Vagus nerve stimulation increases neurotrophins gene expression and alters cell proliferation in the rat hippocampus

Vagus nerve stimulation (VNS) is effective in patients with treatment-resistant epilepsy. More recently, VNS has been approved for treatment-resistant depression; nevertheless, the molecular mechanism(s) underlying its therapeutic action remains unclear. The observation that VNS up-regulates cortical GABA receptor in partial epilepsy suggests that VNS can affect brain plasticity. Neuronal plasticity is the result of a number of molecular and neurochenical events, and it is commonly accepted that in the adultbrain synaptic rearrangements and neurogenesis can occur. Thus, physiologicalstinmli mediate synaptic activity that in turn can be regulated by both neurotransmitters and neurotrophic factors, suggesting that neurotrophins participate to morphological and functional changes associated to neuronal plasticity. In light of the anticonvulsant properties of VNS, its modulation of neurochemical systems implicated in major depression and impact on neuronal functional activity and plasticity, we tested the possibility that VNS could promote the synthesis of brain derived neurotrophic factor (BDNF), basic fibroblast growth factor (bFGF) and neuronal growth factor (NGF) that promote survival, maintenance e proliferation of neuronal cells. Moreover, we tested the possibility that VNS could interfere with neurogenesisin the dentate gyms of rat hippocampus. Sprague-Dawley CD rats were implanted with a couple of electrodes aimed at the left vagus nerve, or were sham operated for control. The experiments started three days after surgery, and in treated animals, electrodes were connected to a battery that was activated at the "antiepileptic" parameters (2mA, 30 s on, 5rain off at 30 Hz delivery current, for 3 hours). For RNA extraction animals were sacrificed and the hippocampus was removed. RNA was extracted from whole hippocampus, and quantified by measurement of absorbance at 260 nm. An RNase protection assay was performed to measure the abundance of BDNF, bFGF, NGF and cyclophilin mRNAs. For inmlunohistochemistry and neurogenesis experiments, rats were injected with bromodeoxyuridine (BrdU) prior stimulation. Three hours after VNS animals were euthanized and tissues fixed by intracardiac perfusion with 4% paraformaldehyde, brains were then prepared for histology by using standard procedures. The results of this study show that 3h VNS increases the abundance of BDNF and bFGF mRNAs in the hippocampus by 26 and 23% respectively, and do not significantly alters the abundance of NGF mRNA compared to control animals. Moreover, VNS decreased by 27% the number of BrdU positive cells in the hippocampal dentate gyms. Interestingly, in the granule cells layer of VNSrats several cell positive to BrdU resulted to be neurons as demonstrated by the double labeling with an antibody specific for the nuclear neuronal proteinNeuN. Our results suggest that VNS could trigger neuronal plasticity and demonstrate that such stimulation induced an increase in the gene expression of BDNF and bFGF in the rat hippocampus. These increases in growth factors were associated with adismption of cell proliferation and neurogenesis process in the granule celllayer of the dentate gyms. These new findings contribute to elucidate the molecular mechanisms underlying the action of VNS, a new therapeutictool for the treatment of epilepsy and depression. References [1] Marrosu, E, Santoni, E, Puligheddu, M., Barberini, L., Maleci, A., Ennas, E, Mascia, M., Zanetti, G., Tuveri, A., and Biggio G., 2005. Increase in 20-50 Hz (gamma frequencies) power spectrum and synchronization after chronic vagal nerve stimulation. Clin Neurophysiol 116, 2026-36. [2] Marrosu, E, Serra, A., Maleci, A., Pulicheddu, M., Biggio, G., and Piga, M., 2003. Correlation between GABAA receptor density and vagus nerve stimulation in individuals with drug-resistant partial epilepsy. Epilepsy Res 55, 59-70. [3] Palma, E., Torchia, G., Limatola, C., Trettel, A., Arcella, A., Cantore, G., Di Gennaro, G., Man- fredi, M., Esposito, V., Quarato, RR, Miledi, R., and Eusebi, E, 2005. BDNF modulates GABAA receptors microtransplanted from the human epileptic brain to Xenopus oocites. Proc Natl Acad Sci USA 102, 1667-72.