Dr. Amy Nett: The way I think it’s best to think of it is that epinephrine and norepinephrine—and that’s synonymous with adrenaline and noradrenaline. Epinephrine is adrenaline, and norepinephrine is noradrenaline. They’re synonymous. I’m just personally more familiar with using epinephrine and norepinephrine. Both epinephrine and norepinephrine can, in the broadest sense, be thought of to some extent as both hormones and neurotransmitters. Epinephrine is primarily a hormone, so epinephrine is mostly going to be made in the adrenal medulla, but to a lesser extent, it is made in some of the neurons of the sympathetic nervous system and acts on different classes of adrenergic receptors. Norepinephrine is primarily a neurotransmitter, and it’s the major neurotransmitter of the sympathetic nervous system, though it is also made to a lesser extent by some cells of the adrenal medulla. So you’re going to see both epinephrine and norepinephrine being transmitted by neurons. Again, the way nerves communicate is you have the synapsis where you’re going to have release of the neurotransmitter from basically the end of the nerve cell, and it’s going to release that neurotransmitter—so predominantly norepinephrine, but also epinephrine—so that those compounds, whether you want to call them a hormone or neurotransmitter, are going to then bind to the receptor sites.
It is a little bit tricky if you kind of get lost in the terminology, and so it might be fair just to think of them more broadly as being both hormones and neurotransmitters, where norepinephrine is primarily a neurotransmitter and epinephrine is primarily a hormone. But again, the sympathetic nervous system, those neurons are going to use epi and norepi for communication, which is what you can think of it as a role as neurotransmitters. Hopefully that clarifies that a little bit.