The ON pathway of the visual system, which detects increases in light intensity, is established at the first retinal synapse between photoreceptors and ON-bipolar cells. Photoreceptors hyperpolarize in response to light and reduce the rate of glutamate release, which in turn causes the depolarization of ON-bipolar cells. This ON-bipolar cell response is mediated by the metabotropic glutamate receptor, mGluR6, which controls the activity of a depolarizing current. Despite intensive research over the past two decades, the molecular identity of the channel that generates this depolarizing current has remained elusive. Here, we present evidence indicating that TRPM1 is necessary for the depolarizing light response of ON-bipolar cells, and further that TRPM1 is a component of the channel that generates this light response. Gene expression profiling revealed that TRPM1 is highly enriched in ON-bipolar cells. In situ hybridization experiments confirmed that TRPM1 mRNA is found in cells of the retinal inner nuclear layer, and immunofluorescent confocal microscopy showed that

Glutamate release from photoreceptor terminals is controlled by voltage-dependent calcium channels (VDCCs). In humans, mutations in the Cacna1f gene, encoding the α 1F subunit of VDCCs, underlie the incomplete form of X-linked congenital stationary night blindness (CSNB2). These mutations impair synaptic transmission from rod and cone photoreceptors to bipolar cells. Here, we report anatomical and functional characterizations of the retina in the nob2 (no b-wave 2) mouse, a naturally occurring mutant caused by a null mutation in Cacna1f. Not surprisingly, the b-waves of both the light-and dark-adapted electroretinogram are abnormal in nob2 mice. The outer plexiform layer (OPL) is disorganized, with extension of ectopic neurites through the outer nuclear layer that originate from rod bipolar and horizontal cells, but not from hyperpolarizing bipolar cells. These ectopic neurites continue to express mGluR6, which is frequently associated with profiles that label with the

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In contrast to conventional synapses, which release neurotransmitter transiently, ribbon synapses formed by photoreceptors and bipolar cells of the retina release neurotransmitter continuously and modulate the rate in response to light. Both modes of release are mediated by synaptic vesicles but probably differ in the regulation of docking and fusion of synaptic vesicles with the plasma membrane. We have found that syntaxin 1, an essential component of the core fusion complex in conventional synapses, is absent from ribbon synapses of the retina, raising the possibility that these synapses contain a different type of syntaxin or syntaxin-like protein. By immunoprecipitating syntaxin 1-depleted retina and brain extracts with a SNAP-25 antibody and microsequencing the precipitated proteins, syntaxin 3 was detected in retina complexed with SNAP-25, synaptobrevin, and complexin. Using an anti-syntaxin 3 antiserum, syntaxin 3 was demonstrated to be present at high levels in retina compared to brain. Immunofluorescent staining of rat retina sections confirmed that syntaxin 3 is expressed by photoreceptor and bipolar cells in the retina. Thus, in the retina, expression of syntaxin 3 is correlated with ribbon synapses and may play a role in the tonic release of neurotransmitter.

Ribbon synapses of vertebrate photoreceptors constantly release glutamate in darkness. Transmitter release is maintained by a steady influx of calcium through voltage-dependent calcium channels, implying the presence of a mechanism that is able to extrude calcium at an equal rate. The two predominant mechanisms of intracellular calcium extrusion are the plasma membrane calcium ATPase (PMCA) and the Na+/Ca2+-exchanger. Immunohistochemical staining of retina sections revealed strong immunoreactivity for the PMCA in rod and cone terminals, whereas staining for the Na+/Ca2+-exchanger was very weak. The PMCA was localized to the plasma membrane along the sides of the photoreceptor terminals and was excluded from the base of the terminals where the active zones are located. The amplitude of a calcium-activated chloride current was used to monitor the intracellular calcium concentration. An upper limit for the time required to remove intracellular free calcium is obtained from two time constants measured for the calcium-activated chloride current tail currents: one of 50 msec and a second of 190 msec. Calcium extrusion was inhibited in the absence of intracellular ATP or in the presence of the PMCA inhibitor orthovanadate, but was unaffected by replacement of external Na+ with Li+. The data indicate that the PMCA, rather than the Na+/Ca2+-exchanger, is the predominant mechanism for calcium extrusion from photoreceptor synaptic terminals.

Purpose To develop diagnostic criteria for nonparaneoplastic autoimmune retinopathy (AIR) through expert panel consensus and to examine treatment patterns among clinical experts. Design Modified Delphi process. Methods A survey of uveitis specialists in the American Uveitis Society (AUS), a face-to-face meeting (AIR Workshop) held at the National Eye Institute (NEI), and two iterations of expert panel surveys were utilized in a modified Delphi process. The expert panel consisted of 17 experts including uveitis specialists and researchers with expertise in antiretinal antibody detection. Supermajority consensus was used and defined as 75% of experts in agreement. Results There was unanimous agreement among experts regarding the categorization of autoimmune retinopathies as nonparaneoplastic and paraneoplastic, including cancer-associated retinopathy (CAR) and melanoma-associated retinopathy (MAR). Diagnostic criteria and tests essential to the diagnosis of nonparaneoplastic AIR and multiple supportive criteria reached consensus. For treatment, experts agreed that corticosteroids and conventional immunosuppressives should be used (prescribed) as 1st or 2nd line treatments, though a consensus agreed that biologics and intravenous immunoglobulin were considered appropriate in the treatment of nonparaneoplastic AIR patients regardless of the stage of disease. Experts agreed that more evidence is needed to treat nonparaneoplastic AIR patients with long-term immunomodulatory therapy and that there is enough equipoise to justify randomized, placebo-controlled trials to determine if nonparaneoplastic AIR patients should be treated with long-term immunomodulatory therapy. Regarding antiretinal antibody detection, consensus agreed that a standardized assay system is needed to detect serum antiretinal antibodies. Consensus agreed that an ideal assay should have a two-tier design and that western blot (WB) and immunohistochemistry (IHC) should be the methods used to identify antiretinal antibodies. Conclusions Consensus was achieved using a modified Delphi process to develop diagnostic criteria for nonparaneoplastic AIR. There is enough equipoise to justify randomized, placebo-controlled trials to determine whether patients with nonparaneoplastic AIR should be treated with long-term immunomodulatory therapy. Efforts to develop a standardized two-tier assay system for the detection of antiretinal antibodies have been initiated as a result of this study.

In spiking neurons, phasic, calcium-dependent transmitter release is initiated when a presynaptic action potential activates voltage-dependent calcium channels. Vertebrate photoreceptors are nonspiking neurons that continuously release transmitter. This study uses patch-clamp recording to examine the electrophysiological properties of mammalian cones in intact retina. The cell capacitance was 10+/-1 pF and the input resistance was 0.52+/-0.46 G omega at -65 mV (31 cells). A specific membrane capacitance of 1.2 pF/cm2 was calculated. The cones did not appear to be chemically or electrically coupled. The calcium conductance averaged 3+/-1 nS (five cells). Fifty percent of the calcium channels were active at -40 mV, and at this voltage the number of active channels changed e-fold for a 6-mV voltage change. At 25 degrees C, the current reached a peak within about 1 ms after onset of a step to -35 mV. The calcium influx produced by depolarization activated a chloride conductance with a delay of a few milliseconds. The channels did not completely inactivate during maintained depolarization. The calcium channels were partially blocked by high concentrations of nifedipine, an L-type specific antagonist, and were recognized by an antibody raised against the L-type subunit alpha-1D. The immunohistochemical staining shows that the calcium channels are localized to the synaptic terminals. The immunohistochemical, physiological, and pharmacological properties indicate that the calcium channels in mammalian photoreceptors may represent a novel isoform, possibly with some homologies to the L-type class. The activation range of the channels matches the physiological operating range of photoreceptors.

The genetic locus for incomplete congenital stationary night blindness (CSNB2) has been identified as the CACNA1f gene, encoding the alpha 1F calcium channel subunit, a member of the L-type family of calcium channels. The electroretinogram associated with CSNB2 implicates alpha 1F in synaptic transmission between retinal photoreceptors and bipolar cells. Using a recently developed monoclonal antibody to alpha 1F, we localize the channel to ribbon active zones in rod photoreceptor terminals of the mouse retina, supporting a role for alpha 1F in mediating glutamate release from rods. Detergent extraction experiments indicate that alpha 1F is part of a detergent-resistant active zone complex, which also includes the synaptic ribbons. Comparison of native mouse rod calcium currents with recombinant alpha 1F currents reveals that the current-voltage relationship for the native current is shifted approximately 30 mV to more hyperpolarized potentials than for the recombinant alpha 1F current, suggesting modulation of the native channel by intracellular factors. Lastly, we present evidence for L-type alpha 1D calcium channel subunits in cone terminals of the mouse retina. The presence of alpha 1D channels in cones may explain the residual visual abilities of individuals with CSNB2.