Possible modulatory role of voltage-activated Ca(2+) currents determining the membrane properties of isolated pyramidal neurones of the rat dorsal cochlear nucleus

The role of GABAB receptors in the subcortical pathways of the mammalian auditory system

GABAB receptors are G-protein coupled receptors for the inhibitory neurotransmitter GABA. Functional GABAB receptors are formed as heteromers of GABAB1 and GABAB2 subunits, which further associate with various regulatory and signaling proteins to provide receptor complexes with distinct pharmacological and physiological properties. GABAB receptors are widely distributed in nervous tissue, where they are involved in a number of processes and in turn are subject to a number of regulatory mechanisms. In this review, we summarize current knowledge of the cellular distribution and function of the receptors in the inner ear and auditory pathway of the mammalian brainstem and midbrain. The findings suggest that in these regions, GABAB receptors are involved in processes essential for proper auditory function, such as cochlear amplifier modulation, regulation of spontaneous activity, binaural and temporal information processing, and predictive coding. Since impaired GABAergic inhibition has been found to be associated with various forms of hearing loss, GABAB dysfunction could also play a role in some pathologies of the auditory system.

Activity-Dependent Calcium Signaling in Neurons of the Medial Superior Olive during Late Postnatal Development

The development of sensory circuits is partially guided by sensory experience. In the medial superior olive (MSO), these refinements generate precise coincidence detection to localize sounds in the azimuthal plane. Glycinergic inhibitory inputs to the MSO, which tune the sensitivity to interaural time differences, undergo substantial structural and functional refinements after hearing onset. Whether excitation and calcium signaling in the MSO are similarly affected by the onset of acoustic experience is unresolved. To assess the time window and mechanism of excitatory and calcium-dependent refinements during late postnatal development, we quantified EPSCs and calcium entry in MSO neurons of Mongolian gerbils of either sex raised in a normal and in an activity altered, omnidirectional white noise environment. Global dendritic calcium transients elicited by action potentials disappeared rapidly after hearing onset. Local synaptic calcium transients decreased, leaving a GluR2 lacking AMPAR-mediated influx as the only activity-dependent source in adulthood. Exposure to omnidirectional white noise accelerated the decrease in calcium entry, leaving membrane properties unaffected. Thus, sound-driven activity accelerates the excitatory refinement and shortens the period of activity-dependent calcium signaling around hearing onset. Together with earlier reports, our findings highlight that excitation, inhibition, and biophysical properties are differentially sensitive to distinct features of sensory experience.SIGNIFICANCE STATEMENT Neurons in the medial superior olive, an ultra-fast coincidence detector for sound source localization, acquire their specialized function through refinements during late postnatal development. The refinement of inhibitory inputs that convey sensitivity to relevant interaural time differences is instructed by the experience of sound localization cues. Which cues instruct the refinement of excitatory inputs, calcium signaling, and biophysical properties is unknown. Here we demonstrate a time window for activity- and calcium-dependent refinements limited to shortly after hearing onset. Exposure to omnidirectional white noise, which suppresses sound localization cues but increases overall activity, accelerates the refinement of calcium signaling and excitatory inputs without affecting biophysical membrane properties. Thus, the refinement of excitation, inhibition, and intrinsic properties is instructed by distinct cues.

A biophysical modelling platform of the cochlear nucleus and other auditory circuits: From channels to networks

Models of the auditory brainstem have been an invaluable tool for testing hypotheses about auditory information processing and for highlighting the most important gaps in the experimental literature. Due to the complexity of the auditory brainstem, and indeed most brain circuits, the dynamic behavior of the system may be difficult to predict without a detailed, biologically realistic computational model. Despite the sensitivity of models to their exact construction and parameters, most prior models of the cochlear nucleus have incorporated only a small subset of the known biological properties. This confounds the interpretation of modelling results and also limits the potential future uses of these models, which require a large effort to develop. To address these issues, we have developed a general purpose, biophysically detailed model of the cochlear nucleus for use both in testing hypotheses about cochlear nucleus function and also as an input to models of downstream auditory nuclei. The model implements conductance-based Hodgkin-Huxley representations of cells using a Python-based interface to the NEURON simulator. Our model incorporates most of the quantitatively characterized intrinsic cell properties, synaptic properties, and connectivity available in the literature, and also aims to reproduce the known response properties of the canonical cochlear nucleus cell types. Although we currently lack the empirical data to completely constrain this model, our intent is for the model to continue to incorporate new experimental results as they become available.

Serotonergic regulation of excitability of principal cells of the dorsal cochlear nucleus

The dorsal cochlear nucleus (DCN) is one of the first stations within the central auditory pathway where the basic computations underlying sound localization are initiated and heightened activity in the DCN may underlie central tinnitus. The neurotransmitter serotonin (5-hydroxytryptamine; 5-HT), is associated with many distinct behavioral or cognitive states, and serotonergic fibers are concentrated in the DCN. However, it remains unclear what is the function of this dense input. Using a combination of in vitro electrophysiology and optogenetics in mouse brain slices, we found that 5-HT directly enhances the excitability of fusiform principal cells via activation of two distinct 5-HT receptor subfamilies, 5-HT2A/2CR (5-HT2A/2C receptor) and 5-HT7R (5-HT7 receptor). This excitatory effect results from an augmentation of hyperpolarization-activated cyclic nucleotide-gated channels (Ih or HCN channels). The serotonergic regulation of excitability is G-protein-dependent and involves cAMP and Src kinase signaling pathways. Moreover, optogenetic activation of serotonergic axon terminals increased excitability of fusiform cells. Our findings reveal that 5-HT exerts a potent influence on fusiform cells by altering their intrinsic properties, which may enhance the sensitivity of the DCN to sensory input.

The antifungal protein AFP secreted by Aspergillus giganteus does not cause detrimental effects on certain mammalian cells

The antifungal protein AFP is a small, cystein-rich protein secreted by the imperfect ascomycete Aspergillus giganteus. The protein efficiently inhibits the growth of filamentous fungi, including a variety of serious human and plant pathogens mainly of the genera Aspergillus and Fusarium, whereas AFP does not affect the growth of yeast and bacteria. This restricted susceptibility range makes it very attractive for medical or biotechnological use to combat fungal infection and contamination. We, therefore, analyzed whether AFP affects the growth or function of a number of mammalian cells. Here we show that the protein neither provokes any cytotoxic effects on human endothelial cells isolated from the umbilical vein nor activates the immune system. Moreover, potassium currents of neurons and astrocytes do not change in the presence of AFP and neither excitatory processes nor the intracellular calcium homeostasis of cultured skeletal muscle myotubes are affected by AFP. Our data, therefore, suggest that AFP is indeed a promising candidate for the therapeutic or biotechnological use as a potential antifungal agent.

Norfluoxetine and fluoxetine have similar anticonvulsant and Ca2+ channel blocking potencies

Norfluoxetine is the most important active metabolite of the widely used antidepressant fluoxetine but little is known about its pharmacological actions. In this study the anticonvulsant actions of norfluoxetine and fluoxetine were studied and compared to those of phenytoin and clonazepam in pentylenetetrazol-induced mouse epilepsy models. Pretreatment with fluoxetine or norfluoxetine (20mg/kg s.c.), as well as phenytoin (30 mg/kg s.c.) and clonazepam (0.1mg/kg s.c.) significantly increased both the rate and duration of survival, demonstrating a significant protective effect against pentylenetetrazol-induced epilepsy. These effects of norfluoxetine were similar to those of fluoxetine. According to the calculated combined protection scores, both norfluoxetine and fluoxetine were effective from the concentration of 10mg/kg, while the highest protective action was observed with clonazepam. Effects of norfluoxetine and fluoxetine on voltage-gated Ca2+ channels were evaluated by measuring peak Ba2+ current flowing through the Ca2+ channels upon depolarization using whole cell voltage clamp in enzymatically isolated rat cochlear neurons. The current was reduced equally in a concentration-dependent manner by norfluoxetine (EC50=20.4+/-2.7 microM, Hill coefficient=0.86+/-0.1) and fluoxetine (EC50=22.3+/-3.6 microM, Hill coefficient=0.87+/-0.1). It was concluded that the efficacy of the two compounds in neuronal tissues was equal, either in preventing seizure activity or in blocking the neuronal Ca2+ channels.

GABAB-receptor-mediated suppression of sympathetic outflow from the spinal cord of neonatal rats

Using a splanchnic nerve-spinal cord preparation in vitro that could spontaneously generate sympathetic nerve discharge (SND), we investigated the roles of intraspinal GABAB receptors in the regulation of SND. Despite an age-dependent difference in sensitivity, bath applications of baclofen (Bac; GABAB-receptor agonist) consistently reduced SND in a concentration-dependent manner. The drug specificity of Bac in activation of GABAB receptors was verified by application of its antagonist saclofen (Sac) or CGP-46381 (CGP). Sac or CGP alone did not change SND. However, in the presence of Sac or CGP, the effects of Bac on SND inhibition were reversibly attenuated. The splanchnic sympathetic preganglionic neuron (SPN) was recorded by blind whole cell, patch-clamp techniques. We examined Bac effects on electrical membrane properties of SPNs. Applications of Bac reduced excitatory synaptic events, induced membrane hyperpolarizations, and inhibited SPN firing. In the presence of 12 mM Mg2+ or 0.5 μM TTX to block Ca2+- or action potential-dependent synaptic transmissions, applications of Bac induced an outward baseline current that reversed at −29 ± 6 mV. Because the K+ equilibrium potential in our experimental conditions was −100 mV, the Bac-induced currents could not simply be attributed to an alteration of K+ conductance. On the other hand, applications of Bac to Cs+-loaded SPNs reduced Cd2+-sensitive and high-voltage-activated inward currents, indicating an inhibition of voltage-gated Ca2+ currents. Our results suggest that the activation of intraspinal GABAB receptors suppresses SND via a mixture of ion events that may link to a change in Ca2+ conductance.

Presence and distribution of three calcium binding proteins in projection neurons of the adult rat cochlear nucleus

The presence and distribution of three cytoplasmic calcium binding proteins, calbindin, calretinin, and parvalbumin, have been investigated in the projection neurons of the cochlear nucleus complex in adult rats by using immunohistochemistry in free-floating slices. Identification of the individual cell types was carried out on the basis of their intranuclear localization, morphological characteristics, and (in the cases of pyramidal and bushy neurons) by retrograde labeling with rhodamine-dextran. The most important findings were confirmed by using confocal microscopy. The data obtained in these experiments are the first to demonstrate the presence of parvalbumin in pyramidal neurons and globular and spherical bushy cells of rat cochlear nucleus, whereas octopus and giant cells did not show positivity for parvalbumin. Calretinin was not present in either Purkinje-like cells or giant neurons. According to the double immunolabeling co-localization experiments, the pyramidal neurons, Purkinje-like cells, globular bushy cells, and octopus cells express two different calcium binding proteins in their cytoplasm (although in different combinations) whereas giant cells and spherical bushy cells contain solely calbindin and parvalbumin, respectively. The presence of calretinin in globular bushy cells provides a tool for distinguishing them from spherical bushy cells. The immunolabeling of the fibers and axonal endings of the acoustic nerve in the ventral part of the cochlear nucleus indicated that these structures are also parvalbumin positive. It is concluded that the heterogenous cell composition of the cochlear nucleus is accompanied by a rather complex expression pattern of the cytoplasmic calcium binding proteins.

Localization of the GABAB receptor 1a/b subunit relative to glutamatergic synapses in the dorsal cochlear nucleus of the rat

Metabotropic gamma-aminobutyric acid receptors (GABA(B)) are involved in pre- and postsynaptic inhibitory effects upon auditory neurons and have been implicated in different aspects of acoustic information processing. To understand better the mechanisms by which GABA(B) receptors mediate their inhibitory effects, we used pre-embedding immunocytochemical techniques combined with quantification of immunogold particles to reveal the precise subcellular distribution of the GABA(B1) subunit in the rat dorsal cochlear nucleus. At the light microscopic level, GABA(B1) was detected in all divisions of the cochlear complex. The most intense immunoreactivity for GABA(B1) was found in the dorsal cochlear nucleus, whereas immunoreactivity in the anteroventral and posteroventral cochlear nuclei was very low. In the dorsal cochlear nucleus, a punctate labeling was observed in the superficial (molecular and fusiform cell) layers. At the electron microscopic level, GABA(B1) was found at both post- and presynaptic locations. Postsynaptically, GABA(B1) was localized mainly in the dendritic spines of presumed fusiform cells. Quantitative immunogold immunocytochemistry revealed that the highest concentration of GABA(B1) in the plasma membrane was in dendritic spines, followed by dendritic shafts and somata. Thus, the most intense immunoreactivity for GABA(B1) was observed in dendritic spines with a high density of immunogold particles at extrasynaptic sites, peaking around 300 nm from glutamatergic synapses. This is in contrast to GABAergic synapses, in which GABA(B1) was only occasionally found. Presynaptically, receptor immunoreactivity was detected primarily in axospinous endings, probably from granule cells, in both the active zone and extrasynaptic sites. The localization of GABA(B1) relative to synaptic sites in the DCN suggests a role for the receptor in the regulation of dendritic excitability and excitatory inputs.

Cellular regulatory mechanisms influencing the activity of the cochlear nucleus: a review

The cochlear nucleus is the site in the auditory pathway where the primary sensory information carried by the fibres of the acoustic nerve is transmitted to the second-order neurones. According to the generally accepted view this transmission is not a simple relay process but is considered as the first stage where the decoding of the auditory information begins. This notion is based on the diverse neurone composition and highly ordered structure of the nucleus, on the complex electrophysiological properties and activity patterns of the neurones, on the activity of local and descending modulatory mechanisms and on the presence of a highly sophisticated intracellular Ca2+ homeostasis. This review puts emphasis on introducing the experimental findings supporting the above statements and on the questions which should be answered in order to gain a better understanding of the function of the cochlear nucleus.

Dendritic Ca2+ transients evoked by action potentials in rat dorsal cochlear nucleus pyramidal and cartwheel neurons

Simultaneous fluorescence imaging and electrophysiologic recordings were used to investigate the Ca(2+) influx initiated by action potentials (APs) into dorsal cochlear nucleus (DCN) pyramidal cell (PC) and cartwheel cell (CWC) dendrites. Local application of Cd(2+) blocked Ca(2+) transients in PC and CWC dendrites, demonstrating that the Ca(2+) influx was initiated by dendritic Ca(2+) channels. In PCs, TTX eliminated the dendritic Ca(2+) transients when APs were completely blocked. However, the Ca(2+) influx could be partially recovered during an incomplete block of APs or when a large depolarization was substituted for the blocked APs. In CWCs, dendritic Ca(2+) transients evoked by individual APs, or simple spikes, were blocked by TTX and could be recovered during an incomplete block of APs or by a large depolarization. In contrast, dendritic Ca(2+) transients evoked by complex spikes, a burst of APs superimposed on a slow depolarization, were not blocked by TTX, despite eliminating the APs superimposed on the slow depolarization. These results suggest two different mechanisms for the retrograde activation of dendritic Ca(2+) channels: the first requires fast Na(+) channel-mediated APs or a large somatic depolarization, whereas the second is independent of Na(+) channel activation, requiring only the slow depolarization underlying complex spikes.

Removal of Ca(2+) following depolarization-evoked cytoplasmic Ca(2+) transients in freshly dissociated pyramidal neurones of the rat dorsal cochlear nucleus

Cytoplasmic [Ca(2+)] ([Ca(2+)](i)) was measured using Fura-2 in pyramidal neurones isolated from the rat dorsal cochlear nucleus (DCN). The kinetic properties of Ca(2+) removal following K(+) depolarization-induced Ca(2+) transients were characterized by fitting exponential functions to the decay phase. The removal after small transients (<82 nM peak [Ca(2+)](i)) had monophasic time course (time constant of 6.43 +/- 0.48 s). In the cases of higher Ca(2+) transients biphasic decay was found. The early time constant decreased (from 3.09 +/- 0.26 to 1.46 +/- 0.11 s) as the peak intracellular [Ca(2+)] increased. The value of the late time constant was 18.15 +/- 1.60 s at the smallest transients, and showed less dependence on [Ca(2+)](i). Blockers of Ca(2+) uptake into intracellular stores (thapsigargin and cyclopiazonic acid) decreased the amplitude of the Ca(2+) transients and slowed their decay. La(3+) (3 mM) applied extracellularly during the declining phase dramatically changed the time course of the Ca(2+) transients as a plateau developed and persisted until the La(3+) was present. When the other Ca(2+) removal mechanisms were available, reduction of the external [Na(+)] to inhibit the Na(+)/Ca(2+) exchange resulted in a moderate increase of the time constants. It is concluded that in the isolated pyramidal neurones of the DCN the removal of Ca(2+) depends mainly on the activity of Ca(2+) pump mechanisms.