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chapter 18 notes

chapter 18 notes - CHAPTER 18 Drug Abuse Positive...

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CHAPTER 18: Drug Abuse Positive Reinforcement: If a behaviour is regularly followed by an appetitive stimulus, that behaviour is reinforced. If an addictive drug is taken by fast-acting route (ie IV), the last action (injecting) is particularly reinforced. This reinforcement is powerful and immediate. Effect of a reinforcing stimulus is greatest if it occurs right after a behaviour/ response. Reason: instrumental conditioning involves learning about consequences of behaviour; if cause and effect times are separated, the “learning” is less effective. Logan: hungry rats trained to run through a maze to one of 2 corridors- one where they were given more food but w/ slight delay, or one where they were given less but immediately. Immediacy of reinforcement took precedence over quantity; rats ran to the latter corridor. Drug users prefer heroin to morphine simply because it has a more rapid effect; a “rush”. The use of methadone for opiate addiction and nicotine patches are based on this principle. Our reinforcement system has evolved to deal w/ immediate effects of behaviour, which is why addictive drug effects can often overpower recognition of long-term aversive effects. Cognitive processes of expectation and memory play an important role: if person takes a pill to feel euphoric ten mins later, they remember taking the pill and have an idea of its effects; if a rat is given the same pill, it will feel the euphoric effects, but they won't reinforce any behaviour. Neural mechanisms of positive reinforcement: All natural reinforcers (food for hungry animal, sexual contact, etc) cause release of dopamine in the nucleus accumbens (not the only effect of reinforcing stimuli, but a necessary one). Different addictive drugs stimulate release of dopamine in different ways. Process of addiction begins in mesolimbic dopaminergic system and produces long-term changes in other brain regions that receive input from these neurons. First changes take place in the ventral tegmental area: single administration of an addictive drug increased strength of excitatory synapses on dopaminergic neurons of the VTA in mice. As a result, increased activation is seen in many regions that receive dopaminergic input from the VTA, including the nucleus accumbens (in ventral striatum). Many important behavior-affecting changes occur in the dorsal striatum (caudate/putamen). (These are all in the basal ganglia, which plays a critical role in instrumental conditioning). Early reinforcing effects that take place in the ventral striatum (nucleus accumbens) encourage drug-taking behavior, but changes that make the behavior habitual involve the dorsal striatum. Infusion of dopamine antagonist into dorsal striatum suppressed rat's lever presses that had been reinforced by illumination of a light that had been paired w/ IV injections of cocaine.
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