2013年8月14日水曜日

Diversity and Homogeneity in Responses of Midbrain Dopamine Neurons

Christopher D. Fiorillo, Sora R. Yun, and Minryung R. Song
The Journal of Neuroscience, 13 March 2013, 33(11):4693-4709; doi:10.1523/JNEUROSCI.3886-12.2013

嫌悪刺激に対するドーパミン・ニューロンの活動(DA)。
- DAは嫌悪刺激によって抑制され、報酬刺激によって促進される(従来の報酬予測誤差仮説と一致)。
- 黒質稠密部(SNc)の腹側及び前方にあるニューロンほど嫌悪刺激によって強く抑制される。
- 嫌悪刺激によって活動が促進されるニューロンも見つかっているが、それらは「嫌悪」とは関係のない「刺激の知覚強度」をコードしているだけである。

Dopamine neurons of the ventral midbrain have been found to signal a reward prediction error that can mediate positive reinforcement. Despite the demonstration of modest diversity at the cellular and molecular levels, there has been little analysis of response diversity in behaving animals. Here we examine response diversity in rhesus macaques to appetitive, aversive, and neutral stimuli having relative motivational values that were measured and controlled through a choice task. First, consistent with previous studies, we observed a continuum of response variability and an apparent absence of distinct clusters in scatter plots, suggesting a lack of statistically discrete subpopulations of neurons. Second, we found that a group of “sensitive” neurons tend to be more strongly suppressed by a variety of stimuli and to be more strongly activated by juice. Third, neurons in the “ventral tier” of substantia nigra were found to have greater suppression, and a subset of these had higher baseline firing rates and late “rebound” activation after suppression. These neurons could belong to a previously identified subgroup of dopamine neurons that express high levels of H-type cation channels but lack calbindin. Fourth, neurons further rostral exhibited greater suppression. Fifth, although we observed weak activation of some neurons by aversive stimuli, this was not associated with their aversiveness. In conclusion, we find a diversity of response properties, distributed along a continuum, within what may be a single functional population of neurons signaling reward prediction error.

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