Juan cut his finger on a piece of glass. After a short time, he felt no pain in his finger. Juan's pain was likely reduced by a natural pain reliever in his brain. This natural pain reliever is the neurotransmitter (s) :

Answer is endorphins.

Explanation:

Endorphins are considered or known to be one of the brain chemicals, which are generally referred to as the neurotransmitters.

The endorphins carry out their activities by interacting with the opiate receptors in the brain in order to reduce pain.

Stress and pain are considered as the major factors that cause the release of the endorphins.

It should be noted that, the activity of the endorphins can be related to that of morphine.

That is why they are referred to as natural pain reliever.

They are the ones that reduce Juan's pain.