While cannabinoid receptor agonists have analgesic activity in chronic pain states, they produce a spectrum of central CB₁receptor‐mediated motor and psychotropic side effects. The actions of endocannabinoids, such as anandamide are terminated by removal from the extracellular space, then subsequent enzymatic degradation by fatty‐acid amide hydrolase (FAAH). In the present study, we compared the effect of a selective FAAH inhibitor, URB597, to that of a pan‐cannabinoid receptor agonist HU210 in rat models of chronic inflammatory and neuropathic pain. Systemic administration of URB597 (0.3 mg kg⁻¹) and HU210 (0.03 mg kg⁻¹) both reduced the mechanical allodynia and thermal hyperalgesia in the CFA model of inflammatory pain. In contrast, HU210, but not URB597, reduced mechanical allodynia in the partial sciatic nerve‐ligation model of neuropathic pain. HU210, but not URB597, produced a reduction in motor performance in unoperated rats. The effects of URB597 in the CFA model were dose dependent and were reduced by coadministration with the cannabinoid CB₁antagonist AM251 (1 mg kg⁻¹), or the CB₂and SR144528 (1 mg kg⁻¹). Coadministration with AM251 plus SR144528 completely reversed the effects of URB597. These findings suggest that the FAAH inhibitor URB597 produces cannabinoid CB₁and CB₂receptor‐mediated analgesia in inflammatory pain states, without causing the undesirable side effects associated with cannabinoid receptor activation. British Journal of Pharmacology(2006)147, 281–288. doi:10.1038/sj.bjp.0706510

The actions of the endogenous ORL₁‐receptor (opioid receptor‐like₁) ligand, nociceptin, on the membrane properties of rat dorsal raphe nucleus neurones were examined by use of whole‐cell patch clamp recording in brain slices. Nociceptin produced an outward current in all neurones tested, with an EC₅₀ of 12 ± 2 nM. Dynorphin A (100 nM to 1 μm) produced little outward current. Outward currents reversed polarity near the predicted K⁺ equilibrium potential in both 2.5 mM (measured/predicted = −105 mV/ −104 mV) and 6.5 mM (measured/predicted = −80 mV/ −77 mV) extracellular K₊. The conductance increase was larger between −120 and −130 mV than between −70 and −80 mV, demonstrating that the nociceptin‐induced K current was due to an increased inwardly rectifying K conductance. The outward current produced by nociceptin was similar to, and occluded by, high concentrations of baclofen, demonstrating actions on the same population of K channels. Naloxone (1 μm) failed to inhibit outward currents produced by nociceptin. These results are consistent with the reported high density of ORL₁ receptor mRNA in dorsal raphe nucleus and with inhibitory actions of nociceptin in cells expressing ORL₁.

In this study we have examined the effects of nociceptin, an endogenous ligand for the opioid‐like receptor ORL₁, on the membrane properties of rat locus coeruleus (LC) neurones in vitro, using intracellular and whole cell patch clamp recording. When locus coeruleus neurones were voltage clamped to −60 mV, application of nociceptin caused an outward current in all cells examined (n = 49), with an EC₅₀ of 90 nM. Neither the potency nor the maximal effect of nociceptin was altered in the presence of the peptidase inhibitors, bestatin (20 μm) or thiorphan (2 μm). The outward currents caused by nociceptin in 2.5 mM extracellular K⁺ reversed polarity at −123 mV, more negative than the predicted K⁺ reversal potential of −105 mV. Increasing extracellular K⁺ to 6.5 mM resulted in a shift of the reversal potential of +25 mV, a shift consistent with a K⁺ conductance. The conductance activated by nociceptin showed mild inward rectification. Application of a high concentration of nociceptin (3 μm) occluded the current produced by simultaneous application of high concentrations of Met‐enkephalin (10 μm), (3 μm) somatostatin and UK 14304 (3 μm), indicating that nociceptin activated the same conductance as μ‐opioid and somatostatin receptors and α₂‐adrenoceptors. The actions of nociceptin were weakly antagonized by the opioid antagonist, naloxone, with pK_b′s estimated from 2 cells of −4.23 and −4.33. The μ‐opioid antagonist, CTAP (D‐Phe‐Cys‐Tyr‐D‐Trp‐Arg‐Pen‐Thr‐NH₂, 1 μm), the opioid antagonist, nalorphine (30 μm), or the somatostatin antagonist, CPP (cyclo(7‐aminoheptanoyl‐Phe‐D‐Tip‐Lys‐Thr[Bzl]) 3 μm) did not affect the nociceptin‐induced current. Dynorphin A (3 μm), another putative endogenous ligand for ORL₁, caused a robust outward current in locus coeruleus neurones that was, however, completely antagonized by moderate concentrations of naloxone (300 nM‐1 μm). Continuous application of nociceptin (3 μm) resulted in a decrease of the outward current to a steady level of 70% of the maximum response with a t_1/2 of 120s. Desensitization was largely homologous because simultaneous application of Met‐enkephalin (30 μm) during the desensitized period of the nociceptin response resulted in an outward current that was 92% of control responses to Met‐enkephalin in the same cells. Conversely, continuous application of Met‐enkephalin (30 μm) resulted in a decrease of Met‐enkephalin current to a steady level that was 54% of the initial current. During this desensitized period application of nociceptin (3 μm) resulted in a current that was 78% of the control responses to nociceptin in the same cells. Thus nociceptin potently activates an inwardly rectifying K⁺ conductance in locus coeruleus neurones, with a pharmacological profile consistent with activation of the ORL₁ receptor. Dynorphin A does not appear to be a ligand for ORL₁ in rat locus coeruleus neurones.