[Hyperalgesic effect of subarachnoid administration of phentolamine in mice]

Painful phenomenon is one of the most important and complex experiences. Phentolamine is a non-selective alpha-adrenergic antagonist. The objective of this study was to compare the effect of increasing doses of phentolamine into subarachnoid space in rats in the modulation of painful phenomenon.

84 male Wistar rats were divided into formalin and plantar incision groups, subdivided into six subgroups (n=7). Control group (CG) received only saline (10μL); active subgroups received phentolamine 10μmg (GF10), 20mg (GF20), 30mg (GF30), 40mg (GF40), and 50g (GF50). In formalin group, pain was induced by injection of 50μL of 2% formalin in dorsal region of right posterior paw. In plantar incision group, pain was induced by plantar incision and evaluated using Von Frey filaments. Induction and maintenance of anesthesia were performed with 3% halothane for catheter placement into subarachnoid space and plantar incision. Statistical analysis was performed using the JMP program from SAS with 5% significance level.

Phentolamine at doses of 20 and 30g increased the algesic response in the intermediate phase of the formalin test. In plantar incision test, it had hyperalgic effect on first, third, fifth, and seventh days at a dose of 10g and on first, third, and fifth days at a dose of 20g and on fifth day at a dose of 30g.

Subarachnoid administration of phentolamine showed hyperalgesic effect, possibly due to the involvement of different subclasses of alpha-adrenergic receptors in modulating pain pathways.