Opposing actions of nitric oxide on synaptic inputs of identified interneurones in the central nervous system of the crayfish

Little is known of the action of nitric oxide (NO) at the synaptic level on identified interneurones in local circuits that process mechanosensory sig nals. Here, we examine the action of NO in the terminal abdominal ganglion of the crayfish Pacifastacus leniusculus, where it has modulatory effects o n the synaptic inputs of 17 identified ascending interneurones mediated by electrical stimulation of a sensory nerve. To analyse the role of NO in the processing of sensory signals, we bath-applied the NO donor SNAP, the NO s cavenger PTIO, the nitric oxide synthase (NOS) inhibitor L-NAME, the NOS su bstrate L-arginine, a cyclic GMP (cGMP) analogue, 8-Br-cGMP, and the solubl e guanylate cyclase (sGC) inhibitor ODQ. The effects of these chemicals on the synaptic inputs of the interneurones could be divided into two distinct classes. The NO donor SNAP enhanced the inputs to one class of interneuron e (class 1) and depressed those to another (class 2). Neither the inactive isomer NAP nor degassed SNAP had any effect on the inputs to these same cla sses of interneurone, The NO scavenger PTIO caused the opposite effects to those of the NO donor SNAP, indicating that endogenous NO may have an actio n in local circuits. Preventing the synthesis of NO using L-NAME had the op posite effect to that of SNAP an each response class of interneurone, Incre asing the synthesis of endogenous NO by applying L-arginine Led to effects on both response classes of interneurone similar to those of SNAP, Taken to gether, these results suggested that NO was the active component in mediati ng the changes in amplitude of the excitatory postsynaptic potentials. Fina lly, the effects of 8-Br-cGMP were similar to those of the NO donor, indica ting the possible involvement of a NO-sensitive guanylate cyclase, This was confirmed by preventing the synthesis of cGMP by sGC using ODQ, which caus ed the opposite effects to those of 8-Br-cGMP on the two response classes o f interneurone, The results indicate that a NO-cGMP signal transduction pat hway, in which NO regulates transmitter release from mechanosensory afferen ts onto intersegmental ascending interneurones, is probably present in the local circuits of the crayfish.