Aims and methods: 

Intrinsic firing properties of retinal ganglion cells (RGCs) of adult rat were studied by the whole cell patch-clamp technique in retinal flat-mounted preparations. In response to 500-ms depolarizing current step the majority (93.4%) of the examined RGCs displayed tonic firing that lasted for the duration of depolarization period. Only 6.6% of the RGCs displayed transient firing accommodated during the stimulus. In addition, 60.7% of the examined RGCs displayed sustained high-frequency firing with the steady-state firing frequency over 50Hz. Ionic conductances underlying excitability in tonically firing neurons were studied by applications of selective pharmacological blockers.

Results: 

Application of TTX (1mM) caused reversible disappearance of action potentials (AP) in response to stimulus. Suppression of Ca²⁺influx through voltage-activated Ca²⁺channels by 200mM Cd²⁺lead to increase of steady-state firing frequency, and to increase of single AP repolarization rate, without abolishing the basic pattern of tonic firing. Role of different types of voltage-gated potassium channels were studied using the application of the respective blockers. It was found that potassium conductance highly sensitive to external TEA (1mM) or 4-AP (200mM) is responsible for fast repolarization and after-hyperpolarization of a single AP, providing the cells with the ability for high-frequency firing. Conductances, sensitive to other blockers of voltage-gated potassium channels (a-DTX and CTX) did not play such a role. The known specificity of these drugs and the fast-spiking phenotype of majority of the cells strongly suggested that this 4-AP and TEA-sensitive conductance is mediated by Kv3 potassium channels.

Conclusion: 

Thus, in our cells, the Na⁺ and Kv3-like K⁺ currents generate a basic firing pattern, while Ca²⁺and Ca²⁺-dependent conductances stabilize tonic firing, efficiently regulate discharge frequency.