Nitric oxide modulates a swimmeret beating rhythm in the crayfish

Abstract

Abstract The modulatory effects of nitric oxide (NO) and cAMP on the rhythmic beating activity of the swimmeret motor neurones in the crayfish were examined. The swimmerets are paired appendages located on the ventral side of each abdominal segment that show rhythmic beating activity during forward swimming, postural righting behaviour and egg ventilation in gravid females. In an isolated abdominal nerve cord preparation, swimmeret motor neurones were usually silent or showed continuous low frequency spiking activity. Application of carbachol, a cholinergic agonist, elicited rhythmic bursts of motor neurone spikes. The co-application of L-arginine, the substrate for NO synthesis with carbachol increased the burst frequency of the motor neurones. The co-application of the NO donor, SNAP, with carbachol also increased the burst frequency of the motor neurones. By contrast, co-application of a NOS inhibitor, L-NAME, with carbachol decreased beating frequency of the motor neurones. These results indicated that NO may act as a neuromodulator to facilitate swimmeret beating activity. The facilitatory effect of L-arginine was cancelled by co-application of a soluble guanylate cyclase (sGC) inhibitor, ODQ. These results indicated that NO acted by activating sGC to promote the production of cGMP. The application of L-arginine alone or a membrane-permeable cGMP analogue, 8-Br-cGMP, alone did not elicit rhythmic activity of motor neurones, but co-application of 8-Br-cGMP with carbachol increased bursting frequency of the motor neurones. Furthermore, application of the membrane-permeable cAMP analogue, CPT-cAMP alone produced rhythmic bursting of swimmeret motor neurones, and the bursting frequency elicited by CPT-cAMP was increased by co-application with L-arginine. The co-application of an adenylate cyclase inhibitor, SQ22536 ceased rhythmic bursts of motor neurone spikes elicited by carbachol. These results suggested that a cAMP system enabled the rhythmic bursts of motor neurone spikes and that a NO/cGMP signaling pathway increased cAMP activity to facilitate swimmeret beating activity.