endorphins

Meaning: Endorphins ("endogenous morphine") are peptide neurotransmitters and neuromodulators released in the brain and spinal cord, and also as hormones from the pituitary gland, especially during pain, stress, or prolonged exercise. They bind opioid receptors, which reduces transmission of pain signals and produces a sense of well-being. Opioid drugs such as morphine and heroin mimic them by acting at the same receptors.

On the MCAT: Expect passages on pain perception, stress, exercise, or addiction. Typical questions ask you to predict the effect of a drug that blocks or mimics opioid receptors. An opioid receptor blocker should reduce pain relief and the pleasurable effect of an opioid, and it may blunt the relief that endorphins normally give. You may also be asked to link endorphins to the gate control idea, in which pain signals are dampened in the spinal cord and brain, or to explain the "runner's high" and why people may keep seeking an activity that triggers their release.

Common confusion: Students mix endorphins up with dopamine. Dopamine is tied to reward and motivation (the mesolimbic pathway). Endorphins are tied mainly to pain relief and euphoria, though they can influence dopamine release. Endorphins are also not the same as epinephrine, which drives arousal in the stress response.