Have you seen the videos of cats going bonkers over catnip? An otherwise serious and composed cat would lose its mastery over self-restraint. Although I have been a cat parent for over a decade, I have never had the chance to introduce catnip to my furry friends. Nevertheless, I observed similar behaviour with Taz and Tango, my first fur babies. As a new pet parent, I was highly protective, keeping them indoors for their safety or well-being.
During the month of 'Shravana,' the fifth month of the Hindu calendar marking the beginning of the festive season, my mother would prepare 'Chudis,' small bouquets, on the auspicious days of 'Chudi Pooja.' This 'Chudi' was crafted from flowers not typically used in prayers. Among the blooms of the Shravana month, the 'Durva' grass, commonly known as Bermuda grass, is an important component in the 'Chudi.' The rains remind me of my mother and me, standing under an umbrella while clutching the plastic with flowers, Selaginella delicatissima ferns, and the Bermuda grass collected from strategic spots near our home. Worldwide, Bermuda grass, scientifically known as Cynodon dactylon, is considered a noxious weed. However, urbanization has made its availability scarce.
One sunny morning of Shravana Sunday, I woke up to my mother's yelling. As I stepped out of my bedroom into the living room, I could not believe my eyes! My mother and the kittens were engaged in a tug-of-war over the Bermuda grass she had laboriously collected. Tango had already secured her share of grass, but Taz held onto the grass between his jaws while my mother clasped to the grass, laughing. After examining Tango's behaviour, my mother let Taz win. Taz and Tango had gone crazy over the grass, rolling over its blades, rubbing their heads and faces against it, and constantly sniffing and munching it. During that Shravana season, this episode repeated every Sunday, eventually escalating to the 'Chudi' abduction. The grassless 'Chudis' would be found elsewhere in the house the following day.
Now, speaking of feline behaviour, let us delve into the science behind catnip's effects. On August 8 this year, the day of International Cats Day, I came across an article by Uenoyama et al. (2021) that piqued my interest. It elucidates the impact of catnip and silver vine on most cats and delves into their characteristic response to plant iridoids. But what exactly are iridoids, you might wonder? In this blog, I aim to simplify the science behind cats' response to catnip and silver vine, as discussed in the article.
Some animal behaviours are inherited, while others are influenced by their environment. Behaviours are often acquired to better adapt to the animals' surroundings. However, specific chemical components can elicit species-specific behaviours, as observed in felids (A cat family member) when exposed to catnip and silver vine plants. The article addresses several questions, including the behavioural response. Initially, the animal may be momentarily stunned but quickly starts sniffing, licking, and chewing on the plant while also rubbing its head and face against it. This behaviour is followed by the animal rolling on the ground and becoming unresponsive for hours afterwards. What leads to this intoxication in cats when exposed to catnip? Are there any pathophysiological effects or benefits associated with this behaviour?
The authors of the article attribute this behaviour as a response to the chemical stimulant iridoid nepetalactone found in catnips. Silver vines contain the chemical substances isoiridomyrmecin, iridomyrmecin, isodihydronepetalactone, and dihydronepetalactone which induce the same behavioural response. Do not fret over the lengthy chemical names! Just remember that these plants contain chemicals that stimulate a reaction in the felids. Although this behaviour in cats was documented first in the 18th century by a Japanese Botanist, the functional outcome of the behaviour remains unclear even today.
To unravel the functional outcome and the mechanism behind the behaviour, the researchers ought to have the chemical component nepetalactol on them. Organic solvents were employed to extract the chemical components from the silver vine leaves. Once extracted, the chemical components were purified by the researchers. I shall keep this article simple and not delve into the technical details of the techniques used. However, I will mention that mass spectrometry confirmed that the major extracted chemical component was nepetalactol, among others. Simply put, the researchers obtained the chemical component required for their study.
It is important to note that in research, a few outliers are expected— observations that differ from the norm. In this study, a few cats did not respond to the test situation due to inherent negative responsiveness or unfamiliarity with the scenario. To subject the cats (25 in number) to nepetalactol, 50 µg of this chemical stimulant was introduced into a filter paper and placed on the ground. Eighteen cats exhibited the characteristic response towards the nepetalactol filter paper for ten minutes, after which their interest waned, similar to the reaction to the silver vine plant. The study design included a control where the cats were exposed to filter paper containing only the solvent (Extraction) but no nepetalactol. The cats displayed limited to negligible response to the control. So, we now understand 'what' causes the behaviour and 'how' the study was conducted to uncover the same. Next, let me outline the mechanism driving the felids to display the characteristic response exhibiting extreme pleasure.
Just as humans provide blood samples for various tests, the researchers studied the blood drawn from cats exposed to 200 µg of nepetalactol and found high levels of β-endorphins. To appreciate the mechanism, let me explain a few terms. β-endorphins are opioid neuropeptides (Short protein chains produced by the neurons) with stress management and pain-relieving properties. You are probably familiar with the term 'Opioid'. However, β-endorphins are opiates endogenous to our system or produced naturally by our body. For β-endorphins to have stress or pain-relieving effects, they must bind to specific receptors, known as µ-opioid receptors. These receptors are found throughout the body, brain, and cells of the immune system (Pilozzi et al., 2020). When β-endorphins bind to µ-opioid receptors, they not only provide an analgesic effect (pain relief) but also induce feelings of reward and euphoria (Feeling happy and excited). To sum it up, in the presence of nepetalactol, the olfactory system- sense of smell in the cats picks up nepetalactol that stimulates the production of β-endorphins, activating the µ-opioid system. The high β-endorphins bind to µ-opioid receptors, evoking the behavioural response associated with the sense of pleasure.
Let me address the 'why.' Why does a cat exhibit the rolling behaviour? Does rubbing and rolling in nepetalactol-stimulated cats have any function? To establish if rolling on the ground was a behaviour with a role, the researchers placed the 200 µg nepetalactol filter paper on the ceiling or the walls of the cage that housed the cats. The cats were motivated to come in contact with the nepetalactol filter paper. As the source of nepetalactol was not easily accessible, the cats resorted to rubbing their heads and faces on the paper while standing on their hind legs or climbing the cage to the paper, if necessary. However, no subject cat rolled on the ground in this scenario compared to when the nepetalactol filter paper was on the floor. What did this behaviour change suggest? This behaviour change helped the researchers establish that the behaviour was functional. Cats would rub themselves against or roll over the source of nepetalactol only to transfer it and spread it on the fur.
What I find intriguing about this article is how the researchers have left no stone unturned! To establish the effect of the catnip or silver vine on felids, the researchers conducted similar studies on non-domesticated felids at zoos in Japan, in addition to domestic cats (Felis catus). An Amur leopard (Panthera pardus orientalis), two jaguars (Panthera onca) and two Eurasian lynx (Lynx lynx) exhibited prolonged face rubbing and rolling on nepetalactol-paper (placed on the floor) than on control, confirming their findings. On the other hand, dogs (Canis lupus familiaris) and mice (C57BL/6) exhibited no interest or response to nepetalactol, affirming the nepetalactol-induced behaviour was specific to only felids.
Apart from analgesic and euphoric effects, does nepetalactol in silver vines have any other benefits for cats? Well, the answer is YES! The behaviour of the cats towards nepetalactol suggests an adaptive function. Rolling on and rubbing against the nepetalactol-containing silver vine or nepetalactone-containing catnip helps protect cats from mosquitoes. This study was conducted in Japan, where nepetalactol on a cat's fur acted as a deterrent to Aedes albopictus mosquitoes. Therefore, the researchers concluded that this functional behaviour in cats was acquired as an adaptive function to protect them from insect pests using plant metabolites.
After reading the article by Uenoyama et al. (2021), I began to wonder if there might be a similar explanation for the behavioural response to the Bermuda grass by Taz and Tango. Evidence only shows that eating grass can benefit the cats' gastrointestinal system- relieve indigestion and expel parasites and hairballs. In addition, Hart (2008) clarifies that grass consumption does not necessarily indicate gastrointestinal illness but could be an inherited trait from their wild ancestors. However, I still cannot fathom my kittens' obsessive behaviour towards the grass. Until a study reports an analgesic and euphoric effect of Bermuda grass, I would make peace with the simple fact that the shape and texture of the grass might have triggered the hunting instinct in them.
References
Hart, B. L. (2008). Why do dogs and cats eat grass? Veterinary Medicine, 103(12), 648–649.
Pilozzi, A., Carro, C., & Huang, X. (2020). Roles of Β-Endorphin in stress, behavior, neuroinflammation, and brain energy metabolism. International Journal of Molecular Sciences, 22(1), 338. https://doi.org/10.3390/ijms22010338
Uenoyama, R., Miyazaki, T., Hurst, J. L., Beynon, R. J., Adachi, M., Murooka, T., Onoda, I., Miyazawa, Y., Katayama, R., Yamashita, T., Kaneko, S., Nishikawa, T., & Miyazaki, M. (2021). The characteristic response of domestic cats to plant iridoids allows them to gain chemical defense against mosquitoes. Science Advances, 7(4). https://doi.org/10.1126/sciadv.abd9135
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