Efferent function of vestibular afferent endings? Similar localization of N-type calcium channels, synaptic vesicle and synaptic membrane-associated proteins

Synaptic transmission at the vestibular hair cells of amniotes

A harmonized interplay between the central nervous system and the five peripheral end organs is how the vestibular system helps organisms feel a sense of balance and motion in three-dimensional space. The receptor cells of this system, much like their cochlear equivalents, are the specialized hair cells. However, research over the years has shown that the vestibular endorgans and hair cells evolved very differently from their cochlear counterparts. The structurally unique calyceal synapse, which appeared much later in the evolutionary time scale, and continues to intrigue researchers, is now known to support several forms of synaptic neurotransmission. The conventional quantal transmission is believed to employ the ribbon structures, which carry several tethered vesicles filled with neurotransmitters. However, the field of vestibular hair cell synaptic molecular anatomy is still at a nascent stage and needs further work. In this review, we will touch upon the basic structure and function of the peripheral vestibular system, with the focus on the various modes of neurotransmission at the type I vestibular hair cells. We will also shed light on the current knowledge about the molecular anatomy of the vestibular hair cell synapses and vestibular synaptopathy.

Nociceptin/orphanin FQ peptide receptor mediates inhibition of N-type calcium currents in vestibular afferent neurons of the rat

The vestibular system is modulated by various neuromodulators including opioid peptides. The current study was conducted to determine whether activation of nociceptin/orphanin FQ peptide (NOP) receptors modulates voltage-gated calcium currents and action potential discharge of rat vestibular afferent neurons. We performed whole cell patch-clamp recordings on cultured vestibular afferent neurons from P7-P10 Long-Evans rats. Application of nociceptin/orphanin FQ (N/OFQ), a 17-amino acid neuropeptide that is the endogenous ligand for NOP receptor, inhibits the high-voltage activated (HVA) component of the calcium current in a concentration-dependent manner with a half inhibitory concentration of 26 nM. Said inhibitory action on the calcium current is voltage-dependent, which was made clear by the fact that it was reverted in 80% by a depolarizing prepulse. Furthermore, the effect of N/OFQ was blocked by application of the specific NOP-antagonist UFP101, by preincubation with G-protein blocker pertussis toxin, and by coapplication of the specific N-type calcium-current blocker ω-conotoxin-MVIIA. N/OFQ application causes an increase in the duration and maximum rate of repolarization of action potentials. It also decreases repetitive discharge and discharge elicited by sinusoidal stimulation. These results show that in vestibular afferents, NOP receptor activation inhibits N-type calcium current by activating G proteins, mostly through the Gβγ subunit. This suggests that NOP activation produces a presynaptic modulation of primary vestibular afferent neurons' output into the vestibular nuclei, thus taking part in the integration and gain setting of vestibular information in second-order vestibular nucleus neurons.NEW & NOTEWORTHY Our results show that in primary vestibular afferent neurons, activation of the nociceptin/orphanin FQ peptide receptor inhibits the N-type calcium current by a mechanism mediated by G proteins. We propose that calcium current inhibition modulates neurotransmitter release from vestibular afferents, producing a presynaptic modulation of vestibular input to vestibular nuclei, thus contributing to gain control in the vestibular afferent input.

Molecular microdomains in a sensory terminal, the vestibular calyx ending

Many primary vestibular afferents form large cup-shaped postsynaptic terminals (calyces) that envelope the basolateral surfaces of type I hair cells. The calyceal terminals both respond to glutamate released from ribbon synapses in the type I cells and initiate spikes that propagate to the afferent's central terminals in the brainstem. The combination of synaptic and spike initiation functions in these unique sensory endings distinguishes them from the axonal nodes of central neurons and peripheral nerves, such as the sciatic nerve, which have provided most of our information about nodal specializations. We show that rat vestibular calyces express an unusual mix of voltage-gated Na and K channels and scaffolding, cell adhesion, and extracellular matrix proteins, which may hold the ion channels in place. Protein expression patterns form several microdomains within the calyx membrane: a synaptic domain facing the hair cell, the heminode abutting the first myelinated internode, and one or two intermediate domains. Differences in the expression and localization of proteins between afferent types and zones may contribute to known variations in afferent physiology.

Cellular distribution of d-serine, serine racemase and d-amino acid oxidase in the rat vestibular sensory epithelia

Glutamate is the main neurotransmitter at the synapses between sensory cells and primary afferents in the peripheral vestibular system. Evidence has recently been obtained demonstrating that the atypical amino acid D-serine is the main endogenous co-agonist of the N-methyl-D-aspartate receptors in the CNS. We studied the distribution of D-serine and its synthesizing and degrading enzymes, serine racemase and d-amino acid oxidase in the rat vestibular sensory epithelium using immunocytochemistry. D-serine, serine racemase and D-amino acid oxidase were localized in the transitional cells, which are parasensory cells located between the sensory epithelium and the dark cells. The dark cells expressed only serine racemase. D-Serine was also detected in the supporting cells of the sensory epithelium. These cells, which are in close contact with glutamatergic synapses, express GLAST, a glial specific transporter for glutamate. They may have similar functions to glial cells in the CNS and thus expression of D-serine suggests a neuromodulator role for D-serine at the glutamatergic synapses in the peripheral vestibular system. Our data also indicate that the metabolism of D-serine is not restricted to glial cells suggesting that the amino acid may play an additional role in the peripheral nervous system.

Role of high-voltage-activated calcium channels in glucose-regulated beta-cell calcium homeostasis and insulin release

High-voltage-activated (HVA) calcium channels are known to be the primary source of calcium for glucose-stimulated insulin secretion. However, few studies have investigated how these channels can be regulated by chronically elevated levels of glucose. In the present study, we determined the level of expression of the four major HVA calcium channels (N-type, P/Q-type, L(C)-type, and L(D)-type) in rat pancreatic beta-cells. Using quantitative real-time PCR (QRT-PCR), we found the expression of all four HVA genes in rat insulinoma cells (INS-1) and in primary isolated rat islet cells. We then determined the role of each channel in insulin secretion by using channel-selective antagonists. Insulin secretion analysis revealed that N- and L-type channels are both involved in immediate glucose-induced insulin secretion. However, L-type was preferentially coupled to secretion at later time points. P/Q-type channels were not found to play a role in insulin secretion at any stage. It was also found that long-term exposure to elevated glucose increases basal calcium in these cells. Interestingly, chronically elevated glucose decreased the mRNA expression of the channels involved with insulin secretion and diminished the level of stimulated calcium influx in these cells. Using whole cell patch clamp, we found that N- and L-type channel currents increase gradually subsequent to lower intracellular calcium perfusion, suggesting that these channels may be regulated by glucose-induced changes in calcium.

Vagal innervation of the air sacs in a songbird, Taenopygia guttata

The air sacs of birds are thin-walled chambers connected to the lung that act as bellows in the ventilatory mechanism. Physiological evidence exists to suggest that they may contain receptors that are innervated by the vagus nerve, but no morphological study has examined the vagal innervation of these putative structures. To do this, we injected the cervical vagus nerve with choleragenoid and examined the innervation of the air sacs using light and confocal microscopy. We identified vagally innervated structures in the air sac wall that resemble the neuroepithelial bodies (NEBs) described in the airways of many vertebrates. Although NEBs have been proposed to have a dual chemoreceptive and mechanoreceptive role, their specific function in the air sacs of birds remains unclear.

The effect of hypergravity on the inner ear: CREB and syntaxin are up-regulated

cDNA microarray analysis of differential mRNA expression in the rat inner ear under hypergravity identified 20 up-regulated and 2 down-regulated genes. The results demonstrated that various response and/or adaptation processes occur at the level of the peripheral organs. From among the genes assessed by microarray, up-regulation of CREB and syntaxin was confirmed by real time PCR and these two molecules were found to be immunocytochemically localized in the primary afferent neurons. Since CREB is believed to be involved in the formation of long term memory, and syntaxin is known as one of the synaptic molecules involved in the exocytosis of synaptic vesicles, the up-regulation of CREB and syntaxin may reflect synaptic plasticity occurring in the peripheral vestibular system.

Reciprocal synapses between inner hair cell spines and afferent dendrites in the organ of corti of the mouse

We provide, for the first time, ultrastructural evidence for the differentiation of reciprocal synapses between afferent dendrites of spiral ganglion neurons and inner hair cells. Cochlear synaptogenesis of inner hair cells in the mouse occurs in two phases: before and after the onset of hearing at 9-10 postnatal (PN) days. In the first phase, inner hair cells acquire afferent innervation (1-5 PN). Reciprocal synapses form around 9-10 PN on spinous processes emitted by inner hair cells into the dendritic terminals, predominantly in conjunction with ribbon afferent synapses. During the second phase, which lasts up to 14 PN, synaptogenesis is led by the olivocochlear fibers of the lateral bundle, which induce the formation of compound and spinous synapses. The afferent dendrites themselves also develop recurrent presynaptic spines or form mounds of synaptic vesicles apposed directly across inner hair cell ribbon synapses. Thus, in the adult 2-month mouse, afferent dendrites of spiral ganglion neurons are not only postsynaptic but also presynaptic to inner hair cells, providing a synaptic loop for an immediate feedback response. Reciprocal synapses, together with triadic, converging, and serial synapses, are an integral part of the afferent ribbon synapse complex. We define the neuronal circuitry of the inner hair cell and propose that these minicircuits form synaptic trains that provide the neurological basis for local cochlear encoding of the initial acoustic signals.

Opioid receptors mediate a postsynaptic facilitation and a presynaptic inhibition at the afferent synapse of axolotl vestibular hair cells

This study was designed to determine the effects of opiate drugs on the electrical activity of afferent neurons and on the ionic currents of hair cells from semicircular canals. Experiments were done on larval axolotls (Ambystoma tigrinum). The multiunit spike activity of afferent neurons was recorded in the isolated inner ear under both resting conditions and mechanical stimulation. Ionic currents were recorded using voltage clamp of hair cells isolated from the semicircular canal. In the isolated inner-ear preparation, microperfusion of either non-specific opioid receptor antagonist naloxone (10 nM to 1 mM), mu receptor agonist [D-Ala(2), N-Me-Phe(4),Gly(5)-ol]-enkephalin (1 pM to 10 microM), or kappa receptor antagonist nor-binaltorphimine (10 nM to 100 microM) elicited a dose-dependent long-lasting (>5 min) increase of the electrical discharge of afferent neurons. The mu receptor agonist funaltrexamine (1 nM to 100 microM) and the kappa receptor agonist U-50488 (1 nM to 10 microM) diminished the basal spike discharge of vestibular afferents. The delta receptor agonist D-Pen(2)-D-Pen(5)-enkephalin (1 nM to 10 mM) and the antagonist naltrindole (1 nM to 10 mM) were without a significant effect. The only drug that displayed a significant action on hair-cell ionic currents was trans-(+/-)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]-cyclohexyl) benzeneacetamide methanesulfonate (U-50488) that reduced the Ca(2+) current in a dose-dependent fashion. On its own, mu receptor agonist [D-Ala(2), N-Me-Phe(4),Gly(5)-ol]-enkephalin (0.01 and 10 microM) significantly potentiated the response of afferent neurons to the excitatory amino acid agonist (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (0.1 microM), while synaptic transmission was blocked by the use of high-Mg(2+), low-Ca(2+) solutions. Our data indicate that the activity of vestibular afferent neurons may be regulated in a complex fashion by opioid receptors: mu opioid receptors mediating an excitatory, postsynaptic modulatory input to afferent neurons, and kappa receptors mediating an inhibitory, presynaptic input to hair cells.