Paul Witkovsky

Dopamine and retinal function

This review summarizes the experimental evidence in support of dopamines role as a chemical messenger for light adaptation. Dopamine is released by a unique set of amacrine cells and activates D1 and D2 dopamine receptors distributed throughout the retina. Multiple dopamine-dependent physiological mechanisms result in an increased signal flow through cone circuits and a diminution of signal flow through rod circuits. Dopamine also has multiple trophic roles in retinal function related to circadian rhythmicity, cell survival and eye growth. In a reciprocal way, the health of the dopaminergic neurons depends on their receiving light-driven synaptic inputs. Dopamine neurons appear early in development, become functional in advance of the animals onset of vision and begin to die in aging animals. Some diseases affecting photoreceptor function also diminish day/night differences in dopamine release and turnover. A reduction in retinal dopamine, as occurs in Parkinsonian patients, results in reduced visual contrast sensitivity

Glutamate receptors and circuits in the vertebrate retina

We survey the evidence for L-glutamates 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.

Synaptic relationships in the plexiform layers of carp retina

SummaryThe synaptic contacts made by carp retinal neurons were studied with electron microscopic techniques. Three kinds of contacts are described: (1) a conventional synapse in which an accumulation of agranular vesicles is found on the presynaptic side along with membrane densification of both pre- and postsynaptic elements; (2) a ribbon synapse in which a presynaptic ribbon surrounded by a halo of agranular vesicles faces two postsynaptic elements; and (3) close apposition of plasma membranes without any vesicle accumulation or membrane densification.In the external plexiform layer, conventional synapses between horizontal cells are described. Horizontal cells possess dense-core vesicles about 1,000 Å in diameter. Membranes of adjacent horizontal cells of the same type (external, intermediate or internal) are found closely apposed over broad regions.In the inner plexiform layer ribbon synapses occur only in bipolar cell terminals. The postsynaptic elements opposite the ribbon may be two amacrine processes or one amacrine process and one ganglion cell dendrite. Amacrine processes make conventional synaptic contacts onto bipolar terminals, other amacrine processes, amacrine cell bodies, ganglion cell dendrites and bodies. Amacrine cells possess dense-core vesicles. Ganglion cells are never presynaptic elements. Serial synapses between amacrine processes and reciprocal synapses between amacrine processes and bipolar terminals are described. The inner plexiform layer contains a large number of myelinated fibers which terminate near the layer of amacrine cells.

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

SummaryCerebrospinal 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 β-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.

Synaptic transmission at retinal ribbon synapses.

The molecular organization of ribbon synapses in photoreceptors and ON bipolar cells is reviewed in relation to the process of neurotransmitter release. The interactions between ribbon synapse-associated proteins, synaptic vesicle fusion machinery and the voltage-gated calcium channels that gate transmitter release at ribbon synapses are discussed in relation to the process of synaptic vesicle exocytosis. We describe structural and mechanistic specializations that permit the ON bipolar cell to release transmitter at a much higher rate than the photoreceptor does, under in vivo conditions. We also consider the modulation of exocytosis at photoreceptor synapses, with an emphasis on the regulation of calcium channels.

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.

The Polymodal Ion Channel Transient Receptor Potential Vanilloid 4 Modulates Calcium Flux, Spiking Rate, and Apoptosis of Mouse Retinal Ganglion Cells

Sustained increase in intraocular pressure represents a major risk factor for eye disease, yet the cellular mechanisms of pressure transduction in the posterior eye are essentially unknown. Here we show that the mouse retina expresses mRNA and protein for the polymodal transient receptor potential vanilloid 4 (TRPV4) cation channel known to mediate osmotransduction and mechanotransduction. TRPV4 antibodies labeled perikarya, axons, and dendrites of retinal ganglion cells (RGCs) and intensely immunostained the optic nerve head. Müller glial cells, but not retinal astrocytes or microglia, also expressed TRPV4 immunoreactivity. The selective TRPV4 agonists 4α-PDD and GSK1016790A elevated [Ca2+]i in dissociated RGCs in a dose-dependent manner, whereas the TRPV1 agonist capsaicin had no effect on [Ca2+]RGC. Exposure to hypotonic stimulation evoked robust increases in [Ca2+]RGC. RGC responses to TRPV4-selective agonists and hypotonic stimulation were absent in Ca2+-free saline and were antagonized by the nonselective TRP channel antagonists Ruthenium Red and gadolinium, but were unaffected by the TRPV1 antagonist capsazepine. TRPV4-selective agonists increased the spiking frequency recorded from intact retinas recorded with multielectrode arrays. Sustained exposure to TRPV4 agonists evoked dose-dependent apoptosis of RGCs. Our results demonstrate functional TRPV4 expression in RGCs and suggest that its activation mediates response to membrane stretch leading to elevated [Ca2+]i and augmented excitability. Excessive Ca2+ influx through TRPV4 predisposes RGCs to activation of Ca2+-dependent proapoptotic signaling pathways, indicating that TRPV4 is a component of the response mechanism to pathological elevations of intraocular pressure.

The organization of dopaminergic neurons in vertebrate retinas

A survey of the shapes of dopaminergic (DA) neurons in the retinas of representative vertebrates reveals that they are divisible into three groups. In teleosts and Cebus monkey, DA cells are interplexiform (IPC) neurons with an ascending process that ramifies to create an extensive arbor in the outer plexiform layer (OPL). All other vertebrates studied, including several primate species, have either DA amacrine cells or IPCs with an ascending process that either does not branch within the OPL or does so to a very limited degree. DA neurons of non-teleosts exhibit a dense plexus of fine caliber fibers which extends in the distal most sublamina of the inner plexiform layer (IPL). Teleosts lack this plexus. In all vertebrates, DA cells are distributed more or less evenly and at a low density (10-60 cells/mm2) over the retinal surface. Dendritic fields of adjacent DA neurons overlap. Most of the membrane area of the DA cell is contained within the plexus of fine fibers, which we postulate to be the major source of dopamine release. Thus, dopamine release can be modeled as occurring uniformly from a thin sheet located either in the OPL (teleosts) or in the distal IPL (most other vertebrates) or both (Cebus monkey). Assuming that net lateral spread of dopamine is zero, the fall of dopamine concentration with distance at right angles to the sheet (i.e. in the scleral-vitreal axis) will be exponential. The factors that influence the rate of fall-diffusion in extracellular space, uptake, and transport--are not yet quantified for dopamine, hence the dopamine concentration around its target cells cannot yet be assessed. This point is important in relation to the thresholds for activation of D1 and D2 dopamine receptors that are found on a variety of retinal cells.

Dogfish ganglion cell discharge resulting from extrinsic polarization of the horizontal cells

1. Ganglion cell discharges were evoked by extrinsic polarization of the horizontal cells in the retina of the smooth dogfish (Mustelus canis). Depolarization of the horizontal cell gave rise to a discharge similar to that evoked by a spot of light (centre type response) and hyperpolarization of the horizontal cell, a discharge similar to that by an annulus (surround type response).

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 Ringers 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 μM), a D2 dopamine agonist, increased rod‐cone coupling, whereas spiperone (5 μM), 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. J. Comp. Neurol. 398:529–538, 1998.