Modulatory effects of nitric oxide on synaptic depression in the crayfish neuromuscular system

A characteristic physiological property of the neuromuscular junction betwe en giant motor neurones (MoGs) and fast flexor muscles in crayfish is synap tic depression, in which repetitive electrical stimulation of the MoG resul ts in a progressive decrease in excitatory junction potential (EJP) amplitu de in flexor muscle fibres, Previous studies have demonstrated that L-argin ine (L-Arg) modulates neuromuscular transmission. Since L-Arg is a precurso r of nitric oxide (NO), we examined the possibility that NO may be involved in modulating neuromuscular transmission from MoGs to abdominal fast flexo r muscles, The effect of a NO-generating compound, NOC7, was similar to that of L-Arg, reversibly decreasing the EJP amplitude mediated by the MoG, While NOC7 re duced the amplitude of the EJP, it induced no significant change in synapti c depression. In contrast, a scavenger of free radical NO, carboxy-PTIO, an d an inhibitor of nitric oxide synthase, L-NAME, reversibly increased the E JP amplitude mediated by MoGs, Synaptic depression mediated by repetitive s timulation of MoGs at 1 Hz was partially blocked by bath application of L-N AME. Bath application of a NO scavenger, a NOS inhibitor and NO-generating compounds had no significant effects on the depolarisation of the muscle fi bres evoked by local application of L-glutamate, The opposing effects on EJP amplitude of NOC7 and of carboxy-PTIO and L-NAM E suggest that endogenous NO presynaptically modulates neuromuscular transm ission and that it could play a prominent role at nerve terminals in elicit ing MoG-mediated synaptic depression in the crayfish Procambarus clarkii.