Neocortical layer 2/3 microcircuits

Abstract

Pairs of interconnected neurons form elementary information processing units within neocortical microcircuits. Pyramidal cells within these microcircuits receive synaptic input, both from their connected partners, and from more distant cortical and sub-cortical regions. Consequently, they require a means to identify relevant synaptic signals. This may be achieved by short- and long-term modification of synaptic strength.

This study examines changes in synaptic strength at unitary inhibitory and excitatory connections onto layer 2/3 pyramidal cells, in neocortical microcircuits. Two types of neocortical microcircuit were studied; fast-spiking non-accommodating interneuron (FSN)-pyramidal cell, and pyramidalpyramidal cell microcircuits.

The strength of the inhibitory connection in FSN-pyramidal cell microcircuits is modulated in the short-term by retrograde signaling. It is shown that this rapid negative feedback pathway uses glutamate as a retrograde messenger. A novel role for vesicular glutamate transporter 3 (VGLUT3) as a mediator in this process is suggested. Additionally, it is shown that the strength of inhibitory connections within the neocortex can be modified in the long-term by spike-timing-dependent plasticity (STDP).

STDP at layer 2/3 pyramidal-pyramidal cell unitary synaptic connections was studied in the third part of this thesis. Previously, it was reported that at excitatory synapses onto pyramidal cells, a few milliseconds shift in spike timing, around the point of spike coincidence, induces a dramatic switch between either LTD or LTP induction. Here it is shown, however, that a smooth transition between LTD and UP induction occurs, as a factor of spike timing, at unitary connections between layer 2/3 pyramidal cells. The results show that the direction of synaptic plasticity is determined by spine Ca2+ dynamics, within a critical time period of about 15 ins, immediately following synaptic activation.

Communication between neurons in vivo takes place on a background of network activity. How effective then is unitary synaptic signaling in the context of an active network? This question was examined in the final part of this thesis. Pyramidal-topyramidal cell connectivity was infrequent, and weak. Moreover, firing of a pyramidal cell did not cause a noticeable change in the firing of a connected pyramidal cell. Conversely, pyramidal cell-FSN connections were numerous, strong, and frequently reciprocal. Firing in either the pyramidal cell or FSN interneuron could induce a change the firing rate of the coupled neuron, even within an active network. It is suggested that interneurons within local networks facilitate pyramidal to pyramidal cell communication via pyramidalinterneuron-pyramidal cell connections.

In summary, this study shows that inhibitory synaptic input onto pyramidal cells is controlled by rapid negative feedback, mediated by a retrograde messenger. Long-term changes in both inhibitory and excitatory unitary synaptic strength can be regulated by STDP. It is also shown that unitary synaptic connections have an impact on pyramidal cell firing, even within the context of an active neuronal network.