Noradrenergic therapies in neurodegenerative disease: from symptomatic to disease modifying therapy?

A feature shared by many different neurodegenerative diseases is early pathology and degeneration of the pontine locus coeruleus. The human locus coeruleus contains about 50 000 neurons and is the primary source of the neurotransmitter noradrenaline. We propose the hypothesis that noradrenergic drugs can have broad, transdiagnostic benefit in slowing or preventing the progression of neurodegenerative diseases. There are direct noradrenergic anti-inflammatory effects in vivo, with microglia and astrocytes regulated by adrenoreceptors, and noradrenergic influences on glymphatics. Noradrenaline loss is associated with a pro-inflammatory state, promoting further neurodegeneration. Noradrenergic neuron loss is associated with worsening of both amyloid and tau deposition in animal models. There may be indirect survival benefits arising from alleviating the prognostically detrimental features of apathy and impulsivity, and noradrenergic influences on other neurotransmitters. The evidence base we set out supports the need for clinical trials of noradrenergic treatments for disease-modification.