Science Focus (issue 24)

13 comes into play. After perceiving that we are in pain, hormones like endorphins, which are known as the natural painkillers and the “feel-good” hormones of the body, are released [4]. These hormones also increase the level of a neurotransmitter, dopamine, making us feel even more pleasant and euphoric [4]. Therefore, the secret behind our addiction to spiciness is, in fact, the “feel-good” chemicals that give us a light-hearted rush. If your first instinct when your mouth is “on fire” is to grab a glass of water, think again! Capsaicin is a hydrophobic (“water-hating”, or “fat-loving”), non-polar molecule with a long hydrocarbon tail. Drinking water will only spread capsaicin all over the tongue, and heightens the burning sensation. You should instead consume dairy products because it contains casein, a non-polar protein which can bind to capsaicin. Similar to the cleaning action of detergents to remove grease, casein molecules will surround capsaicin molecules to form tiny droplets which can then be washed away easily [5]. Hence a cup of milk, or a cone of ice cream can likely “cool off” spicy food. Some foods may contain other compounds that can also activate TRPV1, and/or the “wasabi receptor” TRPA1, another pain receptor under the TRP family. Black peppers contain piperine [6], while mustard [7] and wasabi [8] contain compounds belonging to the class of isothiocyanates. Isothiocyanates are volatile small molecules that can be inhaled and stimulate the receptors in the nasal cavity, so mustard and wasabi can burn not only our mouth, but also our nose. Notably, this will eventually lead to the secretion of the “feel-good” chemicals as well. Alongside its ability to elicit a sensation of pain, surprisingly, capsaicin is also used in pharmaceuticals to provide relief to pain. It is commonly sold over the counter in the form of topical ointments and patches (the pain-relieving “hot” patches) [9]. In addition, capsaisin was approved in Europe to treat neuropathic pain [10], which is often described as shooting or burning pain, resulting from nerve injury [11]. Researchers have hypothesized how capsaicin works in relieving pain, with the desensitization of TRPV1 (i.e. decreased responsiveness after repeated exposure) being one of the possible mechanisms [9]. To conclude, the spicy capsaicin molecule can give us pleasure and pain at the same time. While scientists continue to unravel the working mechanisms of capsaicin and other pungent molecules in our body, we can, at least, take the lesson and keep a glass of milk handy next time when we feast on chili peppers! 如果你嚐過一碗地獄拉麵、一抹山葵或一口辣椒,當 時的辛酸你一定不會忘記:舌頭像被火燒一樣,淚水從眼 角滲出,汗水穿透衣服襲來;沒有一種官能刺激比辣更具 威脅性。告訴你一個有趣事實:與其說辣是一種味道,不 如說是一種痛。那到底是甚麼驅使人們烹調辛辣的食物, 自投痛楚的羅網呢? 辣椒含有一種叫辣椒素的生物鹼,它能觸發辣的灼熱 感。咀嚼會使辣椒素分子從食物中釋放,並擴散到舌頭的 每個角落。然而它們會繞過味覺感受器,並與痛覺感受器結 合 [1]。這些名為TRPV1的痛覺感受器原本的作用是在高 溫(>43°C [2])的情況下對腦部作出警告,但它們亦可以 被辣椒素觸發。作為正離子通道,TRPV1感受器被觸發時 會打開,令正離子得以擴散進入痛覺神經元 [3]。這個增加 電勢的過程叫去極化,會觸發神經元發出神經脈衝把信號 傳遞至腦部,腦部在詮釋信號後便誤以為我們的舌頭正與 灼熱的物件接觸,給予我們嘴巴被火燒的錯覺。 如果辣真的是一種痛,為甚麼我們會有多嚐一口的欲 望呢?那正是因為我們身體有一個自我調節系統:身體在 感到痛楚時會釋放安多酚等激素,安多酚既是天然止痛劑, 亦能帶來身心愉悅的感覺 [4]。同時,安多酚亦能提高多巴 胺這種神經遞質的水平,使我們更感愉快和興奮 [4]。因此, 令我們戀上吃辣的原因正是這些使我們自我感覺良好的化 學分子。 假如你在吃辣後的第一個反應是拿起一杯水來「救火」 的話,你可要再想想!辣椒素是帶有碳氫長鏈的疏水(亦即 親脂)非極性分子,喝水只會使辣椒素散落到舌頭的四周, 加劇灼熱感。你應該喝奶類製品,因為它們含有非極性的 酪蛋白,能與辣椒素結合。與清潔劑去除油脂的原理相近, 酪蛋白分子能包圍辣椒素形成微滴,然後能被輕易沖走