The contribution of nitric oxide to vestibular compensation: are there species differences?

Nitric oxide (NO) has been implicated in the processes by which animals recover from peripheral vestibular damage ("vestibular compensation"). However, there is little systematic data available on the effects of NO inhibition on the vestibular compensation process. In the present study we administered the nitric oxide synthase (NOS) inhibitor NG-nitro-L-arginine methyl ester (L-NAME) using a subcutaneous osmotic minipump and examined its effects on the compensation of spontaneous nystagmus (SN), yaw head tilt (YHT) and roll head tilt (RHT) in guinea pigs. Following unilateral labyrinthectomy (UL), treatment with 5, 10, 50 or 100 mM L-NAME had no effect on the expression of any of these symptoms or their rate of compensation. By contrast, pre-UL treatment with 100 mM L-NAME resulted in a decrease in SN frequency at 10 h post-UL and an increase in its rate of compensation. Lower concentrations had no effect on SN. Pre-UL treatment with L-NAME had no significant effect on YHT or RHT at any particular time point. Analysis of NOS activity demonstrated that the highest concentration of L-NAME inhibited NOS activity in the contralateral vestibular nucleus complex, bilateral cerebellum and bilateral cortices. These results suggest that L-NAME may have different effects on vestibular compensation in guinea pigs compared to other species, such as the rat and frog.