The contribution of cationic conductances to the potential of rod photoreceptors

The contribution of cationic conductances in shaping the rod photovoltage was studied in light adapted cells recorded under whole-cell voltage- or current-clamp conditions. Depolarising current steps (of size comparable to the light-regulated current) produced monotonic responses when the prepulse holding potential (Vh) was -40 mV (i.e. corresponding to the membrane potential in the dark). At Vh=-60 mV (simulating the steady-state response to an intense background of light) current injections <35 pA (mimicking a light decrement) produced instead an initial depolarisation that declined to a plateau, and voltage transiently overshot Vh at the stimulus offset. Current steps >40 pA produced a steady depolarisation to ≈-16 mV at both Vh. The difference between the responses at the two Vh was primarily generated by the slow delayed-rectifier-like K+ current (IKx), which therefore strongly affects both the photoresponse rising and falling phase. The steady voltage observed at both Vh in response to large current injections was instead generated by Ca-activated K+ channels (IKCa), as previously found. Both IKx and IKCa oppose the cation influx, occurring at the light stimulus offset through the cGMP-gated channels and the voltage-activated Ca2+ channels (ICa). This avoids that the cation influx could erratically depolarise the rod past its normal resting value, thus allowing a reliable dim stimuli detection, without slowing down the photovoltage recovery kinetics. The latter kinetics was instead accelerated by the hyperpolarisation-activated, non-selective current (Ih) and ICa. Blockade of all K+ currents with external TEA unmasked a ICa-dependent regenerative behaviour.