Figure 1

General methods, experimental design, and VNS parameters. (A) To assess the effects of different VNS intensities in extinction of severe fear, rats were submitted to the single prolonged stress (SPS) and protracted aversive conditioning (PAC). (B) In phase 1 of the experimental design, rats underwent SPS and remained undisturbed for 7 days. Fear conditioning was performed using the PAC over four days. In phase 2, rats were implanted with vagus nerve cuffs and allowed to recover for 6–7 days. In phase 3, rats were submitted to a five-day extinction training where Sham stimulation of VNS was delivered. Seven days later, all rats were tested for extinction retention in the same extinction context. (C) Rats in the Sham group received no stimulation. Rats in the VNS groups were randomly assigned to one of the following groups: during VNS (four 0.5-s bursts, at 8 s intervals and delivered with the onset of each CS trial); between VNS (four 0.5 s bursts, at 8 s interval and delivered between each CS trial); dispersed VNS (twenty 0.5 s stimulations, at 32 s interval distributed between the onset of the first trial and the end of the session); continuous VNS (eighty 0.5 s stimulations, at 8 s interval distributed between the onset of the first trial and the end of the session). All VNS rats received stimulations at 0.8 mA, 100 µs pulse width, at a frequency of 30 Hz. SPS single prolonged stress, PAC protracted aversive conditioning, CS conditioned stimulus, US unconditioned stimulus.