Dopamine modulates the kinetics of ion channels gated by excitatory amino acids in retinal horizontal cells

Similar effects of carbachol and dopamine on neurons in the distal retina of the tiger salamander

Though there is considerable evidence that dopamine is an important retinal neuromodulator that mediates many of the changes in the properties of retinal neurons that are normally seen during light adaptation, the mechanism by which dopamine release is controlled remains poorly understood. In this paper, we present evidence which indicates that dopamine release in the retina of the tiger salamander, Ambystoma tigrinum, is driven excitatorily by a cholinergic input. We compared the effects of applying carbachol to those of dopamine application on the responses of rods, horizontal cells, and bipolar cells recorded intracellularly from the isolated, perfused retina of the tiger salamander. Micromolar concentrations of dopamine reduced the amplitudes of rod responses throughout the rods' operating range. The ratio of amplitudes of the cone-driven to rod-driven components of the responses of both horizontal and bipolar cells was increased by activation of both D1 and D2 dopamine receptors. Dopamine acted to uncouple horizontal cells and also off-center bipolar cells, the mechanism in the case of horizontal cells depending only upon activation of D1 receptors. Carbachol, a specific cholinomimetic, applied in five- to ten-fold higher concentrations, produced effects that were essentially identical to those of dopamine. These effects of carbachol were blocked by application of specific dopamine blockers, however, indicating that they are mediated secondarily by dopamine. We propose that the dopamine-releasing amacrine cells in the salamander are under the control of cells, probably amacrine cells, which secrete acetylcholine as their transmitter.

Short-term effects of dopamine on photoreceptors, luminosity- and chromaticity-horizontal cells in the turtle retina

The effects of dopamine on luminosity-type horizontal cells have been documented in different vertebrate retinas, both in vivo and in vitro. Some of these effects may reflect direct action of dopamine onto these cells, but indirect effects mediated by presynaptic neurons cannot be ruled out. Furthermore, direct effects of dopamine on horizontal cells may affect other, postsynaptic neurons in the outer plexiform layer. To test these possibilities, we studied the effects of dopamine on photoreceptors and all types of horizontal cells in the turtle (Pseudemys scripta elegans) retina. Receptive-field properties, responsiveness to light, and time course of light responses were monitored with intracellular recordings. Dopamine at a concentration of 40 microM exerted effects with two different time courses. "Short-term" effects were fully developed after 3 min of dopamine application and reversed within 30 min of washout of the drug. "Long-term" effects were fully developed after about 7-10 min and could not be washed out during the course of our experiments. Only the "short-term" effects were studied in detail in this paper. These were expressed in a reduction of the receptive-field size of all types of horizontal cells studied; L1 and L2 luminosity types as well as Red/Green and Yellow/Blue chromaticity types. The L1 horizontal cells did not exhibit signs of reduced responsiveness to light under dopamine, while in the L2 cells and the two types of chromaticity cells responsiveness decreased. None of the rods, long-wavelength-sensitive, or medium-wavelength-sensitive cones exhibited any apparent reduction in their receptive-field sizes or responsiveness to light. The present results suggest that the "short-term" effects of dopamine are not mediated by photoreceptors and are probably due to direct action of dopamine on horizontal cells.

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

The patch-clamp technique in combination with a fast liquid filament application system was used to study the effect of dopamine on the glutamate receptor desensitization in horizontal cells of the perch (Perca fluviatilis). Kinetics of ligand-gated ion channels in fish horizontal cells are modulated by dopamine. This modulation is presumably mediated by a cAMP-dependent protein phosphorylation. Before incubation with dopamine, the glutamate receptors of horizontal cells activate and desensitize with fast time constants. In the whole-cell recording mode, fast application of the agonists L-glutamate, quisqualate, or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid prior to the dopamine incubation gives rise to fast transient currents with peak values of about 200 pA that desensitize within 100 ms. Kainate as agonist produced higher steady-state currents but no transient currents. After incubation of the cells with dopamine for 3 min, the desensitization was significantly reduced and the agonists L-glutamate, quisqualate, or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid induced steady-state currents with amplitudes that were similar to the previously observed transient currents. Kainate-induced currents were only slightly affected. Fast desensitizing currents upon fast application of L-glutamate were also recorded from outside-out patches that were excised from horizontal cells before incubation with dopamine. The currents from excised patches desensitized to a steady-state level of about 0.2 of the peak amplitude with time constants of less than 2 ms. When the outside-out patches were excised from cells after dopamine incubation, steady-state currents were enhanced and no transient currents were observed. The results may indicate that the dopamine-dependent modulation of glutamate-induced currents, which is presumably mediated by a protein phosphorylation, is due to an alteration of the desensitization of the glutamate receptors.

Localization of D2 dopamine receptors in vertebrate retinae with anti-peptide antibodies

Dopamine plays an important role in modulating various aspects of retinal signal processing. The morphology of dopaminergic neurons and its physiological effects are well characterized. Two classes of receptor molecules (D1 and D2) were shown pharmacologically to mediate specific actions, with differences between individual groups of vertebrates. In an attempt to better understand dopaminergic mechanisms at the cellular level, we used antisera against D2 receptors and investigated the localization of the dopamine D2 receptor in the retinae of rat, rabbit, cow, chick, turtle, frog, and two fish species with immunofluorescence techniques. Antisera were raised in rabbits to two oligopeptides predicted from rat D2 receptor cDNA; one specific for the splice-variant insertion in the third cytoplasmic loop and the other directed towards the extracellular amino terminal region shared by both short and long isoforms. Preadsorption with the synthetic peptide resulted in a significant reduction of label, indicating the presence of specific binding in all species except turtle and goldfish. The pattern of labelling produced by the two antisera was essentially identical; however, the staining obtained with antiserum to the extracellular motif was always more intense. Specific staining was present in photoreceptor inner and outer segments, and in the outer and inner plexiform layers of all species. In mammals and chick, strongly fluorescent perikarya were observed in the ganglion cell layer and at the proximal margin of the inner nuclear layer. Label may be present in the pigment epithelium but could not be established beyond doubt. This pattern of labelling is in accordance with previous observations on D2 receptor localization by means of radioactive ligand binding and in situ hybridization techniques. It suggests that retinal dopamine acts as a neuromodulator as well as a transmitter. In the distal retina, it may reach its targets via diffusion over considerable distances, even crossing the outer limiting membrane; in the inner and outer plexiform layers, conventional synaptic transmission seems to coexist with paracrine addressing of more distant targets, and D2 receptors are expressed by both amacrine and ganglion cells.

Phosphorylation and regulation of glutamate receptors by calcium/calmodulin-dependent protein kinase II

The major postsynaptic density (PSD) protein at glutaminergic synapses is calcium/calmodulin-dependent protein kinase II (CaM-K II), but its function in the PSD is not known. We have examined glutamate receptors (GluRs) as substrates for CaM-K II because (1) they are colocalized in the PSD, (2) cloned GluRs contain consensus phosphorylation sites for protein kinases including CaM-K II, and (3) several GluRs are regulated by other protein kinases. Regulation of GluRs, which are involved in excitatory synaptic transmission and in mechanisms of learning and memory, by CaM-K II is of interest because of the postulated role of CaM-K II in synaptic plasticity and its known involvement in induction of long-term potentiation. Furthermore, mice lacking the major neural isoform of CaM-K II exhibit deficits in models of learning and memory that require hippocampal input. We report here that CaM-K II phosphorylates GluR in several in vitro systems, including the PSD, and that activated CaM-K II enhances kainate-induced ion current three- to fourfold in cultured hippocampal neurons. These results are consistent with a role for PSD CaM-K II in strengthening postsynaptic GluR responses in synaptic plasticity.

Phosphorylation and modulation of a kainate receptor (GluR6) by cAMP-dependent protein kinase

Ligand-gated ion channels gated by glutamate constitute the major excitatory neurotransmitter system in the mammalian brain. The functional modulation of GluR6, a kainate-activated glutamate receptor, by adenosine 3',5'-monophosphate-dependent protein kinase A (PKA) was examined with receptors expressed in human embryonic kidney cells. Kainate-evoked currents underwent a rapid desensitization that was blocked by lectins. Kainate currents were potentiated by intracellular perfusion of PKA, and this potentiation was blocked by co-application of an inhibitory peptide. Site-directed mutagenesis was used to identify the site or sites of phosphorylation on GluR6. Although mutagenesis of two serine residues, Ser684 and Ser666, was required for complete abolition of the PKA-induced potentiation, Ser684 may be the preferred site of phosphorylation in native GluR6 receptor complexes. These results indicate that glutamate receptor function can be directly modulated by protein phosphorylation and suggest that a dynamic regulation of excitatory receptors could be associated with some forms of learning and memory in the mammalian brain.

Dopamine enhances both electrotonic coupling and chemical excitatory postsynaptic potentials at mixed synapses

The transmitter dopamine reduces electrotonic coupling between retinal horizontal cells and increases their sensitivity to glutamate. Since in other systems single afferents establish mixed electrotonic and chemical excitatory synapses with their targets, dopamine might be expected there to depress one component of excitation while enhancing the other. This hypothesis was tested by applying dopamine locally in the vicinity of the lateral dendrite of the goldfish Mauthner cell (M cell) and monitoring the composite electrotonic and chemical excitatory postsynaptic potentials and currents evoked by ipsilateral eighth nerve stimulation. Dopamine produces persistent enhancements of both components of the postsynaptic response while it also increases input conductance. All these dopamine actions are prevented by superfusing the brain with saline containing the dopamine D1 receptor antagonist SCH-23390. Postsynaptic injections of the cAMP-dependent protein kinase inhibitor (Walsh inhibitor, or PKI5-24) block the dopamine-induced changes in synaptic transmission, implicating a cAMP-dependent mechanism. Furthermore, there is a dopaminergic innervation of the M cell, as demonstrated immunohistochemically with antibodies against dopamine and the rate-limiting enzyme in its synthetic pathway, tyrosine hydroxylase. Varicose immunoreactive fibers lie in the vicinity of the distal part of the lateral dendrite between the large myelinated club endings that establish the mixed synapses. As determined with electron microscopy, the dopaminergic fibers contain small vesicles, and they do not have synaptic contacts with either the afferents or the M cell, remaining instead in the synaptic bed. Taken together, these results suggest that dopamine released at a distance from these terminals increases the gain of this primary sensory input to the M cell, most likely through a phosphorylation mechanism.

Adenosine decreases neurotransmitter release at central synapses

Adenosine, at concentrations ranging from 5 to 100 microM, decreases the efficacy of transmission at the perforant path synapses on dentate granule cells. We have used whole cell recording from these cells in slices to determine the mechanism of the reduced synaptic strength. We find that size of miniature excitatory postsynaptic currents (mepscs) is unaffected by adenosine at concentrations up to 100 microM, an observation that indicates adenosine's mode of action is not through a decreased postsynaptic sensitivity to neurotransmitter. A quantal analysis indicates, however, that the quantity of neurotransmitter released is sufficiently diminished by adenosine to account entirely for the adenosine-produced decrease in synaptic strength. Application of 3-isobutyl-1-methylxanthine (IBMX), a drug that antagonizes the effects of endogenous adenosine, produces an increase in synaptic strength. This observation suggests that the resting level of adenosine in our slices is appreciable, and an analysis of the adenosine dose-response relation is consistent with endogenous adenosine levels of about 10 microM. IBMX application produces only slight changes in the amplitude of mepscs, whereas a quantal analysis demonstrates that the drug significantly increases the amount of neurotransmitter released. Thus IBMX acts as an "anti-adenosine" in our experiments. In some experiments we have been able to record excitatory and inhibitory synaptic currents produced by the same perforant path stimulus. In these instances we find that inhibitory transmission is unaffected by concentrations of adenosine that produce a marked decrease in the strength of excitatory synapses.

Differential expression of glutamate receptor genes (GluR1-5) in the rat retina

The recent isolation of at least five different cDNAs encoding functional subunits of glutamate receptors (GluR1 to GluR5) has revealed a diversity whose function is not understood. To learn more about how these different receptor subunits are used in the brain, we undertook an in situ hybridization study of the retina to define how the different glutamate receptor genes are expressed. We chose the retina because the glutamate sensitivities of its different cell types have been characterized, and these different neurons reside in different laminae. Hybridization of [35S]UTP-labeled cRNA probes with transverse sections and freshly dissociated cells reveals that all five receptor subunits are expressed in the retina. Hybridization signal is detected in different, but overlapping, sets of cells in the retina. GluR1, GluR2, and GluR5 are expressed by many somata, and GluR4 by a few, in the outer third of the inner nuclear layer, where the horizontal cells reside. Transcripts for GluR1, GluR2, and GluR5 are found in the somata within the middle third of the inner nuclear layer, which is where the bipolar cell somata are located, and GluR2 probes label freshly dissociated rod bipolar cells. All of the probes produce labeling over the cells at the inner edge of the inner nuclear layer, which are probably amacrine cells, as well as over the cell bodies in the ganglion cell layer.

Effects of light stimuli on the release of dopamine from interplexiform cells in the white perch retina

Interplexiform cells are centrifugal neurons in the retina carrying information from the inner to the outer plexiform layers. In teleost fish, interplexiform cells appear to release dopamine in the outer plexiform layer after prolonged darkness that modulates the receptive-field size and light responsiveness of horizontal cells (Mangel & Dowling, 1985; Yang et al., 1988a, b). It has been proposed that interplexiform cells may also release dopamine upon steady illumination because horizontal cells' receptive fields shrink in the light (Shigematsu & Yamada, 1988). Here, we report the shrinkage of the receptive fields of horizontal cells seen in the presence of background illumination is not blocked by dopamine antagonists, indicating that dopamine does not underlie the receptive-field size changes observed during steady illumination. Flickering light, however, does appear to stimulate the release of dopamine from the interplexiform cells, resulting in a marked reduction of horizontal cell receptive-field size. Taken together, experiments on horizontal cells indicate that dopamine is released from interplexiform cells in the teleost retina after prolonged darkness and during flickering light, but that dopamine release from interplexiform cells during steady retinal illumination is minimal.

Retinal neuromodulation: the role of dopamine

Dopamine exerts multiple effects on retinal horizontal cells. Dopamine, via cyclic AMP and protein kinase A, reduces the light responsiveness of horizontal cells and the electrical coupling between the cells. The gating kinetics of both gap-junctional and glutamate channels are altered as a result of phosphorylation by protein kinase A. Dopamine also causes a reversible retraction of neurites of horizontal cells maintained in culture. Diacylglycerol analogues as well as phorbol esters mimic this effect of dopamine, but not cyclic AMP analogues or Forskolin. The results suggest that dopamine causes neurite retraction by the activation of protein kinase C via diacylglycerol.