Inter-individual diversity and intra-individual stability of amphetamine-induced sensitization of frequency-modulated 50-kHz vocalization in Sprague-Dawley rats.

Rationale: Propensity for drug dependence shows great diversity that is related to intrinsic neurobiological factors. This diversity is important both for the understanding of these traits and for the development of therapies.

Objectives: The goals of the study were (1) to define, using ultrasonic vocalization characteristics, inter-individual differences in rats' propensity for sensitization to amphetamine, (2) to test whether possible resistance to this effect could be overcome with repetitive treatment, and (3) to seek useful predictors of the propensity.

Methods: Rats were subject to tests meant to characterize their anxiety, pain sensitivity, and responses to novelty and natural rewards. Then they were subject to the so-called two-injection protocol of sensitization (using amphetamine) followed by 2 weeks of daily amphetamine treatment, 2-week withdrawal, and final amphetamine challenge. The development and outcome of sensitization were monitored by measuring 50-kHz vocalization.

Results: The two-injection protocol yielded three patterns of changes in the frequency-modulated 50-kHz vocalization response to amphetamine. These patterns persisted after completion of the extended drug treatment. Rats with lower sensitivity to pain or with longer latency of their vocalization response to the first drug exposure showed an increased propensity for ultrasonic vocalization sensitization.

Conclusion: Vulnerability to sensitization of frequency-modulated 50-kHz vocalization response of Sprague-Dawley rats to amphetamine, which supposedly reflects rats' propensity for amphetamine dependence, shows large inter-individual diversity. Resistance to this effect, which is evident in a majority of the rats, cannot be overcome even with prolonged intermittent drug treatment under the conditions (novelty) that promote sensitization.