T. Philip Malan

Activation of CB2 cannabinoid receptors by AM1241 inhibits experimental neuropathic pain: pain inhibition by receptors not present in the CNS.

We designed AM1241, a selective CB2 cannabinoid receptor agonist, and used it to test the hypothesis that CB2 receptor activation would reverse the sensory hypersensitivity observed in neuropathic pain states. AM1241 exhibits high affinity and selectivity for CB2 receptors. It also exhibits high potency in vivo. AM1241 dose-dependently reversed tactile and thermal hypersensitivity produced by ligation of the L5 and L6 spinal nerves in rats. These effects were selectively antagonized by a CB2 but not by a CB1 receptor antagonist, suggesting that they were produced by actions of AM1241 at CB2 receptors. AM1241 was also active in blocking spinal nerve ligation-induced tactile and thermal hypersensitivity in mice lacking CB1 receptors (CB1-/- mice), confirming that AM1241 reverses sensory hypersensitivity independent of actions at CB1 receptors. These findings demonstrate a mechanism leading to the inhibition of pain, one that targets receptors localized exclusively outside the CNS. Further, they suggest the potential use of CB2 receptor-selective agonists for treatment of human neuropathic pain, a condition currently without consistently effective therapies. CB2 receptor-selective agonist medications are predicted to be without the CNS side effects that limit the effectiveness of currently available medications.

CB2 cannabinoid receptor-mediated peripheral antinociception

&NA; Cannabinoid receptor agonists diminish responses to painful stimuli. Extensive evidence implicates the CB1 receptor in the production of antinociception. However, the capacity of CB2 receptors, which are located outside the central nervous system (CNS), to produce antinociception is not known. Using AM1241, a CB2 receptor‐selective agonist, we demonstrate that CB2 receptors produce antinociception to thermal stimuli. Injection of AM1241 in the hindpaw produced antinociception to a stimulus applied to the same paw. Injection of an equivalent dose of AM1241 into the paw contralateral to the side of testing did not. The antinociceptive actions of AM1241 were blocked by the CB2 receptor‐selective antagonist AM630, but not by the CB1 receptor‐selective antagonist AM251. AM1241 also produced antinociception when injected systemically (intraperitoneally). The antinociceptive actions of systemic AM1241 were blocked by injection of AM630 into the paw where the thermal stimulus was applied, but not the contralateral paw. These findings demonstrate the local, peripheral nature of CB2 cannabinoid antinociception. AM1241 did not produce the CNS cannabinoid effects of hypothermia, catalepsy, inhibition of activity or impaired ambulation, while this tetrad of effects was produced by the mixed CB1/CB2 receptor agonist WIN55,212‐2. Peripheral antinociception without CNS effects is consistent with the peripheral distribution of CB2 receptors. CB2 receptor agonists may have promise clinically for the treatment of pain without CNS cannabinoid side effects.

Spinal and Supraspinal Mechanisms of Neuropathic Pain

Abstract: Neuropathic pain is associated with abnormal tactile and thermal responses that may be extraterritorial to the injured nerve. Importantly, tactile allodynia and thermal hyperalgesia may involve separate pathways, since complete and partial spinal cord lesions have blocked allodynia, but not hyperalgesia, after spinal nerve ligation (SNL). Furthermore, lesions of the dorsal column, and lidocaine microinjected into dorsal column nuclei block only tactile allodynia. Conversely, thermal hyperalgesia, but not tactile allodynia was blocked by desensitizatin of C‐fibers with resiniferotoxin. Therefore, it seems that tactile allodynia is likely to be mediated by large diameter Aβ fibers, and not susceptible to modulation by spinal opioids, whereas hyperalgesia is mediated by unmyelinated C‐fibers, and is sensitive to blockade by spinal opioids. Additionally, abnormal, spontaneous afferent drive in neuropathic pain may contribute to NMDA‐mediated central sensitization by glutamate and by non‐opioid actions of spinal dynorphin. Correspondingly, SNL elicited elevation in spinal dynorphin content in spinal segments at and adjacent to the zone of entry of the injured nerve along with signs of neuropathic pain. Antiserum to dynorphin A(1–17) or MK‐801 given spinally blocked thermal hyperalgesia, but not tactile allodynia, after SNL, and also restored diminished morphine antinociception. Finally, afferent drive may induce descending facilitation from the rostroventromedial medulla (RVM). Blocking afferent drive with bupivicaine also restored lost potency of PAG morphine, as did CCK antagonists in the RVM. This observation is consistent with afferent drive activating descending facilitation from the RVM, and thus diminishing opioid activity, and may underlie the clinical observation of limited responsiveness of neuropathic pain to opioids

Spinal GABA(A) and GABA(B) receptor pharmacology in a rat model of neuropathic pain.

Background This study tests the hypothesis that loss of spinal activity of &ggr;-aminobutyric acid (GABA) contributes to the allodynia and hyperalgesia observed after peripheral nerve injury. Methods Intrathecal catheters were implanted in male Sprague-Dawley rats. Antinociception was assessed by measuring withdrawal latency to immersion of the tail in a 52°C water bath. Nerve injury was produced by ligation of the L5 and L6 spinal nerves. Testing was performed 4–14 days after spinal nerve ligation, when tactile allodynia and thermal hyperalgesia were established. Tactile allodynia was quantitated using the threshold to withdrawal of the hind paw on probing with von Frey filaments. Thermal hyperalgesia was quantitated using the latency to withdrawal of the hind paw from radiant heat. Motor function was tested using a rotarod apparatus. Results Spinal administration of the GABAA receptor antagonist bicuculline or the GABAB receptor antagonist phaclofen produced tactile allodynia and thermal hyperalgesia in normal rats. The GABAB receptor agonist baclofen, administered spinally, produced antinociception in the tail-flick test, whereas the GABAA receptor agonist isoguvacine did not. Isoguvacine and baclofen each reversed tactile allodynia and thermal hyperalgesia produced by spinal nerve ligation. Baclofen but not isoguvacine prolonged thermal withdrawal latency in nerve-injured rats beyond preoperative values. Baclofen but not isoguvacine impaired motor function. Conclusions Pharmacologic inhibition of intrinsic GABA tone in normal rats resulted in tactile allodynia and thermal hyperalgesia, consistent with the hypothesis being tested. Exogenous administration of GABA agonists reversed spinal nerve ligation–induced allodynia and hyperalgesia, also consistent with this hypothesis. Isoguvacine produced specific antihyperalgesic and antiallodynic effects, whereas assessment of the effects of baclofen was complicated by motor dysfunction. Spinal GABAA agonists may provide a specific therapy for neuropathic pain.

Lack of involvement of capsaicin-sensitive primary afferents in nerve-ligation injury induced tactile allodynia in rats

Tactile allodynia and thermal hyperalgesia, two robust signs of neuropathic pain associated with experimental nerve injury, have been hypothesized to be mechanistically distinguished based on (a) fiber types which may be involved in the afferent input, (b) participation of spinal and supraspinal circuitry in these responses, and (c) sensitivity of these endpoints to pharmacological agents. Here, the possibility that nerve-injury induced tactile allodynia and thermal hyperalgesia may be mediated via different afferent fiber input was tested by evaluating these responses in sham-operated or nerve-injured (L5/L6) rats before or after a single systemic injection of resiniferatoxin (RTX), an ultrapotent analogue of the C-fiber specific neurotoxin, capsaicin. Tactile allodynia, and three measures of thermal nociception, tail-flick, paw-flick and hot-plate responses, were determined before and at various intervals for at least 40 days after RTX injection. Nerve-injured, but not sham-operated, rats showed a long-lasting tactile allodynia and thermal hyperalgesia (paw-flick) within 2-3 days after surgery; responses to other noxious thermal stimuli (i.e., tail-flick and hot-plate tests) did not distinguish the two groups at the stimulus intensities employed. RTX treatment resulted in a significant and long-lasting (i.e. essentially irreversible) decrease in sensitivity to thermal noxious stimuli in both sham-operated and nerve-injured rats; thermal hyperalgesia was abolished and antinociception produced by RTX. In contrast, RTX treatment did not affect the tactile allodynia seen in the same nerve-injured rats. These data support the concept that thermal hyperalgesia seen after nerve ligation, as well as noxious thermal stimuli, are likely to be mediated by capsaicin-sensitive C-fiber afferents. In contrast, nerve-injury related tactile allodynia is insensitive to RTX treatment which clearly desensitizes C-fibers and, therefore such responses are not likely to be mediated through C-fiber afferents. The hypothesis that tactile allodynia may be due to inputs from large (i.e. A beta) afferents offers a mechanistic basis for the observed insensitivity of this endpoint to intrathecal morphine in this nerve-injury model. Further, these data suggest that clinical treatment of neuropathic pains with C-fiber specific agents such as capsaicin are unlikely to offer significant therapeutic benefit against mechanical allodynia.

Single intrathecal injections of dynorphin A or des-Tyr-dynorphins produce long-lasting allodynia in rats: blockade by MK-801 but not naloxone.

&NA; Neuropathic pain states are accompanied by increased sensitivity to both noxious and non‐noxious sensory stimuli, characterized as hyperalgesia and allodynia, respectively. In animal models of neuropathic pain, the presence of hyperalgesia and allodynia are accompanied by neuroplastic changes including increased spinal levels of substance P, cholecystokinin (CCK), and dynorphin. N‐Methyl‐ d‐aspartate (NMDA) receptors appear to be involved in maintaining the central sensitivity which contributes to neuropathic pain. In addition to its opioid activities, dynorphin has been suggested to act at the NMDA receptor complex. In an attempt to mimic the increased levels of spinal dynorphin seen in animal models of neuropathic pain, rats received a single intrathecal (i.t.) injection of dynorphin A(1–17), dynorphin A(1–13), dynorphin A(2–17) or dynorphin A(2–13) through indwelling catheters. Tactile allodynia was determined by measuring response threshold to probing with von Frey filaments. Dynorphin A(1–17) administration evoked significant and long‐lasting tactile allodynia (i.e. > 60 days). Likewise, the i.t. administration of dynorphin A(1–13) or dynorphin A(2–17) or dynorphin A(2–13) also produced long‐lasting tactile allodynia. Intrathecal pretreatment, but not post‐treatment, with MK‐801 prevented dynorphin A(1–17)‐induced development of allodynia; i.t. administration of MK‐801 alone had no effect on responses to tactile stimuli. In contrast, i.t. pretreatment with naloxone did not affect the development of tactile allodynia induced by dynorphin A(1–17) or alter sensory threshold when given alone. These results demonstrate that a single dose of dynorphin A, or its des‐Tyr fragments, produces long‐lasting allodynia which may be irreversible in the rat. Further, this effect appears to be mediated through activation of NMDA, rather than opioid, receptors. While the precise mechanisms underlying the development and maintenance of the allodynia is unclear, it seems possible that dynorphin may produce changes in the spinal cord, which may contribute to the development of signs reminiscent of a ‘neuropathic’ state. Given that levels of dynorphin are elevated following nerve injury, it seems reasonable to speculate that dynorphin may have a pathologically relevant role in neuropathic pain states.

Inhibition of inflammatory hyperalgesia by activation of peripheral CB2 cannabinoid receptors.

Background Cannabinoid receptor agonists inhibit inflammatory hyperalgesia in animal models. Nonselective cannabinoid receptor agonists also produce central nervous system (CNS) side effects. Agonists selective for CB2 cannabinoid receptors, which are not found in the CNS, do not produce the CNS effects typical of nonselective cannabinoid receptor agonists but do inhibit acute nociception. The authors used the CB2 receptor–selective agonist AM1241 to test the hypothesis that selective activation of peripheral CB2 receptors inhibits inflammatory hyperalgesia. Methods Rats were injected in the hind paw with carrageenan or capsaicin. Paw withdrawal latencies were measured using a focused thermal stimulus. The effects of peripheral CB2 receptor activation were determined by using local injection of AM1241. CB2 receptor mediation of the actions of AM1241 was shown by using the CB2 receptor–selective antagonist AM630 and the CB1 receptor–selective antagonist AM251. Results AM1241 fully reversed carrageenan-induced inflammatory thermal hyperalgesia when injected into the inflamed paw. In contrast, AM1241 injected into the contralateral paw had no effect, showing that its effects were local. AM1241 also reversed the local edema produced by hind paw carrageenan injection. The effects of AM1241 were reversed by the CB2 receptor–selective antagonist AM630, but not by the CB1 receptor–selective antagonist AM251. AM1241 also inhibited flinching and thermal hyperalgesia produced by hind paw capsaicin injection. Conclusions Local, peripheral CB2 receptor activation inhibits inflammation and inflammatory hyperalgesia. These results suggest that peripheral CB2 receptors may be an appropriate target for eliciting relief of inflammatory pain without the CNS effects of nonselective cannabinoid receptor agonists.

Extraterritorial neuropathic pain correlates with multisegmental elevation of spinal dynorphin in nerve-injured rats

&NA; Neuropathic pain is often associated with the appearance of pain in regions not related to the injured nerve. One mechanism that may underlie neuropathic pain is abnormal, spontaneous afferent drive which may contribute to NMDA‐mediated central sensitization by the actions of glutamate and by the non‐opioid actions of spinal dynorphin. In the present study, injuries to lumbar or sacral spinal nerves elicited elevation in spinal dynorphin content which correlated temporally and spatially with signs of neuropathic pain. The increase in spinal dynorphin content was coincident with the onset of tactile allodynia and thermal hyperalgesia. Injury to the lumbar (L5/L6) spinal nerves produced elevated spinal dynorphin content in the ipsilateral dorsal spinal quadrant at the L5 and L6 spinal segments and in the segments immediately adjacent. Lumbar nerve injury elicited ipsilateral tactile allodynia and thermal hyperalgesia of the hindpaw. In contrast, S2 spinal nerve ligation elicited elevated dynorphin content in sacral spinal segments and bilaterally in the caudal lumbar spinal cord. The behavioral consequences of S2 spinal nerve ligation were also bilateral, with tactile allodynia and thermal hyperalgesia seen in both hindpaws. Application of lidocaine to the site of S2 ligation blocked thermal hyperalgesia and tactile allodynia of the hindpaws suggesting that afferent drive was critical to maintenance of the pain state. Spinal injection of antiserum to dynorphin A(1–17) and of MK‐801 both blocked thermal hyperalgesia, but not tactile allodynia, of the hindpaw after S2 ligation. These data suggest that the elevated spinal dynorphin content consequent to peripheral nerve injury may drive sensitization of the spinal cord, in part through dynorphin acting directly or indirectly on the NMDA receptor complex. Furthermore, extrasegmental increases in spinal dynorphin content may partly underlie the development of extraterritorial neuropathic pain.

CB2 cannabinoid receptor agonists: pain relief without psychoactive effects?

Cannabinoid receptor agonists significantly diminish pain responses in animal models; however, they exhibit only modest analgesic effects in humans. The relative lack of efficacy in man may be because of the dose limitations imposed by psychoactive side effects. Cannabinoid agonists that selectively target CB(2) (peripheral) cannabinoid receptors should be free of psychoactive effects, perhaps allowing for more effective dosing. CB(2) receptor activation inhibits acute, inflammatory and neuropathic pain responses in animal models. In preclinical studies, CB(2) receptor agonists do not produce central nervous system effects. Therefore, they show promise for the treatment of acute and chronic pain without psychoactive effects.

Quantification of the Degradation Products of Sevoflurane in Two CO2 Absorbants during Low-flow Anesthesia in Surgical Patients

Sevoflurane, a new inhalational anesthetic agent has been shown to produce degradation products upon interaction with CO2 absorbants. Quantification of these sevoflurane degradation products during low-flow or closed circuit anesthesia in patients has not been well evaluated. The production of sevof