Inhibitory effect of mouse neocortex layer I on the underlying cellular network

Area of Science:

• Neuroscience
• Cellular Neuroscience
• Cortical Circuitry

Background:

• Mammalian cortical layer I integrates sub- and intracortical inputs and pyramidal neuron dendrites.
• Layer I features a dense plexus of inhibitory axons from stellate cells, whose role in cortical network activity is unclear.

Purpose of the Study:

• To investigate the influence of inhibitory inputs in layer I on the activity of the underlying cellular network.
• To determine the extent to which layer I inhibition modulates cortical excitability and epileptiform activity.

Main Methods:

• Field potentials (FPs) were recorded in layer II/III.
• Inhibition was manipulated via focal application of the GABAA blocker picrotoxin in layer I or by layer I removal.
• Immunocytochemistry was used to identify inhibitory interneuron subtypes (somatostatin- and calretinin- immunoreactive axons).

Main Results:

• Blocking GABAA inhibition in layer I or removing layer I increased field potential amplitudes.
• Partial blockade of inhibition followed by layer I removal reduced the stimulus threshold for epileptiform events and increased their propagation velocity.
• Layer I inhibition is predominantly somatostatin-immunoreactive (SM-ir), constituting about one-third of cortical SM-ir axons.

Conclusions:

• Layer I exerts a significant inhibitory influence on the cortical cellular network.
• This inhibition provides a moderate restraining effect, and its absence enhances cortical circuit excitability.
• While layer I removal increases excitability, it is insufficient to trigger epileptic phenomena on its own.