Experimental autoimmune myasthenia gravis in rats. Modification by thymectomy and prednisolone

[Myasthenia gravis: clinical and surgical treatment]

The indication for thymectomy in myasthenia gravis still is controversial, and it is uncommon to find in the present days studies comparing conservative treatments, due to the widespread surgical treatment adopted in most centers. We studied 65 cases divided into three groups of patients: (1) 15 thymectomized patients and 50 with conservative treatment; (2) 15 thymectomized patients paired with 15 on conservative treatment; (3) 49 patients treated with corticosteroids against 16 without corticosteroids. These three groups where compared regarding age, age when the symptoms began, disease duration, clinical severity and functional scale, studying remission, stability or worsening of the symptoms and death rate after several years of treatment. It was found a reduction of the symptoms (p < 0.05) in the thymectomized patients of group 1; the remaining parameters of all three groups did not show any statistical significance. These results suggest that the type of treatment did not interfere with evolution of myasthenia gravis in this group of patients.

Prednisolone effects upon body and organ weights, water intake, and several behaviors

Various behavioral sequelae have been noted in patients receiving prolonged, high-dose glucocorticoid therapy. The present study assayed several behavioral dimensions of rats receiving daily intramuscular injections of the synthetic glucocorticoid, prednisolone. Specifically, assessments of water intake, nociception, locomotion, and the grasping responses were conducted; measures of gonadal, adrenal, and total body weights were also taken. Twelve daily injections were given to four groups (N = 6/group) of immature male rats with each group receiving a different dose of drugs (0.0, 8.0, 16.0, 32.0 mg/kg). Prednisolone generally suppressed home-cage water intake, and latencies to hind paw-lick in the hot-plate assay. Measures of wheel running and the grasping response were generally enhanced. Absolute gonadal and adrenal weights as well as total body weights were decreased. Relative organ weights suggested that daily prednisolone treatments had produced suppression of the pituitary-adrenal axis. It was concluded that prednisolone is active in these assays and that such measures may be useful in studies of drug interaction with this agent.

Immunopathology of acetylcholine receptors in myasthenia gravis

It is now clear that the muscular weakness and fatigability seen in MG result from an antibody-mediated immune response to AChR. The mechanisms by which antibodies impair transmission are moderately well understood and detection of antibodies in patient's sera is a reliable diagnostic test for the disease. The spectrum of antibody specificities produced in MG is also beginning to be understood, largely through the use of antibodies produced in the experimental model EAMG. Treatment for MG continues to rely heavily on the symptomatic relief afforded by acetylcholinesterase inhibitors. However, the recent recognition of the autoimmune nature of MG has led to increased emphasis on immunosuppression and antibody removal with some beneficial effects. Despite all that has been learned, the level of ignorance has just been pushed back one step--from the neuromuscular junction to the immune system. What initiates the immune response to AChR in MG and how to specifically suppress this aberrant response remain completely unknown.

Facilitatory effects of 4-aminopyridine on neuromuscular transmission in disease states

The in vitro effects of 4-aminopyridine (4-AP) on neuromuscular transmission were determined by microelectrode techniques in intercostal muscles from patients with myasthenia gravis (MG) and the Eaton-Lambert syndrome (ELS), and in forelimb muscles from rats with experimental autoimmune myasthenia gravis (EAMG). In MG and EAMG, the amplitudes of miniature endplate potentials (MEPPs) and endplate potentials (EPPs) were reduced, and there was increased sensitivity to the blocking action of d-tubocurarine (dTc). In ELS, MEPP amplitude was normal but the average number of acetylcholine quanta released by nerve impulses was reduced, causing subthreshold EPPs. In EAMG muscle, 4-AP produced dose-dependent increases in EPP amplitude and in the duration of indirectly elicited muscle action potentials but no changes in MEPP amplitude and resting membrane potential. 4-AP completely reversed the postsynaptic blockade produced by dTc and EAMG. 4-AP appears to facilitate neuromuscular transmission in EAMG, MG, and ELS by increasing the neurally evoked transmitter release, thus overcoming either the pre- or the postsynaptic neuromuscular blockade.

Experimental myasthenia gravis. A murine system

Mice from eight inbred strains were immunized with acetylcholine receptor (AChR) purified from Torpedo californica. All mice developed high concentrations of serum antibodies (10(-6) M) against the immunogen and approximately 80% possessed antibodies reactive with mouse nicotinic AChR. 33% of the mice immunized (n = 236) developed muscular weakness and flaccid paralysis. Behavioral, electrophysiological, and pharmacological similarities were found between the experimentally induced muscular weakness and the disease myasthenia gravis. Susceptibility to experimental myasthenia was found to be strain dependent in that the frequency of paralysis was much greater in some strains than others. The occurrence of muscular weakness and flaccid paralysis did not correlate with the concentration of antibodies reactive with T. californica or mouse AChR. Anti-receptor antibodies which increased the rate of AChR degradation on the mouse muscle cell line, BC3H-1, were found in the serum of both myasthenic and nonmyasthenic mice. 40% of the mice tested possessed antibodies reactive with antigenic determinants present on mouse receptor but not T. californica receptor. The occurrence of antibodies unique to mouse receptor did not correlate with myasthenia. Thus, myasthenia in the mouse does not occur simply as a consequence of the presence of antibodies directed against cell surface antigenic determinants of AChR. If anti-AChR antibodies are both necessary and sufficient for the induction of myasthenia, then these studies suggest that populations of a particular structure and/or specificity are required. It is anticipated that the mouse model of myasthenia gravis will permit the regulation of the anti-receptor immune response to be studied in detail.