Gate neurons inhibit pain using substances called -epinephrine
-serotonin
-nucleopeptides
-endorphins

Endorphins.

Explanation:

Endorphins are a substance produced by the pituitary gland, whose function is to stabilize the feelings of well-being of the body when performing certain activities, such as those that include physical effort, or even when experiencing pain sensation, where endorphins work as analgesics that slow down the sensation of pain.

the answer is not negative reinforcement.