Bitter insects

Melia azadirachta L. (neem or margosa). The bark, according to Cornish, yields a minute amount of a bitter alkaloid, margosine. The fruit is reported to be toxic and to contain an alkaloid, azaridine. The leaves are stated to be insect-repellent and from them an alkaloid paraisine has been prepared. The flowers are alkaloid-free. ((1) Ind. Ann. Med. Sci., 1857, 4, 104 (2) Carratala, Rev. Asoc. med. Arg., 1939, 53, 338 (3) Volkonsky, Arch. Inst. Pasteur Alg rie, 1937, 427 (4) Subramanian and Rangaswamy, Curr. Sci., 1947, 16, 182.)... 

A. indica L. Indian Aristolochia, also known as Indian birthwort, ishvara (Sanskrit), or adagam (Tamil), is a bitter climber native to India. The medicinal material consists of the rhizome, which is to resolve inflammation (India), counteract insect poison, and as an antipyretic (Philippines and Vietnam). The rhizome contains aristolochic acid, which inhibits in vitro and dose-dependent phospholipid hydrolysis by the human synovial fluid phospholipase A2, snake venom phospholipase A2, porcine pancreatic phospholipase A2, and human platelet phospholipase A2 (2). 

Alkaloids are compounds that contain nitrogen in a heterocyclic ring and are commonly found in about 15-20% of all vascular plants. Alkaloids are subclassified on the basis of the chemical type of their nitrogen-containing ring. They are formed as secondary metabolites from amino acids and usually present a bitter taste accompanied by toxicity that should help to repel insects and herbivores. Alkaloids are found in seeds, leaves, and roots of plants such as coffee beans, guarana seeds, cocoa beans, mate tea leaves, peppermint leaves, coca leaves, and many other plant sources. The most common alkaloids are caffeine, theophylline, nicotine, codeine, and indole... 

When disturbed or molested, these insects release small droplets of hemo-lymph from the tibio-femoral joints of their legs, and it is now well established that the deterrency exhibited by many species of coccinellids towards potential predators results from the presence of repellent and bitter alkaloids in that fluid [ 12,13]. In ladybirds, this unpalatability is associated with a bright aposematic coloration and a characteristic smell due to 3-alkyl-2-methoxypyrazines [14, 15]. The beetles use these molecules not only to reinforce the visual alerting signal on an olfactory level, but also as aggregation pheromones [16]. 

Chicory (Cichorium intybus) is cultivated in cool regions such as Northern Europe. Recently, this vegetable has arisen out of claims that it is able to promote good health since no pesticides are used to cultivate chicory in the field, while the plant remains noticeably free from herbivore and microbial attack. The bitter substances, lactupicrin, 8-deoxylactucin and some phenolics had previously been shown to possess insect antifeedant properties in chicory (Rees and Harbome, 1985). Specifically, sesquiterpenoid lactones from chicory leaves, such as 8-deoxylactucin and lactupicrin (Figure 1), were identified as insect antifeedants against desert locust, Schistocerca gregaria. Similarly, we found some biologically active secondary metabolites in the... 

Insect chemosensory organs have been differentially developed for taste and olfactory sensing. The contact and the distant chemosensory sensilla are responsible for nonvolatile and volatile chemical reception, respectively. The CHCs with long carbon chains are non-volatile, and therefore thought to be received by taste sensilla (Ebbs and Amrein, 2007). However, because of their insolubility in water, it was very difficult to obtain response recordings to them from taste sensilla. Success was recently obtained, however, in Drosophila melanogaster, where a male-specific CHC as a sex-pheromone inhibiting male-male courtship was found to stimulate the bitter taste receptor neuron within the... 

In order to make precise kinetic measurements of the relationship between the strength of stimulus and the magnitude of response in each receptor neuron, it is necessary to use adequate stimuli for the targeting receptor neuron. For example, a taste sensillum of flies houses four functionally differentiated chemoreceptor neurons corresponding to insect fundamental tastes sugar, salt, water and bitter taste receptor neurons. These four receptor neurons, when stimulated by adequate stimuli, generate distinguishable impulses by their... 

Alkaloids are generally bitter, which suggests that these compounds could be utilized as either animal deterrents or in intra- or interspecific competition of plant species. In some cases repellent alkaloids (e.g., pyrrolizidine alkaloids) are sequestered by herbivores and converted to compounds that function as sex pheromones while still possessing deterrent activities. The insect derivation of sex pheromones from known repellent alkaloids that accompany the ingested nutrients... 

Guaianolides (C5 C7 C40L) include many cytotoxic and antineoplastic compounds. Various guaianolides are bitter tasting and insect antifeedants. Zaluzanin inhibits bacterial lipopolysaccharide-induced NFKB-mediated expression of iNOS by immune cells and cynaropicrin inhibits similar induction of TNF-a expression through formation of a covalent protein adduct. Costunolide, 7-hydroxycostunolide and 3,4-epoxydehydroleucodin act in a similar manner to inhibit NFkB binding to DNA. 

Limonoids are C2(, nortriterpenoids deriving from a C30 triterpene precursor. The best known limonoids are the Azadirachta indica (neem tree) antifeedant azadirachtin (C50L C40 C6 -C60(epoxide methylene cross-link) furan) and the Citrus species (Rutaceae) bitter antifeedant limonin (G50L G40 G6 G6 C50L(epoxide)-furan). Limonin gives a delayed bitter taste to Citrus fruit. The limonoids are typically bitter compounds with insect antifeedant activity... 

J, adrenocortical mitochondrial PBZ-R expression —> l corticosteroid AI, antiasthmatic, antistress, insect antifeedant, bitter, neuroprotective]... 

Vinyl oxazolidine-2-thione Brassica okracea (Brussels sprouts), anti-insect, insect repellent] Bitter [affects insect... 

Besides these, a number of other alkaloids are known to have a bitter taste, which was generally discovered accidentally, as many are toxins, poisons, and other biologically active compounds. Finally, the bitter taste of alkaloids also has some importance in the interaction between plants and animals, especially insects, which have a gustative system quite different from that of vertebrates these interactions have some important applications in ecology and agriculture. 

The same conclusions were also recorded for vertebrate herbivores. For example rabbits (Cuniculus europaeus) and hares (Lepus europaeus) clearly prefer the sweet plants and leave the bitter plants almost untouched, at least as long as there is an alternative food source. In conclusion, although taste perception in mammals and insects differs in many aspects, there also some similarities both in anatomy and in the function of the bitter taste perception. A comparison of the effects of alkaloids, as well as of other bitter compounds, will be assisted by further advances in the knowledge of the structure of taste genes and receptors. 

Because alkaloids often deter the feeding of insects, such as aphids and bugs (Table I), viral infection rates may be reduced in alkaloid-rich plants. Such a correlation exists for alkaloid-rich lupines (so-called bitter... 

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