Inhibition by actinomycin D of the generation of acetylcholine receptors induced by denervation in skeletal muscle

Neural regulation of mRNA for the alpha-subunit of acetylcholine receptors: role of neuromuscular transmission

Levels of mRNA for acetylcholine receptor (AChR) subunits are relatively low in innervated skeletal muscles. Following denervation they rise rapidly, leading to increased AChR synthesis. The mechanism by which motor nerves normally regulate these mRNA levels is not yet known. In order to determine the possible role of synaptic transmission in this process, we have compared the effect of blockade of cholinergic ACh transmission with that of surgical denervation. Blockade of quantal ACh transmission was produced by injection of type A botulinum toxin into the soleus muscles of rats. We measured mRNA for the alpha-subunit of the AChR (alpha-AChR mRNA) in RNA extracts of botulinum-treated, denervated, and normal control muscles by hybridization with a highly specific cDNA probe. Our findings show that treatment with botulinum toxin resulted in an increase in alpha-AChR mRNA which was similar to the effect of surgical denervation, although slower in its time course. Since botulinum toxin specifically inhibits quantal ACh release, these results support the concept that cholinergic synaptic transmission plays a key role in mediating the neural control of the alpha-AChR message. The difference between the effects of denervation and botulinum-treatment may be explained by the fact that botulinum toxin does not block the spontaneous non-quantal component of ACh transmission, which has previously been shown to have a partial influence in regulating certain properties of muscles. The present results suggest that synaptic transmission has an important influence in regulating gene expression in the target cell.

Effect of verapamil on organophosphorus-induced delayed neuropathy in hens

Verapamil, a calcium channel blocker, was administered to adult white leghorn hens to determine if inhibition of calcium entry could alter delayed neuropathy induced by administration of phenyl saligenin phosphate (PSP). Verapamil was given im in doses of 7 mg/kg/day for 4 days beginning 24 hr before administration of PSP (2.5 mg/kg im). Ataxia was less pronounced in hens given PSP plus verapamil than in hens given PSP alone during observations made 8-28 days after PSP administration. Myelinated fiber lesions were less extensive and regeneration more notable in the biventer cervicis nerve in chickens given PSP plus verapamil, with samples obtained both 17 and 28 days after PSP. In the absence of verapamil, rheobase and chronaxie values of strength-duration curves were higher and shorter, respectively, and sensitivity to acetylcholine was increased in biventer cervicis nerve-muscle preparations from hens given PSP. Verapamil did not alter PSP-induced inhibition of neurotoxic esterase, indicating that the mechanism involved in amelioration of these indices of delayed neuropathy was not associated with initial enzyme inhibition caused by this organophosphorus ester.

Neurotrophic control of RNA synthesis in amphibian striated muscle

When frog sartorius muscles are incubated in culture for 3 days, a 98% increase in RNA synthesis by the muscles is seen. A portion of this increase (60--100% in different experiments) is prevented by the presence of a 2 cm distal nerve stump left attached to the muscle, or by the addition to the culture medium of nerve and spinal cord extracts from frog, rat or cow. The effect of the nerve stump and the extract is seen under conditions of neuro-muscular blockade by 50 mg/l gentamicin sulfate present in the culture medium. Autoradiography of 1 micrometer sections of muscles previously labeled with [3H]uridine shows that isotope incorporation is largely confined to muscle fibers rather than connective tissue. Muscles cultured for 3 days display normal action and resting potentials either in the presence or absence of extract. The results are discussed in terms of control of muscle RNA synthesis by substances present in motor neurons.

Further studies on the binding properties of cobra neurotoxin to cholinergic receptors in mouse skeletal muscle

Preparations of 3H-monoacetylated derivatives of Naja naja siamensis neurotoxin siamensis 3 were found to be resistant to degradation and deacetylation during in vitro muscle incubation. A low rate of free 3H-acetate (less than 0.4%) was present in the preparations, but should not interfere with the binding of toxin to cholinergic receptors in mouse extensor digitorum longus muscles in vitro. The binding of toxin to cholinergic receptors in mouse extensor digitorum longus muscle was essentially irreversible. d-Tubocurarine antagonized the binding of toxin in both innervated and denervated muscles. Administration of actinomycin D one day after denervation prevented the appearance of new toxin binding sites in the muscles.

Synthesis of acetylcholine receptor by denervated rat diaphragm muscle

Acetylcholine receptor was purified by affinity chromatography from denervated rat hemidiaphragms that had been incubated in organ culture for 24 hr in medium containing [35-S] methionine. Radioactive acetylcholine receptor was identified in purified preparations by zone sedimentation in a sucrose gradient, by isoelectric focusing, and by precipitation with an antiserum to the acetylcholine receptor from electric eel. When innervated and denervated hemidiaphragms were incubated with [35-S] methionine in organ culture, and the acetylcholine receptors from each were purified separately, only the preparation from denervated muscles contained radioactive receptor as determined by zone sedimentation. We conclude that newly synthesized receptor is accumulated as a result of muscle denervation.