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

The incidence of focal polyneuronal innervation was measured at end-plates in normal and reinnervated sartorius muscles from Rana pipiens either in the presence of curare to partially block neuromuscular transmission or in normal Ringer solution after pre-treatment with formamide to block excitation-contraction coupling. The incidence of polyneuronal innervation estimated with curare in normal muscles during the winter (19%) was significantly higher than the incidence in summer (9%). After formamide treatment, large end-plate potentials (e.p.p.s) and miniature e.p.p.s (m.e.p.p.s) could be recorded in Ringer solution containing physiological levels of Ca2+. For both normal and reinnervated muscles, higher levels of polyneuronal innervation could be recorded in normal Ringer solution after formamide treatment compared with levels measured in the presence of curare. This was true whether randomly selected end-plates or the same group of identified end-plates were examined before and after formamide treatment. Quantal content in normal Ringer solution containing 1.8 mM-Ca2+ was measured as the ratio of mean e.p.p. amplitude divided by mean m.e.p.p. amplitude. In randomly selected samples of normal end-plates the range of quantal contents seen was 18-112 (mean 50). For reinnervated end-plates 60 days after nerve crush, quantal contents were 6-181 (mean 54). It was concluded that formamide pre-treatment is a more sensitive technique for estimating the incidence of focal polyneuronal innervation and that, with appropriate controls, it can be used to measure quantal transmitter release accurately without using synaptic blocking agents.