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This Article Full Text Full Text (PDF) All Versions of this Article: 566/2/455    most recent jphysiol.2005.090258v1 Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Jarvinen, J. L. P Articles by Lamb, T. D Search for Related Content PubMed PubMed Citation Articles by Jarvinen, J. L. P Articles by Lamb, T. D

¹ Physiological Laboratory, University of Cambridge, Downing Street, Cambridge CB2 3EG, UK
² Division of Neuroscience, The John Curtin School of Medical Research, The Australian National University, Canberra, ACT 2601, Australia

We recorded photocurrent responses of retinal rods isolated from cane toads Bufo marinus and clawed frogs Xenopus laevis. With the outer segment drawn part way into the suction pipette, presentation of flashes to the base of the outer segment (outside the pipette) elicited a slow inverted response. Stimulation of the same region, with the outer segment drawn fully in, gave a response of conventional polarity. For moderate to bright flashes a fast transient preceded the slow inverted response. Upon bleaching the tip of the outer segment, the slow inverted response was abolished but the fast initial transient remained, and we attribute this fast component to a capacitive current. Experiments employing simultaneous whole-cell patch-clamp and suction pipette recording revealed that both the fast and slow components of the inverted responses were absent in voltage-clamped cells. In current-clamped cells the slow inverted current response was delayed substantially with respect to the voltage response. We present a computational model for the slow component, in which hyperpolarization leads to increased activity of the Na⁺–Ca²⁺,K⁺ exchanger, hence lowering the cytoplasmic Ca²⁺ concentration, activating guanylyl cyclase, raising cyclic GMP concentration, opening cyclic nucleotide-gated channels, and increasing circulating current in the unstimulated region. For the measured voltage response to stimulation of the base, we solve these equations to predict the photocurrent in the tip, and obtain an adequate explanation of the inverted responses. Our work suggests a novel role for membrane voltage in accelerating the inactivation phase of the response to light.

(Received 10 May 2005; accepted after revision 23 May 2005; first published online 26 May 2005)
Corresponding author J. Jarvinen: Division of Neuroscience, The John Curtin School of Medical Research, The Australian National University, Canberra, ACT 2601, Australia. Email: jaakko.jarvinen{at}anu.edu.au