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HOW can we tell it鈥檚 cold when a chilly wind blows? Three teams of scientists
say they鈥檙e homing in on the chemistry behind cold sensations.

How we feel hot isn鈥檛 so much of a mystery. Heat activates a family of ion
channels鈥攎olecules that form tiny pores in cell walls. These regulate the
flow of chemicals in and out of sensory neurons, prompting them to fire an
electrical signal.

Now David Julius and colleagues at the University of California in San
Francisco and another team at the Scripps Research Institute in La Jolla have
both discovered an ion channel that persuades mouse neurons to fire in response
to cool sensations, between 8 and 28 掳C. Both teams found that neurons with
the channel also fired when they encountered menthol, explaining why minty
flavours feel cool in the mouth.

Julius says some neurons fire in both hot and cold temperatures, which might
explain why people sometimes perceive icy cold as a burning pain.

A Spanish team led by F茅lix Viana of Miguel Hern谩ndez
University in San Juan de Alicante has found another way neurons respond to the
cold. They showed that mouse neurons responded to drops in temperature from 33
to 15 掳C by closing off a certain potassium channel. This made a small
fraction of the neurons fire. But most did not react because a second potassium
channel was acting as a 鈥渂rake鈥�.

When the team used a drug to block the brake, these neurons became sensitive
to the cold. Viana wonders if similar chemical sabotage might explain a disease
called cold allodynia, which can happen after nerve injuries. Sufferers feel
pain when they鈥檙e even slightly cold, so that a breeze from a ceiling fan can
feel like knives cutting their skin.

Viana and Julius say understanding this mob of sensory molecules could
eventually lead to the development of new painkillers. 鈥淚dentifying different
classes of sensory neurons may be useful for screening the effects of drugs,鈥�
says Viana.