Addiction is commonplace in American society, but a definition of the disorder is often lost among widespread misunderstanding and stigma. The need for factual information about alcohol and substance abuse is equally important. We are burdened as a society by a lack of accurate information about (1) how alcohol and drugs act in the brain; (2) the nature of addictive diseases, and (3) the role of research in improving and eventually conquering substance abuse. Facts are required if we hope to effectively respond to the worst social problem of our time.
Misunderstanding exists over terms like addiction, chemical dependency, substance abuse, physiological dependence, and drug misuse. But there are really only two main classifications:
The primary characteristic of dependence, or addiction, is impaired control over the use of the drug. In today’s definition of addiction, impaired control over drug use includes all drugs that are addicting, as well as both psychological and physical withdrawal characteristics. Psychological issues include the fact that impaired control is an obsessive-compulsive preoccupation with the drug, and physical issues address the primary loss of control as a neurochemical dysfunction.
Neuroscientists who study the brain now believe that the primary site of action for all addicting drugs is probably within the mesolimbic system of the brain, where our instinctual drives and ability to experience emotions and pleasure reside. Within the mesolimbic system is the medial forebrain bundle (MFB), known as the “pleasure pathway”. This pleasure pathway runs from the center of the brain (in an area known as the ventral tegmental area (VTA), through the region above the pituitary gland (known as the lateral hypothalamus (LT), then through a major relay station known as the nucleus accumbens (ACC), and up to the front part of the brain known as the frontal cortex (FC).
As shown in animal studies, other areas, such as the conscious cortex where judgment and intelligence reside, feed into these brain areas. Pleasurable experiences from outside, the taste of chocolate cake, or a romantic kiss, are carried to the pleasure pathway by way of other paths arising in the cortex, and then pass through and stimulate the “emotional” parts of the mesolimbic system on their way to the pleasure pathway. Neuroscientists point to the major site of addicting drugs – the pleasure pathway, not the cortex – as strong evidence that addictions are not under conscious control.
Studies of this pleasure pathway are escalating in efforts to find the neuro-biological roots of addiction. Regardless of the complexity of the pathway, scientists now believe that all drugs which produce euphoria act on this pathway. In particular, drugs such as heroin and alcohol are known to act at least partially on the VTA to produce euphoria. Drugs acting primarily on the ACC include cocaine/crack, heroin, amphetamine and methamphetamine, nicotine, PCP, and tetrahydrocannabinol, the active ingredient in marijuana. These probable sites of drug action have been found within the past 7-10 years, and more are being identified.
Scientists also believe that the neurochemical dysfunction in addictions is best described as a chemical deficiency in one or more parts of the pleasure pathways of the brain. Science has, to date, identified 50 brain chemicals or neurotransmitters, which carry the signals between the nerve cells, or neurons, in the brain. Neurotransmitters play an essential role in all brain activity: thought, emotion, memory and learning. From the perspective of the neurochemist, every pleasurable human experience – hearing, smelling, tasting, seeing, touching and more – is little more than a burst of one or more neurotransmitters in the pleasure pathway.
Drugs of abuse, stimulants such as cocaine or depressants such as heroin or alcohol, all imitate the structure of certain neurotransmitters, thereby interfering with the balance of brain chemicals needed for appropriate physical and neuro-functioning. For example, the neurotransmitter dopamine is associated with pleasure, but has other influences in brain activity: too little dopamine in certain areas produces tremors and paralysis, symptoms associated with Parkinson’s disease, while too much causes hallucinations and thought disorders, symptoms associated with schizophrenia.
Dopamine is also believed responsible for the exhilarating feeling that reinforces the craving for drugs, and some scientists believe dopamine is one among several neurochemical “keys” to understanding addiction. They speculate that the cocaine addict, for example, does not crave cocaine, but instead wants the burst, or “rush”, of dopamine or other neurotransmitters that cocaine produces in the brain. The known effects of addictive drugs on the release of dopamine, as one example of neurotransmitter response, lend support to this theory:
While dopamine is believed to play a critical role in addiction, among other neurotransmitters shown to have an involvement are:
The cause of pleasure pathway chemical dysfunction is still uncertain. Speculation arising from genetics studies suggests abnormal genes passed from generation to generation produces disorders in MFB chemical functioning, which in turn predispose individuals to become drug dependent. Another possibility is that chronic drug use causes specific changes in localized parts of the MFB, creating an imbalance in brain chemistry. This theory is based on significant differences between the addicted brain and the non-addicted brain. The effects of chronic substance abuse are found at all levels of the addicted brain: molecular, cellular, structural and functional.
If you are concerned about chemical addiction, we encourage you to contact the EAP at 866/443-3277 for additional information.
[i] Substance Abuse in Brief, Center for Substance Abuse Treatment, SAMHSA, U.S. Department of Health and Human Services, July 1999 p 1