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. 2020 Sep 1;124(3):962-972.
doi: 10.1152/jn.00243.2020. Epub 2020 Aug 20.

Activation of GABAB receptors results in excitatory modulation of calyx terminals in rat semicircular canal cristae

Yugandhar Ramakrishna  1   2 , Soroush G Sadeghi  1   3
Affiliations

Activation of GABAB receptors results in excitatory modulation of calyx terminals in rat semicircular canal cristae

Yugandhar Ramakrishna et al. J Neurophysiol. .

Abstract

Previous studies have found GABA in vestibular end organs. However, existence of GABA receptors or possible GABAergic effects on vestibular nerve afferents has not been investigated. The current study was conducted to determine whether activation of GABAB receptors affects calyx afferent terminals in the central region of the cristae of semicircular canals. We used patch-clamp recording in postnatal day 13-18 (P13-P18) Sprague-Dawley rats of either sex. Application of GABAB receptor agonist baclofen inhibited voltage-sensitive potassium currents. This effect was blocked by selective GABAB receptor antagonist CGP 35348. Application of antagonists of small (SK)- and large-conductance potassium (BK) channels almost completely blocked the effects of baclofen. The remaining baclofen effect was blocked by cadmium chloride, suggesting that it could be due to inhibition of voltage-gated calcium channels. Furthermore, baclofen had no effect in the absence of calcium in the extracellular fluid. Inhibition of potassium currents by GABAB activation resulted in an excitatory effect on calyx terminal action potential firing. While in the control condition calyces could only fire a single action potential during step depolarizations, in the presence of baclofen they fired continuously during steps and a few even showed repetitive discharge. We also found a decrease in threshold for action potential generation and a decrease in first-spike latency during step depolarization. These results provide the first evidence for the presence of GABAB receptors on calyx terminals, showing that their activation results in an excitatory effect and that GABA inputs could be used to modulate calyx response properties.NEW & NOTEWORTHY Using in vitro whole cell patch-clamp recordings from calyx terminals in the vestibular end organs, we show that activation of GABAB receptors result in an excitatory effect, with decreased spike-frequency adaptation and shortened first-spike latencies. Our results suggest that these effects are mediated through inhibition of calcium-sensitive potassium channels.

Keywords: GABAB; calcium-sensitive potassium channel; efferent; supporting cell.

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Conflict of interest statement

No conflicts of interest, financial or otherwise, are declared by the authors.

Figures

Fig. 1.
Fig. 1.
Activation of GABAB receptors attenuates voltage‐activated currents. A: response of a calyx terminal to the voltage step protocol (20-mV steps between −129 mV and +11 mV) before (black) and after application of baclofen (gray). Calyx recording exhibited delayed rectifier type outward currents during depolarizing voltage steps (left; red box 1). Average responses (right) show a significant decrease in voltage-sensitive currents at the 3 most depolarized steps (marked by asterisks). Note, in all panels, changes in average currents are presented relative to the current at the initial holding potential of −79 mV. B: the amplitude of tail currents after the steps decreased during baclofen application in an example recording (left; red box 2). Average tail currents (right) significantly decreased after the 3 largest depolarization steps (marked by asterisks). C: application of CGP 35348 (CGP; antagonist of GABAB receptors) blocked the effect of baclofen, confirming that the effect is mediated through GABAB receptors. All traces are from the same calyces recorded in the presence of different drug combinations.
Fig. 2.
Fig. 2.
Effect of baclofen could be inhibited by blocking small (SK)- and large-conductance potassium (BK) channels. A: example calyx recording showing a decrease in outward currents during the step protocol in the presence of apamin (Apa; SK blocker) and iberiotoxin (IBTX; BK blocker), which blocked the effect of baclofen (Bac). B: average of the recorded currents during the step protocol shows that apamin and iberiotoxin application blocked some of the voltage-sensitive currents and that application of baclofen resulted in a small nonsignificant further decrease in the currents for the most depolarized steps. *Significant differences by repeated measures ANOVA, post hoc Bonferroni test: control vs. Apa + IBTX, P < 0.001 at +11 mV and P < 0.05at −9 mV; control vs. Apa + IBTX + Bac, P < 0.001 at +11 mV and −9 mV, and P < 0.05 at −29 mV. C: application of a cocktail of apamin, iberiotoxin, and XE-991 (KCNQ channel blocker) resulted in a decrease in voltage-sensitive currents. Addition of baclofen resulted in a further small nonsignificant decrease in the currents at depolarized steps, which was blocked by cadmium chloride (Cdcl2; a general blocker of calcium channels), suggesting that baclofen inhibits voltage-sensitive calcium channels. *Significant differences between control and other conditions with P values similar to those in B.
Fig. 3.
Fig. 3.
The effect of baclofen was blocked in the absence of calcium. A: example calyx recording showing a decrease in voltage-sensitive currents during the step protocol and in tail currents in 0-Ca2+ external solution, most likely due to inhibition of Ca2+-dependent K+ currents. The effect of baclofen was blocked under these conditions. B: average currents recorded from calyces during and after (tail currents) the step protocol. Application of 0-Ca2+ external solution decreased the currents and almost completely blocked the effect of baclofen. C: application of calcium channel blocker cadmium chloride resulted in a decrease in voltage-sensitive currents and tail currents and almost completely blocked the effect of baclofen. D: application of the combination of 0-Ca2+ external solution and cadmium chloride resulted in a complete block of baclofen effect on voltage-sensitive currents and tail currents. *Significant differences from control condition (repeated measures ANOVA, post hoc Bonferroni test, P < 0.03).
Fig. 4.
Fig. 4.
Calyces become more sensitive and respond faster following activation of GABAB receptors. A: examples of current-clamp recordings from calyces that at best fire a single action potential (AP) in the control condition in response to injection of 100- to 500-pA current steps, but in the presence of baclofen fired many APs during the step. B: the average number of spikes during steps was higher after baclofen application for all current steps. The control condition had 0 or 1 spikes for all steps, while after baclofen, calyces fired at least 1 spike, even for the smallest current step. C: the amplitude of the current required for AP generation (i.e., current threshold) decreased significantly during baclofen application. D: the latency of the first spike decreased during application of baclofen compared with control condition. The decreases were significant for the two largest steps. E: some calyces showed repetitive AP firing during baclofen application (dashed line represents 0 mV). In A–E, control results are shown in blue and baclofen condition in red. *Significant changes from control condition (P < 0.05).
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References

    1. Annoni JM, Cochran SL, Precht W. Pharmacology of the vestibular hair cell-afferent fiber synapse in the frog. J Neurosci 4: 2106–2116, 1984. doi:10.1523/JNEUROSCI.04-08-02106.1984. - DOI - PMC - PubMed
    1. Attwell D, Barbour B, Szatkowski M. Nonvesicular release of neurotransmitter. Neuron 11: 401–407, 1993. doi:10.1016/0896-6273(93)90145-H. - DOI - PubMed
    1. Beraneck M, Straka H. Vestibular signal processing by separate sets of neuronal filters. J Vestib Res 21: 5–19, 2011. doi:10.3233/VES-2011-0396. - DOI - PubMed
    1. Contini D, Price SD, Art JJ. Accumulation of K+ in the synaptic cleft modulates activity by influencing both vestibular hair cell and calyx afferent in the turtle. J Physiol 595: 777–803, 2017. doi:10.1113/JP273060. - DOI - PMC - PubMed
    1. Cullen KE, Minor LB. Semicircular canal afferents similarly encode active and passive head-on-body rotations: implications for the role of vestibular efference. J Neurosci 22: RC226, 2002. doi:10.1523/JNEUROSCI.22-11-j0002.2002. - DOI - PMC - PubMed

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