Voltage‐gated transient outward currents in neurons with different firing patterns in rat superior colliculus

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Abstract
1. We investigated the electrophysiological properties of transient outward currents (TOCs) in neurons with different firing patterns, regular-spiking, fast-spiking and late-spiking neurons, in the intermediate layer (SGI) of the superior colliculus using the whole-cell patch clamp technique in slice preparations obtained from young rats (post-natal days 17-22). 2. Analysis of inactivation kinetics and normalized amplitude revealed that TOCs in regular-and fast-spiking neurons had fast inactivation kinetics (decay time constants (mean +/- s.e.m.) of 13.8 +/- 1.5 and 11.4 +/- 1.2 ms, respectively) and low current densities (36.6 +/- 3.3 and 32.1 +/- 4. 9 pA pF-1, respectively). TOCs in late-spiking neurons, on the other hand, displayed a wide range of both inactivation kinetics (36.7 +/- 2.4 ms, with a range from 11.3 to 147.8 ms) and current density (54. 0 +/- 2.9 pA pF-1, with a range from 9.8 to 131.2 pA pF-1). 3. In regular-, fast- and late-spiking neurons having TOCs with slow time constants (> 50 ms, class II late-spiking neurons), the TOCs were sensitive to 4-aminopyridine (4-AP), with IC50 values of 2.9, 2.4 and 1.2 mM, respectively. In late-spiking neurons having TOCs with fast decay time constants (< 30 ms, class I late-spiking neurons), the TOCs were composed of at least two 4-AP-sensitive components (IC50 values of 0.2 microM and 3.6 mM). 4. Class I late-spiking neurons displayed non-inactivating outward currents which were highly sensitive to 4-AP. They changed their firing patterns to the regular-spiking mode, not only in response to low concentrations of 4-AP (< 50 microM), but also in response to dendrotoxin (200 nM), suggesting that non-inactivating outward currents contribute to the late-spiking property. However, the components of TOCs which were highly sensitive to 4-AP were also sensitive to dendrotoxin. These results suggest that both or either of the two currents contribute to the late-spiking property of class I late-spiking neurons. 5. Although class II late-spiking neurons also displayed non-inactivating outward currents, the late-spiking property was not abolished by low concentrations of 4-AP and dendrotoxin. They changed to a regular firing pattern in response to a high concentration of 4-AP (5 mM), suggesting that TOCs contribute to late-spiking property of class II late-spiking neurons. 6. The results suggest that TOCs with different properties contribute to the different firing patterns of SGI neurons.