Frog rod outer segment shedding in vitro: histologic and electrophysiologic observations

Rod outer segment shedding in the frog, Rana pipiens, has been studied using an in vitro eyecup method. Control experiments have shown that shedding responses in vitro are comparable to those in vivo and, like the situation in vivo, shedding in isolated eyecups requires a dark period followed by light onset. We found an initial, rapid and light-evoked component of the shedding response to be critically dependent upon bicarbonate concentration, supporting the initial discovery of a bicarbonate requirement for Xenopus rod shedding by Besharse et al. In Rana, in vitro shedding occurs in the presence of 20 mM aspartate, suggesting that functional integrity of the inner retina is not a prerequisite for rod shedding. Additionally, shedding was found to be suppressed completely in the presence of the local anesthetic MS-222 and the phosphodiesterase inhibitor IBMX. In the case of IBMX, electrophysiologic recording indicated changes in photoreceptor sensitivity in the presence of the drug. Such changes may play a role in the observed inhibition of shedding.