Figure 1.
Matrix TC projections terminate in outer L1 of mPFC, and their activation drives robust synaptic responses in L1 interneurons. A, Example of matrix TC terminal labeling in cortical L1. Left, Drawing of coronal slice at the focus of the thalamic virus injection (1.82 mm posterior from bregma). The injection was centered on the ventromedial (VM) thalamus (symbolized by green shading). Right, Drawing of coronal slice at plane that received dense TC projections (1.94 mm anterior to bregma). Dashed red rectangle indicates region shown at high magnification in adjacent bright-field (DIC) and fluorescence (EYFP) images. Location of L1–L2 boundary in fluorescence image indicated by dashed line. Intense EYFP/ChR2 labeling of TC arbors was concentrated in outer L1 of the mPFC areas IL, PrL, and Cg, as well as M2. B, Robust TC synaptic responses from an L1 interneuron recorded ∼38 μm from the pia (near position 2 in A). High-intensity laser flashes (11.3 mW, 1 ms, 10 Hz train) were centered ∼30 μm from the pia, near the soma. Green arrows indicate flash times. Spikes were evoked in 4 of 10 sweeps. Inset shows time-locked EPSPs after synaptic delay for two sweeps, one of which leads to a spike. C, Lower-intensity flashes (0.54 mW) evoked subthreshold EPSPs. Short-term synaptic depression was apparent during 10 Hz trains. EPSPs were far stronger when stimuli were delivered near TC terminals in outer L1 (same location as in B) than at L1–L2 border (position 4 in A, right). Similar patterns were observed in 18 of 18 tested L1 cells; PSP responses to matching low-intensity stimuli (mean, 0.76 ± 0.13 mW) averaged 6.4 ± 1.5 mV versus 0.7 ± 0.1 mV for stimuli to outer L1 and the L1/L2 border, respectively. D, Functional evidence that matrix TC synapses are located in outer L1. TTX (1 μm) and 4-AP (1 mm) were applied to block axon conduction but allow optogenetic excitation of ChR2-expressing presynaptic boutons (Petreanu et al., 2009; Cruikshank et al., 2010), and then EPSPs to laser stimuli (2.6 mW) covering a range of positions (positions 1–7 in A) were recorded (from the same interneuron as in B, C) to map the laminar locations of the TC synapses. Responses were strongest for stimuli in outer L1 (positions 1–2), consistent with the EYFP fluorescence pattern. Steady-state Vm was −79 mV for B–D. CL, Centrolateral thalamic nucleus; CM, central medial thalamic nucleus; Ins, insular cortex; M1, primary motor cortex; MD, mediodorsal thalamic nucleus; Pir, piriform cortex; Po, posterior thalamic nucleus; Re, reuniens thalamic nucleus; S1, primary somatosensory cortex; VM, ventromedial thalamic nucleus; VP, ventral posterior thalamic nucleus.