Biomedical subjects

P Witkovsky

Publications and source records attributed to P Witkovsky.

At least 19 recordsLinked to original sources

PubMed2000

Somatostatin modulates voltage-gated K(+) and Ca(2+) currents in rod and cone photoreceptors of the salamander retina.

We investigated the cellular localization in the salamander retina of one of the somatostatin [or somatotropin release-inhibiting factor (SRIF)] receptors, sst(2A), and studied the modulatory action of SRIF on voltage-gated K(+) and Ca(2+) currents in rod and cone photoreceptors. SRIF immunostaining was observed in widely spaced amacrine cells, whose perikarya are at the border of the inner nuclear layer and inner plexiform layer. sst(2A) immunostaining was seen in the inner segments and terminals of rod and cone photoreceptors. Additional sst(2A) immunoreactivity was expressed by presumed bipolar and amacrine cells. SRIF, at concentrations of 100-500 nM, enhanced a delayed outwardly rectifying K(+) current (I(K)) in both rod and cone photoreceptors. SRIF action was blocked in cells pretreated with pertussis toxin (PTX) and was substantially reduced by intracellular GDP(beta)S. Voltage-gated L-type Ca(2+) currents in rods and cones were differently modulated by SRIF. SRIF reduced Ca(2+) current in rods by 33% but increased it in cones by 40%, on average. Both effects were mediated via G-protein activation and blocked by PTX. Ca(2+)-imaging experiments supported these results by showing that 500 nM SRIF reduced a K(+)-induced increase in intracellular Ca(2+) in rod photoreceptor terminals but increased it in those of cones. Our results suggest that SRIF may play a role in the regulation of glutamate transmitter release from photoreceptors via modulation of voltage-gated K(+) and Ca(2+) currents.

Ambystoma↗

PubMed1999

Caffeine-sensitive calcium stores regulate synaptic transmission from retinal rod photoreceptors.

We investigated the role of caffeine-sensitive intracellular stores in regulating intracellular calcium ([Ca(2+)](i)) and glutamatergic synaptic transmission from rod photoreceptors. Caffeine transiently elevated and then markedly depressed [Ca(2+)](i) to below prestimulus levels in rod inner segments and synaptic terminals. Concomitant with the depression was a reduction of glutamate release and a hyperpolarization of horizontal cells, neurons postsynaptic to rods. Caffeine did not affect the rods' membrane potentials indicating that caffeine likely acted via some mechanism(s) other than a voltage-dependent deactivation of the calcium channels. Most of caffeine's depressive action on [Ca(2+)](i), on glutamate release, and on I(Ca) in rods can be attributed to calcium release from stores: (1) caffeine's actions on [Ca(2+)](i) and I(Ca) were reduced by intracellular BAPTA and barium substitution for calcium, (2) other nonxanthine store-releasing compounds, such as thymol and chlorocresol, also depressed [Ca(2+)](i), and (3) the magnitude of [Ca(2+)](i) depression depended on basal [Ca(2+)](i) before caffeine. We propose that caffeine-released calcium reduces I(Ca) in rods by an as yet unidentified intracellular signaling mechanism. To account for the depression of [Ca(2+)](i) below rest levels and the increased fall rate of [Ca(2+)](i) with higher basal calcium, we also propose that caffeine-evoked calcium release from stores activates a calcium transporter that, via sequestration into stores or extrusion, lowers [Ca(2+)](i) and suppresses glutamate release. The effects of store-released calcium reported here operate at physiological calcium concentrations, supporting a role in regulating synaptic signaling in vivo.

Ambystoma↗

PubMed1999

Glutamate receptors and circuits in the vertebrate retina.

We survey the evidence for L-glutamate's role as the primary excitatory neurotransmitter of vertebrate retinas. The physiological and molecular properties of glutamate receptors in the retina are reviewed in relation to what has been learned from studies of glutamate function in other brain areas and in expression systems. We have focused on (a) the evidence for the presence of L-glutamate in retinal neurons, (b) the processes by which glutamate is released, (c) the presence and function of ionotropic receptors for L-glutamate in retinal neurons, (d) the presence and function of metabotropic receptors for L-glutamate in retinal neurons, and (e) the variety and distribution of glutamate transporters in the vertebrate retina. Modulatory pathways which influence glutamate release and the behavior of its receptors are described. Emphasis has been placed on the cellular mechanisms of glutamate-mediated neurotransmission in relation to the encoding of visual information by retinal circuits.

Animals↗

PubMed1998

Dopamine D2 receptor-mediated modulation of rod-cone coupling in the Xenopus retina.

We studied the responses of rod photoreceptors that were elicited with light flashes or sinusoidally modulated light by using intracellular recording. Dark-adapted Xenopus rod photoreceptors responded to sinusoidally modulated green lights at temporal frequencies between 1 Hz and 4 Hz. In normal Ringer's solution, 57% of the rods tested could follow red lights that were matched for equal rod absorbance to frequencies >5 Hz, indicating an input from red-sensitive cones. Quinpirole (10 microM), a D2 dopamine agonist, increased rod-cone coupling, whereas spiperone (5 microM), a selective D2 antagonist, completely suppressed it. D1 dopamine ligands were without effect. Neurobiotin that was injected into single rods diffused into neighboring rods and cones in quinpirole-treated retinas but only diffused into rods in spiperone-treated retinas. A subpopulation of rods (ca. 10% total rods) received a very strong cone input, which quickened the kinetics of their responses to red flashes and greatly increased the bandpass of their responses to sinusoidally modulated light. Based on electron microscopic examination, which showed that rod-rod and cone-cone gap junctions are common, whereas rod-cone junctions are relatively rare, we postulate that cone signals enter the rod network through a minority of rods with strong cone connections, from which the cone signal is further distributed in the rod network. A semiquantitative model of coupling, based on measures of gap-junction size and distribution and estimates of their conductance and open times, provides support for this assumption. The same network would permit rod signals to reach cones.

Animals↗

PubMed1998

Cholinergic, but not the rod pathway-related glycinergic (All), amacrine cells contain calretinin in the rat retina.

Double-label immunocytochemistry was carried out on cryostat sections of rat retina to test for the presence of calretinin in cholinergic starburst and the rod pathway-related glycinergic (All) amacrine cells. All cholinergic cells contained calretinin, but calretinin-immunoreactive cells were much more numerous in both the inner nuclear and ganglion cell layers than the cholinergic cells. Glycinergic All amacrine cells have been found to contain calretinin in cat, monkey and rabbit retinas. Since All amacrine cells in rat can be selectively labeled with antibodies against parvalbumin, in a second experiment we attempted to colocalize these proteins. We found that calretinin- and parvalbumin-immunoreactive neurons belonged to distinct amacrine cell populations permitting the conclusion that, in the rat retina, All amacrine cells do not contain calretinin. The results indicate that even those amacrine cells of the mammalian retina that are highly conserved with respect to morphology and transmitter content, may differ with respect to other neurochemical characteristics, such as their calcium-binding proteins.

Animals↗

PubMed1998

Calcium released from intracellular stores inhibits GABAA-mediated currents in ganglion cells of the turtle retina.

We studied spiking neurons isolated from turtle retina by the whole cell version of the patch clamp. The studied cells had perikaryal diameters > 15 microns and fired multiple spikes in response to depolarizing current steps, indicating they were ganglion cells. In symmetrical [Cl-], currents elicited by puffs of 100 microM gamma-aminobutyric acid (GABA) were inward at a holding potential of -80 mV. All of the GABA-evoked current was blocked by SR95331 (20 microM), indicating that it was mediated by a GABAA receptor. The GABA-evoked currents were unaltered by eliciting a transmembrane calcium current either just before or during the response to GABA. On the other hand caffeine (10 mM), which induces Ca2+ release from intracellular stores, inhibited the GABA-evoked current on average by 30%. The caffeine effect was blocked by introducing the calcium buffer bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) into the cell but was unaffected by replacing [Ca2+]o with equimolar cobalt. Thapsigargin (10 microM), an inhibitor of intracellular calcium pumps, and ryanodine (20 microM), which depletes intracellular calcium stores, both markedly reduced a caffeine-induced inhibition of the GABA-evoked current. Another activator of intracellular calcium release, inositol trisphosphate (IP3; 50 microM), also progressively reduced the GABA-induced current when introduced into the cell. Dibutyryl adenosine 3'5'-cyclic monophosphate (cAMP; 0.5 mM), a membrane-permeable analogue of cAMP, did not reduce GABA-evoked currents, suggesting that cAMP-dependent kinases are not involved in suppressing GABAA currents, whereas calmidazolium (30 microM) and cyclosporin A (20 microM), which inhibit Ca/calmodulin-dependent phosphatases, did reduce the caffeine-induced inhibition of the GABA-evoked current. Alkaline phosphatase (150 micrograms/ml) and calcineurin (300 micrograms/ml) had a similar action to caffeine or IP3. Antibodies directed against the ryanodine receptor or the IP3 receptor reacted with the great majority of neurons in the ganglion cell layer. We found that these two antibodies colocalized in large ganglion cells. In summary, intracellular calcium plays a role in reducing the currents elicited by GABA, acting through GABAA receptors. The modulatory action of calcium on GABA responses appears to work through one or more Ca-dependent phosphatases.

Alkaline Phosphatase↗

PubMed1997

Gain of rod to horizontal cell synaptic transfer: relation to glutamate release and a dihydropyridine-sensitive calcium current.

We related rod to horizontal cell synaptic transfer to glutamate release by rods. Simultaneous intracellular records were obtained from dark-adapted rod-horizontal cell pairs. Steady-state synaptic gain (defined as the ratio of horizontal cell voltage to rod voltage evoked by the same light stimulus) was 3.35 +/- 0.60 for dim flashes and 1.50 +/- 0.03 for bright flashes. Under conditions of maintained illumination, there was a measurable increment of horizontal cell hyperpolarization for each light-induced increment of rod hyperpolarization over the full range of rod voltages. In separate experiments we studied glutamate release from an intact, light-responsive photoreceptor layer, from which inner retinal layers were removed. Steady light reduced glutamate release as a monotonic function of intensity; spectral sensitivity measures indicated that we monitored glutamate release from rods. The dependence of glutamate release on rod voltage was well fit by the activation function for a high-voltage-activated, dihydropyridine-sensitive L-type calcium current, suggesting a linear dependence of glutamate release on [Ca]i in the synaptic terminal. A simple model incorporating this assumption accounts for the steady-state gain of the rod to horizontal cell synapse.

Animals↗

PubMed1997

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.

6-Cyano-7-nitroquinoxaline-2,3-dione↗

PubMed1997

Dependence of photoreceptor glutamate release on a dihydropyridine-sensitive calcium channel.

A "reduced retina" preparation, consisting of the photoreceptor layer attached to the pigment epithelium in the eyecup, was used to study the pharmacology of the calcium channels controlling glutamate release by photoreceptors in Xenopus. Glutamate release was evoked either by dark adaptation or by superfusion with elevated (20 mM) potassium medium. Both darkness- and potassium-induced release were blocked by cadmium (200 microM). The N-type calcium channel blocker, omega-conotoxin GVIA (500 nM), the P-type calcium channel blocker, omega-agatoxin IVA (20 nM), and the P- and Q-type channel blocker omega-conotoxin MVIIC (1 microM) had no effect on glutamate release. In contrast, the dihydropyridines, nifedipine (10 microM) and nitrendipine (10 microM), which affect L-type calcium channels, blocked both darkness- and potassium-induced release. Bay K 8644 (10 microM), which promotes the open state of L-type calcium channels, enhanced glutamate release. These results indicate that photoreceptor glutamate release is controlled mainly by dihydropyridine-sensitive calcium channels. A dependence of glutamate release on L-type calcium channels also has been reported for depolarizing bipolar cells of a fish retina. Thus, it appears that non-inactivating L-type calcium channels are appropriate to mediate transmitter release in neurons whose physiological responses are sustained, graded potentials.

Animals↗

PubMed1996

Glutamate release by the intact light-responsive photoreceptor layer of the Xenopus retina.

In order to study glutamate release from light responsive photoreceptors, we used an eyecup preparation treated with detergent and distilled water, which permitted removal of the inner retina. The remaining 'reduced' retina consists mainly of photoreceptors attached to the pigment epithelium. The viability of the preparation was established by exclusion of trypan blue, light and electron microscopic examination of the photoreceptor layer and by intracellular recordings from rods. The 'reduced' retina was superfused at 1 ml/h and overflow samples were analyzed for their glutamate content by a fluorimetric enzyme assay. We tested the response to dark and light adaptation and to treatment with 100 microM CdCl2. We found a baseline glutamate level in light-adapted preparation which was not affected by cadmium. Dark adaptation induced a 2-fold increase of glutamate release, which was completely blocked by cadmium.

Animals↗

PubMed1996

D2 dopamine receptor-mediated inhibition of a hyperpolarization-activated current in rod photoreceptors.

1. Using the whole cell patch clamp method, we investigated the effect of dopamine on a hyperpolarization-activated current (Ih) in the inner segments of rod photoreceptors of the Xenopus retina. 2. Ih was elicited by hyperpolarizing voltage steps to -120 mV from a holding potential of -40 mV. Dopamine reversibly reduced Ih in a dose-dependent manner. Dopamine-mediated inhibition of Ih was blocked by the D2 dopamine antagonist sulpiride. 3. The D2 dopamine agonist quinpirole (0.1-20 microM) inhibited Ih whereas the D1 agonist SKF-38393 (100 microM) had no effect on Ih. Quinpirole-induced inhibition of Ih was blocked by sulpiride, but not by the D4 antagonist, clozapine. The D3 agonists (+/-)-7-hydroxy-2-dipropylaminotetralin hydrochloride and trans-7-hydroxy-2[N-propyl-N-(3'-iodo-2'-propenyl)amino]-tetralin maleate were, respectively, 5 and 100 times less effective than quinpirole in inhibiting Ih. 4. Quinpirole failed to reduce Ih when the internal solution contained GDP beta S (500 microM). Internal application GTP gamma S (300 microM) progressively and irreversibly reduced Ih and blocked a further reduction by quinpirole, indicating that the inhibition of Ih by quinpirole involves a G protein. 5. The inhibition of Ih by quinpirole was not affected by intracellularly applied adenosine 3',5'-cyclic monophosphate (cAMP) or by the protein kinase inhibitor H-7, indicating that a cAMP-mediated second messenger cascade does not participate in the dopamine-mediated inhibition. 6. Ih was not altered when the patch pipette contained a nominally Ca(2+)-free internal solution, but the inhibition of Ih by quinpirole was abolished, suggesting an involvement of Ca(2+) in the quinpirole-induced effect. 7. We conclude that a D2 dopamine receptor modulates Ih through the activation of a G protein and that intracellular Ca2+, but not cAMP, plays a key role in this process. 8. The reduction of Ih by dopamine may reduce the ability of rods to signal time-modulated light stimuli.

1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine↗

PubMed1995

Feedback from luminosity horizontal cells mediates depolarizing responses of chromaticity horizontal cells in the Xenopus retina.

It has been proposed that the depolarizing responses of chromaticity horizontal cells (C-HCs) to red light depend on a feedback signal from luminosity horizontal cells (L-HCs) to short-wavelength-sensitive cones in the retinas of lower vertebrates. In this regard we studied the C-HCs of the Xenopus retina. C-HCs and L-HCs were identified by physiological criteria and then injected with neurobiotin. The retina then was incubated with peanut agglutinin, which stains red-but not blue-sensitive cones. Electron microscopic examination revealed that L-HCs contact all cone classes, whereas C-HCs contact only blue-sensitive cones. Simultaneous recordings from C-HC/L-HC pairs established that when the L-HC was saturated by a steady bright red light, C-HCs alone responded to a superimposed blue stimulus. In response to red test flashes, the C-HC response was delayed by approximately 30 msec with respect to the L-HC response. Isolated HCs of both subtypes were examined by whole-cell patch clamp. Both responded to kainate with sustained inward currents and to quisqualate or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) with desensitizing currents from a negative holding potential; i.e., both have AMPA-type glutamate receptors. gamma-Aminobutyric acid or glycine opened a chloride channel in the L-HC, whereas the C-HC was unresponsive to either inhibitory amino acid. Since glycine has been shown to abolish selectively the depolarizing response of the C-HC, this finding and other pharmacological data strongly implicate the L-HC in the underlying circuit. Moreover, because the C-HC does not respond to gamma-aminobutyric acid, the neurotransmitter of the L-HC, by elimination, a feedback synapse from L-HC to blue cone is the most plausible mechanism for the creation of depolarizing responses in C-HCs.

Animals↗

PubMed1994

Dopaminergic neurons in the retina of Xenopus laevis: amacrine vs. interplexiform subtypes and relation to bipolar cells.

Presumed dopaminergic neurons were visualized in the retina of the clawed frog, Xenopus laevis, by anti-tyrosine hydroxylase (TH) immunoreactivity. The studied cells constitute a uniform population with perikarya at the junction of inner nuclear (INL) and inner plexiform (IPL) layers. Each cell body gives rise to 4-6 relatively stout processes (0.5-2.0 microns in diameter) which run for up to 1.2 mm in strata 4-5 of the IPL. These processes have a very asymmetric distribution in the horizontal plane of the retina. A dense plexus of TH fine fibers is distributed uniformly in stratum 1 of the IPL. TH cells are distributed evenly but sparsely (16-20 cells/mm2) across the retina. About 20% of the TH neurons emit 1-3 distally directed fine processes, the majority of which extend < 20 microns, which barely suffices to reach the outer plexiform layer (OPL). Other longer processes are typically unbranched; some reach the OPL, others run tangentially in the INL. The axon terminals of Golgi-impregnated bipolar cells are characterized according to the strata of the IPL in which they arborize. About 80% are confined either to strata 1-2 or 3-5, conforming to the 'off' and 'on' zones defined by Famiglietti and Kolb (1976). The remainder appear to end in both zones, some extending across the entire width of the IPL. EM examination showed that TH processes receive bipolar synaptic input in both distal and proximal portions of the IPL.

Animals↗

PubMed1994

Modulation of transient outward potassium current by GTP, calcium, and glutamate in horizontal cells of the Xenopus retina.

1. Membrane currents of luminosity horizontal cells (L-HCs) and chromatic horizontal cells (C-HCs) isolated from the Xenopus retina were characterized using the whole-cell patch-clamp technique. 2. The current-voltage curve for the L-HC had a characteristic negative slope conductance in the voltage range of -30 to -10 mV that was not evident in the C-HC. 3. A transient outward 4-aminopyridine-sensitive potassium current (A-current) was the most prominent current in C-HCs but was also present in L-HCs. A-current characteristics in the two horizontal cell (HC) classes were closely similar. Its threshold of activation was above -45 mV. The half-voltage of inactivation was close to -70 mV. The decay of the A-current was fit by a single exponential with time constants of 30 and 40 ms at depolarizing voltage steps to -10 and +30 mV, respectively. 4. The voltage for 50% A-current inactivation shifted toward negative potentials shortly after we established the whole-cell configuration. This shift was changed to more positive potentials by internal application of guanosine 5'-triphosphate, resulting in a significant overlap of A-current activation and inactivation functions near -40 mV, which is well within the normal operating range of the HC. 5. Internal application of the G-protein activator GTP gamma S shifted the voltage-dependent inactivation of the A-current toward positive potentials by +15 mV. In contrast, GDP beta S shifted the inactivation curve by about -10 mV, similar to what was observed in untreated cells. 6. GTP and GTP gamma S increased the rate of recovery from inactivation and slowed down the rate of inactivation of the A-current enabled by a depolarizing prepulse. 7. Glutamate superfused in the bath solution significantly accelerated the rate of inactivation of A-current induced by depolarizing prepulses. The rate of A-current recovery from inactivation, however, was not affected by glutamate. 8. Removal of calcium from the bath solution reversibly decreased the amplitude of the A-current without a significant shift in its threshold of activation.

Animals↗

PubMed1994

The effects of L-glutamate, AMPA, quisqualate, and kainate on retinal horizontal cells depend on adaptational state: implications for rod-cone interactions.

We studied the responses of isolated and intact luminosity-type horizontal cells (L-HC) in the Xenopus retina to L-glutamate (L-glu) and its analogs. Isolated L-HCs studied with whole-cell patch clamp responded to L-glu, kainate (KA), AMPA, or quisqualate (quis) with inward currents from a holding potential of -60 mV, associated with a conductance increase. The current elicited by KA was relatively large and sustained, whereas AMPA or quis evoked a desensitizing current. Coapplication of quis and KA resulted in a smaller current and conductance change than that evoked by a pulse of either alone at the same concentration. This finding suggests that the L-HC has a single subtype of glutamate receptor that responds to both quis and KA. Prior exposure to dopamine enhanced the KA-evoked current about twofold. In the superfused eyecup we found that L-HC responses to quinoxalinediones (CNQX or DNQX) and to L-glu, KA, AMPA, and quis varied as a function of adaptational state. When driven exclusively by either cones or by rods, CNQX/DNQX hyperpolarized the L-HC and reduced its light response, without altering response kinetics, indicating that both rods and cones communicate with L-HCs at ionotropic glutamatergic synapses. Under mesopic conditions, however, as CNQX or DNQX reduced cone input, the rod input to the L-HC increased up to fivefold in magnitude and had slowed kinetics. The depolarizing response of the L-HC to L-glu, AMPA, or quis was relatively small and transient under photopic conditions, but was much larger and sustained when the eyecup was dark adapted. The D1 dopamine antagonist SCH 23390 potentiated the response to quis. In contrast, responses to KA were largest in light-adapted eyecups, were potentiated by a D1 dopamine agonist, SKF 38393, and were reduced by SCH 23390. We hypothesize that the segregated populations of glutamate receptors in the L-HC opposite cone and rod synaptic endings can be separately modulated to respond differentially to the native transmitter, glutamate. In photopic and mesopic states the dominant cone input tonically inhibits rod to L-HC communication. This inhibition appears to occur at the postsynaptic membrane and may be mediated by second messengers.

6-Cyano-7-nitroquinoxaline-2,3-dione↗

PubMed1993

Effects of submicromolar concentrations of dopamine on photoreceptor to horizontal cell communication.

Dopamine has been postulated to act as an intraretinal messenger for light adaptation by biasing retinal circuits to favor cone over rod inputs to second- and third-order neurons. As an experimental test, we studied the effects of dopamine and related ligands on rod to horizontal cell synaptic transfer. Rods and horizontal cells (HC) were recorded from simultaneously. Red and green light-emitting diodes were modulated sinusoidally in counterphase at 1 Hz and their relative intensities adjusted to elicit a rod null. Dark-adapted HC's also showed a null, but exposure to 0.5-1.0 microM dopamine, which corresponds to the endogenous levels, elicited a large imbalance in the HC response while the rod null was maintained. Similar effects were achieved with either a D1 dopamine agonist, SKF 38393 (20 microM) or a D2 dopamine agonist, quinpirole HCl (1 microM). Correspondingly, the frequency range over which the HC responded to red flickering lights increased very substantially. Exposure to a D2, but not a D1 dopamine agonist increased rod flicker, but not as dramatically as in the HC. These data indicate that the synaptic gains of rod and cone transmission to a second order retinal neuron are modifiable by endogenous levels of dopamine. Secondly, the bandpass of rod flicker is altered by dopamine, acting through a D2 dopamine receptor.

Animals↗

PubMed1993

Extracellular dopamine concentration in the retina of the clawed frog, Xenopus laevis.

Dopamine reaches targets in the outer retina of the clawed frog (Xenopus laevis) by diffusion from a network of dopaminergic cells and processes located predominantly at the junction of inner nuclear and inner plexiform layers. We obtained values for the steady-state release, uptake, and extracellular concentration of dopamine in the retina by a combination of HPLC (with electrochemical detection), scintillation spectroscopy, and fast-scan cyclic voltammetry. Vitreal concentrations of dopamine varied from 564 +/- 109 nM in light-adapted eyes near the time of subjective dawn to 156 +/- 12 nM in dark-adapted eyes. The data are consistent with a simple model for steady-state dopamine diffusion from an appropriately sited thin-sheet source. This model was used to generate a profile of extracellular dopamine concentration as a function of retinal depth. The model predicted an increase in the dopamine concentration from the vitreous to the layer of dopaminergic cells, remaining constant from that layer to the distal tips of the photoreceptors. This prediction was borne out by comparing fast-scan voltammetric measures of dopamine at the distal tips of the receptors with the vitreal concentrations determined by HPLC using electrochemical detection.

Animals↗

PubMed1992

Light-evoked contraction of red absorbing cones in the Xenopus retina is maximally sensitive to green light.

To test the hypothesis that light-evoked cone contraction in eye cups from Xenopus laevis is controlled through a direct mechanism initiated by the cone's own photopigment, we conducted spectral-sensitivity experiments. We estimate that initiation of contraction of red absorbing cones (611 nm) is 1.5 log units more sensitive to green (533 nm) than red (650 nm) light stimuli. The difference is comparable to that predicted from the spectral-sensitivity function of the green absorbing, principal rod (523 nm). Furthermore, 480-nm and 580-nm stimuli which are absorbed nearly equally by the principal rod have indistinguishable effects on cone contraction. We also found that light blockade of nighttime cone elongation is much more sensitive to green than to red light stimuli. Our observations are inconsistent with the hypothesis tested, and suggest that light-regulated cone motility is controlled through an indirect mechanism initiated primarily by the green absorbing, principal rod.

Animals↗