Inverted Responses of Amphibian Rod Photoreceptors

Abstract

Abstract: : Purpose: To investigate light responses of inverted polarity obtained by stimulating the part of the rod outer segment outside the suction pipette. Methods: Retinal rods were obtained from dark-adapted cane toads (Bufo marinus) and clawed frogs (Xenopus laevis). The outer segment of an isolated rod was drawn part way into a suction pipette and stimulated with flashes of 500 nm light. In some experiments, a whole-cell patch pipette was also used either to monitor or to clamp the intracellular voltage. Results: The amplitude of the dim-flash response decreased with displacement of a slit-shaped stimulus from the recorded to the unrecorded part of the outer segment. Correspondingly, a late component with reversed polarity emerged. Stimulation of the region outside the suction pipette gave a pure inverted response. Its time course was slower than that of the conventional light response obtained by stimulating the same region when the outer segment was fully drawn in. In responses to flashes of moderate or high intensity, the late inverted component was preceded by a fast negative-going transient. The peak of the initial transient was coincident with the steepest part of the rising phase of the voltage response. Both the initial transient and the slow inverted component were abolished when the membrane voltage was clamped near the resting level, whereas under current-clamp conditions both components of the inverted responses were present. The peak of the voltage response clearly preceded the peak of the slow inverted component. Conclusions: The fast initial transient of the inverted response is a capacitive current caused by hyperpolarization of the cell membrane. The slow inverted component appears to be mediated indirectly by membrane voltage. A possible mechanism involves a hyperpolarization-induced increase in Na⁺/Ca²⁺,K⁺ exchange, leading to a decrease in [Ca²⁺]_i, an increase in cGMP production by guanylate cyclase, and the opening of cyclic nucleotide-gated channels.

Keywords: photoreceptors • electrophysiology: non-clinical • calcium