Reverse tolerance to amphetamine of mice bearing unilateral striatal lesions: effect upon the circling response to apomorphine

Methamphetamine-induced behavioral alterations following repeated administration of methamphetamine

Repeated administration of a large dose of methamphetamine (MA) (25 mg/kg, i.p. twice daily for 4 days) to mice enhanced locomotor activity and decreased stereotyped behavior following a subsequent injection of MA. Simultaneous determinations of catecholamines revealed a depletion of brain dopamine. The moderate doses of haloperidol significantly enhanced MA-induced locomotor activity in mice. A significant enhancement of MA-induced locomotor activity was observed in the rats pretreated with 6-hydroxydopamine into the striatum, and this effect correlated negatively with the striatal dopamine level. These results suggest that hypofunction of striatal dopaminergic neuron systems induced by repeated administration of MA may be one of possible mechanisms of the enhancement of MA-induced locomotor activity due to the decrease of stereotyped behavior.

Behavioral sensitization: characterization of enduring changes in rotational behavior produced by intermittent injections of amphetamine in male and female rats

Factors influencing the behavioral sensitization ("reverse tolerance") produced by intermittent amphetamine (AMPH) injections were studied by quantifying rotational behavior in rats that had a unilateral 6-hydroxy-dopamine lesion of the substantia nigra. The results indicate that: a single injection of a low dose of AMPH enhances rotational behavior induced by a second injection of AMPH for up to 12 weeks; multiple, weekly injections of AMPH produce a progressive enhancement in rotational behavior, over-and-above that produced by a single injection; female rats show more robust sensitization than males following single or multiple injections of AMPH; this sex difference may be due to the suppression of sensitization by an androgen, because removal of testicular hormones potentiates sensitization; the long-lasting sensitization of rotational behavior produced by infrequent injections of AMPH is not due to drug-environment conditioning effects, but perhaps to a persistent AMPH-induced change(s) in brain catecholamine systems; and a simple change in DA receptors is probably not involved, because the sensitization produced by infrequent injections of AMPH does not influence the rotation produced by a subsequent injection of apomorphine. The results illustrate an intriguing example of neuroplasticity that may have clinical relevance.