To what extent is evolutionarily beneficial brain circuitry enabling the manifestation of addiction in the modern environment?
Word count: 3870
Table of contents
1. Introduction
2. The dopamine system
3. The evolutionary advantage of the dopamine system
a. The hunter gatherer lifestyle
i. Feeding behaviors
ii. Social connections
iii. Novelty seeking
iv. Homeostatic needs
4. The evolutionary mismatch
a. Food addiction
b. Social drugs
c. Depression
5. Conclusion
1. Introduction
Genetic patterns are the consequence of natural selection. That is, the organisms that are the best equipped genetically for their environment have the most offspring, and the advantageous genes proliferate to incoming generations. When a particular characteristic persists throughout many organisms, over multiple generations, it is safe to assume it is functionally important to maintaining life in a given environment. However, the last few decades have exhibited such rapid environmental alterations that it seems to have exceeded the rate at which the process of natural adaptations can compensate. That is, characteristics that were advantageous in the historical environment are disadvantageous in the modern environment. The epicenter of this discussion is specifically focused on the dopaminergic system, and its behavioral effects in the past and present.
2. The dopamine system:
Dopamine is a neurotransmitter that has many implications within human physiology (Beeler 170), and the presence of dopamine predates humans, and is seen in a multitude of non-human organisms (Hidaka 205-214). The neurotransmitter motivates an organism to maximize the rewards of any given environment, oftentimes despite the repercussions. The activity of dopamine neurons can be categorized into two different phases: tonic firing and phasic firing. Tonic firing refers to the rate of dopamine secretion in the absence of stimuli (Wanat 195-213). Phasic pulses are transient increases in dopamine synthesis in response to a perceived reward. A phasic pulse increases the concentration of synaptic dopamine, increasing ligand interactions with the postsynaptic receptor. Dopamine receptors generate an increase in arousal, motivation, and movement. One cannot derive physical energy from a molecule of
dopamine, rather the neurotransmitter stimulates interneuronal pathways that instigates goal oriented behaviors by altering their perceived investment value. (Nitsche 2648-53).
Moreover, the fluctuations of dopamine that occur while pursuing a reward dictate one's memory of a behavior. Dopamine will fluctuate as we expect, pursue and obtain a reward, then return to tonic levels. Dopamine begins to rise when there is an environmental cue that is associated with a reward, and arousal is increased acutely. Permitting the phasic firing is substantial enough, one will engage in the goal-oriented behaviors necessary to obtain the reward. When the reward is obtained, phasic dopamine firing will adjust in accordance with our perception of gratification from the reward. That is, if the outcome is more rewarding than we expected, phasic firing will increase in magnitude to compensate. If the outcome is less rewarding, phasic firing will transiently decrease. However, the latter circumstance has much less favorable outcomes than the former. If one's anticipatory response to a reward is higher than that of its true value, one is much more likely to develop a learned aversion to the behavior, despite its true value. Moreover, if one underestimates the value of a reward, the compensatory increase associated with acquiring the reward magnifies the learned value of the reward, greater than if one had accurately predicted its value. Simply, the fluctuation in dopamine production, up or down, when a reward is acquired is the strongest dictator of one's motivation to engage in the previous behaviors in the future (Puig).
3.The Evolutionary advantage of the dopamine system a. Assessing our evolutionary lifestyle
Humans lived as hunter gatherers for approximately 190,000 years (95% of human evolution) , and gradually acclimated genetically to flourish in such an environment (Bankoff
1-7). Many of the characteristics that disparage humans from our close ancestors, such as apes, are adaptations to increase our ability to thrive hunting in a non-industrialized environment, particularly expanding the size and capability of the human brain. Given the environment, the growth of the human brain generated neuronal systems that enhanced humans' ability to reproductive fitness as hunter-gatherers. The dopamine system adapted to unconsciously inspire behaviors that enhanced survival and reproductive fitness in a much different environment than we find ourselves in currently. Hunter-gatherers' actions illustrate the fundamental rewards that entice the human brain. Evolutionarily humans spent the majority of the day focusing on subsistence hunting and farming to acquire daily nutrition and supplies. Simply, having the motivation to acquire food, water, and shelter was a necessity. Additionally, hunter-gatherers deliberately expended energy to socialize and connect with other communities, and had the intrinsic motivation for reproductive behaviors. The external environment was not as abundant in resources as it currently is in the twenty-first century, thus the majority of one's daily time and energy was spent pursuing resources. The dopaminergic system allowed these behaviors to be gratifying and salient (Pontzer 24-35) Hunter-gatherers were forced to be intentional about the behaviors they gave their attention to so that they acquired the resources they needed for survival. If one expended their energy on unprofitable pursuits, they put themselves and their community at a greater risk of not sufficiently meeting their daily survival needs. Individuals who had the most motivation to seek external rewards had superior survival, and the neuronal genetics that contributed proliferated.
i. Food behaviors
Interestingly, it is very likely that the most “food-addicted” men and women who had the greatest success as hunter-gatherers. Moreover, the men and women with the greatest propensity to overeat and store body fat when they acquired food would have the greatest success during the frequent famines. The Hadza are a modern-day hunter-gatherer tribe whose lifestyle is the best modern-day equivalent to preindustrial man. The prevalence of obesity is recorded at less than five percent, and the average body fats are between twenty-four percent and twenty-eight percent in women, and between nine percent and eighteen percent in men (Pontzer 24-35). For comparison, the average American is estimated at a body fat of forty percent in women and twenty-eight percent in men. Such discrepancies are present despite superior knowledge of nutritional sciences and human metabolism (St-Onge 2067-8). No longer do most industrialized communities suffer from undernutrition, instead, it is evident that the preponderance of the population is suffering from issues caused by the overconsumption of food. Conversely, the prevalence of excess body weight in the Hadza, and our hunter-gatherer ancestors, is not seen when living from the natural abundance, or lack thereof, of the environment. Hunter-gatherers have the same dopaminergic reward system controlling their feeding patterns. However, they live in an environment that compliments the reward system such that they effortlessly attenuate overconsumption.
Naturally growing foods inadvertently limit the energy density and hedonic pleasure of one's diet, compared to what can be obtained from food processing. The dopamine system interacts with a multitude of other psychological and physiological systems to discern caloric and nutrient density. As insufficient food would have been a much more likely cause of ill health and death, the more caloric and nutrient dense a food was perceived to be, the greater the phasic
release of dopamine it would produce. To intrinsically discern the constituents of a food, humans developed physiological tastes and sensations that signal the potential value or potential toxicity of a food (Gómez-A 593-642). Information from the gustatory system interacts with dopamine neurons to evaluate unknown foods based on taste (de Araujo S34-43). Humans developed five distinct tastes that the gustatory system can sense: sweet, bitter, sour, salty, and savory (Gravina 217-22). Each taste is associated with specific characteristics of food used to assume its constituents, in terms of caloric and nutrient content. A sweet flavor would indicate the presence of glucose, therefore that food would contain energy in the form of carbohydrates. A savory taste would indicate the presence of protein, which would also be accompanied by many essential nutrients. A salty taste would signify the presence of the obligatory electrolytes, which govern all physical and neurological functions. Bitter and sour tastes did not indicate positive attributes of food, rather they indicate the presence of toxicity in the ingested food (Katz). Sweet, savory, and salty tastes invoke phasic increases in dopamine, as they indicate the food contains essential nutrients. Moreover, bitter and sour tastes would invoke a phasic decrease in dopamine firing to establish a future aversion to potentially toxic foods. As foods in nature do not come with nutrition labels, the dopamine system adapted to increase one's ability to discern the value of a food and the value of pursuing it in the future. Furthermore, the most energy-abundant foods invoked the greatest phasic increases in dopamine, therefore we would prioritize them over inferior sources of energy density. For example, large animals likely had the greatest caloric yield in relation to the caloric expenditure of obtaining them. Therefore, humans would have developed a robust motivation to expend energy on large animals, rather than smaller fruits and vegetables. Such a pattern is confirmed through carbon isotope testing and it has been
determined that humans have evolved as hypercarnivores, and it is estimated that Neandertals' diets were derived from no less than 80% of their energy obtained from animals (Pushkina 769-782, Fragiadakis 216-227 ). As we spent the majority of evolution in a food-scarce environment, the dopamine system prioritizes the food-seeking patterns that have the largest ratio of energy yield to energy expenditure
ii. Social Connections
Social connectivity is also an intrinsically rewarding behavior, therefore the reward system upregulates such that humans are highly motivated to crave socialization. Furthermore, negative social interactions invoke decreases in dopamine, therefore feelings of aversion for certain social interactions in the future (Krach). Social interactions had a myriad of benefits that could enhance one's survival and fitness. Different communities and individuals had their own unique set of knowledge and wisdom, and socialization was the only mechanism for obtaining the knowledge of others. There was not a large database to document and store valuable knowledge, and much of what was documented was difficult to access. Thus, the acquisition of knowledge was highly contingent upon placing oneself in the correct circumstance and environment to learn from others. The greater propensity one had to engage with others, the greater opportunity we had to procure information. Further, the more diverse those interactions the more exposure one would have to differential information. Novel social interactions could provide insight into different locations and methods for acquiring beneficial resources. Moreover, pertinent information about unknown dangers of engaging in certain behaviors could
also be shared, therefore mitigating potential deaths from experimenting with unfamiliar behaviors. Social behaviors were the predominant way to learn new and beneficial information, which would inevitably decrease accidental deaths, and increase one's ability to maximize the resources within the environment.
iii. Dopamine and novelty seeking
One of the very important nuances of the dopamine system is its tendency to downregulate to a given reward upon repeated exposure. That is, known rewards become less salient after repeated exposures because they begin to invoke a more subtle phasic increase in dopamine firing. Therefore, humans have the propensity to seek novel behaviors that have the potential to be more rewarding. After entertaining a specific behavior for a prolonged period of time, the reward of that behavior begins to become predictable. As we continuously experience a situation, the value of the reward begins to diminish, and predictive dopamine release diminishes (Murray 137-145, Imperato 231-237). Consequently, motivation to continually pursue the same behavior diminishes, making more novel behaviors seem more salient, as they have a greater ability to attract one's attention. Such an adaptation is likely adapted to encourage the pursuit of greater rewards, despite having inferior rewards in the immediate environment. In addition to habituating to a learned reward, novel behaviors invoke abnormally large anticipatory increases in dopamine, therefore enhancing the likelihood they are the most incentivizing rewards in one's environment (Costa 556-566). The propensity to seek novelty was an adaptation similar to a
propensity for social connections. As discussed, the way in which a hunter-gatherer would learn about their resources was a cultivation of experiences. The more diverse and abundant one could make their behaviors, the greater their potential to be exposed to important information. Specifically, hunter-gatherers were enticed to explore novel foods that may contain nutrients they had been previously unable to obtain. Further, it may not be seeking novel foods, but rather seeking novel environments through travel. Therefore, one could locate, and propagate within, an environment of the utmost abundance.
iv. Dopamine regulated our homeostatic needs
While habituation is a recognized pattern of the reward system, it occurs permitted we have met all of our homeostatic requirements. The ability of the reward pathway to perceive and adjust to our current state of homeostasis is necessary to prevent death from dehydration and starvation. Eating and drinking are behaviors that humans likely engage with more than any other behavior. One may habituate to the rewarding aspect of a food if they are meeting their energetic and nutrient requirements. However, a state of energy deficiency is a negative homeostatic condition, and that very same food becomes more incentivizing. Likewise, despite not containing any calories, water-associated cues invoke a phasic increase in dopamine firing in a state of dehydration, and essential mineral-associated cues do the same in a state of deficiency (Mietlicki-Baase). Simply, habituation patterns are reversed after a period of abstinence from a behavior or reward. Thus, we have adapted such that we constantly maintain the motivation to pursue behaviors that allow us to meet our immediate survival requirements. Additionally, one would unconsciously diversify their behaviors. One may periodically habituate to the reward of a
food, then seek more incentivizing foods elsewhere. A period of abstinence provided sufficient time to resensitize to the rewarding sensation of food and begin eating it once again. It would be beneficial to eat in such a cyclical pattern to obtain nutrients unique to specific foods and avoid toxicities from a single food.
Another homeostatic regulator of the reward system is referred to as social homeostasis. Individuals adapt to a specific degree of social interactions through neuroplastic changes. If one's exposure to socialization decreases for a period of time, lower than what they have become accustomed to, tonic dopamine levels begin to decrease, which induces a craving and motivation to increase our dopamine levels. Moreover, habituation to socialization reduces, and social behaviors become more incentivizing. Thus, cues that indicate the prospect of socialization become more salient and more rewarding as a mechanism to maintain social homeostasis (Matthews 5-25). Humans, and the majority of other organisms, have developed genetic adaptations such that we unconsciously attain our homeostatic requirements for survival. Moreover, humans adapted a degree of dependency on a certain degree of socialization, and have specific dopaminergic signals if one's social exposures acutely decrease.
4. The evolutionary mismatch
An evolutionary mismatch is when an adaptation that has evolved to a statistically recurrent problem in an accustomed environment, fails to function properly in the modern environment. A mismatch will typically occur if there is a sudden and drastic change in the environment, such as what has occurred over the most recent Millenia. Simply, the speed at which the modern environment is changing has produced a time gap between a change in environment, and the proper adaptation to the novel environment. The dopamine pathway has
evolved to aid humans in facing the evolutionary familiar dangers of dehydration and starvation. However, we lack an adaptation that alerts us of the danger of overconsuming food, media, and especially drugs. The modern western environment provides a remarkable diversity of rewarding behaviors, often in overwhelming abundance. Humankind has transformed the world from a place of scarcity to a place of overflowing abundance, and many of us are deficient in the adaptations to combat the unfavorable consequences of overconsumption.
Although it is quite counterintuitive, as the wealth of an economy increases, the rates of clinically diagnosed addiction and depression in their community appear to increase concurrently (Smart 297-307). Addiction can be ostensibly defined as a disease of overabundance, thus it requires an environment that can facilitate the persistence of an addictive behavior. As for the latter, the rate of depression increases in both currently addicted individuals, and individuals recovering from addictions (Davis 14-8, Brenner 768-777).
An addiction can be broadly described as the persistent urge to partake in a behavior despite the negative ramifications that accompany it. All rewarding behaviors have an addictive potential, each behavior having more or less potential. In tandem with other physiological processes, dopaminergic firing increases the addictive potential of a behavior (Roy, Volkow 748-755). However, to be constituted as an addiction, it must first be overexploited to the degree by which it begins to manifest negative repercussions. As the modern environment grows in wealth and abundance, humans are exposed to a plethora of supernormal rewards, and with far fewer constraints. However, the speed at which the modern environment is progressing appears to be faster than our ability to adapt to the environmental abundance.
a. Food addiction
The food environment is a term used to describe the foods an individual has access to. Food environments vary throughout the world, and there are specific dietary changes that appear to underpin one's likelihood of overconsumption. Addictive eating behaviors were discussed in literature for the very first time in 1956 (Randolph 198-224) and is currently a clinical disorder with a prevalence of approximately one in five people (da Siiva Junior 2027-2035). Concurrently, 42% of U.S. adults are considered obese (Hales), which can be compared to the <5% prevalence in modern hunter-gatherers (Pontzer 24-35). U.S. rates of obesity rates are climbing despite a rapid increase in the knowledge of, and ability to clinically treat, excess body fatness.
The discrepancy in health seen between hunter-gatherers and western communities is not a variation in food motivation, rather it is the abundance and accessibility of food. As discussed, the characteristics of food that increase its reward value are taste sensations that indicate energy density. Additionally, the immediacy of food increases its saliency. Thus, the most rewarding foods require the least amount of time and energy to procure and provide the most energetic yield. The modern food environment exploits humans' adapted feeding drives.
Phasic increases in Dopamine occur when one associates an environmental cue with the presence of a reward. As the abundance of food in the environment increases, the probability that a food-associated cue gains one's attention is magnified. An increase in phasic dopamine firing increases one's motivation to eat, and we begin to consume foods out of habit rather than hunger (Blanco 874-880). On average, Americans reported eating approximately six times per day (Dietary Guidelines for Americans). The availability of food increases the number of occasions one is motivated to eat daily.
Additionally to continuous access to food, and food-related cues, the food environment is enriched in foods that perpetuate overeating through hyper palatability. As the knowledge of human psychology and food science begins to discover the factors that make foods rewarding, food producers deliberately curate their products such that they elicit the highest increase in dopamine. A hyper-palatable food is a food that contains a mixture of fat, sugar, and salt, and is devoid of any substantial amount of protein (Fazzino 1761-1768). An estimated 62% of the foods in the US food system meet the criteria of hyper palatability (Fazzino 1761-1768). Sugar is used to excite sweet gustatory receptors, which indicate caloric density. The correct ratio of fat provides a pleasurable texture, and the presence of salt indicates the presence of essential electrolytes. Moreover, it has been demonstrated that humans have a specific protein appetite, and we are driven to continue to consume foods until we reach a threshold of protein (Wells 292-294). Thus, protein is removed from hyper-palatable foods to minimize its influence on one's satiety.
Furthermore, after hypertonic dopamine activity, tonic levels of dopamine are reduced, inducing feelings of dissatisfaction and craving. Lowered tonic dopamine firing induces feeding absent of hunger or physical need, rather as psychological compensation for the aversive state of lowered tonic dopamine.Along with their ease of overconsumption acutely, hyper palatable foods persuade chronic overconsumption as well. Hyper-palatable foods produce the largest tonic deficit of dopamine after they are consumed (Johnson 635-641). Moreover, hyper-palatable foods make naturally palatable foods less desirable, therefore the behavior change of avoiding hyper-palatable foods has multiple arduous components.
Unfortunately, our expanding knowledge of human behavior has changed the practices of food manufacturers and food marketers. Food industries can deliberately tailor their products to maximize consumption of their products. Furthermore, the distribution of candy and other snack products has become a revenue strategy for non-food manufacturers, such as clothing and appliance stores. The modern environment over-exploits humans' adaptations to combat starvation, inducing food behaviors that meet the criteria for an addiction.
b. Social drugs
The prior fifty years can be broadly described as the digital revolution. The internet is a modern luxury with an innumerable list of benefits. However, the internet can also be misused, and elicit various inadvertent consequences. It is estimated that adults, on average, spend a little under seven hours a day on the internet (Kemp). As more people are being exposed to the internet, incidences of addictions to gaming, social media, internet gambling, and internet pornography consistently increase annually (Cash 292-298).
Entertainment applications exploit a pattern of reward referred to as intermittent reward, a pattern that maintains one's engagement with a given behavior for a longer period than a single continuous reward (Lerman 153-71, Field 283-295). A positive outcome, such as winning a game or viewing something perceived as rewarding, increases phasic dopamine firing. Thereafter, an outcome that is less rewarding decreases the firing of dopamine neurons. Even a subtle decrease in dopamine stimulates a sense of aversion toward one's internal state, therefore is driven to continue to engage with the media. Moreover, if one acutely habituates to a specific internet behavior, internet use supplies alternative behaviors to ascertain one's attention.
Identical to food, hypertonic dopamine stimulation using the internet lowers one's tonic level of dopamine. Internet use becomes salient not to induce pleasure, rather to evade an abated level of tonic dopamine stimulation. Internet addiction decreases one's motivation to pursue effortful rewards due to low tonic dopamine, thus low motivation. However the internet provides an outlet to stimulate the dopamine system without physical effort as a prerequisite.
c. Dopamine as a cause of depression
As addiction and depression often coexist, it is evident that abnormal elevations of dopamine have downstream consequences for our feeling of well-being (Conner 127-137). An addiction induces abnormally elevated levels of phasic dopamine over an acute period of time, however, such spikes lower one's tonic rate of dopamine secretion. Low tonic dopamine influences the onset of many depressive symptoms. (Belujon 1036-1046). Moreover, constant less extreme phasic increases in dopamine can also decrease one's tonic dopamine. In addition to lower tonic dopamine, unnaturally high levels of dopamine secretion causes the downregulation of postsynaptic receptors. Decreasing the number of receptors is an adaptation to maintain normal dopamine signaling in an environment of abnormally elevated volumes of dopamine (Volkow 748-755). Simply, humans have not yet adapted to cope with the abundance of the modern environment, and the concentration of dopamine is greater than homeostatic requirements. Consequently, one adapts to return to evolutionary familiar levels. When the reward system is downregulated, motivation to pursue rewards is reduced, familiar rewards become unrewarding, and one's baseline state begins to feel aversive and discomforting. The theoretical mechanism of the modern environmental underpinning depression is illustrated by the fact that westernized cultures seem to have the highest rates of depression. Moreover, the
introduction of wealth and westernization increases levels of depression in a non-westernized society (Hidaka 205-214).
5. Conclusion
Individuals with a propensity for naturally high dopamine presumably would have flourished living a lifestyle of early humans. Conversely, such a trait may very well be disadvantageous in the modern environment. The environment around us has developed so rapidly that some of our ancestral proclivities are giving rise to the development of unfavorable outcomes. Humans' inclination for calorically dense food gives rise to unintentional over consumption, and metabolic disease. The impulse to exploit rewarding behavior makes one vulnerable to develop addiction if the environment is sufficiently abundant. Furthermore, the super physiological stimuli humans have created precipitates a compensatory downregulation , which we refer to as depression. Simply, the speed at which the modern environment is changing has produced a time gap between a change in environment, and the proper adaptation to the novel environment.
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