Glutamate (neurotransmitter)
In neuroscience, glutamate refers to the anion of glutamic acid in its role as a neurotransmitter: a chemical that nerve cells use to send signals to other cells. It is by a wide margin the most abundant excitatory neurotransmitter in the vertebrate nervous system. It is used by every major excitatory function in the vertebrate brain, accounting in total for well over 90% of the synaptic connections in the human brain. It also serves as the primary neurotransmitter for some localized brain regions, such as cerebellum granule cells.
Diabetic hypoglycemia
If a person has frequent episodes of hypoglycemia (even mild ones), the brain becomes "used to" the low glucose and no longer signals for epinephrine to be released during such times. More specifically, there are glucose transporters located in the brain cells (neurons). These transporters increase in number in response to repeated hypoglycemia (this permits the brain to receive a steady supply of glucose even during hypoglycemia). As a result, what was once the hypoglycemic threshold for the brain to signal epinephrine release becomes lower. Epinephrine is not released, if at all, until the blood glucose level has dropped to even lower levels. Clinically, the result is hypoglycemic unawareness.