Action potentials in amacrine cells are important for lateral
propagation of signals across the inner retina, but it is unclear how many
subclasses of amacrine cells contain voltage-gated sodium channels or can
fire action potentials. This study investigated the ability of amacrine
cells with narrow (< 200 μm) and wide (> 200 μm) dendritic
fields to fire action potentials in response to depolarizing current
injections and light stimulation. The pattern of action potentials evoked
by current injections revealed two distinct classes of amacrine cells;
those that responded with a single action potential (single-spiking cells)
and those that responded with repetitive action potentials
(repetitive-spiking cells). Repetitive-spiking cells differed from
single-spiking cells in several regards: Repetitive-spiking cells were
more often wide field cells, while single-spiking cells were more often
narrow field cells. Repetitive-spiking cells had larger action potential
amplitudes, larger peak voltage-gated NaV currents lower action potential
thresholds, and needed less current to induce action potentials. However,
there was no difference in the input resistance, holding current or time
constant of these two classes of cells. The intrinsic capacity to fire
action potentials was mirrored in responses to light stimulation;
single-spiking amacrine cells infrequently fired action potentials to
light steps, while repetitive-spiking amacrine cells frequently fired
numerous action potentials. These results indicate that there are two
physiologically distinct classes of amacrine cells based on the intrinsic
capacity to fire action potentials.