Repeated electroconvulsive stimulation, but not antidepressant drugs, induces mossy fibre sprouting in the rat hippocampus

Electroconvulsive stimulation (ECS) has been shown recently to induce axonal sprouting of granule cells in the rodent hippocampus. This may relate to the clinical efficacy of electroconvulsive therapy (ECT) in humans. We compared the effects of three different clinically effective antidepressant treatments on mossy fibre sprouting in the rat dentate gyrus using Timm’s histochemistry: (1) repeated spaced ECS; (2) daily administration for 4 weeks of the serotonin re-uptake inhibitor fluoxetine (1 mg/kg); and (3) daily administration for 4 weeks of the noradrenaline re-uptake inhibitor desipramine (5 mg/kg). The effect of subconvulsive electrical stimulation was also examined. Repeated ECS-induced sprouting while subconvulsive stimulation (which is ineffective clinically) did not. The two well-established chemical antidepressant therapies were also ineffective, indicating that induction of mossy fibre sprouting is not a common property of effective antidepressant agents. It is possible that the ability to induce sprouting might relate to the superior efficacy of ECT when compared to chemical antidepressants in clinical practice. Alternatively, it may contribute to the transient cognitive impairment that accompanies ECS in humans and other species.