Antihyperalgesic effects of a novel muscarinic agonist (LASSBio-873) in spinal nerve ligation in rats

Role of Muscarinic Receptors in Hypoalgesia Induced by Crocin in Neuropathic Pain Rats

Objective

Crocin as an important constituent of saffron has antineuropathic pain properties; however, the exact mechanism of this effect is not known. The aim of this study was whether the hypoalgesic effect of crocin can be exerted through muscarinic receptors.

Materials and Methods

In the present project, 36 male Wistar rats (200 ± 20 g) were used. Animals randomly divided into six groups (sham, neuropathy, neuropathy + crocin, neuropathy + atropine 0.5 mg/kg, neuropathy + atropine 1 mg/kg, and neuropathy + atropine 1 mg/kg + crocin). Neuropathy was induced by the chronic constriction injury (CCI) method on the sciatic nerve. Crocin and atropine was administered intraperitoneally during 14 days following the 14^th day after surgery. Pain response was detected every three days, two hours after each injection and 3 days following last injection. Mechanical allodynia and thermal hyperalgesia were detected using the Von Frey filaments and plantar test device, respectively.

Results

CCI significantly reduced the paw withdrawal response to mechanical and thermal stimulus (P < 0.01 and P < 0.05, respectively). Crocin therapy significantly reduced mechanical allodynia and thermal hyperalgesia induced by CCI (P < 0.05). Atropine pretreatment significantly blocked the hypoalgesic effect of crocin (P < 0.05 in mechanical allodynia and P < 0.01 in thermal hyperalgesia). Fourteen days administration of atropine alone at a dose of 0.5 mg/kg but not 1 mg/kg significantly reduced CCI-induced mechanical allodynia at day 30 after surgery.

Conclusion

Crocin significantly decreased CCI-induced neuropathic pain. The hypoalgesic effect of crocin was blocked by atropine pretreatment, which indicates an important role for muscarinic receptors in the effect of crocin.

Antinociception induced by a novel α2A adrenergic receptor agonist in rodents acute and chronic pain models

The mechanisms and antinociceptive effects of a novel α_2A adrenoceptor agonist, 3-(2-chloro-6-fluorobenzil)-imidazolinide-2,4-dione (PT-31) were investigated using animal models of acute and chronic pain. The effects of PT-31 on pain responses were examined using hot plate and formalin tests in mice and spinal nerve ligation (SNL)-induced hyperalgesia in rats. The effects of antagonists acting on α adrenoceptor were assessed to investigate the interaction of these pathways upon PT-31 induced antinociception. PT-31 effects on motor activity/skills and on hemodynamic parameters were also evaluated. PT-31 had dose-dependent antinociception effects on hot-plate and formalin-injection induced pain responses. Thermal hyperalgesia and mechanical allodynia were reduced following a 7 d treatment with PT-31 (1, 5, and 10mg/kg/d, p.o.), and those effects were attenuated by yohimbine (5mg/kg), atropine (2mg/kg), L-nitro arginine methyl ester (L-NAME; 30mg/kg), or naloxone (2mg/kg). In contrast to clonidine, PT-31 did not have locomotor or hemodynamic effects in rats. The present results suggest that PT-31 represents a candidate for pain treatment with advantages over clonidine, namely no locomotor or hemodynamic impairments.

Antinociceptive effects of hydroalcoholic extract from Euterpe oleracea Mart. (Açaí) in a rodent model of acute and neuropathic pain

Background

Plants rich in flavonoids, such as açaí (Euterpe oleraceae Mart.), can induce antinociception in experimental animals. Here, we tested an extract obtained from the stones of açaí fruits (açaí stone extract, ASE), a native plant from the Amazon region of Brazil, in models of acute/inflammatory and chronic pain.

Methods

Antinociceptive effects of ASE were evaluated in the hot plate, formalin, acetic acid writhing, carrageenan, and neuropathic pain models, as well as in thermal hyperalgesia and mechanical allodynia models induced by spinal nerve ligation. Antinociceptive activities were modulated by the administration of cholinergic, adrenergic, opioid, and L-arginine-NO antagonists.

Results

Oral administration of ASE (30, 100, or 300 mg.kg⁻¹) dose-dependently reduced nociceptive responses to acute/inflammatory pain in mice, including thermal hyperalgesia, acetic acid-induced writhing, and carrageenan-induced thermal hyperalgesia. Moreover, ASE reduced the neurogenic and inflammatory phases after intraplantar injection of formalin in mice. The antinociceptive effect of ASE (100 mg · kg⁻¹) in a hot plate protocol, was inhibited by pre-treatment with naloxone (1 mg · kg⁻¹), atropine (2 mg · kg⁻¹), yohimbine (5 mg · kg⁻¹), or L-NAME (30 mg · kg⁻¹). Furthermore, ASE prevented chronic pain in a rat spinal nerve ligation model, including thermal hyperalgesia and mechanical allodynia.

Conclusion

ASE showed significant antinociceptive effect via a multifactorial mechanism of action, indicating that the extract may be useful in the development of new analgesic drugs.