Neuropsychopharmacology (2007) 32, 1805–1812 & 2007 Nature Publishing Group All rights reserved 0893-133X/07 $30.00 www.neuropsychopharmacology.org Combination of Rimonabant and Donepezil Prolongs Spatial Memory Duration 1 2 1 ,1 Laura E Wise , Philip A Iredale , Rene J Stokes and Aron H Lichtman* 1 2 Department of Pharmacology and Toxicology, Virginia Commonwealth University, Richmond, VA, USA; Central Research Division, Pfizer, Inc., Groton, CT, USA The observations that the cannabinoid (CB ) receptor antagonist/inverse agonist, rimonabant, and the selective noncompetitive inhibitor 1 1 of acetylcholinesterase (AChE), donepezil, improve performance in a variety of animal memory models, suggest that these neurochemical systems play integral roles in cognition. The present study tested whether each of these agents administered alone or in combination will prolong the duration of spatial memory. Rats were trained in a two-phase radial-arm maze procedure, consisting of acquisition and retrieval tests, which were separated by an 18 h delay. Each drug was administered 30 min before the acquisition phase, immediately after the acquisition phase, or 30 min before the retrieval test to assess acquisition/consolidation, consolidation, and retrieval mnemonic processes, respectively. Rimonabant or donepezil administered before the acquisition phase, but not immediately after acquisition or before retrieval, led to a significant decrease in the number of errors committed during the retrieval test. Combined administration of subthreshold doses of rimonabant and donepezil that had no discernable effects on performance when given alone, enhanced memory. These results taken together demonstrate that the delay radial-arm maze task is sufficiently sensitive to detect memory enhancing effects of these drugs. Moreover, these findings suggest that combined administration of subthreshold doses of rimonabant and donepezil can improve memory and may represent a novel approach to treat cognitive deficits associated with neurodegenerative disorders. Neuropsychopharmacology (2007) 32, 1805–1812; doi:10.1038/sj.npp.1301297; published online 10 January 2007 Keywords: radial-arm maze; SR141716 (Rimonabant); donepezil (Aricept); endogenous cannabinoid; acetylcholinesterase inhibitor; spatial memory INTRODUCTION 1996), and an elevated T-maze task (Takahata et al, 2005). Interestingly, intrahippocampal administration of rimona- The centrally occurring endocannabinoid system consists of bant in food-storing birds enhanced memory for the the G-protein coupled receptor cannabinoid (CB ) (Mat- 1 1 location of a hidden food reward (Shiflett et al, 2004). suda et al, 1990) and endogenous ligands, anandamide Additionally, CB receptor knockout mice performed better (Devane et al, 1992) and 2-arachidonyl glycerol (2-AG) 1 than wild-type control mice in a two-trial object recognition (Mechoulam et al, 1995; Sugiura et al, 1995), that bind to task (Reibaud et al, 1999). These findings taken together and activate these receptors. The high concentration of CB 1 suggest that the CB receptor antagonist rimonabant may receptors (Herkenham et al, 1991) and the presence of 1 enhance mnemonic processes. anandamide and 2-AG in the hippocampus and other The central cholinergic system has long been known to forebrain regions associated with memory function (Felder play a key role in learning and memory. Cholinergic et al, 1996; Di Marzo et al, 2000) are consistent with the neurons degenerate in patients with Alzheimer’s disease as notion that the endocannabinoid system modulates cogni- well as senile dementia, and, importantly, the degree of tive processes. Administration of the CB receptor antago- 1 degeneration highly correlates with functional loss in these nist rimonabant (SR 141716) (Rinaldi-Carmona et al, 1995) disorders (Davies and Maloney, 1976; Perry et al, 1978a, b). has been reported to enhance memory in delayed radial- Clinical trials revealed that donepezil (Aricept, previously arm maze tasks (Lichtman, 2000; Wolff and Leander, 2003), E2020, Pfizer), a selective noncompetitive inhibitor of a social recognition memory paradigm (Terranova et al, acetylcholinesterase (AChE; see Sugimoto et al (2002) for a review), had cognitive benefits in Alzheimer’s disease *Correspondence: Dr AH Lichtman, Department of Pharmacology and patients afflicted with mild to moderate degrees of dementia Toxicology, Virginia Commonwealth University, Medical College of Virginia Campus, PO Box 980613, Richmond, VA 23298-0613, USA, (Mintzer and Kershaw, 2003). Presently donepezil is used to Tel: + 1 804 828 8480, Fax: + 1 804 828 2117, treat cognitive deficits associated with Alzheimer’s disease E-mail: [email protected] (Rogers and Friedhoff, 1998; Doody, 1999; Grutzendler and Received 31 July 2006; revised 20 September 2006; accepted 3 Morris, 2001). Additionally, this drug has been found to November 2006 reverse memory deficits in variety of rodent models of Rimonabant and donepezil prolong spatial memory duration LE Wise et al 1806 learning and memory. Donepezil reversed scopolamine- injections were given through the i.p. route of administra- induced impairments in spontaneous alternation in the T- tion in a volume of 1 ml/kg. maze in young and aged mice as well as in a delayed non- match to place task in the radial-arm maze (Bontempi et al, 2003). Repeated administration of donepezil improved the Apparatus and Training memory performance of aged rats in the Morris water maze (Hernandez et al, 2006) and reversed amyloid b-induced The initial radial-arm maze training procedures were memory impairments in a delayed matching to position identical to that previously described (Lichtman and paradigm (Yamada et al, 2005). Additionally, this agent Martin, 1996; Lichtman, 2000). Each of the eight arms was reversed learning impairments in the passive avoidance task baited with a 45 mg rodent formula, dustless precision induced by lesions in the nucleus basalis magnocellularis pellets (Bioserve, Frenchtown, NJ) placed 5 cm from the and scopolamine (Cheng et al, 1996; Ogura et al, 2000), and end. After a subject visited each arm and obtained all the ethylcholine mustard aziridinium ion (AF64A) (Cheng and food pellets, with one or no re-entries into a previously Tang, 1998) induced impairments in eight-arm radial maze visited arm in three consecutive sessions, it was trained in a tasks (Braida et al, 1996). repeated acquisition version of the task that incorporated Although there is evidence demonstrating that rimona- acquisition and retrieval test phases. During the acquisition bant or donepezil enhances performance in rodent models phase, one of the arms was randomly selected and a of learning and memory and that the cannabinoid and Plexiglas barricade blocked its entryway. Each of the cholinergic receptor systems have important roles in remaining seven arms was baited with a food pellet before learning and memory, no study has examined whether the subject’s placement in the maze. After the subject coadministration of these drugs would further enhance entered the seven available arms and consumed all the food memory. Therefore, the main goal of the present study pellets, it was removed from the maze and returned to its was to determine whether concomitant administration of home cage. While undergoing training, the subject returned inactive doses of rimonabant and donepezil would enhance to the maze for the test phase 10 min after the acquisition memory as assessed in a rat radial-arm maze delay phase. During the test phase, all eight arms were available; paradigm. The task employed a two-phase procedure, however, only the previously blocked arm was baited with a acquisition, and test phases, which were separated by an food pellet. The number of entries and the duration of time 18 h delay in order to increase the task difficulty. The dose– required for each subject to enter the seven available arms response relationship of each drug administered 30 min and consume the food pellet(s) were recorded for each before the acquisition phase was evaluated. In addition, phase. Subjects qualified for drug testing once they each drug was given immediately after the acquisition phase committed one or no errors in both the acquisition and or 30 min before retrieval testing to infer whether these test phases on three consecutive sessions. Subjects were compounds influenced consolidation and retrieval pro- given a maximum of two tests/week. cesses, respectively. Finally, inactive doses of these drugs An initial experiment evaluated the relationship between were given in combination to determine whether they would the duration of the delay and performance during the enhance memory performance. retrieval test. Subjects were given repeated acquisition training and given delays of various durations (ie 10 min, 1, 2, 6, or 18 h) before the retrieval test. Based on the results of MATERIALS AND METHODS this initial study, all subsequent experiments imposed an 18 h delay between the two phases in order to optimize the Subjects likelihood of detecting a drug-induced enhancement in The subjects consisted of two groups of Sprague–Dawley performance. (Harlan, IN) male rats that were between 3 and 11 months In Experiment 2, we evaluated the effects of rimonabant of age. The animals were individually housed in a (0.3, 1, and 3 mg/kg) and donepezil (0.1, 0.3, and 1 mg/kg) temperature-controlled (20–221C) environment, with a given 20 min before the acquisition phase on performance. 12 h light/dark cycle. Subjects were placed