Long-term plasticity mediated by mGluR1 at a retinal reciprocal synapse

Short-term plasticity and context-dependent circuit function: Insights from retinal circuitry

Changes in synaptic strength across timescales are integral to algorithmic operations of neural circuits. However, pinpointing synaptic loci that undergo plasticity in intact brain circuits and delineating contributions of synaptic plasticity to circuit function remain challenging. The whole-mount retina preparation provides an accessible platform for measuring plasticity at specific synapses while monitoring circuit-level behaviors during visual processing ex vivo. In this review, we discuss insights gained from retina studies into the versatile roles of short-term synaptic plasticity in context-dependent circuit functions. Plasticity at single synapse level greatly expands the algorithms of common microcircuit motifs and contributes to diverse circuit-level behaviors such as gain modulation, selective gating, and stimulus-dependent excitatory/inhibitory balance. Examples in retinal circuitry offer unequivocal support that synaptic plasticity increases the computational capacity of hardwired neural circuitry.

Glycine Release Is Potentiated by cAMP via EPAC2 and Ca2+ Stores in a Retinal Interneuron

Neuromodulation via the intracellular second messenger cAMP is ubiquitous at presynaptic nerve terminals. This modulation of synaptic transmission allows exocytosis to adapt to stimulus levels and reliably encode information. The AII amacrine cell (AII-AC) is a central hub for signal processing in the mammalian retina. The main apical dendrite of the AII-AC is connected to several lobular appendages that release glycine onto OFF cone bipolar cells and ganglion cells. However, the influence of cAMP on glycine release is not well understood. Using membrane capacitance measurements from mouse AII-ACs to directly measure exocytosis, we observe that intracellular dialysis of 1 mm cAMP enhances exocytosis without affecting the L-type Ca²⁺ current. Responses to depolarizing pulses of various durations show that the size of the readily releasable pool of vesicles nearly doubles with cAMP, while paired-pulse depression experiments suggest that release probability does not change. Specific agonists and antagonists for exchange protein activated by cAMP 2 (EPAC2) revealed that the cAMP-induced enhancement of exocytosis requires EPAC2 activation. Furthermore, intact Ca²⁺ stores were also necessary for the cAMP potentiation of exocytosis. Postsynaptic recordings from OFF cone bipolar cells showed that increasing cAMP with forskolin potentiated the frequency of glycinergic spontaneous IPSCs. We propose that cAMP elevations in the AII-AC lead to a robust enhancement of glycine release through an EPAC2 and Ca²⁺ store signaling pathway. Our results thus contribute to a better understanding of how AII-AC crossover inhibitory circuits adapt to changes in ambient luminance.SIGNIFICANCE STATEMENT The mammalian retina operates over a wide dynamic range of light intensities and contrast levels. To optimize the signal-to-noise ratio of processed visual information, both excitatory and inhibitory synapses within the retina must modulate their gain in synaptic transmission to adapt to different levels of ambient light. Here we show that increases of cAMP concentration within AII amacrine cells produce enhanced exocytosis from these glycinergic interneurons. Therefore, we propose that light-sensitive neuromodulators may change the output of glycine release from AII amacrine cells. This novel mechanism may fine-tune the amount of tonic and phasic synaptic inhibition received by bipolar cell terminals and, consequently, the spiking patterns that ganglion cells send to the upstream visual areas of the brain.

Quantifying the effect of light activated outer and inner retinal inhibitory pathways on glutamate release from mixed bipolar cells

Inhibition mediated by horizontal and amacrine cells in the outer and inner retina, respectively, are fundamental components of visual processing. Here, our purpose was to determine how these different inhibitory processes affect glutamate release from ON bipolar cells when the retina is stimulated with full-field light of various intensities. Light-evoked membrane potential changes (ΔV_m ) were recorded directly from axon terminals of intact bipolar cells receiving mixed rod and cone inputs (Mbs) in slices of dark-adapted goldfish retina. Inner and outer retinal inhibition to Mbs was blocked with bath applied picrotoxin (PTX) and NBQX, respectively. Then, control and pharmacologically modified light responses were injected into axotomized Mb terminals as command potentials to induce voltage-gated Ca²⁺ influx (Q^Ca ) and consequent glutamate release. Stimulus-evoked glutamate release was quantified by the increase in membrane capacitance (ΔC_m ). Increasing depolarization of Mb terminals upon removal of inner and outer retinal inhibition enhanced the ΔV_m /Q^Ca ratio equally at a given light intensity and inhibition did not alter the overall relation between Q^Ca and ΔC_m . However, relative to control, light responses recorded in the presence of PTX and PTX + NBQX increased ΔC_m unevenly across different stimulus intensities: at dim stimulus intensities predominantly the inner retinal GABAergic inhibition controlled release from Mbs, whereas the inner and outer retinal inhibition affected release equally in response to bright stimuli. Furthermore, our results suggest that non-linear relationship between Q^Ca and glutamate release can influence the efficacy of inner and outer retinal inhibitory pathways to mediate Mb output at different light intensities.

The Contribution of L-Type Cav1.3 Channels to Retinal Light Responses

L-type voltage-gated calcium channels (LTCCs) regulate tonic neurotransmitter release from sensory neurons including retinal photoreceptors. There are three types of LTCCs (Ca_v1.2, Ca_v1.3, and Ca_v1.4) expressed in the retina. While Ca_v1.2 is expressed in all retinal cells including the Müller glia and neurons, Ca_v1.3 and Ca_v1.4 are expressed in the retinal neurons with Ca_v1.4 exclusively expressed in the photoreceptor synaptic terminals. Mutations in the gene encoding Ca_v1.4 cause incomplete X-linked congenital stationary night blindness in humans. Even though Ca_v1.3 is present in the photoreceptor inner segments and the synaptic terminals in various vertebrate species, its role in vision is unclear, since genetic alterations in Ca_v1.3 are not associated with severe vision impairment in humans or in Ca_v1.3-null (Ca_v1.3^-/-) mice. However, a failure to regulate Ca_v1.3 was found in a mouse model of Usher syndrome, the most common cause of combined deafness and blindness in humans, indicating that Ca_v1.3 may contribute to retinal function. In this report, we combined physiological and morphological data to demonstrate the role of Ca_v1.3 in retinal physiology and function that has been undervalued thus far. Through ex vivo and in vivo electroretinogram (ERG) recordings and immunohistochemical staining, we found that Ca_v1.3 plays a role in retinal light responses and synaptic plasticity. Pharmacological inhibition of Ca_v1.3 decreased ex vivo ERG a- and b-wave amplitudes. In Ca_v1.3^-/- mice, their dark-adapted ERG a-, b-wave, and oscillatory potential amplitudes were significantly dampened, and implicit times were delayed compared to the wild type (WT). Furthermore, the density of ribbon synapses was reduced in the outer plexiform layer of Ca_v1.3^-/- mice retinas. Hence, Ca_v1.3 plays a more prominent role in retinal physiology and function than previously reported.

Retinoschisin Facilitates the Function of L-Type Voltage-Gated Calcium Channels

Modulation of ion channels by extracellular proteins plays critical roles in shaping synaptic plasticity. Retinoschisin (RS1) is an extracellular adhesive protein secreted from photoreceptors and bipolar cells, and it plays an important role during retinal development, as well as in maintaining the stability of retinal layers. RS1 is known to form homologous octamers and interact with molecules on the plasma membrane including phosphatidylserine, sodium-potassium exchanger complex, and L-type voltage-gated calcium channels (LTCCs). However, how this physical interaction between RS1 and ion channels might affect the channel gating properties is unclear. In retinal photoreceptors, two major LTCCs are Cav1.3 (α1D) and Cav1.4 (α1F) with distinct biophysical properties, functions and distributions. Cav1.3 is distributed from the inner segment (IS) to the synaptic terminal and is responsible for calcium influx to the photoreceptors and overall calcium homeostasis. Cav1.4 is only expressed at the synaptic terminal and is responsible for neurotransmitter release. Mutations of the gene encoding Cav1.4 cause X-linked incomplete congenital stationary night blindness type 2 (CSNB2), while null mutations of Cav1.3 cause a mild decrease of retinal light responses in mice. Even though RS1 is known to maintain retinal architecture, in this study, we present that RS1 interacts with both Cav1.3 and Cav1.4 and regulates their activations. RS1 was able to co-immunoprecipitate with Cav1.3 and Cav1.4 from porcine retinas, and it increased the LTCC currents and facilitated voltage-dependent activation in HEK cells co-transfected with RS1 and Cav1.3 or Cav1.4, thus providing evidence of a functional interaction between RS1 and LTCCs. The interaction between RS1 and Cav1.3 did not change the calcium-dependent inactivation of Cav1.3. In mice lacking RS1, the expression of Cav1.3 and Cav1.4 in the retina decreased, while in mice with Cav1.4 deletion, the retinal level of RS1 decreased. These results provide important evidence that RS1 is not only an adhesive protein promoting cell-cell adhesion, it is essential for anchoring other membrane proteins including ion channels and enhancing their function in the retina.

Proton-mediated block of Ca2+ channels during multivesicular release regulates short-term plasticity at an auditory hair cell synapse

Synaptic vesicles release both neurotransmitter and protons during exocytosis, which may result in a transient acidification of the synaptic cleft that can block Ca(2+) channels located close to the sites of exocytosis. Evidence for this effect has been reported for retinal ribbon-type synapses, but not for hair cell ribbon synapses. Here, we report evidence for proton release from bullfrog auditory hair cells when they are held at more physiological, in vivo-like holding potentials (Vh = -60 mV) that facilitate multivesicular release. During paired recordings of hair cells and afferent fibers, L-type voltage-gated Ca(2+) currents showed a transient block, which was highly correlated with the EPSC amplitude (or the amount of glutamate release). This effect was masked at Vh = -90 mV due to the presence of a T-type Ca(2+) current and blocked by strong pH buffering with HEPES or TABS. Increasing vesicular pH with internal methylamine in hair cells also abolished the transient block. High concentrations of intracellular Ca(2+) buffer (10 mm BAPTA) greatly reduced exocytosis and abolished the transient block of the Ca(2+) current. We estimate that this transient block is due to the rapid multivesicular release of ∼600-1300 H(+) ions per synaptic ribbon. Finally, during a train of depolarizing pulses, paired pulse plasticity was significantly changed by using 40 mm HEPES in addition to bicarbonate buffer. We propose that this transient block of Ca(2+) current leads to more efficient exocytosis per Ca(2+) ion influx and it may contribute to spike adaptation at the auditory nerve.

Nitric oxide mediates activity-dependent plasticity of retinal bipolar cell output via S-nitrosylation

Coding a wide range of light intensities in natural scenes poses a challenge for the retina: adaptation to bright light should not compromise sensitivity to dim light. Here we report a novel form of activity-dependent synaptic plasticity, specifically, a "weighted potentiation" that selectively increases output of Mb-type bipolar cells in the goldfish retina in response to weak inputs but leaves the input-output ratio for strong stimuli unaffected. In retinal slice preparation, strong depolarization of bipolar terminals significantly lowered the threshold for calcium spike initiation, which originated from a shift in activation of voltage-gated calcium currents (ICa) to more negative potentials. The process depended upon glutamate-evoked retrograde nitric oxide (NO) signaling as it was eliminated by pretreatment with an NO synthase blocker, TRIM. The NO-dependent ICa modulation was cGMP independent but could be blocked by N-ethylmaleimide (NEM), indicating that NO acted via an S-nitrosylation mechanism. Importantly, the NO action resulted in a weighted potentiation of Mb output in response to small (≤-30 mV) depolarizations. Coincidentally, light flashes with intensity ≥ 2.4 × 10(8) photons/cm(2)/s lowered the latency of scotopic (≤ 2.4 × 10(8) photons/cm(2)/s) light-evoked calcium spikes in Mb axon terminals in an NEM-sensitive manner, but light responses above cone threshold (≥ 3.5 × 10(9) photons/cm(2)/s) were unaltered. Under bright scotopic/mesopic conditions, this novel form of Mb output potentiation selectively amplifies dim retinal inputs at Mb → ganglion cell synapses. We propose that this process might counteract decreases in retinal sensitivity during light adaptation by preventing the loss of visual information carried by dim scotopic signals.

Effects of a metabotropic glutamate 1 receptor antagonist on light responses of retinal ganglion cells in a rat model of retinitis pigmentosa

Background

Retinitis pigmentosa (RP) is a progressive retinal degenerative disease that causes deterioration of rod and cone photoreceptors. A well-studied animal model of RP is the transgenic P23H rat, which carries a mutation in the rhodopsin gene. Previously, I reported that blocking retinal GABA_C receptors in the P23H rat increases light responsiveness of retinal ganglion cells (RGCs). Because activation of metabotropic glutamate 1 (mGlu1) receptors may enhance the release of GABA onto GABA_C receptors, I examined the possibility that blocking retinal mGlu1 receptors might in itself increase light responsiveness of RGCs in the P23H rat.

Methodology/Principal Findings

Electrical recordings were made from RGCs in isolated P23H rat retinas. Spike activity of RGCs was measured in response to brief flashes of light over a range of light intensities. Intensity-response curves were evaluated prior to and during bath application of the mGlu1 receptor antagonist JNJ16259685. I found that JNJ16259685 increased light sensitivity of all ON-center RGCs and most OFF-center RGCs studied. RGCs that were least sensitive to light showed the greatest JNJ16259685-induced increase in light sensitivity. On average, light sensitivity increased in ON-center RGCs by 0.58 log unit and in OFF-center RGCs by 0.13 log unit. JNJ16259685 increased the maximum peak response of ON-center RGCs by 7% but had no significant effect on the maximum peak response of OFF-center RGCs. The effects of JNJ16259685 on ON-center RGCs were occluded by a GABA_C receptor antagonist.

Conclusions

The results of this study indicate that blocking retinal mGlu1 receptors in a rodent model of human RP potentiates transmission of any, weak signals originating from photoreceptors. This augmentation of photoreceptor-mediated signals to RGCs occurs presumably through a reduction in GABA_C-mediated inhibition.

Glycine transporter 1 modulates GABA release from amacrine cells by controlling occupancy of coagonist binding site of NMDA receptors

The occupancy of coagonist binding sites of NMDA receptors (NMDARs) by glycine or d-serine has been thought to mediate NMDAR-dependent excitatory signaling, as simultaneous binding of glutamate and a coagonist is obligatory for NMDAR activation. Amacrine cells (ACs) mediating GABAergic feedback inhibition of mixed bipolar cells (Mbs) in the goldfish retina have been shown to express NMDARs. Here we studied whether NMDAR-mediated GABAergic inhibitory currents (IGABA) recorded from the axon terminals of Mbs are influenced by experimental manipulations altering retinal glycine and d-serine levels. Feedback IGABA in Mb axon terminals was triggered by focal NMDA application or by synaptically released glutamate from depolarized Mb terminals. In both cases, blocking the coagonist binding sites of NMDARs eliminated the NMDAR-dependent IGABA, demonstrating that coagonist binding is critical in mediating NMDAR activity-triggered GABA release. Glycine transporter 1 (GLYT1) inhibition increased IGABA, indicating that coagonist binding sites of NMDARs on ACs providing GABAergic feedback inhibition to Mbs were not saturated. Focal glycine application, in the presence of the ionotropic glycine receptor blocker strychnine, triggered a GLYT1-dependent current in ACs, suggesting that GLYT1 expressed by putative glycinergic ACs controls the saturation level of NMDARs' coagonist sites. External d-serine also increased NMDAR activation-triggered IGABA in Mbs, further substantiating that the coagonist sites were unsaturated. Together, our findings demonstrate that coagonist modulation of glutamatergic input to GABAergic ACs via NMDARs is strongly reflected in the AC neuronal output (i.e., transmitter release) and thus is critical in GABAergic signal transfer function in the inner retina.

"mGlu Receptors in the Retina" - WIREs Membrane Transport and Signaling

Glutamate, a key neurotransmitter in the vertebrate retina, acts via ionotropic and metabotropic receptors. Retina expresses mRNA for all metabotropic glutamate receptors and proteins for all but mGluR3. Every retinal cell class expresses one or more of these receptors. In general, these receptors are present presynaptically and serve to modulate synaptic transmission. While mGluRs on the photoreceptor terminal act as autoreceptors to titer glutamate levels, those on horizontal cell processes seem to shape the light response. Similarly, autoreceptors on bipolar axon terminals modulate glutamate release and the receptors on amacrine and ganglion cells modulate feedforward signals by modulating K+ or Ca2+ current to fine tune light responses. Since most of the mGluR sub-types are present in amacrine and ganglion cells that belong to many cell types, the pathways downstream of mGluRs are highly diverse with primarily modulatory effects. An exception to most mGluRs which have modulatory function is mGluR6 because it plays a key role in the feedforward transmission from photoreceptors to ON bipolar cells and is also required for the correct localization of the synaptic proteins in the dendritic tips. In humans, mutations in the gene encoding mGluR6 cause autosomal recessive night blindness. In addition, mGluRs appear to play a trophic role in development and after retinal damage, suggesting potential future therapeutic implications.

Light-evoked lateral GABAergic inhibition at single bipolar cell synaptic terminals is driven by distinct retinal microcircuits

Inhibitory amacrine cells (ACs) filter visual signals crossing the retina by modulating the excitatory, glutamatergic output of bipolar cells (BCs) on multiple temporal and spatial scales. Reciprocal feedback from ACs provides focal inhibition that is temporally locked to the activity of presynaptic BC activity, whereas lateral feedback originates from ACs excited by distant BCs. These distinct feedback mechanisms permit temporal and spatial computation at BC terminals. Here, we used a unique preparation to study light-evoked IPSCs recorded from axotomized terminals of ON-type mixed rod/cone BCs (Mb) in goldfish retinal slices. In this preparation, light-evoked IPSCs could only reach axotomized BC terminals via the lateral feedback pathway, allowing us to study lateral feedback in the absence of overlapping reciprocal feedback components. We found that light evokes ON and OFF lateral IPSCs (L-IPSCs) in Mb terminals having different temporal patterns and conveyed via distinct retinal pathways. The relative contribution of rods versus cones to ON and OFF L-IPSCs was light intensity dependent. ACs presynaptic to Mb BC terminals received inputs via AMPA/KA- and NMDA-type receptors in both the ON and OFF pathways, and used TTX-sensitive sodium channels to boost signal transfer along their processes. ON and OFF L-IPSCs, like reciprocal feedback IPSCs, were mediated by both GABA(A) and GABA(C) receptors. However, our results suggest that lateral and reciprocal feedback do not cross-depress each other, and are therefore mediated by distinct populations of ACs. These findings demonstrate that retinal inhibitory circuits are highly specialized to modulate BC output at different light intensities.

The influences of metabotropic receptor activation on cellular signaling and synaptic function in amacrine cells

Amacrine cells receive glutamatergic input from bipolar cells and GABAergic, glycinergic, cholinergic, and dopaminergic input from other amacrine cells. Glutamate, GABA, glycine, and acetylcholine (ACh) interact with ionotropic receptors and it is these interactions that form much of the functional circuitry in the inner retina. However, glutamate, GABA, ACh, and dopamine also activate metabotropic receptors linked to second messenger pathways that have the potential to modify the function of individual cells as well as retinal circuitry. Here, the physiological effects of activating dopamine receptors, metabotropic glutamate receptors, GABAB receptors, and muscarinic ACh receptors on amacrine cells will be discussed. The retina also expresses metabotropic receptors and the biochemical machinery associated with the synthesis and degradation of endocannabinoids and sphingosine-1-phosphate (S1P). The effects of activating cannabinoid receptors and S1P receptors on amacrine cell function will also be addressed.

Disruption of metabotropic glutamate receptor signalling is a major defect at cerebellar parallel fibre-Purkinje cell synapses in staggerer mutant mice

Staggerer mutant mice have functional loss of a transcription factor, retinoid-related orphan receptor α (RORα), which is abundantly expressed in Purkinje cells (PCs) of the cerebellum.Homozygous staggerer (sg/sg)mice show cerebellar hypoplasia and congenital ataxia. Sg/sg mice serve as an important extreme mouse model of the hereditary spinocerebellar ataxia type 1 (SCA1), since it has been shown that RORα dysfunction is strongly correlated with SCA1 pathogenesis. However, synaptic abnormalities, especially at parallel fibre (PF)-PC synapses, in SCA1-related sg/sg mice have not been examined in detail electrophysiologically. In this study, we report that PFs can still establish functional synapses onto PCs in sg/sg mice in spite of reduction in the number of PF-PC synapses. Compared with PF-evoked EPSCs in the wild-type or heterozygotes, the success rate of the EPSC recordings in sg/sg was quite low (∼40%) and the EPSCs showed faster kinetics and slightly decreased paired pulse facilitation at short intervals. The prominent synaptic dysfunction is that sg/sg mice lack metabotropic glutamate receptor (mGluR)-mediated slow EPSCs completely. Neither intense PF stimulation nor an exogenously applied mGluR agonist, DHPG, could elicit mGluR-mediated responses.Western blot analysis in the sg/sg cerebellum revealed low-level expression of mGluR1 and TRPC3, both of which underlie mGluR-mediated slow currents in PCs. Immunohistochemical data demonstrated marked mislocalization of mGluR1 on sg/sg PCs.We found that mGluR-mediated retrograde suppression of PF-PC EPSCs by endocannabinoid is also impaired completely in sg/sg mice. These results suggest that disruption of mGluR signalling at PF-PC synapses is one of the major synaptic defects in sg/sg mice and may manifest itself in SCA1 pathology.

Interneuron circuits tune inhibition in retinal bipolar cells

While connections between inhibitory interneurons are common circuit elements, it has been difficult to define their signal processing roles because of the inability to activate these circuits using natural stimuli. We overcame this limitation by studying connections between inhibitory amacrine cells in the retina. These interneurons form spatially extensive inhibitory networks that shape signaling between bipolar cell relay neurons to ganglion cell output neurons. We investigated how amacrine cell networks modulate these retinal signals by selectively activating the networks with spatially defined light stimuli. The roles of amacrine cell networks were assessed by recording their inhibitory synaptic outputs in bipolar cells that suppress bipolar cell output to ganglion cells. When the amacrine cell network was activated by large light stimuli, the inhibitory connections between amacrine cells unexpectedly depressed bipolar cell inhibition. Bipolar cell inhibition elicited by smaller light stimuli or electrically activated feedback inhibition was not suppressed because these stimuli did not activate the connections between amacrine cells. Thus the activation of amacrine cell circuits with large light stimuli can shape the spatial sensitivity of the retina by limiting the spatial extent of bipolar cell inhibition. Because inner retinal inhibition contributes to ganglion cell surround inhibition, in part, by controlling input from bipolar cells, these connections may refine the spatial properties of the retinal output. This functional role of interneuron connections may be repeated throughout the CNS.

Slow excitation of cultured rat retinal ganglion cells by activating group I metabotropic glutamate receptors

As in many CNS neurons, retinal ganglion cells (RGCs) receive fast synaptic activation through postsynaptic ionotropic receptors. However, the potential role of postsynaptic group I metabotropic glutamate receptors (mGluRs) in these neurons is unknown. In this study we first demonstrated that the selective group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG) increased intracellular calcium concentration in neurons within the ganglion cell layer of the rat retina. This prompted us to use an immunopanned-RGC and cortical astroglia coculture preparation to explore the effect of group I mGluR activation on the electrophysiological properties of cultured RGCs. Using perforated patch-clamp recordings in current-clamp configuration, we found that application of DHPG increased spontaneous spiking and depolarized the resting membrane potential of RGCs. This boosting effect was attributed to an increase in membrane resistance due to blockade of a background K(+) conductance. Further experiments showed that the group I mGluR-sensitive K(+) conductance was not blocked by 3 mM Cs(+), but was sensitive to acidification. Pharmacological studies indicated that the effect of DHPG on RGCs was mediated by the mGluR1 rather than the mGluR5 receptor subtype. Our results suggest a facilitatory role for group I mGluR activation in modulating RGC excitability in the mammalian inner retina.

Activation of the tonic GABAC receptor current in retinal bipolar cell terminals by nonvesicular GABA release

Within the second synaptic layer of the retina, bipolar cell (BC) output to ganglion cells is regulated by inhibitory input to BC axon terminals. GABA(A) receptors (GABA(A)Rs) mediate rapid synaptic currents in BC terminals, whereas GABA(C) receptors (GABA(C)Rs) mediate slow evoked currents and a tonic current, which is strongly regulated by GAT-1 GABA transporters. We have used voltage-clamp recordings from BC terminals in goldfish retinal slices to determine the source of GABA for activation of these currents. Inhibition of vesicular release with concanamycin A or tetanus toxin significantly inhibited GABA(A)R inhibitory postsynaptic currents and glutamate-evoked GABA(A)R and GABA(C)R currents but did not reduce the tonic GABA(C)R current, which was also not dependent on extracellular Ca(2+). The tonic current was strongly potentiated by inhibition of GABA transaminase, under both normal and Ca(2+)-free conditions, and was activated by exogenous taurine; however inhibition of taurine transport had little effect. The tonic current was unaffected by GAT-2/3 inhibition and was potentiated by GAT-1 inhibition even in the absence of vesicular release, indicating that it is unlikely to be evoked by reversal of GABA transporters or by ambient GABA. In addition, GABA release does not appear to occur via hemichannels or P2X(7) receptors. BC terminals therefore exhibit two forms of GABA(C)R-mediated inhibition, activated by vesicular and by nonvesicular GABA release, which are likely to have distinct functions in visual signal processing. The tonic GABA(C)R current in BC terminals exhibits similar properties to tonic GABA(A)R and glutamate receptor currents in the brain.

Retinal ON bipolar cells express a new PCP2 splice variant that accelerates the light response

PCP2, a member of the GoLoco domain-containing family, is present exclusively in cerebellar Purkinje cells and retinal ON bipolar cells. Its function in these tissues is unknown. Biochemical and expression system studies suggest that PCP2 is a guanine nucleotide dissociation inhibitor, although a guanine nucleotide exchange factor has also been suggested. Here, we studied the function of PCP2 in ON bipolar cells because their light response depends on Galpha(o1), which is known to interact with PCP2. We identified a new splice variant of PCP2 (Ret-PCP2) and localized it to rod bipolar and ON cone bipolar cells. Electroretinogram recordings from PCP2-null mice showed a normal a-wave but a slower falling phase of the b-wave (generated by the activity of ON bipolar cells) relative to the wild type. Whole-cell recordings from rod bipolar cells showed, both under Ames medium and after blocking GABA(A/C) and glycine receptors, that PCP2-null rod bipolar cells were more depolarized than wild-type cells with greater inward current when clamped to -60 mV. Also under both conditions, the rise time of the response to intense light was slower by 28% (Ames) and 44% (inhibitory blockers) in the null cells. Under Ames medium, we also observed >30% longer decay time in the PCP2-null rod bipolar cells. We conclude that PCP2 facilitates cation channels closure in the dark, shortens the rise time of the light response directly, and accelerates the decay time indirectly via the inhibitory network. These data can most easily be explained if PCP2 serves as a guanine nucleotide exchange factor.

Short-term depression at the reciprocal synapses between a retinal bipolar cell terminal and amacrine cells

Visual adaptation is thought to occur partly at retinal synapses that are subject to plastic changes. However, the locus and properties of this plasticity are not well known. Here, we studied short-term plasticity at the reciprocal synapse between bipolar cell terminals and amacrine cells in goldfish retinal slices. Depolarization of a single bipolar cell terminal for 100 ms triggers the release of glutamate onto amacrine cell processes, which in turn leads to GABAergic feedback from amacrine cells onto the same terminal. We find that this release of GABA undergoes paired-pulse depression (PPD) that recovers in <1 min (single exponential time constant, tau approximately = 12 s). This disynaptic PPD is independent of mGluR-mediated plasticity and depletion of glutamatergic synaptic vesicle pools, because exocytosis assayed via capacitance jumps (deltaC(m)) recovered completely after 10 s (tau approximately = 2 s). Fast application of GABA (10 mM) onto outside-out patches excised from bipolar cell terminals showed that the recovery of GABA(A) receptors from desensitization depends on the duration of the application [fast recovery (<2 s) for short applications; slow (tau approximately = 12 s) for prolonged applications]. We thus blocked GABA(A) receptors and retested the GABAergic response mediated by nondesensitizing GABA(C) receptors to two rapid glutamate puffs onto the bipolar cell terminal. These responses consistently displayed PPD. Furthermore, blocking AMPA-receptor desensitization with cyclothiazide, or evoking GABA release with NMDA receptors, did not reduce PPD. We thus suggest that depletion of synaptic vesicle pools in GABAergic amacrine cells is a major contributor to PPD.

The synaptic mechanism of direction selectivity in distal processes of starburst amacrine cells

Patch-clamp recordings revealed that distal processes of starburst amacrine cells (SACs) received largely excitatory synaptic input from the receptive field center and nearly purely inhibitory inputs from the surround during both stationary and moving light stimulations. The direct surround inhibition was mediated mainly by reciprocal GABA(A) synapses between opposing SACs, which provided leading and prolonged inhibition during centripetal stimulus motion. Simultaneous Ca(2+) imaging and current-clamp recording during apparent-motion stimulation further demonstrated the contributions of both centrifugal excitation and GABA(A/C)-receptor-mediated centripetal inhibition to the direction-selective Ca(2+) responses in SAC distal processes. Thus, by placing GABA release sites in electrotonically semi-isolated distal processes and endowing these sites with reciprocal GABA(A) synapses, SACs use a radial-symmetric center-surround receptive field structure to build a polar-asymmetric circuitry. This circuitry may integrate at least three levels of interactions--center excitation, surround inhibition, and reciprocal inhibitions that amplify the center--surround antagonism-to generate robust direction selectivity in the distal processes.

The insulin/PI 3-kinase pathway regulates salt chemotaxis learning in Caenorhabditis elegans

The insulin-like signaling pathway is known to regulate fat metabolism, dauer formation, and longevity in Caenorhabditis elegans. Here, we report that this pathway is also involved in salt chemotaxis learning, in which animals previously exposed to a chemoattractive salt under starvation conditions start to show salt avoidance behavior. Mutants of ins-1, daf-2, age-1, pdk-1, and akt-1, which encode the homologs of insulin, insulin/IGF-I receptor, PI 3-kinase, phosphoinositide-dependent kinase, and Akt/PKB, respectively, show severe defects in salt chemotaxis learning. daf-2 and age-1 act in the ASER salt-sensing neuron, and the activity level of the DAF-2/AGE-1 pathway in this neuron determines the extent and orientation of salt chemotaxis. On the other hand, ins-1 acts in AIA interneurons, which receive direct synaptic inputs from sensory neurons and also send synaptic outputs to ASER. These results suggest that INS-1 secreted from AIA interneurons provides feedback to ASER to generate plasticity of chemotaxis.

Receptor and transmitter release properties set the time course of retinal inhibition

Synaptic inhibition is determined by the properties of postsynaptic receptors, neurotransmitter release, and clearance, but little is known about how these factors shape sensation-evoked inhibition. The retina is an ideal system to investigate inhibition because it can be activated physiologically with light, and separate inhibitory pathways can be assayed by recording from rod bipolar cells that possess distinct glycine, GABA(A), and GABA(C) receptors (R). We show that receptor properties differentially shape spontaneous IPSCs, whereas both transmitter release and receptor properties shape light-evoked (L) IPSCs. GABA(C)R-mediated IPSCs decayed the slowest, whereas glycineR- and GABA(A)R-mediated IPSCs decayed more rapidly. Slow GABA(C)Rs determined the L-IPSC decay, whereas GABA(A)Rs and glycineRs, which mediated rapid onset responses, determined the start of the L-IPSC. Both fast and slow inhibitory inputs distinctly shaped the output of rod bipolar cells. The slow GABA(C)Rs truncated glutamate release, making the A17 amacrine cell L-EPSCs more transient, whereas the fast GABA(A)R and glycineRs reduced the initial phase of glutamate release, limiting the peak amplitude of the L-EPSC. Estimates of transmitter release time courses suggested that glycine release was more prolonged than GABA release. The time course of GABA release activating GABA(C)Rs was slower than that activating GABA(A)Rs, consistent with spillover activation of GABA(C)Rs. Thus, both postsynaptic receptor and transmitter release properties shape light-evoked inhibition in retina.

GABA transporters regulate a standing GABAC receptor-mediated current at a retinal presynaptic terminal

At the axon terminal of goldfish retinal bipolar cells, GABA(C) receptors have been shown to mediate inhibitory reciprocal synaptic currents. Here, we demonstrate a novel standing GABAergic current mediated exclusively by GABA(C) receptors. Selective inhibition of GAT-1 GABA transporters on amacrine cells increases this tonic current and reveals a specific functional coupling between GAT-1 transporters and GABA(C) receptors. We propose that this GABA(C) receptor-mediated standing current serves to regulate synaptic gain by shunting depolarizing potentials that can produce Ca2+-dependent action potentials at the bipolar cell terminal. Furthermore, we find that the amount of GABA(C) receptor-mediated reciprocal feedback between bipolar cell terminals and amacrine cells is greatly increased when GAT-1 transporters are specifically blocked by NO-711 (1-[2-[[(diphenylmethylene)imino]oxy]ethyl]-1,2,5,6-tetrahydro-3-pyridinecarboxylic acid hydrochloride). The involvement of GAT-1 transporters in regulating this standing (or tonic) GABA(C) current implicates them in a novel role as major determinants of presynaptic excitability.

Prolonged reciprocal signaling via NMDA and GABA receptors at a retinal ribbon synapse

AMPA and GABAA receptors mediate most of the fast signaling in the CNS. However, the retina must, in addition, also convey slow and sustained signals. Given that AMPA and GABAA receptors desensitize quickly in the continuous presence of agonist, how are sustained excitatory and inhibitory signals transmitted reliably across retinal synapses? Reciprocal synapses between bipolar and amacrine cells in the retina are thought to play a fundamental role in tuning the bipolar cell output to the dynamic range of ganglion cells. Here, we report that glutamate release from goldfish bipolar cell terminals activates first AMPA receptors, followed by fast and transient GABAA-mediated feedback. Subsequently, prolonged NMDA receptor activation triggers GABAA and a slow, sustained GABAC-mediated reciprocal inhibition. The synaptic delay of the NMDA/GABAC-mediated feedback showed stronger dependence on the depolarization of the bipolar cell terminal than the fast AMPA/GABAA-mediated response. Although the initial depolarization mediated by AMPA receptors was important to prime the NMDA action, NMDA receptors could trigger feedback by themselves in most of the bipolar terminals tested. This AMPA-independent feedback (delay approximately 10 ms) was eliminated in 2 mm external Mg2+ and reduced in some terminals, but not eliminated, by TTX. NMDA receptors on amacrine cells with depolarized resting membrane potentials therefore can mediate the late reciprocal feedback triggered by continuous glutamate release. Our findings suggest that the characteristics of NMDA receptors (high agonist affinity, slow desensitization, and activation/deactivation kinetics) are well suited to match the properties of GABAC receptors, which thus provide part of the prolonged inhibition to bipolar cell terminals.

GABA(A), GABA(C) and glycine receptor-mediated inhibition differentially affects light-evoked signalling from mouse retinal rod bipolar cells

Rod bipolar cells relay visual signals evoked by dim illumination from the outer to the inner retina. GABAergic and glycinergic amacrine cells contact rod bipolar cell terminals, where they modulate transmitter release and contribute to the receptive field properties of third order neurones. However, it is not known how these distinct inhibitory inputs affect rod bipolar cell output and subsequent retinal processing. To determine whether GABA(A), GABA(C) and glycine receptors made different contributions to light-evoked inhibition, we recorded light-evoked inhibitory postsynaptic currents (L-IPSCs) from rod bipolar cells mediated by each pharmacologically isolated receptor. All three receptors contributed to L-IPSCs, but their relative roles differed; GABA(C) receptors transferred significantly more charge than GABA(A) and glycine receptors. We determined how these distinct inhibitory inputs affected rod bipolar cell output by recording light-evoked excitatory postsynaptic currents (L-EPSCs) from postsynaptic AII and A17 amacrine cells. Consistent with their relative contributions to L-IPSCs, GABA(C) receptor activation most effectively reduced the L-EPSCs, while glycine and GABA(A) receptor activation reduced the L-EPSCs to a lesser extent. We also found that GABAergic L-IPSCs in rod bipolar cells were limited by GABA(A) receptor-mediated inhibition between amacrine cells. We show that GABA(A), GABA(C) and glycine receptors mediate functionally distinct inhibition to rod bipolar cells, which differentially modulated light-evoked rod bipolar cell output. Our findings suggest that modulating the relative proportions of these inhibitory inputs could change the characteristics of rod bipolar cell output.

Calcium From Internal Stores Triggers GABA Release From Retinal Amacrine Cells

The Ca2+ that promotes transmitter release is generally thought to enter presynaptic terminals through voltage-gated Ca2+channels. Using electrophysiology and Ca2+ imaging, we show that, in amacrine cell dendrites, at least some of the Ca2+ that triggers transmitter release comes from endoplasmic reticulum Ca2+ stores. We show that both inositol 1,4,5-trisphosphate receptors (IP3Rs) and ryanodine receptors (RyRs) are present in these dendrites and both participate in the elevation of cytoplasmic [Ca2+] during the brief depolarization of a dendrite. Only the Ca2+ released through IP3Rs, however, seems to promote the release of transmitter. Antagonists for the IP3R reduced transmitter release, whereas RyR blockers had no effect. Application of an agonist for metabotropic glutamate receptor, known to liberate Ca2+ from internal stores, enhanced both spontaneous and evoked transmitter release.