Some factors affecting spontaneous transmitter release in dystrophic mice

Finite element simulations of acetylcholine diffusion in neuromuscular junctions

A robust infrastructure for solving time-dependent diffusion using the finite element package FEtk has been developed to simulate synaptic transmission in a neuromuscular junction with realistic postsynaptic folds. Simplified rectilinear synapse models serve as benchmarks in initial numerical studies of how variations in geometry and kinetics relate to endplate currents associated with fast-twitch, slow-twitch, and dystrophic muscles. The flexibility and scalability of FEtk affords increasingly realistic and complex models that can be formed in concert with expanding experimental understanding from electron microscopy. Ultimately, such models may provide useful insight on the functional implications of controlled changes in processes, suggesting therapies for neuromuscular diseases.

Electrophysiology of the neuromuscular junction of the laminin-2 (merosin) deficient C57 BL/6J dy2J/dy2J dystrophic mouse

The C57 BL/6J dy2J/dy2J dystrophic mouse expresses an abnormal truncated form of the alpha2 subunit of the protein laminin-2 (or merosin), which is unable to form a stable link between the extracellular matrix and the dystrophin-associated proteins, resulting in muscular dystrophy. Morphological abnormalities of the peripheral nervous system and neuromuscular junction have also been reported. The electrophysiological properties of the neuromuscular junctions of diaphragm, extensor digitorum longus (EDL), and soleus from C57 BL/6J dy2J/dy2J mice and controls are described. No evidence for the presence of denervated fibres were found. Mean MEPP amplitudes were significantly increased in EDL and soleus but reduced in the diaphragm from affected mice. Mean MEPP frequencies were raised in all the dy2J/dy2J muscles studied. dy2J/dy2J muscles were paralysed by low concentrations of mu-conotoxin suggesting that embryonic (tetrodotoxin and mu-conotoxin resistant) sodium channels are not widespread on dy2J/dy2J muscle as has previously been reported. EPP latencies were significantly prolonged in the diaphragm and EDL but not soleus from dy2J/dy2J mice. Quantal contents were higher in all dy2J/dy2J muscles. In the dy2J/dy2J diaphragm failures in neurotransmission occurred and a faster rate of rundown of EPPs were apparent. Some changes appear from a direct effect of dystrophy, whilst increased MEPP frequency and quantal content, and failures in neurotransmission indicate neuronal abnormalities.

Effect of uncouplers of oxidative phosphorylation on transmitter release in dystrophic mice

Intracellular recording was used to study the effect of uncouplers of oxidative phosphorylation on miniature endplate potentials (m.e.p.p.s) in skeletal muscles from dystrophic mice and their clinically normal littermates. Control m.e.p.p. frequency in muscles from dystrophic mice was not significantly different from normal. In the presence of the inhibitors 2,4-dinitrophenol (10(-4) M) or guanidine (5 X 10(-3) M) m.e.p.p. frequency was increased less in muscles from dystrophic mice than that in muscles from normal littermates. In contrast, raising the extracellular calcium concentration, depolarising nerve terminals with potassium or motor nerve stimulation all caused a similar increase in m.e.p.p. frequency in normal and dystrophic muscles. It is suggested that there is a difference in the way in which calcium is stored in dystrophic nerve terminals but that their ability to regulate free calcium is normal.