Patch-clamp investigations and compartmental modeling of rod bipolar axon terminals in an in vitro thin-slice preparation of the mammalian retina

To extend the usefulness of rod bipolar cells for studies of chemical synaptic transmission, we have performed electrophysiological recordings from rod bipolar axon terminals in an in vitro slice preparation of the rat retina. Whole cell recordings from axon terminals and cell bodies were used to investigate the passive membrane properties of rod bipolar cells and analyzed with a two- compartment equivalent electrical circuit model developed by Mennerick et al. For both terminal- and soma- end recordings, capacitive current decays were well fitted by biexponential functions. Computer simulations of simplified models of rod bipolar cells demonstrated that estimates of the capacitance of the axon terminal compartment can depend critically on the recording location, with terminal- end recordings giving the best estimates. Computer simulations and whole cell recordings demonstrated that terminal- end recordings can yield more accurate estimates of the peak amplitude and kinetic properties of postsynaptic currents generated at the axon terminals due to increased electrotonic filtering of these currents when recorded at the soma. Finally, we present whole cell and outside- out patch recordings from axon terminals with responses evoked by GABA and glycine, spontaneous inhibitory postsynaptic currents, voltage- gated Ca2+ currents, and depolarization- evoked reciprocal synaptic responses, verifying that the recorded axon terminals are involved in normal pre- and postsynaptic relationships. These results demonstrate that axon terminals of rod bipolar cells are directly accessible to whole cell and outside- out patch recordings, extending the usefulness of this preparation for detailed studies of pre- and postsynaptic mechanisms of synaptic transmission in the CNS.