Nitric oxide in the afferent synaptic transmission of the axolotl vestibular system

This study was performed using intracellular and multiunit extracellular recording techniques in order to characterize the role of nitric oxide in the afferent synaptic transmission of the vestibular system of the axolotl (Ambystoma tigrinum). Bath application of nitric oxide synthase inhibitors N-G-nitro-L-arginine (0.01 muM to 10 muM) and N-nitro-L-arginine methyl ester hydrochloride (0.1 muM to 1000 muM) elicited a dose-dependent decrease in the basal discharge of the semicircular canal afferent fibers. NG Nitro-L-arginine also diminished the response to mechanical stimuli. Moreover, NG-nitro-L-arginine (1 muM) produced a hyperpolarization associated with a decrease in the spike discharge and diminished the frequency of the excitatory postsynaptic potentials on afferent fibers recorded intracellularly. Nitric oxide donors were also tested: (i) S-nitroso-N-acetyl-DL-penicillamine (0.1 muM to 100 muM) increased the basal discharge and the response to mechanical stimuli. At the maximum effective concentration (100 muM) this drug affected neither the amplitude nor the frequency of the excitatory postsynaptic potentials. However, it slightly depolarized the afferent neurons and decreased their input resistance. (ii) 3-Morpholino-sydnonimine hydrochloride did not significantly affect the basal discharge or the mechanically evoked peak response of afferent neurons at any of the concentrations used (1 muM to 1000 muM). However, after 10 min of perfusion in the bath, 1 muM and 10 muM 3-morpholino-sydnonimine hydrochloride significantly modified the baseline of the mechanically evoked response, producing an increase in the mean spike discharge of the afferent fibers. These results indicate that nitric oxide may have a facilitatory role on the basal discharge and on the response to mechanical stimuli of the vestibular afferent fibers. Thus, nitric oxide probably participates in the sensory coding and adaptative changes of vestibular input in normal and pathological conditions. (C) 2001 IBRO. Published by Elsevier Science Ltd. All rights reserved.