TY - JOUR T1 - An ultrastructural study of the thalamic input to layer 4 of primary motor and primary somatosensory cortex in the mouse JF - The Journal of Neuroscience JO - J. Neurosci. DO - 10.1523/JNEUROSCI.2557-16.2017 SP - 2557-16 AU - Rita Bopp AU - Simone Holler-Rickauer AU - Kevan A.C. Martin AU - Gregor F.P. Schuhknecht Y1 - 2017/01/30 UR - http://www.jneurosci.org/content/early/2017/01/30/JNEUROSCI.2557-16.2017.abstract N2 - The traditional classification of primary motor cortex (M1) as an agranular area has recently been challenged, when a functional layer 4 (L4) was reported in M1. L4 is the principal target for thalamic input in sensory areas, which raises the question how thalamocortical synapses formed in M1 in the mouse compare with those in neighboring sensory cortex (S1). We identified thalamic boutons by their immunoreactivity for the vesicular glutamate transporter 2 (VGluT2) and performed unbiased disector counts from electron micrographs. We discovered that the thalamus contributed proportionately only half as many synapses to the local circuitry of L4 in M1 as compared to S1. Furthermore, thalamic boutons in M1 exclusively targeted spiny dendrites, while in S1 about 9% of synapses were formed with dendrites of smooth neurons. VGluT2+ boutons in M1 were smaller and formed fewer synapses per bouton on average (1.3 vs. 2.1) than those in S1, but VGluT2+ synapses in M1 were larger than in S1 (median postsynaptic density areas of 0.064μm2 vs. 0.042μm2). In M1 and S1 thalamic synapses formed only a small fraction (12.1% and 17.2%, respectively) of all the asymmetric synapses in L4. The functional role of the thalamic input to L4 in M1 has been largely neglected, but our data suggest that, as in S1, the thalamic input is amplified by the recurrent excitatory connections of the L4 circuits. The lack of direct thalamic input to inhibitory neurons in M1 may indicate temporal differences in the inhibitory gating in L4 of M1 vs. S1.SIGNIFICANCE STATEMENTClassical interpretations of the function of primary motor cortex (M1) emphasize its lack of the granular layer 4 typical of sensory cortices. We show here, however, that like S1, mouse M1 also has the canonical circuit motif of a core thalamic input to the middle cortical layer and that thalamocortical synapses form a small fraction (M1:12%; S1:17%) of all asymmetric synapses in layer 4 of both areas. Amplification of thalamic input by recurrent local circuits is thus likely to be a significant mechanism in both areas. Unlike M1, where thalamocortical boutons typically form a single synapse, thalamocortical boutons in S1 usually formed multiple synapses, which means they can be identified with high probability in the electron microscope without specific labeling. ER -