Figure 4.

T-current confers robustness to the neuronal response and stabilizes the transfer function of TC neurons across a large range of membrane potentials. A, Voltage traces recorded in a TC neuron maintained at mean membrane potentials of −58, −63, and −68 mV while being submitted to AMPA conductances (gAMPA) of different amplitudes. The same sequences of gAMPA and excitatory/inhibitory noise were injected in every condition. As indicated by asterisks, which point to the loss of spike upon hyperpolarization, the firing probability was barely affected in the control condition (CTR) while it was highly sensitive to membrane potential changes in the presence of TTA-P2. B, Transfer functions were quasi-invariant in the presence of the T-current but drastically shifted toward larger gAMPA values upon hyperpolarization when the T-current was blocked. C, Similar voltage dependence of the transfer functions was observed in TC neurons recorded in Ca_v3.1^−/− knock-out mice devoid of T-current. D, Mean values of gAMPA_0.5 were determined in TC neurons successively maintained between −72 and −67 mV, −66 and −61 mV, and −60 and −55 mV. In the control condition (CTR, n = 16), gAMPA_0.5 values remained stable when the neurons were hyperpolarized from −60/−55 to −66/−61 mV and decreased at a more hyperpolarized potential. Conversely, when the T-current was blocked (TTA-P2, n = 12) or absent (Ca_v3.1^−/−, n = 9), the required AMPA inputs drastically increased with hyperpolarization of the TC neurons (paired t test; NS, nonsignificant; **p < 0.01; ***p < 0.001). Note that a subset of cells was recorded at different membrane potentials in only one condition, either CTR (n = 8) or TTA-P2 (n = 4), and that absolute mean values cannot be quantitatively compared across graphs in D. E, TC neuron model: gAMPA_0.5 calculated at different membrane voltages for various T-conductances (g_T). Note the stability conferred by the increasing g_T.