Drugs can be used to alter the conscious state of an organism. Depressants, stimulants, hallucinogens all have varying effects on conscious states.

Depressants do not directly reduce arousal in the brain; they enhance the activity of the neurotransmitter gamma-aminobutyric acid (GABA), which is responsible for regulating (specifically, decreasing) neuronal excitability throughout the nervous system. Depressants inhibit the next neuron from sending impulses by binding to receptor molecules. Although the different classes of depressants work in unique ways, it is through their ability to increase GABA and thereby inhibit brain activity that they produce a drowsy or calming effect. The main types of depressants are alcohol, barbiturates, benzodiazepines, cannabinoids, and opioids.

Opioids, like heroin or morphine, work by activating opioid receptors in the brain. These receptors are normally activated by endogenous opioids, such as endorphins, after natural rewarding stimuli (ie exercise). 

Stimulants increase the activity of the central nervous system, the sympathetic nervous system, or both. Stimulants exert their effects through many different mechanisms. Some stimulants facilitate the activity of certain neurotransmitters, specifically norepinephrine and/or dopamine. Others block the action of specific receptors (such as the adenosine receptors) in a process known as receptor antagonism. Still, others cause action in other receptors (such as nicotinic acetylcholine) in a process known as receptor agonism.

Stimulants are often used medically to boost endurance, counteract fatigue, promote weight loss, improve mood, or relieve anxiety; they are also commonly used to treat conditions such as narcolepsy, ADHD, and certain forms of depression. Examples of well-known stimulants include amphetamines, MDMA, NDRIs, cocaine, caffeine, and nicotine.

Hallucinogens are drugs that alter sensory input to the brain. They are divided into three categories: psychedelics, dissociatives, and deliriants. Psychedelics work by interacting with serotonin receptors in the brain and produce a state of empathetic well-being and visual distortion. Dissociatives are a subclass of hallucinogens that work by blocking or altering sensory perception. Deliriants are very similar to dissociatives; they are considered to be “true hallucinogens” because the visuals they produce are hard or impossible to distinguish from reality. Common hallucinogens include MDMA, PCP, and LSD.

Addiction

Often abuse of drugs can lead to addiction. The rewarding effects of drugs abuse come from significant and rapid upsurges in dopamine, a neurochemical critical to stimulating feelings of pleasure and to motivating behavior. The fast dopamine “rush” from drugs of abuse mimics but greatly exceeds in intensity and duration the feelings that occur in response to such pleasurable stimuli as the sight or smell of food, for example. Repeated exposure to large, drug-induced dopamine surges has the insidious consequence of ultimately blunting the response of the dopamine system to everyday stimuli. Thus the drug disturbs a person’s standard hierarchy of needs and desires and substitutes new priorities concerned with procuring and using the drug.

Drug abuse also disrupts the brain circuits involved in memory and control over behavior. Memories of the drug experience can trigger craving as can exposure to people, places, or things associated with former drug use. Stress is also a powerful trigger for craving. Control over behavior is compromised because the affected frontal brain regions are what a person needs to exert inhibitory control over desires and emotions. Additionally, many drugs cause physiological withdrawal symptoms once a person is physically dependent. Alcohol withdrawal, for example, can become so severe that it is life-threatening.

That is why addiction is a brain disease. As a person’s reward circuitry becomes increasingly dulled and desensitized by drugs, nothing else can compete with them; food, family, and friends lose their relative value, while the ability to curb the need to seek and use drugs evaporates. Ironically and cruelly, eventually, even the drug loses its ability to reward, but the compromised brain leads addicted people to pursue it, anyway; the memory of the drug has become more powerful than the drug itself.

Practice Questions

Khan Academy

MCAT Official Prep (AAMC)

Official Guide P/S Section Passage 3 Question 12

Official Guide P/S Section Passage 3 Question 13

Section Bank P/S Section Passage 2 Question 7

Section Bank P/S Section Passage 2 Question 11

Practice Exam 2 P/S Section Passage 5 Question 24

Practice Exam 2 P/S Section Passage 5 Question 25

Practice Exam 2 P/S Section Passage 9 Question 55

Practice Exam 4 P/S Section Passage 8 Question 40

Practice Exam 4 P/S Section Passage 9 Question 51

Practice Exam 4 P/S Section Question 27

Key Points

• Consciousness altering drugs come in three classes, stimulants, depressants and hallucinogens.

• Stimulants increase the activity of the nervous system through several different mechanisms and are often used medically to boost endurance and counteract fatigue.

• Depressants inhibit the next neuron from sending impulses by binding to receptor molecules, decreasing the excitability of the nervous system.

• Hallucinogens are drugs that alter sensory input to the brain.

• The rewarding effects of drugs of abuse come from significant and rapid upsurges in dopamine, a neurochemical critical to stimulating feelings of pleasure and to motivating behavior.

• Repeated exposure to large, drug-induced dopamine surges has the insidious consequence of ultimately blunting the response of the dopamine system to everyday stimuli. Thus the drug disturbs a person’s typical hierarchy of needs and desires and substitutes new priorities concerned with procuring and using the drug.

Key Terms

Serotonin: an indoleamine neurotransmitter (5-hydroxytryptamine) that is crucial in maintaining a sense of well-being and security and is involved in depression

Antagonism: a chemical that binds to a receptor but does not produce a physiological response, thereby blocking the action of agonist chemicals

Neurotransmitter: any substance, such as acetylcholine or dopamine, responsible for sending nerve signals across a synapse between two neurons

Gamma-aminobutyric acid (GABA): neurotransmitter responsible for regulating (specifically, decreasing) neuronal excitability throughout the nervous system

Depressants: inhibit neuronal activity

Endorphin: endogenous opioid receptor agonist

Sympathetic nervous system: directs the body’s rapid involuntary response to dangerous or stressful situations

Central nervous system: the brain and spinal cord

Hallucinogens: drugs that alter sensory input to the brain

Addiction: a psychological and physical inability to stop consuming a chemical

Dopamine system: is a group of nerve cells, most of which originate in the midbrain responsible for the pleasure system in the brain