[kazinator]

Melissa O'Brien at UBC has directly observed the effects of MSG on the blood flow in the dura in rat brains:

https://open.library.ubc.ca/cIRcle/collections/ubctheses/24/... [2016]

She basically takes it for granted that MSG causes headaches and is looking at the mechanisms.

A recent study found that a single oral dose of 150mg/kg taken consecutively for five days resulted in headache and muscle tenderness when given to healthy young men (Shimada et al. 2013). Older work concluded that MSG consumption did not induce symptoms of pain or sensitivity but many of these studies have been scrutinized for their poor methodology (Tarasoff and Kelly 1993). Without definitive proof that MSG is harmful, it has been cleared in the United States and Canada as safe for human consumption and can be added to foods without regulation from Health Canada.

Lots more in this paper; very good work.