What does it mean to be sweet? Have you noticed that when you’re dealing with a tasty food, you can’t refuse to eat it even though you’re already satisfied? Perhaps not many have asked the following questions, but more than some feel identified with them. Today there is a great variety and availability of foods high in fats and sugars, which are partly to blame for our sweet behaviors, but they are also behind more serious situations, such as the significant increase in prevalence of obesity in the last time.
This year’s study by FAO (United Nations Food and Agriculture Organization) found that Chile has the second highest rate of adult obesity in Latin America and the Caribbean, with a prevalence of 28.8% of the population.
This increase in obesity can involve a number of factors, both physiological and environmental. An important factor is the change in the food industry over the past few decades. In developed countries, the availability of tasty foods that are high in sugar, fat and calories has transformed the modern food environment, which, coupled with low physical activity, has contributed to the high numbers of obese people in our country and in the world.
However, more than an industry issue it is necessary to take into account that food as such is a complex behavior, which is finely regulated by various signals of our nervous system, so alterations in these signals and in the regions regulations can also trigger in pathologies and eating disorders such as the aforementioned obesity, bulimia or binge eating disorder. Both binge eating disorder and obesity are characterized by an excess in calorie intake relative to energy expenditure; in fact, obesity can be one of the long-term consequences of binge eating disorder. But beyond suffering one of these disorders, overeating highly caloric foods above our energy needs is not trivial. And it is that everyone at some point in our lives or rather on a large number of occasions in our lives has consumed tasty food after we have been satisfied, and we have even been tempted to try a snack without even being hungry.
It is not surprising that this happens, because there are usually several factors that contribute to this behavior, such as social events where delicious food abounds, whether it is birthdays, together with friends, marriages, cocktails, etc. But what happens in our brains in the face of this behavior?
Food for pleasure and gratification
Under the premise that humans consume food for pleasure and gratification rather than to maintain the energy balance, it is known that the stimuli that induce food intake and the desire that is generated by wanting to consume them involve different regions Brain. Various studies in animals and people using neuroimaging techniques, which allow to see live images of the central nervous system and the brain, have revealed that highly caloric and delicious foods are able to activate brain circuits present in the amygdala, which is one of the regions of our brain responsible for processing emotions, mainly associated with fear, but which also plays a role in the sense of reward generated by food. Previous research has shown that consuming tasty foods increases the number of genes that depend on neural activity in the central amygdala. However, it has been a complex task to specifically study what happens in individual populations of neurons in the amygdala at the time of eating and even more study what happens in our brain when we cannot stop eating delicious snacks.
A recent study published in the journal Neuron and led by Thomas L. Klash and colleagues managed to finely elucidate the neural circuit behind this hedonic behavior versus tasty foods. Researchers showed that the central amygdala is strongly activated in response to tasty food consumption and that a subgroup of neurons containing the prepronociceptin molecule (related to increased appetite) are responsible for promote the consumption of delicious foods.
To do this, the researchers genetically modified mice by injecting a virus into the brain region corresponding to the central amygdala, in order to specifically mark the neurons that expressed the prepronociceptin molecule (Pnoc+ neurons from now on) and study its activity. A group of animals were exposed to a high-fat diet of delicious taste (because of course, just like we rodents love food rich in fat and calories) for intermittent periods of one hour, and another group of animals was fed the normal diet ( control group).
As a first result the researchers observed a large increase in the activity of the Pnoc+ neurons of the central amygdala in animals exposed to the diet high in fat tasty, compared to the control group. In addition, the activation of Pnoc+ neurons was proportional to the amount of calories ingested, i.e. the more calories consumed by mice, the greater the activity of the Pnoc+ neurons, which was also correlated with an increase in weight Body.
Specific function of Pnoc+ neurons
To more specifically understand the function of this neural group during food consumption, scientists used a technique known as optogenetics which involves controlling the activity of specific groups of neurons using pulses of light. This is achieved by incorporating proteins that respond to light in these neurons. The most studied is derived from rodopsine (ChR2), a protein that in the presence of light is activated and promotes that the inside of the neuron becomes more positive, favoring the generation of electrical impulses that allow neurons to communicate.
To study the activity of Pnoc+ neurons versus food consumption, researchers were able to incorporate ChR2 specifically into this type of neurons in the central amygdala. The number of electrical impulses of Pnoc+ neurons was analyzed in animals that were fed for a short period of time with normal diet, high-fat diet of delicious flavor and normal diet adulterated with quinine (bitter taste). The results showed that there was a significant increase in the number of electrical impulses of Pnoc+ neurons, during consumption of each type of food. These results in conjunction with the above showed that specifically the Pnoc+ neurons of the central amygdala were dynamically activated during food consumption of various flavors, but that their increased degree of activity occurred when the animals were fed a delicious, high-fat, high-calorie diet.
But what if these neurons are inhibited? Would the animals stop eating? The researchers also wanted to address these questions, in order to know the two scenarios and to know in a concrete way what the function of these neurons was.
To inhibit Pnoc+ neurons, scientists used DREADD technology, which involves modifying specific sites in neurons in a particular brain region to control their activity (increase or inhibit). These sites respond or activate only in the presence of a synthetic molecule, which can be administered by a simple injection into the study animal.
In this work the inhibitory DREADD was used, which in the presence of the synthetic molecule produces inhibition of neuronal activity, that is, it shuts down the neuron whose sites were modified. This inhibitory DREADD receptor was located only in the Pnoc+ neurons of the central amygdala of the mice under study, so only this neural type could be manipulated (inhibited on this occasion). Inhibiting Pnoc+ neurons with this technology significantly reduced consumption of delicious, high-fat food. Interestingly the consumption of the normal diet was not altered.
In short, the importance of this work lies in the fact that at the first one the specific function of the Pnoc+ neurons of the central amygdala is discovered, which promote the constant consumption of tasty foods, causing various consequences metabolic measures such as increased body weight, amount of body fat, etc.). But this topic also leads us to question certain things about eating delicious food, such as what foods do we consider tasty? What defines a food as tasty? Are they always the ones with the highest number of calories? Regarding tastes, an important factor that determines that something is tasty for me and for another not, is culture. And an explanation already accepted by the experts for this, is that the attraction for certain foods involves an emotional factor in which the tendency to prefer one food over another is determined in part because it reminds us of something familiar. For example, if we see a delicious piece of cake or an appetizing plate of fries, without thinking too much we will taste it, this because in our childhood this type of food was frequent to find it in birthdays and family celebrations.
With all that is mentioned, one thing is clear: Our brains seem to be connected to respond positively to delicious caloric food and the constant availability of fatty foods does not help much in the controlL of this answer, which means that we should monitor or control what we eat and above all from time to time have great willpower when it comes to saying “I’m not hungry” when they offer us a tempting snack.
Link to the original article: https://www.cell.com/neuron/fulltext/S0896-6273(19)30326-5
* Text of the agreement with the Interdisciplinary Center for Neuroscience of the University of Valparaiso (CINV).
