Rat ultrasonic vocalizations (USVs) are ethologically-essential social signals. Under natural conditions, 22kHz USVs and 50kHz USVs are emitted in association with negative and positive emotional states, respectively. Our first experiment examined freezing behavior elicited in naïve Sprague-Dawley rats by a 22kHz USV, a 50kHz USV, and frequency-matched tones. None of the stimuli elicited freezing, which is the most commonly-used index of fear. The second experiment examined single-unit responses to these stimuli in the amygdala (AM), which is well-known for its role in innate and acquired fear responses. Among 127 well-discriminated single units, 82% were auditory-responsive. Elicited firing patterns were classified using a multi-dimensional scheme that included transient (phasic) responses to the stimulus onsets and/or offsets as well as sustained (tonic) responses during the stimulus. Tonic responses, which are not ordinarily evaluated in AM, were 4.4-times more common than phasic responses. The 22kHz stimuli tended to elicit tonic increases in the firing rates, whereas the 50kHz stimuli more often elicited tonic decreases in firing rates. These opposing tonic responses correspond with the ethological valence of USVs in the two frequency bands. Thus, a relatively-small sample of single-unit responses in AM furnished a more sensitive index of emotional valence than freezing behavior. Latency analysis suggested that stimuli in the two frequency bands are processed through different pathways to AM. One possible interpretation is that phasic responses in AM reflect the detection of a stimulus change, whereas tonic responses indicate the valence of the detected stimulus.
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