# Antidiabetic Potential of Monoterpenes: A Case of Small Molecules Punching above Their Weight
## 3 General Function in Nature  
Terpenoids play important roles in the organisms (bacteria, fungi, plants, and animals) that produce them. The large volume of volatile compounds that plants utilize to attract pollinator insects or deter herbivores belong to the monoterene class of compounds. As such, the fragrances of herbs, spices, flowers and fruits are attributed to the essential oils which are dominated by monoterenes. The various roles played by monoterenes as signaling molecules in plant metabolism, plant-plant and plant-animal interactions have been extensively reviewed [12, 17, 18, 19]. The monoterenes are also among the best exploited group of compounds by mankind. The most prominent use once again relates to them being the major component of essential oils that make them valuable in the perfumery industry. Within the food industry, the flavoring and preservative potential of essential oils have been well understood along with their general antimicrobial and antioxidant effects. They are also exploited as pharmaceutical agents [19, 20, 21] and in agrochemical industries primarily as insecticides [22].  
The general mechanism of action of monoterenes such as their antimicrobial and cough therapy have been mainly associated to their volatile nature allowing them to freely move through space inclining biological membranes and interact with various biomolecules. Some of these actions have long been perceived as non-specific that lack drug-like selectivity to particular receptors. For example, the hydrophobicity of these compounds and essential oils in general have been shown to account for  
Figure 2: Structures of monoterenes discussed in this review as antidiabetic agents.  
their disruptive effect on bacterial cellular structures (e.g., cell membrane) and hence leading to cell death [23]. There are also reports on non-specific and non-competitive mechanism of action for the muscle relaxant effects of some monoterenes. For example, [PERSON] and [PERSON] [24] have shown that the relaxant effect of carvacrol on tracheal smooth muscles of guinea pigs could not be accounted by effects via \(\beta\)2-adrenergic stimulatory, histamine H1, and muscarinic blocking. On the other hand, there are reports on specific effects of monoterenes on receptors, such as in the cooling effect of menthol through action on thermoreceptors, even though the lack of specific mechanism for nasal decongestant action by monoterenes is still the subject of intense discussion [25; 26]. A non-competitive manor of receptor inhibition by monoterenes have also been reported and exemplary findings include the direct interaction of linalool with the \(N\)-methyl-d-aspartate receptors [27] and nicotinic acetylcholine receptor inhibition by borneol [28]. Not surprisingly, the antidiabetic effects of monoterenes were only coming to light in the last decade but now appear to gain lots of momentum. In the following sections, some exemplary effects collected from in vitro and in vivo studies are presented to highlight their promise in diabetes as well as insulin resistance and obesity.