The analgesic and toxic effects of nornicotine enantiomers alone and in interaction with morphine in rodent models of acute and persistent pain

Neuronal nicotinic acetylcholinic receptors (nAChR) are promising targets for the development of novel analgesics. Nicotine and other nAChR-agonists produce profound analgesia in rodent models of acute and persistent pain. However, significant side-effects are of concern. Nornicotine (N-desmethyl-nicotine) appears to activate different nAChR subtypes, has a better pharmacokinetic profile, and produces less toxicity than nicotine. Little is known about its analgesic properties. In the present study, the S(-)- and R(+)-enantiomers of nornicotine were characterized with regard to analgesia and side-effects profile. Efficacy was demonstrated in rat models of pain where central sensitization is involved: i.e. the chronic constriction nerve injury model of peripheral neuropathy and the formalin model of tonic inflammatory pain. The desirable (analgesic) properties resided predominantly in the S(-)- rather than the R(+)-enantiomer. In contrast, undesirable effects (motor in-coordination, reduced locomotor activity, ataxia) were more pronounced with the R(+)-enantiomer. This is an interesting finding, which may suggest separation of toxicity from analgesia by utilization of S(-)-enantiomer of nornicotine. Maximum analgesic effectiveness without significant side-effects was achieved when S(-)-nornicotine (sub-analgesic dose) was combined with a low-dose of the micro-opioid, morphine. These preclinical data suggest that S(-)-nornicotine may be of value, either alone or in combination with an opioid, for treatment of a broad-spectrum of pain (i.e. nociceptive, neuropathic, and mixed pain).