Psychopharmacology is the study of the effects of psychoactive substances on emotions, cognition and behaviour and the mechanisms through which these substances exert their effects. Although an understanding of the mechanisms through which psychoactive substances have an effect is not essential to supporting people with substance use problems, it can be helpful in understanding why people use legal and illegal substances, and why they may experience problems with them.
In the human body, the Central Nervous System (CNS) comprises the brain and the spinal cord. Psychoactive substances exert their effects by altering the connections between certain nerve cells in the brain, known as neurons. Thinking, feeling, mood and behaviour are all a consequence of the way that billions of such neurons in the brain communicate with each other. Each neuron communicates with the next neuron through ‘chemical keys’ it produces, called neurotransmitters, which fit into specific matching ‘locks’ in the next neuron called receptors. Neurons may produce and release one or more type of neurotransmitter. Psychoactive substances modify thinking and behaviour by influencing this neurotransmission and exert their effect by either increasing or reducing neurotransmitter activity. Usually, once chemical communication has occurred the neurotransmitters are reabsorbed in to the original cell, which is known as ‘reuptake’, or alternatively they may be broken down and deactivated, known as ‘catabolism’. However, by mimicking neurotransmitters or by disrupting or altering the cycle of neurotransmitters, psychoactive substances are able to alter the way the brain functions and exert their effects.
Some psychoactive substances, including LSD and heroin, fit into the same receptor sites as naturally occurring neurotransmitters. LSD activates the receptors in the brain that are usually activated by serotonin. Similarly, opiates such as heroin and synthetic opioids such as methadone fit into the brain’s opiate receptors in the same way that endorphins, the body’s natural painkillers would, and produce similar feelings of pleasure and pain relief.
Neurotramsmitter Activity Increase
Other psychoactive substances work by causing more of a neurotransmitter to be produced, for example amphetamines cause the release of a greater amount of the neurotransmitters norepinephrine and dopamine. The increases in the levels of these neurotransmitters in the gap between the neurons known as the synaptic cleft increases (or stimulates) receptor activity. This results in potent mental and physical stimulation including increased alertness, excitement, euphoria, energy and mood elevation. However, an increase in neurotransmitter activity does not always have a stimulating effect on the CNS. Gamma-amino butyric acid (GABA) is an amino acid which acts as an inhibitory neurotransmitter binding to the receptor sites on neurons and restricting activity. Increases in GABA can be triggered by the use of several psychoactive substances including alcohol, barbiturates and benzodiazepines and it is closely linked to regulating anxiety levels. GABA is the most predominant neurotransmitter in the brain and “the widespread distribution of GABA receptors suggests that the brain is generally in a state of inhibition.
Neurotransmitter Reuptake Inhibition
Instead of causing an increase in the release of neurotransmitters, other psychoactive substances such as cocaine, prevent their reuptake (in this case serotonin, dopamine and norepinephrine) which also results in an increase in these neurotransmitters between the synapses. There is a group of prescribed anti-depressants referred to as ‘Selective Serotonin Reuptake Inhibitor’ or SSRIs of which the best known is probably Prozac (Fluoxetine). These increase the amount of serotonin in the synaptic cleft through a similar mechanism. Serotonin is influential on mood, appetite, sleep, hallucinations, pain processing and emotional processing. Emotional processing is the way in which emotional disturbances such as stress and anxiety are absorbed and decline, allowing further experiences and behaviour to proceed without disruption.
There are some substances, such as Naltrexone and Subutex (also known as Buprenorphine), that partially or totally block certain receptors and can prevent neurotransmission. Both Naltrexone and Subutex are used in the treatment of heroin dependency as they prevent people who take it from experiencing any affects if they do use heroin.
One of the reasons that people can develop problems with psychoactive substances is that they can ‘hijack’ the brains chemical reward system. Dopamine in particular is very influential on the reinforcement of certain behaviours through its role as the primary neurotransmitter in the reward system. It is critical for normal survival as it provides the pleasure drives for eating, drinking and sexual reproduction. However, dopamine is also responsible for both the euphoric effects of many psychoactive substances, and their potential to cause psychological dependence. In the same way that food stimulate the rewards pathways producing feelings of pleasure and encouraging continuation of the behaviour, dependence-causing drugs such as cocaine and heroin stimulate the same pathways. However, repeated use of such substances can ‘progressively supplant behaviors optimal for survival’ and undermine rather than promote species survival.