Lesson 1.8 · Unit 1 · CED topic 1.3 (part 2)
Neurotransmitters, hormones, and psychoactive drugs
An action potential cannot jump across the synapse. To pass a message to the next neuron, the sending neuron releases chemicals, and these chemical messengers are what the exam asks you to connect to behavior and mental processes: which messenger does what, how hormones act like neurotransmitters but elsewhere in the body, and how drugs change behavior by changing the way these chemicals work. The exam limits its questions to the neurotransmitters and hormones listed in this lesson, so you can study them with confidence.
Learning objectives.
- 1.3.B. Explain how the basic process of neural transmission is related to behavior and mental processes.
- 1.3.C. Explain how psychoactive drugs affect behavior and mental processes.
Essential knowledge.
- 1.3.B.2. Each neurotransmitter has specific function(s) related to behavior and mental processes, which may depend on the neurotransmitter’s location in the nervous system. Neurotransmitters generally communicate either excitatory (making an action potential more likely) or inhibitory (making an action potential less likely) messages. Neurotransmitters related to behavior and mental processes for study in AP Psychology are limited to dopamine, serotonin, norepinephrine, glutamate, GABA, endorphins, substance p, and acetylcholine. Exclusion Statement: The AP Psychology Exam will only address the listed neurotransmitters in EK 1.3.B.2.
- 1.3.B.3. Outside of the nervous system, hormones perform actions similar to neurotransmitters. Hormones related to behavior and mental processes for study in AP Psychology are limited to adrenaline, leptin, ghrelin, melatonin, and oxytocin. Exclusion Statement: The AP Psychology Exam will only address the listed hormones in EK 1.3.B.3. Exclusion Statement: Specific information about the glands of the endocrine system (with the exception of the pituitary gland as referenced in EK 1.4.A.4) is outside the scope of the AP Psychology Exam.
- 1.3.C.1. Psychoactive drugs can influence neurotransmitter function in various ways throughout the neural communication process. Some act as agonists which encourage neural firing. Some act as antagonists which discourage neural firing. Some act as reuptake inhibitors which block the reabsorption of neurotransmitters back into the cell.
- 1.3.C.2. Psychoactive drugs have psychological and physiological effects.
- 1.3.C.2.i. Stimulants, such as caffeine and cocaine, typically cause increased neural activity.
- 1.3.C.2.ii. Depressants, such as alcohol, typically cause decreased neural activity.
- 1.3.C.2.iii. Hallucinogens, such as marijuana, typically cause distortions in perception and/or cognition.
- 1.3.C.2.iv. Opioids, such as heroin, typically act as pain relievers.
- 1.3.C.3. Psychoactive drug use can lead to tolerance and/or addiction. Addiction can create significant withdrawal symptoms if the psychoactive drugs are no longer consumed.
Crossing the synapse
When an action potential reaches the terminal buttons, the neuron releases neurotransmitters into the synapse. These molecules drift across the gap and fit into receptor sites on the next neuron's dendrites, somewhat like a key fitting a lock. A neurotransmitter generally sends one of two kinds of message. An excitatory message makes the receiving neuron more likely to fire an action potential, and an inhibitory message makes it less likely to fire. The receiving neuron adds up all the excitatory and inhibitory messages it receives and fires only if it reaches threshold. Afterward the neurotransmitter is cleared from the synapse, most often by reuptake, when the sending neuron reabsorbs it.
Each neurotransmitter has specific functions related to behavior and mental processes, and these may depend on where in the nervous system it is released. The ones to know are the following.
| Neurotransmitter | Main roles | Links to disorders and drugs |
|---|---|---|
| Dopamine | Reward and motivation, pleasure, voluntary movement, attention | Too little is linked to the tremors and movement problems of Parkinson's disease; excess activity is hypothesized to be linked to schizophrenia; many addictive drugs raise dopamine |
| Serotonin | Mood, sleep, appetite, and arousal | Low levels are linked to depression; common antidepressants raise serotonin activity |
| Norepinephrine | Alertness, arousal, attention, and the stress response | Involved in the body's response to stress; stimulants increase it |
| Glutamate | The main excitatory transmitter of the brain; learning and memory | Too much can overexcite and damage neurons |
| GABA | The main inhibitory transmitter of the brain; calms neural activity | Low GABA is linked to anxiety, and alcohol and antianxiety drugs boost GABA |
| Endorphins | The body's natural pain relievers; pleasure, such as the "runner's high" | Opioid drugs mimic them |
| Substance P | Carries pain messages to the brain | Part of how pain is felt |
| Acetylcholine | Muscle contraction, learning, and memory | Lower levels are linked to Alzheimer's disease; myasthenia gravis (see below) |
Myasthenia gravis is the second example the course gives of a disrupted neural process. In this disease the immune system attacks the receptors for acetylcholine on muscles, so the signal from motor neurons cannot be received properly and muscles become weak and tire easily. It shows that a problem at the synapse, not in the neuron itself, can disrupt behavior.
Hormones
The endocrine system is a set of glands that release hormones, chemical messengers that travel through the bloodstream to affect distant organs and behavior. Outside the nervous system, hormones perform actions similar to neurotransmitters. They act more slowly than neurotransmitters, which work across a synapse in milliseconds, but their effects last longer. The exam limits hormones to the following, and does not require specific glands except the pituitary, which you will meet in the brain lessons (the hypothalamus controls the pituitary, which in turn signals other glands).
- Adrenaline (epinephrine) is released during stress and arousal, speeding the heart and mobilizing energy for the fight-or-flight response.
- Leptin is released by fat cells and signals that you are full (satiety), reducing hunger.
- Ghrelin is released by the stomach and signals that you are hungry.
- Melatonin is released in response to darkness and helps regulate the sleep and wake cycle by making you sleepy.
- Oxytocin is involved in bonding and trust, in childbirth, and in the bond between mother and infant.
The physiologist Otto Loewi wanted to know whether nerves influence organs by electricity or by chemicals. He prepared two frog hearts in separate baths of fluid. Stimulating the vagus nerve of the first heart slowed its beat. Loewi then collected the fluid bathing the first heart and transferred it to the second heart, which also slowed down, even though no nerve to it had been stimulated. He concluded that stimulation had released a chemical (later identified as acetylcholine) and that the chemical, not electricity, carried the message.
Reading the research: a controlled laboratory experiment. The independent variable was the fluid (from a stimulated heart or not), and the dependent variable was the heart rate of the second heart. The study provided the first evidence of chemical neurotransmission. A limitation is that frog hearts are far simpler than the human brain, and the finding had to be extended to the central nervous system by later studies.
Psychoactive drugs and the synapse
A psychoactive drug is a chemical that changes perception, mood, thinking, or behavior by altering the activity of neurotransmitters. Drugs can interfere with neural communication at several points in the process.
- An agonist mimics a neurotransmitter or increases its effect, so it encourages neural firing. For example, opioid drugs mimic endorphins, binding to endorphin receptors and relieving pain.
- An antagonist blocks a neurotransmitter's receptors or reduces its effect, so it discourages firing. An example is curare, a poison that blocks acetylcholine receptors on muscles and causes paralysis.
- A reuptake inhibitor blocks the reabsorption of the neurotransmitter by the sending neuron, so the neurotransmitter stays in the synapse longer and keeps stimulating the receiving neuron. Common antidepressants (selective serotonin reuptake inhibitors, SSRIs) block the reuptake of serotonin. Cocaine blocks the reuptake of dopamine, producing a rush of pleasure.
Types of psychoactive drugs
The exam classifies psychoactive drugs by their typical effects:
- Stimulants, such as caffeine and cocaine, typically increase neural activity, raising alertness, energy, and heart rate.
- Depressants, such as alcohol, typically decrease neural activity, slowing reaction time and judgment, and lower inhibitions at first.
- Hallucinogens, such as marijuana, typically distort perception and cognition. (The course places marijuana in this category.)
- Opioids, such as heroin, typically act as pain relievers and also produce euphoria and drowsiness.
Tolerance, addiction, and withdrawal
With repeated use, the body adapts to many drugs. Tolerance means a person needs larger doses to get the same effect, because the nervous system adjusts. Addiction is a compulsive need to use a drug despite harmful consequences. When a person who has become dependent stops using the drug, withdrawal symptoms, such as anxiety, sweating, tremors, or intense cravings, can result, and these can be severe. The brain's reward circuits, which depend on dopamine, are involved in addiction, which is why quitting is so hard and why treatment is a medical and psychological matter, not a matter of willpower alone.
For each, say whether the drug is acting as an agonist, antagonist, or reuptake inhibitor. (a) A pain medication binds to endorphin receptors and relieves pain. (b) A drug binds to acetylcholine receptors on muscle cells without activating them, so acetylcholine cannot get in. (c) A medication prevents the sending neuron from reabsorbing serotonin, so more remains in the synapse.
Step 1, (a) the drug fits the receptors and produces the effect the neurotransmitter would, so it is an agonist.
Step 2, (b) the drug occupies the receptors and blocks the natural messenger, so it is an antagonist.
Step 3, (c) the serotonin stays in the synapse longer because reuptake is blocked, so it is a reuptake inhibitor.
Step 4, effect on firing: an agonist and a reuptake inhibitor encourage firing at the affected synapse; an antagonist discourages it.
Maya has been drinking several cups of strong coffee each morning for years. Now it takes four cups to feel as alert as one cup once did, and when she skipped coffee on a trip she had a headache, fatigue, and irritability. Identify the concepts.
Step 1, caffeine is a stimulant: it increases neural activity, which explains the alertness.
Step 2, needing four cups for an effect that once took one is tolerance: her nervous system has adapted to the drug.
Step 3, the headache, fatigue, and irritability after stopping are withdrawal symptoms, which show physical dependence.
Step 4, whether to call this addiction depends on whether she uses it compulsively in spite of harm; the key idea is that tolerance and withdrawal are separate, observable outcomes of repeated use.
After skipping lunch, a student feels hungry. The stomach releases ghrelin into the bloodstream, which signals the brain. Later, after a big meal, fat cells release leptin and the student feels full. Meanwhile, at the synapse between two neurons in the brain's reward circuit, dopamine is released and received in milliseconds. Compare the two kinds of chemical messengers.
Step 1, ghrelin and leptin are hormones. They are released by glands or cells outside the nervous system and travel in the bloodstream to distant targets.
Step 2, dopamine is a neurotransmitter: it is released by a neuron into a synapse and acts on the neighboring neuron.
Step 3, speed and duration: neurotransmitter messages are very fast and brief, hormone messages are slower but last longer.
Step 4, similarity: both are chemical messengers that bind to receptors and change the activity of the target cells.
- Know only the listed messengers: dopamine, serotonin, norepinephrine, glutamate, GABA, endorphins, substance P, acetylcholine; hormones: adrenaline, leptin, ghrelin, melatonin, oxytocin.
- Neurotransmitters are excitatory or inhibitory (glutamate and GABA are the main examples).
- Agonist = mimics or boosts; antagonist = blocks; reuptake inhibitor = keeps the transmitter in the synapse.
- Stimulants increase neural activity, depressants decrease it, hallucinogens distort perception, opioids relieve pain. Tolerance and withdrawal accompany repeated use.
- Match each to its role: (a) ghrelin, (b) melatonin, (c) endorphins, (d) GABA.
Show answer
(a) signals hunger; (b) helps regulate sleep and wake cycles by promoting sleepiness; (c) the body's natural pain relievers; (d) the main inhibitory neurotransmitter, which calms neural activity.
- What is the difference between a neurotransmitter and a hormone?
Show answer
A neurotransmitter is released by a neuron into a synapse and acts quickly on the neighboring neuron. A hormone is released by a gland or cell into the bloodstream, acts more slowly on distant targets, and its effect lasts longer.
- A drug increases the amount of serotonin in the synapse by blocking its reabsorption. Name the action and one likely benefit.
Show answer
It is a reuptake inhibitor. Because serotonin is linked to mood, it is used to treat depression.
- Why do heroin and other opioids relieve pain?
Show answer
They act as agonists for endorphin receptors: they mimic the body's natural pain relievers and bind to the same receptors.
- A person who has used a depressant for years now needs a larger amount to feel the same effect, and gets shaky and anxious when stopping. Name the two concepts.
Show answer
Tolerance (needing more for the same effect) and withdrawal (symptoms when stopping).
- What does Loewi's experiment show about how a nerve signal affects an organ?
Show answer
It showed that the signal is carried by a chemical: fluid from a stimulated heart slowed a second heart, so the chemical released by the nerve (acetylcholine) carried the message, not electricity alone.