Repeated intravenous amphetamine exposure: rapid and persistent sensitization of 50-kHz ultrasonic trill calls in rats

Abstract

Short 50-kilohertz (kHz) range frequency-modulated ultrasonic vocalizations (USVs) produced by rats and mice are unconditionally elicited by drugs of abuse or electrical stimulation that increase dopamine activity in the nucleus accumbens, and it has been suggested that they reflect "positive affect" or incentive motivational states associated with appetitive behavior. The repeated administration of amphetamine is known to not only produce "psychomotor" sensitization, but also to facilitate a number of appetitive behaviors, including conditioned drug pursuit behavior. We were interested, therefore, in whether amphetamine-induced 50-kHz USVs would also increase with repeated drug exposure. USV recordings were made during 5-min sessions immediately after a saline infusion, and again 4-5h later after 1.0mg/kg intravenous amphetamine exposure. These sessions took place every other day over a 5-day period. A challenge dose of 1.0mg/kg amphetamine was administered 2 weeks later to determine whether sensitization would persist. The initial amphetamine infusion increased 50-kHz USVs relative to the saline infusion. This effect was enhanced over trials and during the amphetamine challenge 2 weeks later. Classification of 50-kHz range call types revealed that complex frequency-modulated trill calls were sensitized by amphetamine, but not flat 50-kHz calls. It is possible that 50-kHz USV recordings could provide a potentially valuable behavioral measure of sensitization linked to enhanced incentive salience and increased tendency to self-administer drugs of abuse.

Fig. 1

Example sonograms of a) frequency-modulated “trill” 50-kHz call, and b) flat 50-kHz call. Colors indicate relative level of signal intensity (dB). Audio samples of 50 kHz calls (slowed down 10 ×) can be heard at the lab website: www.schallertlab.org

Fig. 2

Repeated amphetamine increases the production of 50-kHz USVs. Results are mean ± SEM of number of 50-kHz USVs recorded in 5 minutes after i.v. infusions of saline and amphetamine. In Trial 1, acute amphetamine elicited more 50-kHz USVs than the preceding saline infusion († paired samples t-test, t = −3.18; p = .015, two-tailed). In Trial 2 and Trial 3 amphetamine elicited a greater increase in 50-kHz USVs than on Trial 1 (* p < .05; N = 8). Two weeks after Trial 3 (day 19), a challenge dose of amphetamine elicited a greater increase in calls than on Trial 1 (# p < .05; n = 5).

Fig 3

Frequency-modulated 50-kHz USVs are increased by repeated amphetamine. Results are mean ± SEM of the number of 50-kHz USVs categorized as frequency-modulated (trill type calls). In Trial 1, amphetamine elicited more frequency-modulated calls than saline († paired samples t-test, t = −3.28; p = .014, two-tailed). Re-exposure to amphetamine in Trials 2 and 3 elicited a greater increase in frequency-modulated 50-kHz USVs than Trial 1 (* p < .05, ** p < .01; N = 8). Two weeks after Trial 3, a challenge dose of amphetamine elicited a greater increase in frequency-modulated calls than the initial exposure (# p < .05; n = 5).

Fig 4

Flat 50-kHz USVs were not significantly increased by the initial exposure to amphetamine or by multiple exposures to amphetamine. Results are mean ± SEM of the number of 50-kHz USVs categorized as flat.

Fig 5

Mean (± SEM) of percentage of time spent rearing in 5 min post-infusion. More rearing behavior was elicited by amphetamine in Trial 3 compared to Trial 1 (* p < .05, N = 8).