Soluble Guanylate Cyclase As the Key Enzyme in the Modulating Effect of NO on Metabotropic Glutamate Receptors

Abstract

The synaptic plasticity of the afferent synapse of the vestibular apparatus is defined by the dynamic interaction of ionotropic and metabotropic glutamate receptors and the modulators of synaptic transmission. It was shown that nitric oxide modulates iGluR responses. In this paper, the effect of NO on the function of the afferent synapse mGluR was investigated. Inhibitor of nitric oxide synthase lowered the level of background activity but increased the amplitude of the responses of groups I and II mGluR agonist ACPD. Donor NO SNAP increased the level of background activity. Short-term perfusion of the synaptic region with low concentrations of SNAP led to a decrease in the amplitude of the answers of mGluR agonists ACPD and DHPG. The inhibitory effect of the NO donor was eliminated under blockade of soluble guanylate cyclase with a specific inhibitor ODQ. A prolonged application of NO did not cause a statistically significant change in the amplitude of the ACPD response. However, SNAP at concentrations of 10 and 100 μM increased the amplitude of the mGluR agonist responses 30 and 15 minutes, respectively, after termination of the NO donor exposure. The obtained data show the multidirectional effect of NO on the function of mGluR and testify to the existence of a complex modulating mechanism of the afferent flow from vestibular organs to the central nervous system.

Keywords: metabotropic glutamate receptors; nitric oxide; soluble guanylate cyclase; synaptic plasticity; vestibular apparatus.

Fig. 1

Effects of long-term application of different concentrations of NO donor SNAP on the background activity in posterior semicircular canal nerve fibers of a frog. A – original recording of firing activity in the same experiment. Ordinate: spike frequency, imp/s; abscissa: time, s. The horizontal lines above the recording indicate the duration of drug applications. B – dose-response curves of the positive (1) and the negative (2) response waves for NO donor SNAP. N=5–6

Fig. 2

Modification of the ACPD-evoked response before, combined and a short-time impact of NO donor SNAP (ACPD 100 μM, SNAP 1 μM). A – time course of firing activity in a typical experiment. The designations are the same as in Fig 1A. B – diagram of the decrease of the ACPD-evoked response combined short-term SNAP application. Abscissa: left to right – ACPD-control; SNAP+ACPD; ACPD recovery after 15 min in a normal solution. Ordinate: value of the response to ACPD, %, mean ±SEM. (ANOVA F(2.18) = 3.9, p = 0.039, Post hoc test p = 0.03)

Fig. 3

Effects of NO donor SNAP (1 μM) on the amplitude of mGluR I agonist DHPG (200 μM). A – typical recording of the experiment. The designations are the same as in Fig. 1 A,B – bar graph of reversible suppression of the DHPG-evoked response before, during and after SNAP application. Abscissa: left to right: DHPG – control; SNAP+DHPG; DHPG recovery in 15 min in a normal solution. Ordinate: mean ±SEM for amplitude of DHPG-induced responses (%). (Wilcoxon p = 0.025)

Fig. 4

Effects of a prolonged application of NO donor SNAP on the amplitude of group I and II mGluR agonist ACPD (50 μM). A – original recording (SNAP 10 μM). The designations are the same as in Fig. 1 A,B – the increase in the amplitude of ACPD-evoked responses after prolonged exposure of different concentrations of NO donor SNAP in the vestibular. Abscissa: left to right: 1 – control; 2 – over 5-min application of SNAP; 3 – recovery 15 min after the end of drug application in normal solution; 4 – recovery in 30 min in a normal solution. Ordinate: mean ±SEM for the amplitude of the ACPD-induced response (%). SNAP 10 μM (ANOVA F(3.18) = 4.4, p = 0.017, Post hoc test p = 0.047) and SNAP 100 μM (ANOVA F(2.16) = 4.58, p = 0.027, Post hoc test p = 0.027)

Fig. 5

Effects of the specific inhibitor of sGC ODQ on SNAP (1 μM) suppression of the ACPD-induced response. A, B – multiunit recording of firing activity in the posterior semicircular canal nerve before (A) and after (B) a 20-min perfusion on vestibular synapse by 15 μM ODQ. The designations are the same as in Fig 1A. C – lack of suppression of ACPD-evoked responses after inhibition of sGC by ODQ. Abscissa: left to right: control; ACPD against SNAP; recovery 15 min after the end of drugs application in a normal solution; the same steps against ODQ application. Ordinate: mean ±SEM for the amplitude of the ACPD-induced response (%)