FosTRAP (Fos the targeted recombination in active populations) mice were a gift of Dr. Fan Wang (MIT) and used as recently reported [9 ; 10 (link)]. The FosTRAP mice were kept in single cages with at least two environmental enrichments for a minimum of 10 days prior the day of 4-Hydroxytamoxifen (4-OHT) injection. The 4-OHT (4-OHT; Sigma, MO, USA Cat# H6278) was dissolved in ethanol to create a 20mg/ml stock solution, which was then divided and frozen in −20°C. On the day of administration, the 4-OHT was added to a sunflower seed oil solution (Sigma, MO, USA Cat # S5007) and heated to 50°C to remove the ethanol, resulting in a final 4-OHT solution with a concentration of 10 mg/ml. Any unused solution was discarded at the end of the day. The mice received an intraperitoneal 4-OHT injection 30 minutes before undergoing 30 minutes of pVNS treatment or sham. Both pVNS and sham group received sevoflurane (UPS, Covetrus North America, OH, USA) anesthesia. The mice were returned to the same single-housing cage for 10 days to minimize any sources of distress before termination.