Opioid and nicotinic medullary hyperalgesic influences in the decerebrated rat

The effects of ethylketazocine (EKC) administered intraperitoneally and the nicotinic ligands (−)- and (+)-nicotine, (−)-cytisine, (−)-lobeline, and (+)-2-methylpiperidine administered into the 4th ventricle on the latency of the thermally evoked withdrawal reflex of the decerebrate rat were investigated. EKC administered intraperitoneally produced both hyperalgesia and analgesia. (−)-Nicotine administered into the 4th ventricle produced a biphasic dose related effect on the latency of the withdrawal reflex; low doses produced a dose related analgesia while higher doses produced hyperalgesia. (−)-Cytisine and (−)-lobeline administered into the 4th ventricle produced biphasic effects. (+)-2-Methylpiperidine administered into the 4th ventricle produced a significant degree of hyperalgesia. Both the analgesic and hyperalgesic effects of (−)-nicotine were antagonized by mecamylamine (1 mg/kg) and naltrexone (5 mg/kg). The hyperalgesic action of (+)-2-methylpiperidine was antagonized by naltrexone but not by mecamylamine. These observations suggest that there are both medullary opioidergic and nicotinic cholinergic mechanisms for modulating both analgesic and hyperalgesic processes and that nicotinic ligands have multiple mechanisms of action in the brain.