Electrophysiological studies of thymectomized and nonthymectomized acetylcholine receptor-immunized animal models of myasthenia gravis

Abstract

White New Zealand rabbits were immunized with acetylcholine receptor (AChR) purified from the electric organs of either the Electrophorous electricus (eel) or Torpedo californica (torpedo). Approximately 4 days after a second injection of 100 μg AChR in complete Freund's adjuvant, the immunized animals developed weakness of the peripheral muscles which was aggravated by repetitive activity and was partially alleviated by anticholinesterases such as edrophonium or pyridostigmine. Electromyograms taken from the anterior tibialis muscle demonstrated a decrement of the compound action potentials at a stimulation frequency of 5 Hz which was reversed by the intravenous administration of 1 mg pyridostigmine. If the animals were not treated with anticholinesterases they died within a couple of days. Biopsies of the intercostal muscle were done under Nembutal anesthesia, and intracellular recordings revealed subliminal end-plate potentials (EPPs) and miniature end-plate potentials (MEPPs) of greatly diminished amplitudes. At some neuromuscular junctions no MEPPs were detected, and in other junctions MEPPs were seen but no EPP was evoked upon nerve stimulation. Muscles from two immunized rabbits showed spontaneous muscle action potentials occurring at 1 to 2 Hz. Serum from the rabbits immunized with torpedo or eel AChR, when applied to in vitro preparations of the eel electroplaque or frog cutaneous pectoris muscle, caused a decrease in postjunctional membrane sensitivity to carbamylcholine and reduced the MEPP and EPP amplitudes even after heat inactivation of the serum complement (56°C for 30 min). In addition to a decrease in chemosensitivity, an enhancement of receptor desensitization occurred in the neuromuscular junctions of serum-treated frog muscles. Thymectomies on adult rabbits prior to the immunization had the effect of delaying the onset of myasthenia in two of six rabbits, and in three of six rabbits only mild myasthenia occurred. The blood antibody titers to AChR were similar in both the thymectomized and nonthymectomized rabbits.