ANTINOCICEPTIVE EFFECT OF INTRATHECAL NEOSTIGMINE EVALUATED IN RATS BY 2 DIFFERENT PAIN MODELS

The analgesic efficacy of cholinergic agonists and anticholinesterase agents has been widely recognized. The analgesic effect obtained by ac tivating cholinergic mechanisms, however, seems to depend on the exper imental pain model utilized for its evaluation, The antinociceptive ef fect of intraspinal neostigmine was examined in rats submitted concurr ently to the tail flick and formalin tests. Neostigmine (8.25 and 16.5 nmol) produced a dose-dependent antinociceptive effect in the tail fl ick test (a model of phasic pain) and reduced the first phase (phasic pain) of the animal response to formalin also in a dose; dependent man ner. Tile second phase (tonic pain) of the response to formalin, howev er, was slightly reduced after a longer period of time only by the hig her dose of the anticholinesterase. The effect of neostigmine was not significantly different when the drug was injected into rats submitted exclusively to the tail flick test. The second phase of the animal re sponse to formalin was slightly reduced by neostigmine (8.25 nmol) and strongly inhibited by the higher dose of the anticholinesterase when injection was made after the first phase, We conclude that phasic and tonic pain can both be controlled by high doses of neostigmine. In add ition, we show that inhibition by a lower dose of neostigmine of the f ormalin-induced phasic pain did not prevent the subsequent occurrence of tonic pain produced by the irritant.