Research by the Harvard Medical School and reported in the November, 2008 issue of Behavioral Brain Research found significant evidence that endogenous opioids are central to the experience of pain and acupuncture analgesia.
Acupuncture analgesia (AA) is defined as acupuncture used to relieve pain and regulate the physiologic status of the body.  Using an integrative imaging approach during acupuncture point stimulation, scientists found functional magnetic resonance imaging signal changes in the orbitofrontal cortex, insula, and pons and diprenorphine PET signal changes in the orbitofrontal cortex, medial prefrontal cortex, insula, thalamus, and anterior cingulated cortex. 

According to a recent joint MIT-Harvard Medical School clinical study, published in the November 2008 issue of the peer-reviewed science journal Behavioural Brain Research, utilized functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) to examine the effects of acupuncture in relieving pain.

The effect of manual acupuncture in 12 healthy "acupuncture-naive" subjects (6 male, 6 female) was observed by monitoring fMRI of the brain and [11C]diprenorphine PET. [11C]Diprenorphine is used with PET to measure endogenous opioid release. Endogenous opioids have a morphine-like action in the body. Currently, "…there is strong evidence that acupuncture analgesia is mediated at least in part by opioid systems" (Dougherty, et. al. p.1).

By comparing the two treatments, the study concluded that "… the reduction in pre- and post-treatment pain ratings was significantly greater in the acupuncture group when compared to the placebo group" (Dougherty, et. al. p.3).

As reported in August 2009 issue of NeuroImage, researchers at the University of Michigan's Chronic Pain and Fatigue Research Center used a positron emission tomography (PET) scanner to view the brains of 20 women diagnosed with fibromyalgia who reported suffering nerve and muscle pain at least 50% of the time. The PET scans were conducted during each woman's first acupuncture session and, a month later, her eighth. 

In the regions of the brain that process and dampen pain signals -- the amygdala, caudate, cingula, thalamus and insula -- the PET scans showed an increase in the receptivity -- and possibly the number -- of brain cells to which opioid substances bind. Study author Richard E. Harris said that suggests that acupuncture appears to make the body more responsive to opioid painkillers.