Aim: 

In hatchling Xenopus laevis tadpoles 7 classes of neuron have been identified anatomically and functionally in the developing spinal cord. These neurons control swimming behaviour and demonstrate 4 firing phenotypes to current injection: single action potential (S); repetitive firing without adaptation (R); repetitive firing with rapid adaptation (RA); and delayed-onset firing (DO). We explore membrane currents underlying the firing of S and R neurons.

Method: 

In vivo whole-cell patch recordings were made from neuron somata exposed in the spinal cord of animals ~40hours post-fertilisation. Neurons were identified by anatomy, and responses to current.

Results: 

Current-clamp recordings investigated firing patterns to current injection. S neurons fire a single long-duration action potential, the duration decreasing with reduced external calcium. Firing was unaltered by Muscarine (M-current antagonist) or 4-AP (A-current antagonist). In contrast, TEA (delayed rectifier antagonist) converts S neurons to multiple-firing R-like phenotypes. Voltage-clamp recordings revealed outward currents in S neurons activate more slowly than those in R neurons; both cell-types show limited current inactivation. The outward current reversed close to the potassium equilibrium potential, and was eliminated by TEA. These characteristics are typical of delayed rectifiers. In both neuron classes an inactivating outward current (A-current) was seen in a minority of cells, and calcium currents were small.

Conclusion

We suggest that in S neurons the slow outward current balances late stage inward currents, preventing further depolarisation. This slow activation may underlie the long duration action potential seen in S neurons. Kinetic differences may be sufficient to explain divergent firing phenotypes.