Long-Term K⁺ Channel-Mediated Dampening of Dopamine Neuron Excitability by the Antipsychotic Drug Haloperidol

Chronic in vivo haloperidol treatment slows DA neuron intrinsic pacemaker activity. A, B, Whole-cell current-clamp recording from a single DA neuron from rats treated with vehicle (A) and haloperidol (B) for 5 or 6 d (chronic Hal). Spontaneous firing activity was recorded without current injection. Lower traces show pacemaker activity on an expanded time scale. Vertical bars, 25 mV; horizontal bars, 20 sec (top) or 2 sec (bottom). C, Histogram of interspike interval from the trace recorded in experiment A. D, Histogram of interspike interval from the trace in experiment B (bin width, 25 msec). E, The mean interspike interval. F, The coefficient of variation of interspike interval from independent DA neurons (vehicle, n = 4; chronic Hal, n = 4). Chronic treatment with haloperidol increased the mean value of ISI and CV_ISI, respectively. ^**p < 0.01; ^*p < 0.05. G, Cumulative histograms for ISI data combined from vehicle and chronic Hal DA neurons (p < 0.001; Kolmogorov-Smirnov test). Note the rightward shift and decreased slope after chronic haloperidol treatment.

Chronic in vivo haloperidol treatment dampens DA neuron excitability. A, Whole-cell current-clamp recordings from single DA neurons from rats treated with vehicle or haloperidol for 5 or 6 d. Spontaneous firing activity was recorded when clamped current was released to 0 pA from a holding potential of -80 mV. B, Chronic haloperidol treatment decreases spontaneous spike frequency and increases first spike latency and interspike interval between first and second spikes. (control, n = 5; vehicle, n = 6; haloperidol, n = 6). Broken lines indicate -80 mV. C, Whole-cell current-clamp recordings from single DA neurons from rats treated 1 d with vehicle or haloperidol. D, One day haloperidol treatment does not affect spike frequency, first spike latency, and first interspike interval (vehicle, n = 5; haloperidol, n = 6). ^*p < 0.05.

TEA does not influence the effect of chronic haloperidol on DA pacemaker activity. A, Whole-cell current-clamp recordings in the presence of 50 mm TEA from DA neurons of rats treated chronically with vehicle or haloperidol (chronic Hal). Decreased firing frequency and increased latency by chronic haloperidol is preserved after acute perfusion of 50 mm TEA. Broken lines indicate -80 mV. B, Summary of 50 mm TEA effects on spike frequency and first spike latency of DA neuron spontaneous activity (control, n = 4; vehicle, n = 5; chronic Hal, n = 5). ^*p < 0.05.

Chronic haloperidol treatment increases Kv4 A-type K⁺ current in DA neurons. A, Whole-cell voltage-clamp recordings and mean current density of transient K⁺ currents from DA neurons of rats treated chronically with vehicle or haloperidol (chronic Hal). Left, Middle, Transient currents were obtained by subtracting noninactivating currents from total outward currents (vehicle, 21.2 pF; chronic Hal, 22.3 pF). Inset, Total outward currents were evoked by depolarizing steps (-20 to 60 mV) from a holding potential of -90 in 20 mV steps. Noninactivating currents were evoked by the same pulse protocol after a prepulse to -40 mV for 500 msec. Right, Mean transient K⁺ current density from each group. Transient K⁺ current density was obtained by dividing peak transient K⁺ currents at 60 mV by cell capacitance (control, n = 6; vehicle, n = 9; chronic Hal, n = 10). Transient current density is increased in DA neurons from a haloperidol-treated rat compared with DA neurons from a vehicle-treated rat. Horizontal bars, 500 msec; vertical bars, 2.5 nA. B, Whole-cell voltage-clamp recordings and mean current density of K⁺ currents in the presence of 50 mm TEA. Left, Middle, Transient currents in DA neurons (vehicle, 26.8 pF; chronic Hal, 27.3 pF) were activated by a pulse to 20 mV from a holding potential of -90 mV. Right, Summary from eight cells in each cell group. ^**p < 0.01. C, D, Block of transient current in representative DA neurons from rats chronically treated with haloperidol by 4-AP (C) and HpTx₃ (D). Currents were recorded at 20 mV (C) and 0 mV (D) from a holding potential of -90 mV.

Specific effect of chronic in vivo haloperidol treatment. A, Mean voltage-dependent Na⁺ current density from DA neurons. Current density was obtained by dividing peak Na⁺ currents at 0 mV by cell capacitance (control, n = 5; vehicle, n = 6; chronic Hal, n = 6). B, Mean hyperpolarization-activated cation current density from DA neurons. Current density was obtained by dividing peak I_h currents at the end of the pulse (-100 mV) by cell capacitance (control, n = 5; vehicle, n = 5; chronic Hal, n = 6). C, Mean delayed rectifier K⁺ current density from DA neurons. Current density was obtained by dividing K⁺ currents at the end of the pulse (60 mV) by cell capacitance (control, n = 29; vehicle, n = 25; chronic Hal, n = 30). D, Mean A-type K⁺ current density from DA neurons does not change during the time period studied here. Current density was obtained by the same protocol described in Figure 5 (P5, n = 6; P10, n = 4; P11, n = 6; P12, n = 6).

Chronic haloperidol treatment does not affect gating of A-type K⁺ current in DA neurons. A, Mean activation kinetics. B, Mean inactivation kinetics. Averaged activation and inactivation time constants are plotted as a function of the test pulse voltage for each cell group. Triangle, Control; square, vehicle treatment; circle, haloperidol treatment. C, Voltage-dependent inactivation of A-type K⁺ currents in a DA neuron from a rat treated chronically with haloperidol. Currents were evoked by a test pulse to 20 mV after conditioning prepulses at various voltage levels (from -140 to 0 mV; 500 msec) in the presence of 50 mm TEA. D, Steady state activation and inactivation curves from DA neurons of rats treated chronically with vehicle and haloperidol. For activation curves, the peak amplitudes were measured from transient K⁺ currents generated by the same protocols described in Figure 5. Membrane conductances (g) at different voltage levels were obtained by dividing the peak transient K⁺ currents by the current driving force and normalized to conductance (g_max) at 80 mV. The mean data from each group were fitted with Boltzmann functions, which yielded half-activation potentials of control (triangle, 24.8 ± 1.3 mV; n = 7), vehicle (square, 23.8 ± 2.2 mV; n = 7), and haloperidol treatment (circle, 23.6 ± 1.1 mV; n = 8). For inactivation curves, the peak current amplitudes (I) obtained from experiment C were normalized to current amplitude with a -140 mV conditioning prepulse (I_max). The data from each group were described by Boltzmann functions with a half-inactivation potential of control (triangle, -78.7 ± 1.4 mV; n = 5), vehicle (square, -78.0 ± 1.1 mV; n = 7), and haloperidol treatment (circle, -77.2 ± 1.7 mV; n = 7). E, Recovery from inactivation of A-type K⁺ currents in a DA neuron from a haloperidol-treated animal. Currents were recorded with a double-pulse protocol with various interpulse intervals at -90 mV in the presence of 50 mm TEA. The test pulses were to 20 mV. F, The mean recovery from inactivation in DA neurons of rats treated chronically with vehicle and haloperidol. The data from each group were fitted with a single exponential function with a time constant of control (triangle, 30.3 msec; n = 5), vehicle (square, 32.2 msec; n = 7), and haloperidol treatment (circle, 30.3 msec; n = 7).

Chronic haloperidol treatment slows intrinsic pacemaker activity by increasing A-type K⁺ current. A, Whole-cell current-clamp recording showing the effect of 100 nm HpTx₃ on spontaneous firing activity of DA neurons from a rat chronically treated with haloperidol. Spontaneous activity was recorded after releasing the current clamp from a holding potential of -80 mV. HpTx₃ increases spike frequency and decreases first spike latency and first interspike interval in DA neurons. Broken lines indicate -80 mV. B, Summary of 100 nm HpTx₃ effects on spike frequency, first spike latency, and first interspike interval of spontaneous activity in haloperidol-treated DA neurons (chronic Hal, n = 7; +HpTx₃, n = 7; -HpTx₃, n = 5). Note that acutely reversing the haloperidol effect on A-type current with HpTx₃ normalizes pacemaker activity. ^**p < 0.01; ^*p < 0.05; NS, no significance. C, The effect of 100 nm HpTx₃ on tonic firing activity by a chronic haloperidol-treated DA neuron.