Dopamine alters glutamate receptor desensitization in retinal horizontal cells of the perch (Perca fluviatilis)

Modulation of horizontal cell function by dopaminergic ligands in mammalian retina

Light responses of rabbit horizontal cell somata (HC) to flickering light stimuli recorded with sharp electrodes consist of a distinctive flicker component superimposed on a sustained hyperpolarisation. Activation of dopamine D1/D5 receptors depolarises HC dark membrane potential and suppresses the flicker component of responses to photopic stimuli without affecting the sustained hyperpolarising response component. Waveforms of responses to scotopic stimuli are preserved. Similar response modulation was observed in depolarising cells of the inner retina, suggesting that activation of D1/D5 receptors of HC causes modification of cone signal transmission to higher order neurons. The impact of dopamine D1/D5 receptor activation on the function of HC in the light stimulated retina is discussed.

Glutamate-evoked alterations of glial and neuronal cell morphology in the guinea pig retina

Neuronal activity is accompanied by transmembranous ion fluxes that cause cell volume changes. In whole mounts of the guinea pig retina, application of glutamate resulted in fast swelling of neuronal cell bodies in the ganglion cell layer (GCL) and the inner nuclear layer (INL) (by approximately 40%) and a concomitant decrease of the thickness of glial cell processes in the inner plexiform layer (IPL) (by approximately 40%) that was accompanied by an elongation of the glial cells, by a thickening of the whole retinal tissue, and by a shrinkage of the extracellular space (by approximately 18%). The half-maximal effect of glutamate was observed at approximately 250 mum, after approximately 4 min. The swelling was caused predominantly by AMPA-kainate receptor-mediated influx of Na+ into retinal neurons. Similar but transient morphological alterations were induced by high K+ and dopamine, which caused release of endogenous glutamate and subsequent activation of AMPA-kainate receptors. Apparently, retinal glutamatergic transmission is accompanied by neuronal cell swelling that causes compensatory morphological alterations of glial cells. The effect of dopamine was elicitable only during light adaptation but not in the dark, and glutamate and high K+ induced strong ereffects in the dark than in the light. This suggests that not only the endogenous release of dopamine but also the responsiveness of glutamatergic neurons to dopamine is regulated by light-dark adaptation. Similar morphological alterations (neuronal swelling and decreased glial process thickness) were observed in whole mounts isolated immediately after experimental retinal ischemia, suggesting an involvement of AMPA-kainate receptor activation in putative neurotoxic cell swelling in the postischemic retina.

Suppression by zinc of AMPA receptor-mediated synaptic transmission in the retina

Zinc is strikingly co-localized with glutamate-containing vesicles in the synaptic terminals of retinal photoreceptors, and it is thought to be co-released with glutamate onto postsynaptic neurons such as horizontal cells and bipolar cells. Here we examined exogenous zinc modulation of glutamate receptors on cultured retinal horizontal cells using patch-clamp recording and endogenous zinc effect on intact horizontal cells using intracellular recording techniques. Application of 3, 30, and 300 microM zinc reduced the whole cell peak current of response to 200 microM glutamate by 2, 30, and 56%, respectively. Zinc suppression of glutamate response persisted in the presence of 10 microM cyclothiazide (CTZ). Glutamate responses of outside-out patches were completely abolished by 30 microM 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI 52466), and the receptor desensitization was blocked by 30 microM CTZ, indicating that receptor target for the zinc action on horizontal cells is alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproponic acid (AMPA) receptors. Zinc decreased the amplitude of outside-out patch peak current without an effect on either its 10-90% rise time or the rate of receptor desensitization. Dose-response curves for glutamate show that zinc reduced the maximal current evoked by glutamate and increased EC(50) from 50 +/- 3 to 70 +/- 6 microM without changing the Hill coefficient. Chelation of endogenous zinc with 1 mM Ca-EDTA depolarized horizontal cells in the intact retina by 3 mV, consistent with relief of the partial glutamate receptor inhibition by zinc. Overall, the results describe a unimodal form of zinc modulation of AMPA-type glutamate receptor responses not previously described in native neuronal preparations and a novel role for endogenous zinc in modulating neurotransmission.

Blocking AMPA receptor desensitization prolongs spontaneous EPSC decay times and depolarizes H1 horizontal cells in carp retinal slices

Desensitization of H1 horizontal cell (H1 HC) glutamate receptors was investigated in carp retinal slices using cyclothiazide (CTZ), an inhibitor of AMPA receptor desensitization. 100 microM CTZ depolarized H1 HCs and increased the amplitude of light responses, without any prominent changes in their kinetics. Spontaneous EPSCs (sEPSCs) in H1 HCs were observed in the presence of 2.5 mM heptanol, an uncoupling agent of gap junctions. 20 microM GYKI52466 (an AMPA receptor antagonist) blocked the sEPSCs, consistent with the sEPSCs being mediated by AMPA receptors. 100 microM cobalt suppressed the frequency of sEPSCs without changing their mean peak amplitude, suggesting that calcium-dependent transmitter release from cones was not affected by heptanol. CTZ increased the total inward charge transferred per sEPSC by increasing the sEPSC decay time constant twofold, without any significant change in their frequency and mean peak amplitude. This suggests that the depolarizing effect of CTZ on H1 HCs was due to blocking desensitization of AMPA receptors, increasing the inward current induced by glutamate released from cone synaptic terminals. The desensitization of glutamate receptors may function to extend the dynamic range of H1 HC light responses.

Analysis of spontaneous EPSCs in retinal horizontal cells of the carp

Spontaneous excitatory postsynaptic currents (sEPSCs) were recorded under Whole-cell voltage clamp from carp type 1 horizontal cells (H1 cells) uncoupled by dopamine in retinal slices. Red light steps, which hyperpolarise cones and reduce glutamate release, induced outward current responses accompanied by a suppression of sEPSCs. sEPSCs decayed exponentially with a mean time constant of 0.71+/-0.07 ms and had a reversal potential near 0 mV. Power spectral analysis of sEPSCs revealed a similar decay time constant. They were suppressed by a non-NMDA receptor antagonist, CNQX at 10 microM, and a relatively specific AMPA receptor antagonist, GYKI52466 at 20 microM. The presence of sEPSCs suggests that the release of glutamate from cone synaptic terminals is vesicular. The reduction in mean sEPSC frequency with red light was not accompanied by a significant change in the mean sEPSC conductance increase (482+/-59 pS), suggesting that a decrease in the vesicular release rate from cones does not alter the vesicular glutamate concentration (quantal contents). The results suggest that the spontaneous events in H1 cells were contributed by non-NMDA (possibly AMPA) type glutamate receptors modulated by the red cone input.

Direct inhibition of the N-methyl-D-aspartate receptor channel by dopamine and (+)-SKF38393

1. Dopamine is known to modulate glutamatergic synaptic transmission in the retina and in several brain regions by activating specific G-protein-coupled receptors. We have examined the possibility of a different type of mechanism for this modulation, one involving direct interaction of dopamine with ionotropic glutamate receptors. 2. Ionic currents induced by fast application of N-methyl-D-aspartate (NMDA) were recorded under whole-cell patch-clamp in cultured striatal, thalamic and hippocampal neurons of the rat and in retinal neurons of the chick. Dopamine at concentrations above 100 microM inhibited the NMDA response in all four neuron types, exhibiting an IC50 of 1.2 mM in hippocampal neurons. The time course of this inhibition was fast, developing in less than 100 ms. 3. The D1 receptor agonist (+)-SKF38393 mimicked the effect of dopamine, with an IC50 of 58.9 microM on the NMDA response, while the enantiomer (-)-SKF38393 was ineffective at 50 microM. However, the D1 antagonist R(+)-SCH23390 did not prevent the inhibitory effect of (+)-SKF38393. 4. The degree of inhibition by dopamine and (+)-SKF38393 depended on transmembrane voltage, increasing 2.7 times with a hyperpolarization of about 80 mV. The voltage-dependent block by dopamine was also observed in the presence of MgCl2 1 mM. 5. Single-channel recordings showed that the open times of NMDA-gated channels were shortened by (+)-SKF38393. 6. These data suggested that the site to which the drugs bound to produce the inhibitory effect was distinct from the classical D1-type dopamine receptor sites, possibly being located inside the NMDA channel pore. It is concluded that dopamine and (+)-SKF38393 are NMDA channel ligands.

Divalent cations modulate glutamate receptors in retinal horizontal cells of the perch (Perca fluviatilis)

Divalent cations had two effects on concentration-response relations of glutamate induced membrane currents recorded from retinal horizontal cells. The first effect was a reduction of maximum currents. Barium, magnesium, cobalt, nickel and an increased calcium concentration caused reductions of maximum currents between 14% and 70%. The second effect of divalent cations was related to the dopamine dependent modulation of glutamate receptors in horizontal cells. The dopamine dependent enhancement of glutamate gated currents requires the presence of divalent cations besides calcium in the extracellular solution. Without such divalent cations application of dopamine caused no increase of the maximum currents induced by glutamate, and only a slight shift of the half maximal saturation concentration was observed. Addition of magnesium or barium cations in millimolar concentration was sufficient to completely restore the dopamine dependent modulation.

AMPA-preferring receptors with high Ca2+ permeability mediate dendritic plasticity of retinal horizontal cells

The synaptic complex formed by the cone photoreceptor pedicles and the dendrites of horizontal cells in the teleost retina undergoes structural changes during light adaptation. Numerous spinules are formed by the terminal dendrites, and they are subsequently retracted during dark adaptation. In a retina kept under continuous illumination, the retraction process can be initiated by analogues of the neurotransmitter glutamate acting at AMPA/kainate receptors. On the other hand, the retraction process depends on calcium influx and the subsequent activation of CaMkII. We show here that the retraction of spinules induced by AMPA or kainate is not impaired in the presence of cobalt, making an involvement of voltage-gated calcium channels unlikely. Using calcium imaging techniques with isolated horizontal cells, we demonstrate that AMPA and kainate, but not NMDA, increase [Ca2+]i in the presence of nicardipine, caffeine and thapsigargin. The increase of [Ca2+]i under these conditions depends on [Ca2+]o and on the agonist in a dose-dependent manner, suggesting that the increase of [Ca2+]i is largely due to calcium influx through the agonist-gated channel. Pharmacological studies were performed to determine whether AMPA- and/or kainate-preferring receptors mediate the calcium influx. The AMPA-preferring receptor antagonist LY303070 blocked glutamate- and kainate-evoked increases of [Ca2+]i in a concentration-dependent manner, indicating that kainate-preferring receptors contributed little or nothing to the observed [Ca2+]i increase. This was supported by experiments where cyclothiazide (which blocks the desensitization of AMPA receptors) and concanavalin A (which potentiates responses mediated by kainate receptors) were applied. In all cases, LY303070 blocked the agonist-evoked increase of [Ca2+]i. The presence of AMPA-preferring receptors with high Ca2+ permeability on horizontal cells was also supported by measuring agonist-induced currents using whole-cell recording techniques. Furthermore, LY303070 was able to impair the retraction of spinules during dark adaption in the in vivo situation.

Submillisecond kinetics of glutamate release from a sensory synapse

Exocytosis-mediated glutamate release from ribbon-type synaptic terminals of retinal bipolar cells was studied using AMPA receptors and simultaneous membrane capacitance measurements. Release onset (delay <0.8 ms) and offset were closely tied to Ca2+ channel opening and closing. Asynchronous release was not copious and we estimate that there are approximately 5 Ca2+ channels per docked synaptic vesicle. Depending on Ca2+ current amplitude, release occurred in a single fast bout or in two successive bouts with fast and slow onset kinetics. The second, slower bout may reflect a mobilization rate of reserve vesicles toward fusion sites that is accelerated by increasing Ca2+ influx. Bipolar cell synaptic ribbons thus are remarkably versatile signal transducers, capable of transmitting rapidly changing sensory input, as well as sustained stimuli, due to their large pool of releasable vesicles.

Desensitization of AMPA receptors on horizontal cells isolated from crucian carp retina

In horizontal cells freshly dissociated from crucian carp (Carassius auratus) retina, we recorded the whole-cell responses to rapid application of glutamate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate. Currents induced by glutamate and AMPA, but not by kainate, usually showed extremely rapid desensitization. 1-(4-aminophenyl)-3-methylcarbamyl- 4-methyl-7,8-methylenedioxy-3,4-dihydro-5H-2,3-benzodiazepine (GYKI 53655), a selective AMPA receptor antagonist, was found to completely block glutamate- and kainate-induced currents, which were supposed to be mediated by activation of AMPA receptors. We further extensively studied the kinetics of desensitization of glutamate- and AMPA-induced currents in horizontal cells. The time constants for decay of whole-cell currents induced by glutamate and AMPA were 1.9 and 1.4 ms, respectively, and the equilibrium responses to glutamate and AMPA at concentrations over 1 mM were invariably less than 10% of the corresponding peak responses. We have determined the values of EC50 for glutamate and AMPA as 1.08 and 1.05 mM, respectively, which are nearly 100-fold higher than that reported previously. Dose dependence of desensitization was also investigated and the glutamate concentration for a half desensitization was 26 microM, much lower than the EC50. Furthermore, kainate and AMPA interacted at AMPA receptors of horizontal cells in a dual competitive manner: the response to kainate of low concentration (10 microM) was potentiated by the addition of 300 microM AMPA, while the responses induced by kainate of relatively higher doses (300 microM or more) were reduced. We conclude that crucian carp horizontal cells may exclusively express the AMPA subtype of glutamate receptors, which is characterized by extremely rapid desensitization.

Atypical effect of dopamine in modulating the functional inhibition of NMDA receptors of cultured retina cells

Cultured retina cells released accumulated [3H]GABA (gamma-aminobutyric acid) when stimulated by L-glutamate, N-methyl-D-aspartate (NMDA) and kainate. In the absence of Mg2+, dopamine at 200 microM (IC50 60 microM), inhibited in more than 50% the release of [3H]GABA induced by L-glutamate and NMDA, but not by kainate. This effect was not blocked by the D1-like dopamine receptor antagonist, R-(+)-7-chloro-8-hydroxy-3-methyl- -phenyl-2,3,4,5-tetrahydro- H-3-benzazepine hydrochloride (SCH 23390), neither by haloperidol nor spiroperidol (dopamine D2-like receptor antagonists). The dopamine D1-like receptor agonist R(+)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,diol hydrochloride (SKF 38393) at 50 microM, but not its enantiomer, also inhibited the release of [3H]GABA induced by NMDA, but not by kainate; an effect that was not prevented by the antagonists mentioned above. (+/-)-6-Chloro-7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepin e hydrobromide (SKF 812497) had no effect. Neither 8BrcAMP (5 mM) nor forskolin (10 microM) inhibited the release of [3H]GABA. Our results suggest that dopamine and (+)-SKF 38393 inhibit the glutamate and NMDA-evoked [3H]GABA release through mechanisms that seem not to involve known dopaminergic receptor systems.

Dopaminergic modulation of NMDA-induced whole cell currents in neostriatal neurons in slices: contribution of calcium conductances

The present experiments were designed to examine dopamine (DA) modulation of whole cell currents mediated by activation of N-methyl-D-aspartate (NMDA) receptors in visualized neostriatal neurons in slices. First, we assessed the ability of DA, D1 and D2 receptor agonists to modulate membrane currents induced by activation of NMDA receptors. The results of these experiments demonstrated that DA potentiated NMDA-induced currents in medium-sized neostriatal neurons. Potentiation of NMDA currents occurred at three different holding potentials, although it was more pronounced at -30 mV. It was mediated by D1 receptors, because it was mimicked by D1 agonists and blocked by exposure to a D1 antagonist. Activation of D2 receptors produced inconsistent effects on NMDA-induced membrane currents. Either decreases, increases, or no effects on NMDA currents occurred. Second, we examined the contributions of intrinsic, voltage-dependent conductances to DA potentiation of NMDA currents. Blockade of K+ conductances did not prevent DA enhancement of NMDA currents. However, voltage-activated Ca2+ conductances provided a major contribution to DA modulation. The dihydropyridine L-type Ca2+ channel blockers, nifedipine, and methoxyverapamil (D-600), markedly reduced but did not totally eliminate the ability of DA to modulate NMDA currents. The D1 receptor agonist SKF 38393 also enhanced Ba2+ currents in neostriatal neurons. Together, these findings provide evidence for a complex interplay between DA, NMDA receptor activation and dihydropyridine-sensitive Ca2+ conductances in controlling responsiveness of neostriatal medium-sized neurons.

Effects of nitric oxide on the horizontal cell network and dopamine release in the carp retina

In the teleost retina the intercellular messenger nitric oxide can be synthesized by several cell types including cone photoreceptors and H1 horizontal cells, indicating a modulatory role within the outer plexiform layer, the first stage of the visual information processing. Therefore, the aim of this study was to elucidate the effects of nitric oxide on the physiology of cone horizontal cells in the intact retina. The nitric oxide donor sodium nitroprusside (0.5-2.5 mM) enhanced the light responsiveness of cone horizontal cells and reduced the degree of electrical coupling in the network. Furthermore, the spread of intracellularly injected Lucifer Yellow was restricted. The effects on light responsiveness and electrical coupling were qualitatively mimicked by 8-bromo-cGMP (0.5 mM) and could not be achieved by ferrocyanide (1 mM), the byproduct of nitric oxide liberation from nitroprusside. The effects of NO on the responsiveness of horizontal cells may be due to an action on green- and red-sensitive cones. Nitroprusside (0.1 mM) diminished the K(+)-stimulated release of endogenous dopamine by 50%, whereas the basal dopamine release was not affected, indicating that the effects on electrotonic horizontal cell coupling were not elicited by an NO-induced release of dopamine. With respect to the morphologic plasticity of the cone-horizontal cell synapse the inhibitor of endogenous nitric oxide synthesis L-nitroarginine (0.1 mM) had no influence on the formation or retraction of spinules. These results show that NO affects the responsiveness and coupling of the horizontal cell network in a dopamine-independent way.

Rapid AMPA receptor desensitization in catfish cone horizontal cells

AMPA and NMDA type glutamate receptors were studied in isolated catfish cone horizontal cells using the whole-cell and outside-out patch-recording techniques. In whole-cell recordings, cyclothiazide (CTZ) enhanced the peak current in response to glutamate (in the presence of NMDA receptor antagonists). In patch recordings, currents evoked by rapid and maintained applications of glutamate desensitized with a time constant of one millisecond. CTZ blocked this rapid desensitization. Recovery from desensitization of the AMPA receptors was rapid, having a time constant of 8.65 ms. In contrast, the whole-cell and patch responses to applications of NMDA were much smaller than the AMPA receptor responses and did not desensitize. The relative contribution of these two receptor subtypes depends critically on the condition of the synapse; if glutamate levels are tonically present, the NMDA receptors contribute significantly to the postsynaptic response. If glutamate levels fall rapidly following the release of a single quantum of glutamate, then AMPA receptor currents will dominate the postsynaptic response.

Modulation of amino acid-gated ion channels by protein phosphorylation

The major excitatory and inhibitory amino acid receptors in the mammalian central nervous system are considered to be glutamate, gamma-aminobutyric acid type A (GABAA), and glycine receptors. These receptors are widely acknowledged to participated in fast synaptic neurotransmission, which ultimately is responsible for the control of neuronal excitability. In addition to these receptors being regulated by endogenous factors, including the natural neurotransmitters, they also form target substrates for phosphorylation by a number of protein kinases, including serine/threonine and tyrosine kinases. The process of phosphorylation involves the transfer of a phosphate group(s) from adenosine triphosphate to one or more serine, threonine, or tyrosine residues, which are invariably found in an intracellular location within the receptor Phosphorylation is an important means of receptor regulation since it represents a covalent modification of the receptor structure, which can have important implications for ion channel function. This chapter reviews the current molecular and biochemical evidence regarding the sites of phosphorylation for both native neuronal and recombinant glutamate, GABAA and glycine receptors, and also reviews the functional electrophysiological implications of phosphorylation for receptor function.