Density of acetylcholine esterase (AchE) and tyrosine hydroxylase (TH) containing fibers in the amygdala of roman high- and low-avoidance rats

The cholinergic and dopaminergic innervation of the amygdala plays an important role in attention, emotional arousal, aversive forms of associative learning, conditioned responses, and stress responsivity. Roman High- (RHA) and Low-Avoidance (RLA) rats are an ideal model to study the potential impact of this innervation on behavioral responses, because they were selected bidirectionally for differences in their two-way active avoidance performance. RHA rats are known to quickly acquire two-way active avoidance and show indications of enhanced impulsive behavior, novelty seeking, and vulnerability to substance abuse, whereas RLA rats exhibit a passive coping style with high levels of immobility and enhanced stress responsivity. In the present study, the density of acetylcholine esterase (AchE)-positive cholinergic fibers and tyrosine hydroxylase immunoreactive (TH-ir) fibers were analyzed in various amygdala nuclei. In comparison to RLA rats, RHA rats displayed a significantly higher density of AchE-positive fibers in the lateral nucleus (La), the major sensory input area of the amygdala. In contrast, RLA rats showed a higher density of TH-ir fibers in the lateral division of the central nucleus (CeL), which modulates amygdala output and is known to contain more corticotropin-releasing hormone (CRH) positive neurons in RLA than in RHA rats. The findings suggest that a higher density of AchE-positive fibers in the La of RHA rats may facilitate attentional mechanisms and aversive forms of associative learning in RHA rats, whereas the increased density of TH-ir fibers in the CeL of RLA rats may be involved in the regulation of enhanced CRH expression and stress responsivity.