Synaptically driven spikes and long-term potentiation in neocortical layer 2/3.

Area of Science:

• Neuroscience
• Synaptic Plasticity
• Neocortical Circuitry

Background:

• Neocortical layer 2/3 synaptic strength estimation often assumes minimal postsynaptic spiking.
• Established hippocampal models inform neocortical research paradigms.

Purpose of the Study:

• To test the assumption that postsynaptic spiking does not significantly contribute to neocortical extracellular potentials.
• To investigate the role of spiking in long-term potentiation (LTP) induction and expression in layer 2/3.

Main Methods:

• Cell-attached recordings in rats to quantify unit spike rates in layer 2/3.
• Analysis of extracellular potentials and intracellular synaptic currents.
• Induction of LTP using theta-burst stimulation (TBS) and waveform analysis.

Main Results:

• Spike responses in layer 2/3 were elicited at latencies matching field potential peaks and synaptic currents.
• Spiking activity was sufficient to significantly influence response amplitudes.
• LTP induction by TBS required a strong initial spike component.
• LTP expression was consistently associated with enhanced spiking activity.

Conclusions:

• Neocortical LTP induced by TBS relies on modifying synaptically driven spike activity, unlike in hippocampal CA1.
• Mechanisms may include enhanced synchronization, recruitment of units, or modifications of recurrent connections within layer 2/3.