Isolation-induced changes in ultrasonic vocalization, fear-potentiated startle and prepulse inhibition in rats.

Isolation causes important changes in the behavioral reactivity of rats to environmental stimuli. These changes include deficit in sensorimotor gating and altered fear-like responses to aversive stimuli. Measures of ultrasound vocalizations at 20-22 kHz when rats are exposed to threatening conditions, such as novelty, have been taken as a good measure of fear. The fear-potentiated startle to loud sounds and the prepulse inhibition tests have been considered reliable indicators of anxiety and attention impairments, respectively. Rats reared under conditions of isolation from weaning display clear deficits in prepulse inhibition. Taking into account that housing condition changes the emotional state of the animals, we evaluated in this work the performance of rats in the fear-potentiated startle test, prepulse inhibition and emission of ultrasound vocalizations to novelty when isolated for 10 days and after resocialization for 1 week in comparison to grouped rats. Isolated rats showed greater reactivity to loud sounds in the fear-potentiated startle test than grouped animals. They also emitted less ultrasound vocalizations at 20-22 kHz than grouped animals when exposed to a novel environment. In contrast to the well-known deficit in prepulse inhibition displayed by isolation-reared animals, in the present study isolation for 10 days caused a significant increase in prepulse inhibition. Resocialization was not able to counteract the effects of isolation in all three tests. The results suggest that the emotional state of the animals is altered by 10 days of isolation; they do not vocalize characteristically as grouped rats when submitted to novelty; unconditioned responses to loud sounds are enhanced and increased prepulse inhibition is shown rather than a deficit as largely documented in studies with isolation-reared animals. It is suggested that the assessment of the emotional state of the animals is a prerequisite in the evaluation of prepulse inhibition. The level of defensive reactivity displayed by isolated animals is crucial for the functioning of sensory gating and, by extension, to the expression of prepulse inhibition.