Temporal patterning of ultrasonic distress calls in the adult rat: effects of morphine and benzodiazepines.

Opioids and benzodiazepines modulate the ultrasounds that rats emit before being presented with aversive stimulation. Mild, intermittent 10 s electrical tail stimulation induces rats to emit 20-30 kHz ultrasounds before and after the stimulation. Analysis of the temporal parameters of the sounds via a customized computer system reveals them to be emitted in bouts of about five sounds, each sound being separated from the next by 0.2-0.35 s pauses. Morphine decreases the per cent time vocalizing during the pre-stimulation period, with 6 mg/kg being fully suppressive. This effect is reversed by 1 mg/kg naloxone pre-treatment. Chlordiazepoxide and morphine have opposite effects on the temporal structure of the pre-stimulation calls; chlordiazepoxide induces longer bouts with more pulses, and morphine dissolves the bout structure into a series of single pulses. These differential and selective effects of morphine and benzodiazepines on the occurrence and temporal structure of ultrasounds may be relevant to characteristics of different affective expressions.