Invited Article: Autonomic ganglia: target and novel therapeutic tool

Abstract

Nicotinic acetylcholine receptors (AChR) are ligand-gated cation channels that are present throughout the nervous system. The muscle AChR mediates transmission at the neuromuscular junction; antibodies against the muscle AChR are the cause of myasthenia gravis. The ganglionic (alpha 3-type) neuronal AChR mediates fast synaptic transmission in sympathetic, parasympathetic, and enteric autonomic ganglia. Impaired cholinergic ganglionic synaptic transmission is one important cause of autonomic failure. Pharmacologic enhancement of ganglionic synaptic transmission may be a novel way to improve autonomic function. Ganglionic AChR antibodies are found in patients with autoimmune autonomic ganglionopathy (AAG). Patients with AAG typically present with rapid onset of severe autonomic failure. Major clinical features include orthostatic hypotension, gastrointestinal dysmotility, anhidrosis, bladder dysfunction, and sicca symptoms. Impaired pupillary light reflex is often seen. Like myasthenia, AAG is an antibody-mediated neurologic disorder. The disease can be reproduced in experimental animals by active immunization or passive antibody transfer. The patient may improve with plasma exchange treatment or other immunomodulatory treatment. Antibodies from patients with AAG inhibit ganglionic AChR currents. Other phenotypes of AAG are now recognized based on the results of antibody testing. These other presentations are generally associated with lower levels of ganglionic AChR antibodies. A chronic progressive form of AAG may resemble pure autonomic failure. Milder forms of dysautonomia, such as postural tachycardia syndrome, are associated with ganglionic AChR in 10-15% of cases. Since ganglionic synaptic transmission is a common pathway for all autonomic traffic, enhancement of autonomic function through inhibition of acetylcholinesterase is a potential specific therapeutic strategy for autonomic disorders. Increasing the strength of ganglionic transmission can ameliorate neurogenic orthostatic hypotension without aggravating supine hypertension. Recent evidence also suggests a potential role for acetylcholinesterase inhibitors in the treatment of postural tachycardia syndrome.

Figure 1. Membrane current in IMR-32 cells at −70 mV holding potential

(A) Currents from IMR-32 cell recorded at baseline and at 20 and 50 minutes after addition of human control IgG (1 mg/mL) to the bath (superimposed traces) remain stable over time, indicating no effect of the control IgG on the AChR current (0% current reduction). (B) Exposure to rabbit ganglionic AChR IgG (0.2 mg/mL) for 20 minutes results in a reduction of peak AChR current by 39%. The steady-state current was similarly reduced. (C) Exposure to autoimmune autonomic ganglionopathy (AAG) IgG (1 mg/ml) produces a gradual decrease in AChR current. (From Wang Z, Low PA, Jordan J, Freeman R, Gibbons CH, Schroeder C, et al. Autoimmune autonomic ganglionopathy: IgG effects on ganglionic acetylcholine receptor current. Neurology 2007;68:1917–1921. Reproduced with permission.)

Figure 2. Diastolic blood pressure (DBP) before and after administration of study drug

Each group (placebo, pyridostigmine bromide, pyridostigmine and 2.5 mg of midodrine hydrochloride, pyridostigmine and 5.0 mg of midodrine hydrochloride) averaged for the supine position and standing position. Asterisk indicates P_.05; dagger, P_.01. Error bars represent mean±SD. (From Singer W, Sandroni P, Opfer-Gehrking TL, Suarez GA, Klein CM, Hines S, et al. Pyridostigmine treatment trial in neurogenic orthostatic hypotension. Arch Neurol 2006;63:513–518. Reproduced with permission.)