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

Abstract

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).

FIG 1

Time course of responsiveness to mechanical noxious stimuli (vocalization threshold, VT, g) in nerve-injured paw (chronic constriction nerve injury, CCI) and sham-operated paw after intraperitoneal (IP) administration: S(−)-nornicotine [Panel A]; R(+)-nornicotine [Panel B] and oral (PO) administration: S(−)-nornicotine [Panel C]. Saline (IP, PO) served as control. Mean ± SEM (n = 8 rats).

FIG 2

Time course of responsiveness to non-noxious mechanical stimuli (50% von Frey paw withdrawal threshold, PWT, g) in nerve-injured paw (chronic constriction nerve injury, CCI) and sham-operated paw after intraperitoneal (IP) administration: S(−)-nornicotine [Panel A]; R(+)-nornicotine [Panel B]. Saline served as control. Mean ± SEM (n = 4 rats).

FIG 3

Time course of formalin-induced flinching behavior (number of flinches; the 1^st and 2^nd phases) after intraperitoneal (IP) administration: S(−)-nornicotine [Panel A]; R(+)-nornicotine [Panel B]. Saline served as control. Mean ± SEM (n = 6 rats/dose).

FIG 4

Time course of responsiveness to thermal noxious stimuli (paw withdrawal latency, PWL, s) after intraperitoneal (IP) administration: S(−)-nornicotine. Saline served as control. Mean ± SEM (n = 8 rats).

FIG 5

Time course of the motor effect (latency to fall, s) in rotarod performance test after intraperitoneal (IP) administration: S(−)-nornicotine [Panel A]; R(+)-nornicotine [Panel B]. Saline served as control. Mean ± SEM (n = 6 rats).

FIG 6

Dose-response curves for: [Panel A] locomotor activity (AUC_0–120min is area under the curve, 0–120 min, for a drug or saline); [Panel B] ataxia (total score, 0–120 min); [Panel C] activity level (total score, 0–120 min) after intraperitoneal (IP) administration: S(−)-nornicotine, R(+)-nornicotine, saline (control). Mean ± SEM (n = 8 rats). * Significantly different from saline (P<0.05; post-hoc SNK).

FIG 7

Dose-response curves for S(−)-nornicotine [Panel A] and R(+)-nornicotine [Panel B]: mechanical hyperalgesia (chronic constriction nerve injury model, CCI; n = 8); tactile allodynia (CCI; n = 4); formalin-induced flinching (the 2^nd phase in the formalin test; n = 6); motor coordination (rotarod; n =6). Saline served as control. Percent maximum possible effect (%MPE) was calculated as described in the Methods section. Data are mean ± SEM (n rats). * Significantly different from saline (P<0.05; post-hoc SNK).

FIG 8

Responsiveness to thermal noxious stimuli (the tail-flick test) after intraperitoneal (IP) administration: S(−)-nornicotine (2.5 mg/kg), morphine (3 mg/kg), S(−)-nornicotine (2.5 mg/kg) - morphine (3 mg/kg) combination [Panel A]; R(+)-nornicotine (2.5 mg/kg), morphine (3 mg/kg), R(+)-nornicotine (2.5 mg/kg) - morphine (3 mg/kg) combination [Panel B]. The AUC_0–180min values are areas under the curves (0–180 min). Mean ± SEM (n = 8 rats). * Significantly different from morphine alone (P<0.05; t-test).

FIG 9

Dose-response curves for the antinociceptive effects (the tail-flick test) after intrathecal (IT) administration: S(−)-nornicotine (10–100 μg) [Panel A]; morphine (0.5–30 μg) [Panel B]; S(−)-nornicotine (10–100 μg) in combination with a fixed dose of morphine (0.5 μg) [Panel C]. Percent maximum possible effect (%MPE) was calculated as described in the Methods section. Dashed lines represent the effects of saline and morphine (0.5 μg) alone, respectively. Mean ± SEM (n = 6–8 rats). * Significantly different from morphine alone (P<0.05; post-hoc SNK test).

FIG 10

Time courses of responsiveness to mechanical noxious stimuli (vocalization threshold, VT, g) in nerve-injured paw (chronic constriction nerve injury model, CCI): S(−)-nornicotine (S- NNIC); morphine (MOR); S(−)-nornicotine and morphine combination after intraperitoneal (IP) [Panel A] and oral (PO) [panel B] administration. Dose-response curves for the antihyperalgesic effects: S(−)-nornicotine (0.01–0.5 mg/kg, IP) in combination with morphine (3 mg/kg, IP) [Panel C]; S(−)-nornicotine (0.1–7.5 mg/kg, PO) in combination with morphine (5 mg/kg, PO) [panel D]. Percent maximum possible effect (%MPE) was calculated as described in the Methods section. Dashed lines are effects of morphine alone (3 mg/kg IP; 5 mg/kg PO). Mean ± SEM (n = 8 rats). * Significantly different from morphine alone (P<0.05; post-hoc SNK).