ABSTRACT
Objective: To study the effects of capsaicin applied locally to infraorbital nerve on primary afferent nerve inadult rats. Methods:Capsaicin solution (2% ) was directly applied onto one lateral intraorbital nerve, and the contralateralnerve was used as self-control. Both experimental and control infraorbita1 nerves were observed under electron tnicroscope on3, 7, 14, 21, 28, 35 or 42 d after local administration. Substance P(SP)--like immunoreactive positive granules in both lateralnucleus caudalis spinalis nervi trigemini (CNV) were examined by immunohistochemical method. Results: Degenerated un-myelinated fibers in capsaicin-treated nerves were found under electron microscope, and no myelinated fibers changes werefound. The gray scale value of SP-like immunoreactive positive granules in the experimental side was markedly lower thanthat in the control side (P＜0. 01), while no significant difference was observed between different time groups. Conclusion:Local administration of capsaicin can produce selective destruction of unmyelinated fibers in primary afferent nerves, and canreduce the levels of SP in regions of CNV. The results suggest that local administration of capsaicin has potential therapeuticvalue for trigeminal neuralgia.