tetrodotoxin and Myasthenia-Gravis

Topics: Animals; Botulinum Toxins; Bungarotoxins; Electromyography; Fatty Acids; Humans; Mice; Motor Neurons; Muscle Denervation; Muscles; Myasthenia Gravis; Nerve Degeneration; Nerve Regeneration; Neuromuscular Diseases; Neuromuscular Junction; Peripheral Nerve Injuries; Receptors, Cholinergic; Schwann Cells; Tetrodotoxin

Muscle electrical activity has been studied in mice after intraperitoneal injection of sera from myasthenia gravis (MG) patients. Myasthenic serum did not modify the electrical properties of innervated muscle fibres. The resting membrane potential and the action potential parameters remained unchanged. However, tetrodotoxin (TTX)-resistant action potentials of denervated muscles were reduced by myasthenic serum, possibly in association with receptor endocytosis induced by the immunoglobulin. However, a direct effect of MG serum on TTX-resistant sodium channels cannot be ruled out.

Topics: Action Potentials; Adolescent; Adult; Aged; Animals; Autoantibodies; Humans; Membrane Potentials; Mice; Middle Aged; Muscle Denervation; Muscles; Myasthenia Gravis; Receptors, Cholinergic; Tetrodotoxin

1. Human intercostal nerve-muscle obtained from normal and myasthenia gravis affected patients has been organ cultured for up to 5 weeks at 23 degrees C. In addition normal nerve-muscle has been cultured for up to 2 weeks at 36 degrees C. Muscle fibres had normal resting and overshooting action potentials. Input resistances dropped markedly after 21 days at 23 degrees C or 10 days at 36 degrees C.2. Muscle fibre action potentials became partially resistant to tetrodotoxin (10(-7) g/ml.) after culturing for 8 days at 36 degrees C.3. Extrajunctional acetylcholine (ACh) sensitivity was compared in fresh and cultured muscle. Fresh normal fibres possessed extrajunctional ACh sensitivity covering several hundred micrometres around the end-plate and at the muscle-tendon junction. Myasthenia gravis affected fibres had reduced extrajunctional sensitivity at the end-plate and no detectable ACh sensitivity near the tendon.4. Less than one third of normal muscle bundles showed an increased area of ACh sensitivity after several days in organ culture. Under the same conditions myasthenic muscle bundles did not show an increase in extrajunctional ACh sensitivity,5. M.e.p.p.s were present for 2-4 days in normal fibres cultured at 36 degrees C. In normal and myasthenic muscles cultured at 23 degrees C m.e.p.p.s disappeared after 6-8 days and re-appeared in some fibres (50%) after approximately two weeks in organ culture. These m.e.p.p.s were abolished by curare and increased in frequency by hypotonic solution suggesting they are due to the release of ACh-packets from Schwann cells.6. Electron microscopic examination of cultured human muscle indicates that the disappearance of m.e.p.p.s corresponds with degeneration of nerve terminals.7. In muscle bundles shown to possess m.e.p.p.s after 13-14 days, the synaptic gutter, which had been vacated by the nerve terminal, was usually occupied by a Schwann cell or projections of Schwann cell cytoplasm. This indicates that the Schwann cell at denervated human end-plates may be capable of releasing packets of ACh.8. It is concluded that the organ culture system described here is suitable for studying normal and diseased human muscle fibres. Using this system we find that the denervation changes which follow nerve transection appear to be similar in most respects at normal and myasthenic end-plates.

Topics: Action Potentials; Electric Conductivity; Humans; Membrane Potentials; Microscopy, Electron; Motor Endplate; Muscles; Myasthenia Gravis; Neuromuscular Junction; Organ Culture Techniques; Tetrodotoxin; Time Factors