The antinociceptive effects of nicotinic partial agonists varenicline and sazetidine-A in murine acute and tonic pain models

Abstract

Nicotinic agonists display a wide-range profile of antinociceptive activity in acute, tonic, and chronic pain models. However, their effectiveness is limited by their unacceptable side effects. We investigated the antinociceptive effects of two new α4β2* nicotinic partial agonists, varenicline and sazetidine-A, in acute thermal and tonic pain mouse models. Both drugs failed to induce significant effects in the tail-flick and hot-plate tests after subcutaneous administration. However, they blocked nicotine's effects in these tests at very low doses. In contrast to acute pain tests, varenicline and sazetidine-A dose-dependently induced an analgesic effect in the mouse formalin test after systemic administration. Their antinociceptive effects were mediated, however, by different nicotinic acetylcholine receptor (nAChR) subtypes. Sazetidine-A effects were mediated by β2* nAChR subtypes, whereas varenicline actions were attributed to α3β4 nAChRs. Moreover, low inactive doses of varenicline blocked nicotine's actions in phase II of the formalin test. Overall, our results suggest that the antagonistic actions of varenicline at low doses are mediated by β2*-nAChRs and at higher doses as an agonist by α3β4*-nAChRs. In contrast, both actions of sazetidine-A are mediated by β2*-nAChR subtypes. These results suggest that nicotinic partial agonists possess analgesic effects in a rodent tonic pain model and may provide a potential treatment for the treatment of chronic pain disorders.

Fig. 1.

Effects of varenicline (Var) and sazetidine-A (Saz) in the tail-flick and hot-plate tests. A, effects of varenicline (3 mg/kg s.c.), sazetidine-A (2 mg/kg s.c.), and nicotine (Nic) (2.5 mg/kg, s.c.) in the tail-flick and hot-plate tests in mice are shown. Mice were tested 5 and 15 min after nicotine and varenicline/sazetidine-A, respectively. Each group represents the mean ± S.E. of 8 to 12 mice. *, p < 0.05 versus vehicle (Veh). B and C, the ability of varenicline and sazetidine-A to antagonize a 2.5 mg/kg dose of nicotine in the tail-flick (B) and hot-plate (C) tests was also determined. The two drugs were given subcutaneously 15 min before nicotine, and mice were tested 5 min later. Each group represents the mean ± S.E. of 8 to 12 mice. *, p < 0.05 versus nicotine.

Fig. 2.

Effects of varenicline and sazetidine-A in the mouse formalin test. The effects of various doses of varenicline (1, 2, and 3 mg/kg) (A) and sazetidine-A (0.1, 0.5, 1, and 1.5 mg/kg) (B) after subcutaneous administration on formalin-induced pain behavior in the mouse are shown. Mice were treated subcutaneously with varenicline and sazetidine-A 15 min before formalin (2.5%; 20 μl) injection into the plantar region of the right hind paw. The cumulative pain response of time of licking was measured for 0 to 5 min (first phase) and 20 to 40 min (second phase). Data are expressed as mean ± S.E.M. of licking time. Each group represents the mean ± S.E. of 8 to 12 mice.

Fig. 3.

Nicotinic receptors subtypes involved in varenicline-induced antinociception in the formalin test. A, blockade of the antinociceptive effect of varenicline in the formalin test by different nicotinic antagonists is shown. Mice were pretreated with various nicotinic antagonists [MEC, 2 mg/kg s.c.; DHβE, 2 mg/kg s.c.; MLA, 10 mg/kg s.c.; α-conotoxin AuIB (α-ctx AuIB), 700 pmol/mouse i.t.] 15 min (5 min for α-conotoxin AuIB) before an active dose of 3 mg/kg varenicline. Fifteen minutes later, mice were injected with formalin (2.5% intraplantary; 20 μl) and then observed for pain behaviors. B, effects of different doses of the α3β4* antagonist α-conotoxin AuIB on varenicline-induced antinociception in the formalin test are shown. Mice were injected intrathecally with different doses of α-conotoxin AuIB (70, 140, and 700 pmol/mouse), and 5 min later they received a dose of 3 mg/kg varenicline. Fifteen minutes later, mice were injected with formalin (2.5% intraplantary; 20 μl) and then observed for pain behaviors. C, antinociceptive effects of vareniciline in the β2 WT and KO mice are shown. Mice received a dose of 3 mg/kg varenicline and 15 min later were tested in the formalin test. Data are expressed as mean ± S.E.M. of licking time. Each group represents the mean ± S.E. of 8 to 12 mice. *, p < 0.05 versus vehicle.

Fig. 4.

Blockade of nicotine-induced antinociception by varenicline and sazetidine-A in the formalin test. Effects of varenicline (A) and sazetidine-A (B) on the antinociceptive effect of nicotine in the formalin test are shown. Mice were pretreated with varenicline (0.04 and 0.2 mg/kg s.c.) or sazetidine-A (0.2 mg/kg s.c.) 15 min before an active dose of nicotine (1.5 mg/kg s.c.). Five minutes later, mice were injected with formalin (2.5% intraplantary; 20 μl) and then observed for pain behaviors. Data are expressed as mean ± S.E.M. of licking time. Each group represents the mean ± S.E. of 8 to 12 mice. *, p < 0.05 versus vehicle.

Fig. 5.

Effects of nicotine in β2 KO mice using the formalin test. Antinociceptive effects of nicotine in the β2 WT and KO mice are shown. Mice received a dose of 1.5 mg/kg s.c. nicotine and 5 min later were tested in the formalin test. Data are expressed as mean ± S.E.M. of licking time. Each group represents the mean ± S.E. of 8 to 12 mice.

Fig. 6.

Nicotinic receptor subtypes involved in sazetidine-A-induced antinociception in the formalin test. A, blockade of the antinociceptive effect of sazetidine-A in the formalin test by different effects of nicotinic antagonists is shown. Mice were pretreated with various nicotinic antagonists (MEC, 2 mg/kg s.c.; DHβE, 2 mg/kg s.c.; MLA, 10 mg/kg, s.c.; α-conotoxin AuIB, 700 pmol/mouse i.t.) 15 min (5 min for α-conotoxin AuIB) before a dose of 1.5 mg/kg sazetidine-A. Fifteen minutes later, mice were injected with formalin (2.5% intraplantary; 20 μl) and then observed for pain behaviors. B, antinociceptive effects of sazetidine-A in the β2 WT and KO mice are shown. Mice received an active dose of sazetidine-A (1.5 mg/kg s.c.), and 15 min later they were tested in the formalin test. Data are expressed as mean ± S.E.M. of licking time. Each group represents the mean ± S.E. of 8 to 12 mice. *, p < 0.05 versus vehicle.

Fig. 7.

Effects of nicotinic partial agonists on motor coordination. Dose-response curves of nicotine, varenicline, and sazetidine-A in the rotarod test after subcutaneous administration in mice are shown. Mice were tested 5 and 15 min after nicotine and varenicline/sazetidine-A injections, respectively, for 5 min on the rotarod. Each point represents the mean ± S.E. of 8 to 12 mice.