Is the Impulsive Behaviour that Precedes Addiction Hardwired into the Brain?

In various blogs we have forwarded the idea that emotional and stress dysregulation are that the heart of addiction and alcoholism and are also possible present in those at risk to these disorders.

Essentially we suggest that the behavioural endpoint of addictive behaviours, the distress based impulsivity (negative urgency) seen in alcoholics and addicts which shapes decision making may be the consequence of chronic neurotoxic activity (as the consequence of chronic alcohol and drug use)  on brain areas which have a pre-existing impairments or vulnerability such as brain regions involved in emotional regulation, processing, inhibition and stress and reward response.

Here we cite an article (1) which looks at some of these brain regions, specifically those involved in emotional regulation and impulsivity and considers whether these deficits may be “hardwired” into the brain in terms of white and grey matter impairments.

 

Brain areas actively developing during adolescence include the prefrontal cortex, limbic system areas, and white matter myelin ( electrically insulating material that forms a layer, the myelin sheath – the yellow insulation below), usually around only the axon of a neuron. It is essential for the proper functioning of the nervous system.)

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These areas serving cognitive, behavioral, and emotional regulation may be particularly vulnerable to adverse alcohol effects.

Alternatively, deficits or developmental delays in these structures and their functions may underlie liability to accelerated alcohol use trajectories in adolescence.

The prefrontal cortex, limbic brain regions, white matter ( composed of bundles of myelinated nerve cell axons which connect various grey matter areas (the locations of nerve cell bodies) of the brain to each other (see below – grey on outside, white inside) and carry nerve impulses between neurons. Myelin acts as an insulator, increasing the speed of transmission of all nerve signals, and reward circuits undergo active development during adolescence (Chambers et al., 2003; Spear, 2000).

 

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These structures and their functions, involving behavioral, emotional and cognitive regulation, may be particularly vulnerable to the adverse effects of alcohol exposure during adolescence.

Delays or deficits in the development of neural substrates necessary for these psychological regulation abilities to fully develop may be termed neurodevelopmental dysmaturation.

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Psychological Dysregulation

The development of accelerated alcohol involvement in adolescence is not an isolated phenomenon, but is typically presaged by persistent behavioral characteristics including attentional deficits, conduct problems, and irritability (Chassin et al., 1999; Clark et al., 1997a, 2005; Tapert et al., 2002).

Two main psychological factors have been identified: (1) Behavioral Undercontrol, comprised of conduct disorder symptoms and personality characteristics including aggression and diminished constraint, and (2) Negative Emotionality, comprised of depression, anxiety and stress reactivity variables (Martin et al., 2000).

These two factors were significantly correlated. These correlated characteristics have been hypothesized to comprise the early phenotypic manifestations of a core liability for SUDs (Tarter et al., 1999).

The proposed construct manifested by these psychopathologic features has been termed psychological dysregulation (Clark and Winters, 2002). Psychological dysregulation is a deficiency in the ability to regulate attention, emotions and behavior in response to environmental challenges. Psychological regulation is thus the ability to modulate prepotent responses in order to optimize reward opportunities. The skills involved in psychological regulation include executive cognitive functioning (ECF), behavioral inhibition and emotional management.

Deficiencies in psychological regulation may be the result of delays or persistent deficits in the acquisition of behavioral, emotional, and cognitive regulation skills.

Adolescents at risk for developing SUDs exhibit deficits in psychological regulation. Childhood psychological dysregulation, or neurobehaviour disinhibition, correlates with parental substance use disorders (SUDs) and prospectively predicts adolescent alcohol and other substance use as well as related disorders (Clark et al., 2005; Tarter et al., 2003).

The psychological dysregulation dimension integrates several psycho patholological dimensions heretofore considered distinct, including affective disorders and SUDS themselves (Krueger et al., 2002).

Neurobiological Basis of Psychological Dysregulation

The functions subsumed under the construct of psychological dysregulation are thought to be served by the prefrontal cortex (Koechlin and Summerfield, 2007). The capabilities that comprise psychological regulation improve during adolescence (Levin et al., 1991; Welsh et al., 1991). The ongoing development of the prefrontal cortex has been hypothesized to be the primary neurobiological foundation for the advancement of these abilities (Happaney et al., 2004; Spear, 2000). Developmental abnormalities in the frontal cortex have been found in children and adolescents with behavioral problems reflecting psychological dysregulation (Rubia et al., 2000; Spear, 2000).

Diffusion tensor imaging (DTI) studies  indicated that white matter organization increases from early childhood to young adulthood (Klingberg et al., 1999; Nagy et al., 2004; Schmithorst et al., 2002; Zhang et al., 2005).White matter development may underlie advancing executive functioning. The prefrontal cortex is a brain region undergoing relatively late gray matter pruning, and volumes of gray matter appear to decrease over adolescence (Gogtay et al., 2004; Lenroot and Giedd, 2006; Sowell et al., 2001, 2004). Unlike grey matter volume, white matter volume appears to increase during adolescence, particularly in the prefrontal area (Ashtari et al., 2007;Barnea-Goraly et al., 2005; Lenroot and Giedd, 2006).

 

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White Matter Development and Alcohol Exposure

Selective white matter loss has been reported among adults with Alcohol Use Disorders (AUDs) (Carlen et al., 1978, 1986) and with fMRI (Agartz et al., 2003), and postmortem specimens (Krill et al., 1997).  Compared with controls, adolescents with AUDs have been found to have smaller prefrontal white matter volumes (DeBellis et al., 2005). Prefrontal grey and white matter volumes were compared in adolescents with AUDs. Compared with control subjects, subjects with AUDs had significantly smaller prefrontal white matter volumes.Marijuana use has also been found to be associated with smaller white matter volumes in adolescents (Medina et al., 2007b). While these volumetric findings suggest problematic frontal development among adolescents with AUD, the emergence of neuroimaging techniques developed to examine white matter organization may prove to be more specifically relevant to understanding the effects of alcohol on neurodevelopmental maturation.

Changes in gene expression may be involved in alteration of white matter structure in AUDs.  In a postmortem study, myelin-related genes were found to be down-regulated in the AUD group (Lewohl et al., 2000).

While evidence has been presented that alcohol consumption may disrupt white matter organization, the possibility remains that delayed or diminished white matter organization may presage alcohol involvement and constitute a risk factor for AUDs. Immaturity of white matter development and the related deficits in the functional integration of brain areas may in part explain individual differences in psychological regulation during adolescence. For example, disruptive behavior disorders in childhood, particularly conduct disorder, have been found to predict accelerated trajectories of alcohol use, cannabis use, and substance-related problems in adolescence (Clark et al., 1999).

Limbic System Development and Alcohol Exposure

The limbic system is central to the processing of affective stimuli, the successful formation of new memories, and the implementation of related responses. Limbic system structures, including the hippocampus and amygdala, may be susceptible to alcohol-induced dysmaturation.

Smaller hippocampal volumes have been reported in adults with AUDs compared with control adults (Sullivan et al., 1995). As hippocampal development progresses in adolescence (Gogtay et al., 2006), this brain area may be particularly susceptible to the adverse effects of alcohol involvement during this developmental period.

DeBellis et al. (2000) compared the hippocampal volumes of 12 adolescents and young adults with adolescent-onset AUD to those of 24 control subjects. Both left and right hippocampi were significantly smaller in AUD subjects compared to the volumes in controls. Specifically, left hippocampal volumes were smaller in teens with AUD than demographically similar controls, and youth with greater severity of AUD had the smallest left hippocampal volumes (Medina et al., 2007a; Nagel et al., 2005).

The amygdala may also be important for understanding the neurodevelopmental effects of alcohol exposure. The amygdala, along with ventral striatum, has been hypothesized to be involved in reward mechanisms and thereby critical for understanding alcohol use trajectories (Koob, 1999). Amygdala volumes have been found to be relatively smaller in high-risk older adolescents and adults with SUDs compared to that in control subjects (Hill et al., 2001; Makris et al., 2004). Lack of correlation with use levels has led to the suggestion that this may be a predisposing characteristics rather than a substance effect.

Interacting brain areas are involved in reward processing (McClure et al., 2004), motivation (Chambers et al., 2003), and decision-making (Verdejo-Garcia et al., 2006).  The interactions between the prefrontal cortex and subcortical areas, including the amygdala and nucleus accumbens, constitute the neurocircuitry involved in reward responding. In the affective component of reward responding, the amygdala appears to be a network node involved in reactivity to emotional stimuli (Hariri et al., 2006; Schwartz et al., 2003). An understanding of the adolescent development of neural circuits underlying reward processing and decision making is central to considering the role of these systems in the development of alcohol involvement.

Impulsivity, defined as acting without forethought, progressively decreases from childhood into adulthood. This change has been thought to occur as a result of neuromaturation in the prefrontal cortex (Casey et al., 2005).

The generation of behaviors optimizing long-term reward opportunities often involves behavioral inhibition. The activation of prefrontal cortical areas during response inhibition tasks has been found to increase from childhood through adolescence, a change corresponding to the development of abilities to suppress prepotent behaviors (Luna and Sweeney, 2004; Luna et al., 2004). The ability to select an optimally adaptive behavioral response while suppressing a predominant or prepotent response with problematic consequences defines impulse control and is fundamental to psychological regulation skills. Improved abilities in response inhibition and related prefrontal activation during adolescence are thought to involve maturation of functional connectivity subserved by ongoing myelination.

Adolescents with psychopathology predictive of SUDs, similar to adults with alcohol dependence, have difficulty with behavioral inhibition during laboratory tasks (Bjork et al., 2004a; Dougherty et al., 2003; Schweinsburg et al., 2004). Furthermore, adolescents with histories of substantial marijuana use, compared with control adolescents, showed more activation in frontal cortical areas during behavioral inhibition tasks (Tapert et al., 2007). More activitation suggests greater effort was required by the marijuana using group.

 

References

1.  Clark, D. B., Thatcher, D. L., & Tapert, S. F. (2008). Alcohol, psychological dysregulation, and adolescent brain development. Alcoholism: Clinical and Experimental Research, 32(3), 375-385.

 

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