الفهرس 
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Abstract
Attention-deficit/hyperactivity disorder (ADHD) is recognized as a common childhood psychiatric disorder . ADHD is characterized by the early onset of age-inappropriate hyperactivity, impulsivity and inattentiveness, and a world-wide pooled population-prevalence of 5.3%. One of the current psychiatric disease classification system, DSM-IV-TR, distinguishes three subtypes: a mainly inattentive, a mainly hyperactive–impulsive and a combined subtype . It is estimated that up to 60% of childhood cases of ADHD will continue to have clinically significant symptoms of ADHD as adults .
Substance use disorders can be subdivided according to the abuse/dependence state and the type of substance abused. According to the DSM-IV substance abuse is a maladaptive, non-medical use of psychoactive drugs that leads to functional impairments or distress, whereas substance dependence involves tolerance to the effects of the drug and the presence of withdrawal symptoms when the use of the drug is reduced or stopped.
Substance use disorders (SUD) (i.e., abuse and dependence) also constitute a substantial clinical, public health, and economic concern in the United States and globally . In 2000, substance dependence was estimated to account for $67 billion in economic loss due to crime, social problems, foster care, and other health services .
The association of ADHD and SUD has become an increasing focus of investigation over the past decade. Studies of clinical samples of individuals with SUD seeking treatment have demonstrated that ADHD is a common co-occurring mental disorder. There are several reasons that ADHD and substance problems may be related.
Dopamine (DA) neurotransmission is central to current models of ADHD and SUD, all drugs of abuse directly or indirectly target the brain’s reward system by flooding the circuit with dopamine. dopamine is a neurotransmitter present in regions of the brain that regulate movement, emotion, cognition, motivation, and feelings of pleasure. The over stimulation of this system, which rewards our natural behaviors, produces the euphoric effects sought by people who abuse drugs and teaches them to repeat the behavior .
When there is a dysfunction in the “brain reward cascade,” especially in the dopamine system, causing a low or hypo-dopaminergic trait, the brain may require dopamine for individuals to avoid unpleasant feelings. It has been proposed that genetic variants of dopaminergic genes and other “reward genes” are important common determinants of reward deficiency syndrome (RDS), which we hypothesize includes ADHD as a behavioral subtype. Moreover, it is concluded that dopamine releasers might be useful therapeutic adjuncts for the treatment of other RDS behavioral subtypes, including addictions.
Impulsivity, broadly defined as action without foresight, is a component of numerous psychiatric illnesses including attention deficit/hyperactivity disorder (ADHD) and substance abuse. Indeed, ADHD patients show elevated levels of impulsivity as measured by a variety of tasks. The relation between impulsivity and substance use has been shown across several classes of drugs, including stimulants, opiates, and alcohol, and across multiple indices of impulsivity, including self-report, behavioral, and neuropsychological measures.
Most neurocognitive models of ADHD have implicated a simple linear pathway in which a single causal factor is hypothesized to give rise to a core cognitive deficit that is both necessary and sufficient to account for all cases of ADHD. The most prominent models proposed that ADHD is due to deficits in overall executive functions (EFs) or specific aspects of executive functions such as response inhibition , aversion to delay , difficulty modulating behavior in response to reward and punishment cues, response inconsistency , and overall slow processing speed .
With the growth of cognitive neuroscience research, the importance of executive function and inhibitory cognitive control in addictive disorders is becoming increasingly apparent. Disruption in inhibitory control is a critical element of most theories of addiction and is closely linked to a number of prefrontal cortex functions, including attention and working memory.
There are several groups of drugs used in treatment of attention deficit hyperactivity disorder(ADHD) and substance use disorder (SUD) (e.g stimulant medication, atomoxetine, clonodine ) through cognitive enhancement.
The prevalence of psychopathology, including SUD, is higher in first-degree relatives of ADHD probands than in healthy controls ,Therefore, ADHD and SUD may share common etiological influences, including similar genetic factors .
Family, twin, and adoption studies indicate that a substantial inherited component exists in the background of attention deficit hyperactivity disorder (ADHD) and substance use disorder(SUD). In this review, we present dopaminergic genetic findings of attention deficit hyperactivity disorder (ADHD) and of substance use disorder (SUD).