Repeated electroconvulsive shock promotes the sprouting of serotonergic axons in the lesioned rat hippocampus

This study reports the effect of repeated electroconvulsive shock on the sp routing of 5-hydroxytryptamine neurons in the partly lesioned rat dorsal hi ppocampus. We have adopted a 5-hydroxytryptamine homotypic collateral sprou ting model to examine whether electroconvulsive shock administration altere d the rate of 5-hydroxytryptamine axonal reinnervation of the dorsal hippoc ampus. The 5-hydroxytryptamine innervation of hippocampus originates from t he median raphe via the cingulum bundle and the fimbria-fornix. Lesioning o f the cingulum bundle has previously been shown to cause sprouting of intac t 5-hydroxytryptamine afferents originating from the unharmed fimbria-forni x. Rats were unilaterally injected with the 5-hydroxytryptamine neurotoxin, 5,7-dihydroxytryptamine, into the right cingulum bundle and 5-hydroxytrypt amine immunoreactivity in the dorsal hippocampus was investigated 1, 3, 6 a nd 12 weeks after the injection. The lowest level of 5-hydroxytryptamine-im munoreactivity in the hippocampus was detected at three weeks after the les ion. At six weeks, 5-hydroxytryptamine immunoreactive fibres started to rea ppear, and at 12 weeks the level of 5-hydroxytryptamine immunoreactivity wa s similar to that observed on the unlesioned side. Based on this time-cours e, six weeks was chosen as the time-point to investigate the action of a co urse of repeated electroconvulsive shock administrations. Repeated electroc onvulsive shock (five shocks over 10 days) doubled the number of sprouting 5-hydroxy-tryptamine-immunoreactive fibres and significantly increased leve ls of the 5-hydroxytryptamine metabolite, 5-hydroxyindoleacetic acid. The present data provide the first direct evidence that electroconvulsive s hock enhances 5-hydroxytryptamine axon sprouting in the partly lesioned hip pocampus. This is an effect which may contribute to the therapeutic effect of electroconvulsive therapy in major depression. (C) 2000 IBRO. Published by Elsevier Science Ltd.