Background and objective The aim of the study was to test the peripheral analgesic action of neostigmine. Intraplantar administration of neostigmine was studied in carrageenan-injected rats.
Methods After assessing baseline values (T0) for paw circumference and the paw withdrawal threshold on both hind paws, the right hind paw was injected with 0.2mL of 1% carrageenan and the left hind paw with 0.2mL of saline (0.9% NaCl). Two hours later (T1), both hindpaws were tested for inflammation (paw circumference) and for hyperalgesia (paw withdrawal threshold) and subsequently 20 mg of neostigmine was injected in the right hind paw. The paw withdrawal threshold was tested again 10 (T2), 20 (T3) and 60 min (T4) and paw circumference 60 min (T4) after neostigmine injection.
Results Carrageenan injection in the right hindpaw was associated with a decrease in the paw withdrawal threshold reflecting mechanical hyperalgesia (P < 0.001) and an increase in paw circumference reflecting oedema (P < 0.001) in the right hind paw when compared with the left side from T1 to T4. Neostigmine had no effect on paw circumference and the paw withdrawal threshold at measuring points T2, T3 and T4 when compared with T1 in the right hindpaw. Conclusion Our results support the lack of peripheral analgesic effect of neostigmine.