Theories of Addiction II

Question 1

positive reinforcement models

Answer 1

incentive motivational model and hedonia model

Question 2

incentive motivational model

Answer 2

suggests there’s a central mechanism that is activated by almost all stimuli we consider rewarding. every incentive stimuli activates the central motivational state and this state drives the behaviour. when this exposure happens repeatedly, the system learns and that stimulus becomes more salient. this process causes an increase in the attractiveness of the incentive stimuli

Question 3

hedonia model

Answer 3

a system that identifies positive outcomes. wise suggested that drugs are addictive because they produce euphoric effects. system is tuned to identify pleasure and to drive the proper behaviour to get that pleasure again. includes the mesolimbic DA pathway. when blocking DA transmission, almost any behaviour associated with reward will be reduced.
- experiment using ICSS and pimozide: animals trained to self administer resulting in increased responses. injected with DA receptor antagonist resulting in drop in responses until it reaches 0 - find the intracranial
stimulation no longer rewarding or due to motor effect. added a door that blocked lever, no response since the lever is blocked. when removed door, increased responses but drops to 0 again. portrays spontaneous recovery.
- experiment using food and pimozide: training rats with food resulting in increased responses. injected DA antagonist leading to decreased responses. used 2 doses of pimozide - higher dose had stronger effect on food taking. group with no reward resulting in true extinction. 4th day, given pimozide and food together leading to decreased response (theres no rewarding effect even with food since the DA system was blocked). should see decreased response in home environment but see very high response.
- experiment using heroin and haloperidol: haloperidol is a DA receptor antagonist. experiment is about animals running. running much slower post haloperidol injected (ran fast they day before) since it’s wasnt rewarding before. even increasing the dose of haloperidol on the first dose, the next day they run even slower. demonstrates the importance of DA on reward. reason why people take drugs = want this euphoric effect and its mediated by DA.

Question 4

problems with the hedonia model (9)

Answer 4

1. some degree of reward function possible even without DA (seen in DA deficient mice)
2. for opiates, suggested there are 2 different reward systems: 1 for drug dependent (DA dependent) and the other for drug independent (DA independent) - can show reward for opiates without DA system
3. opiates act at 2 points: VTA (modulating DA) and NAc (medium spiny neurons). euphoric effects due to opiates and not DA
4. first exposure to drug is experienced as not euphoric yet still abused.
5. under certain conditions, aversive stimuli can increase DA release in NAc
6. people work for low cocaine/morphine doses that have no subjective effect (no pleasure)
7. drug taking may increase over time, but pleasure not reported to increase (something else drives drug taking)
8. DA signal associated with cues that predict arrival of reward and not the consumption of reward. EXPERIMENT: implanted electrodes in VTA of monkeys to record firing of DA neurons. trained monkeys to obtain reward. there is an immediate increase when the monkeys get the reward. exposed to conditioned stimulus associated with reward resulted in an increased firing of DA neurons. there is a increase when CS is presented but not after reward delivery. response if after cue predicting reward. presenting CS without reward, DA neurons are inhibited and fire less. suggested DA is a prediction error signal. when something is surprising (reward w/o CS) DA system identified it as a positive error. after conditioning, CS is presented and its surprising so theres an increase in firing but the reward isnt surprising so theres no increase. when the reward isnt given, expect something and didnt get it, its a negative error so theres less DA firing.
9. DA function might be important for reward and addiction but it is not the mesolimbic DA system only that counts. EXPERIMENT: animals need to respond many times to get the stimulus. lots of responses. light associated with the cocaine fusion which is why they press many times for the infusion of cocaine. DA receptor antagonist (flupenthixol) injected into dorsal striatum. see a decrease in the number of responses - indicates that DA transmission in dorsal striatum important in drug reward. D1 receptor antagonist injected into substantia nigra. got an increase in response for cocaine when injecting DA receptor antagonist. Cocaine is considered less rewarding now and they compensate by increasing responses. increasing antagonist doses, cocaine no longer becomes rewarding and there will be no more responses.

Question 5

modified hedonia model

Answer 5

wise suggested that DA neurons fire at regular rate. that baseline levels of DA necessary for any kind of motivation. subjective pleasure is correlated with events that increase DA firing. not unique since aversive stimuli also increase DA firing. pleasure not a necessity of DA elevation. suggests that DA has a role in reward and not necessarily pleasure.