Transient and permanent effects of androgen during synapse elimination in the levator ani muscle of the rat

Young male rats were castrated at 7 days of age, and treated with testosterone propionate daily from 7 to 34 days of age. At 13 months of age, motor axons and terminals innervating the levator ani (LA) muscle were stained with tetranitroblue tetrazolium (TNBT). The number of separate axons innervating individual muscle fibers was counted, and muscle fiber diameter was measured. Previous studies have shown that this androgen treatment increases muscle fiber diameter and delays synapse elimination, measured as (1) a greater percentage of muscle fibers innervated by multiple axons and (2) larger motor units. The present results indicate that the androgenic effect on synapse elimination is permanent, in that high levels of multiple innervation persisted for 12 months after the end of androgen treatment. In contrast, the effect on muscle fiber diameter was not maintained for this period. This dissociation of androgenic effects on the pattern of innervation from androgenic effects on muscle fiber diameter offers further evidence that the androgenic maintenance of multiple innervation is not dependent on muscle fiber size. In addition, circulating testosterone levels were measured at 50 and 60 days of age in animals similarly treated with androgen or oil from 7 to 34 days of age. By 60 days of age, testosterone levels in hormone-treated animals had dropped below detectability, comparable to levels in oil-treated controls. This provides additional evidence that androgen treatment during juvenile development can have permanent effects on the adult pattern of innervation in the LA muscle.     

The locust abdominal muscle receptor organ: response characteristics and its role in the control of segmental distance

A single mutipolar receptor cell is located at the dorsal edge of the lateral internal dorsal muscle in each abdominal segment of the locust (Locusta migratoria). Muscle and receptor cell form the abdominal muscle receptor organ. The receptor cell monitors length changes in the intersegmental muscle, and as a consequence also detects the length of an abdominal segment (cuticule and intersegmental membrane).The muscle receptor organ responds in a phasictonic fashion. The phasic component encodes the rate of change in the stimulus independent from the prevailing length of the muscle receptor organ. The tonic component monitors the absolute length of the muscle.Stimulation of a single muscle receptor organ leads to reflex effects on the ipsilateral longitudinal muscles in at least three adjacent segments. Muscles that shorten the abdomen are activated while their extending antagonists receive reduced activity.The reflex activation of the muscles is polysynaptic. Monosynaptic connections between the receptor and the motoneurones were not found.We identified an interneurone that receives monosynaptic input from the muscle receptor organs in at least three adjacent segments. The interneurone excites motorneurones to the longitudinal muscles of the next posterior segment.Abbreviations aMROII abdominal muscle receptor interneurone 1 - AS3 third abdominal segment - AS4 fourth abdominal segment - AS5 fifth abdominal segment - AS6 sixth abdominal segment - EPSP excitatory postsynaptic potential - MN median nerve - MR multipolar receptor cell - MRO muscle receptor organ - N1 tergal nerve - N2 sternal nerve     

Correlation of electrophysiological,histochemical, and mechanical properties in fibres of the coxa rotator muscle of the locust,Locusta migratoria

Summary The fibre composition of the anterior coxa rotator muscle of the locust middle leg (M92) was examined. The muscle is composed of 90–100 fibres. Muscle fibres were characterized with regard to innervation pattern, electrophysiological properties, and morphological parameters. Activity and isoenzyme composition of myofibrillar ATPase, succinic acid dehydrogenase (SDH) activity and glycogen content were examined employing histochemical techniques. Shortening velocity and the dependence of tension on intracellular Ca²⁺ were determined in skinned fibre experiments. A close match was observed between the innervation pattern of the muscle fibres and their histochemical and physiological properties. The combination of all parameters examined allowed an accurate classification of the muscle fibres into three types. Within a given type, broad variability of some properties was observed (SDH activity, Ca²⁺ sensitivity) while others assumed distinct values (innervation pattern, shortening velocity). The comprehensive characterization of muscle fibre properties permits a functional interpretation of fibre heterogeneity with regard to muscle performance. Fibres with the same innervation pattern may be recruited specifically, according to their electric properties and Ca²⁺ sensitivities. The resulting specific recruitment of fibres with different mechanical responses should allow a subtle control of muscular force, with regard to force amplitude, temporal characteristics of contraction, and metabolic cost.Abbreviations CI₁ common inhibitory neurone one - ejp ijp excitatory, inhibitory junctional potential - EGTA ethylene glycol-bis[-aminoethyl ether] N,N,N,N-tetraacetic acid - mATPase myofibrillar adenosinetriphosphatase - MOPSO 3-[N-morpholino]-2-hydroxypropanesulfonic acid - M92 anterior rotator muscle of the coxa - n Hill coefficient - pCa₅₀ pCa corresponding to half-maximal tension - P₀ maximal isometric tension - SDH succinic acid dehydrogenase - V _max maximal shortening velocity     

Comparative studies on the mode of action of proctolin and phorbol-12,13-dibutyrate in their ability to contract the locust mandibular closer muscle.

The role of proctolin has been further investigated in the locust (Locusta migratoria) mandibular closer muscles. Radioactive calcium uptake measurements were made using protease-dissociated muscle cells. Both the phorbol ester, phorbol-12,13-dibutyrate, and proctolin produce tonic contractions which are associated with the influx of extracellular calcium. The thresholds for proctolin and the phorbol ester to contract the muscle were 1-10 nM and 10-100nM, respectively, while their respective thresholds for evoking measurable calcium influx into the muscle cells were 0.1-1 nM for proctolin, and 0.1-1 pM for phorbol-12,13-dibutyrate. The effect of phorbol-12,13-dibutyrate is blocked by a number of protein kinase inhibitors (at a concentration of 0.1 mM), suggesting that an activation of a protein kinase can lead to calcium influx. These inhibitors, however, do not block the effect of proctolin, indicating that these two compounds work through different pathways, possibly converging on the same final target. In light of this finding, a number of other compounds have been tested to try to ascertain how proctolin mediates an increased calcium influx.     

Migration of myogenic cells in the rat extensor digitorum longus muscle studied with a split autograft model

Summary The ability of myogenic cells to migrate perpendicular to the long axis of freely autografted muscles was examined. Rat extensor digitorum longus muscles were divided, and one half was devitalized by repeated freezing in liquid nitrogen while the other half was kept viable in physiologic saline. The halves were reunited with sutures and grafted back into the original muscle bed. At intervals between 5 and 25 days the grafts were removed and examined histologically for the presence of myotubes within the devitalized region. Myotubes were first seen in the devitalized half 10 days postgrafting with the maximum number of myotubes observed after 12 to 15 days. These results indicate that myogenic cells are capable of migration perpendicular to the long axis of the muscle fibers in an autograft.     

Morphology and organization of muscle fibres in the thoracic coxal muscle receptor organ and the associated promotor muscle, in a crayfish, Cherax destructor, and mud crab, Scylla serrata

Using confocal laser scanning and conventional light microscopy, the morphology and organization of the muscle fibres in a proprioceptor, the thoracic coxal muscle receptor organ (TCMRO), and the associated 'extrafusal' promotor muscle were investigated in two species of decapod crustacea, the crayfish Cherax destructor and the mud crab Scylla serrata . The diameter of the TCMROs was shown to increase distally, with an increase up to 350% recorded for the crayfish. The tapered shape of the crayfish TCMRO was demonstrated to amplify movements mechanically at the transducer region where the afferent nerves attach. Serial sectioning of the TCMROs, showed that the fibre number increased in the proximal to distal direction from 14 to 30 fibres in the crayfish and from 7 to 20 in the crab. Optical sectioning with the laser scanning confocal microscope revealed that the increase in fibre numbers was the result of muscle fibres branching in the distal third section of the TCMRO. The percentage of muscle tissue in the cross-sectional area in the TCMRO was found to be only 35.2% and 64.6% in the crayfish and crab, respectively. Longitudinal sectioning using laser scanning confocal microscopy revealed the average sarcomere length of the TCMRO muscle fibres of both species to be in the intermediate range for crustacean muscle fibres (4.1 ± 0.1 µm and 4.55 ± 0.34 µm for the crayfish and crab) compared with the long sarcomere muscle fibres in the associated promotor muscles (7.87 ± 0.2 and 10.6 ± 0.6 µm). The distinct morphology of the TCMRO muscle fibres – smaller diameter, intermediate sarcomere length and branching of fibres compared to the larger, long sarcomere promotor fibre muscle fibres – suggest that the TCMRO muscle fibres are specialized in their role of proprioception.     

Neuroanatomical,immunocytochemical, and physiological studies of the pharyngeal retractor muscle and its putative regulatory neurons playing a role in withdrawal and feeding in the snail, <Emphasis Type="Italic">Helix pomatia</Emphasis>

We describe the neurons regulating two separate functions of the pharyngeal retractor muscle (PRM), namely sustained contraction during body withdrawal and rhythmic phasic contractions during feeding, in the snail, Helix pomatia. The distribution of central neurons innervating the PRM is organized into two main units; one in the buccal-cerebral ganglion complex, the other in the subesophageal ganglion complex. Serotonin- (5-HT-), FMRFamide- (FMRFa-), and tyrosine-hydroxylase-immunostained neurons are present among the PRM neurons that densely innervate the PRM. 5HT both decreases and increases the amplitude of the electrically evoked contraction between concentrations of 0.1 M and 1 M. Dopamine (DA) only decreases the amplitude of contraction at a 1-M threshold concentration. In contrast, FMRFa increases the amplitude of the contraction and slightly elevates the tone of the PRM but requires a higher threshold (10 M). Assay by high-performance liquid chromatography of 5HT and DA in the PRM has shown that the 5HT level decreases during locomotion but increases during feeding, whereas the DA level increases during locomotion but slightly decreases during feeding. Thus, different segments of the PRM are innervated by neurons from different loci within the central nervous system. The segments of the PRM distal to the pharynx are innervated from loci of the subesophageal ganglion complex suggesting that they mediate withdrawal. The proximal segment of the PRM is innervated from cerebral and buccal loci indicating that these neurons mediate the feeding rhythm produced by buccal and cerebral feeding central pattern generators to induce rhythmic phasic contractions in the PRM during feeding.This work was supported by Hungarian Scientific Research Fund (OTKA) grants (T034106, T037389, T037505), the Wellcome Trust CRIG Programme, and the Wellcome Trust Travel Grant.