Addiction, in general, is a chronic disorder of the brain that has several characteristics including having a compulsory dependence on a given stimuli, loss of control in limiting the stimuli, and the experience of a negative emotional state upon denial of the stimuli (Koob & Volkow, 2016). An addict, therefore, has a craving, compulsion, and an inability to quit the stimuli. Addiction can either be psychological or behavioral. Psychological addiction usually the dependence on a substance such as alcohol, opioids, tobacco, cocaine, cannabis, amphetamines, among others classified as addictive substances. Behavioral addiction, on the other hand, consists of dependence on behaviors such as eating, playing video games, watching pornography, shopping, sex, and disorders such as kleptomania, pyromania, intermittent explosive disorders, and gambling. This classification is according to the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV-TR). Most articles on addiction issues tend to focus on the psychological and physical aspects of addiction, but this paper seeks to explain the neurobiology of addiction.
To understand the neurobiology of addiction, it is important to understand the neuro-adaptive and neuro-pharmacological mechanisms within given neuron circuits that oversee a change from an occasional use of substances such as drugs to the loss of behavioral control of the use of drugs that characterizes addiction (Koob, 2015). The study of the neurobiology of addiction has mainly been focused on drug abuse such as that of opiate drugs, although other drugs such as nicotine have been studied and lately, the neurobiology of behavioral addictions such as gambling and video gaming. In understanding the neural biology of addiction, it is important first to understand the concept of addiction vulnerability. This increases the risk for dependence on a substance and consists of genetic, physiological, and psychological factors. Of these, genes have been found to be the highest risk factor for most of the drug addictions. It is also of importance to note that neurons function in an organized manner referred to as neural circuits which provide specific kinds of information. Although different circuits vary according to the intended functions, they all contain glial cell processes, axon terminals, and dendrites (Koob & Volkow, 2016).
In vulnerable individuals and the case of drug abuse, addiction is as a result of the interaction of the drugs with factors such as genetic, psychosocial, and environmental. These factors cause alterations which are long-lived in the functional as well as the biochemical properties of specific neurons in the brain. These alterations allow for the formation of deeply ingrained emotional memories that manifest themselves in the form of drug cravings thus resulting in a relapse (Koskela et al., 2017). Neural circuits intimately involved in the control of emotions and motivation tends to be the ones ‘commandeered’ by addictive drugs especially when frequently taken and in an adequate dose. Certain primary human psychological functions such as volition and insight are impaired. It has however been discovered that the neurobiology of addiction cannot be generalized. The analysis must, therefore, be done for various specific cases, and most importantly taking note of the properties of the drug involved, the neural circuit that the drug operates, the presence or absence of psychological or psychiatrist issues, the genetic make-up, and the social-cultural aspects.
The Reward System
The reward system is of great importance in understanding the neurobiology of addiction. The brain reward system consists of neural circuits responsible for positive reinforcers such as positive emotions such as joy and ecstasy, incentive salience including motivation and desire, and associative learning such as classical conditioning. Studies have shown that the reward system changes with the development of addiction (Koob & Volkow, 2016). These findings have led to a focus on the mesolimbic dopamine system which is a pathway in which dopamine is transported in the brain. Dopamine is essential in controlling the reward centers hence its relevance. Drug abuses such as those of opiates, cocaine, amphetamines, nicotine, alcohol, and THC cause positive reinforcement in certain neural systems in the reward center (Robbins, 2017). Upon withdrawal, they lead to dysphoria, and this has been interpreted as being due to a disruption of the same neural systems resulting from neuro-adaptive mechanisms. This disruption is characterized by a decrease in the dopamine and serotonin transmission in the nucleus accumbens and an increased sensitivity of opioid receptor transduction mechanisms during opiate withdrawal. Also during an alcohol withdrawal, there is decreased GABAergic and increased N-methyl-D-aspartate (NMDA) glutamatergic transmission. These decreases in reward neurotransmitters are highly associated with acute drug withdrawal and may trigger long-lived changes in certain neurons in the brain as earlier stated hence increasing the vulnerability to relapse (Koskela et al., 2017).
The Neurobiology of Behavioral Addictions
Behavioral addictions involve non-drug compulsive behavior such as that of gambling disorder. Although the neurobiology of both drug and behavioral addiction shares some similarities, each case has its peculiarity. Genetic factors play a significant role in both addictions with the FBJ murine osteosarcoma viral oncogene homolog B (∆FosB) gene from the Fos gene family playing a crucial role. Excessively high levels of ∆FosB gene expression can lead to addiction-related neuroplasticity in the reward system hence resulting in pathological behavior. This then leads to addictions to natural rewards such as playing video games, eating, and having sex. Certain neurotransmitters have also been found to be related to addictive behaviors. For instance, in people with the internet gaming disorder also known as video game addiction, changes in the neural circuits of the reward system lead to cravings for gaming that makes stopping the habit of playing video games very difficult. In the case of pathological gambling, people with this addiction have been found to have higher levels of noradrenaline that lead to anticipation of a natural reward often characterized by an elevated heart rate and noradrenergic measures (Kiefer, Fauth-Bühler, Heinz & Mann, 2013). Dopamine and serotonin have also been implicated in behavioral addictions. The field of behavioral addiction has been labeled as controversial due to the differing opinions. However, most of the studies tend to agree on the fact that the reward system and genetics play a big role in these addictions (Robbins & Clark, 2015).
The neurobiology of addiction deals with the neural activities that are associated with addictive behavior rather than the physical aspects such as side effects. The brain reward system is at the heart of this understanding, consisting of neural circuits with neurotransmitters playing a significant role. Addictive substances such as opioids and alcohol cause long-lived alterations in neurons in the reward system which can lead to alterations in the functioning of hormones such as dopamine and serotonin. Upon withdrawal of these substances, changes in the neurons lead to a decrease in various neurotransmitters associated with positive rewards causing feelings of dysphoria. This causes a high vulnerability to relapse in using the drug. The effects on the brain reward system by drug use are influenced by a mixture of conditions including genetic, psychological and environmental. More studies, however, need to be done on the neurobiology of behavioral to have a clear answer on the relationship between the neurons and addictive behaviors such as playing video games, eating, using internet, gambling, watching pornography, and exercising.
Kiefer, F., Fauth-Bühler, M., Heinz, A., & Mann, K. (2013). Neurobiology of behavioral addictions. Der Nervenarzt, 84(5), 557-562.
Koob, G. F. (2015). Neurobiology of addiction. Substance Abuse Treatment Fifth Edition. Galanter M, Kleber HD, Brady KT eds. American Psychiatric Association, Arlington, VA.
Koob, G. F., & Volkow, N. D. (2016). Neurobiology of addiction: a neurocircuitry analysis. The Lancet Psychiatry, 3(8), 760-773.
Koskela, M., Bäck, S., Võikar, V., Richie, C. T., Domanskyi, A., Harvey, B. K., & Airavaara, M. (2017). Update of neurotrophic factors in neurobiology of addiction and future directions. Neurobiology of disease, 97, 189-200.
Robbins, T. W. (2017, August). Neurobiology of addiction. in journal of Neurology Neurosurgery and Psychiatry (vol. 88, no. 8, pp. e5-e5). british med assoc house, tavistock square, london wc1h 9jr, england: bmj publishing group.
Robbins, T. W., & Clark, L. (2015). Behavioral addictions. Current Opinion in Neurobiology, 30, 66-72.