Cortical fosGFP expression reveals broad receptive field excitatory neurons targeted by POm.

Neighboring cortical excitatory neurons show considerable heterogeneity in their responses to sensory stimulation. We hypothesized that a subset of layer 2 excitatory neurons in the juvenile (P18 to 27) mouse whisker somatosensory cortex, distinguished by expression of the activity-dependent fosGFP reporter gene, would be preferentially activated by whisker stimulation. In fact, two-photon targeted, dual whole-cell recordings showed that principal whisker stimulation elicits similar amplitude synaptic responses in fosGFP-expressing and fosGFP(-) neurons. FosGFP(+) neurons instead displayed shorter latency and larger amplitude subthreshold responses to surround whisker stimulation. Using optogenetic stimulation, we determined that these neurons are targeted by axons from the posteromedial nucleus (POm), a paralemniscal thalamic nucleus associated with broad receptive fields and widespread cortical projections. We conclude that fosGFP expression discriminates between single- and multi-whisker receptive field layer 2 pyramidal neurons.