Long-term synaptic potentiation (LTP) is thought to be a key process in cortical synaptic network plasticity and memory formation. Hebbian forms of LTP depend on strong postsynaptic depolarization, which in many models is generated by action potentials that propagate back from the soma into dendrites. However, local dendritic depolarization has been shown to mediate these forms of LTP as well. As pyramidal cells in supragranular layers of the somatosensory cortex spike infrequently, it is unclear which of the two mechanisms prevails for those cells in vivo. Using whole-cell recordings in the mouse somatosensory cortex in vivo, we demonstrate that rhythmic sensory whisker stimulation efficiently induces synaptic LTP in layer 2/3 (L2/3) pyramidal cells in the absence of somatic spikes. The induction of LTP depended on the occurrence of NMDAR (N-methyl-d-aspartate receptor)-mediated long-lasting depolarizations, which bear similarities to dendritic plateau potentials. In addition, we show that whisker stimuli recruit synaptic networks that originate from the posteromedial complex of the thalamus (POm). Photostimulation of channelrhodopsin-2 expressing POm neurons generated NMDAR-mediated plateau potentials, whereas the inhibition of POm activity during rhythmic whisker stimulation suppressed the generation of those potentials and prevented whisker-evoked LTP. Taken together, our data provide evidence for sensory-driven synaptic LTP in vivo, in the absence of somatic spiking. Instead, LTP is mediated by plateau potentials that are generated through the cooperative activity of lemniscal and paralemniscal synaptic circuitry.
Rhythmic whisker stimulation (RWS, 8Hz/1min) induces LTP in L2/3 neurons through the recruitment of POm axons. Top left, in vivo whole-cell recording and 2-photon imaging of L2/3 pyramidal neuron. Top right, Single-cell whisker-evoked PSP amplitudes before and after RWS. Bottom, POm neurons were transfected with the genetically-encoded calcium indicator GCaMP5G and their projecting axons were imaged in L1 of the cortex through a chronically-implanted cranial window.
Gambino F., Pagès S., Kehayas V., Baptista D., Tatti R., Carleton A, Holtmaat A.
Frédéric Gambino, tel. +33 (0) 557 575 748