Effects of L-arginine on the afferent resting activity in the cephalopod statocyst

The effects of bath application of the nitric oxide (NO) precursor L-argini ne (L-ARG) on the resting activity (RA) of afferent crista fibers were stud ied in isolated statocysts of the cuttlefish Sepia officinalis under variou s experimental conditions. L-ARG (threshold 10(-7) M) had three different e ffects: inhibition, excitation, and excitation followed by an inhibition; o nly the inhibitory effect of L-ARG was dose-dependent. D-Arginine (D-ARG) h ad no effect. When the preparation was pre-treated with NO synthase inhibit ors (N-G-Nitric-L-arginine methyl ester HCl (L-NAME), N-G-Nitro-L-arginine (L-NOARG)), both the inhibitory and the excitatory effects of L-ARG signifi cantly decreased at higher concentrations (10(-5 to -4) M), or were complet ely blocked at lower concentrations (10(-7) to (-6) M), of L-ARG. When the preparation was pre-treated with guanylate cyclase inhibitors (1H-[1,2,4]ox adiazolo[4,3,-a]quinoxalin-1-one (ODQ), methylene blue (M-BLU), cystamine ( CYS)), L-ARG had only excitatory effects, whereas its effects were only inh ibitory when the preparation was pre-treated with adenylate cyclase inhibit ors 2',3'-dideoxyadenosine (DDA), MDL-12330A (MDL), nicotinic acid (NIC-A)) . L-ARG had no effects when the pre-treatment was with a guanylate cyclase inhibitor and an adenylate cyclase inhibitor combined; in that situation, t he RA of the afferent fibers remained. These data indicate that in cephalop od statocysts, a cGMP and a cAMP signal transduction pathway (presumably vi a the generation of NO) are responsible for the effects of L-ARG on the RA of crista afferent fibers. They also indicate that the L-ARG-cGMP pathway i s the dominant pathway and is inhibitory, and that both pathways have only modulatory effects on, but are not essential for, the generation of the RA. (C) 1999 Elsevier Science B.V. All rights reserved.