GtetP acts prior to opening of cyclic nucleotide-gated channels. (A) Injection from a microelectrode containing 25 mM GtetP was used to desensitize cells to a test flash by 90% (left panel). Data points are the average response with error bars (std. dev.) to seven consecutive test flashes. (B) The response to injection from a microelectrode containing 250 uM Rp-8pCPT-cGMPS (cGMP) in the same three cells was qualitatively unaffected by GtetP (left panel). Respective responses before (control) and after injection (GtetP) are matched by lines and symbols (+, *, and open circles). The voltage traces represent averaged responses from one cell (*) to light (left) and Rp-8pCPT-cGMPS (right) before (control) and after GtetP intracellular injections.