Question

Artificial sweeteners, such as NutraSweet and Splenda, have no nutritional value in terms of calories, yet they taste sweet to human subjects.

a) Explain how compounds with no nutritional value can be perceived as ‘sweet’ when ingested.

b) For most of their adult lives, mosquitoes feed on nectar (i.e. sugar water). When you run taste preference tests on lab mosquitoes, you find that though they drink large amounts when presented with sugar water, they avoid drinking water that has been sweetened with Splenda. Describe what might explain this pattern of results, in terms of gustatory receptors in insects.

Answer 1

a) Sweetness is a taste sensation perceived by tongue. Tongue have sweet taste sensory cells in the taste buds. These cells express receptor (T1r3) on their surface. Binding of artificial sweeteners, such as NutraSweet and Splenda to the receptor triggers the cells in the taste buds. This releases a signaling molecule to an adjoining sensory neuron which sends impulse to brain. Within the brain these signals cause actual sensation of sweetness. Binding of molecule to the receptor and the signaling cascade in the brain is not concerned with whether the molecule is metabolized or whether it has calories. The perception of sweet taste only depends upon the binding of artificial sweeteners to the receptor (T1r3).

b) The insect gustatory receptors (GRs) are members of a large G-protein coupled receptor family. The detection of high calorie sweett compounds is a fundamental property of the taste system. In insects 8 GRs have been mapped to sweet taste neurons. Artificial sweeteners like Splenda offer sweet taste of sugar but does not contain carbohydrates or calories whereas sugars contain carbohydrates. When mosquito ingest natural sugar with high carbohydrate content, the compound binds the receptor secreting neurotransmitter and transfers the signal to brain. Then brain process the information and gives appropriate ingestive response. The artificial sweetener inhibits post ingestive response. So mosquitoes avoid drinking water that has been sweetened with Splenda.