Depending on the dose, acute exposure to many addictive drugs increases or decreases motor activity. Repeated drug administration in specific patterns can produce psychomotor sensitization, an increase in locomotor activity, or stereotypy that has been observed with stimulants39 and alcohol.40 Psychomotor sensitization is associated with increased drug reward,41 relapse,42 and consumption43 in rodents. Pretreatment of rodents with systemic SL327 prevents the hyperlocomotion induced by acute injections of cocaine,44 amphetamine,20,44 and methamphetamine.35 The development of psychomotor sensitization is prevented by pretreatment with intra-VTA injections of SL327 in rats given repeated injections of cocaine45 and by systemic pretreatment with SL327 in mice given repeated injections of amphetamine.21 These data indicate that ERK activity is necessary for the expression of hyperlocomotion and for the psychostimulant induction of psychomotor sensitization.
