50-kHz ultrasonic vocalization subtypes emitted by female rats anticipating same-sex social interaction.

Adult rats possess a wide repertoire of 50-kHz ultrasonic vocalizations (USVs), with the "trill" and "flat" call subtypes appearing to capture positive or negative affect, respectively. USVs emitted during reward anticipation remain poorly characterised but might provide a measure of anticipatory anhedonia, a common feature of major depressive disorder. The present study investigated whether USV measures (specifically 50-kHz call rate and call subtype prevalence) would be modified in adult female Long-Evans rats anticipating access to a familiar social partner vs. no partner. The study also examined whether chronic treatment with oral corticosterone, intended to induce a depressive-like state, would inhibit the anticipation-associated changes in USV measures; chronic oral fluoxetine (antidepressant) was subsequently tested in the same animals. Corticosterone and fluoxetine treatment left 50-kHz call emission largely unchanged, while the independent measure of a depressive-like state (sucrose preference) proved inconclusive. Independent of drug treatment, rats emitted more 50-kHz calls while anticipating access to a social partner vs. access to an empty compartment. As predicted, social anticipation increased the preferential emission of the 50-kHz trill call subtype. Three less studied 50-kHz call subtypes also changed in relative prevalence during social anticipation: short calls were suppressed, while the flat-trill and trill with jumps subtypes were promoted. These findings suggest that 50-kHz call subtypes other than the widely studied flat and trill calls can potentially serve as markers of affect.