Axonal contribution to subthreshold currents in Aplysia bursting pacemaker neurons

Area of Science:

• Neuroscience
• Cellular Electrophysiology

Background:

• Bursting pacemaker activity in neurons is crucial for rhythmic physiological processes.
• The precise contribution of axonal electrical activity to somatic ionic currents in bursting neurons remains incompletely understood.

Purpose of the Study:

• To investigate the role of axonal activity in generating ionic currents underlying bursting pacemaker activity in Aplysia neurons.
• To differentiate between somatic and axonal contributions to the overall ionic current dynamics.

Main Methods:

• Utilized the two-electrode voltage-clamp technique on Aplysia bursting neuron somata.
• Performed simultaneous intra-axonal voltage recordings.
• Applied pharmacological agents like tetrodotoxin (TTX) and manipulated extracellular ion concentrations (Ca2+, Co2+).
• Injected hyperpolarizing current into the axon to eliminate axonal spikes.

Main Results:

• Depolarizing voltage-clamp pulses in the soma triggered axonal spikes, observed as inward current notches.
• Tetrodotoxin (TTX)-sensitive fast axonal spikes and slower calcium (Ca2+) spikes were identified.
• Eliminating axonal spikes altered the time course and magnitude of somatic inward currents.
• Axonal Ca2+ spikes were followed by Ca2+-dependent afterpotentials, including a long-lasting hyperpolarization not attributable to Ca2+-activated K+ current.
• A slow inward tail current was observed, partially dependent on axonal activity, suggesting a mixed somatic-axonal origin.

Conclusions:

• Axonal activity, particularly Ca2+ spikes and their afterpotentials, significantly contributes to the ionic currents governing bursting pacemaker activity.
• The slow inward tail current likely arises from an ionic conductance mechanism shared between the axon and soma.
• These findings highlight the importance of considering axonal electrical events in understanding neuronal bursting dynamics.