Spontaneous quantal and subquantal transmitter release at the Torpedo nerve-electroplaque junction

Abstract

Focal electrodes were used to record the spontaneous miniature potentials generated on delimited patches of innervated membrane in the Torpedo electric organ. The main population of miniature potentials followed a bell-shaped amplitude distribution. In addition, we observed a second class of spontaneous events that were smaller and whose amplitude distribution was skewed. These subminiatures formed an homogenous population together with the regular miniatures with respect to their time course versus amplitude relationship. They were thus probably generated at the same sites. The proportion of potentials that were subminiature was less than 10% in resting, freshly excised tissue, but it increased markedly: (i) when the tissue was kept for 24-28 h in vitro after excision; (ii) in the period following a brief heat challenger or (iii) stimulation to exhaustion; and (iv) in the presence of dinitrophenol or dinitrofluorobenzene. In all these conditions, we measured the acetylcholine, adenosine 5'-triphosphate and creatine phosphate content of the tissue and found a correlation between the relative number of subminiature potentials and the lack of energy rich molecules. It is concluded that subminiature potentials are present in the electric organ as in neuromuscular junctions. They are probably produced at the same sites as the regular miniature potentials and their relative occurrence seems to increase greatly when the nerve terminals are in a state of energy deficiency.