Elemental And Structural Organization Of Neurons And Gli

Neurological Disorders And Stroke

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Abstract

A remarkable feat of neural circuits is the extraction of information from light. The vertebrate retina receives processes and transforms visual images. Light absorbed in photoreceptors is transformed into membrane polarizations that evoke light modulated release of the neurotransmitter glutamate onto horizontal and bipolar cells. Horizontal cells feed back onto photoreceptor synapses, regulating transmitter release according to conditions of ambient illumination. Bipolar cells transfer light signals forward to amacrine cells and ganglion cells. Amacrine cells are mainly regulatory retinal interneurons with inhibitory and modulatory roles. Ganglion cells transform light signals into trains of action potentials that propagate through the optic nerve to brain visual centers. As images pass through retinal circuitry, they are decomposed into component parts, so that at the retinal output specialized sets of ganglion cells report different features of the image. Some signal highlights, others shadows, movement, direction or color. This is retinal image processing. It is achieved by sets of specialized neural circuits. Such circuits are composed of patterns of connections among neurons, and specialized interactions between neurotransmitters and their receptors. This research program studies the relationships between receptor expression on retinal neurons, the neural circuitry of the retina, and retinal information processing tasks.[unreadable] [unreadable] Zebrafish is a visual system model that provides behavioral, molecular, anatomical and physiological measures. Through studies of this model in normal and perturbed states, the visual process may be better understood. We examine zebrafish retinal function using acutely dissociated retinal neurons, in vitro eyecup preparations, and zebrafish retinal slices. These preparations provide information about structural neural pathways, the responses of neural types to stimulation by neurotransmitters, and by light. Through integrating and synthesizing such information, it is possible to infer the physiological transformations that the retina imposes on the image and the synaptic mechanisms that implement these changes.[unreadable] [unreadable] Cell structure, a key element in discerning retinal circuits, is beautifully delineated in the zebrafish model. In retinal slices, the retinal cell body and synaptic layers that process visual information are crisply defined. Neurons in these retinal slices can be stained individually by spraying DiI coated microcarriers onto the cut surface with a gene gun, a method also known as the diolisitic technique. Diffusion of DiI along membranes reveals the shapes of individual neurons. Recent results suggest 3 horizontal cell types, each with processes ramifying in the retinal outer plexiform layer. Some of these types bear axons, others do not. Retinal bipolar cells, which receive signals from photoreceptors in the distal retina and transfer them to ganglion cells and amacrine cells in the proximal retina, may be classified based on branching patterns of axons within the retinal inner plexiform layer, and dendrites within the outer plexiform layer. Seventeen bipolar cell morphologies were identified. Seven amacrine cell morphologies were also identified by branching patterns within the inner plexiform layer. In the inner retina, cells with ON-type (branching in IPL sublamina b), OFF-type (branching in sublamina a) morphological signatures were about equally prevalent. Mixed a/b branching patterns were also observed. [unreadable] [unreadable] In previous studies, whole-cell patch recording and puff pipette techniques identified glutamate receptor mechanisms on the dendrites of many of the morphological types of bipolar cells, seen also by the diolistic method. These studies revealed a distribution of 3 basic glutamate receptor types: AMPA/kainate (OFF cells), mGluR6 (ON cells) and glutamate-gated chloride currents (Iglu, ON cells). The latter current appears generated by a glutamate-transporter-like mechanism.[unreadable] [unreadable] The 3 distinct glutamate receptors expressed on bipolar cells can be isolated in the light-evoked field potential of the zebrafish eye. These field potentials are called electroretinograms or ERG's. CNQX, an antagonist of AMPA/kainate glutamate receptors, blocks the light responses of OFF type bipolar cells and their resultant field potential, the d-wave of the ERG. Input to horizontal cells, amacrine cells, and ganglion cells is also blocked by this treatment, leaving virtually an isolated ON-bipolar-cell system response in the field potential, to which only photoreceptors also contribute. These photoreceptor contributions can be taken into account by isolating the photorecptor response with treatment by further agents such as L-aspartate, which blocks all synaptic transmission in the outer retina. ON-type bipolar signals can be studied in virtual isolation in the ERG by such methods. Isolated ON-bipolar responses are partially blocked by the metabotropic glutamate agonist L-AP4 and the metabotropic glutamate antagonist CPPG. They are more completely blocked by TBOA, an antagonist of glutamate transporters. Glutamate transporters generate the responses of many ON-type bipolar cells, as these transporters combine to form chloride channels when activated by photoreceptor glutamate. However they also greatly slow uptake of glutamate from the photoreceptor synaptic cleft altering post-synaptic response kinetics. The ultimate goal of these studies is to dissect the glutamate mechanisms used by different cone types in transmitting information forward to ON-type bipolar cells, or to OFF bipolar cells. For instance recent studies suggest that there is a very high gain system for transmitting blue cone and UV cone signals to ON-type bipolar cells. This high-gain synaptic system is readily blocked by L-AP4 and CPPG. Work is ongoing to isolate the various mechanisms in bipolar-cell light responses, and it appears that ERG responses can yield information selective to particular cell types.[unreadable] [unreadable] Bipolar cell and horizontal cell glutamate pathways are modulated by neurotransmitters such as GABA glycine and dopamine. These neuromodulators are expressed in amacrine cells and horizontal cells. In isolated bipolar and horizontal cells, AMPA-type glutamate responses are enhanced by treatment with dopamine, the D1 agonist SKF38393, or the D2 agonist quinpirole. Among bipolar cells, AMPA type responses are normally found only on OFF types, those cells excited by shadows. ON type bipolar cells, those cells excited by highlights, do not normally express AMPA receptors, however in some ON bipolar cells, AMPA-like responses can be induced by dopamine. In both ON and OFF bipolar cells dopamine appears to shift the balance between ON and OFF systems towards the OFF system. Dopamine is often associated with the shifting of retinal circuitry from night vision to day vision. It would appear that increased shadow detection capabilities, as provided by the OFF system, might be part of this shift.[unreadable] [unreadable] GABA receptors and GABA transporters are also found on the dendrites of isolated cells of the distal retina. These GABA mechanisms modify the glutamate currents generated by photoreceptor synapses. Ionotropic GABA receptors are expressed on zebrafish bipolar cells, but not on zebrafish horizontal cells. Many horizontal cells and rarely, some bipolar cells, express an Na+ -dependent, Cl- dependent, picrotoxin-insensitive, and muscimol resistant membrane transporter for GABA. A depolarized Na+ gradient drives GABA into the cytoplasm, depolarizing the plasma membrane and activating Na+, K+ ATPase. The net result is a biphasic membrane response to GABA, starting with transporter depolarization, but followed by ATPase generated hyperpolarization.