Treatment with systemic choline has moderate antinociceptive efficacy after surgery in mice. Choline-treated animals responded as if they were tested on postoperative day 5. The decreased response to both heat and punctate stimuli is at least in part due to activity at α7-containing nicotinic acetylcholine receptors as it was significantly attenuated in α7 knockout mice.
In electrophysiological experiments, activation of α7 nicotinic receptors with choline results in a rapid inward current conducted by calcium and sodium that quickly desensitizes.11
Pilot experiments suggested that acute treatment with choline may reduce postoperative nociceptive reflexes (data not shown). However, if the effect quickly dissipated, this would be of little clinical utility. Continual exposure to the choline did not appear to dissipate over the 48 h period tested; rather the antinociceptive effect evolved over the 48 h studied. The fact that the antinociceptive effect of choline evolves more than 48 h of treatment may suggest an anti-inflammatory action or otherwise accelerated healing. One limitation of the paw incision model is that the wound that we induced was so small that it was difficult to classify erythema, oedema, and other clinical signs of wound integrity. Formal studies of wound strength and integrity would be interesting to pursue in the future.
α7 nicotinic acetylcholine receptors have been described on macrophages and microglia where they have anti-inflammatory effects through a pathway mediated by the inhibition of TNF release.17,27
The slowly developing antinociception could potentially be due to decreased wound inflammation after surgery. We have shown that TNF release is enhanced even by our relatively minor surgical procedure. Our immunocytochemistry studies show that high-dose choline prevents binding of α-bungarotoxin-488. However, macrophages from α7 knockout mice also had a small amount of residual α-bungarotoxin binding that could be due to the presence of other sensitive receptors, perhaps specific to macrophages. The original investigators that produced this line of α7 nicotinic knockout mice showed that there was no α-bungarotoxin binding in the hippocampus, but macrophages have not been studied in vitro
Other nicotinic subunits are expressed alveolar macrophages on the mRNA level including α3–α6, α9, and α10.28
Indeed, LPS stimulation causes up-regulation of both α7 and α10 nicotinic subunit expression,29
but it is not known what role activation of different receptor combinations by high-dose choline and bind α-bungarotoxin-488 might play in pain or inflammation.
Macrophage TNF release is inhibited by choline in vitro
, but only at millimolar concentrations that are not likely to be achieved in vivo
. Baseline choline concentrations in rodents (and humans) are ~10 mM and have only been reported to be elevated by 10-fold with supplementation.30
The high concentrations of choline that we and others have used in vitro
may have a non-specific action or action at another type of nicotinic receptor expressed by macrophages from α7 knockout mice because choline reduces TNF release irrespective of genotype. The work in sepsis models has shown the importance of α7 receptors in behavioural assays, but the macrophages studied in vitro
were from the RAW macrophage-like cell line; macrophages from α7-receptor null animals have not been studied.31
Moreover, it is possible that the anti-inflammatory action of choline is not systemic, but rather local, in the wound, to prevent peripheral sensitization and perhaps improve wound integrity as suggested by the recent results of Gurun and colleagues.13
Choline loading before surgery may provide a natural analgesic adjuvant with low toxicity. Choline is currently available as a nutritional supplement in the USA and is thought to be important for pregnancy and lactation.32,33
However, as choline is not fully efficacious, evaluation of its interaction with other analgesic agents, including opioids and non-opioid analgesics, will be of interest. Wang and colleagues12
have previously suggested a synergistic anti-hyperalgesic interaction between choline and aspirin in response to a formalin challenge. Further studies will be required to determine whether choline supplementation may have anti-inflammatory activity, reduce pain, and the requirement for other analgesics after surgery in humans.