Structure And Function In Retinal Neurons

脊椎动物视网膜的神经回路处理图像并从光线中提取信息。在视杆细胞和视锥细胞的光色素中吸收的光被转导成膜极化，从而改变向水平细胞和双极细胞（二级视网膜中间神经元）释放的神经递质谷氨酸。水平细胞以反馈方式调节光感受器突触，根据环境光照条件调节发射器释放。双极细胞将光信号传递给无突细胞和神经节细胞。神经节细胞将双极细胞信号转化为动作电位序列，通过视神经传播到大脑视觉中枢，而无突细胞与水平细胞一样，似乎主要起调节作用。当图像通过视网膜电路时，它们被分解成显著元素，因此在视网膜输出端，专门的神经节细胞组报告图像的不同特征。一些信号是高光，另一些是阴影、移动、方向或颜色。视网膜图像处理是由一组专门的神经回路完成的。这种电路由神经元之间的连接模式和突触活动组成，包括囊泡释放、神经递质及其受体。本课题主要研究视网膜神经元受体表达、视网膜神经回路、视网膜神经元的光反应和视网膜信息处理任务之间的关系。

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.