Neuroscience of Reward and Motivation
Activates the Reward System
- Engaging with reward systems leads to a motivator action, such as picking up the phone again.
- Involves the neurochemical dopamine.
- Dopamine levels increase when rewards are activated, contributing to the pleasurable experiences associated with various behaviors.
Important Date
- Final Exam Date and Time:
- Date: Monday, May 4
- Time: 10:30 AM
- Location: Burnham B6 (our lecture room)
Understanding Reward and Motivation
Definitions
- Reward:
- Described as an event producing a pleasant or positive experience.
- Represents an objective assessment of the positive value attached to an object, behavior, or internal state.
- Motivation:
- Defined as the energizing of behavior directed towards achieving a goal.
- Influenced by a combination of internal states (biological, psychological) and external environmental factors.
General Observations
- All animals have motivational drives to fulfill basic needs such as food, water, sex, and social interaction.
Reward vs. Reinforcement
Definitions
- Reinforcement:
- Closely related to observable behaviors.
- Can be understood through activities that increase the likelihood of a behavioral act, particularly through negative reinforcement, which involves the removal of a negative state of need.
- Rewards:
- Categorized as primary or secondary.
- Primary Rewards:
- Examples include sex and food, fundamental to survival.
- Secondary Rewards:
- Examples include money or social status, which may acquire value through conditioning.
Dopamine (DA)
Properties of Dopamine
- Class of neurotransmitters known as monoamines, particularly from the catecholamine family.
- Acts through G protein-coupled receptors.
- Involved in several physiological and psychological functions:
- Mood
- Reward processing
- Learning and attention
- Movement
- Heart rate
- Kidney function
- Blood vessel function
- Sleep regulation
- Pain processing
- Lactation
- Primary locations found in:
- Ventral tegmental area (A10)
- Substantia nigra pars compacta (A9)
- Retrorubral field (A8)
- Reticular formation
- Hypothalamus
Pleasure Centers in the Brain
Historical Context
- Research from the 1950s by Olds and Milner showed that self-stimulation in rats led them to prefer stimulation sites in the septa region of the brain, particularly in the septum and nucleus accumbens.
- This discovery led to the proposal of brain centers responsible for positive drives related to pleasure.
Experimental Design
- Rats would favor brain stimulation over food, pressing stimulation pedals numerous times (up to 100) to achieve brain stimulation, indicating willingness to work for neural rewards.
Notable Researchers
Nobel Prize Contributions in 2000
- Awardees:
- Arvid Carlsson
- Paul Greengard
- Eric R. Kandel
- Contribution:
- Discoveries regarding signal transduction in the nervous system and specifically in dopamine mechanisms.
Dopamine Pathways in the Brain
Classification of Pathways
- Nigrostriatal Dopamine Pathway
- Mesolimbic Dopamine Pathway
- Mesocortical Dopamine Pathway
- Tuberoinfundibular Dopamine Pathway
Visual Representation of Pathways
- Nigrostriatal Pathway: Originating from the substantia nigra.
- Mesolimbic Pathway: Involved with reward mechanisms linking to the VTA.
- Mesocortical Pathway: Connects to areas associated with cognitive processes.
- Tuberoinfundibular Pathway: Regulates hormone release.
Mesolimbic Pathway and Reward System
Significance
- The mesolimbic pathway is vital for mediating pleasure and rewarding experiences through dopamine release.
- Connects brain regions including:
- Midbrain
- Basal ganglia
- Hippocampus
- Amygdala
- Prefrontal cortex (associated with mesocortical pathways).
Anatomical Details
- Key components:
- Ventral tegmental area (VTA)
- Medial forebrain bundle (MFB)
- Nucleus accumbens (NAC)
- The mesolimbic pathway is involved in motivational salience and reward mechanisms.
Conditions Associated with VTA Abnormalities
- Schizophrenia
- Parkinson's Disease
- ADHD
- Addiction
- Substance Use Disorders:
- Approximately 23 million Americans (ages 12+) require treatment for substance abuse disorders, yet only 10% receive necessary care.
- By contrast, 85% of the 29 million people in the U.S. with diabetes receive treatment. (Source: CDC, 2014 JAMA 2015)
Schizophrenia Overview
Definitions and Symptoms
- Definition:
- Often referred to as “split mind,” indicating a severe thought disorder characterized by dissociative thinking.
- Onset:
- Effects on Functionality:
- Leads to deteriorating daily function, accompanied by various symptoms such as delusions, hallucinations, and movement disorders.
Types of Schizophrenia Development
- Two forms of onset:
- Acute: Sudden onset with good recovery prospects.
- Chronic: Gradual onset requiring long-term treatment; most patients manage their condition with medication and occasional hospitalization.
Prevalence
- Approximately 1% of the population experiences schizophrenia at any given time.
Schizophrenia Symptoms
Positive Symptoms
- Indicate abnormal behaviors that are present, including:
- Thought Disorder: Disorganized and irrational thinking.
- Delusions: False beliefs contrary to fact.
- Hallucinations: False perceptions of stimuli.
- Inappropriate Emotionality
Negative Symptoms
- Reflect an absence of normal behaviors, manifesting as:
- Flattened emotional response
- Poverty of speech
- Lack of initiative
- Anhedonia
- Social withdrawal
Laboratory Models for Schizophrenia Symptoms
Emotional and Behavioral Responses
- Inappropriate Affect: Failure to respond emotionally in expected ways to events.
- Testable in laboratory settings using both humans and animal models.
- Pre-Pulse Inhibition (PPI) Test: Used to assess sensory processing and responsiveness.
- Normally, a quieter sound before a loud sound reduces the startle response, which is significantly impaired in individuals with schizophrenia.
Dopamine Theory of Schizophrenia
Neurochemical Basis
- Schizophrenia is associated with excessive activity at dopamine receptors.
- Treatment Mechanism:
- Antipsychotic drugs reduce symptoms by lowering dopamine receptor activation in the brain.
Amphetamine Psychosis and Dopamine
Conceptual Implications
- Sustained use of amphetamines can generate symptoms resembling schizophrenia:
- Hallucinations
- Delusions
- Thought disturbances
- Reinforces the role of heightened dopamine levels in underscoring these symptoms.
Effective Medication and Neurobiology
Treatment Mechanisms
- Chlorpromazine: Acts on the dopamine system via dopamine receptor blockade.
- Treatment efficacy highlights the neurological substrate underpinning schizophrenia.
Importance of Dopamine Receptor Subtypes in Treatment
Receptor Classifications
- Dopamine receptors: Subdivided into D1 and D2 subtypes.
- Chlorpromazine: Acts on both subtypes.
- Haloperidol: Targets D2 receptors specifically.
- Research indicates that D2 over-activation is a key characteristic in schizophrenia.
Attention Deficit Hyperactivity Disorder (ADHD)
Overview and Statistics
- ADHD: One of the most common neurodevelopmental disorders in children, with over 3 million cases annually.
- Frequently occurs alongside other disorders, about 67% of cases.
- Associated with altered brain structures and connectivity in mesolimbic and cortical pathways.
Types of ADHD
- Predominantly Inattentive (PI or I):
- Challenges with organization and maintaining attention.
- Predominantly Hyperactive-Impulsive (PH or HI):
- Characteristics include excessive movement and difficulty waiting.
- Combined Type (C):
- Displays symptoms from both categories; most prevalent.
Description
- A neural pathway connecting various brain regions involved in emotional and reward processes, primarily composed of dopaminergic fibers leading from the VTA to the nucleus accumbens.
- Functions linked to grief, sadness, and the potential to improve dyskinesia induced by L-DOPA.
Nucleus Accumbens (NAC)
Structural Outline
- Located in the basal forebrain, part of the ventral striatum, positioned rostral to hypothalamus.
- Subdivided into multiple areas, crucial for both motor control associated with rewards and cognitive processing.
- Inputs:
- DAergic drive from VTA, along with inputs from the prefrontal cortex, amygdala, hippocampus, and thalamus.
- Outputs:
- Connects with basal ganglia and VTA.
Limbic System
Structure and Function
- Comprises structures located near the thalamus, fundamental in processing motivation, emotion, learning, and memory.
- Significantly interrelated with the NAC, indicating its foundational role in behavioral outcomes.
Mesocorticol Pathway
Description
- A dopaminergic pathway that links the VTA to the prefrontal cortex, associated with cognition and decision-making processes.
- Additionally implicated in negative symptoms of schizophrenia, such as flattened emotional expression.
Habenula: Anti-Reward Systems
Overview
- Habenula: A small region of the epithalamus functioning as an integrator of information from limbic systems and various reward pathways.
- Composed of lateral and medial habenula, where specific roles pertain to modulation of reward and aversion systems.
- Lateral Habenula: Responds negatively to rewards, inhibiting DA neuron activity.
- Medial Habenula: Involved in nicotine addiction, demonstrating a concentration of nicotinic receptors.
Dopamine Mechanisms
Understanding Dopamine Functionality
- Reward Prediction Error: Initially fires in response to rewards; however, cues signaling potential rewards cause DA neurons to respond rather than the rewards themselves.
- If rewards are not received, a decrease in DA neuron activity follows.
- Impacts learning processes: unexpected rewards elevate DA releases, fostering greater learning capacity.
Incentive Salience
Concept Definition
- A motivation-enhancing signal derived from cues predicting rewards, significantly influencing behaviors related to drug craving and other reward-seeking activities.
Reward Euphoria
Phenomena Associated with Rewarding Experiences
- Euphoric states correspond with synchronized DA increases within the NAC.
- Research indicates cocaine and amphetamines' role in significantly raising synaptic DA levels, with blockade of DA resulting in diminished reinforcing effects.