In the guinea pig ileum myenteric plexus--longitudinal muscle preparation, dynorphin-(1--13) and the prototypical kappa agonist ethylketocyclazocine had equally poor sensitivity to naloxone antagonism and showed selective cross protection in receptor inactivation experiments with the alkylating antagonist beta-chlornaltrexamine. In binding assays with membranes from guinea pig brain, ethylketocyclazocine and dynorphin-(1--13) amide were more potent in displacing tritium-labeled ethylketocyclazocine than in displacing typical mu and delta opioid receptor ligands. In the two preparations studied, the dynorphin receptor appears to be the same as the kappa opioid receptor.

1 Capsazepine is a synthetic analogue of the sensory neurone excitotoxin, capsaicin. The present study shows the capsazepine acts as a competitive antagonist of capsaicin.2 Capsazepine (10 JAM) reversibly reduced or abolished the current response to capsaicin (500 nM) of voltage-clamped dorsal root ganglion (DRG) neurones from rats. In contrast, the responses to 50 JM y-aminobutyric acid (GABA) and S JM adenosine 5'-triphosphate (ATP) were unaffected. 3 The effects of capsazepine were examined quantitatively with radioactive ion flux experiments.Capsazepine inhibited the capsaicin (500 nM)-induced 45Ca2" uptake in cultures of rat DRG neurones with an IC50 of 420 ± 46 nM (mean ± s.e.mean, n = 6). The 45Ca2" uptake evoked by resiniferatoxin (RTX), a potent capsaicin-like agonist was also inhibited. (Log concentration)-effect curves for RTX (0.3 nM-1 JAM) were shifted in a competitive manner by capsazepine. The Schild plot of the data had a slope of 1.08 ± 0.15 (s.e.) and gave an apparent Kd estimate for capsazepine of 220 nM (95% confidence limits, 57-400 nM). 4 Capsazepine also inhibited the capsaicin-and RTX-evoked efflux of 86Rb+ from cultured DRG capsazepine the inhibition by Ruthenium Red (10-500nM in DRG and 0.5-10AM in vagus nerve experiments) was not consistent with a competitive antagonism, but rather suggested a more complex, non-competitive inhibition.

We have examined the effects of cannabinoid agonists on hyperalgesia in a model of neuropathic pain in the rat and investigated the possible sites of action. The antihyperalgesic activity of the cannabinoids was compared with their ability to elicit behavioural effects characteristic of central cannabinoid activity. WIN55,212-2 (0.3-10 mg kg(-1)), CP-55,940 (0.03-1 mg kg(-1)) and HU-210 (0.001-0.03 mg kg(-1)) were all active in a 'tetrad' of tests consisting of tail-flick, catalepsy, rotarod and hypothermia following subcutaneous administration, with a rank order of potency in each of HU-210 > CP-55,940 > WIN55,212-2. The effects of WIN55,212-2 in each assay were blocked by the Cannabinoid1 (CB1) antagonist SR141716A. In the partial sciatic ligation model of neuropathic pain WIN55,212-2, CP-55,940 and HU-210 produced complete reversal of mechanical hyperalgesia within 3 h of subcutaneous administration with D50 values of 0.52, 0.08 and 0.005 mg kg(-1), respectively. In this model WIN55,212-2 was also effective against thermal hyperalgesia and mechanical allodynia. WIN55,212-2 produced pronounced reversal of mechanical hyperalgesia following intrathecal administration that was blocked by the CB1 antagonist SR141716A. Following intraplantar administration into the ipsilateral hindpaw, WIN55,212-2 produced up to 70% reversal of mechanical hyperalgesia, although activity was also observed at high doses following injection into the contralateral paw. The antihyperalgesic effect of WIN55,212-2 injected into the ipsilateral paw was blocked by subcutaneously administered SR141716A, but was not affected by intrathecally administered SR141716A. These data show that cannabinoids are highly potent and efficacious antihyperalgesic agents in a model of neuropathic pain. This activity is likely to be mediated via an action in both the CNS and in the periphery.

1 Vanilloid receptors (VR1) were cloned from human and rat dorsal root ganglion libraries and expressed in Xenopus oocytes or Chinese Hamster Ovary (CHO) cells. 2 Both rat and human VR1 formed ligand gated channels that were activated by capsaicin with similar EC 50 values. Capsaicin had a lower potency on both channels, when measured electrophysiologically in oocytes compared to CHO cells (oocytes: rat=1.90+0.20 mM; human=1.90+0.30 mM: CHO cells: rat=0.20+0.06 mM; human=0.19+0.08 mM). 3 In CHO cell lines co-expressing either rat or human VR1 and the calcium sensitive, luminescent protein, aequorin, the EC 50 values for capsaicin-induced responses were similar in both cell lines (rat=0.35+0.06 mM, human=0.53+0.03 mM). 4 The threshold for activation by acidic solutions was lower for human VR1 channels than that for rat VR1 (EC 50 pH 5.49+0.04 and pH 5.78+0.09, respectively). 5 The threshold for heat activation was identical (428C) for rat and human VR1. 6 PPAHV was an agonist at rat VR1 (EC 50 between 3 and 10 mM) but was virtually inactive at the human VR1 (EC 50 410 mM). 7 Capsazepine and ruthenium red were both more potent at blocking the capsaicin response of human VR1 than rat VR1. 8 Capsazepine blocked the human but not the rat VR1 response to low pH. Capsazepine was also more e ective at inhibiting the noxious heat response of human than of rat VR1.