Neural circuits of the vertebrate retina process images and extract information from light. Light absorbed in the photopigments of rods and cones is transduced into membrane polarizations that alter the release of the neurotransmitter glutamate onto horizontal and bipolar cells, second order retinal interneurons. Horizontal cells regulate photoreceptor synapses in feedback fashion, adjusting transmitter release according to conditions of ambient illumination. Bipolar cells transfer light signals forward to amacrine cells and ganglion cells. Ganglion cells transform bipolar-cell signals into trains of action potentials that propagate through the optic nerve to brain visual centers, while amacrine cells, like horizontal cells, appear to be mainly regulatory. As images pass through retinal circuitry, they are decomposed into salient elements, 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. Retinal image processing is achieved by sets of specialized neural circuits. Such circuits are composed of patterns of connections among neurons, and synaptic actions involving vescicle release, neurotransmitters and their receptors. This research program studies the relationships between receptor expression on retinal neurons, the neural circuitry of the retina, the light responses of retinal neurons, and retinal information processing tasks.? ? Zebrafish is a visual system model that provides behavioral, molecular, anatomical and physiological access. It is a particularly favorable model for studies of color vision. 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. ? ? Retinal structure is a key element in discerning retinal circuits. In zebrafish this is beautifully delineated. Four cone types (red, green, blue and UV) form a perfect mosaic. Blue (B) or UV (U) cones alternate between each pair of red (R) and green (G) double cones in exact 1B:1U:2R:2G ratio. 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. Alternatively these cells can be penetrated and stained in wholemount using sharp electrodes, or by patch electrodes in retinal slice. ? ? 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 the glutamate-transporter EAAT5.? ? The 3 distinct glutamate receptors expressed on bipolar cells can be isolated in the light-evoked field potential of the zebrafish eye (also known as the electroretinogram or ERG). 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 are 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 agents such as L-aspartate, which blocks all synaptic transmission. ON-type bipolar signals studied in virtual isolation in the ERG by such methods are partially blocked by the metabotropic glutamate agonist L-AP4 and the metabotropic glutamate antagonist CPPG. The massed ON-bipolar signal is thus dissected into metabotropic and non-metabotropic (transporter-like) components. The ultimate goal of these studies is to dissect glutamate mechanisms used by different cone types to transmit information forward to ON-type bipolar cells. 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 system is metabotropic. ? ? Sharp electrodes penetrate neurons in perfused zebrafish retinal wholemounts revealing light responses from different neural types. Horizontal cell responses are composed of synaptic signals originating with R, G, B or U cones. Six different spectral types have been identified. Two are L-types, or luminosity types. These cells hyperpolarize with all stimulus wavelengths. L-type signals contain input from both R and G cones. In L1 types the G cone dominates and the spectral peak is in the green. In L2 types, the R cone dominates and the spectral peak is in the red. Four C-type, or chromatic types were found. C-type cells hyperpolarize or depolarize to light, depending on wavelength and intensity of stimulation. The most common of these, the C-type biphasic, combines R cone depolarization with B and G cone hyperpolarization. There are two sorts of C-type triphasic cells: the B-type triphasic and the U-type triphasic. In the B-type triphasic, R cone activation is hyperpolarizing; G-cone, depolarizing; and B-cone, hyperpolarizing. The latter provides the largest input. In the U-type triphasic, R cone stimulation is hyperpolarizing, both G and B cone stimulation are depolarizing, and U cone stimulation is hyperpolarizing. The U-cone provides the largest input. Finally there is a C-type tetraphasic response. This is similar to the B-type triphasic response, except that U cones contribute depolarization. For this cell the sequence is R hyperpolarization, G depolarization, B hyperpolarization, and U depolarization. Thus in zebrafish, even after just one synapse of image processing, there is already a very acute coding of color. ? ? In both synaptic and extrasyanptic responses, retinal neurons express neurotransmitter receptors. With widely distributed types such as GABA-A or GABA-C, cell isolation procedures are required to localize receptors to membrane sites on specific neurons, as inferring the location of neurotransmitter actions from the light responses of intact behaving tissue is problematic. Nonetheless receptor location is critical to the development of neural circuitry models that provide insight into retinal function. Previously, glutamate responses of bipolar and horizontal cells isolated from zebrafish were reported using both voltage probe techniques and patch recording in slice. The GABA responses of these same cells have recently been documented. Inhibitory GABA receptors are particularly dense on bipolar cell axon terminals; but GABA actions on bipolar cell dendrites may be either excitatory or inhibitory. Expression of GABA receptors on horizontal cells varies among species, and GABA transporters can generate significant response components. In zebrafish we find ionotropic GABA receptors expressed on bipolar cells, both axons and dendrites, but not on horizontal cells. The receptor responses were invariably hyperpolarizing. Many horizontal cells and rarely, some bipolar cells, did however express an Na+ -dependent, Cl- dependent, picrotoxin-insensitive, and muscimol resistant membrane transporter for GABA. This response is depolarizing, excitatory and biphasic. Following transporter depolarization, a delayed ATPase activation hyperpolarizes the cell.
| Connaughton, V P; Graham, D; Nelson, R (2004) Identification and morphological classification of horizontal, bipolar, and amacrine cells within the zebrafish retina. J Comp Neurol 477:371-85 |
| Kolb, H; Nelson, R; Ahnelt, P et al. (2001) Cellular organization of the vertebrate retina. Prog Brain Res 131:3-26 |
| Nelson, R; Janis, A T; Behar, T N et al. (2001) Physiological responses associated with kainate receptor immunoreactivity in dissociated zebrafish retinal neurons: a voltage probe study. Prog Brain Res 131:255-65 |
| Connaughton, V P; Nelson, R (2000) Axonal stratification patterns and glutamate-gated conductance mechanisms in zebrafish retinal bipolar cells. J Physiol 524 Pt 1:135-46 |
| Nelson, R; Schaffner, A E; Li, Y X et al. (1999) Distribution of GABA(C)-like responses among acutely dissociated rat retinal neurons. Vis Neurosci 16:179-90 |
