Conditioned place preference
[1] This motivation comes from the pleasurable aspect of the experience, so that the brain can be reminded of the context that surrounded the "encounter".
In comparison, the unbiased method does not allow the animal to choose what compartment they get the drug and vehicle in.
In conditioned place preference, the unconditioned stimulus could be any number of things including food pellets,[9] water,[10] sweet fluid,[11] novel toys,[12] social interaction,[13] drug intoxication, drug withdrawal, foot shock, illness, wheel running[14] or copulation.
[15] The initially neutral environmental cues become associated with the motivational properties of the unconditioned stimulus leading to either approach or avoidance of the environment.
Often in practice, there is a control and treatment group used to strengthen the ability to make causal claims from the results.
[16] In the conditioning phase, the unconditioned stimulus (e.g. morphine) is administered to the animal (usually a mouse or rat) in the treatment group.
[16] Strength of conditioning is inferred by the magnitude of the difference or in the amount of time taken for the response to show extinction.
[16] In the standard conditioned place preference procedure, when the unconditioned stimulus is rewarding, rodents will be more likely to approach the compartment that contains cues associated with it.
[16] Alternatively, when the unconditioned stimulus is aversive, rodents will be more likely to escape and avoid the compartment that contains cues associated with it.
Perhaps the most significant disadvantage is that despite experimenters' best attempts to habituate animals to the procedure before conditioning, novelty-seeking effects can skew the data.
[30] Reinstatement is the rapid reacquisition of an extinct behavior, which is caused by either the presentation of the unconditioned stimulus, by stress, or by context cues.
[1] Primed-induced Reinstatement is a test in CPP whereby the unconditioned stimulus is given to the animal after the association between the UCS and CS has been extinguished.
Drug-primed reinstatement is thought to renew the incentive value of the place compartment because of the motivational effects of the drug.
Stress-induced reinstatement in CPP occurs when the animal is exposed to stress after a place preference has been extinguished.
Common stressors used in these paradigms include foot shock and noise[35] Some studies have shown that when drugs of abuse are used as appetitive stimuli, exposure to stress can reinstate place preference that has been extinguished over two weeks.
[36] When rats experience stress in the form of foot shock or noise, changes occur in the norepinephrine system and the hypothalamic-pituitary-adrenal axis.
CRH acts as a neurotransmitter in regions of the brain including the bed nucleus of the stria terminalis and the amygdala.
Reinstatement of conditioned place preference has shown to be blocked when antagonists for CRH receptors are injected into the BNST.
[37] In other words, the effects of stress on reinstatement can be inhibited by blocking the receptor sites for CRH in certain areas of the brain.
Furthermore, disinhibition of areas of the brain that inhibit the release of noradrenaline also nullifies the effect of stress-induced reinstatement.
Reinstatement studies on stress and drug primes provide evidence for their role in relapse behavior in humans.
[30] In addition to conditioned placed preference, animal testing using self-administration procedures has also been used to examine potential causes of relapse in humans.
Drug priming is thought to induce relapse in humans because of its effects on the reward circuits of the brain.
[42] Genetic knockouts of nicotinic receptor subunit β2 in mice resulted in a lack of conditioned place preference.
Studies also show a lack of conditioned place preference in CB1 receptor knockout mice,[44] implicating a possible contribution of the endocannabinoid system.
