Mechanisms of synaptic transmission in the retina

Tissue culture studies of retinal development

Because of a limited number of cell types, a series of well-described cell-type-specific markers and a stereotyped sequence of cell development, the retina has been a valuable model of CNS development. Dissociated and explant cultures have been used to help define some of the requirements for differentiation of each major cell class. In addition to mixed-cell cultures it is now possible to use cell purification or selective growth methods to give cultures of single cell types. Alteration of gene expression by viral infection has proved to be a valuable method to help elucidate developmental pathways.

Diversity of neuronal phenotypes expressed in monolayer cultures from immature rabbit retina

We have used monolayer cultures prepared from early postnatal rabbit retinae (days 2-5) by the sandwich technique to study the capacity of immature neurons to express specific neuronal phenotypes in a homogeneous in vitro environment. Applying morphological, immunocytochemical, and autoradiographic criteria, we demonstrate that a variety of phenotypes could be distinguished after 7-14 days in vitro, and correlated with known retinal cell types. Bipolar cell-like neurons (approximately 4% of total cell number) were identified by cell type-specific monoclonal antibodies (115A10) and their characteristic bipolar morphology. Small subpopulations (about 1%) of GABA-immunoreactive neurons acquired elaborate morphologies strikingly similar to those of A- and B-type horizontal cells. Amongst putative amacrine cells several different subpopulations could be classified. GABA-immunoreactive amacrine-like neurons (6.5%), which also showed high affinity [3H]-GABA uptake, comprised cells of varying size and shape and could be subdivided into subpopulations with respect to their response to different glutamate receptor agonists (NMDA, kainic acid, quisqualic acid). In addition, a small percentage of [3H]-GABA accumulating cells with large dendritic fields showed tyrosine-hydroxylase immunoreactivity. Presumptive glycinergic amacrine cells (18.5%) were rather uniform in shape and had small dendritic fields. Release of [3H]-glycine from these neurons was evoked by kainic and quisqualic acid but not by NMDA. Small [3H]-glutamate accumulating neurons with few short processes were the most frequent cell type (73%). This cell type also exhibited opsin immunoreactivity and probably represented incompletely differentiated photoreceptor cells. Summing the numbers of characterized cells indicated that we were able to attribute a defined retinal phenotype to most, if not all of the cultured neurons. Thus, we have demonstrated that immature neuronal cells growing in monolayer cultures, in the absence of a structured environment, are capable of maintaining or producing specific morphological and functional properties corresponding to those expressed in vivo. These results stress the importance of intrinsic factors for the regulation of neuronal differentiation. On the other hand, morphological differentiation was far from perfect indicating the requirement for regulatory factors.

GABA and glycine channels in isolated ganglion cells from the goldfish retina

1. Adult goldfish retinas were enzymatically dissociated and ganglion cells were maintained in culture for periods of 1-5 days. Ganglion cells could be identified by their morphology, and this identification was confirmed by retrograde transport of the fluorescent dye Fast Blue injected into the optic nerve stub. 2. All the ganglion cells tested responded to 30 microM-GABA or 100 microM-glycine between 2 and 30 h after enzymatic dissociation of the retina. 3. Whole-cell responses to 30 microM-GABA or glycine declined over a period of seconds during sustained applications of the agonists, probably as a result of desensitization. There was an irreversible decline in the peak whole-cell response to repeated applications of 30 microM-GABA unless the pipette-filling solution contained 2 mM-ATP, 4 mM-Mg2+, 10 mM-EGTA and no added Ca2+. Both GABA and glycine responses also showed an irreversible decline in outside-out patches but, in this case, Mg2+, ATP, and very low Ca2+ failed to stabilize the response. 4. Whole-cell currents activated by both GABA and glycine were demonstrated to be chloride-selective by investigating the dependence of reversal potential (Vr) on internal chloride concentration ([Cl-]i). For GABA responses, the dependence of Vr on [Cl-]i could not be distinguished from that predicted by the Nernst relation. For glycine, deviations from Nernstian dependence were observed, but the permeability to Cl- was at least 20 times greater than to isethionate, SO4(2-), or monovalent cations (Na+ and Cs+). 5. Bicuculline methochloride (10 microM) selectively blocked responses to 3-30 microM-GABA without affecting responses to 30 microM-glycine. Bicuculline itself was not as selective. At agonist concentrations of 30 microM, 3 microM-bicuculline partially blocked the response to GABA but not that to glycine, but bicuculline at 10 microM blocked responses to both GABA and glycine. Strychnine (0.3-1 microM) blocked responses to 30 microM-glycine but also competitively antagonized GABA responses. Picrotoxinin (10 microM) blocked responses to 3 microM-GABA in some cells but also partially antagonized responses to 30 microM-glycine. 6. GABA channels had at least two conductance states at 10-12 degrees C in nearly symmetrical (141 mM in, 142 mM out) chloride. The slope conductance of the most frequently observed (main) state was 16 +/- 2 pS. The reversal potential for the main state was not significantly different from the chloride equilibrium potential (0 mV).(ABSTRACT TRUNCATED AT 400 WORDS)

The importance of cnidarian synapses for neurobiology

Despite being the most primitive organisms to possess a nervous system, cnidarians afford rare opportunities for studying various, general aspects of chemical synaptic transmission. This is made possible by the unique organization of their nervous systems and by the fact that in certain species the neurons and synapses are readily accessible for intracellular recordings and voltage clamp. The results obtained from such studies are summarized here, with particular emphasis on work with two species, Cyanea capillata (Scyphozoa) and Polyorchis pennicilatus (Hydrozoa). The potential of these preparations for providing additional data is also discussed.

Calcium and calcium-dependent chloride currents generate action potentials in solitary cone photoreceptors

Vertebrate rod and cone photoreceptors hyperpolarize when illuminated. However, synaptic input from horizontal cells can depolarize cones and even elicit action potentials. Using the whole-cell tight-seal recording technique, we determined that, in solitary cones isolated from a lizard retina, action potentials can be generated by depolarizing current steps under conditions where only two ionic currents are activated. A dihydropyridine-sensitive, inward Ca2+ current that activates at potentials positive to -40 mV can regeneratively depolarize the cell. Subsequently, a SITS-sensitive, Ca2(+)-dependent outward Cl- current repolarizes the cell. We suggest that these ionic currents may help explain lateral inhibition in the retina.

The presence of glutamic acid decarboxylase and gamma-aminobutyric acid immunoreactivity in photoreceptors of hatching quail retina

The presence of glutamic acid decarboxylase (GAD) and gamma-aminobutyric acid (GABA) was investigated in neuroretina sections from hatching quail embryos by immunocytochemistry. The photoreceptors were found to be intensely immunoreactive to anti-GAD antiserum and to two distinct anti-GABA antisera.

Light-evoked and kainic-acid-induced disc shedding by rod photoreceptors: differential sensitivity to extracellular calcium

In order to study the light and Ca2+ dependence of disc shedding by rod photoreceptors, we have used eyecups prepared from adult Rana pipiens frogs that had been kept in constant light for 4 days. Disc shedding was initiated by a treatment involving 1 hour of darkness followed by exposure to light or by treatment with kainic acid. Maximal L-evoked disc shedding occurred quickly (within 30-60 minutes) after light onset and could be triggered by brief (15 minutes) exposure to light. L-evoked disc shedding was completely blocked by omission of Ca2+ from culture medium or by treatment with 3mM Co2+ or 12 mM Mg2+ in the presence of Ca2+ (2 mM). The response was also blocked by the organic Ca2+ antagonist nifedipine. Experiments designed to distinguish between Ca2+ dependence of the dark- or light-dependent processes necessary for shedding suggest that voltage-sensitive channels mediate a Ca2+-dependent process involved in light-triggering. Kainic acid caused a dose-dependent stimulation of disc shedding under lighting conditions (continuous culture in light or darkness) that did not normally result in a significant response in the absence of the drug. Disc shedding induced by kainic acid was similar in time course and magnitude to that induced by light. However, kainic-acid-induced disc shedding was not inhibited by medium Ca2+ reduction or by the presence of Co2+. The latter observation suggests that kainic acid activates disc shedding directly, by-passing the Ca2+-dependent process involved in the L-evoked response. The Ca2+-dependent process may involve release of an effector of disc shedding that is mimicked by kainic acid.

Electrical feedback mechanism in the processing of signals in the outer plexiform layer of the retina

In any chemical synapse there is an electrical feedback initiated by the electrical current generated by the postsynaptic neurone. We argue that this feedback can be rather effective in the case of invaginating synapses in the retina. A model of a dyad synapse (cone-horizontal cell-bipolar cell) is developed, in which the transfer function between the cone and bipolar cell is modulated by the synaptic current of the horizontal cell. This modulation originates by the change of the potential drop along the intercellular gap between the cone and the horizontal cell. The model takes into account the electrical coupling between the horizontal cells as well as the nonlinearity of their nonsynaptic (somatic) membrane. The model reproduces qualitatively the steady-state responses of an hyperpolarizing bipolar cell to the light spot and an annulus. It gives also the adaptational (with a light background) shift of the cone-bipolar cell transfer function. The model can be applied to depolarizing bipolar cells and to C-type horizontal cells.