Strain differences to the effects of aversive frequency ultrasound on behaviour and brain topography of c-fos expression in the rat.

Previous studies have shown that ultrasound at 20 kHz produces an escape (defence) response in the hooded Lister rat. This study compares the ultrasound-induced behavioural response in the hooded Lister and albino Wistar rat. Ultrasound (continuous tone, square wave, 20 kHz) produced an initial characteristic startle response (brisk running) in the hooded Lister rat that was followed immediately after cessation of the ultrasound by a period of freezing behaviour. In contrast, Wistar rats showed no initial escape response but a prolonged period of freezing that started during the ultrasound and continued for a period after the end of the ultrasound. Immunohistochemical assessment of c-fos expression also showed a difference between the two strains with preferential expression in the dorsal region of the rostral and caudal periaqueductal grey (PAG) in the hooded Lister rat, while the expression occurred in the ventral PAG in the Wistar rats. In summary, the two strains exhibit distinct defensive behaviours and patterns of neuronal activation in response to the same aversive signal. It remains to be determined whether these differences relate to neuronal circuitry or perception of the signal, but analysis of the mechanisms involved may help our understanding of the heterogeneity of anxiety disorders.