Morphological observations on motor end-plates in rabbits with experimental myasthenia

Evidence of Schwann cell degeneration in the aging mouse motor end-plate region

An ultrastructural evaluation was made of the changes encountered in the motor endplates of the gastrocnemius muscle of old mice in comparison with young-adult mice. In old, 27-month-old mice, a great variety of changes were recorded. The main abnormalities occurred in the axon terminals, the myoneural junctional folds, and the endomysial space. Loss of synaptic vesicles and mitochondria and vacuolization of axon terminals were common. Shrinkage and degenerative changes of axon terminals were noticed. The end-plate abnormalities were closely associated with an increased amount of collagen tissue, degenerated Schwann cells, and cell debris. Degenerated Schwann cells were not encountered in young-adult mice. In the old group, endplates of normal appearance occurred in the proportion of 40%. The young group had normally structured end-plates in the proportion of 85%. The lesions described are not specific for the aging process.

Immunisation with Torpedo acetylcholine receptor

Acetylcholine mediates the transfer of information between neurons in the electric organ of, for example, Torpedo as well as in vertebrate skeletal muscle. The nicotinic acetylcholine receptor complex translates the binding of acetylcholine into ion permeability changes. This leads to an action potential in the muscle fibre. The nicotinic acetylcholine receptor protein has been purified from Torpedo by use of affinity chromatography. The receptor is an intrinsic membrane glycoprotein composed of five polypeptide chains. When various animals are immunised with the receptor they demonstrate clinical signs of severe muscle weakness coincident with high antibody titres in their sera. The symptoms resemble those found in the autoimmune neuromuscular disease myasthenia gravis in humans. This animal model has constituted a unique model for studying autoimmune diseases. This paper reviews some of the work using Torpedo acetylcholine receptor in order to increase the understanding of the motor nervous system function and myasthenia gravis. It is now known that the nicotinic acetylcholine receptor protein is the antigen involved in myasthenia gravis. The mechanism of immune damage involves a direct block of the receptor function. This depends on the presence of antibodies which crosslink the postsynaptic receptors leading to their degradation. The questions to be answered in the future are; (a) what initiates or triggers the autoimmune response, (b) how do the antibodies cause the symptoms--is there a steric hindrance of the interaction of acetylcholine and the receptor, (c) why is there not a strict relationship between antibody titre and severity of symptoms, and (d) why are some muscles affected and other spared? With help of the experimental model, answers to these questions may result in improved strategies for the treatment of the autoimmune disease myasthenia gravis.

Morphological changes observed in rats immunized with the Torpedo acetylcholine receptor alpha-chain

Lewis rats were immunized with nicotinic acetylcholine receptor (nAChR) polypeptides purified from the electric tissue of Torpedo marmorata. Animals immunized with the acetylcholine binding, 41.500 daltons, polypeptide showed comparable fragmentation of the postsynaptic membrane and indications of rearrangement of the adjacent muscle fiber as earlier found in rabbits and rats immunized with the nAChR complex. Thus, these results demonstrate that the alpha-polypeptide chain carries all determinants necessary for onset of Experimental Autoimmune Myasthenia Gravis (EAMG).

Antibodies to the nicotinic acetylcholine receptor, obtained from serum of myasthenic patients, may decrease acetylcholine release from rat hippocampal nerve endings in vitro

Release of [3H]-ACh from [3H]-Ch loaded nerve endings from rat hippocampus is dependent on Ca2+ and K+-concentration. [3H]-ACh release, evoked by a depolarizing K+ concentration is decreased in the presence of IgG isolated from the serum of some myasthenic patients but not in that of IgG from controls. Myasthenic IgG contains antibodies to the nicotinic acetylcholine receptor (nAChR). Isolated nerve endings were shown not to be leaky. Curare-sensitive alpha-bgt receptor was found at a concentration of 60-90 fmol/mg protein. It is discussed if a presynaptic nAChR exists in the membrane of hippocampal nerve endings and is involved in an autoregulation of ACh-release or if myasthenic IgG in a manner independent of the receptor.

Myasthenia gravis: further electrophysiological and ultrastructural analysis of transmission failure in the mouse passive transfer model

Using the mouse passive transfer model the mean amplitude of miniature endplate potentials and endplate potentials of mice treated with myasthenic immunoglobulins was markedly decreased. Miniature endplate potential frequency and quantum content of endplate potentials were normal, arguing against a major presynaptic disarrangement. Under electron-microscopy no gross structural alterations of endplates were demonstrated. It is concluded that the mouse passive transfer model closely resembles human myasthenia gravis of recent onset.

Neuromuscular transmission in myasthenic single motor units

In patients with myasthenia gravis neuromuscular transmission has been tested in individual hypothenar and thenar motor units using trains of near threshold electrical stimuli delivered to the motor nerve. The most important observations included: (1) the proportion of motor units with pathological decrements varied from 0 to 90%, (2) the decrements in surface voltage were frequently much more normal in individual motor units than in the corresponding maximum compound potentials evoked by supramaximal nerve stimulation, and (3) the most abnormal decrements were observed in motor units at the low end of the surface voltage range. These observations can be interpreted to suggest important neurogenic factors in the pathogenesis of myasthenia gravis. An attractive alternative would be to suggest that the small motor units have the most abnormal neuromuscular transmission because they normally have a lower margin for safe neuromuscular transmission and, as a consequence, fail first in diseases of neuromuscular transmission.