Discuss the evidence for a dopaminergic system that is involved in reward and drug addiction.

University Degree Subjects allied to Medicine

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Word Count: 1,962 PS2060B Biological Psychology

Discuss the evidence for a dopaminergic system that is involved in reward and drug addiction

Most drugs affect the brain and behaviour by changing synaptic transmissions (Rosenzweig 1999). In order to discuss the evidence for a dopaminergic system that is involved in reward and drug addiction, we need to establish the relationship that a dopaminergic system has with drug reward and drug addiction. In essence when we are discussing a dopaminergic system we are making reference to neurotransmitter systems. The intricacies involving the brain, neurochemically have become more prevalent through processes such as immunocytochemistry. Through such processes we are able to locate substances within the nervous system as well as identifying neurotransmitters. Dopamine and seratonin are neurotransmitters, which are integral in relation to drugs whether it is used for drug reward or drug addiction. This essay will focus on such neurotransmitters in order to discuss evidence for a dopaminergic system that is involved in reward and drug addiction.

Neurotransmitters such as dopamine and seratonin originate from the family of monoamines. Monoamines are classified into to groups, catecholamine and indolemine. Dopamine is within the catecholamine class and seratonin along with another prominent neurotransmitter melatonin are from the indolemine class. Dopamine is essential because it has an impact on drugs and in relation to the question it has an impact on drug addiction and drug reward. Dopamine affects brain processes that control movement, emotional response, and the ability to experience pleasure and pain. Regulation of dopamine plays a crucial role in our mental and physical health.

Many of the concepts that apply to dopamine apply to other neurotransmitters as well. As a chemical messenger, dopamine is similar to adrenaline. Dopamine affects brain processes that control movement, emotional response, and ability to experience pleasure and pain.

Regulation of dopamine plays a crucial role in our mental and physical health. Neurons containing the neurotransmitter dopamine are clustered in the midbrain, in an area called the substantia nigra. In Parkinson's disease, the dopamine- transmitting neurons in this area die. As a result, the brains of people with Parkinson's disease contain almost no dopamine. When referring to an illness such as Parkinson’s disease, we must consider the importance of drug reward. For Parkinson’s disease, drugs are crucial. Some drugs are known as dopamine agonists. These drugs bind to dopamine receptors in place of dopamine and directly stimulate those receptors. Some dopamine agonists are currently used to treat Parkinson's disease. These drugs can stimulate dopamine receptors even in someone without dopamine neurons.

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Dopamine antagonists are drugs that bind but don't stimulate dopamine receptors. Antagonists can prevent or reverse the actions of dopamine by keeping dopamine from attaching to receptors. A very early development in dopamine antagonist was made to reduce self-stimulation. The feeling of craving was increased when taking the drug antagonist. This may provide some evidence to support a dopaminergic system existing in drug reward and drug addiction. However we must understand the relationship between drug use and receptors. What is argued is that drugs will have a strong relationship with particular receptors, not all receptors. The relationship is not specific. As well as establishing that drugs or more precisely drug molecules have a relationship with receptors we must realise that the length of time they bind with receptors vary as well. Pharmacologists would define this as the drugs ‘affinity’ to the receptor. The affinity can be measured based on the concentration of the drug upon the receptor.

Drugs such as cocaine and amphetamine produce their effects by changing the flow of neurotransmitters. These drugs are defined as indirect acting because they depend on the activity of neurons. In contrast, some drugs bypass neurotransmitters altogether and act directly on receptors. Such drugs are direct acting. Use of these two types of drugs can lead to very different results in treating the same disease. As mentioned earlier, people with Parkinson's disease lose neurons that contain dopamine. To compensate for this loss, the body produces more dopamine receptors on other neurons. Indirect agonists are not very effective in treating the disease since they depend on the presence of dopamine neurons. In contrast, direct agonists are more effective because they stimulate dopamine receptors even when dopamine neurons are missing.

Evidence, which supports a dopaminergic system in reward and drug addiction, can be characterised examining how drug addiction affects the body. Taking drugs such as amphetamines can generate a feeling of ecstasy and joy and create perceptive changes such as being more alert. This occurs when the dopamine receptors are stimulated through, as already mentioned, a process of direct and indirect action. This theory was put forward by Wise and Bozarth (1987).

The most important thing when discussing evidence for a dopaminergic system with reward and drug addiction are the effects of stimulant drugs on dopamine synapses. In recent years many researchers have come to believe that reinforcement, drug addiction play a role in inhibiting certain cells in the sub cortical area known as the nucleus accumbens. (Kalat:66).

Drugs such amphetamines and cocaine are important as it will provide evidence for the relationship between drugs and the dopaminergic system. Amphetamines stimulate the dopamine synapses by increasing the activity of dopamine from the presynaptic terminal as the terminal allows it to reabsorb the dopamine after its release (Giros et al 1996). Cocaine has a different effect on dopamine levels as it blocks this reabsorbing of dopamine thus prolonging the effects. Although the effects are temporary the drugs will actually increase the level of dopamine accumulated in the synaptic cleft. Drugs such as Amphetamines and cocaine are all self-stimulated drugs. To support the idea of increasing and decreasing the levels of dopamine, Olds and Milner carried out an experiment involving the use of rats.

The experiment on rats was conducted in Skinner boxes, with electric levers for stimulation. The stimulation acted as a reinforcement measure. Olds and Milner found that the rats worked very hard in order to stimulate themselves, as much as 2000 times an hour till the rats faced exhaustion. Olds in particular believed that the rats kept on administering the stimulations because it triggered areas of limbic system to stimulate. Olds Milner conducted further experiments and found that the stimulations were in fact took place in axons which release dopamine. This does suggest some evidence of drug use whether it is on a reward or addiction purposes with a dopaminergic system.

There also needs to be a distinction between the different types of drugs. Drug types such Hallucinogenic and opiate drugs are very similar in that they are self administered drugs but their effects are contrastingly different. This adds further support to the work done by Giros et al. because dependent on the type of drug intake the effects on the release of dopamine will be different.

So far we have looked at the effects drugs have with dopamine and specifically dopamine receptors, however there is an argument that suggests that it may be dopamine levels that cause drug reward and drug addiction. The theory can be related when considering schizophrenia and dopamine levels.

With more reference to drug reward, Weeks (1962) discovered that chemical self-stimulation led to a drug reward system. In support Deneau (1969) argues that animals will administer agonist drugs when there is a limited supply but if there were a plentiful supply then the animals would suffer an overdose. This of course can be related to humans in that addicts would use drugs in relation with the amount of drugs they have. When drugs such as cocaine are sustained in our bodies for a period of time, it is argued that there will be a craving based on withdrawal symptoms from the drug. This relates to an earlier point of drugs affecting dopamine levels or the dopamine affecting levels of drug usage. Will the sustained effects from drug use in our system affect our levels of dopamine to the extent that dopamine levels are being inhibited?

When discussing drug addiction and establishing the evidence on whether it has a relationship with dopamine, we need to draw out the aspect of sensitisation and desensitisation of drugs. One important aspect of drug addiction is how cells adapt to previous drug exposure. For example, long-term treatment with dopamine antagonists increases the number of dopamine receptors. This occurs as the nervous system tries to subsidise for less stimulation of the receptors by dopamine itself. The receptors themselves become more sensitive to dopamine. These are two example of sensitisation.

Desensitisation occurs after dopamine or dopamine agonists repeatedly stimulate dopamine receptors. Moreover stimulation decreases the number of receptors, and the remaining receptors become less sensitive to dopamine. Desensitisation is better known as tolerance, where exposure to a drug causes less response than previously caused. Tolerance reflects the actions of the nervous system to maintain homeostasis -a constant degree of cell activity in spite of major changes in receptor stimulation. The nervous system maintains this constant level in an attempt to keep the body in a state of equilibrium, even when foreign chemicals are present.

Sensitisation and desensitisation do not take place only after long-term stimulation or over stimulation of dopamine receptors. Both sensitisation and desensitisation can occur after only a single exposure to a drug. In fact, they may develop within a few minutes. The sensitisation of drug and dopamine receptors provides evidence that supports a dopaminergic system that involves drug reward and addiction.

Other neurotransmitters have an affect on drugs. Seratonin is normally involved in temperature regulation, sensory perception, and mood control. However, it plays a major role in emotional disorders such as depression, suicide, impulsive behaviour, and aggression. Drugs such as LSD interact with seratonin receptors, as do most other anti depressants. Glutamate and GABA are amino acids that act as neurotransmitters. The majority of synapses within the brain use glutamate or GABA. They also have other functions in the body such as making energy-rich molecules in cells. The fact that GABA and glutamate are so widely present makes it likely that they will be altered during drug addiction. This also makes it difficult to treat addiction with drug therapy without causing side effects. So as well as dopamine, other neurotransmitters interact with drugs thus proving more evidence of a dopaminergic system that involves drug reward and addiction.

Drugs can interfere with just about every step in the work of neurotransmitter. Each receptor is designed to bind only a certain neurotransmitter. A way to illustrate the discussion could be that the drug acts like a key and if it is structurally similar to the neurotransmitter then it will fit the receptor, which acts as the lock. In this way drugs can damage your intellectual property by blocking nerve impulses, preventing neurotransmitters from getting where they are supposed to be, or producing too many or too little neurotransmitters. As a result, neurons may be over stimulated or not stimulated at all, crippling the nervous system's ability to carry out its functions. There has been plenty of evidence to suggest a dopaminergic system involving drug abuse and reward. Dopaminergic agonists such as cocaine have shown us that through drug use, there is an increase of dopamine, which is brought on by the sensation felt by the drug. It is essential that we remember that the relationship between drugs and a dopaminergic system is reversible but there is substantial evidence to suggest that drug usage has an effect with a dopaminergic system which is useful in explaining the use of drugs to reward and drug addiction.