Myotonic dystrophy (DM) is commonly associated with CTG repeat expansions within the gene for DM-protein kinase (DMPK). The effect of altered expression levels of DMPK, which is ubiquitously expressed in all muscle cell lineages during development, was examined by disrupting the endogenous Dmpk gene and overexpressing a normal human DMPK transgene in mice. Nullizygous (-/-) mice showed only inconsistent and minor size changes in head and neck muscle fibres at older age, animals with the highest DMPK transgene expression showed hypertrophic cardiomyopathy and enhanced neonatal mortality. However, both models lack other frequent DM symptoms including the fibre-type dependent atrophy, myotonia, cataract and male-infertility. These results strengthen the contention that simple loss- or gain-of-expression of DMPK is not the only crucial requirement for development of the disease.

SUMMARY1. Rats were injected once every 48 h with a-bungarotoxin (aBTX) for periods up to 6 weeks. Injections caused weakness of facial muscles which lasted about 8 h. Hemidiaphragms were dissected for biochemical and electrophysiological measurements.2. In muscles from animals treated for 2-3 weeks with toxin, the binding of 125-aBTX was reduced to 58 %, and the ACh content to 81 % of control values. Choline acetyltransferase activity was unchanged. ACh release evoked by 3 Hz nerve stimulation was increased to 175 % of control values.3. The use of ,u-conotoxin, which specifically blocks muscle action potentials, enabled the recording of full-sized endplate potentials (EPPs) and miniature endplate potentials (MEPPs) at normal muscle membrane potentials ( -70 to -80 mV). The amplitude of MEPPs was decreased to 57 % in muscles from animals treated for 3 weeks with aBTX. The mean of the quantal contents, calculated from the ratio of the corrected EPPs and the MEPPs, was increased to 154 %.4. Within individual muscles of both acBTX-treated and control rats, there was an inverse relationship between the quantal content of an endplate and its MEPP amplitude.5. The MEPP frequency of endplates from control muscles was positively correlated with the quantal content. However, this correlation was not found in aBTX-affected muscles.6. Three hours after a single injection of aBTX the amplitude of the MEPPs was reduced to about 60 % of control values but no increase of the quantal content was found. During the first few days of aBTX treatment the quantal content gradually increased; it reached a plateau between 20 and 30 days.7. The results suggest the existence of an adaptive mechanism, operating at individual endplates, in which retrograde signals at the motor nerve terminals modulate ACh release when neuromuscular transmission is endangered by block of acetylcholine receptors. MS 1036 J. J. PLOMP AND OTHERS

In myasthenia gravis, loss of acetylcholine receptors at motor end-plates is induced by antireceptor autoantibodies. At end-plates in rats in which myasthenia gravis-like symptoms are induced by chronic treatment with alpha-bungarotoxin, acetylcholine release is increased. Within muscles from such rats there is a strong correlation between the increase of acetylcholine release at an end-plate and the loss of postsynaptic acetylcholine receptors, caused by the toxin. The question is whether upregulation of acetylcholine release is a clinically relevant compensatory mechanism in myasthenia gravis or only a feature of the animal model using alpha-bungarotoxin. We investigated electrophysiologically the in vitro acetylcholine release at end-plates of muscles from patients with myasthenia gravis and rats with experimental autoimmune myasthenia gravis where acetylcholine receptor reduction is caused by autoantibody attack. In both human and rat autoimmune myasthenic muscle, the mean quantal content was considerably increased compared with control levels. At each individual myasthenic end-plate, the increase in quantal content appeared to be correlated with the reduction of the amplitude of the miniature end-plate potential. This finding suggests the existence of an important compensatory mechanism in myasthenia gravis, in which retrograde acting factors (i.e., from muscle fiber to nerve terminal) upregulate acetylcholine release.

SUMMARY1. Frog sartorius muscles were treated with an irreversible cholinesterase inhibitor and then incubated in Ringer with 2 mM-LaCl3. The amounts of ACh in the tissue and medium were assayed by mass fragmentography, miniature end-plate potentials (min. e.p.p.s) were recorded and the end-plate was investigated by electron microscopy.2. Addition of Las+ caused in normal, but not in denervated, muscles a discharge of both min. e.p.p.s and chemically detectable ACh. After .30 min both min. e.p.p.s and ACh release decreased. Between 4 and 5 hr after the addition of Las+ min. e.p.p.s had practically ceased and the rate of ACh release was almost back to that in the absence of LaS+.3. La3+ caused a 50 % reduction in the ACh content of the tissue within the first 30 min; thereafter ACh gradually increased to 110 % by 5 hr. At this time synaptic vesicles were practically absent in most terminals. The ACh was predominantly located in the end-plate regions of the muscles, before as well as after the incubation with Las+. ACh in end-plate free parts of the muscles was unchanged by La +.4. Hemicholinium-3 inhibited the synthesis of ACh in the muscles, but it had almost no influence on La3+-induced ACh release.5. From these and other results, it is concluded that the ACh released by La3+ originates exclusively from the nerve terminals, that most likely this ACh is released via exocytosis from synaptic vesicles, and that the synthesis of ACh following the release of ACh takes place in the nerve terminals. The results further indicate that in freshly excised muscle the greater part (80-90 %) of the ACh contained in the nerve terminals is located in the vesicles.

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