The Pharmacology and Dynamics of the Light-induced Calcium Signal in Limulus Ventral Photoreceptors

Title of Document: THE PHARMACOLOGY AND DYNAMICS OF THE LIGHT-INDUCED CALCIUM SIGNAL IN LIMULUS VENTRAL PHOTORECEPTORS Youjun Wang, Doctor of Philosophy, 2007 Directed By: Professor Richard Payne, Neuroscience and Cognitive Neuroscience program The Limulus ventral photoreceptor is a model system for understanding inositol (1,4,5) trisphosphate (IP3) induced calcium (Ca ) release and the light response of invertebrate photoreceptors. Light-induced Ca release via the phosphoinositide cascade is thought to activate the photocurrent in Limulus. The pharmacology and dynamics of the light-induced Ca signal was investigated. Application of 1-100 μM 2 aminoethoxydiphenyl borate (2-APB), a non-specific IP3 receptor inhibitor, reversibly inhibited the photocurrent in a concentration-dependent manner, acting on at least two processes thought to mediate the visual cascade. 2-APB reversibly inhibited both light and IP3-induced calcium release, consistent with its role as an inhibitor of the IP3 receptor. 2-APB also reversibly inhibited the activation of depolarizing current flow through the plasma membrane by released calcium ions. In addition, 100 μM 2-APB reversibly inhibited voltage-activated potassium currents. This lack of specificity of 2-APB’s action in Limulus suggests that the effects of 2APB need to be interpreted with caution. Dynamics: The light-induced Ca release begins at the light sensitive plasma membrane of the R lobe. Consistent with its role in mediating light responses, this Ca signal rises to its peak with a similar time course to that of the electrical signals. Then the Ca signal spreads into the interior of the cell with two phases. The initial fast phase is a diffusion driven process that needs both the diffusion of IP3 and of Ca 2+ released by IP3, since both manipulations that alter the apparent diffusion coefficient (D) of Ca ions or those that change the life span of IP3 molecules can change the spread of the fast phase. A model of simple diffusion of IP3 molecules (or Ca 2+ ions) and a diffusion model for IP3 and Ca 2+ released by IP3 were constructed using estimated D values of Ca 2+ and IP3 inside cells obtained through the Graham’s Law. Simulation results indicate that diffusion of IP3 and released Ca is both necessary and sufficient to determine the initial fast phase. The expected spread of excitation following absorption of one photon can be predicted by this model. THE PHARMACOLOGY AND DYNAMICS OF THE LIGHT-INDUCED CALCIUM SIGNAL IN LIMULUS VENTRAL PHOTORECEPTORS