Heriberto P. Mata

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 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.

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.

Cardiovascular Effects of Sevoflurane Compared with Those of Isoflurane in Volunteers

Background Sevoflurane is a new inhalational anesthetic with desirable clinical properties. In some clinical situations, an understanding of the detailed cardiovascular properties of an anesthetic is important, so the authors evaluated the hemodynamic effects of sevoflurane in healthy volunteers not undergoing surgery.

CB2 cannabinoid receptor mediation of antinociception

Abstract Management of acute pain remains a significant clinical problem. In preclinical studies, CB2 cannabinoid receptor‐selective agonists inhibit nociception without producing central nervous system side effects. The CB2 receptor‐selective agonist AM1241 produces antinociceptive effects that are antagonized by CB2, but not CB1, receptor‐selective antagonists, suggesting that activation of CB2 receptors results in antinociception. However, it has not been possible to definitively demonstrate that these effects are mediated by CB2 receptors, because we have lacked the pharmacological tools to confirm the in vivo receptor selectivity of the antagonists used. Further, recent evidence for cannabinoid‐like receptors beyond CB1 and CB2 raises the possibility that AM1241 exerts its antinociceptive effects at uncharacterized CB2‐like receptors that are also inhibited by AM630. The experiments reported here further test the hypothesis that CB2 receptor activation inhibits nociception. They evaluated the antinociceptive actions of AM1241 and the less‐selective CB2 receptor agonist WIN55,212‐2 in wild‐type Symbol mice and in mice with genetic disruption of the CB2 receptor (Symbol mice). AM1241 inhibited thermal nociception in Symbol mice, but had no effect in Symbol littermates. WIN55,212‐2 produced equivalent antinociception in Symbol and Symbol mice, while its antinociceptive effects were reduced in Symbol compared to Symbol mice. The effects of morphine were not altered in Symbol compared to Symbol mice. These data strongly suggest that AM1241 produces antinociception in vivo by activating CB2 cannabinoid receptors. Further, they confirm the potential therapeutic relevance of CB2 cannabinoid receptors for the treatment of acute pain. Figure. No Caption available. Figure. No Caption available. Figure. No Caption available. Figure. No Caption available. Figure. No Caption available. Figure. No Caption available. Figure. No Caption available. Figure. No Caption available. Figure. No Caption available. Figure. No Caption available.

Regular exercise reverses sensory hypersensitivity in a rat neuropathic pain model: Role of endogenous opioids

Background:Exercise is often prescribed as a therapy for chronic pain. Short-term exercise briefly increases the production of endogenous analgesics, leading to transient antinociception. In limited studies, exercise produced sustained increases in endogenous opioids, sustained analgesia, or diminished measures of chronic pain. This study tests the hypothesis that regular aerobic exercise leads to sustained reversal of neuropathic pain by activating endogenous opioid-mediated pain modulatory systems. Methods:After baseline measurements, the L5 and L6 spinal nerves of male Sprague–Dawley rats were tightly ligated. Animals were randomized to sedentary or 5-week treadmill exercise–trained groups. Thermal and tactile sensitivities were assessed 23 h after exercise, using paw withdrawal thresholds to von Frey filaments and withdrawal latencies to noxious heat. Opioid receptor antagonists were administered by subcutaneous, intrathecal, or intracerebroventricular injection. Opioid peptides were quantified using immunohistochemistry with densitometry. Results:Exercise training ameliorated thermal and tactile hypersensitivity in spinal nerve–ligated animals within 3 weeks. Sensory hypersensitivity returned 5 days after discontinuation of exercise training. The effects of exercise were reversed by using systemically or intracerebroventricularly administered opioid receptor antagonists and prevented by continuous infusion of naltrexone. Exercise increased &bgr;-endorphin and met-enkephalin content in the rostral ventromedial medulla and the mid-brain periaqueductal gray area. Conclusions:Regular moderate aerobic exercise reversed signs of neuropathic pain and increased endogenous opioid content in brainstem regions important in pain modulation. Exercise effects were reversed by opioid receptor antagonists. These results suggest that exercise-induced reversal of neuropathic pain results from an up-regulation of endogenous opioids.

The cannabinoid agonist WIN55,212-2 suppresses opioid-induced emesis in ferrets.

Background Cannabinoid receptor agonists reverse nausea and vomiting produced by chemotherapy and radiation therapy in animals and humans but have not been tested against opioid-induced emesis. This study tests the hypothesis that cannabinoid receptor agonists will prevent opioid-induced vomiting. Methods Twelve male ferrets were used. They weighed 1.2–1.6 kg at the beginning and 1.8–2.3 kg at the end of the experiments. All drugs were injected subcutaneously. WIN55,212-2, a mixed CB1–CB2 cannabinoid receptor agonist, was administered 25 min before morphine. Retches and vomits were counted at 5-min intervals for 30 min after morphine injection. Results Retching and vomiting responses increased with increasing morphine doses up to 1.0 mg/kg, above which the responses decreased. Previous administration of naloxone prevented morphine-induced retching and vomiting. WIN55,212-2 dose-dependently reduced retching and vomiting. The ED50 was 0.05 mg/kg for retches and 0.03 mg/kg for vomits. At 0.13 mg/kg, retching decreased by 76% and vomiting by 92%. AM251, a CB1 receptor-selective antagonist, blocked the antiemetic actions of WIN55,212-2, but AM630, a CB2 receptor-selective antagonist, did not. Conclusions These results demonstrate that WIN55,212-2 prevents opioid-induced vomiting and suggest that the antiemetic activity of WIN55,212-2 occurs at CB1 receptors. This is consistent with findings that CB1 receptors are the predominant cannabinoid receptors in the central nervous system and that antiemetic effects of cannabinoids appear to be centrally mediated.

Further examination of the selective toxicity of CCl4 in rat liver slices

Lipid peroxidation and loss of enzymes located predominantly in either periportal or centrilobular hepatocytes were investigated in precision-cut liver slices from male Sprague-Dawley rats. Pretreatment of animals with 80 mg/kg phenobarbital for the site-specific enzyme studies enhanced and accelerated CCl4 toxicity in slices resulting from increased radical formation. Liver slices were exposed to 0.57 mM CCl4 by vaporization using a roller incubation system at 37 degrees C for a total of 9 hr. Conjugated diene formation, an index of lipid peroxidation, was detected 15 min following CCl4 administration and increased over time. Loss of cytochrome P450 occurred in a time-dependent manner relative to controls where levels in treated slices were 42% of controls at 9 hr. A 48-hr fast prior to termination increased intracellular K+ leakage relative to that present in slices from fed animals. Significant leakage of glucose-6-phosphate dehydrogenase and beta-glucuronidase from centrilobular hepatocytes occurred 9 hr following CCl4 administration. The content of the periportal enzymes (lactate dehydrogenase and sorbitol dehydrogenase) was unchanged in the same slices over the duration of the experiment. Reduction of 3-(4,5-dimethylthiazol-2-yl)-2,5- diphenyltetrazolium bromide, a mitochondrial selective dye and indicator of viability, was significantly lower in treated slices from phenobarbital-treated animals at 9 hr relative to controls. These studies demonstrate that precision-cut slices are an ideal in vitro system for mechanistic studies and the investigation of site-specific toxicants since the integral architecture of the liver and cellular identity are maintained.

Spinal NK‐1 receptor‐expressing neurons and descending pathways support fentanyl‐induced pain hypersensitivity in a rat model of postoperative pain

The clinically important opioid fentanyl, administered acutely, enhances mechanical hypersensitivity in a model of surgical pain induced by plantar incision. Activity of neurokinin‐1 (NK‐1) receptor‐expressing ascending spinal neurons, descending pathways originating in the rostral ventromedial medulla (RVM), and spinal dynorphin are necessary for the development and maintenance of hyperalgesia during sustained morphine exposure, suggesting that these mechanisms may also be important in opioid enhancement of surgical pain. Therefore, we examined the roles of these mechanisms in sensory hypersensitivity produced by acute fentanyl administration in rats not undergoing surgical incision and in rats undergoing plantar incision. In non‐operated rats, fentanyl induced analgesia followed by immediate and long‐lasting sensory hypersensitivity, as previously described. Fentanyl also enhanced pain sensitivity induced by plantar incision. Ablation of NK‐1‐expressing spinal neurons by pre‐treatment with substance P–Saporin reduced sensory hypersensitivity in fentanyl‐treated rats and, to a lesser extent, in fentanyl‐treated rats with a surgical incision. Microinjection of lidocaine into the RVM completely reversed fentanyl‐induced sensory hypersensitivity and fentanyl enhancement of incision‐induced sensory hypersensitivity. RVM lidocaine injection resulted in a slight reduction of incision‐induced sensory hypersensitivity in the absence of fentanyl pre‐treatment. Spinal dynorphin content increased by 30 ± 7% and 66 ± 17% in fentanyl‐ and fentanyl/incision‐treated rats. Spinal administration of antiserum to dynorphin attenuated sensory hypersensitivity in fentanyl‐treated rats. These data support a partial role of NK‐1 receptor‐containing ascending pathways and a crucial role of descending facilitatory pathways in fentanyl‐induced hyperalgesia and in the enhanced hyperalgesia produced by fentanyl treatment following surgical incision.