Passive Transfer of Lambert-Eaton Myasthenic Syndrome Induces Dihydropyridine Sensitivity ofI_Cain Mouse Motor Nerve Terminals

Abstract

Mice were injected for 30 days with plasma from three patients with Lambert-Eaton Myasthenic Syndrome (LEMS). Recordings were made from the perineurial sheath of motor axon terminals of triangularis sterni muscle preparations. The objective was to characterize pharmacologically the identity of kinetically distinct, defined potential changes associated with motor nerve terminal Ca2+ currents (ICa) that were affected by LEMS autoantibodies. ICa elicited at 0.01 Hz were significantly reduced in amplitude by approximately 35% of control in LEMS-treated nerve terminals. During 10-Hz stimulation, ICa amplitude was unchanged in LEMS-treated motor nerve terminals, but was depressed in control. During 20- or 100-Hz trains, facilitation of ICa occurred in LEMS-treated nerve terminals whereas in control, no facilitation occurred during the trains at 20 Hz and marked depression occurred at 100 Hz. Saturation for amplitude and duration of ICa in control terminals occurred at 2 and 4-6 mM extracellular Ca2+, respectively; in LEMS-treated terminals, the extracellular Ca2+ concentration had to increase by two to three times of control to cause saturation. Amplitude of the two components of ICa observed when the preparation was exposed to 50 microM 3,4-diaminopyridine and 1 mM tetraethylammonium were both reduced by LEMS plasma treatment. The fast component (ICa,s) was reduced by 35%, whereas the slow component (ICa, s) was reduced by 37%. omega-Agatoxin IVA (omega-Aga-IVA; 0.15 microM) and omega-conotoxin-MVIIC (omega-CTx-MVIIC; 5 microM) completely blocked ICa in control motor nerve terminals. The same concentrations of toxins were 20-30% less effective in blocking ICa in LEMS-treated terminals. The residual ICa remaining after treatment with omega-Aga-IVA or omega-CTx-MVIIC was blocked by 10 microM nifedipine and 10 microM Cd2+. Thus LEMS plasma appears to downregulate omega-Aga-IVA-sensitive (P-type) and/or omega-CTx-MVIIC-sensitive (Q-type) Ca2+ channels in murine motor nerve terminals, whereas dihydropyridine (DHP)-sensitive (L-type) Ca2+ channels are unmasked in these terminals. Acute exposure (90 min) of rat forebrain synaptosomes to LEMS immunoglobulins (Igs; 4 mg/ml) did not alter the binding of [3H]-nitrendipine or [125I]-omega-conotoxin-GVIA (-omega-CgTx GVIA) when compared with synaptosomes incubated with an equivalent concentration of control Igs. Conversely, LEMS Igs significantly decreased the Bmax for [3H]-verapamil to approximately 45% of control. The apparent affinity of verapamil (KD) for the remaining receptors was not significantly altered. Thus acute exposure of isolated central nerve terminals to LEMS Igs does not increase DHP sensitivity, whereas it reduces the number of binding sites for verapamil but not for nitrendipine or omega-CgTx-GVIA. These results suggest that chronic but not acute exposure to LEMS Igs either upregulates or unmasks DHP-sensitive Ca2+ channels in motor nerve endings.

Acetylcholine Release at Neuromuscular Junctions of Adult Tottering Mice Is Controlled by N-(Cav2.2) and R-Type (Cav2.3) but Not L-Type (Cav1.2) Ca2+Channels

“…These concentrations were chosen based on literature determining their effectiveness at murine neuromuscular junctions (cf. Atchison, 1989;Xu et al, 1998;Santafe et al, 2000;Urbano et al, 2001Urbano et al, , 2003Flink and Atchison, 2002;Kaja et al, 2006).…”

“…Up-regulation of L-type channels is a well reported form of plasticity at murine neuromuscular junction. It occurs in adult mice treated chronically with Lambert-Eaton myasthenic syndrome plasma (Smith et al, 1995;Xu et al, 1998;Flink and Atchison, 2002), reinnervating motor endplates following acute nerve damage (Katz et al, 1996) or botulinum toxin-poisoned nerve terminals (Santafe et al, 2000). L-type channels are also involved in ACh release in neonatal rats (Sugiura and Ko, 1997).…”

Calcium channel subtypes contributing to acetylcholine release from normal, 4‐aminopyridine‐treated and myasthenic syndrome auto‐antibodies‐affected neuromuscular junctions

“…However, little is known about the VDCC subtypes involved in synaptic transmission at the LEMS IgGs‐treated NMJ. Changes in the pharmacological sensitivity of VDCCs in mice motor nerve terminals after chronic exposure to LEMS IgGs have been demonstrated (Smith et al ., 1995; Xu et al ., 1998).…”

Age-Dependent Contribution of P/Q- and R-Type Ca2+Channels to Neuromuscular Transmission inLethargicMice

“…Nim and Cd 21 were used in concentrations of 10 mM and diluted in 20 ml of physiological saline solution. These concentrations were chosen based on literature determining their effectiveness at murine neuromuscular junctions (Atchison, 1989;Xu et al, 1998;Santafé et al, 2000;Urbano et al, 2001Urbano et al, , 2003Flink and Atchison, 2002;Kaja et al, 2006).…”