Elimination of a Single Axon Changes Synaptic Characteristics of the Other Axons Innervating a Muscle Fibre of Lobster

Abstract

The neuromuscular system of crustaceans is simple enough to allow the study of plastic changes and interactions between neurons innervating the same postsynaptic cell. Each muscle fibre may be innervated by two or more excitatory axons and one or two inhibitory axons (Atwood 1977). The excitatory transmitter is glutamate (Kawagoe et al. 1982), and the inhibitory one γ-amino butyric acid (GABA) (Otsuka et al. 1966). The axons innervate each muscle fibre at many points (multiterminal innervation) and the presynaptic as well as postsynaptic properties of the synapses were studied in great detail (Dudel and Kuffler 1961 a,b; Atwood 1977; Dudel 1981; Parnas H et al. 1982, 1986; Parnas I et al. 1982 b,c, 1986; Dudel et al. 1983; Franke et al. 1986). Modulation of release can be produced by a variety of treatments such as long-term stimulation (Atwood and Wojtowicz 1986), applications of modulators (Kravitz et al. 1985), changes occurring during development (Atwood and Kwan 1976) or after microlesions (Parnas I et al. 1982a). Thus, the neuromuscular system of crustaceans, although being more complicated than the frog neuromuscular junction (see Pecot-Dechavassine 1986), is still simple enough to enable the study of changes that occur after partial denervation of excitatory or inhibitory inputs. For example, if the muscle is denervated by the removal of one excitatory axon, will the fibre show supersensitivity as found in the frog after denervation? If the inhibitory axon is removed, will changes occur in the remaining excitatory axons innervating the same cell? If an axon with a “strong” synaptic input is eliminated, will the weaker axon become stronger? Such questions certainly cannot be answered in the vertebrate neuromuscular system, and are very difficult to analyze in the vertebrate CNS.