As the world changes rapidly, so should our perception of reality. This is the key in seeking the ultimate truth. Ayurveda, as I see is a path for seeking this truth, and the depth of my comprehension of its principles has changed over the years, as I dwell on its thoughts often. In this quest, I yet again attempt to put forward a number of observations made by contemporary scientists which explicitly coincide with the principles mentioned in Ayurveda. Taste as a mere gustatory perception is an idea which is long gone in the contemporary scientific community. In my previous articles I have briefed about how bitter and sweet tastes affect us. Here we shall try to understand the role of the spicy, hot and pungent taste and its role in physiology and pathology.

Ayurveda has explained a vast number of functions for these rasas. Katu rasa is one among the 6 rasas in Ayurveda which corresponds to spicy, hot or pungent sensations. We all love to add up a little spice in our food for that extra punch. But interestingly, this taste provides us with more than just a gustatory punch! Earlier the perception of spicy and hot taste was considered as a mere irritation of the mucous membrane which increased secretions. This wouldn’t have been possible if rasa was only a sensation on the tongue.

Ayurveda explains that Indriyas are responsible for sensory perception. And the sense organs like eyes, ears, nose, tongue and skin are the Adhishthana or seats of perception. However, these may not be the only places where perception takes place, but they may be the primary sites of their respective perceptions. With the advent of new research concepts and techniques, we are now able to dig in deeper to solve the mysteries of nature. Extra gustatory taste perception is one such discovery which uncovered a whole new dimension in comprehending the human body.

Extra gustatory taste perception is the perception of taste in parts of the body other than the tongue. It is mediated by many families of receptor channels involved in multi-systemic functions. These receptors are present all over the body in blood vessels, tissues and organs and respond to stimulant molecules which circulate in their vicinity.

Ayurveda has extensively explained about the process of digestion, absorption and assimilation of different food and medicines in the body. All the ingested matter undergoes Paka or digestion by the Agni or the Digestive Process. During paka, a number of chemical changes take place in the gut either by the breakdown of the components to simpler substances or by the formation of entirely new compounds which are in absorbable form. The gut has receptors for various tastes which are active throughout the process of digestion and respond to the taste molecules and produce systemic effects by initiating a cascade of reactions right from the gut. And the molecules which are absorbed and enter the circulation activate the peripheral extra gustatory taste receptors to bring about their effects.

Pungent tastes referred to as spicy or hot taste has been explained under the context of Katu rasa in Ayurveda. It is Agni and Vayu mahabhuta pradhana and is teekshna (pungent) by nature and it is usually perceived as a burning sensation in the mouth. The characteristic perception and the systemic effects of Katu rasa are explained in Ayurvedic Treatises as follows:

उद्वेजयति जिह्वाग्रं कुर्वंश्चिमिचिमां कटुः ।
स्रावयत्यक्षि नासास्यं कपोलौ दहतीव च ॥
कटुर्गलामयोदर्दकुष्ठालसकशोफजित् ।
व्रणावसादनः स्नेहमेदःक्लेदोपशोषणः ॥
दीपनः पाचनो रुच्यः शोधनोऽन्नस्य शोषणः ।
छिनत्ति बन्धान् स्रोतांसि विवृणोति कफापहः ॥
कुरुते सोऽतियोगेन तृष्णां शुक्रबलक्षयम् ।
मूर्च्छामाकुञ्चनं कम्पं क्टिपृष्ठादिषु व्यथाम् ॥

- अ. हृ. सू. 10

कटुको रसो वक्त्रं शोधयति, अग्निं दीपयति, भुक्तं शोषयति, घ्राणमास्रावयति, चक्षुर्विरेचयति, स्फुटीकरोतीन्द्रियाणि, अलसकश्वयथूपचयोदर्दाभिष्यन्दस्नेहस्वेदक्लेदमलानुपहन्ति, रोचयत्यशनं, कण्डूर्विनाशयति, व्रणानवसादयति, क्रिमीन् हिनस्ति, मांसं विलिखति, शोणितसंघातं भिनत्ति, बन्धांश्छिनत्ति, मार्गान् विवृणोति, श्लेष्माणं शमयति, लघुरुष्णो रूक्षश्च ।

- च. सु. 26/ 43

I will now attempt to explain the activities of Katu rasa using some research findings of contemporary science.

As mentioned above, taste is not only perceived by the tongue in the body, but also by extra sensory taste receptors present all over the body. A group of receptors has been discovered to be responding to stimuli and coordinating various events in the body. The Transient Receptor Potential (TRP) is a huge family of receptors subdivided in to 7 groups, responsible for perceiving extra gustatory taste and sensation. Members of this class of receptors are involved with many functions but only some of them are involved with perception of pungent taste and noxious stimuli viz., TRPV1, TRPV2, TRPV3, TRPV4 and TRPM2, TRPM3, TRPM8 up to some extent.

TRPV1 & 2 receptors respond to pain, heat and extracellular pH and cause changes in the Osmotic potential, enzymatic activity, endo/exocytosis, fertilization, signal transduction cell survival and development. TRPV3 are responsive to warmth or mild heat in particular. Pungent molecules Capsaicin (found in chilli and capsicum) , Piperine (found in black and long peppers) are perceived by TRPV1, Diallyl sulfide (found in garlic and onion) by TRPA1 & TRPV1, Eugenol (found in basil) by TRPV4, Menthol (found in various types of mint) by TRPM8 and Gingerol (found in ginger and its types) by TRPA1. These are some of the most frequently used pungent substances. Many more pungent molecules are being discovered.

The following functions of katu rasa can be explained well on the basis of TRP receptors.

· TRP channels have a temperature dependent activation. This means they can be regarded as a component of Agni Mahabhuta in the body. As katu rasa is also agni and vayu mahabhuta pradhana, it is well perceived by the receptors. This means that, not just Ushna guna of the rasa is perceived by them, but also temperature dependent activities of substances like Ushna jala in jwara, etc., can help in treating many diseases as rightly mentioned in Ayurveda.

· They activate salivary, esophageal and gastric secretions and thus help in increasing appetite and assisting digestion. In this sense they serve as Agni deepaka. They stimulate taste buds and are hence very useful in treating Aruchi or anorexia.

· They also increase the blood flow to the gastrointestinal tract and regulate intestinal peristalsis. In pathological states like Irritable bowel, Gastroenteritis, etc., upregulation of TRP receptors seems to precipitate the condition. This is due to the abnormal irritant quality of the agni tatva of katu rasa and hence katu rasa dravyas are usually contraindicated.

· Agni deepana not only applies to the qualitative enhancement of process of food digestion (jatharagni) but also to the overall metabolism of the body. The Liver is the largest metabolic organ in the body and TRPV1 receptor activation enhances the functional capacity of the liver. Thus it can be said that Katu rasa performs not only Jatharagni deepana but also Dhatvagni and Bhutagni deepana directly.

· Activation of TRP channels causes local dilation of vessels due to their vital role in calcium ion influx and vasodilation or dilation of any related channels. This is rightly explained in Ayurveda as Vivarana of Srotas or Margas.

· TRPV channels also perform heat dependent protein denaturation. This may be researched as the mamsa lekhana karma of Katu rasa.

· Osmoregulation is one of the primary functions performed by Katu rasa. Dilation of srotas is an important function as seen above. This leads to a potential difference in both ion channels and osmolarity which can be experienced as sweating which serves as a thermoregulatory mechanism in the short term and on the long term, it results in excessive thirst and syncope. Changes in electric potential on upregulation of the receptors results in symptoms like Kampa (tremors/ fasciculation), Akunchana (abnormal muscular contraction), etc. But these symptoms may also be as a result of toxic neurodegeneration.

· Ayurveda says that katu rasa has Kushthahara and vrana avasadana effects. Interestingly, TRPV1 channels have become a curious target for treatment of wounds and skin ailments. They contribute to vasodilation in the skin and anti-inflammatory effect on dendritic cells. They also play a major role in regeneration of sensory neurons. This also helps in relieving and itching. These properties are crucial to undo the pathology of skin ailments like dermatitis, contact dermatitis, allergic dermatitis, psoriasis, urticaria, ulcers, etc. in addition to this, these receptors also produce anti-bacterial and anti-fungal effects when activated which is vital for quick wound healing. However, this activity varies based on the condition.
Ayurveda says that all diseases are due to mandagni, which gives rise to ama. Based on the Dosha associated with the ama, pachana drugs have to be prescribed. Katu rasa is Agni and Vayu mahabhuta pradhana. Hence using Katu rasa as dipana in case of mandagni or diseases caused by pitta or vata will not serve the purpose. Katu rasa best works against the ama associated with kapha, irrespective of the site of the dosha.

· Their activation initiated the degradation of epidermal growth factor receptor, reducing malignant transformation of cells proving their potential role in the treatment of cancer, due to their teekshna guna.

· Ayurveda mentions kleda and meda shoshana activity. This is evidenced as reduced lipid synthesis & accumulation and reduction in triglyceride levels by increasing the activity of the enzyme lipoprotein lipase by TRPV1 & 3 receptors. They are also capable of reducing obesity induced inflammatory responses.

· Due to their anti-inflammatory property, they may be used locally to cure throat ailments. The common practice is the usage of mint, basil or ginger in case of a sore throat.

· Katu rasa being pungent is capable of releasing adhesions or obstructions in channels by increasing motility and secretions. This makes them upregulated in fecal urgency. Activation of these receptors at specific sites in the alimentary tract also induces emesis.

· In Ayurveda, the use of pungent drugs like vacha and hingu has been advised in case of loss of consciousness. This may be due to the fact that TRPV1 receptors are linked with cortical excitability in the brain and maybe a useful tool in managing loss of consciousness due to various reasons.

· TRPV1 receptors as seen above are activated by heat. This state is also characterized by germ cell apoptosis which is explained to be shukrakshaya in the excessive usage of katu rasa.

· TRPV4 receptors in breasts increase vascularity and permeability thereby increasing breast milk production and secretion. This supports the statement made by Arka praksha.

· TRPV4 receptors are also expressed by the sweat and lacrimal glands. Hence katu rasa dravyas are even given internally to treat diseases of the eye.

· Just as TRP receptors are activated by a number of naturally occurring pungent molecules, in pathological conditions, when the ama stays in the body for a very long time, it may turn into amavisha, an asadhya avastha caused by a vicious cycle of autoimmunity. In this state, the non-native molecules may mimic the natural compounds and activate these receptors. One such case is observed when ROS (Reactive Oxygen Species) activate the receptors resulting in death of Beta cell lines. This results in poor glucose uptake and cellular energy levels due to lack of Insulin. This may be identified as Balakshaya.
This concept of amavisha and molecular mimicry may also be the reason why TRP receptors also become pro-inflammatory in certain diseased states like Crohn’s disease, Ulcerative colitis, etc. This is just my theory and I don’t have any evidence to support the same. If the mechanism of this aberrant behavior is discovered, I shall update it here.
There may be many such conditions in which non-self-molecules react with not just TRP receptors but also other kinds of receptors in the body. Exploring this area would give a whole new perspective to the concept of Dosha Dushya Samurchana.

· TRPV1 & 4 knockout in mice resulted in better bone growth and differentiation. This shows that TRPV1 receptors have an inverse relation with bone health. Vata also has an inverse relation with bone: asthi kshaya happens in vata vrddhi. Hence it can be said that over activation of TRPV1 & 4 receptors may result in bone and joint problems and decrease in bone density and growth. This may manifest as pain in various joints of the body as pointed out rightly by Acharya Vagbhata.

· Katu rasa is mentioned as a deepana and not a pachana. This is an important point to be noted. Deepana is necessary to keep the metabolism active. But in cases where the deranged metabolism has already caused certain harm to the body, it is necessary to convert those chemicals to a non-reactive form to eliminate them from the body- ama pachana. Fire needs a constant supply of wind or air to keep burning. Similarly, to keep the metabolism up and running on the long run, Vayu is essential along with agni. This ama pachana and sustenance of agni is done by Tikta rasa dravyas which are Vayu and akasha mahabhuta pradhana. This can be attributed to its pachana quality. This is why Katu rasa is primarily a deepaka and not a pachaka.

Aptopadesha is the first among Pramanas in Ayurveda. But with the advent of contemporary science, our purview has become restricted to only Pratyaksha jnana or perceived knowledge. As a result, we are losing out on understanding the more complex systems existing in nature. Ayurveda has been regarded as pseudoscience by many and traditional Vaidyas have been called Quacks for not agreeing with the methods of contemporary science. But the theories put forward by the contemporary scientists have been constantly changing and researches conducted by them reveal more compelling evidences which support the principles of traditional medicine. Science relies on physical entities and parameters to express it. But the purview of Ayurveda extends far beyond this. Small attempts can be made to understand the physical implications of this wonderful science by using the tools and research evidences formulated by contemporary science and spreading the Science of Ayurveda to the world.