The Reward Pathway

Brain Structures Involved Ventral tegmental area Nucleus accumbens Prefrontal cortex

Located deep inside the brain Regulates feelings of pleasure that are related to our survival. For example, eating, sex, (what gives you pleasure?) These activities create feelings called REWARD and are very powerful. The brain makes these actions pleasurable so they will be repeated over and over The chemical responsible is a neurotransmitter called dopamine

Drugs of Abuse Drugs of abuse activate this same brain area However, drugs create dopamine surges This is the reason why drugs produce feelings called euphoria These INTENSE pleasurable feeling leads the user to repeat the action over and over. This may be the beginnings of drug abuse This euphoria created in the pleasure center is so powerful that over time they override the frontal cortex in controlling behavior, or knowing right from wrong.

How Do Drugs Cause Their Effects on the Brain and Behavior? Drugs contain chemicals that tap into the brain’s communication system and disrupt the way nerve cells normally send, receive, and process information. This disruption occurs at the synapse

Drugs can affect synapses at a variety of sites and in a variety of ways, including: 1. Increasing number of impulses 2. Release neurotransmitters (NT) from vesicles with or without impulses 3. Block reuptake or block receptors 4. Produce more or less NT 5. Prevent vesicles from releasing NT

There are at least two ways that drugs cause this disruption: By imitating the brain’s natural chemical messengers Some drugs (e.g., marijuana and heroin) have a similar structure to neurotransmitters This similarity allows the drugs to “fool” the brain’s receptors and activate nerve cells to send abnormal messages.

Example: nicotine Nicotine binds to a nicotine receptors on the axon terminal. This increases the number of electrical signals in the pre- synaptic neuron Causing the vesicles to release more dopamine than normal. In turn this causes an increase in electrical signals in the post-synaptic neuron The extra impulses cause feelings of euphoria in the drug user.

Nicotine EUPHORIA

2. By overstimulating the “reward circuit” of the brain. Other drugs, such as cocaine or methamphetamine, can cause the nerve cells to release abnormally large amounts of natural neurotransmitters (mainly dopamine) or to prevent the normal recycling of these brain chemicals, which is needed to shut off the signaling between neurons. The result is an increase of dopamine in the synapse The overstimulation of this reward system produces euphoric effects in response to psychoactive drugs. This reaction sets in motion a reinforcing pattern that “teaches” people to repeat the rewarding behavior of abusing drugs.

Example 1: Increase the amount of neurotransmitter that is released from the presynaptic neuron. methamphetamines (crystal, speed, crank) The drug enters the vesicles This causes the release of the vesicle without an electrical signal. The drug also blocks protein channels This causes more dopamine in the synapse than what is normal. This then causes an increase of electrical impulses created in the post-synaptic neuron. This then causes increased feelings of euphoria for the drug user.

methamphetamines 1 EUPHORIA

Problems…… 1. After the drug wears off, dopamine levels drop, and the user “crashes”. The euphoric feeling will not return until the user takes more methamphetamine   2. Long-term use of methamphetamine causes dopamine axons to wither and die.

Example 2: Drugs of abuse alter the removal of dopamine from the synapse. Cocaine Blocks the protein channels This results in more dopamine in the synapse. This causes an increase in electrical impulses in the post-synaptic neuron Which causes feelings of euphoria

Cocaine EUPHORIA

How does alcohol affect synapses? • Alcohol has multiple effects on neurons. It alters neuron membranes, ion channels, enzymes, and receptors. • It binds directly to receptors for acetylcholine, serotonin, and gamma aminobutyric acid (GABA), and glutamate. • We will focus on GABA and its receptor.

GABA and the GABA Receptor • GABA is a neurotransmitter that has an inhibitory effect on neurons. • When GABA attaches to its receptor on the postsynaptic membrane, it allows Cl- ions to pass into the neuron. • This causes the postsynaptic neuron to inhibit transmission of an impulse. When alcohol enters the brain, it binds to GABA receptors and amplifies the effect of GABA. The neuron activity is further diminished This accounts for some of the sedative effects of alcohol

The Adolescent Brain and Alcohol The brain goes through dynamic change during adolescence, and alcohol can seriously damage long- and short-term growth processes. Frontal lobe development and the refinement of pathways and connections continue until age 16, and a high rate of energy is used as the brain matures until age 20-25. Damage from alcohol at this time can be long term and irreversible In addition, short-term or moderate drinking impairs learning and memory far more in youth than adults. Adolescents need only drink half as much as adults to suffer the same negative effects.