In 1709, a Japanese botanist became the first person to describe how cats go wild for catnip and silver vine — plants that put felines into a state of drug-like euphoria. However, while it's very established cats are happy to rub their faces against these plants, what caused the evolution of this behavior — and why it triggers feline brains so powerfully — was not known.
In a new study, scientists suggest they've found those answers. Engaging with a newly identified chemical in silver vine activates a cat's opioid reward system, the study claims. This encourages the cat to interact with the stimulating substance and, in turn, possibly protect itself from an all-too-familiar pest.
Co-author Masao Miyazaki, a researcher at Iwate University, tells Inverse that scientists have known about the intoxicating chemical nepetalactone. This is produced by catnip, or Nepeta cataria. However, he says, "nobody mentioned that 'nepetalactol' in silver vine [also] has the potential effect on cats."
Nepetalacol is a similar-sounding but entirely different chemical compound produced by silver vine, or Actinidia polygama. Its chemical compounds seemingly serve as an important biological defense against mosquitos, the study claims.
This finding was published Wednesday in the journal Science Advances.
The experiment — The researchers tested the effects of the silver vine plant on 25 domesticated cats, several feral cats, and other wild felines such as the Amur leopard. The researchers also conducted the experiment on domestic dogs and lab mice.
The effects of the silver vine were tested by extracting and applying one of its chemical compounds, nepetalactol, to filter paper. The researchers also set up a filter paper lacking nepetalactol as a control group.
The researchers then observed whether the felines rubbed their faces against the filter paper. Cats naturally react to catnip and silver vine by rolling over and brushing their faces against the plant — the question was whether they would have the same reaction to nepetalactol in a controlled setting.
They observed, in turn, what's described as a "positive reaction that has often been interpreted as extreme pleasure."
Based on cats' similar reaction to catnip, the researchers hypothesized that nepetalactol in silver vine stimulates the μ-opioid system in cats, which also regulates euphoric sensations in humans.
What was discovered — Among the 18 domesticated cats that responded positively to silver vine extract, the researchers selected 15 for the filter paper experiment.
All 15 cats positively responded to the nepetalactol filter paper by rubbing their faces against it. Likewise, 17 out of 30 feral cats gleefully rubbed their faces against the nepetalactol filter paper. Researchers observed a similar euphoric response in big jungle cats, including the leopard as well as a jaguar and lynx.
Meanwhile, the dogs and lab mice were totally uninterested in the nepetalactol filter paper, suggesting this silver vine response remains somewhat unique to felines.
By observing the felines' response to this pleasure-producing compoud, the researchers' proved their hypothesis correct — that the cats would respond positively to the silver vine chemical.
But the researchers wanted to go even further and test out another hypothesis: the possibility that nepetalactol may ward off mosquitoes, based on anecdotal reports of humans successfully repelling mosquitoes when applying a similar-sounding compound, nepetalactone, from catnip.
The researchers discovered that cats applied with nepetalactol received fewer mosquito visits than control cats, which lacked the nepetalactol application. Mosquitoes also shied away from cats in the wild that responded to silver vine positively.
Therefore, researchers concluded that nepetalactol possesses a mosquito-repelling function.
Miyazaki summarizes the researchers' four key findings in an email to Inverse.
"Our new findings are that 1) nepetalactol induces the response to cats, 2) the bioactivity of nepetalactol is higher than other iridoid compounds 3) the nepetalactol content of silver vine leaves was much higher than those of other active iridoid compound and 4) nepetalactol has the mosquito-repellent activity."
Why it matters — This evidence is the first to provide an understanding as to why cats behave in this playful fashion when they come into contact with silver vine.
Rubbing catnip and silver vine doesn't just provide a euphoric release for cats, similar to the way humans use drugs for recreational purposes. Instead, cats have also evolved to rub silver vine as a way of guarding against one of nature's most harmful insects. Mosquitoes carry diseases, including West Nile virus.
What's next — After learning that nepetalactol can protect cats against mosquitoes, the scientists wondered: Can this chemical help humans ward off mosquitoes, too?
Preliminary findings indicate yes, though scientists require further research. Miyazaki tested nepetalactol on his own body, finding that it effectively repelled mosquitoes for a short period of time on the arm containing nepetalactol.
"I tested mosquito repellent activity of nepetalactol to humans using my arms. Nepetalactol protected my right arm for 30 minutes, but seven mosquitoes bit my left arm with only solvent ethanol," Miyazaki says.
The researchers have applied for a patent to test out this finding, but they're mostly focused on the cats — for now.
"Our major interest is focusing on studies of cats," Miyazaki explains. "We hope that companies are interested in our findings to develop pesticides in the near future."
Abstract: Domestic cats and other felids rub their faces and heads against catnip (Nepeta cataria) and silver vine (Actinidia polygama) and roll on the ground as a characteristic response. While this response is well known, its biological function and underlying mechanism remain undetermined. Here, we uncover the neurophysiological mechanism and functional outcome of this feline response. We found that the iridoid nepetalactol is the major component of silver vine that elicits this potent response in cats and other felids. Nepetalactol increased plasma endorphin levels in cats, while pharmacological inhibition of opioid receptors suppressed the classic rubbing response. Rubbing behavior transfers nepetalactol onto the faces and heads of respondents where it repels the mosquito, Aedes albopictus. Thus, self-anointing behavior helps to protect cats against mosquito bites. The characteristic response of cats to nepetalactol via the opioid system provides an important example of chemical pest defense using plant metabolites in nonhuman mammals.