Abstract
Purpose: :
Retinal amacrine cells are instrumental in forming the center–antagonistic surround receptive fields of ganglion cells. Multiple studies have shown that wide field amacrine cells (>200 µm) contain voltage–gated sodium channels (NaVs) and utilize action potentials to propagate their signals to the ganglion cells. Despite the presence of NaVs in narrow field amacrine cells (<200 µm), and the detection of action potentials in some narrow field cells, it is not known if all narrow field cells fire action potentials. This study will show that amacrine cells with diverse morphologies fire action potentials in response to current injection and light stimulation and attempt to correlate action potential firing to morphological characteristics.
Methods: :
Using patch clamp techniques, we recorded whole–cell voltages from amacrine cells in the tiger salamander retinal slice. Spiking was evoked by current steps or full field illumination. The recording pipette contained either tetramethyrhodamine or Alexa Fluor 488 to characterize cell morphology. Slices were fixed overnight and imaged using an Olympus confocal microscope.
Results: :
Current steps evoked action potentials in 98% of the recorded amacrine cells (N=36) whether wide field (N= 12), or narrow field (N=23), sparsely ramifying (N=15) or densely ramifying (N=14). Cells responded with a single spike (33%), multiple spikes that rapidly adapted (repetitive) (54%), or sustained spiking (12%). Single spiking cells typically had higher thresholds and smaller peak spike amplitudes than the repetitive spiking cells. Cells were classified according to two characteristics: dendritic extent (lateral spread) and dendritic density (amount of processes). There was no direct correlation between the spiking characteristics and dendritic morphology. Spiking characteristics evoked by current injection frequently did not correspond to the spiking evoked by illumination. For example a cell that fired a single small action potential with depolarizing current fired sustained action potentials during a step of illumination.
Conclusions: :
This study revealed that both wide and narrow field amacrine cells fire action potentials in response to current injection and light stimulation. The pattern of firing did not correspond with the length or density of the amacrine cell processes. Single spiking cells may localize NaV channels in electrotonically distant axodendritic processes, since somatic depolarization was less effective at evoking action potentials. This suggests that spiking characteristics may be better correlated to the location of the NaV channels.
Keywords: amacrine cells • electrophysiology: non-clinical • anatomy