Why Do Chili Peppers Feel Hot? Capsaicin And The Sensory Gate

Two Sensory Routes Behind One Word

In Turkish, acı can mean both a bitter taste and the heat of chilli peppers. Sweet, sour, salty, bitter and umami are taste qualities. Pepper heat belongs to another sensory domain, chemesthesis. Sensory science explains these experiences, described by the same Turkish word, through different processes.

Chemesthesis is the stimulation of neural pathways associated with touch, temperature and irritation by chemical substances. The burn of chilli, the cooling effect of menthol and tingling sensations are different examples of this wider domain. During a meal we experience them alongside taste and smell: a bite’s sourness, herbal aroma and pungency can carry separate sensory information within one experience.

TRPV1: How A Chemical Stimulus Becomes An Electrical Response

In David Julius’s laboratory, Michael Caterina and colleagues cloned a capsaicin-responsive channel from rodent sensory neurons in 1997. Their Nature paper showed that the protein, then called VR1, is an ion channel that allows cations—positively charged ions—to pass. Today it is known as TRPV1.

To find the channel, the researchers transferred pools of clones from a cDNA library made from messenger RNA of rodent sensory ganglia into laboratory cells called HEK293. After adding capsaicin, they used a fluorescent indicator to follow the rise in calcium inside the cells. Repeatedly subdividing and testing the responsive pool led them to a single cDNA clone encoding the channel that made the cells sensitive to capsaicin. The decisive step was to show that introducing this clone could confer capsaicin responsiveness on previously unresponsive cells.

An ion channel opens a passage through the cell membrane. Ion movement changes the cell’s electrical state, helping turn a chemical stimulus into a response the nervous system can process. The channel in the study also responded to noxious heat. Its activation by capsaicin helps explain why a cool food can feel burning, while the threshold for heat activation depends on experimental conditions.

From A Kitchen Ingredient To Sensory Physiology

The 2021 Nobel Prize in Physiology or Medicine was awarded to David Julius and Ardem Patapoutian for discoveries concerning receptors for temperature and touch. Capsaicin was the experimental tool that revealed a heat-sensitive sensor in Julius’ research. Patapoutian’s work identified PIEZO channels responding to mechanical force. The two lines of investigation helped explain how temperature and mechanical stimuli are represented in the nervous system.

Julius’s research began by turning a familiar sensation into a question that could be asked at the cellular level: How does a cell respond to capsaicin? Investigating the everyday experience of a food that ‘burns’ through a specific channel, ion movement and electrical response made a kitchen ingredient a tool for sensory physiology.

Fruit Pungency: Consumers, Seeds And Fungi

The role of pepper pungency in plant life can be investigated through the relationships different consumers have with the fruit. In Tewksbury and Nabhan’s 2001 study, capsaicin deterred the seed-eating mammals examined while leaving seed-dispersing birds undeterred. The researchers considered the difference as directed deterrence: fruit chemistry could discourage consumers that damage seeds while remaining accessible to dispersers.

In 2008, Tewksbury and colleagues approached wild Bolivian peppers through fungal pressure. Their findings supported a role for capsaicinoids in protecting fruits and seeds against fungal damage. Pungency thus opens onto the plant’s relationships with both animal consumers and fungi: the same compound group can have different roles in those encounters.

Describing Sensations, Explaining Mechanisms, Measuring Compounds

What kind of sensation is it? What is the molecular mechanism? How much of a compound is in the sample? These questions complement one another. Chemesthesis names a sensory domain. TRPV1 explains part of a molecular mechanism. Capsaicinoid analysis measures compounds in a particular sample. Although all concern the same pepper, they produce information at different scales.

Separate sensory pathways can interact in a bite. In 2021, Yang and colleagues compared aqueous solutions of nutty-smelling 3-methylbutanal, with and without 5 mg/L capsaicin, in fifteen participants. In triplicate experiments, in-nose aroma release and perceived aroma were monitored over 60 seconds. Capsaicin did not significantly change the measured aroma concentration in the nose, while average perceived aroma increased by 45 percent and saliva flow by 92 percent. The amount reaching the nose and the intensity perceived were therefore investigated separately.

Pepper aroma, sourness, texture and burn are experienced together in a hot-sauce bite. Yang and colleagues’ experiment offers a specific example of a difference between measured aroma release and perceived intensity. Capsaicinoid analysis answers another question: how much of those compounds is present in a sample. Moving to SHU then involves following how that amount is calculated and which sample it describes.

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• Aroma and retronasal smell