The Effects of the alpha2-Adrenergic Agonist, Dexmedetomidine, in the Spinal Nerve Ligation Model of Neuropathic Pain in Rats

Cannabigerol (CBG) attenuates mechanical hypersensitivity elicited by chemotherapy-induced peripheral neuropathy

BACKGROUND

Cannabigerol (CBG) is a non-psychoactive phytocannabinoid produced by the plant Cannabis sativa with affinity to various receptors involved in nociception. As a result, CBG is marketed as an over-the-counter treatment for many forms of pain. However, there is very little research-based evidence for the efficacy of CBG as an anti-nociceptive agent.

METHODS

To begin to fill this knowledge gap, we assessed the anti-nociceptive effects of CBG in C57BL/6 mice using three different models of pain; cisplatin-induced peripheral neuropathy, the formalin test, and the tail-flick assay.

RESULTS

Using the von Frey test, we found that CBG-attenuated mechanical hypersensitivity evoked by cisplatin-induced peripheral neuropathy in both male and female mice. Additionally, we observed that this CBG-induced reduction in mechanical hypersensitivity was attenuated by the α₂ -adrenergic receptor antagonist atipamezole (3 mg/kg, i.p.) and the CB₁ R antagonist, AM4113 (3 mg/kg, i.p.), and blocked by the CB₂ R antagonist/inverse agonist, SR144528 (10 mg/kg, i.p.). We found that the TRPV1 antagonist, SB705498 (20 mg/kg, i.p.) was unable to prevent CBG actions. Furthermore, we show that CBG:CBD oil (10 mg/kg, i.p.) was more effective than pure CBG (10 mg/kg) at reducing mechanical hypersensitivity in neuropathic mice. Lastly, we show that pure CBG and CBG:CBD oil were ineffective at reducing nociception in other models of pain, including the formalin and tail flick assays.

CONCLUSIONS

Our findings support the role of CBG in alleviating mechanical hypersensitivity evoked by cisplatin-induced peripheral neuropathy, but highlight that these effects may be limited to specific types of pain.

SIGNIFICANCE

There are few effective treatments for neuropathic pain and neuropathic pain is projected to increase with the aging population. We demonstrate that CBG (cannabigerol) and CBG:CBD oil attenuate neuropathy-induced mechanical hypersensitivity mice. Second, we identify receptor targets that mediate CBG-induced reduction in mechanical hypersensitivity in neuropathic mice. Third, we demonstrate that an acute injection of CBG is anti-nociceptive specifically for neuropathic pain rather than other forms of pain, including persistent pain and thermal pain.

Descending antinociception induced by secondary somatosensory cortex stimulation in experimental neuropathy: role of the medullospinal serotonergic pathway

Stimulation of the secondary somatosensory cortex (S2) has attenuated pain in humans and inflammatory nociception in animals. Here we studied S2 stimulation-induced antinociception and its underlying mechanisms in an experimental animal model of neuropathy induced by spinal nerve ligation (SNL). Effect of S2 stimulation on heat-evoked limb withdrawal latency was assessed in lightly anesthetized rats that were divided into three groups based on prior surgery and monofilament testing before induction of anesthesia: 1) sham-operated group and 2) hypersensitive and 3) nonhypersensitive (mechanically) SNL groups. In a group of hypersensitive SNL animals, a 5-HT_1A receptor agonist was microinjected into the rostroventromedial medulla (RVM) to assess whether autoinhibition of serotonergic cell bodies blocks antinociception. Additionally, effect of S2 stimulation on pronociceptive ON-cells and antinociceptive OFF-cells in the RVM or nociceptive spinal wide dynamic range (WDR) neurons were assessed in anesthetized hypersensitive SNL animals. S2 stimulation induced antinociception in hypersensitive but not in nonhypersensitive SNL or sham-operated animals. Antinociception was prevented by a 5-HT_1A receptor agonist in the RVM. Antinociception was associated with decreased duration of heat-evoked response in RVM ON-cells. In spinal WDR neurons, heat-evoked discharge was delayed by S2 stimulation, and this antinociceptive effect was prevented by blocking spinal 5-HT_1A receptors. The results indicate that S2 stimulation suppresses nociception in SNL animals if SNL is associated with tactile allodynia-like hypersensitivity. In hypersensitive SNL animals, S2 stimulation induces antinociception mediated by medullospinal serotonergic pathways acting on the spinal 5-HT_1A receptor, and partly through reduction of the RVM ON-cell discharge.NEW & NOTEWORTHY Stimulation of S2 cortex, but not that of an adjacent cortical area, induced descending heat antinociception in rats with the spinal nerve ligation-induced model of neuropathy. Antinociception was bilateral, and it involved suppression of pronociceptive medullary cells and activation of serotonergic pathways that act on the spinal 5-HT_1A receptor. S2 stimulation failed to induce descending antinociceptive effect in sham-operated controls or in nerve-ligated animals that had not developed mechanical hypersensitivity.

High-dose remifentanil prevents development of thermal hyperalgesia in a neuropathic pain model

BACKGROUND

Intraoperative nerve lesions can lead to chronic postoperative pain. There are conflicting data as to whether or not anaesthetics administered intraoperatively are beneficial. We investigated if remifentanil administered at the time of nerve injury was able to attenuate neuropathic hypersensitivity.

METHODS

Rats were anaesthetized with isoflurane, endotracheally intubated, and a tail vein catheter was inserted. Rats received an i.v. infusion of either saline or low- or high-dose remifentanil (2 or 20 μg kg(-1) min(-1), respectively) for 20 min. During this time, rats received a spinal nerve L5 transection to induce neuropathic pain or a sham procedure. Behavioural tests to assess mechanical and cold allodynia and heat hyperalgesia were performed on postoperative days 1, 3, 7, 14, 21, and 28.

RESULTS

Sham-operated animals exhibited no hypersensitivity regardless of the intraoperative remifentanil dose. In rats which received spinal nerve L5 transection, mechanical and cold allodynia developed with no significant differences between treatment groups. However, thermal hyperalgesia was reduced in rats given high-dose remifentanil: mean (standard deviation) area under the curve 426 (53) compared with 363 (34) and 342 (24) in saline or low-dose remifentanil treated rats, respectively (P<0.05).

CONCLUSIONS

High-dose remifentanil administered at the time of transection of the spinal nerve at L5 prevents subsequent thermal hyperalgesia.

Attenuation of pain-related behavior evoked by injury through blockade of neuropeptide Y Y2 receptor

Neuropeptide Y (NPY) has an important but still insufficiently defined role in pain modulation. We therefore examined the ability of NPY to modulate experimentally induced neuropathic pain by injecting it directly into dorsal root ganglion (DRG) immediately following spinal nerve ligation (SNL) injury. We have found that this application exacerbates pain-related behavior induced by SNL in a modality-specific fashion. When saline was injected after SNL, the expected increase in hyperalgesia responses to needle stimulation was present on the 8th postoperative day. When we injected NPY, hyperalgesic responses were increased in a manner similar to the SNL/saline group. To characterize NPY action, specific Y1 and Y2 antagonists were also delivered directly to DRG, which revealed that behavioral actions of NPY were abolished by Y2 receptor antagonist. We tested whether NPY effects were the result of its role in immunity by immunohistochemical staining for glial fibrillary acidic protein, in order to identify activation of DRG satellite cells and dorsal horn astrocytes. Exacerbation of pain-related behavior following NPY injection was accompanied by astrocyte activation in ipsilateral dorsal horn and with satellite cells activation in the DRG proximal to injury. This activation was reduced following Y2 receptor antagonist application. These findings indicate an important link between pain-related behavior and neuroimmune activation by NPY through its Y2 receptor.

Electrophysiologic characterization of the antinociceptive actions of S18616, a novel and potent alpha2-adrenoceptor agonist, after acute and persistent pain states

alpha (2)-Adrenoceptor (AR) agonists are active in behavioral models of persistent pain involving tissue and nerve damage. We evaluated the spinal effect of a novel, potent, and selective alpha (2)-AR agonist, [7,8](2-chlorobenzo)-2-amino-1-aza-3-oxa[4,5]spirodeca-1,7-diene (S18616), on the responses of dorsal horn neurons in halothane-anesthetized rats. Intrathecal administration of S18616 (0.1 to 3.0 microg) dose-dependently suppressed C- and A delta-fiber evoked responses but not the A beta-fiber evoked response. Drug effects were reversed by the alpha (2)-AR antagonists, atipamezole and idazoxan (100 microg). In rats with unilateral spinal nerve (L5-L6) ligation performed 2 weeks before study, S18616 (0.1 to 3.0 microg) dose-dependently suppressed the C- and A delta-fiber evoked responses and blocked "wind-up" in these neurons. The potency was comparable between nerve-injured and sham-operated rats, and S18616 was equally effective against responses to thermal and high-intensity mechanical stimuli. Interestingly, the effectiveness of S18616 on the low-intensity mechanical evoked response was significantly enhanced after nerve injury. Finally, S18616 (0.3 and 3.0 microg) reduced the neuronal responses produced by intraplantar injection of formalin. In conclusion, S18616 dose-dependently and potently inhibits the responses of dorsal horn neurons to peripheral stimulation in normal, inflamed, and neuropathic rats. These data support the use of spinal S18616 and other alpha (2)-AR agonists in the management of clinical pain.