Figure 4.
The sign of intrinsic plasticity correlates with baseline level of spontaneous discharge and excitability. A, Potentiated and depressed neurons exhibit similar passive membrane properties. A1, Average voltage responses (n ≥ 10) to hyperpolarizing (−0.4 nA) current pulses obtained during control periods in a potentiated (LLPIE) and a depressed (LLDIE) neuron. The inset is the superimposed expansion of the initial part of responses shown below. A2, Pooled data of control Rin and membrane time constant values from potentiated (n = 18) and depressed (n = 12) cells. B, Pyramidal neurons expressing LLPIE are more hyperpolarized and less active than neurons expressing LLDIE. B1, Representative epochs of spontaneous activity (1.5 s recordings) recorded during control periods in barrel cortex pyramidal neurons exhibiting either LLPIE or LLDIE after induction. B2, Corresponding summary data (18 potentiated and 12 depressed neurons) of baseline spontaneous firing rate and mean Vm. C, Current threshold is lower in neurons experiencing LLDIE. C1, Examples of control voltage responses in a potentiated and a depressed neuron to the injection of current pulses of same intensity. C2, Population data of control current threshold and gain in LLPIE (n = 18) and LLDIE (n = 12) neurons. D, Relationship between control current threshold values and baseline spontaneous firing rate (left) and mean membrane potential (right), for control cells recorded under barbiturate (Crtl-b, black symbols, n = 9), control cells under fentanyl (Ctrl-f, gray symbols, n = 11), and for neurons experiencing potentiation (LLPIE, blue symbols, n = 18) or depression (LLDIE, orange symbols, n = 12) of their excitability. Dashed gray lines are best curve fits. E, Population data (18 potentiated and 12 depressed neurons) showing that the bidirectional changes in threshold current after induction correlated well (r = −0.65) with the initial threshold current value. *p < 0.05; ***p < 0.001; n.s., nonsignificant.