Dendritic backpropagation and synaptic plasticity in the mormyrid electrosensory lobe

Area of Science:

• Neuroscience
• Electrophysiology
• Cellular Biology

Background:

• GABAergic, medium ganglionic layer neurones (MG-cells) in the mormyrid electrosensory lobe (ELL) exhibit broad action potentials essential for spike timing dependent plasticity (STDP).
• Active conductances in MG-cell apical dendrites were hypothesized to underlie these broad potentials.

Purpose of the Study:

• To investigate the origin of backpropagating action potentials in MG-cells.
• To elucidate the role of ion channels in shaping these action potentials and their contribution to STDP.

Main Methods:

• Immunohistochemistry to localize voltage-gated sodium and potassium channels.
• In vitro intracellular recordings from MG-cells combined with voltage-sensitive dye imaging.
• Focal application of tetrodotoxin (TTX) to block sodium channels.
• Application of potassium channel blockers (TEA and 4-AP).

Main Results:

• Voltage-gated sodium channels are abundant in MG-cell dendrites and soma.
• Dendritic backpropagation of action potentials is active in the inner molecular layer.
• Backpropagating action potentials primarily originate in proximal dendrites, while narrow spikes originate in the axon.
• Both A-type and delayed rectifying potassium channels contribute to repolarization of both dendritic and axonal spikes.
• STDP depression was not dependent on GABA(B) receptors.

Conclusions:

• Backpropagating action potentials in MG-cells originate in proximal dendrites, mediated by voltage-gated sodium channels.
• Potassium channels play a significant role in repolarizing these action potentials.
• The findings provide insights into the mechanisms underlying synaptic plasticity in the electrosensory system.