Our five senses (sight, sound, taste, touch, and smell) gather and transmit information about our environment. Our brains must then process and analyze this information. Although the brain takes in and analyzes an extraordinary amount of information, it relies on a relatively simple electrochemical process for communication.
The brain's communication system permits specific areas of the brain to rapidly interact with other brain regions. The brain achieves this communication through a vast, interconnected, network of specialized cells called neurons. Our brains have billions of these neuronal connections. These neuronal connections form the foundation for an electro-chemical communication system.
The brain is composed of many different regions (or sections). Each of these regions serves a different function. Therefore, these different regions of the brain must have a way to communicate with each other. In particular, the brain must communicate with, and coordinate, all the body's life-sustaining systems (respiratory system, digestive system, cardiovascular system, etc.). This is similar to how individual players on a sports team must communicate with other to coordinate their actions together as a team. Thus, the brain's communication system is essential to our health, well-being, and overall functioning. Conversely, when this communication system is altered, it negatively affects us.
This brain's communication system is constantly changing and adapting. These qualities allow us to learn, to remember, and to adjust to our changing circumstances. Various drugs (including prescribed medications) have the ability to alter the brain's communication system. It makes sense that anything that alters the brain's communication system will alter the way the brain functions. We need to understand how this communication system works so we can understand some of the defining characteristics of addiction. These include cravings, withdrawals, compulsions, and the continued use of addictive substances and activities despite harmful consequences.
The neuron is the primary unit of communication within the brain. A single neuron is extremely tiny. Scientists estimate there are over 100 billion neurons in the human brain. You can imagine just how complex and distinct your brain is from the person next to you. As you know, good communication is a two-way street: We both listen (receive information) and we speak (send information). The same is true of the brain's communication system. Neurons have the ability to both send and receive communication signals. The dendrite is the portion of a neuron that typically receives information (listens). The axon is portion of the neuron that sends out information (speaks).
When humans communicate with each other, we typically use words and gestures. The different parts of the brain communicate with each other using electrical signals. Neurons use electrical pulses to send their communication signals. These electrical impulses are called action potentials. When a neuron fires, the action potential travels down the neuron's axon where it ends. At the end of the axon is the axon terminal or pre-synapse. In this area, special chemical messengers called neurotransmitters and neuromodulators lay in wait. These are stored in specialized capsules called vesicles. The action potential causes the release of these chemical messengers into an open space between one neuron's axon and the next neurons' dendrites. This open space is the synaptic cleft. At the other side of the synaptic cleft is the post synapse that is formed by the dendrites of connecting neurons. In the post synapse, there are special receptors that receive the neurotransmitters.
Receptors and neurotransmitters function in a way that is similar to a keyhole and key. Receptors are like keyholes and neurotransmitters are like the keys. When neurotransmitters fit into the receptors it is called binding. Once a neurotransmitter is bound to a receptor, the key turns the lock. Once the lock opens, it communicates with the receiving neuron's dendrites. In the post synapse, there may be many different receptors (many different shaped keyholes). However, a particular neurotransmitter may be able to fit into (bind to) several different receptors types. This is similar to the way a single key can open several different locks. The particular receptor type determines the type of signal that is transmitted. Thus, the receptor type is often more critical to the communication than the particular neurotransmitter.
It may be easiest to visualize this communication as a single chain of events: First, a neuron sends an electrical impulse (action potential) down the axon. Next, the electrical impulse causes chemicals (neurotransmitters and neuromodulators) to be released into the space between two neurons. Then these chemicals can signal the next neuron to send an electrical impulse and so on. This electro-chemical process forms the brain's communication system
Neurotransmitters and receptors sites associated with addiction
Some neurotransmitters are "excitatory." This means they activate a neuron and cause it to produce an action potential. Other neurons are considered "inhibitory." These neurons prevent the next neuron from sending an action potential. The most common excitatory neurotransmitter in the brain is glutamate. The most common inhibitory neurotransmitter is gamma-aminobutyric acid (GABA). Both of these play a role in the addiction process. Some other common neurotransmitters that play an important role in addiction are dopamine, serotonin, and norepinephrine. Besides neurotransmitters, there are also larger neuromodulators and neuropeptides. These also play a distinct role in the addiction process. Some neuropeptides that are relevant to addiction are: 1) opiates made by the brain itself (called endorphins), 2) stress hormones, and 3) peptides associated with feeding and anxiety. These molecules have their own specific types of receptors.
Some neurotransmitters are sensitive to specific drugs. All drugs in varying degrees affect neurotransmitters, particularly dopamine. Stimulants such as cocaine and methamphetamine have a particularly strong effect on dopamine. However, as previously mentioned both neurotransmitters and receptors play a role in the addictive process.
- Cocaine and methamphetamine cause changes to the dopamine system.
- Opiates (heroin, codeine, Oxycontin®, Vicodin®, hydrocodone) cause changes in the dopamine, opiate (endorphin), and GABA systems.
- Alcohol alters dopamine, glutamate, and GABA systems.
- Marijuana activates dopamine and the brains own cannabinoid system.
- Nicotine (cigarettes) causes changes in the acetylcholine system.
- Ecstasy affects both dopamine and serotonin systems.