A preparation of isolated presynaptic terminals of rat
retinal rod bipolar cells was developed. Patch-clamp recordings
were performed on the isolated terminal to determine the
type(s) of voltage-activated Ca2+ channels and
the contribution of GABAA and GABAC
receptor-mediated currents localized in the terminal region.
Both low-voltage-activated (LVA) and high-voltage-activated
(HVA) Ca2+ currents, with properties similar
to those found in intact cell recordings, were observed
in the isolated terminal recordings. Consistent with previous
studies, the HVA Ca2+ currents are L-type since
the currents were blocked by low micromolar concentrations
of nimodipine and potentiated by BayK 8644. Also, both
GABAA and GABAC receptor-mediated
currents were observed in the isolated terminal. The current
density of GABAC receptors in the terminal was
more than three times higher than that in the soma. In
contrast, the current density of GABAA currents
between the terminal and the soma was not significantly
different. Assessed by 100 μM GABA, the contributions
of GABAA and GABAC receptors to the
total GABA-mediated currents at the terminal were comparable.
This study directly demonstrates the localization of LVA
Ca2+ channels at the axon terminal of mammalian
rod bipolar cells, suggesting that LVA Ca2+
channels may play a role in bipolar cell transmitter release.
Results of this study also support the notion that both
types of ionotropic GABA receptors regulate synaptic transmission
in mammalian rod bipolar cells. In addition, this study
reports for the first time the feasibility of direct patch-clamp
recordings of isolated axon terminals of mammalian retinal
bipolar cells. The isolated presynaptic terminal preparation
of mammalian retinal bipolar cells could be a valuable
system for the study of transmitter release in the central
nervous system (CNS).