Figure 5.

LTP is input specific and only present at cortical synapses. A, Interneurons receive independent cortical and thalamic inputs. Left, EPSCs evoked by either cortical (top traces) or thalamic (bottom traces) stimulation at a holding potential of −60 mV. Middle, Response to paired pulses for cortical or thalamic stimulation. Right, Primed pulses for thalamic followed by cortical (Thal-Cort) and cortical followed by thalamic (Cort-Thal) stimulations at an interval of 50 ms. Paired pulses for either Cort-Cort or Thal-Thal stimulations result in PPF, whereas primed pulses for either Cort-Thal or Thal-Cort do not show cross-facilitation. B, Cortical and thalamic inputs stimulated simultaneously sum together showing there are no shared fibers between the two inputs. Plotted is the algebraic sum of separately evoked EPSCs in response to cortical and thalamic stimulation ([Cort] + [Thal]) against the EPSC evoked by simultaneous stimulation of cortical and thalamic inputs ([Cort + Thal]). C, LTP of cortical inputs is homosynaptic. Average data from neurons with fast NMDAR-mediated synaptic currents in which cortical and thalamic inputs were independently stimulated (τ_w cortical input, 50 ± 6 ms; thalamic input, 47 ± 7 ms; n = 5). Tetanic stimulation of cortical inputs leads to potentiation with no effect on thalamic inputs. D, Cortical and thalamic inputs evoked as in C were co-tetanized at time 0. Only cortical inputs are potentiated after paired stimulation of both inputs. τ_w for cortical input was 43 ± 4 ms and for thalamic input was 44 ± 7 ms (n = 5). E, Thalamic inputs to interneurons do not show LTP. The left panel shows averaged data from interneurons in which thalamic inputs (τ_w = 54 ± 2 ms; n = 10) were tetanized at time 0. F, Average data of interleaved recordings from principal neurons in which the same thalamic input was stimulated (n = 10). EPSCs recorded at the indicated times are shown above the graph. C, Cortical input; T, thalamic input; IN, interneuron; PN, principal neuron.