Insects, bitter taste alkaloids

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... 

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. 

It has been suggested that the toxic effect of pyrrolizidine alkaloids does not really protect insects, because the toxicity only comes into effect when the compound is metabolized, which is well separated from the eating or tasting of the insect, rather it is only the bitterness of the taste of the compounds that offers protection. 

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