TASK-3 as a potential antidepressant target

Abstract

Modulation of TASK-3 (Kcnk9) potassium channels affect neurotransmitter release in thalamocortical centers and other sleep-related nuclei having the capacity to regulate arousal cycles and REM sleep changes associated with mood disorders and antidepressant action. Circumstantial evidence from this and previous studies suggest the potential for TASK-3 to be a novel antidepressant therapeutic target; TASK-3 knock-out mice display augmented circadian amplitude and exhibit sleep architecture characterized by suppressed REM activity. Detailed analysis of locomotor activity indicates that the amplitudes of activity bout duration and bout number are augmented in TASK-3 mutants well beyond that seen in wildtypes, findings substantiated by amplitude increases in body temperature and EEG recordings of sleep stage bouts. Polysomnographic analysis of TASK-3 mutants reveals increases in nocturnal active wake and suppressed REM sleep time while increased slow wave sleep typifies the inactive phase, findings that have implications for the cognitive impact of reduced TASK-3 activity. In direct measures of their resistance to despair behavior, TASK-3 knock-outs displayed significant decreases in immobility relative to wildtype controls in both tail suspension and forced swim tests. Treatment of wildtype animals with the antidepressant Fluoxetine markedly reduced REM sleep, while leaving active wake and slow wave sleep relatively intact. Remarkably, these effects were absent in TASK-3 mutants indicating that TASK-3 is either directly involved in the mechanism of this drug's action, or participates in parallel pathways that achieve the same effect. Together, these results support the TASK-3 channel to act as a therapeutic target for antidepressant action.

Figure 1

TASK-3 knock-outs display enhanced amplitude in diurnal locomotor activity and temperature. A. Locomotor assessment by infrared beam break of wildtype (filled symbols, n=8) and TASK-3 knock-outs (open symbols, n=8) averaged from 5 days of continuous monitoring under home cage conditions. Average beam breaks as well as the number and duration of activity bouts (± SEM) from 30 minute intervals are plotted over a 24 hour time course (traces, left). Activity bouts are defined as consecutive 30 second scoring epochs in which 5 beams were broken. Lights on occurred at 14:00 and lights off at 02:00 as indicated by open and dark filled horizontal bars above graphs. TASK-3 knock-outs differed from wildtypes in both total beams broken and activity bout duration (2-way ANOVA: F_1,3666=162.8, P <0.0001, F_1,3666 = 235.28, P <0.0001, respectively), but not in number of beams broken (F_1,3666=0.05, P=0.8267). Average beam breaks/30 minute interval are quantified for total 24 hour period, inactive (light) phase only, or active (dark) phase only (bars, right). *, ***, p < 0.05, 0.001, respectively (unpaired t test for repeated measures). B. Subcutaneous temperature of wildtype (filled symbols, n=7) and TASK-3 knock-outs (open symbols, n=8) monitored continuously over 5 days of baseline home cage conditions. Wildtype and mutant animals exhibited significant differences in their time course of temperature variation (traces, left) over 24 hours (F_1,3358=37.04, P < 0.0001) and during the active phase (F_1,1606=3.04, P < 0.001), but not during the inactive phase (F_1,1679=0.71, P<0.8379). Average temperatures also quantified for the total 24 hour period, inactive and active phases (bars, right). *, **, ***, p < 0.05, 0.01, 0.001, respectively (unpaired t test for repeated measures).

Figure 2

TASK-3 knock-outs show increased amplitude in sleep EEG architecture. Baseline ECoG/EMG of Wildtype (closed symbols, n=5) and TASK-3 homozygous mutant (open symbols, n=4) mice was monitored continuously over 16 days, and scored for both mean time spent in Active Wake, Delta and REM sleep (upper panel) as well as the number of entries into these sleep stages during 30 minute intervals. Each value is the 16 day mean ± SEM plotted on a single 24 hour period. Time points exhibiting significant differences from wildtype are highlighted by grey lines with short, medium and long tics marks representing p values ≤ 0.05, 0.01, ≤ 0.001 (linear mixed effects model for repeated measures).

Figure 3

TASK-3 knock-outs display changes in cognitive behavior. A. Novel object recognition assessment of wildtype (n=10) and TASK-3 mutants (n=12). Recognition index = (time spent exploring a novel object)/(total time spent exploring both novel and familiar objects). *, p < 0.05 (unpaired, two tailed t test) relative to wildtype; #, ##, p < 0.05, 0.01 (paired t test) relative to familiar object condition. B. Y-maze spontaneous alternation of wildtype (n=13) and TASK-3 mutants (n=13) during the Inactive phase (left panels) and Active phase (right panels). Upper panels show overall % correct alternation trios during the 7.5 minute assessment. Bottom panels plot the correct alternation % relative to arm entries during the 7.5 minute trial. Note that TASK-3 knock-outs exhibit an increased number of arm entries in the 7.5 minute assessment period. *, **, p < 0.05, 0.01 (unpaired t test) relative to wildtype condition.

Figure 4

TASK-3 mutants exhibit resistance to despair behavior. A. Tail suspension test assessment of wildtype (n=10) and TASK-3 knock-out (n=12) animals. Mice were suspended by their tails from motion-sensitive pressure transducers for an evaluation period of six minutes during the inactive phase. Mean time (in seconds) of immobility was calculated for each group and plotted at two minute intervals and the mean immobility for the total experiment and for the last 4 minutes of the experiment (right) (*, **, p < 0.05, 0.01, unpaired t test). B. Forced swim test evaluation of wildtype (n=10) and TASK-3 mutants (n=12) placed into a room temperature water bath for 6 minutes and visually scored for struggling versus minimal floating behavior. Mean time (in seconds) of immobility was calculated for each group and plotted at two minute intervals. The mean immobility for the total experiment and for the last 4 minutes of the experiment (right) are also shown (*, **, p < 0.05, 0.01, unpaired t test). C. Locomotor activity following introduction of novel environment (grey arrow). Wildtype (n=8) and Task 3 knock-out (n=8) animals were subjected to a fresh cage change during which locomotor activity was assessed by infrared beam break and averaged over 30 minute intervals (left panel) or quantified over 1, 2 or 3 hours following cage change. No statistical differences were observed at 30 minute intervals (2-way ANOVA) or during 1, 2 or 3 hours following treatment (t test).

Figure 5

REM suppressing effects of the antidepressant Fluoxetine are altered in TASK-3 knock-out animals. Continuous ECoG/EMG recordings from radio telemetry implanted wildtype (n = 5) and TASK-3 mutants (n = 4) were used to evaluate the polysomnographic responses to either vehicle (saline, i.p.; closed circles) or 20 mg/kg Fluoxetine (open circles). A 5 day balanced cross-over paradigm was used to evaluate the response to daily treatment administered at 10:00 (ZT 20:00, block arrow). In each group, time points exhibiting significant differences between vehicle and Fluoxetine treatment conditions are highlighted by grey lines with short, medium and long tics marks representing p values ≤ 0.05, 0.01, ≤ 0.001 (linear mixed effects model for repeated measures).