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

Two types of cone bipolar cells express voltage-gated Na+ channels in the rat retina

Two groups of retinal cone bipolar cells (CBCs) in rats were found to express voltage-gated Na+ channels. The axon terminals of the first group stratify in sublamina 2 of the inner plexiform layer (IPL) and partially overlap with the OFF-cholinergic band. This group was identified as type 3 CBCs. The axon terminals of the second group stratify in sublamina 3 of the IPL, slightly distal to the ON-cholinergic band. Cells of this second group resemble type 5 CBCs. In addition, we observed another group of ON-type CBCs with terminal stratification similar to that of the second group. However, this latter group did not show any Na+ current, instead exhibiting a large hyperpolarization-activated cyclic nucleotide-gated cation current, suggesting the existence of two subclasses of physiologically distinct type 5 CBCs. Both groups of Na+-expressing bipolar cells were capable of generating a rapid tetrodotoxin-sensitive action potential as revealed by current injection. Multiple spike-like potentials were also observed in some of these cells. Results of this study provide valuable insights into the function of voltage-gated Na+ channels of retinal bipolar cells in retinal processing.

Vesicle association and exocytosis at ribbon and extraribbon sites in retinal bipolar cell presynaptic terminals

Synaptic vesicles release neurotransmitter by following a process of vesicle docking and exocytosis. Although these steps are well established, it has been difficult to observe and measure these rates directly in living synapses. Here, by combining the direct imaging of single synaptic vesicles and synaptic ribbons, I measure the properties of vesicle docking and evoked and spontaneous release from ribbon and extraribbon locations in a ribbon-type synaptic terminal, the goldfish retinal bipolar cell. In the absence of a stimulus, captured vesicles near ribbons associate tightly and only rarely undock or undergo spontaneous exocytosis. By contrast, vesicle capture at outlier sites is less stable and spontaneous exocytosis occurs at a higher rate. In response to a stimulus, exocytic events cluster near ribbons, but show no evidence of clustering away from ribbon sites. Together, the results here indicate that, although vesicles can associate and fuse both near and away from synaptic sites, vesicles at synaptic ribbons associate more stably and fusion is more tightly linked to stimuli.

Sophisticated temporal pattern recognition in retinal ganglion cells

Pattern recognition is one of the most important tasks of the visual system, and uncovering the neural mechanisms underlying recognition phenomena has been a focus of researchers for decades. Surprisingly, at the earliest stages of vision, the retina is capable of highly sophisticated temporal pattern recognition. We stimulated the retina of tiger salamander (Ambystoma tigrinum) with periodic dark flash sequences and found that retinal ganglion cells had a wide variety of different responses to a periodic flash sequence with many firing when a flash was omitted. The timing of the omitted stimulus response (OSR) depended on the period, with individual cells tracking the stimulus period down to increments of 5 ms. When flashes occurred earlier than expected, cells updated their expectation of the next flash time by as much as 50 ms. When flashes occurred later than expected, cells fired an OSR and reset their temporal expectation to the average time interval between flashes. Using pharmacology to investigate the retinal circuitry involved, we found that inhibitory transmission from amacrine cells was not required, but on bipolar cells were required. The results suggest a mechanism in which the intrinsic resonance of on bipolars leads to the OSR in ganglion cells. We discuss the implications of retinal pattern recognition on the neural code of the retina and visual processing in general.

Stimulated exocytosis of endosomes in goldfish retinal bipolar neurons

After exocytosis, synaptic vesicle components are selectively retrieved by clathrin-mediated endocytosis and then re-used in future rounds of transmitter release. Under some conditions, synaptic terminals in addition perform bulk endocytosis of large membranous sacs. Bulk endocytosis is less selective than clathrin-mediated endocytosis and probably internalizes components normally targeted to the plasma membrane. Nonetheless, this process plays a major role in some tonic ribbon-type synapses, which release neurotransmitter for prolonged periods of time. We show here, that large endosomes formed after strong and prolonged stimulation undergo stimulated exocytosis in retinal bipolar neurons. The result suggests how cells might return erroneously internalized components to the plasma membrane, and also demonstrates that synaptic vesicles are not the only neuronal organelle that stains with styryl dyes and undergoes stimulated exocytosis.

Capacitance measurements in the mouse rod bipolar cell identify a pool of releasable synaptic vesicles

The mouse is an important model system for understanding the molecular basis of neuronal signaling and diseases of synaptic communication. However, the best-characterized retinal ribbon-style synapses are those of nonmammalian vertebrates. To remedy this situation, we asked whether it would be feasible to track synaptic vesicle dynamics in the isolated mouse rod bipolar cell using time-resolved capacitance measurements. The results demonstrate that membrane depolarization triggered an increase in membrane capacitance that was Ca(2+) dependent and restricted to the synaptic compartment, consistent with exocytosis. The amplitude of the capacitance response recorded from the easily accessible soma of an intact mouse rod bipolar cell was identical to that recorded directly from the small synaptic terminal, suggesting that in the carefully selected cohort of cells presented here, axonal resistance was not a significant barrier to current flow. This supposition was supported by the analysis of passive membrane properties and a comparison of membrane capacitance measurements in cells with and without synaptic terminals and reinforced by the lack of an effect of sine-wave frequency (200-1,600 Hz) on the measured capacitance increase. The magnitude of the capacitance response increased with Ca(2+) entry until a plateau was reached at a spatially averaged intraterminal calcium of about 600 nM. We interpret this plateau, nominally 30 fF, as corresponding to a releasable pool of synaptic vesicles. The robustness of this measure suggests that capacitance measurements may be used in the mouse rod bipolar cell to compare pool size across treatment conditions.

Modulation of Ca(2+)-activated K+ currents and Ca(2+)-dependent action potentials by exocytosis in goldfish bipolar cell terminals

Retinal bipolar cells convey light-evoked potentials from photoreceptors to ganglion cells and mediate the initial stages of visual signal processing. They do not fire Na(+)-dependent action potentials (APs) but the Mb1 class of goldfish bipolar cell exhibits Ca(2+)-dependent APs and regenerative potentials that originate in the axon terminal. I have examined the properties of Ca(2+)-dependent APs in isolated bipolar-cell terminals in goldfish retinal slices. All recorded terminals fired spontaneous or evoked APs at frequencies of up to 15 Hz. When an AP waveform was used as a voltage stimulus, exocytosis was evoked by single APs, maintained throughout AP trains and modulated by AP frequency. Furthermore, feedback inhibition of the Ca2+ current (I(Ca)) by released vesicular protons reduced depression of exocytosis during AP trains. In the absence of K+ current inhibition, step depolarizations and AP waveforms evoked a rapidly activated outward current that was dependent on Ca2+ influx I(K(Ca). I therefore investigated whether proton-mediated feedback inhibition of I(Ca) affected the activation of I(K(Ca)). A transient inhibition of I(K(Ca)) was observed that was dependent on exocytosis, blocked by high-pH extracellular buffer, of similar magnitude to inhibition of I(Ca) but occurred with a delay of 2.7 ms. In addition, the amplitude of APs evoked under current clamp was inhibited by the action of vesicular protons released by the APs. Protons released via exocytosis may therefore be a significant modulator of Ca(2+)-dependent currents and regenerative potentials in bipolar-cell terminals.

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.

Form and Function of on-off Amacrine Cells in the Amphibian Retina

on-off amacrine cells were studied with whole cell recording techniques and intracellular staining methods using intact retina-eyecup preparations of the tiger salamander ( Ambystoma tigrinum) and the mudpuppy ( Necturus maculosus). Morphological characterization of these cells included three-dimensional reconstruction methods based on serial optical sections obtained with a confocal microscope. Some cells had their detailed morphology digitized with a computer-assisted tracing system and converted to compartmental models for computer simulations. The dendrites of on-off amacrine cells have spines and numerous varicosities. Physiological recordings confirmed that on-off amacrine cells generate both large- and small-amplitude impulses attributed, respectively, to somatic and dendritic generation sites. Using a multichannel model for impulse generation, computer simulations were carried out to evaluate how impulses are likely to propagate throughout these structures. We conclude that the on-off amacrine cell is organized with multifocal dendritic impulse generating sites and that both dendritic and somatic impulse activity contribute to the functional repertoire of these interneurons: locally generated dendritic impulses can provide regional activation, while somatic impulse activity results in rapid activation of the entire dendritic tree.

Retinal bipolar cell types differ in their inventory of ion channels

Bipolar cells were recorded in rat retinal slices to study the distribution of hyperpolarization-activated and cyclic nucleotide-gated (HCN) channels. Patch-clamp whole cell measurements were combined with intracellular filling and recorded cells were morphologically identified. HCN channel isoforms HCN1-4 are differentially expressed in bipolar cells. Each bipolar cell type has a characteristic inventory of HCN channels. The combination of HCN channel currents and other voltage-gated currents can be used as a kind of “finger print” to electrophysiologically identify and classify bipolar cell types. Using this approach of combined electrophysiological and morphological classification we could identify a new ON-cone bipolar cell type.

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.

Photoreceptor encoding of supersaturating light stimuli in salamander retina

In the dark-adapted salamander retina, spikes could be elicited from rods under normal physiological conditions. Spike activity was observed in rods during the recovery phase of the response to saturating light. These action potentials were calcium spikes, blocked by cadmium and L-type calcium channel blockers. In response to light stimuli that saturate the rod peak response, calcium action potentials occurred with a delay that depended on light intensity, with stronger light increasing spike latency. Therefore, these spikes encode rod visual information at light intensities beyond rod saturation. Postsynaptic currents of similar time course were observed in second and third order neurones. Since rods exposed to brighter light stimuli produced more delayed spike activity, these signals might contribute to negative afterimages.

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

The flow of information across the retina is controlled by reciprocal synapses between bipolar cell terminals and amacrine cells. However, the synaptic delays and properties of plasticity at these synapses are not known. Here we report that glutamate release from goldfish Mb-type bipolar cell terminals can trigger fast (delay of 2-3 ms) and transient GABA(A) IPSCs and a much slower and more sustained GABA(C) feedback. Synaptically released glutamate activated mGluR1 receptors on amacrine cells and, depending on the strength of presynaptic activity, potentiated subsequent feedback. This poststimulus enhancement of GABAergic feedback lasted for up to 10 min. This form of mGluR1-mediated long-term synaptic plasticity may provide retinal reciprocal synapses with adaptive capabilities.

Heterogeneous expression of voltage-dependent Na+ and K+ channels in mammalian retinal bipolar cells

Retinal bipolar cells show heterogeneous expression of voltage-dependent Na+ and K+ currents. We used whole-cell patch-clamp recordings to investigate the possible roles of these currents in the response properties of bipolar cells in rats. Isolated bipolar cells showed robust spontaneous regenerative activity, but the regenerative potential of rod bipolar cells reached a more depolarized level than that of cone bipolar cells. In both isolated cells and cells in retinal slices, the membrane depolarization evoked by current injection was apparently capped. The evoked membrane potential was again more depolarized in rod bipolar cells than in cone bipolar cells. Application of tetraethylammonium and 4-aminopyridine shifted the spontaneous regenerative potential as well as the evoked potential to a more depolarized level. In addition, a subclass of cone bipolar cells showed a prominent spike in the initial phase of the voltage response when the cells were depolarized from a relatively negative membrane potential. The spike was mediated mainly by tetrodotoxin-sensitive Na+ current. The presence of the spike sped up the response kinetics and enhanced the peak membrane potential. Results of this study raise the possibility that voltage-dependent K+ currents may play a role in defining different membrane operating ranges of rod and cone bipolar cells and that voltage-dependent Na+ currents may enhance the response kinetics and amplitude of certain cone bipolar cells.

Cytosolic Cl- ions in the regulation of secretory and endocytotic activity in melanotrophs from mouse pituitary tissue slices

Cl- ions are known regulators of Ca2+ -dependent secretory activity in many endocrine cells. The suggested mechanisms of Cl- action involve the modulation of GTP-binding proteins, voltage-activated calcium channels or maturation of secretory vesicles. We examined the role of cytosolic Cl- ([Cl-]i) and Cl- currents in the regulation of secretory activity in mouse melanotrophs from fresh pituitary tissue slices by using the whole-cell patch-clamp. We confirmed that elevated [Cl-]i augments Ca2- -dependent exocytosis and showed that Cl- acts on secretory vesicle maturation. The latter process was abolished by a V-type H- -ATPase blocker (bafilomycin), intracellular 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS), a Cl- channel blocker, and tolbutamide, a sulphonylurea implicated in secretory vesicle maturation. In a small subset of cells, block of plasmalemmal Cl- current by DIDS reversibly enhanced endocytosis. The direct activation of G-proteins by GTP-gamma-S, a non-hydrolysable GTP analogue, did not restore the impaired secretion observed in low [Cl-]i conditions. The amplitude of voltage-activated calcium currents was unaffected by the [Cl-]i. Furthermore, two Cl- -permeable channels, calcium-activated Cl- channels and GABAA receptors, appeared as major regulators of intracellular Cl- homeostasis. In conclusion, the predominant underlying mechanism of Cl- action is mediated by intracellular Cl- fluxes during vesicle maturation, rather than activation of G-proteins or modulation of voltage-activated Ca2+channels.

Synaptic cleft acidification and modulation of short-term depression by exocytosed protons in retinal bipolar cells

The release of vesicular protons during exocytosis causes a feedback inhibition of Ca2+ channels in photoreceptor terminals; however, the effect of this inhibition on subsequent exocytosis has not been studied. Here we show that a similar L-type Ca2+ channel inhibition occurs in bipolar cell terminals in slices of goldfish retina, and we investigate the effect that this has on subsequent exocytosis with membrane capacitance measurements. We find that transient Ca2+ current inhibition is correlated with exocytosis and modulated by the concentration of extracellular pH buffer. Ca2+ current inhibition is negligible in acutely dissociated terminals, demonstrating the importance of an intact synaptic cleft. The sensitivity of bipolar cell Ca2+ currents to extracellular pH was assessed: channel conductance is reduced and activation is shifted to more positive potentials by acidification. The effect of Ca2+ current inhibition on subsequent exocytosis was investigated by measuring paired-pulse depression. Under conditions in which there is a large amount of inhibition of Ca2+ influx, the degree of paired-pulse depression is significantly reduced. Finally, we show that under physiological (bicarbonate) buffering conditions, pronounced Ca2+ current inhibition occurs after exocytosis ( approximately 60% peak inhibition), which can decrease subsequent exocytosis during single depolarizations. We estimate that exocytosis is accompanied by a transient change in synaptic cleft pH from 7.5 to approximately 6.9. We suggest that this effect serves as an activity-dependent modulator of exocytosis at ribbon-type synapses where a large and compact coterie of vesicles can fuse at each active zone.

Mammalian Retinal Bipolar Cells Express Inwardly Rectifying K+ Currents (IKir) With a Different Distribution Than That of Ih

Retinal bipolar cells comprise multiple subtypes that are well known for the diversity of their physiological properties. We investigated the properties and functional roles of the hyperpolarization-activated currents in mammalian retinal bipolar cells using whole cell patch-clamp recording techniques. We report that bipolar cells express inwardly rectifying K+ currents ( IKir) in addition to the hyperpolarization-activated cationic currents ( Ih) previously reported. Furthermore, these two currents are differentially expressed among different subtypes of bipolar cells. One group of cone bipolar cells in particular displayed mainly IKir. A second group of cone bipolar cells displayed both currents but with a much larger Ih. Rod bipolar cells, on the other hand, showed primarily Ih. Moreover, we showed that IKir and Ih differentially influence the voltage responses of bipolar cells: Ih facilitates and/or accelerates the membrane potential rebound, whereas IKir counteracts or prevents such rebound. The findings of the expression of IKir and the differential expression of Ih and IKir in bipolar cells may provide new insights into an understanding of the physiological properties of bipolar cells.

Cav1.4alpha1 subunits can form slowly inactivating dihydropyridine-sensitive L-type Ca2+ channels lacking Ca2+-dependent inactivation

The neuronal L-type calcium channels (LTCCs) Cav1.2alpha1 and Cav1.3alpha1 are functionally distinct. Cav1.3alpha1 activates at lower voltages and inactivates more slowly than Cav1.2alpha1, making it suitable to support sustained L-type Ca2+ inward currents (ICa,L) and serve in pacemaker functions. We compared the biophysical and pharmacological properties of human retinal Cav1.4alpha1 using the whole-cell patch-clamp technique after heterologous expression in tsA-201 cells with other L-type alpha1 subunits. Cav1.4alpha1-mediated inward Ba2+ currents (IBa) required the coexpression of alpha2delta1 and beta3 or beta2a subunits and were detected in a lower proportion of transfected cells than Cav1.3alpha1. IBa activated at more negative voltages (5% activation threshold; -39mV; 15 mm Ba2+) than Cav1.2alpha1 and slightly more positive than Cav1.3alpha1. Voltage-dependent inactivation of IBa was slower than for Cav1.2alpha1 and Cav1.3alpha1( approximately 50% inactivation after 5 sec; alpha2delta1 + beta3 coexpression). Inactivation was not increased with Ca2+ as the charge carrier, indicating the absence of Ca2+-dependent inactivation. Cav1.4alpha1 exhibited voltage-dependent, G-protein-independent facilitation by strong depolarizing pulses. The dihydropyridine (DHP)-antagonist isradipine blocked Cav1.4alpha1 with approximately 15-fold lower sensitivity than Cav1.2alpha1 and in a voltage-dependent manner. Strong stimulation by the DHP BayK 8644 was found despite the substitution of an otherwise L-type channel-specific tyrosine residue in position 1414 (repeat IVS6) by a phenylalanine. Cav1.4alpha1 + alpha2delta1 + beta channel complexes can form LTCCs with intermediate DHP antagonist sensitivity lacking Ca2+-dependent inactivation. Their biophysical properties should enable them to contribute to sustained ICa,L at negative potentials, such as required for tonic neurotransmitter release in sensory cells and plateau potentials in spiking neurons.

Synaptic activation of presynaptic glutamate transporter currents in nerve terminals

Glutamate uptake by high-affinity transporters is responsible for limiting the activation of postsynaptic receptors and maintaining low levels of ambient glutamate. The reuptake process generates membrane currents, which can be activated by synaptically released glutamate in glial cells and some postsynaptic neurons. However, less is known about presynaptic transporter currents because the small size of synaptic boutons precludes direct recordings. Here, we have recorded from two giant nerve terminals: bipolar cell synaptic terminals in goldfish retina and the calyx of Held in rat auditory brainstem. Exocytosis was evoked by brief depolarizations and measured as an increase in membrane capacitance. In isolated bipolar cell terminals, exocytosis was associated with an anion (NO3- or Cl-) current. The current peaked 2.8 msec after the start of the depolarization and decayed with a mean time constant of 8.5 msec. It was inhibited by the nontransportable glutamate transporter antagonist sc-threo-beta-benzyloxyaspartate (TBOA) but was insensitive to the GLT1/EAAT2 subtype-selective antagonist dihydrokainate and was affected by extracellular pH buffering. A TBOA-sensitive anion current was also evoked by application of exogenous glutamate to bipolar cell terminals. The large single-channel conductance, derived from noise analysis, and previous immunolocalization studies suggest that synaptically released glutamate activates EAAT5-type transporters in bipolar cell terminals. In contrast, neither exocytosis nor exogenous glutamate evoked a transporter current in the calyx of Held. Glutamate transporter currents with rapid kinetics are therefore identified and characterized in bipolar cell terminals, providing a valuable system for investigating the function and modulation of presynaptic glutamate transporters.

Ionic mechanisms mediating oscillatory membrane potentials in wide-field retinal amacrine cells

Particular types of amacrine cells of the vertebrate retina show oscillatory membrane potentials (OMPs) in response to light stimulation. Historically it has been thought the oscillations arose as a result of circuit properties. In a previous study we found that in some amacrine cells, the ability to oscillate was an intrinsic property of the cell. Here we characterized the ionic mechanisms responsible for the oscillations in wide-field amacrine cells (WFACs) in an effort to better understand the functional properties of the cell. The OMPs were found to be calcium (Ca2+) dependent; blocking voltage-gated Ca2+ channels eliminated the oscillations, whereas elevating extracellular Ca2+ enhanced them. Strong intracellular Ca2+ buffering (10 mM EGTA or bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid) eliminated any attenuation in the OMPs as well as a Ca2+-dependent inactivation of the voltage-gated Ca2+ channels. Pharmacological and immunohistochemical characterization revealed that WFACs express L- and N-type voltage-sensitive Ca2+ channels. Block of the L-type channels eliminated the OMPs, but omega-conotoxin GVIA did not, suggesting a different function for the N-type channels. The L-type channels in WFACs are functionally coupled to a set of calcium-dependent potassium (K(Ca)) channels to mediate OMPs. The initiation of OMPs depended on penitrem-A-sensitive (BK) K(Ca) channels, whereas their duration is under apamin-sensitive (SK) K(Ca) channel control. The Ca2+ current is essential to evoke the OMPs and triggering the K(Ca) currents, which here act as resonant currents, enhances the resonance as an amplifying current, influences the filtering characteristics of the cell membrane, and attenuates the OMPs via CDI of the L-type Ca2+ channel.

Stimulation of sodium pump restores membrane potential to neurons excited by glutamate in zebrafish distal retina

Glutamate either depolarizes or hyperpolarizes retinal neurons. Those are the initial and primary effects. Using a voltage probe (oxonol, DiBaC4 (5)) to study dissociated zebrafish retinal neurons, we find a secondary, longer-term effect: a post-excitatory restoration of membrane potential, termed after-hyperpolarization (AHP). AHP occurs only in neurons that are depolarized by glutamate and typically peaks about 5 min after glutamate application. AHP is seen in dissociated horizontal cells (HCs) and hyperpolarizing, or OFF type, bipolar cells (HBCs). These cells commonly respond with only an AHP component. AHP never occurs in depolarizing, or ON type, bipolar cells (DBCs), which are cell types hyperpolarized by glutamate. AHP is blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). It is evoked by kainate, AMPA and the AMPA-selective agonist (S)-5-fluorowillardiine, but not by NMDA, D-aspartate, the kainate-selective agonist SYM 2081 or by DL-2-amino-4-phosphonobutyric acid (DL-AP4). Cells with exclusively AHP responses are tonically depolarized. Resting potentials can be restored by nifedipine, suggesting a tonic, depolarizing action of L-type Ca2+ channels. However AHP is not blocked by nifedipine and is insensitive to [Cl-]o. AHP is blocked by Li+o substitution for Na+o and by ouabain. A mechanism is proposed in which Na+ entering through ionotropic AMPA channels stimulates Na+,K+-ATPase, which, by electrogenic action, restores membrane potential, generating the AHP response. Patterns of ATPase immunoreactivity support localization in the outer plexiform layer (OPL) as cone pedicles, HCs and BCs were positively labelled. Labelling was weaker in the inner plexiform layer (IPL) than in nuclear layers, though two IPL bands of immunoreactive BC terminals could be discerned, one in sublamina a and the other in sublamina b. Persistent stimulation of distal retina by photoreceptor glutamate may induce increased expression and activity of Na+,K+-ATPase, with a consequent impact on distal glutamate responses.

Timing of quantal release from the retinal bipolar terminal is regulated by a feedback circuit

In isolation, a presynaptic terminal generally releases quanta according to Poisson statistics, but in a circuit its release statistics might be shaped by synaptic interactions. We monitored quantal glutamate release from retinal bipolar cell terminals (which receive GABA-ergic feedback from amacrine cells) by recording spontaneous EPSCs (sEPSCs) in their postsynaptic amacrine and ganglion cells. In about one-third of these cells, sEPSCs were temporally correlated, arriving in brief bursts (10-55 ms) more often than expected from a Poisson process. Correlations were suppressed by antagonizing the GABA(C) receptor (expressed on bipolar terminals), and correlations were induced by raising extracellular calcium or osmolarity. Simulations of the feedback circuit produced "bursty" release when the bipolar cell escaped intermittently from inhibition. Correlations of similar duration were present in the light-evoked sEPSCs and spike trains of sluggish-type ganglion cells. These correlations were suppressed by antagonizing GABA(C) receptors, indicating that glutamate bursts from bipolar terminals induce spike bursts in ganglion cells.

Retinal processing: smaller babies thrown out with bathwater

Rod bipolar cells in the mammalian retina receive synaptic input from many noisy rod photoreceptors. When photons are scarce, linear addition of inputs would swamp signals with noise. A nonlinearity at the synapse optimizes the signal to noise ratio.

Regulation of the retinal bipolar cell mGluR6 pathway by calcineurin

Glutamate produces a hyperpolarizing postsynaptic potential in ON bipolar cells by binding to the metabotropic receptor mGluR6 and subsequently closing a cation-selective channel. It has been proposed that Ca(2+) influx through the cation channel triggers a depression of the synaptic potential. Here we report that this Ca(2+)-mediated depression requires activation of calcineurin, a Ca(2+)/calmodulin-regulated phosphatase. We measured glutamate-evoked currents (I(glu)) with whole cell recordings of ON bipolar cells in light-adapted retinal slices. Depression of I(glu) by Ca(2+) was prevented by inhibitors of calcineurin or by tightly buffering Ca(2+) with bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA). However, when cells were dialyzed with BAPTA and a Ca(2+)-independent form of calcineurin (CaN420), depression of I(glu) was restored. Similarly, CaN420 induced depression of I(glu) during continuous glutamate application, a protocol that ordinarily prevents depression. Analysis of changes in the amplitude of the cation-selective current (I(cat)) of cells that were dialyzed with high Ca(2+) (1 microM), or with BAPTA and CaN420, indicates that Ca(2+) depresses I(glu) by reducing I(cat) and that calcineurin acts via the same mechanism. Ca(2+)-mediated depression of I(glu) was not found to involve CaMKII, as inhibitors of CaMKII did not prevent this depression nor did they affect the sensitivity of the response to small changes in the concentration of mGluR6 agonist. Our data suggest that Ca(2+) and calcineurin may play an adaptive role at the synapse between photoreceptor and ON bipolar cells, closing postsynaptic cation channels that are opened by a drop in synaptic glutamate levels during prolonged photoreceptor illumination.

Increase in the pool size of releasable synaptic vesicles by the activation of protein kinase C in goldfish retinal bipolar cells

Secretion from neurons and neuroendocrine cells is enhanced by the activation of protein kinase C (PKC) in various preparations. We have already reported that transmitter (glutamate) release from Mb1 bipolar cells in the goldfish retina is potentiated by the activation of PKC. However, it is not yet settled whether the potentiation is ascribed to the increase in the pool size of releasable synaptic vesicles or in release probability. In the present study, Ca2+ influx and exocytosis were simultaneously monitored by measuring the presynaptic Ca2+ current and membrane capacitance changes, respectively, in a terminal detached from the bipolar cell. The double pulse protocol was used to estimate separately the changes in the pool size and release probability. The activation of PKC by phorbol 12-myristate 13-acetate (PMA) specifically increased the pool size but not the release probability. PKC was activated by PMA even after the Ca2+ influx was blocked by Co2+. In bipolar cells the releasable pool can be divided into two components: one is small and rapidly exhausted, and the other is large and slowly exocytosed. To identify which component is responsible for the increase in the pool size, the effects of PMA and a PKC-specific inhibitor, bisindolylmaleimide I (BIS), on each component were examined. The slow component was selectively increased by PMA and reduced by BIS. Thus, we conclude that the activation of PKC in Mb1 bipolar cells potentiates glutamate release by increasing the pool size of the slow component.

Non-linear, high-gain and sustained-to-transient signal transmission from rods to amacrine cells in dark-adapted retina of Ambystoma

In darkness, On-Off amacrine cells (ACs) of the tiger salamander retina exhibited large spontaneous transient depolarizing potentials (sTDPs) with average peak amplitude of 5.05 +/- 2.5 mV and average frequency of 0.42 +/- 0.25 s(-1). Under voltage-clamp conditions the cell displayed large spontaneous postsynaptic currents (sPSCs) with average peak amplitude of 98 +/- 39 pA and average frequency of 0.45 +/- 0.22 s(-1). To a light step, ACs gave rise to a transient 'On' response at the light onset and a transient 'Off' response at light offset, followed by a train of TDPs ('After' response). Near the response threshold (0.3 activated rhodopsin molecules per rod per second), light-evoked TDPs (leTDPs) of similar amplitude and kinetics as the large sTDPs observed in darkness were seen, and about half of these leTDPs elicited a regenerative potential (RP). Brighter light steps gave rise to more leTDPs and higher rates of RPs in the On, Off and After responses. Within the linear response range of the rods, the AC response was non-linear, with the highest gain (676 +/- 429) near the dark potential. The amplitude of Off responses increased with the duration of the light step, and ACs may use this to encode speeds of moving stimuli: the faster the light object moves, the smaller the AC Off response. Moreover, the number of leTDPs in the AC After response increased with light intensity, and the onset of the After response coincides with bipolar cell tail response recovery. One possible origin of the large sTDPs and leTDPs is the spontaneous and depolarization-induced regenerative calcium potentials (RCaPs) in bipolar cell synaptic terminals. RCaPs in bipolar cell synaptic terminals cause transient glutamate release that results in the sTDPs in darkness, and leTDPs in On, Off and After responses in ACs.

Modulation of excitatory synaptic transmission by GABA(C) receptor-mediated feedback in the mouse inner retina

In many vertebrate CNS synapses, the neurotransmitter glutamate activates postsynaptic non-N-methyl-D-aspartate (NMDA) and NMDA receptors. Since their biophysical properties are quite different, the time course of excitatory postsynaptic currents (EPSCs) depends largely on the relative contribution of their activation. To investigate whether the activation of the two receptor subtypes is affected by the synaptic interaction in the inner plexiform layer (IPL) of the mouse retina, we analyzed the properties of the light-evoked responses of ON-cone bipolar cells and ON-transient amacrine cells in a retinal slice preparation. ON-transient amacrine cells were whole cell voltage-clamped, and the glutamatergic synaptic input from bipolar cells was isolated by a cocktail of pharmacological agents (bicuculline, strychnine, curare, and atropine). Direct puff application of NMDA revealed the presence of functional NMDA receptors. However, the light-evoked EPSC was not significantly affected by D(-)-2-amino-5-phosphonopentanoic acid (D-AP5), but suppressed by 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide (NBQX) or 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine hydrochloride (GYKI 52466). These results indicate that the light-evoked EPSC is mediated mainly by AMPA receptors under this condition. Since bipolar cells have GABA(C) receptors at their terminals, it has been suggested that bipolar cells receive feedback inhibition from amacrine cells. Application of (1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA), a specific blocker of GABA(C) receptors, suppressed both the GABA-induced current and the light-evoked feedback inhibition observed in ON-cone bipolar cells and enhanced the light-evoked EPSC of ON-transient amacrine cells. In the presence of TPMPA, the light-evoked EPSC of amacrine cells was composed of AMPA and NMDA receptor-mediated components. Our results suggest that photoresponses of ON-transient amacrine cells in the mouse retina are modified by the activation of presynaptic GABA(C) receptors, which may control the extent of glutamate spillover.

Voltage-dependent sodium channels are expressed in nonspiking retinal bipolar neurons

Retinal bipolar neurons transmit visual information by means of graded synaptic potentials that spread to the synaptic terminal without sodium-dependent action potentials. Although action potentials are not involved, voltage-dependent sodium channels may enhance subthreshold depolarizing potentials in the dendrites and soma of bipolar cells, as they do in other CNS neurons. We report here that voltage-dependent sodium currents are observed in a subset of bipolar neurons from goldfish retina. Single-cell reverse transcriptase-PCR identified four different sodium channel alpha subunits in goldfish bipolar cells, putatively corresponding to the mammalian voltage-gated sodium channels Na(v)1.1, Na(v)1.2, Na(v)1.3, and Na(v)1.6. The amount of sodium current was largest in cells with smaller synaptic terminals, which probably represent cone bipolar cells. Localization of sodium channel immunoreactivity in goldfish retina confirmed the expression of voltage-gated sodium channels in cone bipolar cells of both ON and OFF types. Both immunocytochemical and physiological evidence suggests that the sodium channels are localized to the soma and dendrites where they may play a role in transmission of synaptic signals, particularly in the long, thin dendrites of cone bipolar cells.

Na(+) action potentials in human photoreceptors

Mammalian photoreceptors are hyperpolarized by a light stimulus and are commonly thought to be nonspiking neurons. We used the whole-cell patch-clamp technique on surgically excised human retina to examine whether human photoreceptors can elicit action potentials. We discovered that human rod photoreceptors express voltage-gated Na(+) channels, and generate Na(+) action potentials, in response to membrane depolarization from membrane potentials of -60 or -70 mV. Na(+) spikes in human rods were elicited at the termination of a light response that hyperpolarized the potential well below -50 mV. This served to amplify the release of a neurotransmitter when a bright light is turned off, and thus selectively amplify the off response to the light signal.

Bipolar cells use kainate and AMPA receptors to filter visual information into separate channels

Unlike cone photoreceptors, whose light responses have a uniform time course, retinal ganglion cells are tuned to respond to different temporal components in a changing visual scene. The signals in a mammalian cone flow to three to five morphologically distinct "OFF" bipolar cells at a sign-conserving, glutamatergic synapse. By recording simultaneously from pairs of synaptically connected cones and OFF bipolar cells, I now show that each morphological type of OFF bipolar cell receives its signal through a different AMPA or kainate receptor. The characteristic rate at which each receptor recovers from desensitization divides the cone signal into temporal components. Temporal processing begins at the first synapse in the visual system.

Voltage-dependent Na(+) currents in mammalian retinal cone bipolar cells

Voltage-dependent Na(+) channels are usually expressed in neurons that use spikes as a means of signal coding. Retinal bipolar cells are commonly thought to be nonspiking neurons, a category of neurons in the CNS that uses graded potential for signal transmission. Here we report for the first time voltage-dependent Na(+) currents in acutely isolated mammalian retinal bipolar cells with whole cell patch-clamp recordings. Na(+) currents were observed in approximately 45% of recorded cone bipolar cells but not in rod bipolar cells. Both ON and OFF cone bipolar cells were found to express Na(+) channels. The Na(+) currents were activated at membrane potentials around -50 to -40 mV and reached their peak around -20 to 0 mV. The half-maximal activation and steady-state inactivation potentials were -24.7 and -68.0 mV, respectively. The time course of recovery from inactivation could be fitted by two time constants of 6.2 and 81 ms. The amplitude of the Na(+) currents ranged from a few to >300 pA with the current density in some cells close or comparable to that of retinal third neurons. In current-clamp recordings, Na(+)-dependent action potentials were evoked in Na(+)-current-bearing bipolar cells by current injections. These findings raise the possibility that voltage-dependent Na(+) currents may play a role in bipolar cell function.