The early receptor current (ERC) and light–activated Ca2+ release in wild–type and alpha–transducin knockout mouse rods

Abstract

Abstract: : Introduction: The early receptor potential and early receptor current (ERC) are produced by movement of charge resulting from light–induced changes in conformation of the rhodopsin photopigment. Although previously recorded from single photoreceptors in lower vertebrates, the ERC has not yet been observed in mammals. We now describe a method for recording ERC from mouse rods that could be utilized in investigations of models of human disease. Method: Suction electrode recording of currents in isolated rods from wild–type and α–transducin knockout mice (Gnat1^–/–) were made conventionally. Rods were illuminated with a 10 µm spot centered on the outer segment, formed from an argon ion laser at 488 nm. Currents were filtered at 1.5 kHz and acquired at 4.0 kHz. Results: In wild–type rods we recorded the ERC to a brief laser flash after suppression of circulating current with a saturating illumination from the light bench. The ERC consisted of an outward current 5–10 pA in amplitude reaching peak in 3–4 ms and decaying to baseline within 8 ms. The amplitude of the total charge movement declined exponentially with cumulative flash exposure, consistent with the direct proportionality of the ERC to the quantity of pigment bleached. From the constants of exponential decline, we estimate that each Rh^* moved about 0.1 charge, and that the in vivo photosensitivity of mouse rhodopsin is approximately 6 x 10^–9 µm² per molecule. In Gnat^–/– mice a similar ERC was recorded but was followed by a much slower outward current averaging 0.5 pA but as large as 6 nA in some rods, approaching the amplitude of the normal light response. This slower current was suppressed by L–cis diltiazem and by pre–incubation of the rods with BAPTA–AM, and it is probably produced by light–activated Ca²⁺ release independent of the normal transduction pathway. Conclusion: Measurement of ERC should make possible in vivo estimates of photopigment concentration in mouse models of retinal dysfunction. The slow current recorded after the ERC from Gnat1^–/– mice provides the first evidence of light–activated Ca²⁺ release in a mammal and may prove useful for understanding the mechanism of this phenomenon.

Keywords: calcium • photoreceptors • signal transduction