Effects of neostigmine and pyridostigmine at the neuromuscular junction.

The effect of subcutaneous administration of pyridostigmine or neostigmine for 7 to 15 days on neuromuscular transmission has been studied in the rat phrenic nerve-hemidiaphragm preparation. The quantal contents of end-plate potentials at different stimulus rates and the amplitude and frequency of miniature end-plate potentials were compared with those of untreated controls. The rate of release of acetylcholine quanta at high stimulus rates, and the frequency of miniature end-plate potentials, were reduced by pretreatment with both pyridostigmine and neostigmine. Presynaptic effects differed in that the number of quanta released by each nerve impulse at a stimulus rate of 1/sec was not altered by pyridostigmine, but was reduced to 52% of normal by neostigmine. The amplitude of miniature end-plate potentials was reduced to 81% by pretreatment with neostigmine and to 54% by pretreatment with pyridostigmine. The cause appears to be a reduction in the number of acetylcholine receptor sites as a result of disorganisation of the postsynaptic muscle membrane, which may contribute to the muscular weakness associated with the long term use of anticholinesterase agents.