Synaptic repression at crayfish neuromuscular junctions. II. Evidence for calcium involvement

Abstract

The inability of synaptic junctions to generate normalisized postsynaptic potentials under normal physiological conditions was studied at crayfish neuromuscular synapses. Synaptic repression in the superficial flexor muscle system of the crayfish was induced by surgery: the nerve was cut in the middle of the target field, and the lateral muscle fibers were removed. After this surgery, the remaining medial synapses were unable to generate normal-sized junction potentials (jp) over the medial muscle population. In an attempt to study the mechanisms underlying this response, we varied the extracellular calcium concentration of the Ringers solution bathing the preparation, in both repressed and control animals, while monitoring the size of the same junction potential. The junction potential generated by the spontaneous activity of the nerve increased in size with increasing calcium concentrations in control animals, but failed to do so in repressed animals, that is, changes in external calcium concentrations did not affect repressed synapses. However, in the presence of the calcium ionophore A23187, control and repressed synapses both show an increase in the junction potential sizes they generate. Our data suggest that calcium is involved in the mechanisms that underlie synaptic repression in this crustacean neuromuscular system.

Depressing Effects of Caffeine at Crayfish Neuromuscular Synapses I. Dosage Response and Ca++ Gradient Effects

“…These two caffeine concentrations were chosen, based on the dosage response experiments, as representing the low-and high-end spectrum of caffeine effects in this system. The 30 and 200% calcium Ringers were chosen, based on previous experiments in this system (Gupta et al, 1993), as representative of the effects of low and high external calcium on these terminals. Normal (100%) calcium Ringers contained 14 mM CaCl 2 , while 30% calcium Ringers had 4.2 mM CaCl 2 and 200% calcium Ringers was made using 28 mM CaCl 2 (for details on adjustments made to other salts in the 30 and 200% Ringer solutions, see Gupta et al, 1993).…”

“…2. To test whether a single exposure to a different calcium concentration was able to affect how these terminals respond to caffeine, the terminals were first exposed for 10 min to 117% calcium Ringers (made with 16 mM CaCl 2 , Gupta et al, 1993) before changing the solution to one containing 5 mM caffeine, recording JPs as before. The data were then compared to the responses of a new set of animals exposed only to normal Ringers before exposure to 5 mM caffeine.…”

“…This calcium concentration was chosen as one which these terminals respond to and quickly recover from, returning to control JP sizes (Gupta et al, 1993). To test how normal terminals respond to changes in calcium, JPs generated by axon 6 were recorded as before for 6 min in normal Ringers.…”

“…The superficial flexor muscle system (SFM) of the crayfish Procambarus clarkii has been used to study specificity of neuronal connections (Vélez and Wyman, 1978a,b), neuronal regeneration (Ely and Vélez, 1982;Hunt and Vélez, 1989a,b), synaptic repression (Prosser et al, 1993;Gupta et al, 1993) and neuronal transplants (Krause and Vélez, 1995). In this simple system of few cells with position-dependent connectivity, we had shown that during regeneration or nerve transplantation, the SFM neurons follow an intrinsic developmental program not affected by environmental disturbances.…”

“…We had previously shown that these terminals change their transmitter output as a result of diminished calcium entry when synaptic repression is triggered by experimental manipulations of the target (Gupta et al, 1993). Drugs that could alter calcium homeostasis in control animals, when applied to regenerated or transplanted terminals, could provide information on how well the calcium homeostatic mechanisms are reestablished after completion of synaptic contact formation.…”