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First published online October 18, 2006
Journal of Experimental Biology 209, 4273-4282 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02520

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Repeated cocaine effects on learning, memory and extinction in the pond snail Lymnaea stagnalis

Kathleen Carter¹, Ken Lukowiak², James O. Schenk^1,3 and Barbara A. Sorg^1,*

¹ Program in Neuroscience, Department of Veterinary and Comparative Anatomy, Pharmacology and Physiology, Washington State University, Pullman, WA 99164, USA
² Department of Physiology and Biophysics, Neuroscience Research Group, University of Calgary, T2N 4N1, Canada
³ Department of Chemistry, Washington State University, Pullman, WA 99164, USA

^* Author for correspondence (e-mail: sorg{at}vetmed.wsu.edu)

Accepted 4 September 2006

The persistence of drug addiction suggests that drugs of abuse enhance learning and/or impair extinction of the drug memory. We studied the effects of repeated cocaine on learning, memory and reinstatement in the pond snail, Lymnaea stagnalis. Respiratory behavior can be operantly conditioned and extinguished in Lymnaea, and this behavior is dependent on a critical dopamine neuron. We tested the hypothesis that repeated cocaine exposure promotes learning and memory or attenuates the ability to extinguish the memory of respiratory behavior that relies on this dopaminergic neuron. Rotating disk electrode voltammetry revealed a K_m and V_max of dopamine uptake in snail brain of 0.9 µmol l^-1 and 558 pmol s^-1 g^-1 respectively, and the IC₅₀ of cocaine for dopamine was approximately 0.03 µmol l^-1. For operant conditioning, snails were given 5 days of 1 h day^-1 immersion in water (control) or 0.1 µmol l^-1 cocaine, which was the lowest dose that maximally inhibited dopamine uptake, and snails were trained 3 days later. No changes were found between the two groups for learning or memory of the operant behavior. However, snails treated with 0.1 µmol l^-1 cocaine demonstrated impairment of extinction memory during reinstatement of the behavior compared with controls. Our findings suggest that repeated exposure to cocaine modifies the interaction between the original memory trace and active inhibition of this trace through extinction training. An understanding of these basic processes in a simple model system may have important implications for treatment strategies in cocaine addiction.

Key words: cocaine, dopamine, reinstatement, Lymnaea stagnalis, long-term memory, snail

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