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for resistance to cocaine-seeking (rate of lever-pressing) extinction and for cue-induced
reinstatement (rate of lever pressing induced by the conditioned cue). These tests were
conducted after 1, 30 and 90 days of withdrawal. Reinstatement of lever pressing was
assessed in the absence of the discrete tone-light cue. Rats were allowed to lever-press until
they had reached an extinction criterion of <15 responses per session on the active lever.
Then, after the final extinction session, the test for cue-induced reinstatement was conducted.
Each spontaneous lever press resulted in a presentation of the tone-light cue, that then became
conditioned reinforcer. Sucrose trained rats were tested for resistance to extinction and cue-
induced reinstatement after 1, 30, or 90 days of withdrawal from sucrose in the same
conditions used for cocaine (Grimm et al., 2003). The results of this study will be reported
later in this chapter.
Four learning processes can be identified in this experiment that also occur in human
cocaine consumption and which have been found to be critical to the development of human
addiction. The first is Pavlovian conditioning, represented by the discrete tone-light paired
with cocaine. The tone-light begins as a neutral stimulus (NS), which after being associated
with cocaine, the unconditioned stimulus (UC), becomes a conditioned stimulus (CS). The
second process is a stimulus, such as the tone-light, becoming a conditioned reinforcer
through the acquisition of reinforcing properties (positive or negative) by being paired with
other, generally primary reinforcers, such as food, drugs, or sex. The third process taking
place is instrumental reward-related learning (instrumental conditioning), by which rodents
learn to perform a behavior in response a rewarding stimulus. The fourth process is
Pavlovian-instrumental transfer (PIT), the modulation of instrumental performance by
Pavlovian CS. For example, the tone-light cue that predicts the arrival of cocaine will enhance
lever pressing for cocaine. PIT is important because it probably plays a major role in CS-
precipitated reinstatement of instrumental responding, exemplified by cue-induced relapse in
drug addiction (Cardinal et al., 2002).
From the neurobiological point of view, dopamine is essential in the different reward-
related learning processes described above. Midbrain dopamine cells fire bursts of action
potentials as a consequence of stimuli that predict the rewarding events (Mirenowicz and
Schultz, 1994; Schultz, 1998). If dopamine release coincides with the solution of behavioral
problems, such as obtaining food or drink, the appropriate response is to do it again and the
response is learned (Ljungberg et al., 1992). Dopamine release to facilitate further learning is
not necessary and does not occur after biological rewards once the most efficient behavior to
obtain a reward has been learned (Schultz, 2004). However, dopamine continues to be
released after a conditional stimuli once it has become a conditioned reinforcer (Schultz,
1998).
In order to be able to use the learned information the animals need a record of what has
occurred at the arrival of dopamine, which acts on corticostriatal synapses and their respective
striatal partners. Dopamine contributes to the creation of a new memory trace or to the change
of a pre-existing one. This memory trace makes the activity of the involved set of connections
more likely to be activated in similar circumstances and continues to do so for some time
even in the absence of further reward. Once a behavior designed to obtain a reward or avoid a
negative consequence as been learned, the role of dopamine changes. Dopamine then enables
the use of the learned information to execute the adaptive behavioral response in an efficient
manner (Schultz, 2004).
There is a major difference between dopamine release in response to biological reward
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