2.3 Neurotransmitters Influence Your Mental Activity and Behavior

Drugs Alter How Neurotransmitters Function Drugs that enhance the actions of neurotransmitters are called agonists. One example of an agonist is nicotine, a drug found in tobacco. Nicotine acts as an agonist to the neurotransmitter acetylcholine because it is chemically similar and so acetylcholine receptors cannot tell the difference between the two. What happens when a person ingests nicotine, perhaps by smoking a cigarette? The nicotine binds to acetylcholine receptors and causes effects that are typical of acetylcholine (see Table 2.1). So after smoking the cigarette, the person is more alert and may experience changes in motor coordination. The effect is short-lived, however.

By contrast, drugs that inhibit the actions of neurotransmitters are called antagonists. As discussed further in Chapter 3, in the United States an opioid abuse epidemic is leading to many deaths from overdose. A drug that can prevent death by overdose is naloxone, an antagonist that binds with endorphin receptors. Naloxone therefore blocks the ability of opioids to bind with the same receptors. When naloxone is administered—for example, by a nasal spray—it can help save lives (Kahn et al., 2020). Every day, many people are helped by a scientific understanding of neurons and neural communication.

A neurotransmitter fits a receptor the way a key fits a lock. But receptors cannot sense the difference between an ingested drug and the real neurotransmitter released from a presynaptic neuron. That is, the receptor can be affected by either a neurotransmitter or a drug that resembles the neurotransmitter. Thus, addictive drugs such as heroin and cocaine, and even nicotine and caffeine, have their effects because they are chemically similar to naturally occurring neurotransmitters.

To better understand how neurotransmitters affect behavior, researchers often inject agonists or antagonists into animals’ brains. For instance, scientists may want to test the hypothesis that a certain neurotransmitter in a specific brain region leads to increased eating. Injecting an agonist into that brain region should increase an animal’s eating. Injecting an antagonist should decrease its eating. Such studies help in the development of drug treatments for many psychological and medical disorders.

Let’s now examine each of the major neurotransmitters summarized in Table 2.1.

Acetylcholine Maybe you have seen ads for Botox or known someone who received Botox injections to remove wrinkles. Botox treatments depend on the action of acetylcholine, the neurotransmitter responsible for motor control over muscles. After moving across the synapses, acetylcholine binds with receptors on muscle cells. This chemical binding makes the muscles contract.

Where does Botox come in? Botulism is a form of food poisoning that inhibits the release of acetylcholine. The resulting paralysis of muscles leads to difficulty in chewing, difficulty in breathing, and often death. In small, much less toxic doses, the botulism toxin—popularly known as Botox—paralyzes muscles that produce wrinkles in certain areas, such as the forehead (Figure 2.7). Because the effects of Botox wear off over time, a new dose of botulism toxin needs to be injected every 2 to 4 months. But Botox also paralyzes the facial muscles we use to express emotions, as in smiling and frowning. If too much Botox is injected, the result can be an expressionless face.

In addition to regulating motor control, acetylcholine is involved in some complex mental processes, including attention, memory, learning, and sleeping.

Norepinephrine The neurotransmitter norepinephrine is involved in states of arousal and alertness. Norepinephrine is especially important for noticing what is going on around you. In your body, the same chemical is called epinephrine. Epinephrine, originally called adrenaline, can produce an adrenaline rush, that sudden burst of energy that seems to take over your whole body. You may have experienced an adrenaline rush when doing something exciting or when confronted with a dangerous situation (Figure 2.8). The increased heart rate and perspiration of the adrenaline rush are part of a system called the fight-or-flight response, which prepares the body for dealing with threats from the environment.

Serotonin The neurotransmitter serotonin is involved in a wide range of psychological activities. It is especially important for emotional states, impulse control, and dreaming. Serotonin functioning is related to sad and anxious moods, food cravings, and aggressive behavior. One class of drugs that specifically targets serotonin is prescribed widely to treat depression (Jakubovski et al., 2016). These drugs, which include Prozac, are called selective serotonin reuptake inhibitors (SSRIs). As you learned in study unit 2.2, reuptake is the process by which the presynaptic neuron reabsorbs a neurotransmitter, which stops the stimulation of the postsynaptic receptors. SSRIs help to reduce the symptoms of depression by leaving more serotonin in synapses to bind with the receptors on postsynaptic neurons.

Dopamine The neurotransmitter dopamine has many important brain functions. Its most important functions are motivation and reward. Consider that people eat when they’re hungry, drink when they’re thirsty, and have sex when they’re aroused. These drives activate the dopamine system, and the increased dopamine produces the desire to act. Dopamine also plays an important role in drug addiction (Wise & Robble, 2020).

A lack of dopamine may also be associated with specific problems or illnesses. For example, severe loss of dopamine is connected to Parkinson’s disease. First identified by the physician James Parkinson in 1917, Parkinson’s disease is a degenerative and fatal neurological disorder. It affects about 1 in every 200 older adults and occurs in all known cultures. Most people with Parkinson’s do not experience symptoms until after age 50, but the disease can occur earlier in life. For example, the boxer Muhammad Ali was diagnosed with Parkinson’s at age 42, but he had been showing symptoms, such as slurred speech and shuffling walk, for many years. Before he died, Ali’s toughest fight might have been with Parkinson’s disease. He spent decades raising awareness about the disease and aiding others diagnosed with it, helping to create the Muhammad Ali Parkinson Center (Figure 2.9).

In a person who has Parkinson’s disease, the dopamine-producing neurons slowly die off. The resulting lack of dopamine causes disturbances in motor function: rigid muscles, tremors, and difficulty starting an action the person wants to perform. In the later stages of the disorder, people experience severe cognitive and mood disturbances. Injections of one of the chief chemical building blocks of dopamine, L-DOPA, help the surviving neurons produce more dopamine. When L-DOPA is used to treat Parkinson’s disease, patients often have a remarkable, though temporary, recovery.

GABA and Glutamate The main inhibitory neurotransmitter is GABA (gamma-aminobutyric acid). It is more widely distributed throughout the brain than most other neurotransmitters. Without the inhibitory effect of GABA, the excitation of neurons might get out of control and spread through the brain chaotically. In fact, epileptic seizures may be caused by low levels of GABA (Shetty & Upadhya, 2016).

Drugs that are GABA agonists (for example, Xanax and Ativan) are widely used to treat anxiety disorders (Balon & Starcevic, 2020). The increased inhibitory effect provided by these drugs helps calm anxious people. Alcohol has similar effects on GABA receptors. As a result, people typically experience alcohol as relaxing. GABA receptor activation may also be the primary mechanism that causes alcohol to interfere with motor coordination.

In contrast, glutamate is the main excitatory neurotransmitter. It is involved in fast-acting neural transmission throughout the brain. Glutamate receptors aid learning and memory by strengthening synaptic connections. Excessive glutamate release can lead to overexcitement of the brain, which can destroy neurons. For example, much of the damage inflicted to the brain following concussion is caused by the excessive release of glutamate that naturally occurs following brain injury (Cantu et al., 2015).

Endorphins If you are ever injured, you will be grateful for endorphins. In the early 1970s, researchers established that opioid drugs such as heroin and morphine bind to receptors in the brain. This finding led to the discovery of naturally occurring substances in the body that bind to those sites (Pert & Snyder, 1973). Called endorphins (short for endogenous, or naturally occurring, morphine), these substances are a class of neurotransmitters involved in natural pain reduction (Figure 2.10).

Pain is useful because it signals that you are hurt or in danger. That signal should then prompt you to try to escape or withdraw. If you do not experience pain when you touch a hot stove, you would not know that you should pull your hand away to avoid being injured. Have you ever been hurt or had a deep cut? If so, you may have noticed that you did not immediately feel any pain. You were initially spared the pain because the event triggered the release of endorphins. This release is a survival mechanism that lets you function for a short time. After a while, your endorphin levels declined. You probably began to feel the pain. You even may have taken painkilling medicine to replace the reduced endorphins so you could continue to function. In people, drugs that bind with endorphin receptors (for example, morphine) reduce the subjective experience of pain. However, morphine does not block the nerves that transmit pain signals. Instead, it alters the way pain is experienced. In other words, people still feel pain, but they report a sense of detachment that lets them not care about the pain.

Endorphins are also involved with the pleasure you experience when you do something rewarding. Thus that nice piece of chocolate cake that you savor owes some of its pleasure to endorphins. Along with dopamine, endorphins are involved whenever you do something you enjoy (Berridge & Robinson, 2016; Volkow et al., 2019).