Azadirachta indica is a medicinal plant belonging to the family meliaceae. It is mostly seen growing in tropical and subtropical regions of Southeast Asia. This species is native to India and it is known as Neem, Neemtree or Indian lilac. It is primarily used for production of natural insecticides and long being used medicinally for various diseases specifically in India. Its extracts contains various biologically active components commonly known as triterpenoid or limonoid like azadirachtin, nimbin, meliacin etc. from the different parts of the tree. But the prime secondary metabolite is azadirachtin and it is crucial for insecticidal and larvicidal activities and also various other properties like anti-allergenic, anti-fungal, anti-inflammatory and many more.
Neem (azadirachta indica) is a fast growing tree which can easily reach height upto 15 to 20 metres. It has been used medicinally long back due to its remarkable medicinal properties and has a vital importance in India and becoming famous all over the globe. Neem has been considered harmless for human and animals and other beneficial insects. This is approved by US Environmental Protection Agency and thus it is safe for use in agricultural land and on food crops.
The biologically active components of Neem tree are generally obtained from oil, leaf and bark parts of the tree. The formulations are based on the tetracyclic triterpenoid parent compound called ’tirucallol’.
All naturally occurring terpenoids on thermal decomposition gives isoprene (2-methyl-1,3-butadiene) as one of the product. Thus terpenoids are classified on the basis of isoprene (C-5) unit present in them. The triterpene tirucallol has 6 isoprene (C-5) unit whereas mono and diterpenes have 2 and 4 isoprene (C-5) unit respectively.
Azadirachtin itself has 9 isomers (A-I) but notably A, B and H isomers are present in higher percentage. The important biological active components found in neem tree are produced as a result of many chemical processes like oxidation, reduction, rearrangement from the parent tirucallol molecule.
Isolation and Extraction of Azadirachtin
The neem tree contains bitterness in varied extents but the major portion is found in the neem kernel. Neem kernel is valuable source of other major limonoids. The seed coats and pulps are removed from the neem seeds by a process called depulping of neem fruits. It is normally done by hand or by using a mechanical depulper. After depulping and cleaning the dried neem seeds are stored in a cool and dry place.
The outer shell of the dried neem seeds are removed and the kernels present inside the shell are separated and made into powder using a grinder. Previously it was done using a mortar and pestle. It should be pounded such that no oil comes out. For extraction of azadirachtin by solvent process, 500 grams of fine neem kernel powder is weighed and transferred into a thimble placed in a soxhlet apparatus. To it about 600 ml of DCM (Dichloro methane) was added. It was then kept on a heating mantle and heat to reflux for about 12 hours. At this point, liminoids and other constituents get dissolves in it. The solvent from the mixture is recovered by distillation. The distilled solution is kept aside for cooling. Hexane is added to the distilled solution and then filtered using a vacuum pump. The above residue is dried which gives a pale greenish coloured powder. The powder consists of azadirachtin and a very small quantity if nimbin ( a triterpenoid). It is interesting to note that 100 grams of fine neem kernel powder on extraction gives about 1 gram of azadirachtin.
Mechanism of Action
Azadirachtin is very crucial in natural larvicidal or insecticidal activities which includes antifeedancy, insect growth regulator (IGR) and reproductive effect.
The mode of action of azadirachtin primarily consists of effect on the chemoreceptors found on the insects which leads to antifeedancy and effects in juvenile and ecdysteroid hormones e.g. PTTH (prothoracicotropic hormone) and allatotrpoins. The hydroxyl furan fragment is responsible for antifeedant effects. It reduces the food consumption by the insect and causes inhibition in various growth processes of the insect larvae depending on the concentration of azadirachtin used. Hence, by increasing the concentration or amount of azadirachtin the larval stage reduce longevity, fecundity, wingless adults and incomplete moulting process.
Azadirachtin treatment given to male and female insects shows apart from antifeedant and insect growth regulator (IGR) effects followed by the adverse effect on the reproductive system. This includes effect on the ovarian development, oogenesis and fecundity, egg viability, ovarian protein levels, reduced oviposition etc.
Azadirachtin is also have detrimental effects on the endocrine system of the insects and causes reduced or delay in important development hormones. These hormones are mainly ecdysteroid type such as ecdysone, ecdysterone, turkesterone and 20-hydroxyecdysone. Azadirachtin mainly targets the epidermal cells where ecdysteroid hormones are produced.
Juvenile hormones on the other hand, are also affected in the presence of azadirachtin, but this is quite complex to give a plausible mechanism, as it is simultaneously related to other processes like, neurosecretion and ecdysone release during the moulting. The azadirachtin primarily hinders the release of the allatotropins and thereby blocking the synthetic and release processes of the juvenile hormones. This leads to the overall deficiency of juvenile hormones in the body of the insects. This deficiency disturbs the oogenesis process and crochets destruction in the larvae.
It is important to give some details regarding the structure-properties, relationship of the hydroxyfyran ring of the azadirachtin. The decalin and dihydrofuran ring of azadirachtin are important for the activity of the azadirachtin molecule as a pesticide, while hydroxyfuranacetal is responsible for antifeedant effects. Also C7 and C11 hydroxy groups (-OH) play a role for potency and for the transport phenomena hydrophobic moiety is essential. At present more mechanistic studies are being conducted by many authors worldwide to have more detailed perspective at molecular level.
For ease of understanding, below is a table to summarise the azadirachtin effects on the insects and a brief conclusion of its mechanism of action.
| Name of the Effects | Targeted Organs/Tissues | Mechanism of Action |
|---|---|---|
| Antifeedancy | Mouthparts and other chemoreceptor, gut | Sugar cells inhibition and stimulation of deterrent cells, inhibition of peristalsis, reduced production of the enzymes. |
| IGR effects | Cuticle | Reduced ecdysteroid and juvenile hormones with the blockage of morphogenic peptide hormones |
| Cellular Processes | Cell Division | Inhibition of mitosis, meiosis metaphase and blockage of cell division |
| Sterility | Reproduction System | Reduction of ecdysteroid and juvenile hormones and it leads to reduced eggs. |
International Research Journal of Biological Science
Journal of Advanced Pharmacy Education and Research
International Journal of Agriculture and Biology
Herbal Remedies of Azadirachta indica and its Medicinal Applications
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PS The thumbnail image is being made by me via Canva.com using template image from Ninetechno and all other structures are drawn by me through KingDraw app
