Development of frequency tuning in the auditory periphery

Nicotinic acetylcholine receptor structure and function in the efferent auditory system

This article reviews and presents new data regarding the nicotinic acetylcholine receptor subunits alpha9 and alpha10. Although phylogentically ancient, these subunits have only recently been identified as critical components of the efferent auditory system and medial olivocochlear pathway. This pathway is important in auditory processing by modulating outer hair cell function to broadly tune the cochlea and improve signal detection in noise. Pharmacologic properties of the functionally expressed alpha9alpha10 receptor closely resemble the cholinergic response of outer hair cells. Molecular, immunohistochemical, and knockout mice studies have added further weight to the role this receptor plays in mediating the efferent auditory response. Alternate and complementary mechanisms of outer hair cell efferent activity might also be mediated through the nAChR alpha9alpha10, either through secondary calcium stores, second messengers, or direct protein-protein interactions. We investigated protein-protein interactions using a yeast-two-hybrid screen of the nAChR alpha10 intracellular loop against a rat cochlear cDNA library. Among the identified proteins was prosaposin, a precursor of saposins, which have been shown to act as neurotrophic factors in culture, can bind to a putative G0-coupled cell surface receptor, and may be involved in the prevention of cell death. This study and review suggest that nAChR alpha9alpha10 may represent a potential therapeutic target for a variety of ear disorders, including preventing or treating noise-induced hearing loss, or such debilitating disorders as vertigo or tinnitus.

Transplantable sites confer calcium sensitivity to BK channels

Both intracellular calcium and voltage activate Slo1, a high-conductance potassium channel, linking calcium with electrical excitability. Using molecular techniques, we created a calcium-insensitive variant of this channel gated by voltage alone. Calcium sensitivity was restored by adding back small portions of the carboxyl (C)-terminal 'tail' domain. Two separate regions of the tail independently conferred different degrees of calcium sensitivity; together, they restored essentially wild-type calcium dependence. These results suggest that, in the absence of calcium, the Slo1 tail inhibits voltage-dependent gating, and that calcium removes this inhibition. Slo1 may have evolved from an ancestral voltage-sensitive potassium channel represented by the core; the tail may represent the more recent addition of a calcium-dependent modulatory domain.

Slo3, a novel pH-sensitive K+ channel from mammalian spermatocytes

Potassium channels have evolved to play specialized roles in both excitable and inexcitable tissues. Here we describe the cloning and expression of Slo3, a novel potassium channel abundantly expressed in mammalian spermatocytes. Slo3 represents a new and unique type of potassium channel regulated by both intracellular pH and membrane voltage. Reverse transcription-polymerase chain reaction, Northern analysis, and in situ hybridization show that Slo3 is primarily expressed in testis in both mouse and human. Because of its sensitivity to both pH and voltage, Slo3 could be involved in sperm capacitation and/or the acrosome reaction, essential steps in fertilization where changes in both intracellular pH and membrane potential are known to occur. The protein sequence of mSlo3 (the mouse Slo3 homologue) is similar to Slo1, the large conductance, calcium- and voltage-gated potassium channel. These results suggest that Slo channels comprise a multigene family, defined by a combination of sensitivity to voltage and a variety of intracellular factors. Northern analysis from human testis indicates that a Slo3 homologue is present in humans and conserved with regard to sequence, transcript size, and tissue distribution. Because of its high testis-specific expression, pharmacological agents that target human Slo3 channels may be useful in both the study of fertilization as well as in the control or enhancement of fertility.

A novel calcium-sensing domain in the BK channel

The high-conductance Ca2+-activated K+ channel (mSlo) plays a vital role in regulating calcium entry in many cell types. mSlo channels behave like voltage-dependent channels, but their voltage range of activity is set by intracellular free calcium. The mSlo subunit has two parts: a "core" resembling a subunit from a voltage-dependent K+ channel, and an appended "tail" that plays a role in calcium sensing. Here we present evidence for a site on the tail that interacts with calcium. This site, the "calcium bowl," is a novel calcium-binding motif that includes a string of conserved aspartate residues. Mutations of the calcium bowl fall into two categories: 1) those that shift the position of the G-V relation a similar amount at all [Ca2+], and 2) those that shift the position of the G-V relation only at low [Ca2+]. None of these mutants alters the slope of the G-V curve. These mutant phenotypes are apparent in calcium ion, but not in cadmium ion, where mutant and wild type are indistinguishable. This suggests that the calcium bowl is sensitive to calcium ion, but insensitive to cadmium ion. The presence and independence of a second calcium-binding site is inferred because channels still respond to increasing levels of [Ca2+] or [Cd2+], even when the calcium bowl is mutationally deleted. Thus a low level of activation in the absence of divalent cations is identical in mutant and wild-type channels, possibly because of activation of this second Ca2+-binding site.

Development of electrical membrane properties and discharge characteristics of superior olivary complex neurons in fetal and postnatal rats

Although hearing onset occurs relatively late during ontogeny of rats [around postnatal day (P) 12], anatomical brainstem connections are formed much earlier and are present before birth, indicating that a substantial amount of maturation occurs without acoustic input. Electrical activity is thought to influence neuronal development, but the physiological properties of auditory brainstem neurons during perinatal maturation are barely known. The present study focuses on the development of electrophysiological membrane properties of neurons in the rat's superior olivary complex (SOC), the first binaural station in the mammalian auditory brainstem. In in vitro slice preparations, intracellular recordings were obtained from 115 SOC cells from embryonic day (E) 18 to P17, and cells were morphologically identified by intracellular injection of biocytin or neurobiotin. By E18, i.e. 4 days before birth, SOC neurons were capable of generating Na(+)-dependent action potentials. Several passive and active membrane properties, including the resting potential, spike threshold and spike amplitude, did not change with development. In contrast, input resistance, time constant and spike duration decreased significantly, and maximal spike frequency increased significantly during the age period sampled. Our results show that rat SOC neurons display mature as well as immature electrical membrane properties during the same developmental period when anatomical connections are refined and when the soma-dendritic morphology develops. We conclude, therefore, that their membrane properties represent adequate physiological adaptations to the immature auditory brainstem microcircuits and that they form a basis upon which the development of these microcircuits is shaped.

Hair cell development

Neurobiologists have been challenged by the desire to understand how the highly specialized ultrastructure of the sensory hair cells of the ear develops, how patterns of phenotypically distinct hair cells are formed and regenerate, and how their specific neural connections are formed. Recent research has addressed some of these challenges at the level of cell and molecular biology, focusing on cell proliferation in hair cell epithelia, the mechanisms that control hair cell differentiation, and the developmental interdependencies between hair cells and neurons. The initial identification of some of the homeobox genes and growth factors that are involved in hair cell development has occurred during the past year.