Facial Chronic Inflammatory Pain Induces The Phenotypic Switch Of Neurons Expressing P2X3 In Trigeminal Ganglion Of Rat

In the present study, we tried to explore whether peripheral chronic inflammation could lead to the phenotypic change of neurons expressing P2X3 in trigeminal ganglion of rat. The model of facial chronic inflammatory pain was established by hypodermic injection of turpentine oil according to the method of Neumann (1996). The pain threshold of facial skin was measured once a day within 15 days using thermal measurement apparatus of pain. The immunohistochemical staining for P2X3, one subtype of P2X receptor was carried out in the trigeminal ganglions ipsilateral and contralateral to the inflammation 5 days after the facial injection of turpentine oil. The size of the profiles of neurons expressing P2X3 was measured by an Image Analysis System. The changes reached the lowest value at days 5 and recovered to the normal level at days 13 after inflammation. The average size of the profiles of trigeminal ganglion cells expressing P2X3 was 721±12μm2 in the inflammatory group, and significantly increased compared to that in the control group (616±8μm2) (P<0.01). Furthermore, it was found that the area size of the small-sized ganglion cells'profile expressing P2X3 (<950μm2) increased compared with that in the control (582±15μm2 vs 537±13μm2, P<0.05) and the percentage of the small-sized cells expressing P2X3 to the total cells increased also (42.2±3.2% vs 51.8±3.5%, P<0.05), while the percentage of the large-sized of P2X3 immunoreactive positive cells (>950μm2) increased from 6.5±1.9% to 12.8±2.2% (P<0.05) after inflammation. These results indicate that facial chronic inflammatory pain induces the phenotypic switch of neurons expressing P2X3 in trigeminal ganglion of rat, which may play an important role in contributing to hyperalgesia and allodynia in relation to facial chronic pain. Trigeminal ganglion (TG) neurons innervating endodontium are definitely pain sensitive and generally accepted as an ideal model for pain study. In this study we established a method of tracing the cell bodies of nociceptive TG neurons using HRP injected into endodontium. After HRP injected into endodontium was axotransported antidromically to the nociceptive TG neurons somata, the morphological property was analyzed.The area size of the traced nociceptive cells was calculated by photo analyze system. The results showed that the cells of trigeminal ganglion traced by HRP are mainly small-sized cells range from 100-350μm2. This demonstrated that the cells of primary sensory neurons sensing toothache are mainly smaller-sized cells among the small-sized cells. It is helpful for us to study and analyze the properties of the nociceptor.