Role of capsaicin receptors in periaqueductal gray on pain modulation in diabetic rats

Numerous studies have shown the presence of capsaicin (CAP) receptors in periaqueductal gray (PAG) area. The aim of this study was to examine the role of CAP receptors in PAG on pain modulation in diabetic neuropathic (NP) rats. This experimental study was carried out on 24 male rats in 4 groups (n=6): normal – saline; normal – capsaicine; NP - saline and NP - capsaicine. To investigate the effect of intra-PAG injection of capsaicin (10 nmol/ul) or saline on pain modulation، tail flick (TF)، hot plate and Von frey tests were used. To induce NP، a single dose of streptozotocin (55 mg/kg، i. p) was injected. In normal rats، 5 and 15 min after the injection into PAG، CAP showed an analgesic effect in TF test (P<0. 001)، but no analgesic effect in NP rats. Moreover، CAP had neither effect on pain induction in normal rats in thermal (Hot plate) and tactile (Von frey) stimulus nor on allodynia in NP rats. It seems that in NP rats، CAP injection into PAG had no effect on pain modulation.