NAD+ Attenuates Bilirubin-Induced Hyperexcitation in the Ventral Cochlear Nucleus by Inhibiting Excitatory Neurotransmission and Neuronal Excitability.

Min Liang,Xin-Lu Yin,Wei-Hai Yin,Chun-Yan Li,Lu-Yang Wang,Shan-Kai Yin,Ning-Ying Song,Hai-Bo Shi

doi:10.3389/fncel.2017.00021

Abstract

Nicotinamide adenine dinucleotide (NAD+) is an important molecule with extensive biological functions in various cellular processes, including protection against cell injuries. However, little is known regarding the roles of NAD+ in neuronal excitation and excitotoxicity associated with many neurodegenerative disorders and diseases. Using patch-clamp recordings, we studied its potential effects on principal neurons in the ventral cochlear nucleus (VCN), which is particularly vulnerable to bilirubin excitotoxicity. We found that NAD+ effectively decreased the size of evoked excitatory postsynaptic currents (eEPSCs), increased paired-pulse ratio (PPR) and reversed the effect of bilirubin on eEPSCs, implicating its inhibitory effects on the presynaptic release probability (Pr). Moreover, NAD+ not only decreased the basal frequency of miniature EPSCs (mEPSCs), but also reversed bilirubin-induced increases in the frequency of mEPSCs without affecting their amplitude under either condition. Furthermore, we found that NAD+ decreased the frequency of spontaneous firing of VCN neurons as well as bilirubin-induced increases in firing frequency. Whole-cell current-clamp recordings showed that NAD+ could directly decrease the intrinsic excitability of VCN neurons in the presence of synaptic blockers, suggesting NAD+ exerts its actions in both presynaptic and postsynaptic loci. Consistent with these observations, we found that the latency of the first postsynaptic spike triggered by high-frequency train stimulation of presynaptic afferents (i.e., the auditory nerve) was prolonged by NAD+. These results collectively indicate that NAD+ suppresses presynaptic transmitter release and postsynaptic excitability, jointly weakening excitatory neurotransmission. Our findings provide a basis for the exploration of NAD+ for the prevention and treatment of bilirubin encephalopathy and excitotoxicity associated with other neurological disorders.

Highlights

Emerging evidence indicates that nicotinamide adenine dinucleotide (NAD+) plays essential roles in energy metabolism, mitochondrial functions, aging, calcium homeostasis, immune functions and gene expression (Ying, 2006, 2008; Xia et al, 2009)
Differences in the frequency and amplitude of the miniature EPSCs (mEPSCs), the charge of evoked EPSCs and the spike latency were evaluated by one-way analysis of variance (ANOVA) with Student-Newman-Keuls post hoc test, and the rates of spontaneous spikes were evaluated by KruskalWallis non-parametric test, with P < 0.05 used as the level of significance
We focused the present study on stellate cells which represent one of the major cell types in the ventral cochlear nucleus (VCN), and can be visually identified from their large soma size and multipolar shape and long dendrites

Summary

INTRODUCTION

Emerging evidence indicates that nicotinamide adenine dinucleotide (NAD+) plays essential roles in energy metabolism, mitochondrial functions, aging, calcium homeostasis, immune functions and gene expression (Ying, 2006, 2008; Xia et al, 2009). NAD+ was implicated in inhibiting apoptotic neuronal death after glutamate insult (Wang et al, 2014), implicating the potential of NAD+ in protecting neurons from excitotoxicity under pathological conditions. Our previous studies showed that bilirubin could induce neuronal hyperexcitation via the potentiation of presynaptic glutamate release in the ventral cochlear nucleus (VCN) and lateral superior olive nucleus (LSO), which are most vulnerable to bilirubin (Li et al, 2011, 2012). To explore the potential utility of NAD+ for neuroprotection, we made patch-clamp recordings from VCN neurons in acute auditory brainstem slices from early postnatal rats (

Ethics Statement

RESULTS

DISCUSSION