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|Title:||50 KHZ ULTRASONIC VOCALIZATIONS AFTER INJECTION OF SALINE OR AMPHETAMINE INCREASE AFTER REPEATED EXPOSURE TO TEST ARENA IN ADULT, BUT NOT IN ADOLESCENT FEMALES|
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|Authors/Affiliations:||1 Katie Walters; 1 Iva Z. Mathews*; 1 Cheryl M. McCormick; 1 Stefan M. Brudzynski; |
1 Brock University;
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|Content:||Objectives: The production of 50 kHz ultrasonic vocalizations (USV) is associated with elevated dopaminergic transmission in the mesolimbic system. However, individual differences and situational testing factors result in a high variability in the production of these calls, such that the differences in number of calls after injection to amphetamine compared to saline are not always observed. Here we tested whether the number of exposures to the test arena influenced the number of calls observed in female rats. Further, because of our previous findings of differences in locomotor activity to acute and repeated amphetamine in females in late adolescence compared to adulthood, we examined whether such differences would be observed for USVs. |
Materials and methods: USVs were measured twice (once after 1.0 mg / kg amphetamine and once after saline, order counterbalanced) for each rat either during a first exposure to a test arena or after repeated exposure to a test arena. For the repeated exposure condition, rats were exposed to the test arena 6 times before USV measurement (once daily beginning on days 46 or 70 of age), given alternating i.p. injection of amphetamine and saline and then tested on days 52-53 (Adol; n = 12) or 77-78 (Adult; n = 18) of age. Rats in the single exposure condition also were tested on either days 52-53 (n = 24) or 77-78 (n = 16) of age. To keep the environments distinct for amphetamine and saline, specific tactile cues (bar or grid floor tiles) were associated with each treatment.
Results: Rats were placed in the test arena for 10 minutes 15 min after injection of either amphetamine or saline, and calls were recorded using a bat detector. In the single exposure condition, there were low rates of calls for both groups of females irrespective of drug treatment, however Adol > Adult (p = 0.001). Adult, but not adolescent, females called more in the repeated exposure condition than in the single exposure condition (p < 0.0001), such that the age difference was reversed (Adult > Adol, p = 0.001). Both groups called more when given amphetamine than when given saline, but the difference was not statistically significant. In the repeated measure condition, locomotor activity (distance traveled in the arena) also was measured, and there was significantly greater locomotor activity to amphetamine than to saline. Further, only in adult females given amphetamine was there a positive correlation between number of 50 kHz calls and locomotor activity (r = .61, p = 0.01). There were some differences in call characteristics between the groups (bandwith: Adults > Adol, p = 0.006; call duration Adol > Adult p = 0.015; peak frequency of calls made: Adol = Adult).
Conclusion: These results suggest that a novel environment may suppress the production of 50 kHz USVs, and that for adult, but not adolescent females, familiarity alone is enough to increase significantly the number of calls. Further, the results suggest that the neural mechanisms subserving the production of 50 kHz USV are continuing to mature into late adolescence.
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