With repeated exposure to drugs of abuse, animals can show either a heightened or decreased response to a dose of drug. Termed sensitization and tolerance, respectively, the occurrence of one or the other can be dependent on the manner in which the drug of abuse is given. That is, repeated high doses with little time intervening between administrations can lead to tolerance, whereas intermittent exposure can lead to sensitization (63, 64). Sensitization —which in animals is usually studied as increased locomotor activity when the drug is given— has been demonstrated for all drugs of abuse, and the incentive sensitization theory posits that sensitization functions to transition drug ‘liking’ to drug ‘wanting’ (65). Specifically it is the incentive motivational properties of the drug that are sensitized (66) and this is thought to lead to approach to drugs of abuse.
