Thank you for your reply, mrsD.

The stress intolerance I was talking about is related to mTBI/TBI, not ADHD.

I do know that the GABA system dysfunctions in ADHD, although the major problem is the Dopamine metabolism - Dopamine is metabolised too fast and many of it cannot reach the receptors and activate them because of this problem.

As you have said, the GABA system doesn't work well enough in ADHD and this creates a problem with over-excitation, but it's not only GABA. Other inhibitory systems, such as the endocannabinoid system and the opioid system are also thought to be somewhat related to poor inhibition seen in ADHD patients.

The opioid and endocannabinoid systems are similar in their roles, and they are both integral parts of the reward system, together with GABA (dopamine/norepinephrine are related to "wanting" - they are important for reward prediction and motivation but are not part of the reward system). Unfortunatenly, both systems, motivation and reward show some dysfunctioning in ADHD patients (although the low dopamine is the biggest problem of ADHD).

It's important to notice that stress hormones/neurotransmitters such as Dopamine and Norepinephrine are important to self control, even though they are stimulating (too high or too low levels of dopamine will cause hyperactivity, for example, and normal levels are needed for self control).

But back to the main point - I have suffered stress tolerance problems since my mTBI and posted this here since I wanted to know what parts of the brain are exactly responsible for low stress tolerance after suffering a concussion. Also, I think that everyone suffers a cognitive decline in higher stress levels - it's just that it's more severe in PCS patients than in most people.