Systemic postganglionic adrenergic studies do not distinguish Parkinson's disease from multiple system atrophy

Area of Science:

• Neuroscience
• Autonomic Dysfunction
• Pharmacology

Background:

• Multiple system atrophy (MSA) impacts preganglionic adrenergic neurons, while Parkinson's disease (PD) affects postganglionic neurons.
• Widespread denervation in MSA and PD may lead to denervation supersensitivity and impaired norepinephrine release.
• Investigating pharmacological dissection of the adrenergic neuron could differentiate between MSA and PD.

Purpose of the Study:

• To determine if pharmacological dissection of the adrenergic neuron can distinguish between multiple system atrophy (MSA) and Parkinson's disease (PD).

Main Methods:

• Measured blood pressure, heart rate, and plasma norepinephrine responses to direct (phenylephrine) and indirect (tyramine) acting adrenergic agonists.
• Studied 15 patients with probable MSA, 16 patients with idiopathic PD, and 16 age- and gender-matched controls.

Main Results:

• Baroreflex sensitivity was impaired in MSA but intact in PD.
• Pressor responses to phenylephrine were higher in both MSA and PD patients compared to controls.
• Blood pressure responses to tyramine were elevated in MSA patients only, indicating denervation supersensitivity.

Conclusions:

• Excessive pressor responses in MSA are attributed to baroreflex failure, not solely denervation supersensitivity.
• While PD patients exhibit denervation supersensitivity, it is insufficient to alter dose-response curves significantly.
• The diagnostic approach using these adrenergic agonists lacks the sensitivity to reliably differentiate between PD and MSA.