Patch-clamp recordings were used to investigate the properties of the
regenerative activity in acutely isolated bipolar cells from the rat
retina. Spontaneous, pacemaker-like membrane potential oscillations
were observed in all rod bipolar cells and the majority of cone bipolar
cells. The waveform of the regenerative potential was stereotypical but
distinct among different bipolar cell groups, especially between rod
and cone bipolar cells. The spontaneous activity was completely blocked
by Co2+, suggesting that Ca2+ influx through
voltage-dependent Ca2+ channels was required for initiating
such activity. Ca2+-induced Ca2+ release, however,
was not found to be involved. The spontaneous activity was also blocked
by mibefradil, a T-type Ca2+ channel antagonist. In contrast,
application of nimodipine, an L-type Ca2+ current antagonist,
affected mainly the waveform of the regenerative potential. This study
shows that mammalian retinal bipolar cells in isolation are also capable
of generating Ca2+-dependent spontaneous regenerative potential.
However, T-type Ca2+ channels appear to be essential for the
initiation of the spontaneous activity in mammalian bipolar cells.