The sympathetic neurons releases norepinephrine, also called noradrenaline, and the cells of the adrenal medulla release norepinephrine and epinephrine, also called adrenaline.
And some GPCRs in our brain control levels of chemicals like serotonin and noradrenaline, which scientists believe are key players in bipolar disorder.
As people take more opioids to compensate for tolerance, noradrenaline levels become lower and lower, to a point that could impact basic bodily functions.
This increased sensitivity to noradrenaline allows the body to continue functioning normally— in fact, it becomes dependent on opioids to maintain the new balance.