Effect of tramadol on expression of 5-HT1A receptor in distal cerebrospinal fluid contacting neurons in mid-brain in a rat model of neuropatlic pain / 中华麻醉学杂志

ABSTRACT

Objective To investigate the effect of tramadol on the expression of 5-HT1A receptor in the distal'cerebrospinal fluid contacting neurons (CSF-CNs) in mid-brain in a rat model of neuropathic pain. Methods Forty male SPF SD rats weighing 220-280 g were randomly divided into 5 groups (n = 8 each) group Ⅰ normal control (group C); group Ⅱ normal saline (group NS); group Ⅲ tramodol (group T); group Ⅳ neuropathic pain + normal saline (group NP+ NS) and group Ⅴ neuropathic pain + tramadol (group NP + T). Neuropathic pain was induced by chronic constrictive injury (CCI) in group Ⅳ and Ⅴ . Four silk ligatures were placed on the sciatic nerve at 1 mm intervals. In group Ⅱ (NS) and group Ⅲ (T) the sciatic nerve was exposed but not ligated and NS 2 ml/kg and tramadol 10 mg/kg were injected IP respectively, while in group Ⅳ and Ⅴ NS 2 ml/kg and tramadol 10 mg/kg were injected IP respectively on the 7th day after CCI. Paw withdrawal threshold (PWT) to von Frey filament stimulation and paw withdrawal latency (PWL) to noxious thermal stimuli were measured before (T1) and after IP NS or tramadol injection (T2) in group Ⅱ-Ⅴ. The distal CSF-CNs in the mid-brain was labelled with 30% cholera toxin subunit B and horseradish peroxidase compound (CB-HRP) 3 μl injected in left lateral cerebral ventricle. The expression of 5-HT1A receptors was measured by immuno-histochemistry. Results PWT and PWL were significantly decreased after CCI in group Ⅳ (NP + NS) and tramadol significantly inhibited the mechanical and thermal hyperalgesia in group Ⅴ (NP + T). There was no significant difference in the number of distal CSF-CNs among the 5 groups. CCI significantly down-regulated the expression of 5-HT1A in distal CSF-CNs in group Ⅳ(NP+ NS) as compared with group Ⅰ , Ⅱ and Ⅲ and tramadol significantly inhibited the CCI-induced downregulation of 5-HT1A receptor expression. Conclusion Tramadol can ease neuropathic pain by down-regulating the expression of 5-HT1A receptor in distal CSF-CNs in mid-brain.