Abstract
Purpose::
How amacrine cells respond to and process light information influence the bipolar cell output and ganglion cell responses. Previously we showed that amacrine cells respond to current depolarization with either single or repetitive action potentials, and that action potential thresholds are lower in repetitive spiking cells than in single spiking cells. Do these intrinsic limits on action potential firing correlate to transient and sustained responses to light?
Methods::
Using patch clamp techniques, we recorded whole-cell voltages or currents from amacrine cells in the tiger salamander retinal slice. Depolarizing current steps were used to characterize amacrine cells as single or repetitively spiking. Illumination from an LED was passed through a series of neutral density and wavelength (560nm) filters and was transmitted through the 60X objective.
Results::
Repetitive spiking cells responded to light with both transient and sustained depolarization and they typically fired action potentials. Though some repetitive cells did not have inhibitory currents, in those cells that did, the inhibitory post-synaptic currents (IPSC) and excitatory post-synaptic currents (EPSC) were evoked by light from an LED, and the IPSCs always followed the EPSCs with a latency difference greater than 10 ms. Conversely, single amacrine cell responses to light were mostly transient and rarely fired action potentials, even with increased light intensities. The latency difference between EPSC and IPSCs in single spiking cells was short, about 1 ms, and less than the latency difference in repetitive firing cells.
Conclusions::
Repetitive spiking cells tended to respond strongly to light by firing action potentials, in contrast to the single spiking cells which infrequently spiked in response to light. Intrinsic mechanisms such as low threshold for action potential firing may be responsible for the action potential responses in repetitive spiking cells. Single spiking cells that are intrinsically limited may also be limited in their response to light since the coincident arrival of EPSC and IPSCs may shunt the membranes, reducing the likelihood for action potentials firing, indicating that their synaptic input must be very specific to evoke large responses from these cells. The shorter latencies between the EPSCs and IPSCs in single spiking cells suggest that the cells mediating these inputs may be coupled through gap junctions.
Keywords: amacrine cells • retina: proximal (bipolar, amacrine, and ganglion cells) • retinal connections, networks, circuitry