Tonic pain has been difficult to demonstrate in animals. Because relief of pain is rewarding, analgesic agents that are not rewarding in the absence of pain should become rewarding only when there is ongoing pain. We used conditioned place preference to concomitantly determine the presence of tonic pain in rats and the efficacy of agents that relieve it. This provides a new approach for investigating tonic pain in animals and for evaluating the analgesic effects of drugs.

No abstract

Background and Purpose: Effects of locally administered agonists and antagonists for cannabinoid CB 1 and CB 2 receptors on mechanical and thermal hypersensitivity were compared after the establishment of chronic inflammation. Experimental approach: Carrageenan was administered unilaterally to the rat hindpaw on day 1. Prophylactic efficacy of locally administered CB 1 -and CB 2 -selective agonists Àarachidonyl-2-chloroethylamide (ACEA) and (R,S)-(2-iodo-5-nitrophenyl)-[l-(l-methyl-piperidin-2-ylmethyl)-lH-ubdik-3-yl]-methanone ((R,S)-AM1241), respectivelyÀ on mechanical and thermal hypersensitivity were compared on day 2. Pharmacological specificity was evaluated using locally administered CB 1 and CB 2 -selective antagonistsKey Results: Administration of either ACEA or AM1241 to the inflamed but not noninflamed paw suppressed the maintenance of carrageenan-evoked mechanical hyperalgesia and tactile allodynia and attenuated thermal hyperalgesia. The ACEA-induced suppression of mechanical and thermal hypersensitivity was blocked by local injection of SR141716A but not SR144528. AM1241 suppressed mechanical hypersensitivity with the reverse pharmacological specificity. The AM1241-induced suppression of thermal hyperalgesia was blocked by SR144528 and to a lesser extent by SR14176A. Co-administration of ACEA with AM1241 in the inflamed paw increased the magnitude but not the duration of thermal antihyperalgesia compared to intraplantar administration of either agonist alone. Conclusions and Implications: Cannabinoids act locally through distinct CB 1 and CB 2 mechanisms to suppress mechanical hypersensitivity after the establishment of chronic inflammation, at doses that produced modest changes in thermal hyperalgesia. Additive antihyperalgesic effects were observed following prophylactic co-administration of the CB 1 -and CB 2 -selective agonists. Our results suggest that peripheral cannabinoid antihyperalgesic actions may be exploited for treatment of inflammatory pain states.

Rationale-Early, accurate detection of degenerative neurological disorders such as Alzheimer's Disease (AD) is essential for therapies designed to slow disease progression. Performance of a touch-screen mediated visuo-spatial paired-associates learning (vsPAL) task predicts neurocognitive decline in elderly populations presenting with mild cognitive impairment and distinguishes AD patients from elderly depressed individuals. Translation of this cognitive task to a nonhuman model may therefore provide an improved tool for study of the etiology and treatment of dementia.Objective-The goal of the current study was to contrast cholinergic and glutamatergic contributions to performance of this AD-sensitive task by challenging rhesus monkeys performing vsPAL with muscarinic antagonist and non-competitive NMDA antagonist drugs.Methods-Monkeys (7) were trained to perform vsPAL and then serially challenged with acute doses of scopolamine (3, 10, 17 µg/kg, i.m.) and ketamine (0.3, 1.0, 1.78 mg/kg, i.m.).Results-Scopolamine produced a dose × difficulty related impairment of both recognition memory and incremental acquisition aspects of task performance. In contrast, ketamine administration resulted in a dose-dependent impairment of recognition memory but not incremental acquisition.Conclusions-Monkeys' performance of a task sensitive to AD in humans was impaired by two classic pharmacological models of cognitive impairment therefore supporting the use of this nonhuman model to explore mechanisms of AD-associated cognitive decline. The differential pattern of impairment observed is consistent with a hypothesis that muscarinic mechanisms are required for linking external events with an existing internal representation, whereas NMDA mechanisms are required for the formation/strengthening of such an internal representation.

Recent work in our laboratories has demonstrated that an opioid-independent form of stress-induced analgesia (SIA) is mediated by endogenous cannabinoids [Hohmann et al., 2005. Nature 435, 1108. Non-opioid SIA, induced by a 3-min continuous foot shock, is characterized by the mobilization of two endocannabinoid lipidsd2-arachidonoylglycerol (2-AG) and anandamidedin the midbrain periaqueductal gray (PAG). The present studies were conducted to examine the contributions of spinal endocannabinoids to nonopioid SIA. Time-dependent increases in levels of 2-AG, but not anandamide, were observed in lumbar spinal cord extracts derived from shocked relative to non-shocked rats. Notably, 2-AG accumulation was of smaller magnitude than that observed previously in the dorsal midbrain following foot shock. 2-AG is preferentially degraded by monoacylglycerol lipase (MGL), whereas anandamide is hydrolyzed primarily by fatty-acid amide hydrolase (FAAH). This metabolic segregation enabled us to manipulate endocannabinoid tone at the spinal level to further evaluate the roles of 2-AG and anandamide in nonopioid SIA. Intrathecal administration of the competitive CB 1 antagonist SR141716A (rimonabant) failed to suppress nonopioid SIA, suggesting that supraspinal rather than spinal CB 1 receptor activation plays a pivotal role in endocannabinoid-mediated SIA. By contrast, spinal inhibition of MGL using URB602, which selectively inhibits 2-AG hydrolysis in the PAG, enhanced SIA through a CB 1 -selective mechanism. Spinal inhibition of FAAH, with either URB597 or arachidonoyl serotonin (AA-5-HT), also enhanced SIA through a CB 1 -mediated mechanism, presumably by increasing accumulation of tonically released anandamide. Our results suggest that endocannabinoids in the spinal cord regulate, but do not mediate, nonopioid SIA.