Pain is considered both a sensation and an emotion, showing considerable complexity and subjectivity. In clinical and laboratory settings, the perception of pain bears a poor relationship to the intensity of the noxious stimulus . Therefore, strong interest exists in understanding the individual differences in response to pain and analgesics. To elucidate the genetic contributions to such individual variability in animals and humans, researchers are now employing a variety of approaches, such as microarray analysis, epigenetics and human brain imaging [13, 28, 29].
The analgesic effects of EA also show marked individual differences in acute, inflammatory and neuropathic pain rats [5, 8–11]. To identify and characterize the genes that cause these individual differences in response to EA analgesia, we previously conducted cDNA microarray analysis, using the hypothalamus, a main center of EA analgesia and the descending pain inhibitory system . Among several genes that are more abundantly expressed in the responder rats than non-responder rats, AMPK gene is the most differently expressed between the two groups. In the present study, we confirmed this with a real-time RT-PCR (Fiure 3) strongly suggesting that the expression of AMPK in the hypothalamus is closely associated with individual differences in response to EA analgesia. This study further validated the results by using viral gene transfer of AMPK into the hypothalamus (Figure 4). EA-induced analgesic effects were gradually decreased and slightly increased after injection of DN AMPK virus and WT AMPK virus, respectively, producing a significant difference between the two groups at 7 and 14 days post-injection.
The mammalian AMPK is a heterotrimer consisting of an α catalytic subunit and β and γ noncatalytic subunit . Isolation of AMPK to homogenously revealed that the catalytic subunit (α) co-purifies with two other noncatalytic subunit (β and γ). The formation of a trimeric subunit complex is necessary for an optimal AMPK activity and it is known that overexpression of wild type α subunit does not exert any positive effect on an endogenous AMPK activity [24, 26]. Consistent with these reports, there was no significant increase in EA-induced analgesic effect after WT AMPK virus injection. Conversely, the inhibition of AMPK activity by DN AMPK virus injection significantly decreased the EA analgesia (Figure 4).
AMPK is primarily regulated by cellular AMP/ATP and nutrient levels and plays a central role in the regulation of energy homeostasis and metabolic stress . It has emerged as a promising new drug target for treatment metabolic disorders, including obesity, type 2 diabetes and cardiovascular disease . Several studies also suggested that AMPK activation plays a significant role in important neuronal processes, including the regulation of neuronal plasticity and long-term potentiation, and the protection of neurons from neurodegenerative diseases . Although there has been little research on the role of AMPK in nociception, very recent studies demonstrated that AMPK activation significantly alleviates acute, inflammatory and neuropathic pain through the modulation of mammalian target of rapamycin (mTOR) and mitogen activated protein kinase (MAPK) signaling in the periphery and spinal cord that are related to pain hypersensitivity [18, 19, 32]. Our data further demonstrated that the hypothalamic AMPK play a role in mediating individual differences in response to EA-mediated analgesia. Thus, these findings not only provide a clinically useful evidence for the application of acupuncture or EA for analgesia, but also suggest an unexpected role of the hypothalamic AMPK in pain modulation.
It is currently unclear how the hypothalamic AMPK plays a role in EA-induced analgesia as shown in this study. One possible explanation is that AMPK might regulate EA analgesia-related neuropeptides that released in the hypothalamus. AMPK activation in the hypothalamus is positively correlated with neuropeptide Y (NPY) expressions  and this hypothalamic NPY has a significant antinociceptive effect . Interestingly, several reports demonstrated that acupuncture or EA stimulation at ST36 decreases NPY levels in the hypothalamus [35, 36]. Thus, we cautiously assumed that the responder rats with high AMPK levels, but not non-responders, might maintain sufficient NPY levels in the hypothalamus to be involved in antinociception, although EA stimulation decreased NPY expressions. In addition to this, further studies to explore the relationship between the AMPK and beta-endorphin in the hypothalamus, a well-known EA analgesia mediator, are required. Also, it would be interesting to examine the analgesic effects of EA on pathological pain, such as neuropathic pain , the mechanism of which is somewhat different from acute pain (e.g. TFL test). Although the individual differences in the sensitivity of acute nociceptive and chronic neuropathic pain to EA in rats were known to be maintained , we believe that studies using pathological pain models could provide a better understanding of EA-induced analgesia and its responsiveness.