Chronic fluoxetine treatment in vivo enhances excitatory synaptic transmission in the hippocampus.

Depression is a leading cause of mortality and morbidity. Selective serotonin reuptake inhibitors, such as fluoxetine, are the most commonly prescribed antidepressant medication. SSRIs produce their therapeutic effects by elevating extracellular concentrations of serotonin. Although this elevation occurs rapidly, there is a paradoxical delay of weeks-to-months of continuous treatment before most patients experience meaningful relief of their depressive symptoms. Here, we address the effects of chronic fluoxetine treatment and prolonged elevation of serotonin in the rat hippocampus. Previous work has shown that acute administration of fluoxetine rapidly potentiates the excitatory temporoammonic synapse onto CA1 pyramidal cells in the hippocampus via activation of serotonin 1B receptor in brain slices. In contrast to observations in brain slices, we report here that prolonged administration of fluoxetine was required to produce significant changes in temporoammonic-CA1 synaptic strength in ex vivo brain slices. Evidence of potentiation included increases in the ratio of AMPA receptor-to NMDA receptor-mediated temporoammonic-CA1 synaptic responses, occlusion of electrically evoked long-term potentiation, enhanced long-term depression, impaired anpirtoline-mediated potentiation, and impaired memory recall in the Morris water maze task. These synaptic and behavioral changes coincided with the alleviation of anhedonic behavioral state. We conclude that the effects of elevated serotonin accumulate slowly in vivo and may account for the delay to relief of depressive symptoms by selective serotonin reuptake inhibitors. Acceleration of this process should lead to faster therapeutic responses to antidepressants.