Steady-state dynamics and experience-dependent plasticity of dendritic spines of layer 4/5a pyramidal neurons in somatosensory cortex

The steady state dynamics and experience-dependent plasticity of dendritic spines of layer (L) 2/3 and L5B cortical pyramidal neurons have recently been assessed using in vivo two-photon microscopy (Trachtenberg et al., 2002; Zuo et al., 2005; Holtmaat et al., 2006). In contrast, not much is known about spine dynamics in L4/5a neurons, regarded as direct recipients of thalamocortical input (Constantinople and Bruno, 2013). In the adult mouse somatosensory cortex (SCx), the transcription factor Ebf2 is enriched in excitatory neurons of L4/5a, including pyramidal neurons. We assessed the molecular and electrophysiological properties of these neurons as well as the morphology of their apical tufts (Scholl analysis) and cortical outputs (optogenetics) within the SCx. To test the hypothesis that L4/5a pyramidal neurons play an important role in sensory processing (given their key laminar position; soma depth ~450-480 µm), we successfully labeled them in Ebf2-Cre mice with EGFP by expressing recombinant rAAV vectors in utero. Using longitudinal in vivo two-photon microscopy through a craniotomy (Mostany and Portera-Cailliau, 2008), we repeatedly imaged spines in apical dendritic tufts of L4/5a neurons under basal conditions and after sensory deprivation. Under steady-state conditions in adults, the morphology of the apical tufts and the mean spine density were stable at 0.39 ± 0.05 spines/μm (comparable to L5B, Mostany et al., 2011). Interestingly, spine elimination increases 4-8 days after sensory deprivation, probably due to input loss. This suggests that Ebf2+ L4/5a neurons could be involved in early steps of processing of thalamocortical information.