The intense craving experienced by heroin users has a powerful physical basis. Initial drug use scars the addict's cerebral tissue, altering the brain's architecture and leading to compulsive addiction.
"Craving is the kernel of addiction," stated Prof Ciaran Regan, associate professor of pharmacology in UCD. "It is a view not held broadly, but I believe the physical withdrawal can be controlled with drugs. But I do not believe you can be clean of the drug psychologically."
His research, still in its initial stages, found that rats who self-administered heroin became addicted after 10 days, a condition which left them unable to handle new learning experiences.
Several stages occur during the development of an addiction. The initial euphoria of the new experience is followed by a desire to repeat the experience. The amount of drug needed to achieve the level of emotional and physical response gradually goes up, in turn leading to physical dependence. At this stage, the emotional response has become the learned response to taking the drug and thereafter influences the person's behaviour either with or without the drug.
The learned understanding of the pleasures of the drug have been demonstrated experimentally in two ways, Prof Regan says. The rats could no longer perform learning tasks and the molecular mechanism of memory formation, known as LTP, no longer worked correctly. This process showed that if memory is to form, nerve cells must receive a stimulation, but this must be long term. It must endure.
In heroin-addicted animals, the nerve cells do become stimulated, but their stimulation does not endure, and so cannot form new memories such as unlearning their addiction.
Although Prof Regan is not exactly sure how this happens, he says it may have to do with how memory is stored. Memory is not a single faculty in the brain - there are several memory systems, each dealing with a different aspect of memory.
He believes two cerebral regions are important for addiction, the hippocampus and the amygdala. The hippocampus is central to long-term memory, but with only 20 per cent of memory actually retained, the defining feature of much stored information is its powerful emotional content, says Prof Regan.
The part of the brain processing emotional information is the amygdala. Physically close to the hippocampus, these two brain regions have a strong interaction. He says the amygdala stamps the information as being important and tells the hippocampus to store it. This information becomes so enduring that it actually causes a physical change in the brain, as the connective pattern between nerve cells is altered.
The amygdala continues to respond to environmental drug cues long after the addict has overcome the physical effects of drug withdrawal. For example, brain scans of intravenous cocaine addicts show the amygdala becomes highly stimulated when the subject is shown a syringe.
The UCD team is examining the change in pattern of nerve cell connections in the hippocampus. It is working to understand how the structurally changed hippocampus can alter the cortex, the main thinking part of the brain.
Prof Regan has published a book which looks at addiction and the role it has played in human society, entitled Intoxicating Minds. Drawing from the disciplines of pharmacology, anthropology and sociology, the book examines the history of drug use, placing the development of drugs in their historical context.
The book describes how psychoactive substances, which change the action of the brain, have been used in different societies throughout history in highly organised and ritualised ways.
Intoxicating Minds by Ciaran Regan (164 pages), published by Weidenfeld & Nicolson, is on sale, priced at £15.