Analysis of freezing behavior and ultrasonic vocalization in response to foot-shocks, ultrasound signals and GABAergic inhibition in the inferior colliculus: effects of muscimol and midazolam.

Freezing behavior is a common response to distal threatening stimuli. It has been reported that experimenter-presented 20-kHz tones produce freezing in Wistar rats. The present studies were designed to determine the acoustic specificity for induction of the emission of ultrasound vocalizations (USV) and freezing of rats to either ultrasound tones or to a standard stressor, foot-shocks. We also examined whether GABA-benzodiazepine mechanisms, known for modulating anxiety-related processes, are involved in the regulation of defensive responses to these two aversive unconditioned stimuli. It was found that emission of USV was only observed with foot-shocks. Moreover, rats exhibited freezing when they were exposed to foot-shocks and 20-25-kHz ultrasound signals. Likewise, removal of the GABAergic inhibitory control on the neural substrates of aversion in the inferior colliculus by local microinjections of semicarbazide, a blocker of the glutamic acid decarboxylase, caused freezing behavior without emission of USV. All these responses were significantly reduced by midazolam and muscimol. It is suggested that the neural substrates of aversion in the inferior colliculus are under inhibitory control of GABAergic mechanisms and are different from those triggered by foot-shocks.