Acoustic startle and fear-potentiated startle in rats selectively bred for fast and slow kindling rates: relation to monoamine activity

Abstract

The acoustic startle response, prepulse inhibition, fear-potentiated startle and monoamine activity induced by either, a novel stimulus or a cue previously paired with foot-shock (fear-conditioning), were assessed in rats selectively bred for differences in amygdala excitability (Fast vs. Slow kindling epileptogenesis). Comorbid differences of anxiety, which were dependent both on the rats' behavioural style and the kind of stressor, also characterized these strains. In the present investigation, Slow rats exhibited a greater startle reflex to noise relative to Fast rats, suggesting differences in generalized anxiety, but similar rates of startle habituation and prepulse inhibition. The fear-potentiated startle, however, was greater in Fast rats. When movement of the rat was restricted in a new environment, presentation of a novel stimulus (light) increased norepinephrine, dopamine and/or serotonin activity in brain regions typically associated with stressors (e.g. locus coeruleus, paraventricular hypothalamic nucleus). Generally, these effects were more pronounced in Fast rats, and norepinephrine utilization in the central amygdala was particularly highlighted in response to a conditioned fear stimulus. Thus, while generalized anxiety appeared greater in Slow rats, behavioural and central neurochemical reactivity in response to novel stimuli and to fear-eliciting stimuli, was greater in Fast rats. Similarly, basal dopamine activity in the prefrontal cortex was greater in Fast rats, but dopamine utilization elicited by a novel stimulus was more pronounced in Slow rats. This suggested that relative to Slow rats, dopamine neurons in prefrontal cortex of Fast rats do not react normally to environmental stimuli, and this phenomenon could lead to disturbances of attention or impulsivity.