Pharmacological aspects of neuromuscular transmission in the isolated diaphragm of the dystrophic (Rej 129) mouse

Quantal Analysis of Endplate Potentials in Mouse Flexor Digitorum Brevis Muscle

The isolated flexor digitorum brevis (FDB) muscle from mice is extremely well suited to rapid acquisition of data and analysis of neurotransmitter release and action at neuromuscular junctions, because the muscle and its tibial nerve supply are simple to dissect and its constituent muscle fibers are short (<1 mm) and isopotential along their length. Methods are described here for dissection of FDB, stimulation of the tibial nerve, microelectrode recording from individual muscle fibers, and quantal analysis of endplate potentials (EPPs) and miniature endplate potentials (MEPPs). Curr. Protoc. Mouse Biol. 1:429-444 © 2011 by John Wiley & Sons, Inc.

Neuromuscular transmission in the murine mutants "motor end-plate disease" and "jolting"

Mice with the inherited disorder "motor end-plate disease" suffered from a progressive neuromuscular weakness and muscular wasting. The weakness resulted from a failure of evoked transmitter release from the motor nerve terminals. The failure in transmission was all-or-nothing in nature. The numbers of muscle fibres in skeletal muscle and myelinated axons in several major nerve trunks were no different from normal. The loss in muscle bulk was caused by the neuromuscular defect and not from a loss of motor units or muscle fibres. The inherited murine disorder "jolting" was allelic with "motor end-plate disease". Affected "jolting" mice suffered no detectable morphological abnormality in skeletal muscle or peripheral nerve. The physiological properties of skeletal muscle and the characteristics of neuromuscular transmission were indistinguishable from normal.

The origin of (+)-tubocurarine resistance in dystrophic mice

Intracellular recording, twitch responses and radio-ligand binding techniques were used to study the causes of resistance to (+)-tubocurarine (curare) of extensor digitorum longus (EDL) muscles from dystrophic mice (129 ReJ/strain). The indirectly evoked twitch response of muscles from dystrophic mice was more resistant to block by curare than the twitch response of muscles from normal littermates. The IC50 (concentration producing 50% inhibition of stimulus-evoked contractions) values for the curare block of muscle twitch were 0.78 +/- 0.03 microM and 1.32 +/- 0.05 microM (mean +/- 95% confidence limits) for muscles from normal and dystrophic mice, respectively. There was no difference between muscles from normal and dystrophic mice in the number of alpha-bungarotoxin binding sites per endplate. The amplitudes of both spontaneous miniature endplate potentials (m.e.p.ps) in unblocked preparations and of evoked endplate potentials (e.p.ps) in 1.91 microM curare were greater in muscles from dystrophic mice than in muscles from normal mice. The ratio dystrophic/normal was greater for the e.p.p. amplitudes than for the m.e.p.p. amplitudes. The quantum content of e.p.ps in magnesium-blocked and in cut-fibre preparations was greater in muscles from dystrophic mice than in muscles from normal littermates. Calculation of the binomial parameters n and p in the cut-fibre preparations indicated that this increased quantum content was caused by an increase in the value of p. It is concluded that at least part of the increased resistance to curare of the indirectly evoked twitch response of muscles from dystrophic mice is due to an increase in the quantum content of e.p.ps in these muscles.

Presence of immunoreactive alpha-melanotropin and beta-endorphin in spinal motoneurones of the dystrophic mouse

Two derivatives of the stem hormone opiomelanocortin, alpha-melanotropin (alpha-MSH) and beta-endorphin, were detected immunocytochemically in the cell bodies and in the peripheral axon terminals of spinal motoneurones in immature but not in adult healthy mice. Immunoreactivity was demonstrated in motoneurones in both immature and adult mice with inherited muscular dystrophy. These results provide evidence for a motoneurone abnormality in murine dystrophy. The observations are discussed in the light of trophic influences over neuromuscular transmission which have been attributed to this family of neuropeptides.

The uptake of 99mtechnetium diphosphonate into degenerating and regenerating muscle. A correlative histological and biochemical study

The uptake of 99mTc-methylene diphosphonate into rat skeletal muscle has been studied. Uptake did not occur in normal or denervated muscles. Muscles damaged by exposure to the myotoxin notexin did accumulate label. The toxin causes a severe inflammatory, necrotising myopathy and regeneration normally begins 2-3 days after toxin administration. The uptake of label was maximal at 12-24 h, at which time muscle wet weight and cellular infiltration were also maximal. Label was found in the muscle mitochondria, the invading inflammatory cells and the oedema fluid. Regenerating muscles did not accumulate label. The uptake of label was positively correlated with total muscle calcium up to 24 h after the onset of necrosis, but it was suggested that the changes in the two parameters are not causally related.

The membrane morphology of the neuromuscular junction, sarcolemma, sarcoplasmic reticulum and transverse tubule system in murine muscular dystrophy studied by freeze-fracture electron microscopy

'Dystrophic' mice of the 129/ReJ-dy strain have a genetic defect in Schwann cell proliferation and neuromuscular junction formation. The presynaptic membrane specializations associated with vesicle fusion and acetylcholine release, as well as the postsynaptic membrane specializations associated with acetylcholine receptivity, appear normal in these animals when visualized with freeze-fracture techniques. However, there is a reduction in the infolding of the postsynaptic membrane, which forms the secondary synaptic cleft at the motor endplate and is the site of acetylcholinesterase activity. The orthogonal arrays of the non-junctional sarcolemma are found on dystrophic muscles, but at lower than normal densities. These observations are made on muscle fibers in which the membrane molecular organization of the sarcoplasmic reticulum and transverse tubule systems appear normal. Several possible linkages between the deficit in myelination and the altered synaptic morphology are discussed in the context of neuromuscular interaction.

Motor innervation of the gastrocnemius muscle of wobbler and dystrophic mice

Innervation of the gastrocnemius muscle of mice from the wobbler, dystrophic and C57/BL colonies has been studied. It was found that phenotypically normal mice from each of the colonies did not differ in their innervation properties, hence suggesting no heterozygote penetrance. However, the end-plate complexity increased with age of normal mice. Functional terminal innervation ratio for both the wobbler and dystrophic gastrocnemius muscle was raised above that of the normal and many dystrophic end-plates also appeared abnormal. A study of mice from the wobbler colony manifesting a late onset hind-limb muscle degeneration (Wr/HLD) has been included and the results suggest a relatively benign form of spinal muscular atrophy.

The relationship between end-plate size and transmitter release in normal and dystrophic muscles of the mouse

1. The morphology of nerve terminals and sub-neural apparatuses was examined in the muscles of normal and dystrophic adult mice of the Bar Harbor 129 ReJ strain. Nerve terminals were larger in dystrophic muscles than in normal muscles and nerve terminal sprouting was evident in about 50% of the dystrophic muscles fibres. End-plate area was positively correlated with muscle fibre diameter in both normal and dystrophic muscles. 2. Polynueronal innervation was found in only 1% of dystrophic muscle fibres impaled with micro-electrodes. 3. Miniature end-plate potential amplitude was positively correlated with muscle fibre input resistance in both normal and dystrophic muscles. There was however, a greater than normal variation in the amplitudes of m.e.p.p.s recorded from individual dystrophic muscle fibres. 4. Quantum contents of end-plate potentials were estimated in normal and dystrophic mouse nerve-muscle preparations partially blocked with D-tubocurarine. The quantum content of e.p.p.s seemed to be related to muscle fibre diameter, and in dystrophic muscles the characteristics of evoked release were indistinguishable from normal. 5. It was concluded that the nerve terminal sprouting and the expansion of end-plate area which were observed in dystrophic muscles are not a consequence of any form of denervation, but represent an attempt by the axon to expand the area of synaptic contact in hypertrophied muscle fibres.