The noradrenaline pathways in phobia

One hypothesis about the biological basis of phobia suggests that there is an excess of noradrenaline in the principal noradrenergic pathways in the brain and that this causes a down-regulation of post-synaptic adrenergic receptors. Transmission of noradrenaline from the caudal raphe nuclei and the locus coeruleus is increased in phobia.

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References

Drug treatments for obsessive compulsive disorder, panic disorder, and phobia. In: Essential Psychopharmacology. Neuroscientific basis and practical applications. Stahl S. Cambridge, UK: Cambridge University Press, 1996: 216–248.