The DAGL-α inhibitor DO34 impairs acquisition rate, but not expression, extinction, or forgetting, of spatial memory in C57BL/6J mice

L. D. Schurman1, M. C. Carper1, D. Ogasawara2, B. F. Cravatt2, A. H. Lichtman1,. 1Pharmacology and Toxicology, Virginia Commonwealth University, Richmond, 2The Skaggs Institute for Chemical Biology, The Scripps Research Institute, La Jolla

Introduction: A growing body of evidence implicates the importance of the endocannabinoid system in the regulation of spatial learning and memory. Elevation of 2-arachidonyl glycerol (2-AG) by genetic deletion of its catabolic enzyme monoacylglycerol lipase (MAGL) facilitates extinction1, acquisition, and expression of spatial memory2. Pharmacological inhibition of MAGL has revealed divergent effects of spatial memory processing, with impairments in acquisition and expression3,4. Here we examine whether inhibition of the primary biosynthetic enzyme of 2-AG, diacylglycerol lipase-α (DAGL-α), will disrupt acquisition, expression, and extinction, of Morris water maze (MWM) spatial memory tasks in C57BL/6J mice.

Method: During MWM acquisition, mice received a 2h pretreatment of 30 mg/kg DO345 or vehicle (VEH) on each of 10 Fixed Platform training days. In the assessment of expression (using a separate group) mice were given 10 days of drug-free Fixed Platform training, 24h later administered 30 mg/kg DO34 or VEH, and underwent a MWM Fixed Platform Probe Trial 2h later (i.e. the submerged platform was removed). The extinction of spatial memory was then measured by repeated probe trial evaluation at 1, 2, 3, 4 and 6 weeks post-acquisition with an injection of 30 mg/kg DO34 or VEH 2h before each trial. Forgetting was evaluated in mice given 10 days of drug-free Fixed Platform acquisition training followed by one single Fixed Platform Probe trial at 6 weeks post-acquisition with a 2h 30 mg/kg DO34 or VEH pretreatment.

Results: DO34-treated mice (n=10) exhibited impaired acquisition rates (P=.001), but no performance differences during the expression of reference memory (P=.366) compared with VEH-treated mice (n=12). During the acquisition of reference memory DO34-treated mice also demonstrated faster swim speeds (P=.003), and spent a greater proportion of their time in the outer ring of the MWM (P=.038) compared with VEH-treated mice, suggesting that DO34 produced altered murine search strategies. No performance differences between DO34- and VEH-treated mice were evident in either the extinction (P=.180) or forgetting tests (P=.916).

Conclusion: The observations that DAGL-α inhibition produced acquisition performance deficits but no extinction delays, suggest that the tonic availability of 2-AG may be necessary for homeostatic regulation of spatial learning and memory, but not extinction learning.

References:

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3. Wise et al. (2012). ACS Chem Neurosci 3: 369-378.

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5. Ogasawara et al. (2016). PNAS 113: 26-33.