Last week, the FDA approved an inplantable vagus nerve stimulator for the treatment of refractory depression:

The device, called the Vagus Nerve Stimulation (VNS) System, was approved for adult patients with long-term or recurrent major depression that has not responded adequately to four or more antidepressant treatments.

VNS consists of a stopwatch-sized device that's surgically implanted in the upper chest. Tiny wires attach the device to the vagus nerve, which runs from the neck to the brain. The electrical stimulation is thought to alter the chemical transmitters that carry messages between nerve cells involved in regulating mood.

(Smith, "FDA Approves Implanted Device for Depression", WebMD, Jul.15)