The pharmacology and neural circuitry of sensitization to psychostimulants

Behavioral sensitization to amphetamine-like psychostimulants is manifest as a progressive increase in drug-induced anxiety and paranoia which can culminate in psychopathologies, such as paranoid psychosis and panic attacks. Sensitization may also mediate the facilitation of drug relapse in addicts by increasing the reinforcing value of acute drug administration. The primary animal model for psychostimulant-induced psychopathologies involves repeated, non-contingent administration of drug to rodents, which can produce a progressive and enduring augmentation in motor activity and increased susceptibility to drug self-administration. Because of the mature literature implicating mesoaccumbens dopamine transmission in the acute motor and reinforcing effects of amphetamine-like stimulants, investigation into the neural basis of behavioral sensitization has focused on this projection. Over the last decade, with a few exceptions, the neurochemical and molecular literature that has emerged from this effort is replete with inconsistencies. In contrast, the presence of behavioral sensitization is a highly replicable event. It is proposed that behavioral sensitization arises from an alteration in the neural circuitry that subserves the translation of motivationally relevant stimuli into adaptive motor responses. The mesoaccumbens dopamine projection is embedded in this circuit and an enduring change in dopamine transmission may alter the functional state of the circuit to produce behavioral sensitization. However, combinations of alterations in other connections within the circuit can also support behavioral sensitization. The specific changes in the circuit that promote behavioral sensitization are under the control of experimental parameters, such as the drug employed, dosage regimen, withdrawal period and the presence of conditioning cues. Thus, the profile of neurochemical alterations observed after exposure to repeated psychostimulants may vary depending upon the experimental protocol and strain of animals, even though all laboratories report the presence of behavioral sensitization.