Distribution and Metabolism of Amphetamine in Tolerant Animals. In: Ellinwood EH, Kilbey MM, editors. Cocaine and Other Stimulants. Boston: Springer US; 1977. pp. 161-77. [DOI: 10.1007/978-1-4684-3087-5_7]

Effects of crus cerebri lesions and repeated amphetamine treatment on the activity of nigral dopaminergic neurons

Previous research suggests that the firing rate of dopamine (DA) neurons in the substantia nigra pars compacta (SNC) may be altered by repeated DA agonist treatment. Because changes in the frequency of DA activity could reflect the firing patterns (e.g., bursting) of the neurons sampled, this study examined both the firing rate and pattern of SNC DA neurons after long-term amphetamine (AMPH) treatment (5 mg/kg d-AMPH s.c. twice daily for 6 days). To assess the contribution of postsynaptic feedback from the forebrain, unilateral electrolytic lesions were made to the crus cerebri (CC), containing the striatonigral pathway, prior to AMPH treatment. Single-unit activity of presumed SNC DA neurons was recorded in adult male rats under urethane anesthesia. Spontaneous firing rate was reduced by AMPH treatment, relative to saline vehicle, but was unaffected by CC or sham lesions. Neurons categorized as bursting had faster rates of activity than nonbursting cells. AMPH treatment reduced the number of bursts seen in intact rats but increased bursting in lesioned rats. These results suggest that changes in DA firing rate previously found after chronic AMPH may reflect altered patterns of activity. In addition, the effects of long-term AMPH on the firing patterns of DA neurons appear to be mediated by fibers in the CC.

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.