The Antinociceptive and Antiinflammatory Properties of 3-furan-2-yl-N-p-tolyl-acrylamide, a Positive Allosteric Modulator of α7 Nicotinic Acetylcholine Receptors in Mice

Abstract

Background: Positive allosteric modulators (PAMs) facilitate endogenous neurotransmission and/or enhance the efficacy of agonists without directly acting on the orthosteric binding sites. In this regard, selective α7 nicotinic acetylcholine receptor type II PAMs display antinociceptive activity in rodent chronic inflammatory and neuropathic pain models. This study investigates whether 3-furan-2-yl-N-p-tolyl-acrylamide (PAM-2), a new putative α7-selective type II PAM, attenuates experimental inflammatory and neuropathic pains in mice.

Methods: We tested the activity of PAM-2 after intraperitoneal administration in 3 pain assays: the carrageenan-induced inflammatory pain, the complete Freund adjuvant-induced inflammatory pain, and the chronic constriction injury-induced neuropathic pain in mice. We also tested whether PAM-2 enhanced the effects of the selective α7 agonist choline in the mouse carrageenan test given intrathecally. Because the experience of pain has both sensory and affective dimensions, we also evaluated the effects of PAM-2 on acetic acid-induced aversion by using the conditioned place aversion test.

Results: We observed that systemic administration of PAM-2 significantly reversed mechanical allodynia and thermal hyperalgesia in inflammatory and neuropathic pain models in a dose- and time-dependent manner without motor impairment. In addition, by attenuating the paw edema in inflammatory models, PAM-2 showed antiinflammatory properties. The antinociceptive effect of PAM-2 was inhibited by the selective competitive antagonist methyllycaconitine, indicating that the effect is mediated by α7 nicotinic acetylcholine receptors. Furthermore, PAM-2 enhanced the antiallodynic and antiinflammatory effects of choline, a selective α7 agonist, in the mouse carrageenan test. PAM-2 was also effective in reducing acetic acid-induced aversion in the conditioned place aversion assay.

Conclusions: These findings suggest that the administration of PAM-2, a new α7-selective type II PAM, reduces the neuropathic and inflammatory pain sensory and affective behaviors in the mouse. Thus, this drug may have therapeutic applications in the treatment and management of chronic pain.

Scheme 1

New strategy for the synthesis of 3-furan-2-yl-N-p-tolyl-acrylamide. Reagents and conditions: (E)-3-(furan-2-yl)acrylic acid (10.1 mmol, 1.01 Eq), p-toluidine (10 mmol, 1.0 equiv.), N,N-dicyclohexylcarbodiimide (DCC) (10.1 mmol, 1.01 Eq), 4-(dimethylamino)pyridine (10.1 mmol, 1.01 Eq), dichloromethane, 0°C to RT, 12 hours (see Methods for more details).

Figure 1

The antiallodynic and antiinflammatory effects of 3-furan-2-yl-N-p-tolyl-acrylamide (PAM-2) using the carrageenan-induced inflammatory pain model in mice. A, The antiallodynic effects after intraperitoneal (IP) administration of various doses of PAM-2 (2, 6, and 8 mg/kg). B, Antiinflammatory effect of PAM-2, as measured by the difference in the ipsilateral paw diameter before and after carrageenan injection (ΔPD), 1 hour after IP injection of PAM-2 (8 mg/kg). C, Blockade of the antinociceptive effects of PAM-2 by subcutaneous administration of either mecamylamine or methyllycaconitine (MLA). Mecamylamine (2 mg/kg) or MLA (10 mg/kg) was given 15 minutes before an active dose of 8 mg/kg of PAM-2 or vehicle. D, The antinociceptive effects of PAM-2 (8 mg/kg IP) in vehicle of carrageenan-injected*mice. Pain sensitivity was measured by von Frey filament thresholds. Data are given as the mean ± SEM of 5 to 7 animals for each group. *P < 0.01, significantly different from its vehicle group; #P < 0.01, significantly different from PAM-2–treated group. φP = 0.022, t= 2.815 for ΔPD difference between groups.

Figure 2

Effects of PAM-2 and choline on carrageenan-induced inflammatory pain behaviors (A) and paw edema (B) in mice. Mice received PAM-2 (2 mg/kg intraperitoneally) or vehicle, and 30 minutes later mice were given an intrathecal injection of choline (10 μg/mouse) or vehicle. Paw edema, measured by the difference in the ipsilateral paw diameter before and after carrageenan injection (ΔPD), was assessed 1 hour after the last injection. Pain sensitivity was measured by von Frey filament thresholds. Data are given as the mean ± SEM of 6 animals for each group. * P < 0.01, significantly different from vehicle group; #P < 0.01, significantly different from PAM-2 treated group; φP < 0.01, significantly different from cholinetreated group. PAM-2 = 3-furan-2-yl-N-p-tolyl-acrylamide.

Figure 3

The antihyperalgesic and antiinflammatory effects of 3-furan-2-yl-N-p-tolyl-acrylamide (PAM-2) using the complete Freund adjuvant (CFA)-induced chronic inflammatory pain model in mice. A, The antihyperalgesic effects after intraperitoneal (IP) administration of various doses of PAM-2 (2, 6, and 8 mg/kg). B, Antiinflammatory effect of PAM-2, measured by the difference in the ipsilateral paw diameter before and after carrageenan injection (ΔPD), was assessed 1 hour after the injection of PAM-2 (8 mg/kg). C, The antinociceptive effects of PAM-2 (8 mg/kg IP) on vehicle of CFA-injected mice. Pain sensitivity was measured by paw withdrawal latency (PWL) with radiant heat beam. Data were given as the mean ± SEM of 5 to 6 animals for each group. *P < 0.01, significantly different from its vehicle group; φP = 0.0257, t = 2.617 for ΔPD difference between groups.

Figure 4

The antiallodynic effects of PAM-2 using the chronic constriction injury (CCI)-induced neuropathic pain model in mice. A, The antiallodynic effects after intraperitoneal (IP) administration of various doses of PAM-2 (2, 6, and 8 mg/kg) in CCI mice. B, The effects of PAM-2 (8 mg/kg IP) on mechanical sensitivity in sham mice. C, Blockade of the antiallodynic effects of PAM-2 by subcutaneous administration of mecamylamine or MLA. Mecamylamine (2 mg/kg) or MLA (10 mg/kg) was given 15 minutes before an active dose of 8 mg/kg of PAM-2 or vehicle. Pain sensitivity was measured by von Frey filament thresholds. Data are given as the mean ± SEM of 6 animals for each group. *P < 0.01, significantly different from its vehicle group; #P < 0.01, significantly different from PAM-2– treated group. MLA = methyllycaconitine; PAM-2 = 3-furan-2-yl-N-p-tolyl-acrylamide.

Figure 5

Effect of 3-furan-2-yl-N-p-tolyl-acrylamide (PAM-2) on acetic acid (AA)-induced conditioned place aversion. PAM-2 (2 or 8 mg/kg) or vehicle was injected intraperitoneally 15 minutes before administration of AA (1%). Data are given as the mean ± SEM of 6 to 9 ani-mals for each group. *P < 0.01, compared with the vehicle-injected mice; #P < 0.01, compared with the AA-injected mice.