Lack of peripheral analgesia mediated by intraplantar administration of neostigmine in carrageenan-injected rats

Background and objective The aim of the study was to test the peripheral an algesic action of neostigmine. Intraplantar administration of neostigmine w as studied in carrageenan-injected rats. Methods After assessing baseline values (To) for paw circumference and the paw withdrawal threshold on both hind paws, the right hind paw was injected with 0.2 mt of 1% carrageenan and the left hind paw with 0.2 mi of saline (0.9% NaCl). Two hours later (T-1), both hindpaws were tested for inflammat ion (paw circumference) and for hyperalgesia (paw withdrawal threshold) and subsequently 20 mug of neostigmine was injected in the right hind paw. The paw withdrawal threshold was tested again 10 (T-2), 20 (T-3) and 60 min (T -4) and paw circumference 60 min (T-4) after neostigmine injection. Results Carrageenan injection in the right hindpaw was associated with a de crease 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 T-1 to T-4. Neo stigmine had no effect on paw circumference and the paw withdrawal threshol d at measuring points T-2, T-3 and T-4 when compared with T-1 in the right hindpaw. Conclusion Our results support the lack of peripheral analgesic effect of n eostigmine.