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Current rodent models emphasize the joint action of the stress mediators noradrenaline (NE) and cortisol (CORT) in conferring a memory advantage of emotional over neutral stimuli.
Using a pharmacological strategy of tackling this stress-related mechanism to enhance human episodic (autobiographical) memory, we measured amygdala-hippocampal responses during encoding of emotional and neutral stimuli with functional magnetic resonance imaging in 51 healthy subjects under four pharmacological conditions in a double-blind parallel group design: (i) placebo; (ii) the NE-reuptake inhibitor reboxetine (4 mg); (iii) hydrocortisone (synthetic CORT) (30 mg); or (iv) both agents in combination.
Differential drug effects were found in the left hippocampus, whereas hydrocortisone alone selectively decreased hippocampal responses to emotional relative to neutral stimuli, reboxetine potentiated hippocampal responses to these stimuli. Importantly, the inhibitory influence of hydrocortisone was reversed by co-administration of reboxetine.
Our results imply that stress levels of CORT alone attenuate hippocampal responses to emotional stimuli, an effect possibly related to a regulatory negative feedback loop. However, when simultaneously elevated to stress levels, NE and CORT act together to synergistically enhance hippocampal activity during encoding of emotional stimuli, a mechanism that may turn maladaptive under circumstances of traumatic stress.
Animal models of anxiety disorders emphasize the crucial role of locus ceruleus–noradrenergic (norepinephrine, NE) signaling, the basolateral amygdala (BLA) and their interactions in the expression of anxiety-like behavioral responses to stress. Despite clinical evidence for the efficacy of a β-noradrenergic receptor blockade with propranolol in the alleviation of anxiety symptoms and the secondary prevention of post traumatic stress disorder, preclinical evidence for a β-noradrenergic modulation of BLA activity in humans is missing.
We combined functional magnetic resonance imaging in healthy volunteers with probabilistic mapping of intra-amygdalar responses to fearful, neutral and happy facial expressions to test the hypothesis that a β-noradrenergic receptor blockade with propranolol would inactivate the BLA.
Consistent with our a priori hypothesis, propranolol diminished BLA responses to facial expressions, independent of their emotional valence. The absence of activity changes in probabilistically defined visual control regions underscores the specific action of propranolol in the BLA.
Our findings provide the missing link between the anxiolytic potential of propranolol and the biological basis of β-noradrenergic activation in the human BLA as a key target for the pharmacological inhibition of anxiety neurocircuitry. Moreover, our findings add to emerging evidence that NE modulates both the reactivity (sensitivity) and the operating characteristics (specificity) of the BLA via β-noradrenergic receptors.
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