Developmental Changes in the Inhibition of Glycinergic Synaptic Currents by Niflumic Acid in Hypoglossal Motoneurons.

Elena Petukhova,Marat Mukhamedyarov,Piotr Bregestovski,Galyna Maleeva,Daria Ponomareva

doi:10.3389/fnmol.2018.00416

Abstract

Mammalian brainstem hypoglossal motoneurones (HMs) receive powerful synaptic glycinergic inputs and are involved in a variety of motor functions, including respiration, chewing, sucking, swallowing, and phonation. During the early postnatal development, subunit composition of chloride-permeable glycine receptors (GlyRs) changes leading to a decrease of “fetal” alpha2 and elevation of “adult” alpha1 GlyR subunits. It has been recently demonstrated that niflumic acid (NFA), a member of the fenamate class of non-steroidal anti-inflammatory drugs, is an efficient subunits-specific blocker of GlyRs. At a heterologous expression of different GlyR subunits it has been shown that blocking potency of NFA is more than one order higher for alpha2 GlyRs than for receptors formed by alpha1 subunit. To reveal the action of NFA on the synaptic activity we analyzed here the effects of NFA on the glycinergic inhibitory post-synaptic currents in the HMs from mouse brainstem slices. In the whole-cell patch clamp configuration, the amplitude and the frequency of glycinergic synaptic currents from two age groups have been analyzed: “neonate” (P2–P4) and “juvenile” (P7–P12). Addition of NFA in the presence of antagonists of glutamate and GABA receptors caused a decrease in the mean amplitude and frequency of synaptic events. The degree of the inhibition induced by NFA decreased with the postnatal development, being higher on the motoneurons from “neonate” brainstem slices in comparison with the “juvenile” age group. Analysis of the pair-pulse facilitation suggests the post-synaptic origin of NFA action. These observations provide evidence on the developmental changes in the inhibition by NFA of glycinergic synaptic transmission, which reflects increase in the alpha1 and decrease in the alpha2 GlyR subunits expression in synapses to hypoglossal motoneurons during the early stages of postnatal life.

Highlights

In the mammalian spinal cord and some other parts of central nervous system (CNS), inhibitory synaptic transmission is mediated by glycinergic synapses
The average relative values of a I2/I1 ratio in the neonate and juvenile groups in the presence of 100 μM niflumic acid (NFA) increased only by 4.6 ± 6.4% (n = 4) and 4.4 ± 8.1% (n = 7) respectively (Figure 6C). These results suggest that NFA does not modulate the function of the presynaptic voltage-gated Ca2+-channels and the probability of the neurotransmitter release from the presynaptic terminals, supporting the post-synaptic origin of the glycinergic inhibitory post-synaptic currents (IPSCs) inhibition by this compound
Recording from the hypoglossal motoneurones (HM) of brainstem slices of the “neonatal” and the “juvenile” mice under the conditions of constant suppression of the glutamatergic and GABAergic neurotransmission, we have demonstrated that: (i) NFA caused an inhibition of the amplitude and frequency of the spontaneous synaptic glycinergic currents and the amplitude of evoked inhibitory post-synaptic currents (eIPSCs); (ii) inhibitory ability of NFA was weaker in the “juvenile” group; (iii) inhibition was voltage-dependent with the higher efficacy at the positive potentials and this effect was pronounced on the MNs from the “neonatal” group; (iv) NFA did not change the degree of a paired-pulse potentiation of eIPSCs in the both age groups

Summary

Introduction

In the mammalian spinal cord and some other parts of CNS, inhibitory synaptic transmission is mediated by glycinergic synapses. Expression of the different receptor subtypes is developmentally regulated: “neonatal” alpha subunits are predominant at birth, but during the first weeks of postnatal development number of alpha GlyRs decreases, being partially substituted by “adult” alpha subunits (Akagi and Miledi, 1988; Becker et al, 1988; Malosio et al, 1991). This process is accompanied by the functional changes in the glycinergic neurotransmission – acceleration of the decay kinetics of IPSCs over the first weeks of postnatal period (Singer and Berger, 1999; Takahashi, 2005). This phenomenon is due to a shorter single-channel open time of the “adult” alpha receptors, comparing to the “neonatal” alpha GlyRs (Takahashi et al, 1992)

Results

Discussion

Conclusion