Frontiers in Systems Neuroscience (Mar 2014)
The coincident activation of lemniscal and paralemniscal inputs can drive synaptic plasticity in layer 2/3 pyramidal neurons of the mouse somatosensory cortex in vivo
Abstract
Structural plasticity in the somatosensory cortex is maintained throughout life. In adult animals structural changes occur at the level of dendritic spines and axonal boutons in response to alterations in sensory experience. The causal relationship between synaptic activity and structural changes, however, is not clear. Hebbian-plasticity models predict that synapses will be stabilized at the nodes of neuronal networks that display high levels of coincident activity. Here, we aim at studying the effects of a targeted increase in coincident activity between segregated inputs on pyramidal cell synapses of the mouse somatosensory barrel cortex in vivo. Supragranular layers of the barrel cortex receive anatomically distinct inputs from two thalamic pathways: the ‘lemniscal’ pathway that originates in the ventral posteromedial (VPM) nucleus and projects in a whisker-specific fashion to the barrel columns, and the ‘paralemniscal’ pathway that originates in the posteromedial (POm) nucleus and projects to the cortex in a non-specific manner. Previous work from our lab shows that rhythmic (8Hz) whisker stimulation-evoked LTP (RWS-LTP) in layer (L) 2/3 pyramidal cells relies on the combined activity of lemniscal and paralemniscal pathways. Here, we targeted ChR2 expression to POm neurons using AAV-mediated gene transfer in order to optically control the activity of those inputs. As a first step, we show that photostimulation of the POm nucleus induces NMDA-dependent, sub-threshold responses in L2/3 pyramidal cells similar to those that are required for the induction of RWS-LTP. In addition, simultaneous photostimulation of POm neurons together with whisker stimulation at low frequencies (1Hz) can also elicit LTP, suggesting that coincident lemniscal and paralemniscal input can drive LTP induction. Next, we combined the ChR2-tdTomato expression in POm neurons with sparse AAV-mediated eGFP expression in L2/3 pyramidal cells in order to study the effects of coincident activity on dendritic spines. Preliminary data suggest that simultaneous photostimulation of the POm nucleus together with whisker stimulation may also affect spine dynamics. Taken together, these results indicate that the coincident activity of the convergent lemniscal and paralemniscal inputs onto L2/3 pyramidal cells may potentiate synapses and affect their stability.
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