Ion channels in mammalian vestibular afferents may set regularity of firing

Abstract

Rodent vestibular afferent neurons offer several advantages as a model system for investigating the significance and origins of regularity in neuronal firing interval. Their regularity has a bimodal distribution that defines regular and irregular afferent classes. Factors likely to be involved in setting firing regularity include the morphology and physiology of the afferents' contacts with hair cells, which may influence the averaging of synaptic noise and the afferents' intrinsic electrical properties. In vitro patch clamp studies on the cell bodies of primary vestibular afferents reveal a rich diversity of ion channels, with indications of at least two neuronal populations. Here we suggest that firing patterns of isolated vestibular ganglion somata reflect intrinsic ion channel properties, which in vivo combine with hair cell synaptic drive to produce regular and irregular firing.

Figure 1

Vestibular epithelia, zones and afferent classes in the rodent. Confocal images have been combined to produce this picture of part of the vestibular portion of the mouse inner ear. The tissue is labeled with fluorescently-labeled phalloidin (yellow; labels hair bundles), as well as antisera to calretinin (green; stains certain afferents and hair cells) and β–III tubulin (red, stains afferents). Extracellular matrices have been removed to expose the hair cells in three sensory epithelia: the cristae of the anterior and horizontal semicircular canals and the macula of the utricle. The central zones of the cristae and the striolar zone of the utricular macula are outlined in red. At the bottom edge of the tissue is part of the vestibular ganglion. The large neuronal somata that are brightly stained for calretinin give rise to pure-calyx afferents to the central and striolar zones. Scale bar, 50 µm.

Figure 2

Schematic showing classification of vestibular afferent neurons by terminal morphology as pure-calyx (green, C), dimorphic (orange, D) and pure-bouton (red, B). Grey fibres are efferents, arising from neurons in the brainstem. Pure-calyx afferents exclusively innervate the centre and striola and often form complex calyces around multiple type I hair cells, as illustrated. Pure-bouton afferents exclusively innervate the peripheral zone and extrastriola and can innervate tens of type II hair cells. Dimorphic afferents innervate both zones, but have more compact dendritic trees in the centre and striola than in the periphery and extrastriola (not shown). Pure-calyx afferents express calretinin, calbindin and parvalbumin; dimorphic afferents are thought to express calbindin and parvalbumin; and pure-bouton afferents, which are the thinnest, express just parvalbumin. Some differences in ion channel expression have been noted between large and small isolated ganglion somata and are indicated here on the pure-calyx and pure-bouton somata. Whether dimorphic somata, which are likely to be mid-sized, have intermediate expression is not known. Met, mechanoelectrical transduction channels; see Table 2 and text for details on other ion channels. * indicates a possible site of spike initiation on each afferent.

Figure 3

Vestibular ganglion somata produce either transient or sustained firing patterns in response to small depolarizing currents. (A,B) Voltage responses of two isolated somata to steps of +50 pA, recorded in whole-cell current clamp. Small depolarizing currents (4–200 pA) evoke single spikes (transient responses) in some neurons (A) and multiple spikes (sustained responses) in others (B). Somata dissociated from the mouse vestibular ganglion, recorded in the first postnatal week with a standard high-K⁺ pipette solution and a bath of L-15 medium. Similar results have been obtained with perforated patch recordings and from rat vestibular ganglion somata (J. Xue, R. Kalluri and R.A. Eatock, unpublished observations). Arrows point to AHPs after the first spike and the offset of the step; AHPs have longer repolarizing phases in the neuron with the sustained response (B). (C,D) Whole-cell current responses to voltage steps, recorded in voltage clamp from the same neurons as in A,B. Depolarizing steps (bottom panel) evoked large brief Na⁺ currents followed by large steady outward K⁺ currents. The sustained neuron (D) had prominent A and h currents, but it is not established that these differences influence the spike pattern.