Electrical signals that begin in the brain and reach nerve terminals cannot directly stimulate muscle fibers to generate action potentials which subsequently initiate contraction. However, action potentials arriving in the nerve terminals stimulate the release of a chemical transmitter that diffuses across the space between the nerve terminals and muscle fiber and reacts with the membrane of the muscle fiber which in turn initiates the electrical response in the muscle fiber. In the neuromuscular junction this chemical is acetylcholine (AcH). Quanta are released spontaneously with little influence; in response to the action potential in the presynaptic terminal, about 100 quanta are released, and these initiate the action potential in the muscle fiber. The electric organ of the skate is similar to muscle and has the advantage of providing large amounts of nerve terminals. Consequently, the electric organ provides the ideal tissue to study the biochemical, physiological, as well as, anatomical properties of transmitter release. The nerve terminals are readily separated from the other cells so that the calcium channels (located in the membranes) can be electrophysiologically studied and biochemically isolated. In recent years, Kriebel and co-workers have shown that the quantum of transmitter release is composed of 10 subunits. This finding has been confirmed in mammals, amphibians, and electric fish. An important question is the morphological and/or molecular basis of the the subunit. The subunit could represent the release of transmitter from one vesicle or each vesicle or each vesicle could contain 10 subunits. The mammalian and amphibian neuromuscular junctions have been well-suited for electrophysiological studies but are not suited for biochemical studies because of the relatively small amount (less than 0.01%) of nerve terminals available. On the other hand, the Torpedo electric organ (an electric fish) has been the standard preparation for biochemical studies of transmitter release because of its rich supply of nerve terminals (3% of the tissue). However, the Torpedo preparation has been difficult to use for electrophysiological studies. The new skate electric organ preparations are ideal because this tissue is rich in nerve terminals and isolated terminals and electric cells (which have been derived embryologically from muscle cells) can be separated with enzymatic treatment. Electrophysiological studies on the skate have shown that subunits of the quantum appear just after hatching and that several days are required for the subunits to be combined into quanta for release. The purpose of the proposed research involving the skate electric organ is to study the molecular basis of the subunit composing the quantum, the excitation-secretion process and the calcium channels in the presynatic terminals. The skate electric organ provides a unique preparation for basic studies of transmitter release common to vertebrates.
