Personnel: Neal S. Peachey, Sherry Ball, Brett Hanzlicek, Marc Schiavone, Jiang Wu, Li Xu and Minzhong Yu
(a) Cellular Origins of the Cone ERG The electroretinogram (ERG) is used to investigate the origins of visual dysfunction in human hereditary and acquired retinal disorders. The ERG is the mass response of the retina to light that represents the activity of the initial stages in the initiation of the response of the visual system to light. By controlling the conditions under which stimuli are presented, the activity of the rod or cone visual pathways can be monitored independently. Because the ERG allows retinal function to be analyzed non-invasively, the response has been utilized in a variety of situations to follow the progression of retinal diseases. In the research laboratory, the response provides a sensitive means to evaluate experimental therapies for retinal disease, which can be repeated at different time points on the same animal. In addition, the ERG is used to characterize the effects of pharmacological manipulation or introduction of gene defects. Based on contributions from a number of investigators, it is now possible to relate the different components that comprise the rod-mediated ERG to the major cell types of the rod visual pathway. This knowledge has led to a comprehensive model of the rod ERG which finds wide application. The cone-mediated ERG measures activity in the cells that comprise the cone visual pathway. However, while it is clear that the cone ERG waveform includes contributions from at least three classes of retinal neuron (the cone photoreceptors, the cone depolarizing bipolar cells, and the cone hyperpolarizing bipolar cells), the manner in which the responses of these different neurons interact to form the final response waveform is not known. Nevertheless, a model capable of relating the components of the cone ERG to the cells that comprise the cone pathway would be extremely useful. For example, because the response persists at disease stages where the rod ERG is absent, the cone ERG is a useful tool for evaluating therapeutic initiatives for retinal disorders. A quantitative model of the cone ERG would also be useful in the evaluation of abnormal responses recorded from patients with retinal disease, and in the characterization of animals with gene defects that involve the cone pathway. In view of the importance of this measure to both diagnostic and experimental efforts, we propose to identify the cone ERG component generated by each class of retinal neuron and the manner in which the different components combine to form the final response waveform. This work will be carried out in mice, which allow a multidisciplinary approach to this fundamental question.
(b) Calcium Channels and Retinal Information Processing Ca2+ channels exist in the plasma membrane of excitable cells and control mechanisms such as excitation-contraction coupling, modulation of neurotransmitter release, and neuronal plasticity. Many neurological disorders have been linked to channelopathies or mutations in Ca2+ channel subunits. Thus, the study of Ca2+ channel function in neuronal cells can increase our ability to diagnose and treat a wide variety of sensory disorders, a VA medical research priority area. In the retina, L-type Ca2+ channels have been localized at synaptic regions of rod and cone photoreceptors and bipolar cells. These channels are composed of 4 types of subunits: a1, a2d, b, and possibly a neuronal homologue of the skeletal g subunit. b subunits regulate the assembly and distribution of a subunits and single channel properties are determined by the specific pairing of a1 and b subunits. Mutations in the Ca2+ channel a1F subunit in photoreceptors cause the incomplete form of congenital stationary night blindness (CSNB2), a human retinal disease. We have found the electoretinogram (ERG) phenotype of CNS b2-null mice to closely match that of CSNB2 patients. Electroretinography indicated no post-receptoral transmission in CNS b2-null mice while transmission remained unaltered in CNS b1-, b3-, and b4-null mice. Thus, the CNS b2-null mice may provide an animal model for this human condition as well as a system in which to study Ca2+ channel function. We hypothesize that Ca 2+ channel function and synaptic transmission, in the retina will be dependent on interactions and pairings between a and b subunits.