159 Combined with neurochemical imaging (such as receptor imaging), TMS can be used to probe the role of specific neurotransmitter systems.150 Vagus nerve stimulation Vagus nerve stimulation (VNS) uses a programmable electrical stimulator to provide intermittent stimulation to a patient’s left vagus nerve. VNS was originally FDA-approved for treatment-resistant epilepsy160 and was recently approved for the adjunctive treatment of a major depressive episode that has not responded to at least

four antidepressant medication trials. However, the efficacy data on VNS are mixed. Mood improvements have been reported by epileptic patients receiving VNS,161 and one open Inhibitors,research,lifescience,medical and one double-blind study have shown antidepressant efficacy for VNS in depressed epilepsy patients.162,163 A single open study of VNS in 60 nonepileptic patients with treatment-resistant depression

found a 31% response rate and 15% remission rate after 10 weeks164; response and remission Inhibitors,research,lifescience,medical were generally maintained after at least 1 year of treatment165 and showed further increases after 2 years of treatment.166 However, a large, sham-controlled study failed to show statistically significant Inhibitors,research,lifescience,medical antidepressant effects for active VNS167 after 10 weeks of treatment. After 1 year of active VNS (all sham-treated patients received active VNS after the initial 10-week evaluation period), the response rate increased to 27% and remission rate was 16%. 168 These 1-year response and remission rates were better than those in a medication management, observation-only comparison group of similarly treatment-resistant patients Inhibitors,research,lifescience,medical followed for a similar period of time (13% response and 7% remission in the observation-only group).169 Longer-term response, remission, and relapse data are not currently available for this group of patients. Generally, VNS is safe, well-tolerated, and acceptable to patients. The body of Inhibitors,research,lifescience,medical data, taken together in this very refractory patient population, was sufficient to lead to FDA to approve VNS

for the treatment of pharmacoresistant depression. The potential mechanism(s) of action of VNS are not fully understood. The central projections of the vagus nerve via the nucleus tractus solitarius first innervate multiple brain areas implicated in mood regulation, and functional brain imaging studies have confirmed that VNS alters activity of many of these cortical and subcortical regions.170 VNS may affect function of GABA,171,172 DA,173 and NE,174-177 though conflicting data have been reported.173 These neurotransmitter system effects have not been consistently associated with therapeutic response.171 Deep brain stimulation Deep brain stimulation (DBS) medical involves a small electrical stimulator implanted into a defined brain location which typically provides chronic stimulation.