THE LIMULUS VENTRAL PHOTORECEPTOR - LIGHT RESPONSE AND THE ROLE OF CALCIUM IN A CLASSIC PREPARATION

The ventral nerve photoreceptor of the horseshoe crab Limulus polyphem us has been used for many years to investigate basic mechanisms of inv ertebrate phototransduction. The activation of rhodopsin leads in visu al cells of invertebrates to an enzyme cascade at the end of which ion channels in the plasma membrane are transiently opened. This allows a n influx of cations resulting in a depolarization of the photoreceptor cell. The receptor current of the Limulus ventral photoreceptor consi sts of three components which differ in several aspects, such as the t ime course of activation, the time course of recovery from light adapt ation, and the reversal potential. Each component is influenced in a d ifferent, characteristic way by various pharmacological manipulations. In addition, at least two types of single photon-evoked events (bumps ) and three elementary channel conductances are observed in this photo receptor cell. These findings suggest that the receptor current compon ents are controlled by three different light-activated enzymatic pathw ays using three different ligands to increase membrane conductance. Pr obably one of these ligands is cyclic GMP, another one is activated vi a the IP3-cascade and calcium, the third one might be cyclic AMP. Calc ium ions are very important for the excitation and adaptation of visua l cells in invertebrates. The extracellular and intracellular calcium concentrations determine the functional state of the visual cell. A ri se in the cytosolic calcium concentration appears to be an essential s tep in the excitatory transduction cascade. Cytosolic calcium is the m ajor intracellular mediator of adaptation. If the cytosolic calcium le vel exceeds a certain threshold value after exposure to light it cause s the desensitization of the visual cell. On the other hand, from a sl ight rise in cytosolic calcium facilitation results, i.e. increased se nsitivity of the photoreceptor. (C) 1997 Elsevier Science Ltd.