Optogenetic approaches to vision restoration.

Inherited retinal disease (IRD) affects about 1 in 3000 to 1 in 5000 individuals and is now believed to be the most common cause of blindness registration in developed countries. Until recently, the management of such conditions had been exclusively supportive. However, advances in molecular biology and medical engineering have now seen the rise of a variety of approaches to restore vision in patients with IRDs. Optogenetic approaches are primarily aimed at rendering secondary and tertiary neurons of the retina light-sensitive in order to replace degenerate or dysfunctional photoreceptors. Such approaches are attractive because they provide a "causative gene-independent" strategy, which may prove suitable for a variety of patients with IRD. We discuss theoretical and practical considerations in the selection of optogenetic molecules, vectors, surgical approaches and review previous trials of optogenetics for vision restoration. Optogenetic approaches to vision restoration have yielded promising results in pre-clinical trials and a phase I/II clinical trial is currently underway (ClinicalTrials.gov NCT02556736). Despite the significant inroads made in recent years, the ideal optogenetic molecule, vector and surgical approach have yet to be established.

Inner retinal inhibition shapes the receptive field of retinal ganglion cells in primate.

The centre-surround organisation of receptive fields is a feature of most retinal ganglion cells (RGCs) and is critical for spatial discrimination and contrast detection. Although lateral inhibitory processes are known to be important in generating the receptive field surround, the contribution of each of the two synaptic layers in the primate retina remains unclear. Here we studied the spatial organisation of excitatory and inhibitory synaptic inputs onto ON and OFF ganglion cells in the primate retina. All RGCs showed an increase in excitation in response to stimulus of preferred polarity. Inhibition onto RGCs comprised two types of responses to preferred polarity: some RGCs showed an increase in inhibition whilst others showed removal of tonic inhibition. Excitatory inputs were strongly spatially tuned but inhibitory inputs showed more variable organisation: in some neurons they were as strongly tuned as excitation, and in others inhibitory inputs showed no spatial tuning. We targeted one source of inner retinal inhibition by functionally ablating spiking amacrine cells with bath application of tetrodotoxin (TTX). TTX significantly reduced the spatial tuning of excitatory inputs. In addition, TTX reduced inhibition onto those RGCs where a stimulus of preferred polarity increased inhibition. Reconstruction of the spatial tuning properties by somatic injection of excitatory and inhibitory synaptic conductances verified that TTX-mediated inhibition onto bipolar cells increases the strength of the surround in RGC spiking output. These results indicate that in the primate retina inhibitory mechanisms in the inner plexiform layer sharpen the spatial tuning of ganglion cells.

Cannabinoids modulate spontaneous synaptic activity in retinal ganglion cells.

The endocannabinoid (ECB) system has been found throughout the central nervous system and modulates cell excitability in various forms of short-term plasticity. ECBs and their receptors have also been localized to all retinal cells, and cannabinoid receptor activation has been shown to alter voltage-dependent conductances in several different retinal cell types, suggesting a possible role for cannabinoids in retinal processing. Their effects on synaptic transmission in the mammalian retina, however, have not been previously investigated. Here, we show that exogenous cannabinoids alter spontaneous synaptic transmission onto retinal ganglion cells (RGCs). Using whole-cell voltage-clamp recordings in whole-mount retinas, we measured spontaneous postsynaptic currents (SPSCs) in RGCs in adult and young (P14-P21) mice. We found that the addition of an exogenous cannabinoid agonist, WIN55212-2 (5 µM), caused a significant reversible reduction in the frequency of SPSCs. This change, however, did not alter the kinetics of the SPSCs, indicating a presynaptic locus of action. Using blockers to isolate inhibitory or excitatory currents, we found that cannabinoids significantly reduced the release probability of both GABA and glutamate, respectively. While the addition of cannabinoids reduced the frequency of both GABAergic and glutamatergic SPSCs in both young and adult mice, we found that the largest effect was on GABA-mediated currents in young mice. These results suggest that the ECB system may potentially be involved in the modulation of signal transmission in the retina. Furthermore, they suggest that it might play a role in the developmental maturation of synaptic circuits, and that exogenous cannabinoids are likely able to disrupt retinal processing and consequently alter vision.

Light evokes Ca2+ spikes in the axon terminal of a retinal bipolar cell.

Bipolar cells in the vertebrate retina have been characterized as nonspiking interneurons. Using patch-clamp recordings from goldfish retinal slices, we find, however, that the morphologically well-defined Mb1 bipolar cell is capable of generating spikes. Surprisingly, in dark-adapted retina, spikes were reliably evoked by light flashes and had a long (1-2 s) refractory period. In light-adapted retina, most Mb1 cells did not spike. However, an L-type Ca2+ channel agonist could induce periodic spiking in these cells. Spikes were determined to be Ca2+ action potentials triggered at the axon terminal and were abolished by 2-amino-4-phosphonobutyric acid (APB), an agonist that mimics glutamate. Signaling via spikes in a specific class of bipolar cells may serve to accelerate and amplify small photo-receptor signals, thereby securing the synaptic transmission of dim and rapidly changing visual input.

Synaptic currents generating the inhibitory surround of ganglion cells in the mammalian retina.

The receptive field (RF) of retinal ganglion cells (RGCs) consists of an excitatory central region, the RF center, and an inhibitory peripheral region, the RF surround. It is still unknown in detail which inhibitory interneurons (horizontal or amacrine cells) and which inhibitory circuits (presynaptic or postsynaptic) generate the RF surround. To study surround inhibition, light-evoked whole-cell currents were recorded from RGCs of the isolated, intact rabbit retina. The RFs were stimulated with light or dark spots of increasing diameters and with annular light stimuli. Direct inhibitory currents could be isolated by voltage clamping ganglion cells close to the Na(+)/K(+) reversal potential. They mostly represent an input from GABAergic amacrine cells that contribute to the inhibitory surround of ganglion cells. This direct inhibitory input and its physiological function were also investigated by recording light-evoked action potentials of RGCs in the current-clamp mode and by changing the intracellular Cl(-) concentration. The excitatory input of the ganglion cells could be isolated by voltage clamping ganglion cells at the Cl(-) reversal potential. Large light spots and annular light stimuli caused a strong attenuation of the excitatory input. Both GABA(A) receptors and GABA(C) receptors contributed to this inhibition, and picrotoxinin was able to completely block it. Together, these results show that the RF surround of retinal ganglion cells is mediated by a combination of direct inhibitory synapses and presynaptic surround inhibition.

Calcium channel blockers and transmitter release at the normal human neuromuscular junction.

Transmitter release evoked by nerve stimulation is highly dependent on Ca2+ entry through voltage-activated plasma membrane channels. Calcium influx may be modified in some neuromuscular diseases like Lambert-Eaton syndrome and amyotrophic lateral sclerosis. We studied the pharmacologic sensitivity of the transmitter release process to different calcium channel blockers in normal human muscles and found that funnel web toxin and omega-Agatoxin-IVA, both P-type calcium channel blockers, blocked nerve-elicited muscle action potentials and inhibited evoked synaptic transmission. The transmitter release was not affected either by nitrendipine, an L-type channel blocker, or omega-Conotoxin-GVIA, an N-type channel blocker. The pharmacologic profile of neuromuscular transmission observed in normal human muscles indicates that P-like channels mediate transmitter release at the motor nerve terminals.

Long-term neuromuscular dysfunction produced by passive transfer of amyotrophic lateral sclerosis immunoglobulins.

We investigated the role of the immune system in the pathogenesis of amyotrophic lateral sclerosis (ALS) by studying the long-term consequences of ALS immunoglobulin (Ig) application on the levator auris muscle of the mouse. We applied Ig from seven ALS patients, four disease controls, and a pool of normal Ig (6 mg of Ig in 2 weeks) by subcutaneous injection; removed the muscles 4 to 12 weeks after the beginning of treatment; and recorded both spontaneous and evoked release of transmitter. None of the control Ig induced changes in transmitter, whereas five of seven ALS Ig induced a significant increase in the rate of spontaneous release, and all ALS Ig produced significant changes in the quantal content of evoked release. In muscles treated with one of the ALS Igs, synaptic activity was completely absent. Cholinesterase and silver staining demonstrated intact neuromuscular junctions in the control Ig-treated muscles and also in many areas of ALS Ig-treated muscles. Axonal degeneration and denervation were present in most muscles treated with ALS Ig. There was complete denervation when no synaptic activity could be recorded. Thus, ALS Ig appears to lead to long-lasting effects at the neuromuscular junction, and such effects may be an early stage in the immune-mediated pathogenesis of ALS.

Effects of Ca2+ channel blocker neurotoxins on transmitter release and presynaptic currents at the mouse neuromuscular junction.

1. The effects of the voltage-dependent calcium channel (VDCC) blockers omega-agatoxin IVA (omega-AgaIVA), omega-conotoxin GVIA (omega-CgTx), omega-conotoxin MVIIC (omega-MVIIC) and omega-conotoxin MVIID (omega-MVIID) were evaluated on transmitter release in the mouse diaphragm preparation. The effects of omega-AgaIVA and omega-MVIIC were also evaluated on the perineurial calcium and calcium-dependent potassium currents, ICa and IK(Ca), respectively, in the mouse levator auris preparation. 2. The P- and Q-type VDCC blocker omega-AgaIVA (100 nM) and P- Q- and N-type channel blockers omega-MVIIC (1 microM) and omega-MVIID (3 microM) strongly reduced transmitter release (> 80-90% blockade) whereas the selective N-type channel blocker omega-CgTx (5 microM) was ineffective. 3. The process of release was much more sensitive to omega-MVIIC (IC50 = 39 nM) than to omega-MVIID (IC50 = 1.4 microM). After almost completely blocking transmitter release (quantal content approximately 0.3% of its control value) with 3 microM omega-MVIIC, elevating the external [Ca2+] from 2 to 10 mM induced an increase of approximately 20 fold on the quantal content of the endplate potential (e.p.p.) (from 0.2 +/- 0.04 to 4.8 +/- 1.4). 4. Nerve-evoked transmitter release in a low Ca(2+)-high Mg2+ medium (low release probability, quantal content = 2 +/- 0.1) had the same sensitivity to omega-AgaIVA (IC50 = 16.8 nM) as that in normal saline solutions. In addition, K(+)-evoked transmitter release was also highly sensitive to the action of this toxin (IC50 = 11.5 nM; 100 nM > 95% blockade). The action of omega-AgaIVA on transmitter release could be reversed by toxin washout if the experiments were carried out at 31-33 degrees C. Conversely, the effect of omega-AgaIVA persisted even after two hours of toxin washout at room temperature. 5. Both the calcium and calcium-dependent potassium presynaptic currents, ICa and IK(Ca), respectively, were highly sensitive to low concentrations (10-30 nM) of omega-AgaIVA. The ICa and the IK(Ca) were also strongly reduced by 1 microM omega-MVIIC. The most marked difference between the action of these two toxins was the long incubation times required to achieve maximal effects with omega-MVIIC. 6. In summary these results provide more evidence that synaptic transmission at the mammalian neuromuscular junction is mediated by Ca2+ entry through P- and/or Q-type calcium channels.

Transmitter release and presynaptic Ca2+ currents blocked by the spider toxin omega-Aga-IVA.

Mammalian neuromuscular transmission is resistant to L and N type calcium channel blockers but very sensitive to a low molecular weight funnel web spider venom toxin, FTX, which selectively blocks P type calcium channels. To further characterize the calcium channels involved in neuromuscular transmission we studied the effect of omega Agatoxin (omega-Aga-IVA) a polypeptide P type channel blocker from the same spider venom. We show that omega-Aga-IVA is a potent and irreversible inhibitor of the presynaptic Ca2+ currents and of acetylcholine release induced by electrical stimulation or by K+ depolarization. This provides further evidences that transmitter release at the mammalian neuromuscular junction is mediated by P type Ca2+ channels.

Effect of omega-conotoxin GVIA on neurotransmitter release at the mouse neuromuscular junction.

The effect of omega-conotoxin GVIA (omega-CgTx) was studied on spontaneous, K(+)-induced and electrically evoked neurotransmitter release at the neuromuscular junction of mouse diaphragm. omega-CgTx decreased the frequency and amplitude of basal and K(+)-induced miniature end plate potentials. This effect was abolished by raising the extracellular Ca2+ concentration. omega-CgTx had no effect on the quantal content of the electrically evoked release in this preparation.

P/Q-type calcium channels activate neighboring calcium-dependent potassium channels in mouse motor nerve terminals.

The identity of the voltage-dependent calcium channels (VDCC), which trigger the Ca2+-gated K+ currents (IK(Ca)) in mammalian motor nerve terminals, was investigated by means of perineurial recordings. The effects of Ca2+ chelators with different binding kinetics on the activation of IK(Ca) were also examined. The calcium channel blockers of the P/Q family, omega-agatoxin IVA (omega-Aga-IVA) and funnel-web spider toxin (FTX), have been shown to exert a strong blocking effect on IK(Ca). In contrast, nitrendipine and omega-conotoxin GVIA (omega-CgTx) did not affect the Ca2+-activated K+ currents. The intracellular action of the fast Ca2+ buffers BAPTA and DM-BAPTA prevented the activation of the IK(Ca), while the slow Ca2+ buffer EGTA was ineffective at blocking it. These data indicate that P/Q-type VDCC mediate the Ca2+ influx which activates IK(Ca). The spatial association between Ca2+ and Ca2+-gated K+ channels is discussed, on the basis of the differential effects of the fast and slow Ca2+ chelators.

Calcium currents and calcium signaling in rod bipolar cells of rat retinal slices.

Combined electrophysiological and imaging techniques were used to study calcium currents (ICa) and their sites of origin at rod bipolar cells in rat retinal slices. We report here for the first time the successful whole-cell patch-clamp recording from presynaptic boutons that were compared with somatic recordings. TTX-resistant inward currents were elicited in response to depolarization. The kinetic and pharmacological properties of ICa were very similar for recordings obtained from the soma and the presynaptic terminals. ICa activated maximally between -30 and -20 mV was enhanced by Bay K 8644 and was blocked by isradipine and nifedipine. Peak amplitude and time to peak were -31.3 +/- 1.2 pA and 3.2 +/- 0.2 msec with somatic recordings (n = 54), whereas the corresponding values were -31.6 +/- 6.1 pA and 3.2 +/- 0.7 msec in recordings obtained directly from terminals (n = 6). ICa showed little inactivation during sustained depolarizations. No T-type ICa was observed with depolarizations from -90 mV. Concomitant with Ca2+ entry, depolarization induced the appearance of transient outward currents that resembled IPSCs and were blocked by GABA and glycine receptor antagonists, suggesting that they arise from activation of amacrine feedback synapses. Upon depolarization, intracellular Ca2+ ([Ca2+]i) rises were restricted to the presynaptic terminals with no somatic or axonal changes and were linearly dependent on pulse duration when using a low-affinity Ca2+ indicator. In cone bipolar cells, ICa inactivated markedly, and [Ca2+]i rises occurred in the axon, as well as in the presynaptic terminals.

GABAergic and glycinergic IPSCs in ganglion cells of rat retinal slices.

GABAergic and glycinergic IPSCs were studied in identified retinal ganglion cells (RGCs) of light-adapted rat retinal slices, using whole-cell recording techniques. GABAergic IPSCs were blocked specifically by SR95531 (3 microM) and bicuculline (3 microM) and glycinergic IPSCs by strychnine (0.3 microM). From 37 RGCs studied, 25 showed exclusively GABAergic IPSCs, 6 presented only glycinergic IPSCs, and 6 showed both. This distribution may result from differences in amacrine cells input rather than from receptor heterogeneity, because both GABA and glycine elicited Cl--selective currents in all RGCs tested. TTX markedly reduced GABAergic IPSCs frequency, whereas glycinergic IPSCs were unaffected. Ca2+-free media, with or without high Mg2+, blocked TTX-resistant GABAergic and glycinergic IPSCs. These results suggest that GABAergic IPSCs in RGCs can be elicited either by Na+-dependent action potentials or by local Ca2+ influx in medium or large dendritic field GABAergic amacrine cells, whereas glycinergic IPSCs are generated by action potential-independent Ca2+ influx in narrow field glycinergic amacrine cells. Both types of IPSCs had fast rise times and biexponential decays, but glycinergic IPSC decay was significantly slower than that of GABAergic IPSCs. An elementary conductance of 54 pS for the glycine-gated channels was estimated from single-channel events, clearly detected in the falling phase of glycinergic IPSCs, and from responses to exogenous glycine.

P-type voltage-dependent calcium channel mediates presynaptic calcium influx and transmitter release in mammalian synapses.

We have studied the effect of the purified toxin from the funnel-web spider venom (FTX) and its synthetic analog (sFTX) on transmitter release and presynaptic currents at the mouse neuromuscular junction. FTX specifically blocks the omega-conotoxin- and dihydropyridine-insensitive P-type voltage-dependent Ca2+ channel (VDCC) in cerebellar Purkinje cells. Mammalian neuromuscular transmission, which is insensitive to N- or L-type Ca2+ channel blockers, was effectively abolished by FTX and sFTX. These substances blocked the muscle contraction and the neurotransmitter release evoked by nerve stimulation. Moreover, presynaptic Ca2+ currents recorded extracellularly from the interior of the perineural sheaths of nerves innervating the mouse levator auris muscle were specifically blocked by both natural toxin and synthetic analogue. In a parallel set of experiments, K(+)-induced Ca45 uptake by brain synaptosomes was also shown to be blocked or greatly diminished by FTX and sFTX. These results indicate that the predominant VDCC in the motor nerve terminals, and possibly in a significant percentage of brain synapses, is the P-type channel.

Properties of the ryanodine-sensitive release channels that underlie caffeine-induced Ca2+ mobilization from intracellular stores in mammalian sympathetic neurons.

The most compelling evidence for a functional role of caffeine-sensitive intracellular Ca2+ reservoirs in nerve cells derives from experiments on peripheral neurons. However, the properties of their ryanodine receptor calcium release channels have not been studied. This work combines single-cell fura-2 microfluorometry, [3H]ryanodine binding and recording of Ca2+ release channels to examine calcium release from these intracellular stores in rat sympathetic neurons from the superior cervical ganglion. Intracellular Ca2+ measurements showed that these cells possess caffeine-sensitive intracellular Ca2+ stores capable of releasing the equivalent of 40% of the calcium that enters through voltage-gated calcium channels. The efficiency of caffeine in releasing Ca2+ showed a complex dependence on [Ca2+]i. Transient elevations of [Ca2+]i by 50-500 nM were facilitatory, but they became less facilitatory or depressing when [Ca2+]i reached higher levels. The caffeine-induced Ca2+ release and its dependence on [Ca2+]i was further examined by [3H]ryanodine binding to ganglionic microsomal membranes. These membranes showed a high-affinity binding site for ryanodine with a dissociation constant (KD = 10 nM) similar to that previously reported for brain microsomes. However, the density of [3H]ryanodine binding sites (Bmax = 2.06 pmol/mg protein) was at least three-fold larger than the highest reported for brain tissue. [3H]Ryanodine binding showed a sigmoidal dependence on [Ca2+] in the range 0.1-10 microM that was further increased by caffeine. Caffeine-dependent enhancement of [3H]ryanodine binding increased and then decreased as [Ca2+] rose, with an optimum at [Ca2+] between 100 and 500 nM and a 50% decrease between 1 and 10 microM. At 100 microM [Ca2+], caffeine and ATP enhanced [3H]ryanodine binding by 35 and 170% respectively, while binding was reduced by > 90% with ruthenium red and MgCl2. High-conductance (240 pS) Ca2+ release channels present in ganglionic microsomal membranes were incorporated into planar phospholipid bilayers. These channels were activated by caffeine and by micromolar concentrations of Ca2+ from the cytosolic side, and were blocked by Mg2+ and ruthenium red. Ryanodine (2 microM) slowed channel gating and elicited a long-lasting subconductance state while 10 mM ryanodine closed the channel with infrequent opening to the subconductance level. These results show that the properties of the ryanodine receptor/Ca2+ release channels present in mammalian peripheral neurons can account for the properties of caffeine-induced Ca2+ release. Our data also suggest that the release of Ca2+ by caffeine has a bell-shaped dependence on Ca2+ in the physiological range of cytoplasmic [Ca2+].