Characterization of the antinociception induced by nicotine in the pedunculopontine tegmental nucleus and the nucleus raphe magnus.

The antinociceptive effect of subcortically administered nicotine was investigated in the rat using the hot-plate and tail-flick tests. Adult male Sprague-Dawley rats were implanted with guide cannulas aimed at 185 sites in the forebrain, midbrain and hindbrain. After 1 week, nicotine was injected in 0.5 microliter of 50 mM phosphate buffer, pH 7.4. The pedunculopontine tegmental nucleus (PPTg) of the mesopontine tegmentum and the nucleus raphe magnus (NRM) of the ventral medulla were the most sensitive sites of nicotine-induced antinociception. The median effective doses of nicotine to inhibit hot-plate or tail-flick nociception after PPTg or NRM administration ranged between 1.4 and 3 nmol. The lack of effect of s.c. injections of naloxone on the antinociception induced by nicotine in the PPTg or NRM ruled out endogenous opioid mechanisms. Coadministration of mecamylamine or pirenzepine with nicotine into the NRM competitively antagonized nicotine-induced antinociception. The administration of the muscurinic cholinergic type 2 receptor antagonist methoctramine into the NRM produced a strong antinociceptive response which was blocked by prior treatment of the NRM with hemicholinium-3. Hemicholinium-3 pretreatment of either the PPTg or the NRM antagonized the antinociception induced by nicotine at these sites. Hemicholinium-3 pretreatment of the NRM also antagonized the antinociception produced by s.c. administered nicotine. The antinociceptive effects of nicotine injected in the PPTg were blocked by procainamide injections in the NRM; however, the antinociceptive effects of nicotine injected in the NRM were not blocked by bilateral injections of procainamide in the PPTg. Both lesioning the PPTg with ibotenic acid and pretreating the NRM with hemicholinium-3 abolished completely the antinociception induced by nicotine or (+)-cis-dioxolane microinjections into the PPTg. However, neither ibotenic acid-induced nor electrolytic lesions of the PPTg alone altered CRL. The data support the existence of a tonically active cholinergic pathway which is under autoinhibitory control that originates in the PPTg, terminates in the NRM and modulates nociception by activating descending pain inhibitory systems relaying within the NRM.