Dopamine enhances excitatory amino acid-gated conductances in cultured retinal horizontal cells

Dopamine mediates circadian clock regulation of rod and cone input to fish retinal horizontal cells

A circadian (24-hour) clock regulates the light responses of fish cone horizontal cells, second order neurones in the retina that receive synaptic contact from cones and not from rods. Due to the action of the clock, cone horizontal cells are driven by cones in the day, but primarily driven by rods at night. We show here that dopamine, a retinal neurotransmitter, acts as a clock signal for the day by increasing cone input and decreasing rod input to cone horizontal cells. The amount of endogenous dopamine released from in vitro retinae was greater during the subjective day than the subjective night. Application of dopamine or quinpirole, a dopamine D(2)-like agonist, during the subjective night increased cone input and eliminated rod input to the cells, a state usually observed during the subjective day. In contrast, application of spiperone, a D(2)-like antagonist, or forskolin, an activator of adenylyl cyclase, during the subjective day reduced cone input and increased rod input. SCH23390, a D(1) antagonist, had no effect. Application of R(p)-cAMPS, an inhibitor of cAMP-dependent protein kinase, or octanol, an alcohol that uncouples gap junctions, during the night increased cone input and decreased rod input. Because D(2)-like receptors are on photoreceptor cells, but not horizontal cells, the results suggest that the clock-induced increase in dopamine release during the day activates D(2)-like receptors on photoreceptor cells. The resultant decrease in intracellular cyclic AMP and protein kinase A activation then mediates the increase in cone input and decrease in rod input.

Mechanical properties of the dorsal fin muscle of seahorse (Hippocampus) and pipefish (Syngnathus)

The dorsal and pectoral fins are the primary locomotor organs in seahorses (Hippocampus) and pipefish (Syngnathus). The small dorsal fins beat at high oscillatory frequencies against the viscous medium of water. Both species are able to oscillate their fins at frequencies likely exceeding the point of flicker fusion for their predators, thus enhancing their ability to remain cryptic. High-speed video demonstrated that seahorse dorsal fins beat at 30-42 Hz, while pipefish dorsal fins oscillate at 13-26 Hz. In both species, the movement of the fin is a sinusoidal wave that travels down the fin from anterior to posterior. Mechanical properties of seahorse and pipefish dorsal fin muscles were tested in vitro by the work loop method. Maximum isometric stress was 176.1 kN/m(2) in seahorse and 111.5 kN/m(2) in pipefish. Work and power output were examined at a series of frequencies encompassing the range observed in vivo, and at a number of strains (percent length change during a contractile cycle) within each frequency. At a given strain, work per cycle declined with increasing frequency, while power output rose to a maximum at an intermediate frequency and then declined. Frequency and strain interacted in a complex fashion; optimal strain was inversely related to cycle frequency over most of the frequency range tested. Seahorse dorsal fin muscle was able to generate positive work at higher cycling frequencies than pipefish. Both species produced positive work at higher frequencies than have been reported for axial and fin muscles from other fish.

Serotonin potentiation of glycine-activated whole-cell currents in the superficial laminae neurons of the rat spinal dorsal horn is mediated by protein kinase C

The modulatory effects of serotonin (5-HT) on glycine (Gly)-activated whole-cell currents were investigated in neurons acutely dissociated from the superficial laminae (I and II) of the rat spinal dorsal horn using the nystatin-perforated patch recording configuration under voltage-clamp conditions. Our results demonstrate that (1). Gly acted on strychnine (STR)-sensitive Gly receptors and elicited inward Cl(-) currents (I(Gly)) at a holding potential of -40 mV; (2). 5-HT potentiated I(Gly) without affecting the reversal potential of I(Gly); (3). the agonist (alpha-methyl-5-HT) and antagonist (ketanserine) of 5-HT(2) receptor mimicked and blocked the potentiating effect of 5-HT on I(Gly), respectively; (4). bisindolylmaleimide I (BIM), a selective inhibitor of protein kinase C (PKC), reduced the potentiating effect of 5-HT on I(Gly); and (5). 5-HT-induced enhancement of I(Gly) was not affected by pretreatment with 1,2-bis-(2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid tetrakis (acetoxy-methyl) ester (BAPTA AM), a Ca(2+) chelator. These results indicate that (1). the potentiation of 5-HT on I(Gly) is mediated by 5-HT(2) receptor and through Ca(2+)-independent PKC intracellular signal transduction pathway; and (2). the interactions between 5-HT and Gly might modulate the transmission of nociceptive information through the spinal cord.

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.

Activation of Protein Kinase C Modulates Light Responses in Horizontal Cells of the Turtle Retina

The effect of phorbol esters on the light-evoked responses of horizontal cells were studied in the turtle eyecup preparation. Phorbol esters caused a reduction in receptive field size and a significant decrease in the amplitude of responses to annular and full-field illumination; however, they caused only minor changes in responses to small spots in the receptive field centre. The dark membrane potential was not affected. The results suggest that phorbol esters may affect both coupling resistance and membrane resistance in horizontal cells. The effects of phorbol esters were blocked by the protein kinase C inhibitor staurosporine, and inactive phorbol ester had no effect, making it very likely that the phorbol ester effects were mediated through activation of protein kinase C. The above effects of the phorbol esters were considerably reduced by the dopamine antagonists haloperidol and fluphenazine, suggesting that they were in part mediated by release of dopamine.

Dopaminergic Modulation of Transient Neurite Outgrowth from Horizontal Cells of the Fish Retina is not Mediated by cAMP

Horizontal cell dendrites invaginating the cone pedicles in the fish retina exhibit a marked light dependent plasticity in the morphology of their synaptic connections. Upon light adaptation of the retina, numerous spinules are formed which disappear during dark adaptation. This process is paralleled by a strengthening and weakening, respectively, of the horizontal cell's inhibitory output. The formation of spinules during light adaptation requires dopaminergic activity as it does not occur in dopamine-depleted retinas, but can be partially induced in depleted retinas by the exogenous administration of dopamine. Although horizontal cells do have D1 receptors the action of dopamine is not coupled to a stimulation of cAMP. An increase of intracellular cAMP either by injection of a cAMP analogue or by metabolic interference does not result in any spinule formation. The data suggest that dopamine must act through a cAMP independent intracellular mechanism.

Phosphorylation of GluR4 AMPA-type glutamate receptor subunit by protein kinase C in cultured retina amacrine neurons

We have previously reported that the activity of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors is potentiated by protein kinase C (PKC) in cultured chick retina amacrine neurons, and that constitutive PKC activity is necessary for basal AMPA receptor activity (Carvalho et al., 1998). In this study, we evaluated the phosphorylation of the GluR4 subunit, which is very abundant in cultured amacrine neurons, to correlate it with the effects of PKC on AMPA receptor activity in these cells. 32P-labelling of GluR4 increased upon AMPA receptor stimulation or cell treatment with phorbol 12-myristate 13-acetate (PMA) before stimulating with kainate. By contrast, phosphorylation of GluR4 was not changed when PKC was inhibited by treating the cells with the selective PKC inhibitor GF 109203X before stimulation with kainate. We conclude that GluR4 is phosphorylated upon PKC activation and/or stimulation of AMPA receptors in cultured amacrine cells. Additionally, AMPA receptor activation with kainate in cultured chick amacrine cells leads to translocation of conventional and novel PKC isoforms to the cell membrane, suggesting that PKC could be activated upon AMPA receptor stimulation in these cells.

Leitmotifs in the biochemistry of LTP induction: amplification, integration and coordination

Hippocampal long-term potentiation (LTP) is a robust and long-lasting form of synaptic plasticity that is the leading candidate for a cellular mechanism contributing to mammalian learning and memory. Investigations over the past decade have revealed that the biochemistry of LTP induction involves mechanisms of great subtlety and complexity. This review highlights themes that have emerged as a result of our increased knowledge of the signal transduction pathways involved in the induction of NMDA receptor-dependent LTP in area CA1 of the hippocampus. Among these themes are signal amplification, signal integration and signal coordination. Here we use these themes as an organizing context for reviewing the profusion of signaling mechanisms involved in the induction of LTP.

An electrophysiological test of the effect of the temporal pattern of light adaptation on teleost H1 type horizontal cell plasticity

The possible importance of the temporal pattern of photon delivery in light adaptation-induced physiological plasticity in the outer retina of carp was tested by intracellular recording. Steady and flicker (3 Hz) background adaptation was applied whilst recording chromatic voltage responses of H1 type horizontal cells (HCs) to 680 and 440 nm full-field test flashes (generating response amplitudes of V(r) and V(b), respectively). A third parameter V(b)/V(r) (B/R) was calculated as an indicator of the cells' short/long wavelength relative spectral contrast. Steady light adaptation increased V(r) and to a lesser extent V(b), and reduced B/R. Flicker adaptation also increased V(r) (by a similar amount), but, unlike steady adaptation, consistently decreased V(b). The reduction in B/R was statistically greater for flicker than for steady adaptation, although the former delivered half as many photons to the retina. These results suggest that the temporal pattern of light adaptation is indeed an important determinant of qualitative and quantitative aspects of plasticity induced in the outer retina, and complement earlier morphological findings. The effects are discussed in terms of dopamine and nitric oxide as underlying possible neurochemical control mechanisms.

Direct gating by retinoic acid of retinal electrical synapses

Retinoic acid (RA), a signaling molecule derived from vitamin A, controls growth and differentiation of a variety of cell types through regulation of gene transcription. In the vertebrate retina, RA also regulates gap junction-mediated physiological coupling of retinal neurons through a nontranscriptional mechanism. Here we report that RA rapidly and specifically modulates synaptic transmission at electrical synapses of cultured retinal horizontal cells through an external RAR(beta)(/gamma)-like binding site, the action of which is independent of second messenger cascades. External application of all-trans retinoic acid (at-RA) reversibly reduced the amplitude of gap junctional conductance in a dose-dependent manner, but failed to affect non-gap-junctional channels, including glutamate receptors. In contrast, internal dialysis with at-RA was ineffective, indicating an external site of action. Selective RAR(beta)(/gamma) ligands, but not an RAR(alpha)-selective agonist, mimicked the action of at-RA, suggesting that gating of gap junctional channels is mediated through an RAR(beta)(/gamma)-like binding site. At-RA did not act on gap junctional conductance by lowering [pH](i) or by increasing [Ca(2+)](i). A G protein inhibitor and protein kinase inhibitors did not block at-RA uncoupling effects indicating no second messenger systems were involved. Direct action of at-RA on gap junction channels was further supported by its equivalent action on whole-cell hemi-gap-junctional currents and on cell-free excised patch hemichannel currents. At-RA significantly reduced single-channel open probability but did not change unitary conductance. Overall, the results indicate that RA modulates horizontal cell electrical synapses by activation of novel nonnuclear RAR(beta)(/gamma)-like sites either directly on, or intimately associated with, gap junction channels.

Diurnal metabolism of dopamine in dystrophic retinas of homozygous and heterozygous retinal degeneration slow (rds) mice

Dopamine metabolism was studied in dystrophic retinal degeneration slow (rds) mice which carry a mutation in the rds/peripherin gene. RDS mutations in humans cause several forms of retinal degeneration. Dopamine synthesis and utilization were analyzed at various time points in the diurnal cycle in homozygous rds/rds retinas which lack photoreceptor outer segments and heterozygous rds/+ retinas which have short malformed outer segments. Homozygous retinas exhibited depressed dopamine synthesis and utilization while the heterozygous retina retained a considerable level of activity which was, nevertheless, significantly lower than that of normal retinas. By one year, heterozygous rds/+ retinas which had lost half of the photoreceptors still maintained significant levels of dopamine metabolism. Normal characteristics of dopamine metabolism such as a spike in dopamine utilization at light onset were observed in mutant retinas. However, light intensity-dependent changes in dopamine utilization were observed in normal but not rds/+ retinas. The findings of this study suggest that human patients with peripherin/rds mutations, or other mutations that result in abnormal outer segments that can still capture light, might maintain light-evoked dopamine metabolism and dopamine-dependent retinal functions during the progression of the disease, proportional to remaining levels of light capture capabilities. However, visual deficits due to reduced light-evoked dopamine metabolism and abnormal patterns of dopamine utilization could be expected in such diseased retinas.

Regulation of AMPA receptors by phosphorylation

The AMPA receptors for glutamate are oligomeric structures that mediate fast excitatory responses in the central nervous system. Phosphorylation of AMPA receptors is an important mechanism for short-term modulation of their function, and is thought to play an important role in synaptic plasticity in different brain regions. Recent studies have shown that phosphorylation of AMPA receptors by cAMP-dependent protein kinase (PKA) and Ca2+- and calmodulin-dependent protein kinase II (CaMKII) potentiates their activity, but phosphorylation of the receptor subunits may also affect their interaction with intracellular proteins, and their expression at the plasma membrane. Phosphorylation of AMPA receptor subunits has also been investigated in relation to processes of synaptic plasticity. This review focuses on recent advances in understanding the molecular mechanisms of regulation of AMPA receptors, and their implications in synaptic plasticity.

Mesopic state: cellular mechanisms involved in pre- and post-synaptic mixing of rod and cone signals

At least twice daily our retinas move between a light adapted, cone-dominated (photopic) state and a dark-adapted, color-blind and highly light-sensitive rod-dominated (scotopic) state. In between is a rather ill-defined transitional state called the mesopic state in which retinal circuits express both rod and cone signals. The mesopic state is characterized by its dynamic and fluid nature: the rod and cone signals flowing through retinal networks are continually changing. Consequently, in the mesopic state the retinal output to the brain contained in the firing patterns of the ganglion cells consists of information derived from both rod and cone signals. Morphology, physiology, and psychophysics all contributed to an understanding that the two systems are not independent but interact extensively via both pooling and mutual inhibition. This review lays down a rationale for such rod-cone interactions in the vertebrate retinas. It suggests that the important functional role of rod-cone interactions is that they shorten the duration of the mesopic state. As a result, the retina is maintained in either in the (rod-dominated) high sensitivity photon counting mode or in the second mode, which emphasizes temporal transients and spatial resolution (the cone-dominated photopic state). Experimental evidence for pre- and postsynaptic mixing of rod and cone signals in the retina of the clawed frog, Xenopus, is shown together with the preeminent neuromodulatory role of both light and dopamine in controlling interactions between rod and cone signals. Dopamine is shown to be both necessary and sufficient to mediate light adaptation in the amphibian retina.

Serotonin potentiates the response of neurons of the superficial laminae of the rat spinal dorsal horn to gamma-aminobutyric acid

Employing the Nystatin-perforated whole-cell patch-clamp recording technique, the modulatory effects of serotonin (5-HT) on gamma-aminobutyric acid (GABA)-activated whole-cell currents were investigated in neurons acutely dissociated from the superficial laminae (laminae I and II) of the rat spinal dorsal horn. The results showed: (1) GABA acted on GABA(A) receptors and elicited inward Cl(-) currents (I(GABA)) at a holding potential (V(H)) of -40 mV; (2) 5-HT potentiated GABA-induced Cl(-) current without affecting the reversal potential of I(GABA) and the apparent affinity of GABA to its receptor; (3) alpha-methyl-5-HT, a selective agonist of 5-HT(2) receptor, mimicked the potentiation effect of 5-HT on I(GABA), whereas ketanserine, an antagonist of 5-HT(2) receptor, blocked the potentiation effect of 5-HT; (4) Chelerythrine, an inhibitor of protein kinase C, reduced the potentiation effect of 5-HT on I(GABA). The present results indicate: (1) The potentiation of 5-HT on I(GABA) is mediated by 5-HT(2) receptor and through a protein kinase-dependent transduction pathway; (2) The interactions between 5-HT and GABA might play an important role in the modulation of nociceptive information transmission at spinal cord level.

Diurnal metabolism of dopamine in the mouse retina

Dopamine is an important retinal neurotransmitter and neuromodulator that regulates key diurnal cellular and physiological functions. In the present study we carried out a comprehensive analysis of dopamine metabolism during the light phase of the diurnal cycle and evaluated the presence of diurnal and circadian rhythms of dopaminergic activity in the mouse retina. Steady-state levels of dopamine did not change significantly between the dark phase (night) and the light phase (day) of the diurnal cycle, nor did they change between early and late points in the day. Dopamine synthesis and utilization, however, revealed significant alterations between the night and day and between early and late time points in the day. A spike in synthesis and utilization was measured immediately after light onset at the end of the night. Subsequently, dopamine synthesis and utilization partially declined and remained stable throughout the remainder of the day at a level that was significantly higher than that at night. The burst of dopamine synthesis and utilization at the beginning of the day is entirely light evoked and not driven by a circadian clock. Similarly, there was no circadian rhythm in dopamine synthesis and utilization in mice kept in constant darkness. This daily pattern of dopaminergic activity may impact upon a variety of temporally regulated retinal events. Moreover, these data will provide a basis for evaluating the role of dopamine in retinal pathology in mouse models of retinal degeneration where mutations affect light perception.

Relation between brain tissue pO2 and dopamine synthesis of basal ganglia--a 18FDOPA-PET study in newborn piglets

Perinatal hypoxic-ischemic cerebral injury is a major determinant of neurologic morbidity and mortality in the neonatal period and later in childhood. There is evidence that the dopaminergic system is sensitive to oxygen deprivation. However, the respective enzyme activities have yet not been measured in the living neonatal brain. In this study, we have used 18F-labelled 6-fluoro-L-3,4-dihydroxyphenylalanine (FDOPA) together with positron emission tomography (PET) to estimate the activity of the aromatic amino acid decarboxylase (AADC), the ultimate enzyme in the synthesis of dopamine, in the brain of newborn piglets under normoxic and moderate asphyxial conditions. The study was performed on 8 newborn piglets (2-5 days old). In each piglet PET studies were performed under control conditions and during 2-hour asphyxia. Simultaneously, brain tissue pO2 was recorded, cerebral blood flow (CBF) was measured with colored microspheres and cerebral metabolic rate of oxygen (CMRO2) was determined. Asphyxia was induced by lowering the inspired fraction of oxygen from 0.35 to 0.10 and adding about 6% CO2 to the inspired gas. Asphyxia elicited a more than 3-fold increase of the CBF (p < 0.01) so that CMRO2 remained unchanged throughout the asphyxial period. Despite this, brain tissue pO2 was reduced from 19 +/- 4 mm Hg to 6 +/- 3 mm Hg (p < 0.01). Blood-brain transfer of FDOPA as well as permeability-surface area product (PS) from striatum were unchanged. Striatal synthesis rate of fluoro-dopamine from FDOPA (k3) was, however, significantly increased (p < 0.01). This increase of the AADC activity is associated with reduced brain tissue pO2. Asphyxia-induced CBF increase impedes an alteration of brain oxidative metabolism.

Light-adaptive effects of retinoic acid on receptive field properties of retinal horizontal cells

Besides its role in ocular development, retinoic acid (RA), which is a light-correlated byproduct of the phototransduction cycle, was recently shown to affect light-driven synaptic plasticity in the outer plexiform layer of the adult fish retina. Tuning by ambient light conditions of the retinal network properties is very prominent in outer plexiform layer circuits, and we therefore examined whether RA could affect cone horizontal cell physiology similar to ambient light. Performing intracellular recordings and dye injections in the dark-adapted inverted eyecup preparation of the carp, we found that RA reduced the receptive fields of horizontal cell somata and impaired gap junctional communication. This action was not observed among coupled axon terminals of horizontal cells and appeared to be stereospecific because it could only be attributed to all-trans and 13-cis RA but not to the 9-cis isomer and photoisomerized all-trans RA. Modulation of receptive field size occurred independently of the dopaminergic system. Furthermore, RA affected the light responsiveness of cone horizontal cells. Compared to the dark-adapted condition, responsiveness to intense light stimulation was enhanced but decreased when low intensities were used. Moreover, following RA treatment H2-type horizontal cells of dark-adapted retinae which do not give rise to colour-opponent light properties became colour-opponent and performed depolarizing responses to long-wavelength stimulation. In all these cases RA perfectly matched the effects of light adaptation, supporting the notion that RA acts as an endogenous neuromodulator.

Intrinsic cone adaptation modulates feedback efficiency from horizontal cells to cones

Processing of visual stimuli by the retina changes strongly during light/dark adaptation. These changes are due to both local photoreceptor-based processes and to changes in the retinal network. The feedback pathway from horizontal cells to cones is known to be one of the pathways that is modulated strongly during adaptation. Although this phenomenon is well described, the mechanism for this change is poorly characterized. The aim of this paper is to describe the mechanism for the increase in efficiency of the feedback synapse from horizontal cells to cones. We show that a train of flashes can increase the feedback response from the horizontal cells, as measured in the cones, up to threefold. This process has a time constant of approximately 3 s and can be attributed to processes intrinsic to the cones. It does not require dopamine, is not the result of changes in the kinetics of the cone light response and is not due to changes in horizontal cells themselves. During a flash train, cones adapt to the mean light intensity, resulting in a slight (4 mV) depolarization of the cones. The time constant of this depolarization is approximately 3 s. We will show that at this depolarized membrane potential, a light-induced change of the cone membrane potential induces a larger change in the calcium current than in the unadapted condition. Furthermore, we will show that negative feedback from horizontal cells to cones can modulate the calcium current more efficiently at this depolarized cone membrane potential. The change in horizontal cell response properties during the train of flashes can be fully attributed to these changes in the synaptic efficiency. Since feedback has major consequences for the dynamic, spatial, and spectral processing, the described mechanism might be very important to optimize the retina for ambient light conditions.

Dopamine D1-receptor immunolocalization in goldfish retina

Dopamine, a neuromodulator in the vertebrate retina, is involved in numerous functions related to light adaptation. However, unlike in mammals, localization of retinal D1-dopamine receptors in nonmammalian vertebrates has been hampered due to a lack of antisera. To address this problem, an antiserum against the 18 C-terminal amino acids of the goldfish D1 receptor (gfD1r) was generated in chicken eggs and tested in retinae of goldfish and rat, and rat caudate putamen, by using immunoblots and light microscopic immunocytochemistry. No labeling was observed in any tissue or immunoblots with preabsorbed gfD1r antiserum. Immunoblot analysis of goldfish retina revealed a single band at about 101 kDa. The patterns of gfD1r immunoreactivity (gfD1r-IR), found in rat caudate putamen and rat retina were virtually identical to that previously reported with other D1-receptor ligands and antisera. In goldfish retina, gfD1r-IR was most intense over cell bodies in the ganglion cell layer, amacrine cells in the proximal inner nuclear layer (INL), and bipolar cells in the distal INL. Weaker gfD1r-IR was observed over horizontal cell bodies and both plexiform layers. Müller cells and axons of cone photoreceptors were labeled as well. Double labeling showed that all protein kinase C-immunoreactive bipolar cells (ON type) were gfD1r-IR on the soma, axon terminal, and dendrites. All glutamate decarboxylase-immunoreactive (i.e., gamma-aminobutyric acid utilizing) amacrine cells and horizontal cells were gfD1r-IR. Retinal D1r distribution is more extensive than dopamine neuron innervation, but is consistent with physiologic estimates of dopamine function, suggestive of both wiring and volume transmission of dopamine in the retina. The gfD1r antiserum displays cross-reactivity to dopamine receptors in a mammal and a nonmammal and should prove useful in future studies of dopaminergic systems.

Expression of mt1 melatonin receptor in rat retina: evidence for multiple cell targets for melatonin

Melatonin is synthesized in the retina at night and acts as a local modulator within this tissue by mediating the effects of darkness. We investigated the expression and localization of the mt1 (Mel1a) melatonin receptor in rat retina in order to disclose the cellular and molecular bases of melatonin's action in the mammalian retina. Western blotting of the mt1 receptor in rat retina exhibited a single immunoreactive band of approximately 37,000 mol. wt, which corresponds to the predicted molecular size of the receptor. The mt1 receptor was immunocytochemically localized to both the inner and outer plexiform layers. During postnatal development, retina from two-week-old rats showed the highest mt1 immunoreactivity; the outer plexiform layer and horizontal cell bodies were strongly immunolabeled, with weaker labeling in the inner plexiform layer. Expression of mt1 receptor messenger RNA in the rat retina was demonstrated by reverse transcription-polymerase chain reaction and in situ hybridization. mt1 receptor transcripts were localized to ganglion cells, amacrine cells and horizontal cells. These results suggest that melatonin influences retinal physiology by acting on multiple retinal cell types, including ganglion, amacrine and horizontal cells, via the mt1 receptor expressed in their processes.

The feedback pathway from horizontal cells to cones. A mini review with a look ahead

The feedback pathway from HCs to cones forms the basis of the surround responses of the bipolar cells and is essential for the spectral opponency of horizontal cells. The nature of this feedback pathway is an issue of debate. Three hypothesis are presented in literature: (1) a GABAA-ergic feedback pathway; (2) a GABA-independent feedback pathway that modulates the Ca-current in cones; and (3) an electrical feedback pathway. In this review the evidence for the various pathways will be discussed. The conclusion is that the available evidence favors the hypothesis that feedback modulates the Ca-current in the cones in a GABA independent way. An alternative role of GABA in the outer plexiform layer is discussed and finally the functional consequences of the negative feedback pathway from horizontal cells to cones are presented.

Dopamine receptor localization in the mammalian retina

After a short history of dopamine receptor discovery in the retina and a survey on dopamine receptor types and subtypes, the distribution of dopamine receptors in the retinal cells is described and correlated with their possible role in cell and retinal physiology. All the retinal cells probably bear dopamine receptors. For example, the recently discovered D1B receptor has a possible role in modulating phagocytosis by the pigment epithelium and a D4 receptor is likely to be involved in the inhibition of melatonin synthesis in photoreceptors. Dopamine uncouples horizontal and amacrine cell-gap junctions through D1-like receptors. Dopamine modulates the release of other transmitters by subpopulations of amacrine cells, including that of dopamine through a D2 autoreceptor. Ganglion cells express dopamine receptors, the role of which is still uncertain. Müller cells also are affected by dopamine. A puzzling action of dopamine is observed in the ciliary retina, in which D1- and D2-like receptors are likely to be involved in the cyclic regulation of intraocular pressure. Most of the dopaminergic actions appear to be extrasynaptic and the signaling pathways remain uncertain. Further studies are needed to better understand the multiple actions of dopamine in the retina, especially those that implicate rhythmic regulations.

Regulation of ligand-gated ion channels by protein phosphorylation

The studies discussed in this review demonstrate that phosphorylation is an important mechanism for the regulation of ligand-gated ion channels. Structurally, ligand-gated ion channels are heteromeric proteins comprised of homologous subunits. For both the AChR and the GABA(A) receptor, each subunit has a large extracellular N-terminal domain, four transmembrane domains, a large intracellular loop between transmembrane domains M3 and M4, and an extracellular C-terminal domain (Fig. 1B). All the phosphorylation sites on these receptors have been mapped to the major intracellular loop between M3 and M4 (Table 1). In contrast, glutamate receptors appear to have a very large extracellular N-terminal domain, one membrane hairpin loop, three transmembrane domains, a large extracellular loop between transmembrane domains M3 and M4, and an intracellular C-terminal domain (Fig. 1C). Most phosphorylation sites on glutamate receptors have been shown to be on the intracellular C-terminal domain, although some have been suggested to be on the putative extracellular loop between M3 and M4 (Table 1). A variety of extracellular factors and intracellular signal transduction cascades are involved in regulating phosphorylation of these ligand-gated ion channels (Fig. 2). Once again, the AChR at the neuromuscular junction is the most fully understood system. Phosphorylation of the AChR by PKA is stimulated synaptically by the neuropeptide CGRP and in an autocrine fashion by adenosine released from the muscle in response to acetylcholine. In addition, acetylcholine, via calcium influx through the AChR, appears to activate calcium-dependent kinases including PKC to stimulate serine phosphorylation of the receptor. Presently, agrin is the only extracellular factor known to stimulate phosphorylation of the AChR on tyrosine residues. For glutamate receptors, non-NMDA receptor phosphorylation by PKA is stimulated by dopamine, while NMDA receptor phosphorylation by PKA and PKC can be induced via the activation of beta-adrenergic receptors, and metabotropic glutamate or opioid receptors, respectively. In addition, Ca2+ influx through the NMDA receptor has been shown to activate PKC. CaMKII, and calcineurin, resulting in phosphorylation of AMPA receptors (by CaMKII) and inactivation of NMDA receptors (at least in part through calcineurin). In contrast to the AChR and glutamate receptors, no information is presently available regarding the identities of the extracellular factors and intracellular signal transduction cascades that regulate phosphorylation of the GABA(A) receptor. Surely, future studies will be aimed at further clarifying the molecular mechanisms by which the central receptors are regulated. The presently understood functional effects of ligand-gated ion channel phosphorylation are diverse. At the neuromuscular junction, a regulation of the AChR desensitization rate by both serine and tyrosine phosphorylation has been demonstrated. In addition, tyrosine phosphorylation of the AChR or other synaptic components appears to play a role in AChR clustering during synaptogenesis. For the GABA(A) receptor, the data are complex. Both activation and inhibition of GABA(A) receptor currents as a result of PKA and PKC phosphorylation have been reported, while phosphorylation by PTK enhances function. The predominant effect of glutamate receptor phosphorylation by a variety of kinases is a potentiation of the peak current response. However, PKC also modulates clustering of NMDA receptors. This complexity in the regulation of ligand-gated ion channels by phosphorylation provides diverse mechanisms for mediating synaptic plasticity. In fact, accumulating evidence supports the involvement of protein phosphorylation and dephosphorylation of AMPA receptors in LTP and LTD respectively. There has been a dramatic increase in our understanding of the nature by which phosphorylation regulates ligand-gated ion channels. However, many questions remain unanswered. (AB

Plasticity of first-order sensory synapses: interactions between homosynaptic long-term potentiation and heterosynaptically evoked dopaminergic potentiation

Persistent potentiations of the chemical and electrotonic components of the eighth nerve (NVIII) EPSP recorded in vivo in the goldfish reticulospinal neuron, the Mauthner cell, can be evoked by afferent tetanization or local dendritic application of an endogenous transmitter, dopamine (3-hydroxytyramine). These modifications are attributable to the activation of distinct intracellular kinase cascades. Although dopamine-evoked potentiation (DEP) is mediated by the cAMP-dependent protein kinase (PKA), tetanization most likely activates a Ca2+-dependent protein kinase via an increased intracellular Ca2+ concentration. We present evidence that the eighth nerve tetanus that induces LTP does not act by triggering dopamine release, because it is evoked in the presence of a broad spectrum of dopamine antagonists. To test for interactions between these pathways, we applied the potentiating paradigms sequentially. When dopamine was applied first, tetanization produced additional potentiation of the mixed synaptic response, but when the sequence was reversed, DEP was occluded, indicating that the synapses potentiated by the two procedures belong to the same or overlapping populations. Experiments were conducted to determine interactions between the underlying regulatory mechanisms and the level of their convergence. Inhibiting PKA does not impede tetanus-induced LTP, and chelating postsynaptic Ca2+ with BAPTA does not block DEP, indicating that the initial steps of the induction processes are independent. Pharmacological and voltage-clamp analyses indicate that the two pathways converge on functional AMPA/kainate receptors for the chemically mediated EPSP and gap junctions for the electrotonic component or at intermediaries common to both pathways. A cellular model incorporating these interactions is proposed on the basis of differential modulation of synaptic responses via receptor-protein phosphorylation.

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.

Differential expression of ionotropic glutamate receptor subunits in the outer retina

Ionotropic glutamate receptors (iGluRs) are extremely diverse in their subunit compositions. To understand the functional consequences of this diversity, it is necessary to know the subunits that are expressed by known cell types. By using immunocytochemistry with light and electron microscopy, we localized several subunits (GluR2/3, GluR4, and GluR6/7) in cat retinal neurons, postsynaptic to photoreceptors. Type A horizontal cells express all three subunits strongly, whereas type B horizontal cells express GluR2/3 strongly, GluR6/7 weakly, and do not express GluR4. When they are present, the subunits are expressed strongly throughout the cytoplasm of the somata and primary dendrites; however, in the terminals, they are concentrated at the postsynaptic region, just opposite the presumed site of photoreceptor glutamate release. Surprisingly, all bipolar cell classes (OFF cone bipolar cells, ON cone bipolar cells, and rod bipolar cells) express at least one iGluR subunit at their dendritic tips. Cone bipolar cells forming basal contacts with the cones (presumably OFF cells) express all three subunits in association with the electron-dense postsynaptic membrane. Invaginating dendrites of cone bipolar cells (presumably ON cells) express GluR2/3 and GluR4. Rod bipolar cells (ON cells) express GluR2/3 in their invaginating dendrites. The function of iGluRs in horizontal cells and OFF bipolar cells clearly is to mediate their light responses. GluR6/7 subunit in the receptor of these cells may be responsible for the dopamine-mediated enhancement of glutamate responses that have been observed previously in these cells. The function of iGluRs in ON bipolar cells remains an enigma.

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.

A mixed D1 and D2 antagonist does not replay pattern electroretinogram alterations observed with a selective D2 antagonist in normal humans: relationship with Parkinson's disease pattern electroretinogram alterations

The human retina produces a tuned response to stimuli of increasing spatial frequency reversed at a steady state. The peak amplitude response, at medium spatial frequencies, is decreased in Parkinson's disease and in normal subjects (n = 18) treated with a D2 dopaminergic antagonist (l-sulpiride). Here, we report that a mixed D1-D2 receptor antagonist (haloperidol) in normal subjects (n = 18) does not produce an amplitude decrease of medium spatial frequencies (SFs) responses but it decreases low-frequency response. It could argued that the increased dopamine release produced by the presynaptic D2 antagonistic action of haloperidol is subsequently counteracted at postsynaptic level by its D1 antagonistic effect, producing a net counterbalance at medium SFs. These data suggest that the two dopamine receptors may play different roles in the retinal function and in the origin of visual alterations in Parkinson's disease.

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.

Endothelium-derived hyperpolarizing factor--a critical appraisal

Endothelium-derived hyperpolarizing factor is defined as that substance which produces vascular smooth muscle hyperpolarization which cannot be explained by nitric oxide or by a cyclo-oxygenase product such as prostacyclin. The possibility that the factor is an epoxyeicosatrienoic acid or a cannabinoid agonist such as anandamide continues to be investigated, but definitive evidence in favour of either is lacking. The sensitivity of EDHF-mediated responses to charybdotoxin, to apamin or to mixtures of these two toxins may indicate the opening of more than one smooth muscle K-channel, but the possibility that these are located on the vascular endothelium is discussed.

Multiple second-messenger system modulation of voltage-activated calcium currents in teleost retinal horizontal cells

Two voltage-activated calcium currents, a transient T-type and a PL-sustained type, have been measured in isolated, cultured white bass horizontal cells. These two voltage-activated calcium currents were found to be modulated by two independent second-messenger systems. Furthermore, activation of either second-messenger system led to similar changes in calcium current activity. Activation of the cyclic AMP second-messenger pathway or the sn-1,2-diacylglycerol (DAG) second-messenger system resulted in a significant decrease in the amplitude of the transient current and a simultaneous large increase in the amplitude of the sustained current. Both second-messenger systems achieved their effects through protein phosphorylation. The cyclic AMP pathway resulted in the activation of protein kinase A (PKA) and the DAG pathway worked to activate protein kinase C (PKC). Two protein kinase inhibitors were analyzed in this study for their ability to inhibit second-messenger activated protein kinase activity and separate the two pathways. The peptide cyclic AMP-dependent protein kinase inhibitor and staurosporine were found to be nonspecific at high concentrations and inhibited both second-messenger pathways. At low concentrations however, staurosporine specifically inhibited only PKC, whereas adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase inhibitor was selective for PKA. Both second-messenger systems were activated by the neuromodulator, dopamine. Thus one agonist can initiate multiple second-messenger systems leading to similar changes in voltage-activated calcium current activity. The modulatory action on calcium currents produced by one second-messenger system added to the modulatory action resulting from activation of the other second-messenger system. The effect is to alter the magnitude of the horizontal cell calcium currents.

Nitric oxide depresses GABAA receptor function via coactivation of cGMP-dependent kinase and phosphodiesterase

Nitric oxide (NO) is thought to play an essential role in neuronal processing, but the downstream mechanisms of its action remain unclear. We report here that NO analogs reduce GABA-gated currents in cultured retinal amacrine cells via two distinct, but convergent, cGMP-dependent pathways. Either extracellular application of the NO-mimetic S-nitroso-N-acetyl-penicillamine (SNAP) or intracellular perfusion with cGMP depressed GABA currents. This depression was partially blocked by a pseudosubstrate peptide inhibitor of cGMP-dependent protein kinase (PKG), suggesting both PKG-dependent and independent actions of cGMP. cAMP-dependent protein kinase (PKA) is known to enhance retinal GABA responses. 8-Bromoinosine 3',5'-cyclic monophosphate (8Br-cIMP), which activates a type of cGMP-stimulated phosphodiesterase that hydrolyzes cAMP, also significantly reduced GABA currents. 1-Methyl-3-isobutylxanthine (IBMX), a nonspecific phosphodiesterase (PDE) inhibitor, blocked both the action of 8Br-cIMP and the portion of SNAP-induced depression that was not blocked by PKG inhibition. Our results suggest that NO depresses retinal GABAA receptor function by simultaneously upregulating PKG and downregulating PKA.

Does pattern electroretinogram spatial tuning alteration in Parkinson's disease depend on motor disturbances or retinal dopaminergic loss?

Systemic decrease of dopaminergic cells, such as in Parkinson's disease may produce visual alterations in humans. In order to show possible pattern electroretinogram (PERG) spatial tuning function (STF) alterations due to impaired dopaminergic transmission in humans, we studied a group of Parkinson's disease patients before and during treatment with the dopamine precursor, levodopa, and compared their performances with those of an age-matched control group. Moreover, in order to exclude the possible involvement of motor disabilities to produce PERG alterations, we also investigated PERG responses in post-traumatic parkinsonian patients who exhibited motor abnormalities as a consequence of focal lesions of basal ganglia, in the absence of systemic dopaminergic degeneration. Our results showed a clear decrease of PERG responses in Parkinson's disease patients particularly at medium spatial frequency range (2.7-4.0 cycles/degree) with a substantial preservation of responses at low frequencies. Levodopa therapy reversed these alterations in Parkinson's disease patients, resulting in the recovery of a normal tuning function shape. In contrast to Parkinson's disease, the tuning function appeared to be preserved in post-traumatic parkinsonian patients. Our results clearly establish a relationship between retinal alteration in PD patients and dopaminergic retinal function.

Excitatory amino acid receptor antagonists decrease hypoxia induced increase in extracellular dopamine in striatum of newborn piglets

The present study tested the hypothesis that the increase in extracellular striatal dopamine during hypoxia is least partly associated with activation of N-methyl-D-aspartate (NMDA) and/or non-NMDA excitatory amino acid receptors. Studies were performed in anesthetized and mechanically ventilated 2-3 days old piglets. Hypoxic insult was induced by decreasing the oxygen fraction in inspired gas (FiO2) from 22 to 7% for 1 h, followed by 1 h reoxygenation at 22%. Cortical oxygen pressure was measured optically by oxygen dependent quenching of phosphorescence, and extracellular striatal dopamine was measured using in vivo microdialysis. The microdialysis probes were perfused with Ringer solution +/- 50 microM (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801) or 50 microM 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline (NBQX). One hour of hypoxia decreased the cortical oxygen pressure from 46 +/- 3 Torr to 10 +/- 1.8 Torr. In striatum perfused with Ringer, statistically significant increase in extracellular dopamine, to 1050 +/- 310% of control, was observed after 20 min of hypoxia. By 40 min of hypoxia the extracellular level of dopamine increased to 4730 +/- 900% of control; by the end of the hypoxic period the values increased to 18,451 +/- 1670% of control. The presence of MK-801 in the perfusate significantly decreased the levels of extracellular dopamine during hypoxia. At 20, 40 and 60 min of hypoxia extracellular level of dopamine increased to 278 +/- 94% of control, 1530 +/- 339% of control and 14,709 +/- 1095 of control, respectively. The presence of NBQX caused a statistically significant decrease, by about 30%, in the extracellular dopamine compared to control, only at the end of the hypoxic period. It can be concluded that in striatum of newborn piglets, the excitatory NMDA receptors but not the non-NMDA receptors may be modulating the changes in extracellular levels of dopamine. The NMDA receptor antagonist, MK-801, may exert part of its reported neuroprotective effect to hypoxic stress in striatum by decreasing the levels of extracellular dopamine.

Tyrosine hydroxylase- and/or aromatic L-amino acid decarboxylase-containing cells in the suprachiasmatic nucleus of the Syrian hamster (Mesocricetus auratus)

Catecholamines, including dopamine (DA), affect the activity of cells in the suprachiasmatic nucleus (SCN) of the hypothalamus, the principal circadian clock in mammals. This study examined the distribution of dopaminergic cells in the SCN of the male Syrian hamster, using both single- and double-label immunocytochemistry for tyrosine hydroxylase (TH), the rate-limiting enzyme in DA synthesis and for aromatic L-amino acid decarboxylase (AADC), the second enzyme needed to produce DA. Some neurons immunopositive for TH (TH + ) were found in the SCN, but most of the TH + cells of the region were located just outside the borders of the nucleus, as defined by pyronin Y staining. In the SCN, 91% of these cells were also immunopositive for AADC and thus, likely to be dopaminergic. Cells positive for AADC, many of which were not TH +, were found throughout the SCN, with the highest concentration seen in the ventral aspects of the nucleus. Cells containing AADC, but lacking TH may synthesize products other than DA, such as trace amines. These anatomical observations suggest that local neurons that produce DA and perhaps trace amines, may play a role in SCN function and in the neural control of circadian rhythms.

Parallel increase of heterochromatic increment threshold and postadaptation thresholds in Parkinson's disease and in neuroleptic treatment

Following reports on a predominant loss of blue/yellow contrast sensitivity in Parkinson's disease, we revisited the physiological phenomenon of transient tritanopia. Normative data were collected from 33 healthy individuals using different colour and time combinations. Stimuli of 440 nm wavelength (blue) proved optimal, if flashed for 50 msec within the early phase of a 2 sec pause in the 600 nm adaptation light. These conditions were then applied to 15 patients with Parkinson's disease. We found a parallel increase of increment threshold (P < 0.001) and postadaptation thresholds (P < 0.01), with little change in the extent of transient tritanopia. The same tendency at a lower significance level was found in 15 psychiatric patients under chronic treatment with depot neuroleptics.

Neurobiology of retinal dopamine in relation to degenerative states of the tissue

Neurobiology of retinal dopamine is reviewed and discussed in relation to degenerative states of the tissue. The Introduction deals with the basic physiological actions of dopamine on the different neurons in vertebrate retinae with an emphasis upon mammals. The intimate relationship between the dopamine and melatonin systems is also covered. Recent advances in the molecular biology of dopamine receptors is reviewed in some detail. As degenerative states of the retina, three examples are highlighted: Parkinson's disease; ageing; and retinal dystrophy (retinitis pigmentosa). As visual functions controlled, at least in part, by dopamine, absolute sensitivity, spatial contrast sensitivity, temporal (including flicker) sensitivity and colour vision are reviewed. Possible cellular and synaptic bases of the visual dysfunctions observed during retinal degenerations are discussed in relation to dopaminergic control. It is concluded that impairment of the dopamine system during retinal degenerations could give rise to many of the visual abnormalities observed. In particular, the involvement of dopamine in controlling the coupling of horizontal and amacrine cell lateral systems appears to be central to the visual defects seen.

Dopaminergic modulation of excitatory postsynaptic currents in rat neostriatal neurons

Gamma-aminobutyric acid (GABA)-containing medium spiny neurons constitute approximately 90% of the neuronal population in the neostriatum (caudate and putamen) and play an important role in motor programming. Cortical glutamatergic afferents provide the main excitatory drive for these neurons, whereas nigral dopaminergic neurons play a crucial role in regulating their activity. To further investigate the mechanisms underlying the dopaminergic modulation of medium spiny neuronal activity, we tested the effect of dopamine receptor agonists on excitatory synaptic transmission recorded from these neurons. Excitatory postsynaptic currents (EPSCs) were evoked by local stimulation and recorded from medium spiny neurons in postnatal rat striatal thin brain slices. Recordings were made using the whole cell patch-clamp technique under voltage clamp and conditions that selected for the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate- and kainate-type glutamate receptor-mediated components of the EPSC. Incubation of slices in 10 microM dopamine resulted in a 33 +/- 11% (mean +/- SE) decrease in the amplitude of evoked EPSCs, an effect that developed during seconds. The relative variability in amplitude of dopamine's effects on medium spiny neuron EPSCs may reflect activation of different receptor subtypes with opposing effects. In contrast to the results with dopamine, incubation of slices in SKF 38393, a D1-type dopamine receptor selective agonist, resulted in dose-dependent potentiation of the medium spiny neuron EPSC that developed during several minutes. At a concentration of 5 microM, SKF 38393 resulted in a 29 +/- 4.5% increase in EPSC amplitude, an effect that was blocked by preincubation with the D1-selective antagonist, SCH 23390 (10 microM). On the other hand, 5 microM SKF 38393 had no apparent effect on medium spiny neuron currents activated by exogenous application of glutamate or kainate. However, because of the inherent limitations of rapid agonist perfusion in the brain slice preparation (caused by slow agonist diffusion and rapid glutamate receptor desensitization) and because of anatomic evidence that colocalizes D1 and glutamate receptors to medium spiny neuron dendrites, our results leave open the possibility that the effect of D1 receptor activation on the EPSC is mediated via modulation of postsynaptic glutamate receptor responsiveness. The significant potentiation by D1 receptor agonists of EPSC amplitude at the cortico-striatal medium spiny synapse that we observed, in part, may underlie the role of D1 receptors in facilitating medium spiny neuronal firing, with implications for understanding regulation of movement.

Antipsychotic drug effects on glutamatergic activity

Previous work from this laboratory indicated that some antipsychotic drugs possess unique action at N-methyl-D-aspartate (NMDA) receptors. A functional neurochemical assay showed that, at concentrations similar to those found in the cerebrospinal fluid (CSF) of schizophrenics, antipsychotic drugs augment NMDA activity while, at higher concentrations, NMDA activity is suppressed. Using similar analysis, the present paper reports that this pattern of response is also shown by the antipsychotic drugs thioridazine and chlorpromazine. In contrast, promazine, which is structurally similar to chlorpromazine but lacking both D2-effects and antipsychotic potency, had no influence on NMDA receptors. In addition, sulpiride and metoclopramide, drugs with high affinity for D2-dopamine receptors but with weak or no antipsychotic efficacy, also lack effects at the NMDA receptor. Thus, the drugs with clinical efficacy that were tested in the present and previous studies all share unique influence on NMDA receptors. Further work with other antipsychotic agents will be necessary to determine if influence on NMDA receptors contributes to antipsychotic effectiveness.

Properties of glutamate-gated ion channels in horizontal cells of the perch retina

The effect of two different concentrations of L-glutamate and kainate on the gating kinetics of amino acid-sensitive non-NMDA channels were studied in cultured teleost retinal horizontal cells by single-channel recording and by noise analysis of whole-cell currents. When the glutamate agonist kainate was applied clearly parabolic mean-variance relations of whole-cell membrane currents (up to 3000 pA) indicated that this agonist was acting on one type of channels with a conductance of 5-10 pS. The cells were less sensitive when L-glutamate was used as the agonist and in most cases whole-cell currents amounted to less than 200 pA. The mean-variance relation of glutamate induced currents was complex, indicating that more than one type of channel opening could be involved. Power spectra of whole-cell currents were fitted with two Lorentzians with time constants of approx. 1 and 5-20 msec. Effects on amplitudes and time constants of agonist concentrations are demonstrated. Two categories of unitary events with mean open times of approx. 1 and 7 msec and conductances of approx. 7 and 12 pS, respectively, were obtained in single-channel recordings from cell-attached patches at different concentrations of glutamate in the pipette.

Both high- and low voltage-activated calcium currents contribute to the light-evoked responses of luminosity horizontal cells in the Xenopus retina

We examined the contribution of two intrinsic voltage-dependent calcium channels to the light-evoked responses of a non-spiking retinal neuron, the horizontal cell (HC). HC's isolated from the Xenopus retina were studied by the whole cell version of the patch clamp. In a mixture of agents which suppressed Na- and K-dependent currents, we identified a transient, low voltage-activated Ca current suppressed by Ba2+ and blocked by Ni2+ (T-type) and a sustained, high voltage-activated, dihydropyridine-sensitive Ca current that was enhanced by Ba2+ (L-type). We made simultaneous intracellular recordings from rods and HC's in the intact, dark-adapted Xenopus retina. Under certain stimulus conditions, transient oscillations appeared in HC responses but were absent in rod light-evoked waveforms. One type of transient was seen at relatively hyperpolarized potentials (< -45 mV), was enhanced by Sr2+ and inhibited by Ni2+. It thus appears to depend on a T-type Ca-current. A second type of oscillation was seen to be superimposed on a prolonged depolarizing wave following light off in the HC and as spike-like depolarizations in rods. These oscillations were enhanced by Ba2+ and Sr2+, but blocked by the dihydropyridine, nifedipine, indicating their dependence on an L-type calcium conductance. All calcium-dependent oscillations were suppressed by 0.05-0.5 mM Co2+. Suppression of glutamate neurotransmission with CNQX or kynurenate, or glycine neurotransmission with strychnine, enhanced the HC oscillations.

Regionally specific effects of haloperidol and clozapine on dopamine reuptake in the striatum

This study examined the extent to which local application of the typical neuroleptic haloperidol (HAL) or the atypical neuroleptic clozapine (CLOZ) influences dopamine (DA) transporter function in the dorsal and ventral striatum. Using urethane-anesthetized rats, DA was pressure ejected and monitored with in vivo electrochemistry, into the dorsal and ventral striatum to establish regional baseline DA reuptake rates. Haloperidol or CLOZ (10 microM) was then applied, followed 5 min later by DA, in order to assess drug effects on DA reuptake rates. Haloperidol caused a 62% decrease in dorsal striatal DA reuptake rates while CLOZ had no effect on reuptake rates. Neither neuroleptic significantly altered DA reuptake rates in the ventral striatum. It is possible that HAL-induced decrease in DA reuptake in the sensorimotor striatum could be related to the motor side effect profile of this neuroleptic. Additional studies with other typical and atypical neuroleptics are needed to further evaluate the relationship between slowing of DA reuptake and the side effect potential of neuroleptic agents.

Regulation of excitatory and inhibitory neurotransmitter-gated ion channels by protein phosphorylation

Phosphorylation of ligand-gated ion channels is recognised as a potentially important mechanism for short- and long-term modulation of ion-channel function. Following the discovery of numerous sites of phosphorylation on ligand-gated ion channel proteins, recent studies have demonstrated that neurotransmitter-induced activation of serine/threonine, tyrosine and other kinases can result in the modulation of glutamate, type A gamma-aminobutyric acid (GABAA) and glycine receptors. These findings may have important consequences for our understanding of synaptic transmission and neuronal excitability.

Immunocytochemical localization of dopamine D1 receptors in the retina of mammals

Dopamine is one of the major neurotransmitters in the retina. It is released from amacrine and interplexiform cells into both inner (IPL) and outer (OPL) plexiform layers. Several dopaminergic actions are known to occur through D1 receptors (D1R) but the precise location of these receptors has not been established. An antibody that recognizes the intracytoplasmic C-terminal of the rat D1R was used to detect D1R, immunohistochemically, in rats (Wistar and RCS), mouse, hamster, and macaque monkey retinas. The OPL was heavily stained in each species, consistent with the known actions of dopamine on horizontal cells. Three to five bands were observed in the IPL, depending on species. Three were in the a sublayer, the outermost of which was close to the amacrine cell layer, and may represent the massive dopamine input to the AII rod-amacrine cells. As observed in mice, where bipolar cells are D1-immunoreactive, the band located in sublayer 3 of the IPL may contain cone-bipolar cell terminals. A band of D1R-immunoreactivity in the b sublayer of the IPL contains ON-bipolar cell terminals and a second site of interaction between dopaminergic cells and the AII amacrine cells. This sublayer was absent from the RCS rat retina, suggesting a severe impairment of the rod-driven pathway following rod degeneration in these mutant rats. Cells in the ganglion cell layer exhibited relatively heavy staining, and may be ganglion cells or displaced amacrine cells. Some extrasynaptic localizations of D1R in the retina are suggested.