Hippocampal activation of 5-HT receptors and BDNF production by vagus nerve stimulation in rats under chronic restraint stress.

A growing body of evidence shows that the electrical stimulation of the vagus nerve can improve mental illness including depression. Here, we investigated whether the vagus nerve stimulation (VNS) is involved in regulating the responsiveness of hippocampal neurons in rats under chronic restraint stress (CRS). c-Fos protein signals were detected 2 hr after VNS in 5-HT receptor-positive neurons in the dorsal raphe nucleus (DRN) as well as in the nucleus tractus solitarius (NTS). Chronic VNS was performed on a daily basis for 2 weeks using an implanted microelectrode in rats that had undergone CRS for 2 weeks. We found that the levels of both 5-HT receptors and phospho-Erk1/2 were decreased in parallel in the hippocampal neurons of CRS animals and then increased to the baseline levels by chronic VNS. Hippocampal induction of 5-HT receptors and phospho-Erk1/2 by VNS was diminished after the injection of 5,7-dihydroxytryptamine (5,7-DHT), a neurotoxin of serotonergic neurons, into the DRN. Hippocampal production of brain-derived neurotrophic factor (BDNF) was also upregulated by VNS, but the treatment of 5,7-DHT abrogated the effects of VNS on BDNF induction. VNS in CRS animals improved the behavioral scores in forced swimming test (FST) compared to sham-stimulated control. Our results suggest that VNS-mediated serotonergic input via 5-HT receptors into the hippocampal neurons may activate BDNF pathway and improve depressive-like behaviors in CRS animals.