The effects of local anaesthetics on retinal function.

Isolated frog eyecups were incubated in Ringer containing local anaesthetics to study the effects of these drugs on dark-adaptation of the ERG. Relative to controls, dark-adaptation in eyecups treated with millimolar concentrations of MS-222, benzocaine, and procaine HCl was significantly inhibited during 10 to 120 min following the cessation of the adapting light. These drugs also prevented the recovery of the c-wave during dark-adaptation, resulting in ERG waveforms resembling those found in light-adapted eyecups. Measurements of rhodopsin in the retina were consistent with previous findings showing that rhodopsin regeneration in situ is inhibited by local anaesthetics. In vitro regeneration experiments in which bleached rod outer segment fragments were added to 11-cis retinal showed that preincubation of retinal with MS-222 in ethanol prevents rhodopsin regeneration. Evidence was obtained spectrophotometrically for the formation of a complex between MS-222 and 11-cis retinal with a gamma max of 512 nm. We propose that the formation of a Schiff's base between these two compounds blocks the recombination of rhodopsin, and in situ, leads to the inhibition of dark-adaptation.