Sprouting and regression of the nerve at the frog neuromuscular junction in normal conditions and after prolonged paralysis with curare

Abstract

A light microscopical, histochemical and electron microscopical investigation of the frog neuromuscular junction has been performed on muscles from animals in different functional states of activity. The combined staining of axon terminals and cholinesterase (ChE) allows a precise description of the nerve terminal arborization and its synaptic contacts. Most terminal arborizations form long continuous contacts with the muscle cell. Distinquishable from these are nerve branches (usually of small diameter)d or distal endings of branches with one or several small and isolated contacts. It is assumed that these are sprouts with newly-formed synaptic sites. Other sprouts end without apparent synaptic contact. At the uttrastructural levet, nerve sprouts end without apparent synaptic contact. At the uttrastructural levet, nerve sprouts growing into empty, well-differentiated synaptic gutters or inducing the formation of new synaptic sites were observed. In other sites, ChE is apparently located at postsynaptic gutters with no nerve present. Similarly, in the electron microscope, well-differentiated synaptic gutters lacking any nerve or Schwann cell elements were observed. In addition, synaptic gutters only partially occupied by the nerve were frequently seen. These features have been interpreted as signs of regression of the nerve terminals. Nerve regression and sprouting were found in animals chronically paralysed with curare over several weeks as well as in untreated frogs (winter and summer frogs, laboratory frogs, fed and unfed). When quantitatively evaluating the occurence of presumed features of nerve sprouting and nerve regression, differences were found between different experimental groups. From this it is concluded that, in addition to developmental changes, the degree of nerve sprouting and regression is controlled by external factors such as muscle activity and seasonal variations. Signs of sprouting and nerve regression can be simultaneously present in a single synapse. It appears that the frog neuromuscular synapse is not a static structure, but is in a state of permenent remodelling.

Precision of Reinnervation and Synaptic Remodeling Observed in Neuromuscular Junctions of Living Frogs

“…In adult mice, the loss of AChRs from parts of the junction renders such sites unattractive to innervation (Rich and Lichtman, 1989;Balice-Gordon and Lichtman, 1994). Long-term denervated frog junctions also show a progressive decline in agrin (Reist et al, 1987) and cholinesterase (Wernig et al, 1980;.…”

“…It is possible that some of these new sites previously were innervated, but terminals retracted before the first observation. However, terminal retraction is relatively infrequent in normal junctions, as evidenced both by histology (Wernig et al, 1980) and repeated observation . We conclude, therefore, that most of these new sites were innervated de novo.…”

Effect of Sciatic Nerve Transection on acetylcholinesterase activity in spinal cord and skeletal muscles of the bullfrog Lithobates catesbeianus

“…However, other factors should be considered, such as seasonal temperature changes. In Rana temporaria, signs of nerve sprouting and regression are infrequent in winter and frequent in summer (Wernig et al, 1980). In our study, AChE activity was measured in summer, when the mean ambient temperature was 26.5 °C, whereas in Pytasz et al (1977) study it was measured in frogs maintained at 15-18 °C.…”

Polyneuronal innervation and quantal transmitter release in formamide‐treated frog sartorius muscles.

“…It is also likely that seasonal variation might contribute since, in confirmation of our earlier observations , present results show that polyneuronal innervation is higher in the winter. This higher incidence of multiple component e.p.p.s in winter may be the physiological correlate of the higher incidence of sprouting seen in winter Rana temporaria (Wernig, Pecot-Dechavassine & Stover, 1980). The results obtained with formamide-treated muscles, however, suggest that all estimates of polyneuronal innervation obtained with curare may be underestimates.…”