Large conductance Ca(2+)-activated K+ channels are involved in both spike shaping and firing regulation in Helix neurones

Area of Science:

• Neuroscience
• Cell Physiology
• Ion Channel Function

Background:

• BK-type calcium-dependent potassium channels (K+Ca) are critical for regulating neuronal excitability.
• U cells in Helix pomatia were chosen for study due to their significant K+Ca current contribution to repolarization.

Purpose of the Study:

• To investigate the role of K+Ca channels in regulating the firing patterns of Helix pomatia U cells.
• To determine the activation kinetics and calcium dependency of K+Ca channels during neuronal activity.

Main Methods:

• Whole-cell voltage clamp and patch clamp electrophysiology were employed on U cell neurons.
• Experiments involved manipulating calcium influx and voltage protocols to assess K+Ca channel gating.
• Hybrid current-clamp and voltage-clamp techniques were used to analyze channel activity during action potentials.

Main Results:

• K+Ca channel opening is primarily limited by the calcium binding step, not voltage-dependent conformational changes.
• Despite slow activation under voltage clamp, 25-30% of K+Ca current was active during isolated spikes.
• A rapid surge of calcium during action potentials accelerates K+Ca channel opening via local calcium increases (50-80 μM).

Conclusions:

• K+Ca channels do not shape interspike trajectories due to their voltage dependence but contribute to firing adaptation.
• Fast adaptation in U cells is linked to cumulative calcium entries and K+Ca current overlap with calcium currents, ultimately stopping firing.
• Residual calcium accumulation enables rapid K+Ca channel activation during subsequent depolarization, influencing firing patterns.