Autonomic ganglia, acetylcholine receptor antibodies, and autoimmune ganglionopathy

Abstract

Nicotinic acetylcholine receptors (AChR) are ligand-gated cation channels that are present throughout the nervous system. The ganglionic (alpha3-type) neuronal AChR mediates fast synaptic transmission in sympathetic, parasympathetic and enteric autonomic ganglia. Autonomic ganglia are an important site of neural integration and regulation of autonomic reflexes. Impaired cholinergic ganglionic synaptic transmission is one important cause of autonomic failure. Ganglionic AChR antibodies are found in many patients with autoimmune autonomic ganglionopathy (AAG). These antibodies recognize the alpha3 subunit of the ganglionic AChR, and thus do not bind non-specifically to other nicotinic AChR. Patients with high levels of ganglionic AChR antibodies typically present with rapid onset of severe autonomic failure, with orthostatic hypotension, gastrointestinal dysmotility, anhidrosis, bladder dysfunction and sicca symptoms. Impaired pupillary light reflex is often seen. Like myasthenia gravis, AAG is an antibody-mediated neurological disorder. Antibodies from patients with AAG inhibit ganglionic AChR currents and impair transmission in autonomic ganglia. An animal model of AAG in the rabbit recapitulates the important clinical features of the human disease and provides additional evidence that AAG is an antibody-mediated disorder caused by impairment of synaptic transmission in autonomic ganglia.

Figures

Figure 1. The autonomic ganglionic synapse

A) A simplified schematic showing the anatomy of the peripheral autonomic nervous system. The autonomic ganglia receive input from cholinergic motor neurons in the brainstem or spinal cord. Fast ganglionic synaptic transmission is mediated by acetylcholine acting on neuronal nicotinic acetylcholine receptors. The postganglionic fibers extend to innervate numerous target organs (a few examples are shown) and release acetycholine acting on muscarinic receptors (m) or norepinephrine acting on alpha and beta adrenergic receptors (α and β). B) Electron micrograph showing the ultrastructure of a ganglionic synapse in rabbit superior cervical ganglia. The presynaptic terminal contains mitochondria, numerous small clear vesicles containing acetycholine, and larger dense core vesicles presumably containing neuropeptides and other transmitters. The synapse (lower right) is characterized by a short area of close apposition of the nerve terminal and dendrite membranes. Vesicles are poised on the presynaptic side, ready for release. The thickened postsynaptic membrane is the area of synaptic specialization that contains the neurotransmitter receptors. C) Microelectrode recording of synaptic potentials from a neuron in isolated mouse superior cervical ganglia. Stimulation of the preganglionic nerve (arrowhead) leads to a fast excitatory postsynaptic potential (fEPSP) in the neuron. The y-axis indicates the change in membrane potential from the resting potential (which is usually around -50 to -60mV). Gradually increasing stimulus intensity produces discrete fEPSPs indicating the presence of multiple preganglionic inputs to this single ganglia neuron (a typical single fEPSP causes about a 5mV depolarization in the neuronal soma in this case). The simultaneous activation of several inputs is required to reach threshold and produce an action potential in the neuron. In this example, at least three distinct synaptic inputs combine to reach the action potential threshold.

Figure 2. Effects of ganglionic AChR antibodies on nicotinic AChR responses

A) Patch-clamp recording of whole-cell membrane current in IMR-32 cells at baseline, 20 minutes and 50 minutes after addition of IgG (1 mg/ml) purified from the plasma of a patient with AAG. Application of agonist (DMPP 50 μM, 4 second horizontal bar) produces an inward current. Exposure to ganglionic AChR IgG results in a progressive reduction of peak AChR current (Wang et al., 2007). B) Microelectrode recording from ganglia neuron in isolated mouse superior cervical ganglia. The fast excitatory postsynaptic potential (fEPSP) is produced by a fixed stimulus to the preganglionic nerve. The amplitude of the fEPSP progressively decreases after exposure to ganglionic AChR IgG (1 mg/ml added to the bath solution). Each response represents the average of 5 fEPSP recorded at baseline or 20 minutes after bath application of IgG.

Figure 3. Preserved cardiac sympathetic innervation in rabbits with EAAG

Single-photon computer tomography (SPECT) using 123I-metaiodobenzylguanidine (MIBG) was used to assess cardiac sympathetic innervation in awake, gently restrained rabbits. Images were obtained five hours after intravenous injection with 3mCi of MIBG. 99Tc-MIBI scans (not shown) were performed simultaneously to confirm adequate cardiac perfusion. Cardiac uptake of MIBG (shown as the bright area within the rabbit thorax) indicates the presence of intact sympathetic nerve terminals in the myocardium. There is no difference in the intensity of MIBG uptake in EAAG rabbits (with low or high ganglionic AChR antibody levels) compared to control rabbits. In the setting of low plasma catecholamine levels, this finding suggests pathology at the level of ganglionic neurotransmission (Goldstein et al., 2002; Vernino et al., 2003).

Figure 4. Cardiovascular response to tyramine in rabbits with EAAG

Rabbits were implanted with radiotelemetry catheters to allow continuous recording of arterial blood pressure and heart rate in the awake rabbit. Tyramine, an indirect sympathetic agonist, was infused intravenously over one minute (0.5mg/kg). A) Blood pressure and heart rate response to tyramine infusion. A rise in blood pressure (as seen in both control and EAAG rabbits) indicates intact postganglionic sympathetic nerve terminals. In the control rabbits, the rise in blood pressure leads to a baroreflex-mediated decrease in heart rate. In the EAAG rabbit, both the blood pressure and heart rate increase. This observation indicates failure of the baroreflex as well as direct activation of cardiac beta receptors via intact cardiac sympathetic terminals. B) A summary of the tyramine response in control rabbits (n=9), rabbits with chronic EAAG (n=13), and normal rabbits after treatment with the ganglionic blocker mecamylamine (3 mg/kg, n=3). In all rabbits, tyramine causes a rise in blood pressure (open bars). The magnitude of blood pressure increase is greater in EAAG and mecamylamine-treated rabbits, most likely due to failure of the sympathetic baroreflex to buffer the rise in blood pressure. In control rabbits, there is a marked baroreflex-mediated decrease in heart rate (shaded bar) while the EAAG and mecamylamine-treated rabbits show an increase in heart rate. This unique response to tyramine shows the similarity in autonomic physiology between EAAG and pharmacological blockade of ganglionic transmission.