Figure 7.

Synaptic responses during repetitive TC activation are more sustained for matrix than core pathways. Short-term synaptic dynamics to 10 Hz optical stimulus trains are compared for matrix TC projections to L1 of mPFC (data from Fig. 3C) versus core sensory TC projections from the ventrobasal thalamus to L4–L6 of somatosensory cortex. The sensory TC data were obtained using the same optogenetic methods applied in the present study and are adapted from Cruikshank et al. (2010), their Figure 6c. A, Group synaptic dynamics for excitatory cortical cells in response to 10 Hz optical stimulation of TC arbors. Responses to matrix TC input (in L2/L3 pyramidal cells) typically exhibited short-term facilitation followed by weak depression, whereas responses to core sensory TC input (in L4 excitatory spiny stellate cells and L5/L6 pyramidal cells) depressed strongly. B, Same configuration as in A except postsynaptic cells are inhibitory interneurons. Responses to matrix TC input (in L1 interneurons) generally exhibited weak depression, whereas responses to core sensory TC input (in L4 and L5/L6 fast-spiking inhibitory interneurons) depressed strongly. Note that short-term synaptic depression is likely to be artificially enhanced and short-term facilitation blunted because of desensitization of ChR2 in the presynaptic axons/terminals (discussed by Cruikshank et al., 2010). Thus, matrix TC synapses are probably even more capable of maintaining sustained transmission than indicated by the dynamics plotted here.