Dr Gaurav Das : ‘Fruit flies remember components of food’

Food does not merely supply calories, but also provide specific components like carbohydrates, proteins, lipids, water, or minerals that are essential for proper functioning. At times, adulterated food could result in sickness and is better avoided subsequently. However, very little is known about the neural circuit basis of how discrete components of food are learned and remembered to help us make correct food choices.

To define food component specific circuits and their interactions, I use the sophisticated genetic toolbox in Drosophila to control specific neurons while they learn and recall compound food cues. Further, I study how physiological states like hunger or thirst influence the processes of componentspecific
learning and memory recall.

My recently published work showed that in flies trained to associate an odour with an edible sugarbitter
tastant mix, the separation of positive and negative components of food could take place at the level of mushroom body afferent dopaminergic neurons in the brain. Fly dopamine neurons assigning reward or
punishment values can be physically segregated, based on component quality. This segregated layout allows the compound mixture of bitter tasting sugar to be encoded as two opposing bitter and sugar memories through the parallel recruitment of both the negatively and positivelyreinforcing
subsets of dopaminergic neurons.

I am currently studying the influence of internal states on memories reinforced by two positive food components: sugar and water. A default view in the fly learning field is that a stimulus could only be rewarding if it redresses a deficiency state; thus flies seem to learn sugar as a reward only when hungry. Surprisingly, I show that thirsty but nonhungry flies can learn the rewarding value of sugar when constrained to consume sugarwater mix during associative olfactory training. This enables me to determine whether learning under different states (hungry or thirsty) engages distinct circuit
elements in the fly brain.

Lastly, hunger and thirst specifically control the expression of sugaronly or wateronly reinforced memories. Strikingly, when flies are trained with compound sugarwater reward, the resultant memory is expressed when flies are both hungry or thirstyirrespective of the state of the flies when trained. I intend to explore how such specific connections are made between deprivation states and the reward (hunger with sugar memory or thirst with water memory).