Neurotransmitters

Neurotransmitters are genes that promote or inhibit the release and transmission of electrical impulses between a neuron and other cells. These chemicals include around 10 “small-molecule neurotransmitters” (e.g., acetylcholine, dopamine, norepinephrine, serotonin) and over 50 neuropeptides (such as neuropeptide Y [NPY] and agouti-related protein [AgRP]), and are related to the control of a subject's physiological processes. They, therefore, play an important role in normal functioning. As many neurotransmitters are involved in food intake and in energy expenditure, they are also implicated in eating disorders and obesity.

Indeed, humans have an excellent system which allows them to conserve energy in time of plenty, storing it in the form of body fat, but very few mechanisms aimed at decreasing unnecessary food intake. In fact, as humans are more dependent than most animals on social cues and timetables, sometimes they eat even if they are not hungry. However, as a general rule, food intake is related to the perception of hunger and satiety, which originate in the brain through processing signals which can be activated by dietary breakdown, or produced by distension of the stomach and intestines. This process is regulated by interactions between neuronal networks and neurotransmitters.

Neurotransmitters are produced in the cell body of the neuron and then transported to the end of a presynaptic axon or synthesized directly within the axon terminal. When nerve impulses reach this point, the small vesicles that contain the molecules of neurotransmitters fuse with the membrane of the axon terminal, so that the molecules of neurotransmitters are released into the synaptic cleft. Here, they can bind to specific receptors and start their excitatory/inhibitory action. In fact, some neurotransmitters (e.g., glutamic acid) promote the transmission of electrical impulses, while others (e.g., gamma aminobutyric acid) discourage it.

Various mechanisms have the task of stopping this stimulating/inhibitory action. The neurotransmitter can be removed from the synaptic cleft, its structure can be changed by specific enzymes, or the whole neurotransmitter molecule can be re-taken up into the axon terminal which released it. Furthermore, the effectiveness of the neurotransmitter action can be hindered by vesicles which are too small, and/or by their too rapid closure.

Many neurotransmitters are involved in the process of food intake. Some of them (e.g., dopamine, norepinephrine, and serotonin) suppress the appetite, while others (e.g., neuropeptide Y) stimulate it and promote overeating. Overeating, which may lead to obesity, could thus be caused by a lack of anorexigenic (food intake reducing) neurotransmitters, or a failure in the reception mechanism of their signals.

As neurotransmitters can easily be modulated for treatment purposes, scientists are developing antiobesity medications whose goals are to increase the level of anorexigenic neurotransmitters in the body (enhancing their production and preventing their destruction or reabsorption by the nerve endings after they have performed their function) and optimize the reception mechanisms of their messages.