The cholinesterase inhibitor donepezil has antidepressant-like properties in the mouse forced swim test

Abstract

Finding new antidepressant agents is of high clinical priority given that many cases of major depressive disorder (MDD) do not respond to conventional monoaminergic antidepressants such as the selective serotonin reuptake inhibitors (SSRIs), tricyclic antidepressants, and monoamine oxidase inhibitors. Recent findings of effective fast-acting antidepressants indicate that there are biological substrates to be taken advantage of for fast relief of depression and that we may find further treatments in this category. In this vein, the cholinergic system may be a relatively overlooked target for antidepressant medications, given its major role in motivation and attention. Furthermore, the classically engaged monoaminergic neurotransmitter systems in depression treatment-serotonin, norepinephrine, and dopamine-interact directly at times with cholinergic signaling. Here we investigate in greater detail how the cholinergic system may impact depression-related behavior, by administering widely ranging doses of the cholinesterase inhibitor drug, donepezil, to C57BL/6J mice in the forced swim test. First, we confirm prior findings that this drug, which is thought to boost synaptic acetylcholine, promotes depression-like behavior at a high dose (2.0 mg/kg, i.p.). But we also find paradoxically that it has an antidepressant-like effect at lower doses (0.02 and 0.2 mg/kg). Further this antidepressant-like effect is not due to generalized hyperactivity, since we did not observe increased locomotor activity in the open field test. These data support a novel antidepressant-like role for donepezil at lower doses as part of an overall u-shaped dose-response curve. This raises the possibility that donepezil could have antidepressant properties in humans suffering from MDD.

Fig. 1. Repeated forced swims with the same donepezil groups do not show strong antidepressant-like effects.

Shown are the six FSTs from Experiment 1, where a gap of 7 or 14 days was used for shorter versus longer washout periods, respectively, between swims. In this figure and the subsequent ones, statistically significant inverted u (climbing or swimming) or u-shaped (immobility) dose-response curves are depicted, based on their p values from the randomization test. Colored text (C1, C2, C3, and C4) below each bar denotes which cohort of eight mice was given that dose of donepezil for a given FST. Error bars represent mean ± SEM. For bars: *adjusted p < 0.05 versus vehicle, **adjusted p < 0.01 versus vehicle (or adjusted p value is explicitly shown for a statistical trend). *p < 0.05 for inverted u-shape.

Fig. 2. Crossing over drug groups in repeated forced swims yields antidepressant-like effects.

Shown are the four FSTs from Experiment 2, where a gap of 7 or 14 days was used for shorter versus longer washout periods, respectively, between swims. After FST1, the vehicle and 0.02 mg/kg dose groups were switched (crossed over), as were the 0.2 and 2.0 mg/kg groups, for FST2. Such crossing over was also carried out in the subsequent FSTs, where maroon color indicates the original drug grouping, and green indicates the crossed over grouping. As shown, the maroon grouping (FST1 = FST3) was not significantly antidepressant-like to donepezil, whereas the green grouping (FST2 = FST4) was during FST2 (and also in FST4 based on the u-shaped randomization test). Error bars represent mean ± SEM. For bars: *adjusted p < 0.05 versus vehicle, **adjusted p < 0.01 versus vehicle (or adjusted p value is explicitly shown for a statistical trend). *p < 0.05 for u-shape, **p < 0.01 for inverted u-shape.

Fig. 3. Extending the effects from Experiment 2 in a new cohort yields robust antidepressant-like effects.

Shown are the results from Experiment 3. A gap of 7 or 17 days (slightly different from Experiment 2) was used for shorter versus longer washout periods, respectively, between FSTs. Maroon (FST1 = FST3 = FST6) indicates the original drug grouping, and green (FST2 = FST4 = FST5) indicates the crossed over grouping. Error bars represent mean ± SEM. For bars: *adjusted p < 0.05 versus vehicle (or adjusted p value is explicitly shown for a statistical trend). *p < 0.05 for u-shape or inverted u-shape, **p < 0.01 for u-shape or inverted u-shape.

Fig. 4. Prior exposure to donepezil is not necessary for its antidepressant-like effects observed in later swims.

Shown are the results from Experiment 4, consisting of a new cohort of mice that was given vehicle only injections for FST1, followed 7 days later by donepezil treatment (FST2). Immobility shown here was qualitatively u-shaped, and swimming behavior showed antidepressant-like effects in FST2. Error bars represent mean ± SEM. For bars: *adjusted p < 0.05 versus vehicle, **adjusted p < 0.01 versus vehicle.

Fig. 5. Antidepressant-like effects of low dose donepezil are not due to generalized hyperactivity.

Shown are the results from Experiment 5, consisting of a new cohort of mice that was given the open field test (OFT) twice, followed by two FSTs. A gap of 7 or 14 days was used for shorter versus longer washout periods, respectively, between OFTs or FSTs. Crossing over of drug groups was the same as in Experiments 2 and 3 (maroon grouping: OFT1 = FST1; green grouping: OFT2 = FST2). Error bars represent mean ± SEM. For bars: **adjusted p < 0.01 versus vehicle (or adjusted p value is explicitly shown for a statistical trend). **p < 0.01 for inverted u-shape.