The serotonergic modulation of synaptic plasticity in the rat hippocampo-medial prefrontal cortex pathway

The ability of the serotonergic (5-HTergic) system to affect the hippocampo-medial prefrontal cortex (mPFC) synaptic properties was examined in rats with lesions of 5-HTergic neurons. Intracerebroventricular injections of 5,7-dihydroxytryptamine (5,7-DHT) resulted in selective depletion of 5-HT and 5-hydroxyindoleacetic acid in the cerebral cortex, hippocampus and raphe regions. The 5,7-DHT-lesioned rat had no changes in basal synaptic transmission in the hippocampo-mPFC pathway. Conversely, we observed the augmentation of short-term synaptic plasticity, i.e. paired-pulse facilitation, when compared with sham-operated rats in this pathway. The magnitude of long-term potentiation (LTP) was significantly augmented in 5,7-DHT-lesioned rats. This augmentation of hippocampo-mPFC LTP had a significant correlation with the degree of cortical 5-HT levels. These results suggest that the 5-HTergic system may modulate plastic properties at the hippocampal-mPFC synapses in vivo.