Acupuncture has been shown in clinical studies to have analgesic effects [2
]. Several neuroimaging studies showed that acupuncture modulate activities in areas of the brain that are involved in pain signal processing. However, correlation between other physiological effects induced by acupuncture and the corresponding neuroimaging changes has not been well studied. Here we used a different experimental model, the acupuncture/xerostomia model, to investigate the possible mechanisms of action of acupuncture.
The pattern of fMRI activation we observed was interesting. By "pattern", we mean the totality of activation/deactivation areas. We observed some overlap with areas involved in pain perception. Pain can elicit activation of the sensorimotor cortices, rostral anterior cingulate cortex, insula, cerebellum, hippocampus, brain stem, etc. [9
]. However, despite unilateral (on the left hand only) stimulation of subjects included in the final data analysis, we saw bilateral activation. We can not fully explain this finding by attributing it to painful stimulation alone.
The mechanisms of action of acupuncture in stimulating saliva production are unknown. It is possible that acupuncture at points in the head and neck area directly stimulates nerves innervating salivary glands. Another possibility is the placebo effect because expectation is well known to induce saliva production as in Pavlovian conditioning. A third possibility is that acupuncture acts by interacting with certain components of the neuronal network involved in salivation.
The neuronal matrix that controls salivation is not fully understood. Our current understanding indicates that it results from a complex interaction among many components of the peripheral and central nervous system, instead of a simple local reflex. Gustatory, olfactory and visual stimuli lead to activation of the insula, frontal operculum and rolandic operculum [30
]. The insula and rolandic operculum receive a direct projection from the thalamic gustatory relay through bifurcate neurons[31
]. Neuronal activity in this region is modulated by sensory input from taste receptors and lingual somatosensory receptors[30
]. Pure gustatory stimuli and somato-gustatory stimuli both activate the insular lobe, the rolandic operculum, the frontal operculum and the temporal operculum[32
]. Hypersalivation is observed during temporal lobe seizures [33
]. During intraoperative mapping of a case of ictal hypersalivation, the seizures were identified as arising from the left anterior frontal operculum. After resection of epileptogenic opercular cortex, the seizures disappeared with no additional neurological deficits[34
Our data shows that areas activated by acupuncture at LI-2 overlap those involved in gustation/salivation[30
]. But our study was descriptive in nature. We can not conclude that there is a causal relationship, i.e. such activation led to increased salivation. Nonetheless, our observations serve as a foundation for further hypothesis testing studies. A hypothesis we are proposing is illustrated in Figure . In this hypothesis, the insula and adjacent operculi are where gustatory, olfactory, visual stimuli and signals from expectation/suggestion are integrated. After the integration, signals are sent to the salivary nuclei in the pons which then go to the salivary glands via cranial nerves. Acupuncture, by activating the insula and adjacent operculi, taps into this circuit and produces the down-steam event of increased saliva production.
Figure 4 Schematic representation of the gustation-salivation neuronal circuitry and how acupuncture may interact with it. Green lines represent input neural signals, red lines output signals. Solid lines represent physiological response of salivation to gustatory (more ...)
There are several limitations in our study. They were due to the exploratory nature of the study. Our original goal was to identify areas of interest, if any, that warrant further studies. It was not to test point specificity or other hypothesis. First, we included only one type of control intervention – sham acupuncture at a non-acupuncture point. We had considered using sham acupuncture at LI-2 or real needles at a non-acupuncture point as controls. We thought that the former might generate similar, although weaker, stimulation at the same location. We would then be comparing dose responses. In the latter case, we would be comparing point specificity which was not our original goal. To establish point specificity, we would have to compare true acupuncture at different acupoints and non-acupoints in one experiment, which would be a logical next step. Secondly, we did not ask the subjects to describe sensations, especially gustatory sensations, experienced during acupuncture. If acupuncture stimulates salivation via the gustatory cortices, one would expect to evoke such sensation. We also did not asked the subjects to describe "deqi" sensation, which is thought to be important for responses to acupuncture. A correlation between "deqi" sensation and fMRI changes would be an important finding.