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

The acoustic startle response, prepulse inhibition, fear-potentiated startl e and monoamine activity induced by either, a novel stimulus or a cue previ ously paired with foot-shock (fear-conditioning), were assessed in rats sel ectively bred for differences in amygdala excitability (Fast vs. Slow kindl ing epileptogenesis). Comorbid differences of anxiety, which were dependent both on the rats' behavioural style and the kind of stressor, also charact erized these strains. In the present investigation, Slow rats exhibited a g reater startle reflex to noise relative to Fast rats, suggesting difference s in generalized anxiety, but similar rates of startle habituation and prep ulse inhibition. The fear-potentiated startle, however, was greater in Fast rats. When movement of the rat was restricted in a new environment, presen tation of a novel stimulus (light) increased norepinephrine, dopamine and/o r serotonin activity in brain regions typically associated with stressors ( e.g. locus coeruleus, paraventricular hypothalamic nucleus). Generally, the se effects were more pronounced in Fast rats, and norepinephrine utilizatio n in the central amygdala was particularly highlighted in response to a con ditioned 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. Sim ilarly, basal dopamine activity in the prefrontal cortex was greater in Fas t rats, but dopamine utilization elicited by a novel stimulus was more pron ounced in Slow rats. This suggested that relative to Slow rats, dopamine ne urons in prefrontal cortex of Fast rats do not react normally to environmen tal stimuli, and this phenomenon could lead to disturbances of attention or impulsivity.