Attention-deficit hyperactivity disorder (ADHD) is characterized by hyperactivity, impulsivity and inattention. Dysfunction of mesocorticolimbic dopaminergic system such in the amygdala (Amy) and prefrontal cortex (PFC) is related to emotional regulation in ADHD. Spontaneously hypertensive rat (SHR) is known as an ADHD model. We previously revealed that oral intake of monosodium L- glutamate (MSG), a taste substance for umami, for 5 weeks from postnatal day 25 (P25) to P60 altered social behavior in SHR that was grown in single isolated condition: parameters for sociality (sniffing time and riding) were significantly reduced in the MSG group in social interaction test. However, the mechanism of the effect of MSG intake during the period of development on enhanced sociality in adulthood is still unknown. We first investigate the changes of dopamine receptor expression (D1R, D2R) in the Amy and PFC using real-time PCR. Expression of D1R and D2R in the Amy was significantly increased in MSG-treated group, while that in the PFC was not changed by MSG intake. We then investigate the effect of brain-gut communication via vagus nerve on the MSG effect. The vagotomy at stomach level was performed in isolated SHR at P25 followed by MSG (0.6% solution) intake for 5 weeks. Social interaction test revealed that the vagotomy in MSG-treated group caused the increase of sniffing time and riding number to the same level as H₂O-treated control group. However, no difference was observed in open-field by the vagotomy. Data suggest that MSG effect on social behavior during the period of development was mediated by the afferent of vagus nerve from the gastrointestinal tract receptor, regulating the dopaminergic system in the Amy.