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P Witkovsky

Publications and source records attributed to P Witkovsky.

At least 37 records · Page 2Linked to original sources
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↗
PubMed1992

Membrane currents of horizontal cells isolated from turtle retina.

1. Membrane currents of horizontal cells isolated from the retina of the turtle, Pseudemys, were characterized by the whole-cell patch-clamp technique. 2. Four membrane currents were identified: an anomalous rectifier blocked by barium, a transient A-current, a sustained L-type calcium current enhanced by Bay K 8644, and a fast, tetrodotoxin-sensitive sodium current. Each of these four currents was found in both horizontal cell somata and axon terminals. 3. The current-voltage relations of axon terminals and somata were similar, but, in the normal operating range of the cell (-30 to -50 mV), the mean slope resistance of the axon terminal was higher (1.38 G omega) than that of the soma (0.26 G omega). 4. Exposure to either glutamate, kainate, or quisqualate induced a sustained inward current in horizontal cell axon terminals. The reversal potential for this current was -3 mV when tested with voltage steps and +9.1 mV when measured by a voltage ramp. The same horizontal cells were insensitive to N-methyl-D-aspartate. 5. A continuum model was developed to compute the degree of signal transfer between a horizontal cell body and its axon terminal. The model consisted of a network of electrically coupled somata that communicates with a network of electrically coupled axon terminals through the connecting axons. The specific membrane resistances used for the model derived from the patch-clamp measures. 6. We computed the voltage change elicited in either the layer of somata or of axon terminals by a static light stimulus of arbitrary dimensions. The amplitude of a spot response as a function of its radius was given by the weighted sum of two Bessel functions with different space constants. 7. The computed responses of the cell body were dominated by the Bessel function with the smaller space constant, whereas those of the axon terminal depended primarily on the Bessel function with the larger space constant. 8. The model predicts that, in contrast to the findings in teleost retina, there is little signal transfer between the somata and axon terminals of horizontal cell in the turtle retina.

Animals↗
PubMed1991

Dopaminergic interplexiform cells and centrifugal fibres in the Xenopus retina.

Putative dopaminergic neurons in the Xenopus retina were identified using an immunoreaction against tyrosine hydroxylase. A single class of cell was stained whose perikaryon (12-15 microns in diameter) was located at the border of the inner nuclear and inner plexiform layers. About 2% of the stained cell bodies were located in the ganglion cell layer, but the distribution of the processes of displaced cells had the same geometry as for the majority of stained cells. Tyrosine hydroxylase-like immunoreactive perikarya gave rise to one to four stout processes that descended to the most proximal level of the inner plexiform layer, within which they branched repeatedly to generate a diffuse network of fine processes. Secondary branches ascended to the most distal sublayer of the inner plexiform layer where they ramified into fine processes that joined other fibres arising horizontally from the cell body and confined to the distal inner plexiform layer throughout their course. The diameter of the dendritic arbor of stained cells was in the range of 350-600 microns. The dense network of fine fibres within the distal inner plexiform layer was arrayed in rings that surrounded other amacrine cells; using an antiserum against glycine we found that at least some of these were glycinergic neurons. Most tyrosine hydroxylase-positive neurons emitted one or two fine ascending processes that arose from the perikaryon, traversed the inner plexiform layer and arborized within the outer plexiform layer. Additionally, fine varicose fibres arising from the sublayer 1 of the inner plexiform layer and running to the outer retina were observed. Thus, based on light microscopic criteria, dopaminergic neurons in the Xenopus retina appeared to be interplexiform cells. A few tyrosine hydroxylase-immunoreactive fibres were observed in the optic nerve, some of which entered the inner retina where they ramified, thus indicating that they were centrifugal axons. In addition, a small number of stout smooth processes were observed to traverse the entire inner nuclear layer and course laterally at the level of the photoreceptor bases. Whether this second class of ascending process arises from the tyrosine hydroxylase-like immunoreactive efferents remains to be determined. The total number of dopaminergic neurons per retina was 750-800, equivalent to an average density of 30 cells mm-2. The dendritic fields of adjacent cells strongly overlapped, with an estimated coverage factor of 4.8.

Animals↗
PubMed1990

Membrane currents of spiking cells isolated from turtle retina.

We examined the membrane properties of spiking neurons isolated from the turtle (Pseudemys scripta) retina. The cells were maintained in culture for 1-7 days and were studied with the whole cell patch clamp technique. We utilized cells whose perikaryal diameters were greater than 15 microns since Kolb (1982) reported that ganglion cell perikarya in Pseudemys retina are 13-25 microns, whereas amacrine perikarya are less than 14 microns in diameter. We identified 5 currents in the studied cells: (1) a transient sodium current (INa) blocked by TTX, (2) a sustained calcium current (ICa) blocked by cobalt and enhanced by Bay-K 8644, (3) a calcium-dependent potassium current (IK(Ca)), (4) an A-type transient potassium current (IA) somewhat more sensitive to 4-AP than TEA, (5) a sustained potassium current (IK) more sensitive to TEA than 4-AP. The estimated average input resistance of the cells at -70 mV was 720 +/- 440 M omega. When all active currents were blocked, the membrane resistance between -130 and +20 mV was 2.5 G omega. When examined under current clamp, some cells produced multiple spikes to depolarizing steps of 0.1-0.3 nA, whereas other cells produced only a single spike irrespective of the strength of the current pulse. Most single spikers had an outward current that rose to a peak relatively slowly, whereas multiple spikers tend to have a more rapidly activating outward current. Under current clamp, 4-AP slowed the repolarization phase of the spike thus broadening it, but did not always abolish the ability to produce multiple spikes. TEA induced a depolarized plateau following the initial spike which precluded further spikes. It thus appears that the spiking patterns of the retinal cells are shaped primarily by the kinetics of INa, IK and IA and to a lesser extent by IK(Ca).

Action Potentials↗
PubMed1990

Serotonin-like immunoreactivity in the retina of the clawed frog Xenopus laevis.

Using an antiserum directed against serotonin, we have studied the morphology and distribution of serotonin-containing and serotonin-accumulating neurons in the retina of Xenopus laevis. Endogenous serotonin-like immunoreactivity was found in two classes of amacrine cell, one class of bipolar cell and a few centrifugal fibres. Kainic acid-induced depletion of serotonin, under various conditions, enabled us to determine the distribution of stained bipolars, amacrine cells and centrifugal fibres within the meshwork of serotonin-like immunoreactivity-labelled processes. Kainic acid-induced release of serotonin by bipolar cells is calcium dependent. Stimulation of release by kainic acid as well as the fact that all serotonin-like immunoreactive bipolar cells ramify in sublayer 1 of the inner plexiform layer suggest that serotonergic bipolars are OFF centre cells. Release of serotonin from amacrine cells is largely calcium independent. Serotonin-containing amacrines send primary dendrites into layer 1 of the inner plexiform layer; short off-shoots from the primary dendrites descend into sublayers 3-5 in which they ramify into a fine network. Serotonergic amacrines have an uneven distribution in the Xenopus retina. Their highest density occurs in the posterolateral quadrant, whereas large portions of the anteromedial quadrant lack serotonin-like immunoreactivity altogether. The uneven distribution of serotonin-containing elements in the Xenopus retina with its peak falling onto the retinal area which generates binocular vision, suggests its involvement in binocular perception.

Animals↗
PubMed1990

Slow light and dark adaptation of horizontal cells in the Xenopus retina: a role for endogenous dopamine.

A role for endogenous dopamine in the control of rod and cone contributions to a second-order retinal neuron, the horizontal cell (HC) was studied in the Xenopus retina. Relative rod and cone contributions were estimated from HC responses to scotopically balanced 491- and 650-nm flashes. In eyecups prepared in light then placed in darkness, cone input to the HC slowed and diminished on a time scale of hours. The decline in cone input was balanced by a slow growth of rod input to the HC. Administration of D-amphetamine, a dopamine releasing agent, restored the light-adapted waveform. The kinetics of slow light adaptation were examined by recording HC responses from eyecups that had been dark-adapted previously for 11-14 h. When test flashes fell on a dark field, cone input to the HC grew for 2-4 h, reached a plateau, and later declined. If, however, flashes were superimposed on a weak background field, cone input to the HC continued to increase monotonically at about 10%/h. This increase was abolished by superfusion with a nonspecific dopamine receptor blocker, cis-flupenthixol (50 microM), resulting in the complete suppression of cone-to-horizontal cell synaptic transfer and the enhancement of rod-to-horizontal cell communication. Subcutaneous injection of reserpine, a drug that depletes dopamine stores (2 mg/kg on 1-4 successive days), or intraocular injection of the dopamine neurotoxin, 6-hydroxydopamine (10-30 micrograms) slowed and reduced the amplitude of cone input to the HC, even in completely light-adapted eyes.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1990

A chromatic horizontal cell in the Xenopus retina: intracellular staining and synaptic pharmacology.

1. We identified a chromatic-type horizontal cell (C-cell) in the Xenopus retina by intracellular dye injection with Lucifer yellow or horseradish peroxidase (HRP). C-cells hyperpolarized in response to blue light and depolarized in response to red light. 2. In either photopic or mesopic states, moderate-intensity blue and red stimuli evoked responses that were inverted with respect to each other but of similar waveform and latency. In the presence of a bright green adapting field, the maximal voltage (Vmax) of the hyperpolarizing and depolarizing response component approached 30 mV; the kinetics of both waveforms were fast, and the hyperpolarizing response was followed by a small depolarizing overshoot at light OFF. Thus the blue-sensitive photoreceptor is capable of initiating large visual signals under photopic conditions when transmission from green-sensitive rods is suppressed. Under mesopic conditions (no adapting field) the kinetics of both waveforms were slower. The Vmax of the hyperpolarizing response reached 30-40 mV, whereas the cone-mediated depolarization saturated at 15 mV. 3. Both response components of the C-cell showed large receptive fields with no center-surround antagonism. 4. The C-cell perikaryon was located in the distal inner nuclear layer. It emitted four to seven long, tapering processes that ran horizontally for 90-100 microns. Two kinds of terminal dendrites, short and long, extended from the tapering processes toward the layer of photoreceptor bases. 5. Glycine (5-10 mM) completely eliminated the depolarizing response of the C-cell, whereas the hyperpolarizing component was unaffected. In contrast, gamma-aminobutyric acid (GABA; 5-10 mM) had no obvious effect on either component. 6. The C-cell light response was modified in two stages by cis-2,3-piperidine dicarboxylic acid (cis-PDA; 0.5-5 mM): first the depolarizing response disappeared; then the membrane potential hyperpolarized concomitant with a large reduction or elimination of the hyperpolarizing light response. In contrast, DL-2-amino-4-phosphonobutyric acid (APB) had no obvious effect on either response component or the membrane potential of the cell. 7. Our pharmacological findings are consistent with the view that the hyperpolarizing response in the C-cell is mediated by direct synaptic input from a blue-sensitive photoreceptor. The depolarizing response mediated by the red-sensitive cone could be explained by a direct synapse from the red cone or an indirect pathway involving luminosity (L-type) horizontal cells.

Adaptation, Ocular↗
PubMed1989

Photoreceptor to horizontal cell synaptic transfer in the Xenopus retina: modulation by dopamine ligands and a circuit model for interactions of rod and cone inputs.

1. In the Xenopus retina, the effects of selective D1 and D2 dopamine ligands on photoreceptor to horizontal cell transfer were studied by intracellular recording from horizontal cell axons. Rod and cone inputs to the horizontal cell were estimated by adjusting the intensities of red and green flashes to elicit equal rod tails. The resultant waveforms were digitized and subtracted, and their difference was taken to reflect solely cone input to the horizontal cell. 2. It was found that both D1 (SKF 38393) and D2 (LY 171555) agonists increased the amplitude and quickened the kinetics of cone-to-horizontal cell transfer; they also depolarized the horizontal cell by 8-10 mV. In contrast, either D1 or D2 agonists reduced the rod input to the horizontal cell without altering its kinetics. 3. Type D2 antagonists reduced and slowed the cone input and hyperpolarized the horizontal cell. D2 antagonists increased the rod input but left its kinetics unchanged. 4. Although both D1 and D2 agonists elicited qualitatively similar effects, the D1 agonist evoked a greater increase in the amplitude and a greater acceleration of the kinetics of the cone input than did the D2 agonist. Moreover, the action of the D1 agonist was blocked by SCH 23390 but not by spiroperidol or metoclopramide, whereas the reverse was true for the D2 agonist. These data indicate that D1 and D2 agonists probably act at different sites. 5. The pharmacologic findings are interpreted to indicate that dopamine ligands act primarily through the cone pathway and that rod-to-horizontal cell transfer is shunted to a variable degree. 6. An equivalent circuit model was developed for a spine-bearing portion of a horizontal cell axon of the Xenopus retina. Anatomic study shows that such spines branch, making contact with both rod and cone photoreceptor bases. Thus there are two conductance pathways in parallel for rod-to-horizontal cell and cone-to-horizontal cell transmission. The model is used to test the hypothesis that mutual shunting in the two pathways can account for the physiological effects observed. 7. The values of the purely resistive elements of the pathway are based on their dimensions. Membrane resistance was taken to be 5,000 omega/cm2 and axial resistance 200 omega/cm. The photoreceptor-to-horizontal cell synaptic battery was taken to be composed of glutamate-sensitive channels, with unitary channel conductance of 6 pS. Channel density was estimated from freeze-fracture data at 5,000 microns-2. A potassium battery and a glycine-sensitive synaptic input from an interplexiform cell were modeled to exist in parallel with the light-sensitive battery. 8. Dopamine was assumed to increase the conductance of the cone-to-horizontal cell synapse, but not to affect the conductance of the rod-to-to-horizontal cell synapse, consistent with physiological measures.(ABSTRACT TRUNCATED AT 400 WORDS)

2,3,4,5-Tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-ben↗
PubMed1988

Morphology and synaptic connections of HRP-filled, axon-bearing horizontal cells in the Xenopus retina.

Axon-bearing horizontal cells of the Xenopus retina were studied by intracellular injection of HRP following physiological characterization. The profile of the cell viewed in whole mount consisted of a round or oval perikaryon about 50 microns in diameter and an axon about 1 mm long which lacked a prominent terminal expansion. The axonal diameter was 0.5-1.0 microns in its proximal third but 2-4 microns in its distal portion. Along its course the axon emitted 25-40 branchlets each 0.2 micron in diameter, up to 10 micron long and terminating in a cluster of two to six synaptic knobs. Electron microscopic examination revealed that both perikaryal dendrites and axon branchlets ended in both rod and cone synaptic bases; cone contacts outnumbered rod contacts by two- to threefold. We were unable to document synapses of presumed interplexiform cells onto identified horizontal cells. Horizontal cell axons are joined in their distal portions by numerous, small (0.2 micron long) gap junctions. Other gap junctions were noted between horizontal cell processes within the synaptic endings of photoreceptors. An hypothesis is advanced whereby the cluster of axon branchlet synaptic knobs permits dynamic interaction of rod and cone synaptic inputs to the horizontal cell.

Animals↗
PubMed1988

Dopamine modifies the balance of rod and cone inputs to horizontal cells of the Xenopus retina.

Dopamine (greater than or equal to 2 microM) increased the cone input and suppressed the rod input to axon-bearing horizontal cells of the Xenopus retina. Dopamine (10 microM) also depolarized the horizontal cell by about 9 mV. The D2-dopamine antagonists spiperone and metoclopramide had the opposite action to dopamine, whereas the D1-dopamine antagonist SCH 23390 was without effect. None of the agents tested modified the light-evoked responses of rods.

Animals↗
PubMed1988

Coexpression of opsin- and VIP-like-immunoreactivity in CSF-contacting neurons of the avian brain.

Cerebrospinal fluid-contacting (CSF) cells in both the septal and the tuberal areas in the brain of the ring dove are labeled by RET-P1, a monoclonal antibody to opsin that reacts with inner and outer segment membranes of rod photoreceptors in a variety of vertebrates. Immunoblot analysis of proteins from diverse brain regions, however, revealed bands of anti-RET-P1 immunoreactivity that did not correspond to opsin. Binding of RET-P1 to opsin-containing membranes, was not inhibited by membranes rich in muscarinic and beta-adrenergic receptor proteins (red blood cells, heart, lung) taken from doves. RET-P1-immunoreactive CSF-contacting cells emit a dendritic process that penetrates the ependyma and ends in a knob-like terminal suspended in the ventricle. These cells also possess other processes that penetrate more or less deeply into the neuropil. Additionally, a band of labeled fibers occurs in the external layer of the median eminence. A double-label technique demonstrated that RET-P1-positive cells coexpress VIP-like immunoreactivity. VIP-positive cells in other brain areas are not RET-P1-positive.

Animals↗
PubMed1988

GABA release from Xenopus retina does not correlate with horizontal cell membrane potential.

The relationship between horizontal cell membrane potential and the release of GABA was explored in the retina of Xenopus laevis. The intracellularly recorded membrane potential of horizontal cells was monitored while the retina was exposed to different concentrations of depolarizing agents. The dose-response curves obtained revealed a rise from 5 to 95% maximum depolarization in 0.5-1.5 log unit concentration change. The molar concentrations that elicited a 20 mV depolarization were 40 mM (potassium), 0.8 mM (glutamate), 0.8 mM (glycine), 5 microM (kainate) and 1.3 microM (quisqualate). Autoradiography revealed that radiolabel was accumulated almost exclusively by horizontal cells when isolated retinas were incubated in medium containing 1 microM [3H]GABA. Thus, retinal release of radioactivity was used as a measure of [3H]GABA release from horizontal cells. Endogenous GABA released from retinas was measured using high performance liquid chromatography and was taken to reflect both amacrine and horizontal cell GABA pools. The release of both [3H]GABA and endogenous GABA was stimulated by glutamate, kainate and potassium, but not by glycine or quisqualate. Similar dose-response curves for GABA release and for depolarization were obtained in the case of potassium and kainate but not for glutamate. Potassium-evoked release either of endogenous GABA or [3H]GABA was both calcium- and sodium-dependent, whereas kainate- or glutamate-evoked GABA release was sodium-dependent but calcium-independent. The results indicate that depolarization per se is not necessarily associated with transmitter release in Xenopus retinal horizontal cells. It is suggested that the action of a given neurotransmitter upon the efflux of GABA from horizontal cells may depend on the degree to which it modifies the sodium conductance of the horizontal cell.

Animals↗