Effect of tetrodotoxin on spontaneous potentials at pigeon iris neuromuscular junctions

Abstract

Spontaneous junctional potentials (s.j.p.'s) were continuosly recorded intracellularly from the neuromuscular junctions of the pigeon iris before, during, and after tetrodotoxin (TTX) superfusion. Large potentials (8–18 mV) were observed prior to and during exposure to TTX. TTX (5 × 10 −10 to 4 × 10 −8 g/ml) produced an increase in frequency of the s.j.p.'s. The maximal effect was obtained after 3–5 min of drug superfusion. The frequency returned to control levels after a 15–30 min washout. After repetitive or prolonged exposure to TTX, there was a decrease in the drug effect (tachyphylaxis). Similar concentrations of TTX blocked the ciliary nerve conduction. In 30% of the cells tested, the s.j.p. frequency increase induced by TTX was associated with depolarization of the muscle fiber (2–10 mV). In a few muscle fibers with an abnormally high control frequency, the application of TTX produced an inhibitory effect. Two mechanisms responsible for these TTX effects are suggested: (1) changes on the membrane potential of nerve terminals; (2) direct influence on the transmitter release.