Repeated ECS and fluoxetine administration have equivalent effects on hippocampal synaptic plasticity

Rationale: Recent studies have implicated intracellular transduction pathwa ys and neurotrophic factors in the action of antidepressants. Adaptation in these pathways may ultimately affect electrophysiological and morphologica l properties of neurones. We have previously shown that repeated electrocon vulsive stimulation, a safe and effective antidepressant treatment, has pro found effects on hippocampal synaptic connectivity and plasticity in the ra t. Here, we investigated whether these electrophysiological properties were shared by the chemical antidepressant, fluoxetine. Objectives: To compare the electrophysiological and cognitive effects of two very different antide pressant treatments: repeated electroconvulsive stimulation (rECS); and chr onic administration of the serotonin specific re-uptake inhibitor (SSRI), f luoxetine. Methods: Rats were exposed to either rECS or daily fluoxetine ad ministration for 15 days. The animals were then anaesthetised and dentate f ield excitatory post-synaptic potential (fEPSP) characteristics were measur ed before and after the induction of long-term potentiation (LTP) by high f requency perforant path stimulation. In a separate experiment, the effects of rECS and chronic fluoxetine administration on acquisition and retention of a spatial learning task in the Morris watermaze were determined. Results : Chronic fluoxetine administration and rECS produced equivalent increases in dentate fEPSP compared to respective control groups. LTP induction was a ttenuated in both groups. Spatial learning was, in contrast, unaffected by fluoxetine treatment but significantly impaired following rECS. Conclusions : Given that fluoxetine and rECS share antidepressant properties, but diffe r in their effects on learning and memory, we propose that the common effec ts on dentate connectivity and synaptic plasticity described here are more likely to relate to affective rather than cognitive function. This result i s consistent with other experiments showing that a reduction in dentate con nectivity correlates with stress susceptibility in animals.