The involvement of the TRPA1 receptor in a mouse model of sympathetically maintained neuropathic pain.

Sympathetic fibres maintain some forms of neuropathic pain, but the underlying mechanisms are poorly understood. Therefore, this study investigated the possible involvement of transient receptor potential ankyrin 1 (TRPA1) and the role of the sympathetic nervous system (involved in sympathetically maintained neuropathic pain) in a model of neuropathic pain induced by sciatic nerve chronic constriction injury (CCI) in mice. Systemic injection of the selective TRPA1 antagonist HC-030031 reversed the mechanical and cold allodynia that was induced by sciatic nerve chronic constriction injury (CCI). Nerve injury also sensitised mice to nociception, which was induced by the intraplantar injection of a low dose of the TRPA1 agonist allyl isothiocyanate without changing TRPA1 immunoreactivity in the injected paw. Furthermore, chemical sympathectomy produced by guanethidine largely prevented CCI-induced mechanical and cold allodynia. CCI also induced a norepinephrine-triggered nociception that was inhibited by an α-adrenoceptor antagonist, norepinephrine transporter block and monoamine oxidase inhibition. Finally, the peripheral injection of HC-030031 also largely reduced CCI-induced norepinephrine nociception and mechanical or cold allodynia. Taken together, the present findings reveal a critical role of TRPA1 in mechanical and cold hypersensitivity and norepinephrine hypersensitivity following nerve injury. Finally, our results suggest that TRPA1 antagonism may be useful to treat patients who present sympathetically maintained neuropathic pain.

Anandamide attenuates haloperidol-induced vacuous chewing movements in rats.

Antipsychotics may cause tardive dyskinesia in humans and orofacial dyskinesia in rodents. Although the dopaminergic system has been implicated in these movement disorders, which involve the basal ganglia, their underlying pathomechanisms remain unclear. CB1 cannabinoid receptors are highly expressed in the basal ganglia, and a potential role for endocannabinoids in the control of basal ganglia-related movement disorders has been proposed. Therefore, this study investigated whether CB1 receptors are involved in haloperidol-induced orofacial dyskinesia in rats. Adult male rats were treated for four weeks with haloperidol decanoate (38mg/kg, intramuscularly - i.m.). The effect of anandamide (6nmol, intracerebroventricularly - i.c.v.) and/or the CB1 receptor antagonist SR141716A (30µg, i.c.v.) on haloperidol-induced vacuous chewing movements (VCMs) was assessed 28days after the start of the haloperidol treatment. Anandamide reversed haloperidol-induced VCMs; SR141716A (30µg, i.c.v.) did not alter haloperidol-induced VCM per se but prevented the effect of anandamide on VCM in rats. These results suggest that CB1 receptors may prevent haloperidol-induced VCMs in rats, implicating CB1 receptor-mediated cannabinoid signaling in orofacial dyskinesia.

The antinociceptive effect of reversible monoamine oxidase-A inhibitors in a mouse neuropathic pain model.

Neuropathic pain is a debilitating condition that is often resistant to common analgesics, such as opioids, but is sensitive to some antidepressants, an effect that seems to be mediated by spinal cord 5-HT3 receptors. Because the analgesic potential of monoamine oxidase-A (MAO-A) inhibitors is understudied, we evaluated the potential antinociceptive effect of the reversible MAO-A inhibitors moclobemide and 2-(3,4-dimethoxy-phenyl)-4,5-dihydro-1H-imidazole (2-DMPI) in a mouse neuropathic pain model induced by chronic constriction injury (CCI) of the sciatic nerve. Neuropathic mice showed a decreased mechanical paw withdrawal threshold (PWT) 7 days after lesion compared with the baseline PWT, characterizing the development of hyperalgesia. Moclobemide (100-300 µmol/kg, s.c.) and 2-DMPI (30-300 µmol/kg, s.c.) treatments were able to reverse the CCI-induced hyperalgesia, with 50% inhibitory dose (ID50) values of 39 (18-84) and 11 (4-33) µmol/kg, and maximum inhibition (Imax) values of 88±14 and 98±15%, respectively, at the 300 µmol/kg dose. In addition, we observed a significant increase in the MAO-A activity in the lumbar spinal cord of CCI-submitted mice compared with sham-operated animals. Furthermore, the antihyperalgesic effects of both 2-DMPI and moclobemide were largely reversed by intrathecal injection of the 5-HT3 receptor antagonist ondansetron (10 µg/site). These results suggest a possible involvement of MAO-A in the mechanisms of neuropathic pain and a potential utility of the reversible inhibitors of MAO-A in the development of new therapeutic approaches to treat it.

Triterpene 3ß, 6ß, 16ß trihidroxilup-20(29)-ene protects against excitability and oxidative damage induced by pentylenetetrazol: the role of Na(+),K(+)-ATPase activity.

Administration of the compound triterpene 3ß, 6ß, 16ß-trihidroxilup-20(29)-ene (TTHL) resulted in antinociceptive activity in several pain models in mice. Because pain and epilepsy have common mechanisms, and several anticonvulsants are clinically used to treat painful disorders, we investigated the anticonvulsant potential of TTHL. Behavioral and electrographic recordings revealed that pretreatment with TTHL (30 mg/kg; i.g.) increased the latencies to the first clonic seizure to the tonic-clonic and reduced the duration of the generalized seizures induced by the GABA(A) receptor antagonist PTZ (80 g; i.p.). The TTHL pretreatment also protected against PTZ-induced deleterious effects, as characterized by protein carbonylation, lipid peroxidation, [(3)H] glutamate uptake and the inhibition of Na(+),K(+)-ATPase (subunits α(1) and α(2)/α(3)). Although TTHL did not exhibit DPPH, ABTS radical scavenging activity per se and does not alter the binding of [(3)H]flunitrazepam to the benzodiazepinic site of the GABA(A) receptor, this compound was effective in preventing behavioral and EEG seizures, as well as the inhibition of Na(+),K(+)-ATPase induced by ouabain. These results suggest that the protection against PTZ-induced seizures elicited by TTHL is due to Na(+),K(+)-ATPase activity maintenance. In fact, experiments in homogenates of the cerebral cortex revealed that PTZ (10 mM) reduced Na(+),K(+)-ATPase activity and that previous incubation with TTHL (10 µM) protected against this inhibition. Collectively, these data indicate that the protection exerted by TTHL in this model of convulsion is not related to antioxidant activity or GABAergic activity. However, these results demonstrated that the effective protection of Na(+),K(+)-ATPase elicited by this compound protects against the damage due to neuronal excitability and oxidation that is induced by PTZ.

Involvement of monoamine oxidase B on models of postoperative and neuropathic pain in mice.

In this study we assessed the involvement of monoamine oxidase B (MAO-B), a key enzyme implicated in monoamine metabolism, on postoperative (plantar incision) and neuropathic (partial sciatic nerve ligation) pain models in mice. Paw incision submitted mice showed a significant decrease in mechanical threshold compared with the sham-operated mice, characterizing the development of mechanical allodynia. The selective and irreversible MAO-B inhibitor selegiline, at a dose sufficient to selectively inhibit MAO-B activity (10 mg/kg), showed an anti-allodynic effect from 0.5 to 6 h after incision. Likewise, partial sciatic nerve ligation submitted mice also developed mechanical allodynia, which was reversed by selegiline (10 mg/kg) from 2 to 6 h after treatment. In addition, a significant increase on striatal MAO-B activity was observed in neuropathic mice compared with the sham-operated animals, which was reversed by selegiline treatment. Taken together, our results showed that MAO-B seems to exert a critical role in the development of postoperative and neuropathic pain.

Antidepressant-like effect of the novel MAO inhibitor 2-(3,4-dimethoxy-phenyl)-4,5-dihydro-1H-imidazole (2-DMPI) in mice.

Monoamine oxidase (MAO) inhibitors were the first antidepressant drugs to be prescribed and are still used today with great success, especially in patients resistant to other antidepressants. In this study, we evaluated the MAO inhibitory properties and the potential antidepressant action of 2-(3,4-dimethoxy-phenyl)-4,5-dihydro-1H-imidazole (2-DMPI) in mice. We found that 2-DMPI inhibited both MAO isoforms (K(i) values were 1.53 (1.3-1.8) µM and 46.67 (31.8-68.4) µM for MAO-A and MAO-B, respectively) with 30-fold higher selectivity toward MAO-A. In relation to the nature of MAO-A inhibition, 2-DMPI showed to be a mixed and reversible inhibitor. The treatment with 2-DMPI (100-1000 µmol/kg, s.c.) caused a significant decrease in immobility time in the tail suspension test (TST) without affecting locomotor activity, motor coordination or anxiety-related activities. Conversely, moclobemide (1000 µmol/kg, s.c.) caused a significant increase in immobility time in the TST, which appeared to be mediated by a nonspecific effect on motor coordination function. 2-DMPI (300 µmol/kg, s.c.) decreased serotonin turnover in the cerebral cortex, hippocampus and striatum, whereas dopamine turnover was diminished only in the striatum, and norepinephrine turnover was not changed. The antidepressant-like effect of 2-DMPI was inhibited by the pretreatment of mice with methysergide (2 mg/kg, s.c., a non-selective serotonin receptor antagonist), WAY100635 (0.1 mg/kg, s.c., a selective 5-HT(1A) receptor antagonist) or haloperidol (0.05 mg/kg, i.p., a non-selective dopamine receptor antagonist). These results suggest that 2-DMPI is a prototype reversible and preferential MAO-A inhibitor with potential antidepressant activity, due to its modulatory effect on serotonergic and dopaminergic systems.

The potential antidepressant-like effect of imidazoline I2 ligand 2-BFI in mice.

The compound 2-(2-benzofuranyl)-2-imidazoline (2-BFI) is a 2-imidazoline derivative that selectively inhibits the in vitro activity of monoamine oxidase-A and it is also an imidazoline I(2) agonist. However, the antidepressant potential of this compound and its mechanism of action have not been well defined. Therefore, in this study we investigated the antidepressant-like effect of 2-BFI in mice. 2-BFI (100 and 300µmol/kg, s.c.) significantly reduced the immobility time on the tail suspension test (TST) without changing locomotion in the open field test. The reduced the immobility time of 2-BFI (100µmol/kg, s.c.) was confirmed with the forced swimming test (FST). The antidepressant-like effect of 2-BFI (100µmol/kg, s.c.) in the TST was prevented by pretreatment with idazoxan (0.4µmol/kg, i.p., a I(2) site antagonist), methysergide (4µmol/kg, i.p., a non-selective serotonergic receptor antagonist) and haloperidol (0.1µmol/kg, i.p., a non-selective dopaminergic receptor antagonist). The anxiolytic effect of 2-BFI was also evaluated, using the elevated plus-maze test. 2-BFI (300µmol/kg, s.c.) was able to significantly increase the % of number of entries and the % of time spent in the open arms, indicating that it possesses an anxiolytic effect at high doses. In conclusion, these results suggest that the antidepressant-like effect of 2-BFI might involve serotonergic, dopaminergic and imidazoline systems, and then the imidazoline site could represent a new pharmacological target for the treatment of depression.

Low concentrations of methamidophos do not alter AChE activity but modulate neurotransmitters uptake in hippocampus and striatum in vitro.

AIMS: Methamidophos (Meth) is a toxic organophosphorus compound (OP) that inhibits acetylcholinesterase enzyme (AChE) and induces neurotoxicity. As the mechanism of its neurotoxic effects is not well understood, the aim of the present study was to evaluate the effects of Meth on glutamate and gamma aminobutyric acid (GABA) uptake and correlate with cell viability and AChE and Na(+)/K(+)-ATPase enzyme activities in striatum and hippocampus slices exposed to low concentrations (0.05 to 1.0 µM) of Meth. MAIN METHODS: Hippocampal and striatal slices of rat brain were exposed to Meth for 5 min ([(3)H]Glutamate uptake) or 15 min ([(3)H]GABA uptake) for assays. The enzyme activities and cell viability were also accessed at both times in hippocampal and striatal slices and homogenates. KEY FINDINGS: At concentrations that did not inhibit AChE, Meth caused changes in glutamate uptake in striatal (0.05 and 1.0 µM Meth) and hippocampal (1.0 µM Meth) slices. GABA uptake was increased by the pesticide in striatum at 0.5 and 1.0 µM and in hippocampus at 0.05 µM. After 3.5h of Meth exposure, striatal and hippocampal cells showed no changes in viability as well as no inhibition of Na(+)/K(+)-ATPase were observed after 5 or 15 min exposure to Meth in the same brain structures. SIGNIFICANCE: Results suggest that Meth, even without changing the AChE activity can modify somehow the neurotransmitters uptake. However, further studies are necessary to clarify if this modulation in glutamate or GABA uptake may be responsible to cause some disturbance in behavior or in other neurochemical parameters following low Meth exposure in vivo.

Eriodictyol: a flavonoid antagonist of the TRPV1 receptor with antioxidant activity.

The transient potential vanilloid 1 receptor (TRPV1) is a calcium-permeable channel responsible for the transduction and modulation of acute and chronic pain signaling. As such, this receptor is a potential target for the treatment of a number of pain disorders. However, AMG517, a TRPV1 antagonist, presents several clinical limitations that include the induction of severe hyperthermia. The aim of this study was to investigate the possible interaction of the flavonoid eriodictyol with the TRPV1 receptor and to determine its putative antinociceptive and hyperthermic effects. Eriodictyol was able to displace [(3)H]-resiniferatoxin binding (IC(50)=47; 21-119nM) and to inhibit calcium influx mediated by capsaicin (IC(50)=44; 16-125nM), suggesting that eriodictyol acts as a TRPV1 antagonist. Moreover, eriodictyol induced antinociception in the intraplantar capsaicin test, with maximal inhibition of 49±10 and 64±4% for oral (ID(50)=2.3; 1.1-5.7mg/kg) and intrathecal (ID(50)=2.2; 1.7-2.9nmol/site) administration, respectively. Eriodictyol did not induce any change in body temperature or locomotor activity. Orally administered eriodictyol (4.5mg/kg) prevented the nociception induced by intrathecal injections of capsaicin, as well as the non-protein thiol loss and 3-nitrotyrosine (3-NT) formation induced by capsaicin in spinal cord. Eriodictyol also reduced the thermal hyperalgesia and mechanical allodynia elicited by complete Freund's adjuvant (CFA) paw injection. In conclusion, eriodictyol acts as an antagonist of the TRPV1 receptor and as an antioxidant; it induces antinociception without some of the side effects and limitations such as hyperthermia that are expected for TRPV1 antagonists.