Behavioural sensitisation: A progressive increase in the locomotor-activating effects of drug following repeated exposure to a set drug dose (typically 5-10 days). The expression of behavioural sensitisation is typically assessed during withdrawal following an acute drug challenge at a dose equivalent to that administered during the sensitisation exposure.
Conditioned place preference (CPP): A classical (Pavlovian) conditioning paradigm whereby animals are administered either drug or saline (delivered intraperitoneally). After injection animals are immediately placed into one of two distinct contexts that differ in terms of their contextual cues (e.g. wall colour/pattern, floor texture). Following conditioning, the animals are tested for their preference of the drug- vs. saline- paired chamber, as measured by time spent in each chamber. An increase in preference for the drug-paired context is thought to indicate a positive reinforcing effect of the drug.
Conditioned place aversion (CPA): Similar to CPP, however, one context is associated with an aversive stimulus, whilst the other is associated with a neutral stimulus. Following conditioning, animals are tested for their preference for the contexts. An increase in preference for the neutral context indicates an aversive effect of the test stimulus.
Operant drug self-administration model: A procedure whereby animals are trained to respond (typically lever press or nose poke) for a drug reward. Psychostimulants such as cocaine and amphetamine are typically delivered to the animal via a chronic indwelling jugular catheter, whereas drugs such as alcohol are delivered in small volumes for oral consumption. This model can also be used to train animals to respond for natural rewards, such as a sucrose solution or sucrose/food pellets.
Extinction learning: A reduction in drug-seeking produced by breaking the contingency between drug-seeking behaviour or drug-predictive stimuli and delivery of drug reward. Importantly, extinction learning involves new learning, and is not a ‘forgetting’ of the previous association.
Intracranial self-stimulation (ICSS): A procedure whereby animals learn to respond for direct electrical stimulation via an electrode implanted into a specific region of the brain. Higher rates of responding for ICSS are thought to be caused by decreased ‘reward’ pathway sensitivity whereas lower rates of responding for ICSS are thought to result from increased ‘reward’ pathway sensitivity.
Progressive ratio schedule: A reinforcement schedule applied to the drug self-administration procedure to assess an animals’ motivation to obtain a natural or drug reward. Under this schedule, the number of responses required to obtain a drug reward is increased following every reward. The point at which the animal ceases to press is referred to as the animals’ ‘break point’.
Operant reinstatement models of relapse-like behaviour: In the drug self-administration procedure, animals are trained to self-administer drug before their responding is extinguished to a set criterion e.g. numbers of days of extinction training or number of responses/session. After reaching the extinction criterion, ‘reinstatement’ of responding can be elicited by exposure to drug-associated stimuli, psychological stress or re-exposure to the drug itself (also known as a drug-prime) – analogous stimuli are thought to elicit drug-seeking and relapse in humans.
Reinstatement of a CPP: CPP is induced by drug administration as described above. The CPP is then extinguished by repeatedly placing the animal into the CPP apparatus without drug/saline injections. Reinstatement of a CPP can be induced by drug-priming injections or exposure to stress.
Renewal: Animals are trained to self-administer drug in Context A, before drug-seeking is extinguished in Context B. Context A and B differ from one another in terms of various contextual cues, including wall colour/pattern, floor texture. ‘Renewal’ of drug seeking can then be elicited by placing the animal back into Context A.
Table 1: Glossary of terms.