PSYC62H3 Lecture Notes - Lecture 5: Brain Stimulation Reward, Medial Forebrain Bundle, Nucleus Accumbens

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24 Apr 2012
Lecture Five
Neurobiology of brain reward
o Dependence
o Tolerance
o Sensitization
Neurology of Brain Reward
What is Reward?
Reward in the context of learning
o When a behaviour is followed by a consequence that strengthens the future likelihood
of the behaviour occurring again
o Produces pleasure
o Stimuli that may be neutral under normal circumstances can become capable of
producing pleasure when paired with another stimulus (Pavlovian)
Gains incentive properties and induces approach behaviour by virtue of it
producing a pleasurable state
Different Types of Rewards
Natural Rewards
o Food, water, sex, nurturing, etc
Artificial Rewards
o No adaptive value
o Can strongly control behaviour
Does so by acting on the same structures that natural rewards work on
Drugs, money (gambling), etc
Olds and Milner (1954)
o Discovered that electrical stimulation
of certain areas of the brain was
rewarding and reinforcing
o Brain stimulation reward procedure
o If an electrode is implanted in the
brain reward circuit, they will work
to apply stimulation in that brain
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Brain Stimulation Reward Sites Include:
o Medial forebrain bundle (MFB)
o Ventral Tegmental Area (VTA)
o Nucleus Accumbens
o Frontal Cortex
o Lateral Hypothalamus
o Substantia Nigra
o Striatum, amygdala, hippocampus, thalamus, cerebellum
o Locus coeuleus
DA hypothesis
o Very influential
o The DA hypothesis of reward has instigated a considerable amount of research over the
last few decades
o However, much of that research can lead one to conclude that it’s oversimplified to
consider DA as the sole mediator of primary “reward”, or primary motivation for natural
stimuli such as food, water, and sex
o DA transmission within the reward circuit, especially the nucleus accumbens reinforces
a behaviour
o Drugs of abuse can hijack this system and produce pathological changes that lead to
o All drugs of abuse share the ability to dramatically increase extracellular levels of
dopamine in the nucleus accumbens (NA)
o Dopamine isn’t just a pleasure chemical
Effects of a saline injection or cocaine
o Cocaine produces a dramatic effect
o If we substitute morphine, alchohol, etc,
there would still be an increase in the
dopamine in the NA
o Evidence of its reinforcing effect
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Imaging studies are often used to support the idea that nucleus accumbens mediates pleasure
But imaging research also demonstrates that the nucleus accumbens responds to:
o Stress
o Aversion
o Hyperarousal/irritability
NA lights up during pleasure but also during stress
Thus, concepts about role of DA in drug-induced euphoria, dependence, and addiction have
undergone revision
Animal studies indicate that dopamine neuron activity isn’t just tied to positive reinforcing
If you have a drug that’s reducing anxiety, we would expect an increase in dopamine if it’s a
pleasure chemical. But in fact, the drug is associated with reducing DA transmission
Drugs that induce anxiety are associated with DA increases
We have to move beyond the DA synapse view of drugs, pleasure and reinforcement
o It’s not true that the more DA you have in the NA synapses, the greater the rewarding
A related set of terms describes behaviour patterns of excessive or chronic drug taking that can
result in deleterious effects
These terms include:
o Drug abuse
o Drug dependence
o Drug addiction
Individual shows excessive use of drugs that becomes involuntary/out of control
o Leads to averse effects
o Compromises functioning
Family, employment, etc
Can be diagnosed with drug abuse without being diagnosed with substance dependence
The terms drug dependence and drug addiction can be used interchangeably
o DSM prefers drug dependence
A state in which an organism functions normally only in the presence of drug
Doesn’t only occur in addicted individuals
Can occur with chronic exposure to non-addictive drugs
Refers to a real or perceived need of the drug for normal psych/physiological functioning
Emergence of withdrawal/abstinence syndrome
There are two types of drug dependence:
o A. physiological or physical dependence - when deprived, there’s a physical abstinence
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