**1. Introduction**

Mammalian central nervous system (CNS) consists of brain and spinal cord. The neuronal network extends from the brain to all over the body and various neurotransmitters help transmit the message to target cells in different tissues. The part of the nervous system located in our gut is called the enteric nervous system (ENS). In all animals, gut brain axis is a lesser known nervous system so far. Our gut or "second brain" (ENS) chemically connects with the brain through neurons, secreted chemicals like hormones and neurotransmitters that send messages to the brain. The enteric nervous system's network of neurons and neurotransmitters extends along the entire digestive tract – it starts from the esophagus to the stomach and intestines, and down to the anus. The "gut microbiome" comprises of microorganisms living in the gut (bacteria, viruses, and fungi) that can affect the chemical messages that pass between the gut and the brain. Microorganisms residing in the gut help regulate the body's immune response. Since, the brain and the gastrointestinal (GI) system are intimately connected, therefore, they play key roles in certain diseases and to maintain our overall health to regulate cognitive and digestive behavior. The bidirectional communication between the brain and digestive system hence, are opening up avenues to think about diseases considering this angle.

Gut has recently become a subject of research in medical sciences wherein subjects with depressive symptoms, Parkinson's and Alzheimer's disease, autism, amyotrophic lateral sclerosis, multiple sclerosis, pain, anxiety and other neurodegenerative conditions are beginning to be looked to see what is going on in the gut. Effects of conditions like ulcers, constipation, and other GI problems have been a focus of research on aspects of brain functioning. The enteric microbiota impacts the gut brain axis (GBA), interacts locally with the intestinal cells and ENS as well as with the CNS through neuroendocrine and metabolic pathways. These studies suggest that GBA plays a vital role in maintaining mental health and can affect the feeding behavior when nutrient detection or absorption does not function properly as in case of metabolic conditions.

To ensure stability in the internal environment of body and drive adaptive changes, control mechanisms are key to animal's survival. Recently GI has been recognized as a major source of signals modulating feeding behaviors, food intake, metabolism, insulin secretion and energy balance. Through its interaction with microbiota, it can shape our physiology and behavior in complex and sometimes unexpected ways. A growing scientific community has exploited the genetic amenability of *Drosophila* gut in great and resourceful ways. In this chapter, we are shedding some light on a broad range of biological questions revolving around gut-brain axis, neural connectivity and its role in regulating food preferences by using inter-organ signaling and disease state, especially metabolic and neurodegenerative diseases. Despite being a relatively new research area for fly biologist, many of the mechanisms active in the intestine of flies have already been shown to be more widely applicable to gastrointestinal systems of higher system and humans, and may therefore become relevant in the context of human pathologies such as metabolic disorders, neurodegenerative diseases, gastrointestinal cancers, aging, or bowel disorders. This chapter will be summarizing our current understanding and knowledge of function of the adult *Drosophila* digestive tract with a major focus on gut-brain neural connectivity and role in digestive/absorptive functions.
