reward and addiction (Andrew Young)

What is motivation

• the reason for acting in a certain way

• the driving force for performing a behaviour

In animal studies, what is increased when a reward is received

dopamine

Direct measurement of what in where shows that dopamine release is increased during naturally rewarding behaviours

dopamine in nucleus accumbent using micro dialysis

What are the main pathways

• mesolimbic: from ventral tegmental area (VTA) to nucleus accumbens

• mesocortical: from ventral tegmental area (VTA) to cortex (mainly frontal)

• nigrostriatal: from substantial nigra to dorsal striatum 

Animals will work to

receive natural rewards including food, water, sex

• e.g. pressing a lever

Work rates are affected by drugs which affect the

dopamine system, given either systemically or locally into nucleus accumbens

• e.g. effect of pimozide (dopamine antagonist) on lever pressing for food reward

Describe the study and findings by Olds and Milner, 1954

• carrying out experiments on effect of electrical stimulation of the reticular formation on attention in rats

• observed that a single animal appears to ‘enjoy’ brain stimulation

  • “by the time the third electrical stimulus had been applied the animal seemed indubitably to be coming back for more”

• found that the electrode was in VTA (ventral tegmental area), rather than reticular formation

• showed that animals would press a lever repeatedly in order to receive electrical stimulation of specific brain areas

  • mesolimbic pathway

  • effect in very region specific

What pathway doe animal models show is important in controlling reward

mesolimbic pathway

What increases in the nucleus accumbens during natural reward (e.g. food, water)

dopamine release

How can we be sure that dopamine is what gets released after receiving a reward

A dopamine antagonist is a drug that blocks dopamine receptors.

• When animals are given a dopamine antagonist:

  • they stop pressing the lever as much

  • they lose motivation to work for the reward

  • the reward no longer feels as rewarding

This shows that dopamine is necessary for motivation and reward-seeking behaviour.

What is a reward

• a pleasurable experience (Hedonia)

  • “Hedonia refers to the pursuit of pleasure and positive emotions as a source of wellbeing.”

  • “While hedonia focuses on pleasure and comfort, eudaimonia emphasises meaning and personal growth.”

• may increase the likelihood of repeating the behaviour

How can the reward/reinforcement system create a problem for empirical study

what is pleasurable for one person/animal, may not be pleasurable for another (subjective)

Animals will work (e.g. press a lever) in order to receive electrical stimulation of certain brain areas such as:

• ventral tegmental area

• median forebrain bundle

• nucleus accumbens 

What affects electrical self-stimulation

• effects of stimulation current/frequency on press rate

  • not supported below a certain current

  • as the current or/and frequency increases, so does the press rate

• press rate is increased by drugs which increase dopamine

  • amphetamine, nicotine, morphine, heroine

• pressing abolished by lesions of median forebrain bundle

List and describe what drugs that self-administered by animals

• amphetamine - increases dopamine release, blocks reuptake and inhibits enzymatic breakdown

• cocaine - blocks dopamine reuptake

• morphine - agonist at opiate receptors

• nicotine - agonist at acetylcholine receptors

• ethanol - mechanisms of action uncertain

What happens to self-administration of amphetamine or cocaine when a dopamine antagonist is given?

• Self-administration is potentiated (increased)

• The animal presses the lever more frequently to receive additional drug in an attempt to overcome the blocking effects of the dopamine antagonist

What happens to systemic self-administration of amphetamine or cocaine when high doses of dopamine antagonists are given?

• Self-administration is abolished.

• At high antagonist doses, the drug’s effects cannot be overcome by taking more, so animals stop pressing the lever, similar to the effect seen in self-stimulation experiments.

Give evidence that dopamine release in nucleus accumbens signals reward

• self-administration and effects of dopaminergic drugs

• drug self-administration

• dopamine release in nucleus accumbens during rewarded behaviour

• drugs which enhance these behaviours, all evoke dopamine release in nucleus accumbens when given alone

What is substance abuse

frequent and/or excessive use of a substance for non-medical reasons, bringing damage to relationships, and/or daily functioning

What is substance dependence or addiction

• compulsive substance taking

• lives centred around a substance

• tolerance and/or withdrawal symptoms

State the data found from DiChiara & Imperato (1988)

• brain micro dialysis studies showed that a number of drugs increase dopamine release in nucleus accumbens

• larger responses in nucleus accumbens than striatum

• these are the drugs which

  • animals will self-administer

  • enhance self-stimulation

  • enhance reinforced behaviours

List the pharmacology of addictive drugs

• amphetamine - dopamine reuptake inhibitor, also causes efflux from terminal

• cocaine - dopamine reuptake inhibitor

• nicotine - actelycholine agonist

• alcohol - has effects on opioid, GABA and glutamate systems

• morphine - opiate agonist

• heroine - opiate agonist

• benzodiazepines - allosteric enhancer of GABA function

Mechanisms of action of addictive drugs

Motivation and addiction loops

Many behaviours and/or stimuli associated with drug taking become reinforced such as

• promote drug taking

• induce cravings for drugs

• induce physical symptoms similar to withdrawal symptoms 

Behaviour associated with reward may also become addictive such as

• gambling (addiction of impulse-control disorder)

• exercise

• eating disorders

What system appears to be critically involved in addiction

dopamine systems

Why do drugs reinforce drug taking behaviour

because drugs activate the natural motivation system

• this reinforcement drives further drug-taking, often at the expense of other important behaviours, such as eating and drinking