Endorphins and the Placebo Effect

Persons who receive treatments with agents that possess no pharmacological
activity for various illnesses or conditions have often been known to show
improvement. Such a reaction is called the placebo effect. Whether the placebo
effect is real has long been controversial. A 1955 study published in the
prestigious Journal of the American Medical Association was the first significant
report that the effect was real. More recent work has suggested the placebo
effect may be sometimes more myth than reality. Nevertheless, there is evidence
that such an effect may indeed occur and may be associated with
forms of neurotransmitters called endorphins (endogenous morphines)
and enkephalins. Endorphins and enkephalins represent a class of neurotransmitter-
like chemicals called neuropeptides, small molecules which
consist of between two and forty amino acids.

Enkephalins, discovered in 1975, block pain impulses within the central
nervous system in ways similar to the drug morphine. The second class of
molecules, subsequently called endorphins, was discovered soon afterward.
They appear to act through suppression of pain impulses through suppression
of a chemical called substance P. Substance P is released by neurons in
the brain, the result of pain impulses from receptors in the peripheral nervous
system. By inhibiting the release of substance P, these neuropeptides
suppress sensory pain mechanisms. In support of a physiological basis for
the placebo effect, patients treated with the endorphin antagonist naloxon
produced no discernable response to placebo treatment.
Endorphins have been shown to play a role in a wide variety of body functions,
including memory and learning and the control of sexual impulses.
Abnormal activity of endorphins has been shown to play a role in organic
psychiatric dysfunctions such as schizophrenia and depression. Deficits in
endorphin levels have been observed to correlate with aggressiveness; endorphin
replacement therapy results in the diminishment of such behavior.
Abnormal levels of endorphins in the blood have also been found in individuals
suffering from behavioral disorders such as anorexia or obesity. 569