Some like it in the pot, nine days old

Pease porridge hot, pease porridge cold, pease porridge in the pot, nine days old.

Some like it hot.

The scene: the dining room of a suburban, middle class, American home.
The players: middle aged, depressed, unappreciated MOTHER; middle aged, vaguely unsuccessful, masochistic FATHER; bored, cynical TEENAGER; and impressionable YOUTH.
The event: Dinner.

The normal dining room table clutter had been cleared just enough to allow each of us room for a plate, silverware, and a glass. It had been a long time since we gave up on the idea of eating together regularly (we never were much for traditional American family values). I don't recall why we bothered to gather that evening. Mom had cooked dinner, and there were, among other things, baked potatoes.

FATHER sits surrounded by a semicircular array of bottles. He takes on the appearance of a mad scientist, light reflecting off of the bottles onto the lenses of his glasses, making his expression difficult to judge.

FATHER: baked potatoes, eh? Well, let's just spice these up a bit, shall we?

My dad is one of those people who seeks out spicy food because of the fire instead of the flavor. I suspect he has developed an addiction to the endogenous opioid release associated with consumption of spicy food. That aside, there remains a burning question: What is the origin of that fiery sensation?

FATHER sits, grotesque, sweating and coughing, a half-eaten potato that has clearly been adulterated by the substance contained within the "Scorned Woman Hot Sauce" bottle resting on the plate before him. YOUTH looks on, amazed, as he forces mouthful after mouthful of starchy fire into his system.

FATHER: Oh, I'm going to regret this tomorrow!

The sense of touch can actually be separated into three components: position, pressure, and temperature. Position is determined from the subset of the total population of receptor cells that are stimulated by a given point of contact. The sensation of pressure results from the action of specialized receptors that are sensitive to physical deformation. Likewise, a suite of specialized receptor types underlies our ability to sense temperature.

Buried among all the other cell types and parts that make up your taste buds lie the limbs of neurons that live in your trigeminal ganglia.¹ Within the membranes rest receptors variously called TRPV1 (for Transient Receptor Potential -- Vanilloids) or VR1 (for Vanilloid Receptor). When stimulated, sodium and calcium ions are admitted into the cell. If sufficient amounts of these ions trickle into the cell, then the membrane becomes depolarized.² The important bit of information, though, is the identity of those stimuli that cause these receptors to open. As the names suggest, this receptor is sensitive to vanilloid compounds, including capsaicin, a molecule present in all chiles. They are also receptive to another major stimulus: heat.³ TRPV1 receptors will open when exposed to temperatures between 43 and 52° C.¹

So, both heat and capsaicin stimulate the same receptor. It isn't hard to see why your brain would interpret the signals from these disparate stimuli in the same way. The result? When you eat substances that contain capsaicin, you get the sense that your mouth is burning. Moreover, the capsaicin molecule tends to lock into the receptor, causing prolonged stimulation that would be equivalent to prolonged exposure to heat. Prolonged exposure to heat causes cellular damage (not to mention that sufficient amounts of capsaicin can itself cause cellular damage), so the circulating level of opioids is increased to compensate for any pain resulting from the damage.⁴ The big picture? My father is a chile head. So it goes.

Some like it cold.

The scene shifts. We look in on YOUTH, some ten years in the future. She is in a classroom. The students are restless because they are about to take a final exam. A neighboring student offers her a mint as the tests are handed out.

YOUTH: Altoids? um...sure, thanks.

Admittedly, I inherited some of my father's chile affinity. In fact, it went a little beyond that, and I developed an appreciation for any substance that produces an unusual or unexpected sensation: the palate cleansing powers of acidic substances, the vaguely slick feel of alkaline substances, and, best of all, the cool sensation associated with mint and menthol products.

Five years later we observe YOUTH at home, alone. Her apartment is not big enough for a cluttered dinner table, so she has settled for a cluttered kitchen counter. She is eating Ben and Jerry's Mint Chocolate Cookie ice cream from a half-empty carton. Scattered about the apartment we see the small, rectangular tins characteristic of the mints that have become part of popular culture. We note that she has cut her hair short.

The cool sensation associated with mint products is elicited by a similar set of circumstances. Another receptor in the transient receptor potential family, alternately known as TRPM8 (transient receptor potential - menthol) and CMR1 (cold and menthol receptor)², is typically found in the cell membranes of unmyelinated and thinly myelinated neurons that project from the trigeminal ganglion. These receptors open to calcium ions at temperatures between 8 and 28° C, or when they come in contact with menthol.¹ Furthermore, exposure to menthol in amounts insufficient to cause depolarization will sensitize the receptor to stimulation by cold.⁶ In other words, not only do minty things feel cold, but the presence of mint makes other things seem colder. Those of you who live in warm climates may want to consider rubbing yourselves with crushed mint leaves before turning up the air conditioner.

Some like it in the pot, nine days old.

So what have we learned?

• On a cold day, a little chile powder in your hot chocolate will make it better.
• On a hot day, a little mint in your lemonade will make it better.
• On the remaining days, you won't know what to do. Don't worry. It will be okay.

1. McKemy, Neuhausser, and Julius (2002). Identification of a cold receptor reveals a general role for TRP channels in thermosensation. Nature, 416, 52-58.
2. Clapham (2002). Hot and Cold TRP Ion Channels. Science, 295, 2228-2229.
3. Gunthorpe, Benham, Randall, and Davis (2002). The diversity in the vanilloid (TRPV) receptor family of ion channels. Trends in Pharmacological Sciences, 23(4), 183-191.
4. It is interesting to note that TRPV1 receptors (and its close cousin, TRPV2, which is sensitive to temperatures above 52° C;) also exist in the body skin. These receptors project from the dorsal root ganglion. I'm guessing that they are responsible for the reflex that causes you to pull away from hot surfaces.
5. Menthol, in sufficiently high doses, acts as an irritant to skin and mucous membranes. See McKemy et al. for more detail. 6. Montell, Birnbaumer, and Flockerzi (2002). The TRP Channels, a Remarkably Functional Family. Cell, 108, 595-598.