Function of Neuronal Circuitries
As a model system we use the representation of rodent whiskers, the so-called barrel field in somatosensory cortex. Individual synaptic connections within a cortical column were investigated in acute brain slices using patch clamp recordings combined with intracellular labelling. Our long-term goal is to identify and eventually to model the structural and functional organisation of neurones and their synapses in a cortical column.
To date, it is still rather unclear, how the organisation of the cortical column and its neuronal connectivity develops. In this context, we investigate how transient and persistent neuronal connections are embedded in early cortical networks and which role they play in the establishment of cortical columns.
Neocortical acetylcholine (ACH) release is known to enhance signal processing by increasing the amplitude and signal-to-noise ratio (SNR) of sensory responses. It is widely accepted that the larger sensory responses are caused by a persistent increase in the excitability of all cortical excitatory neurons. Here, contrary to this concept, we show that ACH persistently inhibits layer 4 (L4) spiny neurons, the main targets of thalamocortical inputs. Using whole-cell recordings in slices of rat primary somatosensory cortex, we demonstrate that this inhibition is specific to L4 and contrasts with the ACH-induced persistent excitation of pyramidal cells in L2/3 and L5. We find that this inhibition is induced by postsynaptic M(4)-muscarinic ACH receptors and is mediated by the opening of inwardly rectifying potassium (K(ir)) channels. Pair recordings of L4 spiny neurons show that ACH reduces synaptic release in the L4 recurrent microcircuit. We conclude that ACH has a differential layer-specific effect that results in a filtering of weak sensory inputs in the L4 recurrent excitatory microcircuit and a subsequent amplification of relevant inputs in L2/3 and L5 excitatory microcircuits. This layer-specific effect may contribute to improve cortical SNR.
Using in vitro and in vivo data, a computational model of a synaptic connection within a cortical column, the smallest signal transformation unit of the neocortex, was constructed. This is a first step in describing how the neuronal network in the cortical column processes sensory signals from the periphery.
Unitary synaptic connection in the somatosensory cortex. A presynaptic layer 4 spiny stellate cell (dendrites, red; axon, blue) forms excitatory synapses with its postsynaptic target neuron, a layer 2/3 pyramidal cell (dendrites, black; axon, green). Two adjacent barrels are indicated in grey, cortical lamination is indicated on the left at the end of the movie.