Motor and locomotor responses to systemic amphetamine in three lines of selectively bred Long-Evans rats

The goal of the study was to measure spontaneous and amphetamine-induced motor and locomotor activity in three selectively bred lines of male Long-Evans rats. The number of 50 kHz ultrasonic vocalizations (USVs) emitted in response to heterospecific play with human hand ("tickling") had been measured daily in these lines of rats from 21 to 24 days of age, as a criterion for dividing them into high vocalizing line, low vocalizing line, and random breeding and testing lines. This study sought to determine whether selection of rats based on their affective social-vocalizations also had effects on their locomotor performance and sensitivity to amphetamine. In this study adult animals from the 25th generation (with no further selection) were tested. The results showed that rats, which were selectively bred to emit high numbers of 50 kHz vocalizations, also exhibited elevated levels of spontaneous locomotor activity. After systemic injection of d-amphetamine (1.5 mg/kg), the level of motor and locomotor activity significantly increased further in all the lines as compared to saline controls. The horizontal and vertical activities and the distance covered by rats of the high line, both at the baseline and after amphetamine challenge, were significantly higher than those of the low line animals in absolute scores but not as proportion of relevant saline controls. Since appetitive 50 kHz USVs and locomotor activity are both dependent on the activity of the dopamine system, it is concluded that selection of rats based on the expression of their positive emotional state is also selecting other features than vocalization, namely locomotor behavior. This may help explain why these animals are relatively resistant to depressogenic manipulations.