Richard Hunt
Division of Gastroenterology McMaster University Medical Centre, United Kingdom
Acupuncture has been used empirically in China for centuries and an extensive literature has confirmed its efficacy in anesthesia and analgesia. Studies have also suggested that acupuncture could relieve epigastric pain and even heal peptic ulcer. However, until recently, few systematic studies have been undertaken. Studies have suggested that acupuncture, in conscious dogs, inhibits basal acid output, acid secretion stimulated by sham-feeding, histamine and pentagastrin. Moreover, it also stimulates the non-parietal secretion of bicarbonate and sodium.
In six randomized, placebo controlled studies we investigated the effect of electro-acupuncture at Zusanli on gastric acid secretion in 38 healthy males. Electro-acupuncture reduced significantly basal acid output, sham feeding stimulated acid output, but had no effect on acid output in response to pentagastrin. The inhibitory effect of electro-acupuncture on sham feeding acid output was not affected by local anesthesia at the acupoint but was prevented by prior intravenous injection of naloxone. Plasma gastrin levels were not altered by acupuncture but were increased by the naloxone infusion. We concluded that the antisecretory effects of electroacupuncture are mediated through naloxone-sensitive opioid neural and vagal efferent pathways. These observations, and those of others, emphasize the importance of understanding better the gut-brain-gut axis, and have led to our studying this in much greater detail.
Visceral sensations are transmitted either via afferent nervous fibers in the vagus or through spinal afferent pathways with cell bodies in the dorsal root ganglia, which project to the dorsal horn of the spinal cord. From the dorsal horn, higher order neurons transmit to the brainstem and thalamus, with subsequent impulses eventually reaching the cerebral cortex where conscious perception occurs. However, there are no good objective methods for evaluating the afferent and efferent circuits of the nervous system and studies of the gut-brain-gut communications and connections are in their infancy. However, several new and sophisticated methods are becoming increasingly available. A variety of techniques include study of cortical evoked potentials following electrical stimulus or balloon distention from within the gut. The evoked potentials have a characteristic pattern and the velocity of the signal can be measured. Three-dimensional EEG recordings and dipole techniques permit accurate localization of the signal in the brain and this can be augmented by the use of PET scanning with 2-fluoro-de-oxy-glucose.
Recent work from our group has shown that balloon inflation or electrical stimulation in the esophagus is associated with predictable and reproducible cerebral evoked responses, a finding subsequently confirmed also by others. The velocity of the afferent impulse differs according to transmission either via slow C fibers with balloon distention, or faster, myelinated A delta fibers in response to electrical stimulation.
It has proved more difficult to determine the effect of any of these changes on gastric or intestinal function, probably because the endpoints of gastric acid secretion or gastrointestinal motility are relatively insensitive measures. We have therefore now established an alternative measure to determine the effect of visceral afferent sensory stimuli originating from the gastrointestinal tract on the autonomic efferent output elicited by the brain in response to the sensory stimulus. We have now established power spectral analysis of the beat to beat heart rate variability to determine the impact of visceral sensory stimuli on efferent autonomic outflow to the heart. Balloon or electrical stimulation from the esophagus produces marked and opposing alterations in the respective magnitude of the low and high frequency peaks present in the power spectral distribution of heart rate variability, reflecting a simultaneous decrease in sympathetic cardiac outflow and increase in the parasympathetic (vagal) cardiac tone.
In patients with non cardiac chest pain (NCCP), we have shown that those patients who have a high resting sympathetic tone are much more likely (probability > 95%) to develop symptoms of chest pain in response to esophageal acid perfusion, than in those in whom the resting vagal peak is dominant.
These techniques and other similar approaches now offer the opportunity to study functional disorders of the gastrointestinal tract in a more objective manner than has previously been possible. Indeed, our findings indicate that a spectrum of afferent stimuli arising from the gut or other sensory apparatus may alter efferent autonomic function and modify the relative influence of the sympathetic and parasympathetic nervous systems on visceral organs.
Conceptually, this offers an intriguing opportunity to hypothesize on the origins for a variety of functional gastrointestinal disorders as well as other conditions such as labile hypertension, non-allergic bronchospasm, fibromyalgia, fibromyosytis, etc. Moreover, we believe that these techniques offer the opportunity to explore in an objective scientific way some of the effects of acupuncture and, perhaps, to explain the mechanisms involved in our own studies.