Absorption of light by metarhodopsin modifies the effect of a conditioning light on the barnacle photoreceptor.

We show that the effect of an adapting light on the sensitivity of barnacle photoreceptors depends on the direction of net pigment transfer [rhodopsin (R) to metarhodopsin (M) or reverse] occasioned by the adapting light. For stimuli giving no net pigment transfer the state of the pigment appears irrelevant, R-->R having the same effect as M-->M. With respect to these, R-->M gives enhanced facilitation and M-->R depressed facilitation. This suggests a correlation with the prolonged depolarising after-potential (PDA) and the anti-PDA, which follow R-->M and M-->R stimuli respectively. These effects appear mainly in less sensitive cells and for higher amounts of conditioning light--but still well within the physiological range and well below the threshold for PDA and anti-PDA induction. The special interest of these results is that they appear to be interpretable only by assuming that absorption of light by metarhodopsin exerts an effect on the stimulus coincident response (LRP), the first demonstration of such an effect.