Alterations in neuromuscular junction morphology during fast-to-slow transformation of rabbit skeletal muscles

Abstract

Chronic low frequency stimulation of motor nerves results in transformation of muscle fibre phenotype from fast- to slow-twitch. We examined the light and electron microscopic structure of neuromuscular junctions in normally fast twitch muscles, tibialis anterior and extensor digitorum longus of rabbit after 3 weeks of stimulation to determine whether synaptic structure is also modified during fibre type transformation. Neuromuscular junctions of stimulated and unstimulated (control) tibialis anterior and extensor digitorum longus muscles and unstimulated slow twitch soleus muscle were visualized with rhodamine-conjugated alpha-bungarotoxin. Video light microscopic images of neuromuscular junctions were digitized to allow quantification of their surface areas, perimeters, lengths and widths. Three weeks of stimulation resulted in a decrease in the maximal velocity of muscle fibre shortening and augmentation of mitochondrial volume in fast muscles, demonstrating the efficacy of the stimulation protocol employed in altering muscle fibre phenotype. Neuromuscular junctions of control tibialis anterior and extensor digitorum longus are thin, compact, and continuous, with complex branching patterns. In contrast, those of slow-twitch soleus are thicker and discontinuous. Neuromuscular junctions in control tibialis anterior and extensor digitorum longus are larger than those in soleus. Three weeks of stimulation causes a marked decrease in the size of neuromuscular junctions in tibialis anterior and extensor digitorum longus, as reflected in the significant reduction in neuromuscular junction surface area, length and width. Electron microscopy of these junctions suggests that secondary postsynaptic folds in stimulated muscles are more closely spaced. Also, axon terminals of stimulated muscles appear to contain more densely packed synaptic vesicles and mitochondria than controls. Decreases in neuromuscular junction dimensions can be partly explained by muscle fibre atrophy. However, the decrease in neuromuscular junction size is proportionately greater than that of muscle fibre diameter in both muscles, indicating that factors other than fibre atrophy may contribute to the reduced neuromuscular junction size in stimulated muscles. Neuromuscular junctions of stimulated tibialis anterior and extensor digitorum longus muscles exhibit some features characteristic of normal soleus neuromuscular junctions, indicating structural adaptations consistent with the altered muscle fibre phenotype. On the other hand, neuromuscular junctions of 3 week stimulated tibialis anterior and extensor digitorum longus and their synaptic branches remain as thin and continuous as those of unstimulated controls, suggesting that the transformation of neuromuscular junctions towards a morphology characteristic of slow muscle, is only partial. These results demonstrate that an altered pattern of impulse activity cause significant synaptic remodelling in adult rabbit skeletal muscles.

Long‐term in vivo modulation of synaptic efficacy at the neuromuscular junction of Rana pipiens frogs

“…Furthermore, the transformation induced by chronic nerve stimulation has been correlated with these features (Lnenicka et al 1986;Wojtowicz et al 1994;Somasekhar et al 1996). However, it was concluded at the crayfish NMJ that structural or ultrastructural differences could not explain the large difference in synaptic output of tonic and phasic NMJs (King et al 1996;Msghina et al 1998Msghina et al , 1999.…”

“…Conversely, the seasonal increases in activity of crayfish phasic terminals induces a more tonic-like state (Lnenicka & Zhao, 1991). Comparable synaptic changes were obtained in many different species following chronic electrical stimulation of phasic nerve terminals (Lnenicka & Atwood, 1985;Hinz & Wernig, 1988;Somasekhar et al 1996;Reid et al 2003). Furthermore, morphological transformations, such as vesicle density, NMJ and mitochondrial size were also described (Lnenicka et al 1986;Somasekhar et al 1996).…”

Late onset muscle plasticity in the whisker pad of enucleated rats

“…Metabolic properties, myosin isoform subtypes and plate size in striate muscles are all correlated positively (22)(23)(24). In general, oxidative slow twitch type I muscle fibers have larger motor plates than those observed in association with glycolytic fast twitch type II muscle fibers (25)(26)(27); although see (28)(29)(30). We evaluated the area of the presynaptic and postsynaptic elements of the motor plate in control and enucleated rats at different ages (Fig.…”

In vivolong-term synaptic plasticity of glial cells

“…Also, sprouting of nerve terminals and PSC processes has been observed when synaptic activity was lowered (Snider & Harris, 1979; Wernig et al 1980; Wines & Letinsky, 1988; Diaz et al 1989; Diaz & Pecot‐Dechavassine, 1989; Tsujimoto et al 1990). In contrast, phasic nerve terminals adapt to higher patterns of neuronal activity by decreasing their synaptic efficacy (releasing less neurotransmitter) and increasing their resistance to synaptic depression (Lnenicka & Atwood, 1985; Hinz & Wernig, 1988; Lnenicka & Zhao, 1991; Mercier et al 1992; Wernig et al 1996; Somasekhar et al 1996; Cooper et al 1998; Reid et al 2003; Belair et al 2005). Changes in protein expression and morphological transformations have also been reported (Lnenicka et al 1986; Nguyen & Atwood, 1990; Somasekhar et al 1996; Cooper et al 1998).…”

“…In contrast, phasic nerve terminals adapt to higher patterns of neuronal activity by decreasing their synaptic efficacy (releasing less neurotransmitter) and increasing their resistance to synaptic depression (Lnenicka & Atwood, 1985; Hinz & Wernig, 1988; Lnenicka & Zhao, 1991; Mercier et al 1992; Wernig et al 1996; Somasekhar et al 1996; Cooper et al 1998; Reid et al 2003; Belair et al 2005). Changes in protein expression and morphological transformations have also been reported (Lnenicka et al 1986; Nguyen & Atwood, 1990; Somasekhar et al 1996; Cooper et al 1998). Thus, both presynaptic and postsynaptic as well as some morphological changes of PSCs were observed following alterations of synaptic activity.…”