The evidence of accelerative interaction between cAMP-dependent protein kinase and external calcium for the desensitization of nicotinic acetylcholine receptor channel in mouse skeletal muscle cells

Inhibition of acetylcholine receptor function by seronegative myasthenia gravis non-IgG factor correlates with desensitisation

15% of myasthenia gravis (MG) patients do not have antibodies against the acetylcholine receptor (AChR). Some of these "seronegative" MG patients have antibodies against muscle specific kinase (MuSK), and many have a non-IgG factor that acutely inhibits AChR function in a muscle-like cell line, CN21. Here we show, using mainly one plasma negative for both AChR and MuSK antibodies, that the inhibitory effect of the non-IgG fraction correlates well with the desensitisation caused by 100 microM nicotine, and is found also when AChRs are expressed in a non-muscle cell line (HEK). Moreover, a similar effect was seen with M3C7-a monoclonal antibody against human AChR. The results suggest that, rather than acting indirectly as previously proposed, the SNMG factor may bind directly to an allosteric site that induces or enhances AChR desensitisation.

Extracellular concentrations of catecholamines and amino acids in the dorsomedial hypothalamus of kindled rats. A microdialysis study

Epilepsy affects homeostasis and autonomic nervous system functions. It has been thought that the dysfunction in the autonomic neural mechanisms could be a cause of sudden unexpected death in patients with epilepsy. The kindling model of epilepsy is considered to be an animal model for complex partial seizures with secondary generalization. The objectives of this study were to investigate the extracellular gamma-aminobutyric acid (GABA), glutamate, noradrenaline and dopamine levels in the dorsomedial nucleus of the hypothalamus in non-epileptic and kindled epileptic rats and to explain some of the cardiovascular changes in the kindling model of epilepsy. Stimulation electrodes were stereotaxically implanted into the basolateral amygdala and electrical stimulation was applied 3 times a day at a constant current. The rats were then kindled to full stage 5 seizures. Microdialysis experiments were performed to demonstrate the neurotransmitter levels in the dorsomedial nucleus of the hypothalamus 3-5 days after being kindled. Decreases in noradrenaline and dopamine levels in the dorsomedial nucleus were detected in the conscious kindled animals. This finding is in agreement with prior findings that the noradrenergic system has a negative role in the process of kindling. The basal level of glutamic acid and GABA remained unchanged in the kindled group when compared to non-epileptic animals, and similarly, neither blood pressure nor heart rate responses to bicuculline or N-methyl-D-aspartate were affected by the acute kindled state. These findings suggest that the autonomic changes in kindling require further studies.

Active calcium accumulation underlies severe weakness in a panel of mice with slow-channel syndrome

Mutations affecting the gating and channel properties of ionotropic neurotransmitter receptors in some hereditary epilepsies, in familial hyperekplexia, and the slow-channel congenital myasthenic syndrome (SCCMS) may perturb the kinetics of synaptic currents, leading to significant clinical consequences. Although at least 12 acetylcholine receptor (AChR) mutations have been identified in the SCCMS, the altered channel properties critical for disease pathogenesis in the SCCMS have not been identified. To approach this question, we investigated the effect of different AChR subunit mutations on muscle weakness and the function and viability of neuromuscular synapses in transgenic mice. Targeted expression of distinct mutant AChR subunits in skeletal muscle prolonged the decay phases of the miniature endplate currents (MEPCs) over a broad range. In addition, both muscle strength and the amplitude of MEPCs were lower in transgenic lines with greater MEPC duration. SCCMS is associated with calcium overload of the neuromuscular junctional sarcoplasm. We found that the extent of calcium overload of motor endplates in the panel of transgenic mice was influenced by the relative permeability of the mutant AChRs to calcium, on the duration of MEPCs, and on neuromuscular activity. Finally, severe degenerative changes at the motor endplate (endplate myopathy) were apparent by electron microscopy in transgenic lines that displayed the greatest activity-dependent calcium overload. These studies demonstrate the importance of control of the kinetics of AChR channel gating for the function and viability of the neuromuscular junction.

Calcium-dependent desensitizing function of the postsynaptic neuronal-type nicotinic acetylcholine receptors at the neuromuscular junction

Several subunits that commonly have been regarded as neuronal-type nicotinic acetylcholine receptor (nAChR) subtypes, have been found in the postjunctional endplate membrane of adult skeletal muscle fibres. The postsynaptic function of these neuronal-type nAChR subtypes at the neuromuscular junction has been investigated by using aequorin luminescence and fluorescence confocal imaging. A biphasic elevation of intracellular Ca2+ is elicited by prolonged nicotinic action at the mouse muscle endplates. The fast and slow Ca2+ components are operated by a postsynaptic muscle- and colocalized neuronal-type nAChR, respectively. Neuromuscular functions may be regulated by a dual nAChR system to maintain the normal postsynaptic excitability. Certain neuronal-type nAChR may be endowed with the same functional role in the central nervous system also.

Enhancement by calcitonin gene-related peptide of non-contractile Ca2(+)-induced nicotinic receptor desensitization at the mouse neuromuscular junction

1. Nicotinic acetylcholine receptor (AChR)-operated non-contractile Ca2+ mobilization (unaccompanied by muscle contraction) depressed contractile Ca2+ mobilization (accompanied by muscle contraction) in mouse diaphragm muscles. In the process of nicotinic AChR desensitization, the enhancing role of calcitonin gene-related peptide (CGRP) on the non-contractile Ca2(+)-induced depression of contractile Ca2+ mobilization was investigated by measurement of Ca2(+)-aequorin luminescence in the presence of neostigmine (0.1 microM). 2. When the phrenic nerve was stimulated with paired pulses at intervals of 150, 300, 600, 1000 and 2000 ms, contractile Ca2+ transients were elicited during the generation of non-contractile Ca2+ mobilization. The amplitude of the contractile Ca2 transients elicited by the second pulse (S2) was depressed at the shorter pulse intervals, but not at the longer pulse intervals. 3. The extent of depression of S2 was enhanced when the duration of non-contractile Ca2+ mobilization was prolonged by CGRP (10 nM). However, CGRP failed to enhance the depression of S2 when non-contractile Ca2+ mobilization was not observed at the low external Ca2+ concentration (1.3 mM). 4. The enhancing effect by CGRP on the depression of S2 was counteracted by staurosporine (3 nM), a protein kinase-C inhibitor, despite prolongation of the duration of non-contractile Ca2+ mobilization. 5. When H-89 (1 microM), a protein kinase-A inhibitor, completely blocked non-contractile Ca2+ mobilization, the depression of S2 was diminished. The prolongation of the duration of non-contractile Ca2+ mobilization by AA373 (300 microM), a protein kinase-A activator, enhanced the depression of S2. The enhancing effect was observed neither with CGRP nor with AA373, in the presence of H-89 (0.1 microM). 6. These findings suggest that the CGRP mobilizes non-contractile Ca2+ through activation of protein kinase-A, which in turn may activate protein kinase-C, then enhance the desensitization of postsynaptic nicotinic AChRs at the neuromuscular junction.

Modulation of acetylcholine receptor function in TE671 (rhabdomyosarcoma) cells by non-AChR ligands: possible relevance to seronegative myasthenia gravis

The acetylcholine receptor (AChR) is the main target antigen in myasthenia gravis (MG), but about 15% of patients with typical immunologically mediated MG do not have detectable anti-AChR antibodies. Previous studies showed that plasma from these 'seronegative' patients (SNMG) reduced AChR function in the human AChR-expressing TE671 cell line, and it was proposed that SNMG plasmas may act indirectly via phosphorylation of AChR. We show here that substances such as the beta 2-adrenergic agonist, salbutamol, calcitonin-gene-related-peptide (CGRP), and cholera toxin, that increase intracellular cAMP via binding to specific cell-surface receptors, reduced AChR function in TE671 cells. Moreover, non-specific activation of cell surface proteins by lectins achieved similar results. These observations lead us to hypothesise that SNMG immunoglobulins act in TE671 cells by cross-linking of specific cell surface antigen(s) resulting in generation of intracellular cAMP and/or other second messengers. The role of such antibodies at the neuromuscular junction in vivo could be reduction in AChR function by desensitization and/or damage to the postsynaptic membrane following complement activation.

Accelerated desensitization of nicotinic receptor channels and its dependence on extracellular calcium in isolated skeletal muscles of streptozotocin-diabetic mice

1. To elucidate the influence of the diabetic state on desensitization of nicotinic acetylcholine (ACh) receptor channels, we investigated the time course of the decrease in amplitude of ACh potentials elicited by iontophoretic application to isolated diaphragm muscle of streptozotocin-diabetic mice. We also investigated time- and extracellular Ca(2+)-dependent changes in the channel opening frequency of ACh-activated channel currents and the involvement of protein kinases by use of the cell-attached patch clamp technique in single skeletal muscle cells. 2. When ACh potentials were evoked at 10 Hz, the decline in trains of ACh potentials was accelerated in the diabetic state. 3. The time-dependent decrease in the channel opening frequency of diabetic muscle cells was greatly accelerated compared with normal cells in 2.5 mM Ca2+ medium. 4. This accelerated decrease in channel opening frequency was restored by pretreatment with a protein kinase C inhibitor, staurosporine (10 nM) but neither a protein kinase A inhibitor, H-89 (3 microM) nor a calmodulin kinase II inhibitor, KN-62 (5 microM) were able to restore the fall in opening frequency. 5. These results demonstrate that in the diabetic state the desensitization of nicotinic ACh receptor channels may be greatly accelerated by activating protein kinase C, which is caused by an increase in the amount of available intracellular Ca2+.