Neurotrophic control of RNA synthesis in amphibian striated muscle

Temporal relationship between nerve-stump-length-dependent changes in the autophosphorylation of a cyclic AMP-dependent protein kinase and the acetylcholine receptor content in skeletal muscle

The acetylcholine receptor (AChR) content and the autophosphorylation of the regulatory subunit of cyclic AMP-dependent protein kinase type II (R-II) were evaluated in rats soleus muscles at 24, 30 and 66 hr after surgical denervation by cutting the nerve at a short distance (short-nerve-stump) and at a long distance (long-nerve-stump) from the muscle. AChR content was based on the specific binding of [125I]alpha-bungarotoxin (BUTX); changes in the autophosphorylation of R-II were based upon the predominant in vitro 32P-phosphorylation of a 56-Kd soluble protein in cytosolic fractions of solei. The AChR content and the 32P-autophosphorylation of R-II were increased in samples from short-nerve-stump solei, but not from long-nerve-stump solei, after a denervation-time of 30 hr. This nerve-stump-length dependency indicates that the two denervation effects are not related to the immediate halt of impulse-evoked muscle contractility. Furthermore, the results show that alterations in the 32P-autophosphorylation of R-II occurred before, as well as whenever, increases in the AChR content were found. Speculatively, this temporal relationship may be significant with respect to the potential role of R-II in gene expression.

Trophic capacity of experimentally lengthened gustatory axons

Long nerve stumps will maintain end-organs for a longer period of time than short nerve stumps. However, longer maintenance might result not from a longer nerve stump per se but from uncontrolled factors such as less interference with the target's blood supply when transecting a nerve far from its target. To control for the site of nerve transection and to evaluate the trophic capacity of lengthened axons, we extended the gerbil's (Meriones unguiculatus) IXth nerve (glossopharyngeal) by splicing in a 5.0 mm segment of the superior laryngeal nerve. Thus, when assessing nerve stump-length dependency 3-5 months later, both the control regenerated nerve and the lengthened experimental nerve could be transected at the same site on the IXth nerve trunk. The peak magnitude of the summated taste discharges of the experimentally lengthened IXth nerve declined at a rate exactly predicted by an earlier study of nerve stump-length dependency. We conclude that maintenance of synaptic transmission depends upon taste axon stump-length and not upon factors uncontrolled in conventional experimental designs of stump-length dependency which transect nerves at different locations. Control regenerated and experimentally lengthened nerves maintained similar numbers of taste buds. Thus, neither the added burden of maintaining longer axons nor the presence of Schwann cells of the superior laryngeal nerve splice degraded the IXth nerve's trophic performance.

Denervation modulated changes in mouse skeletal muscle RNA concentration

We have examined normal and denervated mouse skeletal muscle for histological evidence of changes in RNA concentration using acridine orange/nucleic acid fluorescence. In denervated muscle, cytoplasmic RNA concentration increases dramatically. The earliest increase is in the subsarcolemmal region, 7 days after denervation. By 28 days after denervation, the increase in RNA is distributed throughout the cytoplasm of the atrophic cells, non-atrophic cells show little or no increase. Prior to the increase in cytoplasmic RNA concentration, many myocyte nuclei exhibit enlarged nucleoli, indicative of active ribosomal RNA synthesis. Such active nucleoli are present throughout the 42 day study period. We conclude that both new RNA synthesis, and preferential accumulation of cytoplasmic RNA are prominent events in denervation atrophy. There is an ordered progression from nucleolar synthesis to subsarcolemmal accumulation to diffuse accumulation in atrophying cells. These observations complement biochemical findings of others which indicate an increase in both RNA and protein synthesis after denervation.

Neurotrophic regulation of rat muscle glucose-6-phosphate dehydrogenase in vitro

In organ culture of rat diaphragm, the presence of a 2-2.5 cm phrenic nerve stump delays the time of failure of miniature endplate potentials and eliminates the increase in glucose-6-phosphate dehydrogenase (G6PD) activity which otherwise occurs at 16.5 h in muscles cultured without nerve stumps. The nerve stump effect persists in the presence of blocking doses of D-tubocurarine but is eliminated by nerve crush. As shown by studies of amino acid incorporation into protein, the effect does not involved an overall change in protein synthesis. Effluents collected over 1-2 h from unstimulated or stimulated phrenic nerve-muscle preparations had no effect on G6PD activity when applied to muscles cultured without nerve stumps. However, medium conditioned by use in organ cultures with long nerve stumps partially countered the denervation-like effect in host cultures. Thus, the nerve maintains muscle G6PD by a humoral mechanism probably unrelated to impulse activity or nicotinic receptor activation.

Induction of extrajunctional acetylcholine sensitivity of rat EDL muscle by peptidic component of peripheral nerve

The induction of extrajunctional ACh-sensitivity was studied electrophysiologically 4 days after implantation of segments of the sciatic nerve, segments of dorsal and ventral spinal roots and silastic plates, containing some components of peripheral nerve onto surface fibres of innervated rat extensor digitorum longus muscle. It has been found that extrajunctional ACh-sensitivity can be induced by a piece of rat sciatic nerve and by peptidic material of about 8,000 and 10,000 mol. wt. No sensitivity was induced by segments of ventral and dorsal roots, by a piece of rabbit sciatic nerve and by rat sciatic nerve sectioned 7 days prior to implantation. The active nerve fractions lost its inducing potency after pepsin pre-treatment and irradiation with UV-light. Peripheral nerve probably contains some specific factor, peptidic in nature, which is able to induce the ACh-sensitivity of muscle fibre membrane.