Introduction

Adolescence has been associated with risk taking behaviors, with at least 50 percent of adolescents in the US taking part in risky behaviors such as drug abuse, unprotected sexual intercourse, driving under influence, and engaging in minor delinquencies, among others. In most cases, this risk-taking behavior does not extend beyond adolescence and tends to reduce as an individual progresses to adulthood. For Anderson & Spear (2014), adolescents normally experience a developmental transition occurring between maturity and immaturity, which is typified by physiological and hormonal changes, as well as sexual maturation and a significant growth spurt.

Research has also pointed out that, during adolescence, marked alterations occur in the brain, which relate to changes shaping the adolescent-typical behaviors. It is fascinating to note that those behavioral, neural, and physiological changes that happen during the immaturity-maturity transition have been evident since evolution, with several similarities with regard to the nature of the adolescent-typical changes in a number of mammalian species. The duration associated with this developmental transition usually depends on the species lifespan; however, owing to the fact that the onset and offset of adolescence are not marked by a single event, it is relatively difficult to establish the specific timing of adolescence for any species.

The similarities in adolescence of various species with regard to behavioral and biological features provide a rationale for using animal models of adolescence in order to evaluate the environmental and neural factors that influence adolescent-typical functionality (Doremus-Fitzwater, Barreto, & Spear, 2012). Certainly, the full complexity associated with the behavioral function and the human brain in the course of adolescence cannot be entirely likened to modeled using other species, which raises validity issues when modeling human adolescent behavior using other species such as rats.

Nevertheless, careful consideration is needed to ascertain the validity of any adolescent model. In addition, this validity depends significantly on the particular aspect of adolescence that is being investigated (Anderson & Spear, 2014). The goal of this research paper is to model adolescent behavior by comparing how adolescents and adults are different with regard to place conditioning with ethanol or other drugs. To this end, this paper provides an overview of the topic followed by a review of relevant empirical literature.

Overview of the Topic

According to Spear & Varlinskaya (2010), the adolescent brain experiences significant molding that is both highly system-specific and regional-specific. In addition, this molding has been observed across species. Some of the development transformations that occur in the course of adolescence include a decline in the density of the gray matter and the cell bodies in sub-cortical and cortical regions of the brain. Those declines in the density of the gray matter can be partly attributed to:

  • the increase in synaptic pruning (whereby about 50% of the synaptic connections in cortical regions are lost when adolescence sets in);
  • cell death that is genetically programmed;
  • developmental decreases in neurogenesis rates;
  • development increases with regard to the amount of white matter in the brain (Spear & Varlinskaya, 2010).

Some of the important regions in the brain that experience transformations in the course of adolescence include brain regions that make up the major nodes used in the modulation of sensitivity towards and motivations for a number of natural rewards, such as risks, novelty, and social stimuli. Those rewards are likely to be modulated and motivated by non-natural rewards like alcohol and other drugs. Examples of these regions include some forebrain areas like dorsal striatum, amygdala, nucleus accumbens, and prefrontal cortex (Spear & Varlinskaya, 2010).

Changes in the adolescent brain have been established to result in several functional consequences. One of the most important functions associated with these changes in the brain in the course of adolescence relate to the transformation of the plastic, immature, and inefficient brain into a less-plastic, efficient, and mature brain that can support adult-typical behavioral and neural functions. Another important function associated with the changes in the adolescent brain relates to triggering increases in pubertal hormones, which leads to sexual maturation as well as to the remodeling of other regions of the brain in ways that are sex-appropriate.

Even when the remodeling occurs rapidly, the adolescent brain ought to do more, that is, be able to accommodate the emerging adult-typical neurobehavioral functions, while at the same time being able to support adolescent functioning (Spear & Varlinskaya, 2010). In addition, owing to the fact that adolescent developmental transitions take place in forebrain regions that play a crucial role in influencing behavioral responses to rewards like risks, novelty, and social stimuli, it is expected that these changes should result in significant increase in social interactions, risk-taking behaviors, novelty-seeking behaviors and peer affiliations, which are age-specific.

According to Spear & Varlinskaya (2010), these age-specific behaviors have been observed in adolescents of diverse species and thought to have result in adaptive advantages for adolescents. One of those adaptive advantages is social interaction, which plays a role in the development of social support, social skills, and choice of behavior. Risk-taking and novelty seeking behaviors have been thought to lead to peer acceptance, increase exploration beyond the home territory, facilitate emigration, thus, circumventing inbreeding together with its associated adverse outcomes.

Besides accommodating adult-typical neural development and at the same tine supporting adolescent functioning, changes in the adolescent brain have also been linked to auxiliary outcomes. For example, some of the brain regions that experience alterations in the course of adolescence are affected by drugs abuse, including ethanol, which is likely to change adaptations and sensitivity to alcohol and drugs abuse in the course of adolescence, as well as influence the likelihood of using those drugs.

Analysis

People usually begin consuming alcohol during adolescence, with some engaging in the consumption of alcohol at 14 years. Pautassi et al. (2011) reported that the per-episode alcohol consumption among 12-20 year-olds is two times higher than that of legal age drinkers. In addition, high adolescent alcohol consumption has not only been observed in humans, but also in adolescents of other species. For example, Doremus-Fitzwater et al. (2005), cited in Spear and Varlinskaya (2010), found out that adolescent rats drink about 2-3 times more when compared to adult rats. Studies involving animals have shown that adolescents voluntarily consume more alcohol when compared to adults and that sensitivity to diverse alcohol differs between adolescents and adults.

For example, adolescent rats have been found to be significantly less sensitive than adult rats to several seemingly undesirable effects of alcohol that are likely to act as cues to moderate alcohol intake (aversive conditioning), such as social inhibition, that is ethanol-induced inhibition, sedation, and hangover effects. In addition, adolescents are less sensitive to dysphoric effects, such as conditioned-taste aversions, when compared to adults, with adolescents needing higher alcohol doses together with more pairings of a new taste in order for the aversion to be effective. Nevertheless, it is impossible to perform similar aversion experiments on human adolescents owing to the ethical constraints associated with giving adolescents alcohol.

However, an early study by Behar et al (1983), cited in Spear and Varlinskaya (2010), reported that 8-15 year-old boys exhibited few intoxication signs when measured clinically, subjectively, or when using the objective physical intoxication tests. Despite the fact that alcohol use by human adolescents and sensitivity have not been vastly explored, the existing scanty evidence indicates that the reduced sensitivity to intoxicating and impairing effects associated with alcohol, observed using simple animal models, are also recorded in human adolescents.

The fact that adolescents are insensitive to the effects of alcohol that are supposed to moderate its consumption is consistent with the increase in the prevalence of binge drinking among youths (Spear & Varlinskaya, 2010).

Studies on adolescent rats indicate that the relative insensitivity to the impairing and intoxicating effects of alcohol is likely to be further reduced by history of prior alcohol consumption or prior stress. A case in point is that, in adolescents, higher levels of exposure to alcohol are needed to restrain social behavior than in adults. This inconsiderateness to the social behavior suppressing effects associated with alcohol is reduced further after days of continued restraint stress. In addition, chronic exposure to alcohol has been established to increase tolerance among adolescents to sedative and aversive effects associated with alcohol consumption. This tolerance further reduces adolescents sensitivity to effects of alcohol.

Nevertheless, whereas it is quite clear that, under particular circumstances, continued alcohol exposure results in chronic tolerance in adolescents, further reducing their insensitivities to alcohol, adults have also showed signs of chronic tolerance. However, there are mixed results as to whether chronic tolerance among adults is higher, lower, or equal relative to adolescents. Overall, it is evident that adolescents often exhibit reducing sensitivity towards the aversive effects associated with alcohol (Spear & Varlinskaya, 2010).

Moreover, this reducing sensitivity increases tolerance to such aversive effects, something that has also been observed in adults, although it is not clear whether adults have significantly higher or lower tolerance to aversive alcohol effects. Besides the aversive effects of alcohol, the interoceptive effects associated with alcohol were explored by Anderson and Spear (2014),who used the rapidly acquired Pavlonian conditioned approach procedure to characterize ethanol discrimination by adolescent and adult rats.

The authors reported that, though both adolescent and adult rats reacted differently to the ethanol and the saline sessions at the end of the acquisition, adult rats had higher discrimination scores than adolescent rates, especially during reinforcement sessions.

As a result, the authors concluded that adolescent rats are less sensitive to the interoceptive effects associated with alcohol when compared to the adult rats, which is an indicator that adults consider ethanol concentration to be a more important cue than adolescents (Anderson & Spear, 2014).

In contrast to the reduced sensitivity towards the aversive, sedating, inhibitory, and impairing effects associated with alcohol consumption that adolescents exhibit relative to adults, adolescents have been found to be more sensitive to selected alcohol effects. For instance, on the one hand, it has been revealed that adolescent rats are likely to be more sensitive than the adult rats to ethanol-induced disruptions in the brain plasticity, as well as to memory performance tested using a spatial water maze (Spear & Varlinskaya, 2010).

Young people who have just attained the legal drinking age (21-25 years) have also demonstrated the same higher sensitivity to alcohol-related memory performance than older adults aged 25-29 years. The higher sensitivity manifested itself with regard to the performance of memory tasks, as well as both nonverbal and verbal learning after moderate alcohol consumption. Spear and Varlinskaya (2010) consider this increased sensitivity to alcohol-induced effects in memory tasks among adolescents as unfortunate, because the reduced sensitivity to the intoxicating and aversive effects of alcohol may not be effective in moderating alcohol consumption.

As a result, the unique patterns that adolescents have with regard to their sensitivity to the effects of alcohol may encourage adolescents to consume more alcohol even though they are highly sensitive to memory disruptive effects associated with alcohol consumption.

In addition, Spear and Varlinskaya (2010)pointed out that increased alcohol intake can also be encouraged in the course of adolescence because of the adolescents sensitivity towards a number of the appealing effects associated with alcohol consumption. Some of the appealing effects of alcohol use include alcohol-induced social facilitation, rewarding effects of the alcohol, and the self-medicating effects associated with alcohol consumption. The most appealing alcohol-induced effect among adolescents is the alcohol-facilitated social behavior.

In this regard, studies have shown that social interactions in adolescent rats increase after low alcohol doses in new, same sex, familiar and nonthreatening situations. However, under the same test environments, adult rats do not exhibit those social facilitation tendencies. In addition, human adolescents have often quoted social facilitation as one of the vital consequences of consuming alcohol. Moreover, adolescent rats are also sensitive to alcohol-induced anxiety, especially when they are placed in environments that raise their baseline anxiety levels because of the history of alcohol use or previous exposure to some kind of a stressor. Chronic alcohol doses do not only reduce sensitivity to the impairing alcohol effects but also boost anxiety.

Specifically, alcohol has been found to increase the baseline anxiety levels of adolescent rats; however, among adults, alcohol reverses the alcohol-induced anxiety. As a result, alcohol is likely to be used to counter the anxiety that is introduced by prior exposure to alcohol or previous stress, especially among adolescents (Philpot, Badanich, & Kirstein, 2003).

The apparent reduced sensitivity to the intoxicating and aversive effects associated with alcohol experienced during adolescence is likely to interact with other factors. Those factors further lessen alcohol sensitivity and increase the likelihood of developing problems with alcohol during adolescence. For instance, lower alcohol sensitivity has been identified as a significant risk factor for future alcoholism, probably because it increases the likelihood of heavy drinking.

One of the major contributing factors to high insensitivity to alcohol is the genetic background, whereby lower sensitivity to intoxicating and aversive effects of alcohol is evident in not only offspring coming from a family with a history of alcoholism but also evident in several lines of rodents that are bred selectively for high voluntary alcohol consumption. As a result, a combination of genetic-based lower sensitivity to alcohol and early alcohol initiation during adolescence form a recipe for high levels of alcohol consumption when the genetically at-risk youth experience stressful situations (Doremus-Fitzwater et al, 2012).

Although numerous factors contribute to the expression of risk-taking behaviors among adolescents, impulsive behaviors have been identified as one of the major contributors, whereby high impulsivity is associated with the likelihood to engage in risky behaviors during adolescence. In this regard, Doremus-Fitzwater et al (2012) examined impulsive behavior using delay discounting. Greater impulsivity was denoted by the more rapid shift in preference for smaller and immediate rewards, whereas lower impulsivity was denoted by delays in order to increase the reward.

In addition, delay discounting has been found to be a function of age in humans, which makes it an ideal framework to compare impulsivity and risk-taking behavior among adolescents and adults. Specifically, human adolescents have been established to exhibit more impulsive human behaviors, that is, they exhibit more delay discounting, when compared to adults.

The researchers found out that adolescent rats showed higher levels of impulsive-like behavior when compared to the adult rats, even after adjusting the data for the baseline differences in the levels of activity across. The authors concluded that impulsivity is a persistent nature of adolescence, which is a significant contributor towards adolescents expressing risky behaviors such as alcohol consumption in the course of the ontogenetic period.

In addition, the authors used their findings to report that similar tendencies have been reported in human studies. Thus, adolescents have been established to be intolerant to delay and more likely than adults to quickly change their preference and opt for behaviors and actions that result in smaller but more immediate rewards, when compared to cases when a delay is initiated.

Responses to aversive effects of alcohol and other drugs have also been explored using the conditioned place preferences (CPP), sometimes referred to as environmental place conditioning. CPP is a method commonly deployed in animal studies to assess animals preferences fir environmental stimuli linked to a negative or positive reward. CPP is normally utilized in determining the addiction potential of a given drug. CPP entails performing a number of trials, whereby the animal is provided with a positive stimulus, such as food, and paired with another placement in a unique environment that has a number of cues, olfactory, visual, or tactile. The subject is then tested in a normal state in order to find out the measure for reward learning as well as indicating preference.

Philpot, Badanich, and Kirsten (2003) used CPP to evaluate the ability of ethanol in supporting aversion or CPP. The authors specifically examined alcohol-induced conditioning in rats in postnatal days 25, 35, 45, and 60, and how they responded to doses of ethanol, which included 0.2, 0.5, 1.0 and 2.0 grams per kilogram intraperitoneally. The results indicated that, at higher doses, aversion to the ethanol-paired compartment was observed; however, this trend differed significantly with respect to age. Specifically, younger rates (PND 25 days) showed signs of CPP at low ethanol doses and aversion at high ethanol doses. Late adolescent rats showed signs of CPP at moderate ethanol doses and CPP at the highest ethanol dose.

Adult rats (PND 35 and 60) showed neither signs of CPP at any ethanol dose, nor signs of progressive aversion when the dose was increased (Philpot, Badanich, & Kirstein, 2003). The authors used these findings to affirm the development processes experienced during adolescence and their respective effects on alcohol responsiveness. Specifically, the authors established that adolescents prefer ethanol doses that are aversive for adults. Similar results were reported by Pautassi et al (2011), who found that adolescents are more insensitive to the aversive effects of ethanol than adults.

There is evidence suggesting that reduced sensitivity to alcohol and high sensitivity to appealing effects of alcohol are also manifested for the case of other drugs. Using CPP, it has been reported that adolescents tend to develop stronger nicotine-induced CPP when compared to adults. Enhanced CPP has also been reported in adolescent rats for psychomotor stimulants, including cocaine. Despite the fact that adolescents are highly sensitive to the appetitive/appealing features of alcohol and other drugs, they are highly insensitive to the aversive effects associated with these drugs than adults (Spear & Varlinskaya, 2010).

Conclusion

This paper has compared adolescent and adult responses to effects of alcohol and other drugs by using animal models. A consistent finding reported in literature is that adolescents are less sensitive to aversive effects associated with alcohol and other drugs than adults; however, the adolescents are more sensitive to appetitive effects of alcohol and other drugs. In addition, chronic exposure to alcohol has been established to increase tolerance among adolescents to sedative and aversive effects associated with alcohol consumption. This tolerance further reduces adolescents sensitivity to effects of alcohol.

However, there are mixed results as to whether chronic tolerance among adults is higher, lower, or equal relative to adolescents. Moreover, adolescents are less sensitive to the interoceptive effects associated with alcohol when compared to adult rats, which is an indicator that adults consider ethanol concentration to be a more important cue than adolescents.

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