2014年2月20日木曜日

Dopamine dependency for acquisition and performance of Pavlovian conditioned response

Martin Darvas, Amanda M. Wunsch, Jeffrey T. Gibbs, and Richard D. Palmiter
PNAS February 18, 2014 vol. 111 no. 7 2764–2769

古典的条件づけにおける"Goal tracking"行動にも、ドーパミンの活動が不可欠である。

【背景】
刺激・報酬の連合を十分に学習させた後に刺激を呈示すると、「刺激に近付いていく個体」と「報酬(が出現する場所)に近付いていく個体」がいる。
前者は"sign tracking"、後者は"goal tracking"と呼ばれるが、「goal trackingにドーパミンが必要か否か」はよく分かっていなかった。
http://www.nature.com/nature/journal/v469/n7328/abs/nature09588.html

During Pavlovian conditioning, pairing of a neutral conditioned stimulus (CS) with a reward leads to conditioned reward-approach responses (CRs) that are elicited by presentation of the CS. CR behaviors can be sign tracking, in which animals engage the CS, or goal tracking, in which animals go to the reward location. We investigated CR behaviors in mice with only ∼5% of normal dopamine in the striatum using a Pavlovian conditioning paradigm. These mice had severely impaired acquisition of the CR, which was ameliorated by pharmacological restoration of dopamine synthesis with L-dopa. Surprisingly, after they had learned the CR, its expression decayed only gradually in following sessions that were conducted without L-dopa treatment. To assess specific contributions of dopamine signaling in the dorsal or ventral striatum, we performed virus-mediated restoration of dopamine synthesis in completely dopamine-deficient (DD) mice. Mice with dopamine signaling only in the dorsal striatum did not acquire a CR, whereas mice with dopamine signaling only in in the ventral striatum acquired a CR. The CR in mice with dopamine signaling only in the dorsal striatum was restored by subjecting the mice to instrumental training in which they had to interact with the CS to obtain rewards. We conclude that dopamine is essential for learning and performance of CR behavior that is predominantly goal tracking. Furthermore, although dopamine signaling in the ventral striatum is sufficient to support a CR, dopamine signaling only in the dorsal striatum can also support a CR under certain circumstances.

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