An excitatory GABAergic plexus in developing neocortical layer 1.

Area of Science:

• Neuroscience
• Developmental Neuroscience
• Cortical Development

Background:

• The developing rodent somatosensory cortex (layer 1) features a transient, dense GABAergic fiber plexus.
• This plexus receives input from the zona incerta (ZI) and intrinsic layer 1 GABAergic cells.
• The function of this early plexus, contacting pyramidal neuron dendrites, remains unclear.

Purpose of the Study:

• To investigate the function of the GABAergic fiber plexus in layer 1 of the developing somatosensory cortex.
• To determine the source and impact of GABAergic inputs on cortical pyramidal neurons.

Main Methods:

• Electrical stimulation of layer 1 and gramicidin perforated patch recordings in neonatal rodents.
• Activation of intrinsic layer 1 neurons using glutamate agonists.
• Recording from identified neonatal incertocortical neurons.

Main Results:

• Electrical stimulation of layer 1 induced GABA(A)-mediated postsynaptic currents (PSCs) in pyramidal neurons.
• The GABAergic synapse in layer 1 was found to be excitatory, capable of triggering action potentials.
• Stimulation of intrinsic layer 1 neurons did not produce PSCs, unlike electrical stimulation.
• Responses were observed even with stimulation at distant sites in layer 1.
• Neonatal incertocortical neurons showed high spontaneous activity and robust cortical neuron activation.

Conclusions:

• The incertocortical pathway is a major contributor to the excitatory GABAergic innervation of layer 1 during synaptogenesis.
• This pathway provides spatially restricted, excitatory GABAergic input to the distal apical dendrites of pyramidal neurons.
• The findings reveal a novel excitatory role for GABA in early cortical circuit formation.