C3Fe-spa/spa) that did not have the retinal degeneration (rd) phenotype. Glycine receptor density in the spastic mutant is greatly reduced in all areas of the CNS that have been examined. Phenotypically normal litter-mates were used as controls. Radial sections from the retinae of both spastic and normal animals were examined with light and electron microscopy and no differences were observed. The planimetric density of the cell bodies in the inner nuclear layer did not differ between the normal and mutant animals, about 400 cm-2. The absolute dark-adapted sensitivity of spastic ganglion cells was greater (271 +/- 69.0 impulses quanta-1 rod-1) than that of normal ganglion cells (47.7 +/- 10.4 impulses quanta-1 rod-1; P < 0.01). 2. Extracellular recordings of retinal ganglion cell responses to circular and annular stimuli, centred on the receptive field, were used to construct peristimulus-time histograms. In normal retinae, an annular stimulus elicited a response that was characteristic of the surround response mechanism of receptive fields with antagonistic centre-surround organization. In the mutant retina, annular stimuli did not elicit a surround-type response; instead, a centre-type response was recorded. 3. Illumination of the receptive field periphery attenuated centre-type responses in ganglion cells from both spastic and normal retinae. Centred circular stimuli of various areas (14, 35, 78, 122, 235, 783 deg2) were presented to the receptive fields. For mutant and normal ganglion cells, the response to the largest stimulus was smaller than that to an intermediate-sized stimulus. 4. The effect of strychnine, a glycine receptor antagonist, on the response to circular stimuli was examined. Very low concentrations of strychnine attenuated the light response in mutant retinae (apparent inhibitory binding constant K(I) = 8.1 x 10(-13) M). In normal animals, the light response was also attenuated by strychnine, but the apparent K(I) was much higher (apparent K(I) = 1 x 10(-7) M). 5. In normal ganglion cells, the sustained component of the light response was much more attenuated by strychnine than was the transient component. Interestingly, ganglion cells from spastic retinae did not exhibit a sustained component, even at stimulus luminances that evoked responses near threshold. 6. These results suggest that the generation of surround-type responses may depend upon a glycine receptor or that a glycine receptor may lie in the pathway of the surround-response mechanism and the mechanism generating the sustained component of the centre response. Perhaps the expression of a novel retinal form of the glycine receptor, which is extremely sensitive to strychnine, is unmasked by the spastic mutation.