Synaptically evoked dendritic action potentials in rat neocortical pyramidal neurons

Area of Science:

• Neuroscience
• Cellular Electrophysiology

Background:

• Previous studies identified dendritic spikes evoked by glutamate iontophoresis in rat neocortical pyramidal neurons.
• These spikes were recorded intracellularly in the soma and originated in the dendrite.

Purpose of the Study:

• To investigate if synaptic inputs, specifically excitatory postsynaptic potentials (EPSPs), can also evoke dendritic spikes.
• To determine if dendritic spikes are unique to direct glutamate application or can be elicited by physiological synaptic activity.

Main Methods:

• Electrical stimulation of presynaptic elements near the apical dendrite of layer 5 pyramidal neurons.
• Recording of excitatory postsynaptic potentials (EPSPs) and dendritic spikes using intracellular microelectrodes.
• Application of glutamate iontophoresis to compare responses.
• Reversible blockade of glutamate receptors to confirm the role of glutamate.

Main Results:

• Electrically evoked EPSPs above a threshold size triggered all-or-none postsynaptic responses similar to iontophoretically evoked spikes.
• Both synaptically and iontophoretically evoked depolarizations were blocked by glutamate receptor antagonists.
• Subthreshold EPSPs and iontophoresis, when combined, evoked dendritic spikes.
• Summation of two independent subthreshold EPSPs could also evoke dendritic spikes.

Conclusions:

• Dendritic spikes are not exclusive to glutamate iontophoresis and can be evoked by physiological EPSPs.
• These findings have significant implications for understanding synaptic integration within neurons.
• The results necessitate a re-evaluation of how recorded synaptic potentials are interpreted in light of dendritic spike generation.