Rectification of cGMP-activated channels induced by phosphorylation in dogfish retinal 'on' bipolar cells

Activation mechanism of a neuromodulator-gated pacemaker ionic current

The neuromodulator-gated current (I_MI) found in the crab stomatogastric ganglion is activated by neuromodulators that are essential to induce the rhythmic activity of the pyloric network in this system. One of these neuromodulators is also known to control the correlated expression of voltage-gated ionic currents in pyloric neurons, as well as synaptic plasticity and strength. Thus understanding the mechanism by which neuromodulator receptors activate I_MI should provide insights not only into how oscillations are initiated but also into how other processes, and currents not directly activated by them, are regulated. To determine what specific signaling molecules are implicated in this process, we used a battery of agonists and antagonists of common signal transduction pathways. We found that the G protein inhibitor GDPβS and the G protein activator GTPγS significantly affect I_MI amplitude, suggesting that its activation is mediated by G proteins. Interestingly, when using the more specific G protein blocker pertussis toxin, we observed the expected inhibition of I_MI amplitude but, unexpectedly, in a calcium-dependent fashion. We also found that antagonists of calcium- and calmodulin-associated signaling significantly reduce I_MI amplitude. In contrast, we found little evidence for the role of cyclic nucleotide signaling, phospholipase C (PLC), or kinases and phosphatases, except two calmodulin-dependent kinases. In sum, these results suggest that proctolin-induced I_MI is mediated by a G protein whose pertussis toxin sensitivity is altered by external calcium concentration and appears to depend on intracellular calcium, calmodulin, and calmodulin-activated kinases. In contrast, we found no support for I_MI being mediated by PLC signaling or cyclic nucleotides.NEW & NOTEWORTHY Neuronal rhythmic activity is generated by either network-based or cell-autonomous mechanisms. In the pyloric network of decapod crustaceans, the activation of a neuromodulator-gated pacemaker current is crucial for the generation of rhythmic activity. This current is activated by several neuromodulators, including peptides and acetylcholine, presumably via metabotropic receptors. We have previously demonstrated a novel extracellular calcium-sensitive voltage-dependence mechanism of this current. We presently report that the activation mechanism depends on intracellular and extracellular calcium-sensitive components.

Voltage Dependence of a Neuromodulator-Activated Ionic Current

The neuromodulatory inward current (IMI) generated by crab Cancer borealis stomatogastric ganglion neurons is an inward current whose voltage dependence has been shown to be crucial in the activation of oscillatory activity of the pyloric network of this system. It has been previously shown that IMI loses its voltage dependence in conditions of low extracellular calcium, but that this effect appears to be regulated by intracellular calmodulin. Voltage dependence is only rarely regulated by intracellular signaling mechanisms. Here we address the hypothesis that the voltage dependence of IMI is mediated by intracellular signaling pathways activated by extracellular calcium. We demonstrate that calmodulin inhibitors and a ryanodine antagonist can reduce IMI voltage dependence in normal Ca(2+), but that, in conditions of low Ca(2+), calmodulin activators do not restore IMI voltage dependence. Further, we show evidence that CaMKII alters IMI voltage dependence. These results suggest that calmodulin is necessary but not sufficient for IMI voltage dependence. We therefore hypothesize that the Ca(2+)/calmodulin requirement for IMI voltage dependence is due to an active sensing of extracellular calcium by a GPCR family calcium-sensing receptor (CaSR) and that the reduction in IMI voltage dependence by a calmodulin inhibitor is due to CaSR endocytosis. Supporting this, preincubation with an endocytosis inhibitor prevented W7 (N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride)-induced loss of IMI voltage dependence, and a CaSR antagonist reduced IMI voltage dependence. Additionally, myosin light chain kinase, which is known to act downstream of the CaSR, seems to play a role in regulating IMI voltage dependence. Finally, a Gβγ-subunit inhibitor also affects IMI voltage dependence, in support of the hypothesis that this process is regulated by a G-protein-coupled CaSR.

Regulation of ON bipolar cell activity

Synaptic transmission from photoreceptors to all types of ON bipolar cells is primarily mediated by the mGluR6 receptor. This receptor, which is apparently expressed uniquely in the nervous system by ON bipolar cells, couples negatively to a nonselective cation channel. This arrangement results in a sign reversal at photoreceptor/ON bipolar cell synapse, which is necessary in order to establish parallel ON and OFF pathways in the retina. The synapse is an important target for second messenger molecules that are known to modulate synaptic transmission elsewhere in the nervous system, second messengers that act on a time scale ranging from milliseconds to minutes. This review focuses on two of these molecules, Ca2+ and cGMP, summarizing our current knowledge of how they modulate gain at the photoreceptor/ON bipolar cell synapse, as well as their proposed sites of action within the mGluR6 cascade. The implications of plasticity at this synapse for retinal function will also be examined.

Distribution of soluble guanylyl cyclase in rat retina

The nitric oxide (NO)-cGMP pathway is implicated in modulation of visual information processing in the retina. Despite numerous functional studies of this pathway, information about the retinal distribution of the major downstream effector of NO, soluble guanylyl cyclase (sGC), is very limited. In the present work, we have used immunohistochemistry and multiple labeling to determine the distribution of sGC in rat retina. sGC was present at high levels in inner retina but barely detectable in outer retina. Photoreceptors and horizontal cells, as well as Müller cells, were immunonegative, whereas retinal ganglion cells exhibited moderate staining for sGC. Strong immunostaining was found in subpopulations of bipolar and amacrine cells, but staining was weak in rod bipolar cells, and AII amacrine cells were immunonegative. Double labeling of sGC with neuronal nitric oxide synthase showed that the two proteins are generally located in adjacent puncta in inner plexiform layer, implying paracrine interactions. Our results suggest that the NO-cGMP pathway modulates the neural circuitry in inner retina, preferentially within the cone pathway.

Efficiency of synaptic transmission of single-photon events from rod photoreceptor to rod bipolar dendrite

A rod transmits absorption of a single photon by what appears to be a small reduction in the small number of quanta of neurotransmitter (Q(count)) that it releases within the integration period ( approximately 0.1 s) of a rod bipolar dendrite. Due to the quantal and stochastic nature of release, discrete distributions of Q(count) for darkness versus one isomerization of rhodopsin (R*) overlap. We suggested that release must be regular to narrow these distributions, reduce overlap, reduce the rate of false positives, and increase transmission efficiency (the fraction of R* events that are identified as light). Unsurprisingly, higher quantal release rates (Q(rates)) yield higher efficiencies. Focusing here on the effect of small changes in Q(rate), we find that a slightly higher Q(rate) yields greatly reduced efficiency, due to a necessarily fixed quantal-count threshold. To stabilize efficiency in the face of drift in Q(rate), the dendrite needs to regulate the biochemical realization of its quantal-count threshold with respect to its Q(count). These considerations reveal the mathematical role of calcium-based negative feedback and suggest a helpful role for spontaneous R*. In addition, to stabilize efficiency in the face of drift in degree of regularity, efficiency should be approximately 50%, similar to measurements.

Modulation by brain natriuretic peptide of GABA receptors on rat retinal ON-type bipolar cells

Natriuretic peptides (NPs) may work as neuromodulators through their associated receptors [NP receptors (NPRs)]. By immunocytochemistry, we showed that NPR-A and NPR-B were expressed abundantly on both ON-type and OFF-type bipolar cells (BCs) in rat retina, including the dendrites, somata, and axon terminals. Whole-cell recordings made from isolated ON-type BCs further showed that brain natriuretic peptide (BNP) suppressed GABAA receptor-, but not GABAC receptor-, mediated currents of the BCs, which was blocked by the NPR-A antagonist anantin. The NPR-C agonist c-ANF [des(Gln18, Ser19, Gln20, Leu21, Gly22)ANF(4-23)-NH2] did not suppress GABAA currents. The BNP effect on GABAA currents was abolished with preincubation with the pGC-A/B antagonist HS-142-1 but mimicked by application of 8-bromoguanosine-3',5'-cyclomonophosphate. These results suggest that elevated levels of intracellular cGMP caused by activation of NPR-A may mediate the BNP effect. Internal infusion of the cGMP-dependent protein kinase G (PKG) inhibitor KT5823 essentially blocked the BNP-induced reduction of GABAA currents. Moreover, calcium imaging showed that BNP caused a significant elevation of intracellular calcium that could be caused by increased calcium release from intracellular stores by PKG. The BNP effect was blocked by the ryanodine receptor modulators caffeine, ryanodine, and ruthenium red but not by the IP3 receptor antagonists heparin and xestospongin-C. Furthermore, the BNP effect was abolished after application of the blocker of endoplasmic reticulum Ca2+-ATPase thapsigargin and greatly reduced by the calmodulin inhibitors W-7 and calmidazolium. We therefore conclude that the increased calcium release from ryanodine-sensitive calcium stores by BNP may be responsible for the BNP-caused GABAA response suppression in ON-type BCs through stimulating calmodulin.

Desensitization of the mGluR6 transduction current in tiger salamander On bipolar cells

Light depolarizes retinal On bipolar cells, opening the cation-selective channels that are responsible for producing the synaptic current. In this study, the basic features of light-induced signals were mimicked by bathing slices of salamander retina with an agonist for the mGluR6 receptor that is expressed on the dendrites of On cells, and then displacing the agonist with the mGluR6 antagonist (RS)-a-cyclopropyl-4-phosphonophenylglycine (CPPG). The transduction current that is activated by this protocol rapidly shuts off, or desensitizes. Desensitization was highly correlated with the concentration and the type of Ca2+ buffer that was dialysed into the cell: When Ca2+ buffering was minimized by dialysing cells with 0.5 mM EGTA, the steady-state response was reduced to approximately 40% of the peak response. Buffering with 10 mM EGTA reduced desensitization, while BAPTA completely eliminated it. Removing external Ca2+ also prevented desensitization, suggesting that entry of Ca2+ through the transduction channel provides the trigger. The time course of desensitization was measured by using a voltage jump protocol to rapidly increase Ca2+ influx, and could be fitted with a single time constant on the order of 1 s, in good agreement with previously published rates of desensitization to steps of light in this species. It is proposed that Ca(2+)-dependent shut-off of the On bipolar cell transduction current may contribute to the conversion of sustained to transient light responses that predominate in the inner retina.

Subtype-specific coupling with ADP-ribosyl cyclase of metabotropic glutamate receptors in retina, cervical superior ganglion and NG108-15 cells

Cyclic ADP-ribose (cADP-ribose) is a putative second messenger or modulator. However, the role of cADP-ribose in the downstream signals of the metabotropic glutamate receptors (mGluRs) is unclear. Here, we show that glutamate stimulates ADP-ribosyl cyclase activity in rat or mouse crude membranes of retina via group III mGluRs or in superior cervical ganglion via group I mGluRs. The retina of mGluR6-deficient mice showed no increase in the ADP-ribosyl cyclase level in response to glutamate. GTP enhanced the initial rate of basal and glutamate-stimulated cyclase activity. GTP-gamma-S also stimulated basal activity. To determine whether the coupling mode of mGluRs to ADP-ribosyl cyclase is a feature common to individual cloned mGluRs, we expressed each mGluR subtype in NG108-15 neuroblastoma x glioma hybrid cells. The glutamate-induced stimulation of the cyclase occurs preferentially in NG108-15 cells over-expressing mGluRs1, 3, 5, and 6. Cells expressing mGluR2 or mGluRs4 and 7 exhibit inhibition or no coupling, respectively. Glutamate-induced activation or inhibition of the cyclase activity was eliminated after pre-treatment with cholera or pertussis toxin, respectively. Thus, the subtype-specific coupling of mGluRs to ADP-ribosyl cyclase via G proteins suggests that some glutamate-evoked neuronal functions are mediated by cADP-ribose.

Metabotropic glutamate receptors in vertebrate retina

A striking feature in visual information processing is the fact that the primary signaling elements, the rods and the cones, are hyperpolarized and thus inhibited by light, the physiological stimulus. Light effectively shuts down neurotransmitter release by the photoreceptors onto the second-order retinal neurons. It has long been recognized that a sign-inverting synapse utilizing a specialized receptor is required to translate the inhibitory photoreceptor response into an excitatory signal suitable for transmission to the visual cortex. Although the first clues to the underlying mechanism were discovered in the 1970s, the actual receptor initiating the sign inversion in the ON bipolar cells was only identified in 1993. This receptor was found to belong to the family of metabotropic glutamate receptors (mGluRs) and is referred to as mGluR6. Subsequent studies have focused on the intracellular transduction pathway allowing mGluR6 to mediate a hyperpolarizing response to the neurotransmitter glutamate. The mGluR family of receptors comprises seven additional members, all of which are also found in retinal cells. Their function is to modulate rather than to transmit visual signals. In this brief overview, I describe the basic properties of mGluRs and summarize their roles in retinal signaling.

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.

Potentiation of 'on' bipolar cell flash responses by dim background light and cGMP in dogfish retinal slices

The high sensitivity of the vertebrate visual system results from amplification inherent in phototransduction in rods and from the amplification of rod signals on their synaptic transfer at the first synapse with 'on' bipolar cells. These cells possess a metabotropic glutamate receptor linked via a cGMP cascade to the control of cGMP-activated channels. In the study presented here, we show that very dim background light, isomerising only one rhodopsin in 1 out of 10 rods per second, potentiates 'on' bipolar cell responses to superimposed flashes. Responses to dim flashes, which were undetectable above the noise in the dark, were boosted above the increased noise level induced by the background. This potentiation could be reproduced by elevating cGMP, which increases with light, or by dialysing the cells with a non-hydrolysable cGMP analogue. Inhibition of tyrosine kinase activity also reproduced the effect and induced a speeding up of the rising phase of the flash response, similar to the action of dim background light. Conversely, inhibition of tyrosine phosphatase activity blocked the potentiation. These results suggest that cGMP promotes tyrosine-site dephosphorylation of 'on' bipolar cell cGMP-activated channels, resulting in a rise in the sensitivity to cGMP, as has recently been demonstrated for rod cGMP-activated channels. This constitutes a positive feedback mechanism such that as cGMP increases with light, the sensitivity of the channels to cGMP increases and boosts the signal above background noise. This mechanism would allow stochastic resonance to occur, facilitating single-photon detection when dark-adapted, and may therefore lead to improved discrimination.