Potentiation of imipramine-induced antinociception by nicotine in the formalin test

Synergistic effect between imipramine and citicoline upon induction of analgesic and antidepressant effects in mice

Imipramine is a tricyclic antidepressant (TCA) drug that is sometimes used to treat neuropathic pain. Citicoline is a dietary supplement that has been used as a neuroprotective agent for neurological disorders. Probable interaction between imipramine and citicoline on pain and depression behaviors was examined in mice using a tail-flick test, open field test (OFT), forced swimming test (FST), and tail suspension test (TST). The results indicated that the intraperitoneal (i.p.) administration of citicoline (50 mg/kg) induced analgesic and antidepressant-like behaviors in mice. Similarly, i.p. injection of imipramine (5 mg/kg) induced dose-dependent anti-nociceptive and anti-depressive effects. Co-administration of different doses of imipramine (1.25, 2.5, and 5 mg/kg) along with an ineffective dose of citicoline (6.25 mg/kg) increased tail-flick latency and decreased immobility time in the FST, suggesting an analgesic and antidepressant-like behaviors. Interestingly, there is a synergistic effect between imipramine and citicoline upon the induction of analgesic and antidepressant effects. All doses of the drugs had no significant effect on the locomotor activity. Based on these results, it can be concluded that the administration of citicoline (as an adjuvant drug) in combination with imipramine increased the efficacy of TCA drugs for modulation of pain and depression behaviors.

Antinociceptive effect of tebanicline for various noxious stimuli-induced behaviours in mice

Tebanicline (ABT-594), an analogue of epibatidine, exhibits potent antinociceptive effects and high affinity for the nicotinic acetylcholine receptor in the central nervous system. We assessed whether tebanicline exerts an effect on various noxious stimuli and mediates the nicotine receptor or opioid receptor through stimulation. The antinociceptive effects of tebanicline were determined by noxious chemical, thermal and mechanical stimuli-induced behaviours in mice. Tebanicline had dose-dependent analgesic effects in formalin, hot-plate and tail-pressure tests. By contrast, the antinociceptive effect of tebanicline was not demonstrated in the tail-flick assay. Pre-treatment with mecamylamine, a nicotinic acetylcholine receptor antagonist, blocked the effects of tebanicline in formalin, tail-pressure and hot-plate tests. Moreover, pre-treatment with naloxone, an opioid receptor antagonist, only partially inhibited the effects of tebanicline in formalin and tail-pressure tests. Tebanicline produced antinociception in persistent chemical (formalin), acute thermal (hot-plate, but not tail-flick) and mechanical (tail-pressure) pain states. Moreover, tebanicline stimulated the nicotinic acetylcholine receptor and opioid receptor.

α4β2* neuronal nicotinic receptor ligands (agonist, partial agonist and positive allosteric modulators) as therapeutic prospects for pain

α4β2* neuronal nicotinic acetylcholine receptor are ligand-gated ion channels and widely expressed throughout the central and peripheral nervous system. α4β2* neuronal nicotinic acetylcholine receptor play crucial role in pain signaling via modulation of multiple neurotransmitters like acetylcholine, dopamine, γ-amino butyric acid (GABA) and norepinephrine. Both spinal and supraspinal pathways are involved in the mechanisms by which α4β2* neuronal nicotinic acetylcholine receptor ligands modulate the neuropathic and inflammatory pain. Selective α4β2* neuronal nicotinic acetylcholine receptor ligands are being developed for the treatment of neuropathic and inflammatory pain as they show considerable efficacy in a wide range of preclinical pain models. Agonists/partial agonists of α4β2* neuronal nicotinic acetylcholine receptor show efficacy in animal models of pain and their anti-nociceptive properties are blocked by nicotinic antagonists. Positive allosteric modulators are being developed with the aim to increase the potency or therapeutic window of agonists/partial agonists. Accumulating evidences suggest that anti-nociceptive effects of nicotinic acetylcholine receptor ligands may not be mediated solely by α4β2* neuronal nicotinic acetylcholine receptor. We have also reviewed the stage of clinical development of various α4β2* neuronal nicotinic acetylcholine receptor ligands.

Enhanced inhibitory synaptic transmission in the spinal dorsal horn mediates antinociceptive effects of TC-2559

Background

TC-2559 is a selective α4β2 subtype of nicotinic acetylcholine receptor (nAChR) partial agonist and α4β2 nAChR activation has been related to antinociception. The aim of this study is to investigate the analgesic effect of TC-2559 and its underlying spinal mechanisms.

Results

1) In vivo bioavailability study: TC-2559 (3 mg/kg) had high absorption rate in rats with maximal total brain concentration reached over 4.6 μM within first 15 min after administration and eliminated rapidly with brain half life of about 20 min after injection. 2) In vivo behavioral experiments: TC-2559 exerts dose dependent antinociceptive effects in both formalin test in mice and chronic constriction injury (CCI) model in rats by activation of α4β2 nAChRs; 3) Whole-cell patch-clamp studies in the superficial dorsal horn neurons of the spinal cord slices: perfusion of TC-2559 (2 μM) significantly increased the frequency, but not amplitude of spontaneous inhibitory postsynaptic currents (sIPSCs). The enhancement of sIPSCs was blocked by pre-application of DHβE (2 μM), a selective α4β2 nicotinic receptor antagonist. Neither the frequency nor the amplitude of spontaneous excitatory postsynaptic currents (sEPSCs) of spinal dorsal horn neurons were affected by TC-2559.

Conclusions

Enhancement of inhibitory synaptic transmission in the spinal dorsal horn via activation of α4β2 nAChRs may be one of the mechanisms of the antinociceptive effects of TC-2559 on pathological pain models. It provides further evidence to support the notion that selective α4β2 subtype nAChR agonist may be developed as new analgesic drug for the treatment of neuropathic pain.

Antinociceptive effect of nicotine in various pain models in the mouse

The antinociceptive effect of nicotine administered intracereboventricularly (i.c.v.) or intrathecally (i.t) in several pain models was examined in the present study. We found that i.t. treatment with nicotine (from 5 to 20 g) dose-dependently blocked pain behavior revealed during the second phase, but not during the first phase in the formalin test. In addition, i.c.v. treatment with nicotine (from 0.1 to 10 microg) dose-dependently attenuated pain behavior revealed during both the first and second phases. In addition to the formalin test, nicotine administered i.c.v. or i.t. attenuated acetic acid-induced writhing response. Furthermore, i.c.v. or i.t. administration of nicotine did not cause licking, scratching and biting responses induced by substance P, glutamate, TNF-alpha (100 pg), IL-1beta (100 pg) and INF-gamma (100 pg) injectied i.t. The antinociception induced by supraspinally-administered nicotine appears to be more effective than that resulting from spinally administered nicotine. Our results suggest that nicotine administration induces antinociception by acting on the central nervous system and has differing antinociceptive profiles according to the various pain models.