Rationale  

In rats, dorsal periaqueductal gray (dPAG) stimulation elicits escape behavior that is thought to be related to fear and panic. A noninvasive technique—exposure to ultrasound—has been reported to stimulate the dPAG and induce escape followed by freezing in Lister-hooded (LH) rats.

Objective  

Further characterize pharmacologically the ultrasound-induced defensive behaviors test with anxiolytics acting via different mechanisms.

Materials and methods  

LH rats, treated with clinically validated anxiolytics, putative anxiolytics, or compounds devoid of anxiolytic properties, were exposed to ultrasound. Baseline locomotion before and duration of escape and freezing behaviors during ultrasound were measured.

Results  

The low-potency benzodiazepine receptor agonists, diazepam and chlordiazepoxide, selectively reduced escape compared to baseline locomotor activity. The high-potency agonist alprazolam, the mGlu2/3 receptor agonist LY 354740, and the mGlu5 receptor antagonist MTEP reduced escape but did not show such a separation. The voltage-dependent calcium channel inhibitors, pregabalin and gabapentin, selectively reduced escape. The nociceptin OFQ peptide receptor agonist Ro 64-6198 did not affect escape but reduced freezing, an effect that was not produced by any of the other compounds. Buspirone and morphine did not affect escape. As expected, haloperidol reduced escape in a nonselective manner.

Conclusions  

The present data demonstrate that ultrasound-induced defensive behaviors in LH rats can be independently modulated by anxiolytics of different classes. In particular, ultrasound-induced escape shows sensitivity to the majority of acute therapeutics effective in panic disorder, although sensitivity to compounds with slow onset of action (e.g., antidepressants) remains to be demonstrated.