The Possible Neuronal Mechanism of Acupuncture: Morphological Evidence of the Neuronal Connection between Groin A-Shi Point and Uterus

Abstract

Somatovisceral reflex suggested that the somatic stimulation could affect visceral function like acupuncture which treats diseases by stimulating acupoints. The neuronal connection between somatic point and visceral organ was not clear. Uterine pain referred to the groin region has long been recognized clinically. Wesselmann, using neurogenic plasma extravasation method, showed that uterine pain was referred to the groin region through a neuronal mechanism (Wesselmann and Lai 1997). This connection could be considered through the somatovisceral reflex pathway. However, the relay center of this pathway is still not clearly identified. In the present study, bee venom was injected in the groin region to induce central Fos expression to map the sensory innervation of groin region. Pseudorabies virus (PrV), a transneuronal tracer, was injected in the uterus to identify the higher motor control of the uterus. Immunohistochemistry staining revealed the Fos expression and PrV-infected double-labeled neurons in the nucleus of solitary tract (NTS), the dorsal motor nucleus of vagus (DMX), and the paraventricular hypothalamic nucleus (PVN). These results suggest a somatoparasympathetic neuronal connection (groin-spinal dorsal horn-NTS/DMX-uterus) and a somatosympathetic neuronal connection (groin-spinal dorsal horn-NTS-PVN-uterus). These two neuronal connections could be the prerequisites to the neuronal basis of the somatovisceral reflex and also the neuronal mechanism of acupuncture.

Figure 1

Fos expression neurons in the spinal cord after bee venom injection in the left groin region (n = 9). (a) Neurons express Fos protein (arrow) in ipsilateral L2 spinal dorsal horn. (b) Higher magnification of Fos expression neurons in (a) (scale bar: 100 um). (c) Mean number of Fos expression neurons in T10 to S3 spinal segments (±SEM). *P < 0.05. (d) Mean number of Fos expression neurons in laminas I to V of L2 spinal segment.

Figure 2

Fos expression neurons (arrow) in the supraspinal area after bee venom injection in the left groin region (n = 9). (a) Nucleus of solitary tract (NTS). (b) Locus coeruleus (LC), parabrachial nucleus (PB). (c) Raphe pallidus nucleus (RPa). (d) Paraventricular hypothalamic nucleus (PVN). (e) Lateral hypothalamic area. (f) Paraventricular thalamic nucleus (PV). 3V: third ventricle; 4V: fourth ventricle; D3V: dorsal third ventricle; d: dorsal; f: fornix; ic: internal capsule; l: lateral; m: medial; opt: optic tract; scp: superior cerebellar peduncle; v: ventral (scale bar: 100 um).

Figure 3

PrV-infected neurons in spinal cord after 6 to 8 d PrV injection in the left uterine horn (n = 8). (a) PrV-infected neuron (arrow) in T12 spinal segment. (b) Higher magnification of PrV-infected neuron in (a) (arrow) (scale bar: 100 um). (c) Mean number of PrV-infected neurons in T10 to S2 spinal segments (±SEM).

Figure 4

PrV-infected neurons (arrow) in the supraspinal area after 7-8 d PrV injection in the left uterus horn (n = 8). (a) PrV-infected neurons in the NTS and DMX. (b) PrV-infected neurons in the A5 noradrenaline cell group. (c) PrV-infected neurons in the RPa and RMg. (d) PrV-infected neurons in the Gi. (e) PrV-infected neurons in the PVN. 3V: third ventricle; AP: area postrema; DMX: motor nucleus of vagus; Gi: gigantocellular reticular nucleus; Gr: gracile nucleus; NTS: nucleus of solitary tract; PVN: paraventricular hypothalamic nucleus; RPa: raphe pallidus nucleus; RMg: raphe magnus nucleus (scale bar: 100 um).

Figure 5

Double-labeled neurons of Fos expression and PrV infection (n = 8). Fos expression neurons ((a), red, arrow) in the PVN (a1), NTS ((a2), (a3)), and DMX ((a2), (a4)). PrV-infected neurons ((b), green, arrow) in the PVN (b1), NTS ((b2), (b3)), and DMX ((b2), (b4)). Merged double labeling Fos expression and PrV-infected neurons ((c), yellow, arrow) in the PVN (c1), NTS ((c2), (c3)), and DMX ((c2), (c4)) (scale bar: 1000 um). (d) The percentage of double labeled neurons in PVN, DMX, and NTS between saline and bee venom injection groups (*P < 0.1).

Figure 6

Schematic drawing of three neuronal pathways of somatovisceral reflex. (a) Somato-parasympathetic reflex pathway through the vagus nerve. (b) Somato-sympathetic reflex pathways through the NTS and PVN. DMX: dorsal motor nucleus of vagus; DRG: dorsal root ganglia; NTS: nucleus of solitary tract; SPN: sympathetic preganglionic neurons; PVN: paraventricular hypothalamic nucleus; blue: somatic afferent; red: visceral efferent.