Retinal neural transmission represents a key function of the eye. Identifying the molecular components of this vital process is helped by studies of selected human genetic eye disorders. For example, mutations in the calcium channel subunit gene CACNA1F cause incomplete X-linked congenital stationary night blindness (CSNB2 or iCSNB), a human retinal disorder with abnormal electrophysiological response and visual impairments consistent with a retinal neurotransmission defect. To understand the subcellular basis of this retinal disorder, we generated a mouse with a loss-of-function mutation by inserting a self-excising Cre-lox-neo cassette into exon 7 of the murine orthologue, Cacna1f. Electroretinography of the mutant mouse revealed a scotopic a-wave of marginally reduced amplitude compared with the wild-type mouse and absence of the post-receptoral b-wave and oscillatory potentials. Cone ERG responses together with visual evoked potentials and multi-unit activity in the superior colliculus were also absent. Calcium imaging in Fluo-4 loaded retinal slices depolarized with KCl showed 90% less peak signal in the photoreceptor synapses of the Cacna1f mutant than in wild-type mice. The absence of post-receptoral ERG responses and the diminished photoreceptor calcium signals are consistent with a loss of Ca((2+)) channel function in photoreceptors. Immunocytochemistry showed no detectable Ca(v)1.4 protein in the outer plexiform layer of Cacna1f-mutant mice, profound loss of photoreceptor synapses, and abnormal dendritic sprouting of second-order neurons in the photoreceptor layer. Together, these findings in the Cacna1f-mutant mouse reveal that the Ca(v)1.4 calcium channel is vital for the functional assembly and/or maintenance and synaptic functions of photoreceptor ribbon synapses. Moreover, the outcome of this study provides critical clues to the pathophysiology of the human retinal channelopathy of X-linked incomplete CSNB.

Ocular growth and refraction are regulated by visual processing in the retina. We identified candidate regulatory neurons by immunocytochemistry for immediate-early gene products, ZENK (zif268, Egr-1) and Fos, after appropriate visual stimulation. ZENK synthesis was enhanced by conditions that suppress ocular elongation (plus defocus, termination of form deprivation) and suppressed by conditions that enhance ocular elongation (minus defocus, form deprivation), particularly in glucagon-containing amacrine cells. Fos synthesis was enhanced by termination of visual deprivation, but not by defocus and not in glucagon-containing amacrine cells. We conclude that glucagon-containing amacrine cells respond differentially to the sign of defocus and may mediate lens-induced changes in ocular growth and refraction.

A high-affinity uptake mechanism for [3H]-gamma-aminobutyric acid (GABA) has been localized to type H1 cone horizontal cells and type Ab pyriform amacrine cells in the retina of the goldfish by light and electron microscopy autoradiography. By stimulating isolated retinas with colored lights during incubation we have been able to use [3H]-GABA uptake as a probe of light-evoked changes in membrane potential. All colors of lights increase and darkness decreases [3H]-GABA uptake by H1 cone horizontal cells. Our model of voltage dependence of GABA uptake predicts that all colors of light should hyperpolarize H1 cone horizontal cells and other investigators have shown by intracellular recording and dye-marking that type H1 cone horizontal cells hyperpolarize to all wavelengths of light. We have also obtained evidence that dark-induced depolarization of cone horizontal cells leads to release of GABA. Type Ab pyriform amacrine cells show maximal [3H]-GABA uptake in darkness and when exposed to green or blue lights, but red lights dramatically suppress uptake. We predict these neurons to be red-depolarizing, and recent intracellular recordings and dye-marking by Famiglietti et al. ('77) support our conclusions. Synaptic relations of apparently GABA-ergic neurons were investigated in the electron microscope. We propose type H1 cone horizontal cells to be both pre- and post-synaptic to red-sensitive cones and type Ab pyriform amacrine cells to be both pre- and post-synaptic to red-sensitive center-depolarizing bipolar cells.

In Golgi preparations of goldfish retina we have observed three types of horizontal cell which receive exclusively from cones and one which receives exclusively from rods. The cone horizontal cells were designated H1, H2 and H3, in order of increasing dendritic spread, increasing separation from the outer synaptic layer, decreasing size of perikaryon, and decreasing density of cone contacts. Slender appendages with knobby terminal enlargements project horizontal cells by alalyzing serial 1 mum sections with the light microscope. The probable inputs, in terms of visual pigments in the cones which contact them, are: H1, red+green+blue; H2, green+blue; H3, blue. Analysis of previously published work suggests (1) that H1 cells generate monophasic or L-type responses, H2 cells generate biphasic or C1-type responses, and H3 cells generate triphasic or C2-type responses; (2) that H1 cells receive direct functional input at least from red-sensitive cones, H2 cells from green-sensitive cones, and H3 cells from blue-sensitive cones, and (3) that H1 constitute pathways from cones to H2 cells, and H2 cells, and H2 cells constitute pathways from cones and H1 cells to H3 cells. The precise location and route of the transfers, from H1 to H2 and from H2 to H3, are not yet known.