Mounting evidence suggests that voltage-gated L-type Ca2+ channels can modulate affective behaviour. We therefore explored the role of CaV1.3 L-type Ca2+ channels in depression- and anxiety-like behaviours using CaV1.3-deficient mice (CaV1.3-/-). We showed that CaV1.3-/- mice displayed less immobility in the forced swim test as well as in the tail suspension test, indicating an antidepressant-like phenotype. Locomotor activity in the home cage or a novel open-field test was not influenced. In the elevated plus maze (EPM), CaV1.3-/- mice entered the open arms more frequently and spent more time there indicating an anxiolytic-like phenotype which was, however, not supported in the stress-induced hyperthermia test. By performing parallel experiments in Claudin 14 knockout mice (Cldn14-/-), which like CaV1.3-/- mice are congenitally deaf, an influence of deafness on the antidepressant-like phenotype could be ruled out. On the other hand, a similar EPM behaviour indicative of an anxiolytic phenotype was also found in the Cldn14-/- animals. Using electroretinography and visual behavioural tasks we demonstrated that at least in mice, CaV1.3 channels do not significantly contribute to visual function. However, marked morphological changes were revealed in synaptic ribbons in the outer plexiform layer of CaV1.3-/- retinas by immunohistochemistry suggesting a possible role of this channel type in structural plasticity at the ribbon synapse. Taken together, our findings indicate that CaV1.3 L-type Ca2+ channels modulate depression-like behaviour but are not essential for visual function. The findings raise the possibility that selective modulation of CaV1.3 channels could be a promising new therapeutic concept for the treatment of mood disorders.

We related rod to horizontal cell synaptic transfer to glutamate release by rods. Simultaneous intracellular records were obtained from dark-adapted rod-horizontal cell pairs. Steadystate synaptic gain (defined as the ratio of horizontal cell voltage to rod voltage evoked by the same light stimulus) was 3.35 Ϯ 0.60 for dim flashes and 1.50 Ϯ 0.03 for bright flashes. Under conditions of maintained illumination, there was a measurable increment of horizontal cell hyperpolarization for each light-induced increment of rod hyperpolarization over the full range of rod voltages.In separate experiments we studied glutamate release from an intact, light-responsive photoreceptor layer, from which inner retinal layers were removed. Steady light reduced glutamate release as a monotonic function of intensity; spectral sensitivity measures indicated that we monitored glutamate release from rods. The dependence of glutamate release on rod voltage was well fit by the activation function for a high-voltage-activated, dihydropyridine-sensitive L-type calcium current, suggesting a linear dependence of glutamate release on [Ca] i in the synaptic terminal. A simple model incorporating this assumption accounts for the steady-state gain of the rod to horizontal cell synapse.Key words: Xenopus; photoreceptor; rod; synaptic gain; horizontal cell; glutamate release; calcium In vertebrate retinas, the photoreceptors (rods and cones) and the second-order retinal neurons (horizontal and bipolar cells) all are nonspiking neurons with light-evoked responses that are slow potentials of complex waveform, graded in amplitude with stimulus intensity. Glutamate, the transmitter used by both rods and cones (Copenhagen and Jahr, 1989;Marc et al., 1990) is released at a steady rate in darkness (Schmitz and Witkovsky, 1996). Light, by hyperpolarizing the photoreceptor membrane, decreases glutamate release (for review, see Wu, 1994).At synapses between spiking cells, types N and P calcium channels are most often implicated in the gating of neurotransmitter release (for review, see Olivera et al., 1994;Regehr and Mintz, 1994;Katz et al. 1995). In contrast, at a tonic retinal synapse for which a depolarizing bipolar cell is the presynaptic element, transmitter release is controlled by a dihydropyridinesensitive L -type calcium current (Heidelberger and Matthews, 1992;Tachibana et al., 1993). Rods and cones also possess an L-type C a current (Bader et al., 1982;Corey et al., 1984;Barnes and Hille, 1989;Lasater and Witkovsky, 1991;Wilkinson and Barnes, 1996), and there is evidence that it underlies exocytosis (Rieke and Schwartz, 1996) and glutamate release (Schmitz and Witkovsky, 1997). In contrast to the depolarizing bipolar cell, however, light hyperpolarizes the photoreceptor, bringing its membrane into a voltage range (ϽϪ45 mV) in which the L-type Ca current, as characterized by whole-cell patch-clamp recordings (Corey et al., 1984;Wilkinson and Barnes, 1996), becomes too small to measure. A main concern of the present study is whether the L-type Ca ...

We investigated the cellular localization in the salamander retina of one of the somatostatin [or somatotropin release-inhibiting factor (SRIF)] receptors, sst(2A), and studied the modulatory action of SRIF on voltage-gated K(+) and Ca(2+) currents in rod and cone photoreceptors. SRIF immunostaining was observed in widely spaced amacrine cells, whose perikarya are at the border of the inner nuclear layer and inner plexiform layer. sst(2A) immunostaining was seen in the inner segments and terminals of rod and cone photoreceptors. Additional sst(2A) immunoreactivity was expressed by presumed bipolar and amacrine cells. SRIF, at concentrations of 100-500 nM, enhanced a delayed outwardly rectifying K(+) current (I(K)) in both rod and cone photoreceptors. SRIF action was blocked in cells pretreated with pertussis toxin (PTX) and was substantially reduced by intracellular GDP(beta)S. Voltage-gated L-type Ca(2+) currents in rods and cones were differently modulated by SRIF. SRIF reduced Ca(2+) current in rods by 33% but increased it in cones by 40%, on average. Both effects were mediated via G-protein activation and blocked by PTX. Ca(2+)-imaging experiments supported these results by showing that 500 nM SRIF reduced a K(+)-induced increase in intracellular Ca(2+) in rod photoreceptor terminals but increased it in those of cones. Our results suggest that SRIF may play a role in the regulation of glutamate transmitter release from photoreceptors via modulation of voltage-gated K(+) and Ca(2+) currents.

The progressive death of retinal ganglion cells and resulting visual deficits are hallmarks of glaucoma, but the underlying mechanisms remain unclear. In many neurodegenerative diseases, cell death induced by primary insult is followed by a wave of secondary loss. Gap junctions (GJs), intercellular channels composed of subunit connexins, can play a major role in secondary cell death by forming conduits through which toxic molecules from dying cells pass to and injure coupled neighbors. Here we have shown that pharmacological blockade of GJs or genetic ablation of connexin 36 (Cx36) subunits, which are highly expressed by retinal neurons, markedly reduced loss of neurons and optic nerve axons in a mouse model of glaucoma. Further, functional parameters that are negatively affected in glaucoma, including the electroretinogram, visual evoked potential, visual spatial acuity, and contrast sensitivity, were maintained at control levels when Cx36 was ablated. Neuronal GJs may thus represent potential therapeutic targets to prevent the progressive neurodegeneration and visual impairment associated with glaucoma.

Secondary cell death via gap junctions (GJs) plays a role in the propagation of neuronal loss under a number of degenerative disorders.Here, we examined the role of GJs in neuronal death in the retina, which has arguably the most diverse expression of GJs in the CNS. Initially, we induced apoptotic death by injecting single retinal ganglion cells and glia with cytochrome C and found that this resulted in the loss of neighboring cells to which they were coupled via GJs. We next found that pharmacological blockade of GJs eradicated nearly all amacrine cell loss and reduced retinal ganglion cell loss by ϳ70% after induction of either excitotoxic or ischemic insult conditions. These data indicate that the GJ-mediated secondary cell death was responsible for the death of most cells. Whereas genetic deletion of the GJ subunit Cx36 increased cell survivability by ϳ50% under excitotoxic condition, cell loss in Cx45 knock-out mouse retinas was similar to that seen in wild-type mice. In contrast, ablation of Cx45 reduced neuronal loss by ϳ50% under ischemic insult, but ablation of Cx36 offered no protection. Immunolabeling of the connexins showed differential changes in protein expression consistent with their differing roles in propagating death signals under the two insults. These data indicate that secondary cell death is mediated by different cohorts of GJs dependent on the connexins they express and the type of initial insult. Our results suggest that targeting specific connexins offers a novel therapeutic strategy to reduce progressive cell loss under different neurodegenerative conditions.