PAIN tolerance seems to be infectious鈥攆or rats, at least. 快猫短视频s in
Sweden have shown that when one rat is injected with a painkilling and
anxiety-reducing hormone, its cage mates start to feel less pain as well. The
message is carried through smell, they believe.
Greta 脜gren and a team of researchers at the Karolinska Institute in
Stockholm made this discovery after injecting rats with a hormone called
oxytocin. This hormone, which is best known for inducing labour and lactation,
can also make rats feel more placid.
After the injections, the temperature of the tail skin of the rats fell, a
telltale sign that the stress levels had been reduced. But strangely, the skin
temperatures of rats in the same cage which had not been injected with the
hormone, also started to decline.
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When 脜 gren and her colleagues duplicated the experiment, this time knocking
out the cage mates鈥� sense of smell with nose drops, these rats no longer
experienced the sympathetic stress relief. This suggests that they reacted to
the injected rat鈥檚 oxytocin levels through smell.
脜gren鈥檚 team knew that oxytocin can have an analgesic effect, possibly by
stimulating natural opioids in the body. So they wondered if a rat鈥檚 pain
threshold, which is also linked to levels of oxytocin, might also be sensed and
鈥渃opied鈥� by other rats.
To test this, they put eight groups of four male rats together in cages. In
four cages they injected one rat with oxytocin. Its three cage mates were
injected either with saline, thought to have no chemical effect, or an
oxytocin-antagonist, which would block any effects of the hormone. In four
control cages, no rats were injected with oxytocin.
Fifteen minutes after the injections, the researchers placed the hind paw of
each rat onto a plate that was heated to 52 掳C 鈥攔oughly the
temperature of very hot beach sand. The scientists measured how long each rat
took to raise its paw in discomfort.
As the team expected, the animals that had been injected with oxytocin
tolerated the heat better, and took longer to raise their paws. Their pain
relief peaked about 30 minutes after the injections. The saline-injected cage
mates also experienced pain relief, the researchers found, but their relief rose
more slowly and turned out to be most pronounced a further 30 to 45 minutes
later.
Rats that had been injected with an oxytocin-antagonist, and rats that had
not shared a cage with an oxytocin-injected rat, experienced no pain relief, the
researchers say. They lifted their paws just as quickly as they had before all
the injections took place.
鈥淩ats smell the condition of their cage mates,鈥� 脜 gren concludes. She says
that it鈥檚 likely that in response to the smell of rats with high oxytocin
levels, cage mates mobilise their own supplies of oxytocin. Her team will
publish their results in the 29 September issue of the journal
Neuroreport.
脜gren thinks that the rats鈥� olfactory systems may help to stop them
over-reacting to everyday stresses. So an anxious rat will be calmed by others in the
group, for example. It is possible that similar mechanisms are at work in
humans, she says鈥攆or instance, in the way a mother calms a baby. 鈥淚t
hasn鈥檛 been shown that it is due to olfaction,鈥� she says, 鈥渂ut why not?鈥�