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¹ Neuroscience Group, Institute for Science and Technology in Medicine, Keele University, Keele ST5 5BG, UK

Retinal bipolar cells convey light-evoked potentials from photoreceptors to ganglion cells and mediate the initial stages of visual signal processing. They do not fire Na⁺-dependent action potentials (APs) but the Mb1 class of goldfish bipolar cell exhibits Ca²⁺-dependent APs and regenerative potentials that originate in the axon terminal. I have examined the properties of Ca²⁺-dependent APs in isolated bipolar-cell terminals in goldfish retinal slices. All recorded terminals fired spontaneous or evoked APs at frequencies of up to 15 Hz. When an AP waveform was used as a voltage stimulus, exocytosis was evoked by single APs, maintained throughout AP trains and modulated by AP frequency. Furthermore, feedback inhibition of the Ca²⁺ current (I_Ca) by released vesicular protons reduced depression of exocytosis during AP trains. In the absence of K⁺ current inhibition, step depolarizations and AP waveforms evoked a rapidly activated outward current that was dependent on Ca²⁺ influx (I_K(Ca)). I therefore investigated whether proton-mediated feedback inhibition of I_Ca affected the activation of I_K(Ca). A transient inhibition of I_K(Ca) was observed that was dependent on exocytosis, blocked by high-pH extracellular buffer, of similar magnitude to inhibition of I_Ca but occurred with a delay of 2.7 ms. In addition, the amplitude of APs evoked under current clamp was inhibited by the action of vesicular protons released by the APs. Protons released via exocytosis may therefore be a significant modulator of Ca²⁺-dependent currents and regenerative potentials in bipolar-cell terminals.

(Received 18 January 2006; accepted after revision 20 February 2006; first published online 23 February 2006)
Corresponding author M. J. Palmer: Huxley Building, School of Life Sciences, Keele University, Keele ST5 5BG, UK. Email: m.j.palmer{at}cns.keele.ac.uk

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