Chemosensory systems are critical for evaluating the caloric value and potential toxicity of food prior to ingestion. While animals can discriminate between 1000s of odors, much less is known about the discriminative capabilities of taste systems. Fats and sugars represent calorically potent and innately attractive food sources that contribute to hedonic feeding. Despite the differences in nutritional value between fats and sugars, the ability of the taste system to discriminate between different rewarding tastants is thought to be limited. In Drosophila, sweet taste neurons expressing the Ionotropic Receptor 56d (IR56d) are required for reflexive behavioral responses to the medium-chain fatty acid, hexanoic acid. Further, we have found that flies can discriminate between a fatty acid and a sugar in aversive memory assays, establishing a foundation to investigate the capacity of the Drosophila gustatory system to differentiate between various appetitive tastants. Here, we tested whether flies can discriminate between different classes of fatty acids using an aversive memory assay. Our results indicate that flies are able to discriminate medium-chain fatty acids from both short- and long-chain fatty acids, but not from other medium-chain fatty acids. Characterization of hexanoic acid-sensitive Ionotropic receptor 56d (Ir56d) neurons reveals broad responsive to short-, medium-, and long-chain fatty acids, suggesting selectivity is unlikely to occur through activation of distinct sensory neuron populations. However, genetic deletion of IR56d selectively disrupts response to medium chain fatty acids, but not short and long chain fatty acids. These findings reveal Ir56d is selectively required for fatty acid taste, and discrimination of fatty acids occurs through differential receptor activation within shared populations of neurons. These findings uncover a capacity for the taste system to encode tastant identity within a taste category.

中文翻译：

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果蝇中依赖Ir56d的脂肪酸反应揭示了不同类别脂肪酸之间的味道区分

化学感应系统对于摄入前评估食物的热值和潜在毒性至关重要。虽然动物可以区分出数千种气味，但对味道系统的区分能力知之甚少。脂肪和糖代表热量丰富和天生具有吸引力的食物来源，有助于享乐性进食。尽管脂肪和糖之间的营养价值存在差异，但人们仍认为口味系统区分不同的有益口味的能力有限。在果蝇中，表达离子性受体56d（IR56d）的甜味神经元是对中链脂肪酸己酸的自反行为反应所必需的。此外，我们发现苍蝇可以在厌恶记忆测试中区分脂肪酸和糖，为研究果蝇味觉系统区分不同食味口味的能力奠定了基础。在这里，我们使用厌恶记忆测试测试了苍蝇是否可以区分不同类别的脂肪酸。我们的结果表明，果蝇能够将中链脂肪酸与短链脂肪酸和长链脂肪酸区分开，但不能与其他中链脂肪酸区分开。己酸敏感性离子型受体56d（Ir56d）神经元的表征揭示了对短，中和长链脂肪酸的广泛响应，这表明选择性不可能通过激活不同的感觉神经元种群而发生。但是，IR56d的基因缺失选择性地破坏了对中链脂肪酸的响应，但不破坏短链和长链脂肪酸。这些发现表明，Ir56d选择性地具有脂肪酸味道，并且通过共享的神经元群体内的差异性受体激活来区分脂肪酸。这些发现揭示了口味系统在口味类别中编码味觉特征的能力。