Acupuncture inhibition on neuronal activity of spinal dorsal horn induced by noxious colorectal distention in rat

Abstract

Aim: To observe how acupuncture stimulation influences the visceral nociception in rat and to clarify the interactions between acupuncture or somatic input and visceral nociceptive inputs in the spinal dorsal horn. These will provide scientific base for illustrating the mechanism of acupuncture on visceral pain.

Methods: Experiments were performed on Sprague-Dawley rats and the visceral nociceptive stimulus was generated by colorectal distention (CRD). Unit discharges from individual single neuron were recorded extracellularly with glass-microelectrode in L(1-3) spinal dorsal horn. Acupuncture stimulation was applied at contralateral heterotopic acupoint and ipsilateral homotopic acupoint, both of which were innervated by the same segments that innervate also the colorectal-gut.

Results: The visceral nociception could be inhibited at the spinal level by the heterotopic somatic mechanical stimulation and acupuncture. The maximal inhibition was induced by acupuncture or the somatic noxious stimulation at spinal dorsal horn level with inhibiting rate of 68.61% and 60.79%, respectively (P<0.01 and <0.001). In reversible spinalized rats (cervical-thoracic cold block) both spontaneous activity and responses to CRD increased significantly in 16/20 units examined, indicating the existence of tonic descending inhibition. The inhibition of acupuncture on the noxious CRD disappeared totally in the reversible spinalized rats (P<0.001).

Conclusion: The inputs of noxious CRD and acupuncture may interact at the spinal level. The nociceptive visceral inputs could be inhibited by acupuncture applied to hetero-topic acupoint. The effect indicates that the spinal dorsal horn plays a significant role in mediating the inhibition of acupuncture and somatic stimulation on the neuronal response to the noxious visceral stimulation and the inhibition is modulated by upper cervical cord and/or supra-spinal center.

Figure 1

Skin receptive fields (A) and histological localization of the spinal dorsal horn neurons (B).

Figure 2

Convergent neuron responses to CRD ranged from 20 to 100 mmHg in a monotonic and accelerating fashion.

Figure 3

Effect of acupuncture and noxious stimuli on the responses of convergent neuron to CRD. Whole recording time covers 60 s, of which first 5 s (0-5 s) is the duration for recording spontaneous background (bg) activity; from 5^th to 55^th s the CRD was consecutively administered, during 15^th to 45^th s the stimulations of acupuncture, pinch or hotwater was given (CRD + testing); from 55^th to 60^th s was for recording only the post-stimuli bg activity. Inhibitory effects induced by the stimuli of pinch in hind paw (A), 48 °C-hotwater on tail (B) and acupuncture at contralateral St-36 acupoint (C) in non-receptive fields (NRFs), and facilitated effect produced by acupuncture at ipsilateral St-36 acupoint (D) in receptive field (RF). The top traces in each section are original computer-captured spike records, and the bottom traces are the corresponding peri-stimulus time histograms of the discharges.

Figure 4

Statistical graph showing the effects of acupuncture and pinch on the responses of convergent neurons to CRD. Control CRD: CRD induced the neuron’s responses; CRD+Pinch NRF: effect of pinch in non-receptive field on the responses of the neurons to CRD; CRD+Pinch RF: effect of pinch in the receptive field; CRD+Acup. NRF: effect of acupuncture in non-receptive field; CRD+Acup. RF: effect of acupuncture in receptive field. ^bP<0.01 and ^dP<0.001 vs Control CRD respectively.

Figure 5

Effects of acupuncture on the responses of the convergent neurons to CRD before and after reversible spinalization. All the experimental procedure in this figure is the same as those in Figure 3.

Figure 6

Statistical graph showing the effects of acupuncture on the responses of convergent neurons to CRD in normal and spinalized rats. Open columns represent the CRD induced the neuron’s responses; hatched columns exhibit the effects of acupuncture at the acupoint of non-receptive field on CRD-induced responses before and after reversible spinalization. ^bP<0.001 vs CRD-induced response and acupuncture effect of post-spinalization compared with that of normal animals respectively.