Vagal nerve stimulation rapidly activates brain-derived neurotrophic factor receptor TrkB in rat brain

Abstract

Background: Vagal nerve stimulation (VNS) has been approved for treatment-resistant depression. Many antidepressants increase expression of brain-derived neurotrophic factor (BDNF) in brain or activate, via phosphorylation, its receptor, TrkB. There have been no studies yet of whether VNS would also cause phosphorylation of TrkB.

Methods: Western blot analysis was used to evaluate the phosphorylation status of TrkB in the hippocampus of rats administered VNS either acutely or chronically. Acute effects of VNS were compared with those caused by fluoxetine or desipramine (DMI) whereas its chronic effects were compared with those of sertraline or DMI.

Results: All treatments, given either acutely or chronically, significantly elevated phosphorylation of tyrosines 705 and 816 on TrkB in the hippocampus. However, only VNS increased the phosphorylation of tyrosine 515, with both acute and chronic administration causing this effect. Pretreatment with K252a, a nonspecific tyrosine kinase inhibitor, blocked the phosphorylation caused by acute VNS at all three tyrosines. Downstream effectors of Y515, namely Akt and ERK, were also phosphorylated after acute treatment with VNS, whereas DMI did not cause this effect.

Conclusion: VNS rapidly activates TrkB phosphorylation and this effect persists over time. VNS-induced phosphorylation of tyrosine 515 is distinct from the effect of standard antidepressant drugs.

Figure 1. Vagal nerve stimulation (VNS) rapidly induces TrkB activation in rat hippocampus.

Effects of acute (2 hr) VNS, desipramine (DMI, 10 mg/kg, i.p) or fluoxetine (15 mg/kg, i.p) on TrkB phosphorylation at different tyrosine residues in rat hippocampus.Phospho-TrkB values are normalized against total-TrkB values. One-way MANOVA, Student's Newman-Keuls post-hoc test, *P<0.05, n = 4−8 per group.

Figure 2. Vagal nerve stimulation (VNS) induces TrkB activation in rat hippocampus.

Effects of chronic (14 days) VNS, desipramine (DMI, 10 mg/kg/day, i.p) or sertraline (7.5 mg/kg/day, i.p) on TrkB phosphorylation at different tyrosine residues in rat hippocampus.Phospho-TrkB values are normalized against total-TrkB values. One-way MANOVA, Student's Newman-Keuls post-hoc test, *P<0.05, n = 6−10 per group.

Figure 3. K252a inhibits VNS-induced TrkB phosphorylation in rat hippocampus.

The ability of acute (2 hr) VNS to induce phosphorylation of TrkB was abolished by prior i.c.v administration ofK252a. Phospho-TrkB values are normalized against total-TrkB values. Two-way MANOVA, Student's Newman-Keuls post-hoc test, *P<0.05, n = 4−6 per group.

Figure 4. Effects of acute (2 h) VNS (A) or DMI administration (B) on ERK phosphorylation in rat hippocampus.

Phospho-ERK2 values are normalized against total ERK2 values. Data were analyzed by two-way ANOVA followed by Student's Newman-Keuls post-hoc test. *Significantly different from corresponding control value, P<0.05. #Significantly different from VNS value in vehicle group, P<0.05, n = 7 per group for VNS and 6 per group for DMI. The upper bands seen in the representative images correspond to pERK1 or ERK1 and were not analyzed due to poor immunoreactivity of pERK1.

Figure 5. Effects of acute (2 h) VNS (A) or DMI administration (B) on Akt phosphorylation (S473 and T308) in rat hippocampus.

Phospho-Akt values are normalized against total Akt values. Data were analyzed by two-factor MANOVA followed by Student's Newman-Keuls post-hoc test. *Significantly different from corresponding control value, P<0.05. #Significantly different from VNS value in vehicle group, P<0.001; n = 7 per group for VNS and 6 per group for DMI. No significant main effect was obtained for DMI.