Reduction in mitochondrial ATP synthesis mimics the effect of low glucose in depolarizing neurons from the subpostremal nucleus of the solitary tract of rats.

Neurons of the subpostremal nucleus of the solitary tract (NTS) respond to changes in extracellular glucose with alterations in membrane potential with both depolarization and hyperpolarization. From 5 mM glucose, a rapid shift to 0.5 mM glucose produces a membrane depolarization by an unknown mechanism in most neurons. However, the mechanism involved in this response needs to be known. Here, we investigated if the low glucose-induced depolarization could be mimicked by reducing ATP synthesis and possible mediators of this effect. We showed that applying the mitochondrial uncoupler CCCP (1 µM) reproduced the effects of low glucose depolarizing the membrane, generating an inward current, and decreasing membrane resistance. On the other hand, activation of AMPK did not alter these parameters. To test if low glucose and CCCP could depolarize the membrane by affecting the ionic gradient, we inhibited the electrogenic Na/K pump with 10 µM of ouabain. We observed a similar membrane depolarization but not a decrease in membrane resistance. We conclude that perfusion of neurons of the subpostremal NTS with a low glucose solution depolarizes the membrane by probably reducing intracellular ATP, but not by activating AMPK or decreasing the ionic gradient across the membrane.

Development of electrophysiological properties of fusiform neurons from the dorsal cochlear nucleus of mice before and after hearing onset.

The dorsal cochlear nucleus (DCN) in the auditory brainstem integrates auditory and somatosensory information. Mature DCN fusiform neurons fall into two qualitatively distinct types: quiet, with no spontaneous regular action potential firing, or active, with regular spontaneous action potential firing. However, how these firing states and other electrophysiological properties of fusiform neurons develop during early postnatal days to adulthood is not known. Thus, we recorded fusiform neurons from mice from P4 to P21 and analyzed their electrophysiological properties. In the prehearing phase (P4-P13), we found that most fusiform neurons are quiet, with active neurons emerging after hearing onset at P14. Subthreshold properties underwent significant changes before hearing onset, whereas changes to the action potential waveform occurred mainly after P14, with the depolarization and repolarization phases becoming markedly faster and half-width significantly decreased. The activity threshold in posthearing neurons was more negative than in prehearing cells. Persistent sodium current (INaP) was increased after P14, coinciding with the emergence of spontaneous firing. Thus, we suggest that posthearing expression of INaP leads to hyperpolarization of the activity threshold and the active state of the fusiform neuron. At the same time, other changes refine the passive membrane properties and increase the speed of action potential firing of fusiform neurons.NEW & NOTEWORTHY Auditory brainstem neurons express unique electrophysiological properties adapted for their complex physiological functions that develop before hearing onset. Fusiform neurons of the DCN present two firing states, quiet and active, but the origin of these states is not known. Here, we showed that the quiet and active states develop after hearing onset at P14, along with changes in action potentials, suggesting an influence of auditory input on the refining of fusiform neuron's excitability.

ATP-sensitive K+ channels control the spontaneous firing of a glycinergic interneuron in the auditory brainstem.

KEY POINTS: Cartwheel neurons provide potent inhibition to fusiform neurons in the dorsal cochlear nucleus (DCN). Most cartwheel neurons fire action potentials spontaneously, but the ion channels responsible for this intrinsic activity are unknown. We investigated the ion channels responsible for the intrinsic firing of cartwheel neurons and the stable resting membrane potential found in a fraction of these neurons (quiet neurons). Among the ion channels controlling membrane potential of cartwheel neurons, the presence of open ATP-sensitive potassium channels (KATP ) is responsible for the existence of quiet neurons. Our results pinpoint KATP channel modulation as a critical factor controlling the firing of cartwheel neurons. Hence, it is a crucial channel influencing the balance of excitation and inhibition in the DCN. ABSTRACT: Cartwheel neurons from the dorsal cochlear nucleus (DCN) are glycinergic interneurons and the primary source of inhibition on the fusiform neurons, the DCN's principal excitatory neuron. Most cartwheel neurons present spontaneous firing (active neurons), producing a steady inhibitory tone on fusiform neurons. In contrast, a small fraction of these neurons do not fire spontaneously (quiet neurons). Hyperactivity of fusiform neurons is seen in animals with behavioural evidence of tinnitus. Because of its relevance in controlling the excitability of fusiform neurons, we investigated the ion channels responsible for the spontaneous firing of cartwheel neurons in DCN slices from rats. We found that quiet neurons presented an outward conductance not seen in active neurons, which generates a stable resting potential. This current was sensitive to tolbutamide, an ATP-sensitive potassium channel (KATP ) antagonist. After inhibition with tolbutamide, quiet neurons start to fire spontaneously, while the active neurons were not affected. On the other hand, in active neurons, KATP agonist diazoxide activated a conductance similar to quiet neurons' KATP conductance and stopped spontaneous firing. According to the effect of KATP channels on cartwheel neuron firing, glycinergic neurotransmission in DCN was increased by tolbutamide and decreased by diazoxide. Our results reveal a role of KATP channels in controlling the spontaneous firing of neurons not involved in fuel homeostasis.

Loss of Brain-Derived Neurotrophic Factor Mediates Inhibition of Hippocampal Long-Term Potentiation by High-Intensity Sound.

Exposure to noise produces cognitive and emotional disorders, and recent studies have shown that auditory stimulation or deprivation affects hippocampal function. Previously, we showed that exposure to high-intensity sound (110 dB, 1 min) strongly inhibits Schaffer-CA1 long-term potentiation (LTP). Here we investigated possible mechanisms involved in this effect. We found that exposure to 110 dB sound activates c-fos expression in hippocampal CA1 and CA3 neurons. Although sound stimulation did not affect glutamatergic or GABAergic neurotransmission in CA1, it did depress the level of brain-derived neurotrophic factor (BDNF), which is involved in promoting hippocampal synaptic plasticity. Moreover, perfusion of slices with BDNF rescued LTP in animals exposed to sound stimulation, whereas BDNF did not affect LTP in sham-stimulated rats. Furthermore, LM22A4, a TrkB receptor agonist, also rescued LTP from sound-stimulated animals. Our results indicate that depression of hippocampal BDNF mediates the inhibition of LTP produced by high-intensity sound stimulation.

A Negative Slope Conductance of the Persistent Sodium Current Prolongs Subthreshold Depolarizations.

Neuronal subthreshold voltage-dependent currents determine membrane properties such as the input resistance (Rin) and the membrane time constant (τm) in the subthreshold range. In contrast with classical cable theory predictions, the persistent sodium current (INaP), a non-inactivating mode of the voltage-dependent sodium current, paradoxically increases Rin and τm when activated. Furthermore, this current amplifies and prolongs synaptic currents in the subthreshold range. Here, using a computational neuronal model, we showed that the creation of a region of negative slope conductance by INaP activation is responsible for these effects and the ability of the negative slope conductance to amplify and prolong Rin and τm relies on the fast activation of INaP. Using dynamic clamp in hippocampal CA1 pyramidal neurons in brain slices, we showed that the effects of INaP on Rin and τm can be recovered by applying an artificial INaP after blocking endogenous INaP with tetrodotoxin. Furthermore, we showed that injection of a pure negative conductance is enough to reproduce the effects of INaP on Rin and τm and is also able to prolong artificial excitatory post synaptic currents. Since both the negative slope conductance and the almost instantaneous activation are critical for producing these effects, the INaP is an ideal current for boosting the amplitude and duration of excitatory post synaptic currents near the action potential threshold.

Inhibitors of oxidative and hydrolytic endocannabinoid degradation do not enhance depolarization-induced suppression of excitation on dorsal cochlear nucleus glycinergic neurons.

Neurons from the dorsal cochlear nucleus (DCN) present endocannabinoid (EC) dependent short-term synaptic plasticity in the form of depolarization-induced suppression of excitation (DSE). Postsynaptic calcium influx promotes EC synthesis and depression of neurotransmission. ECs can be degraded by a hydrolytic and an oxidative pathway, the latter via the enzyme cyclooxygenase 2 (COX-2). Hyperactivity in the DCN is related to the development of tinnitus, which can be induced by high doses of salicylate, a COX-2 inhibitor. Since EC-dependent plasticity in the DCN can affect its excitation-inhibition balance, we investigated the impact of inhibitors of both oxidative and hydrolytic EC metabolism on the DSE from the synapses between the parallel fibers and cartwheel neurons (PF-CW) in the DCN. We found that inhibitors of COX-2 (ibuprofen and indomethacin) did not alter DSE at the PF-CW synapse. Salicylate also did not alter DSE. However, we found that inhibitors of the hydrolytic pathway did not affect DSE magnitude, but surprisingly speeded DSE decay. We conclude that oxidative EC degradation in the PF-CW synapse is not relevant for termination of DSE and are probably not important for controlling this form of synaptic plasticity in the DCN PF-CW synapse. The lack of effect on DSE of high doses of salicylate also suggests that it is not acting by increasing DSE in the PF-CWC synapse. However, the counter intuitive effect of the hydrolytic inhibitors shows that increasing EC on this synapse have more complex effects on DSE.

Intrinsic properties of rostral ventrolateral medulla presympathetic and bulbospinal respiratory neurons of juvenile rats are not affected by chronic intermittent hypoxia.

The presympathetic neurons in the rostral ventrolateral medulla (RVLM) are considered to be the source of the sympathetic activity, and there is experimental evidence that these cells present intrinsic autodepolarization. There is also evidence that an important respiratory neuronal population located in the RVLM/Bötzinger complex (BötC) corresponds to augmenting expiratory neurons (aug-E), which send projections to the phrenic nucleus in the spinal cord. However, the pacemaker activity of presympathetic neurons and the intrinsic properties of aug-E neurons had not been evaluated in brainstem slices of juvenile rats (postnatal day 35). Chronic intermittent hypoxia (CIH) is a sympathetic-mediated hypertension model, which seems to produce an associated increase in the activity of aug-E neurons. In this study, we evaluated the effects of CIH on the intrinsic properties of RVLM/BötC presympathetic and phrenic nucleus-projecting neurons (aug-E) in brainstem slices of juvenile rats (postnatal day 35). We observed that all presympathetic neurons presented spontaneous action potential firing (n = 18), which was not abolished by ionotropic receptor antagonism. In addition, exposure to 10 days of CIH produced no changes in their intrinsic passive properties, firing pattern or excitability. Most aug-E neurons presented spontaneous firing in control conditions (13 of 15 neurons), and this characteristic was preserved after blocking fast synaptic transmission (12 of 15 neurons), clearly demonstrating their intrinsic pacemaker activity. Chronic intermittent hypoxia also produced no changes in intrinsic passive properties, frequency and pattern of discharge or excitability of the aug-E neurons. The present study shows that: (i) it is possible to record the electrophysiological properties of RVLM/BötC presympathetic and aug-E neurons in brainstem slices from juvenile rats; (ii) these neurons present characteristics of intrinsic pacemakers; and (iii) their intrinsic properties were not altered by chronic intermittent hypoxia.

Chronic intermittent hypoxia depresses afferent neurotransmission in NTS neurons by a reduction in the number of active synapses.

Long-term synaptic plasticity has been recently described in brainstem areas associated to visceral afferent sensory integration. Chronic intermittent hypoxia (CIH), an animal model for studying obstructive sleep apnea in humans, depresses the afferent neurotransmission in nucleus tractus solitarii (NTS) neurons, which affect respiratory and autonomic regulation. Here we identified the synaptic mechanisms of CIH-induced depression of the afferent neurotransmission in NTS neurons in juvenile rats. We verified that CIH reduced the amplitude of both NMDA and non-NMDA glutamatergic excitatory currents (eEPSCs) evoked by tractus solitarii stimulation (TS-eEPSC) of second-order neurons in the NTS. No changes were observed in release probability, evidenced by absence of any CIH-elicited effects on short-term depression and failures in EPSCs evoked in low calcium. CIH also produced no changes in TS-eEPSC quantal size, since the amplitudes of both low calcium-evoked EPSCs and asynchronous TS-eEPSCs (evoked in the presence of Sr(2+)) were unchanged. Using single TS afferent fiber stimulation in slices from control and CIH rats we clearly show that CIH reduced the quantal content of the TS-eEPSCs without affecting the quantal size or release probability, suggesting a reduction in the number of active synapses as the mechanism of CIH induced TS-eEPSC depression. In accordance with this concept, the input-output relationship of stimulus intensity and TS-eEPSC amplitude shows an early saturation in CIH animals. These findings open new perspectives for a better understanding of the mechanisms underlying the synaptic plasticity in the brainstem sensory neurons under challenges such as those produced by CIH in experimental and pathological conditions.

Elucidating the neurotoxicity of the star fruit.

Caramboxin: Patients suffering from chronic kidney disease are frequently intoxicated after ingesting star fruit. The main symptoms of this intoxication are named in the picture. Bioguided chemical procedures resulted in the discovery of caramboxin, which is a phenylalanine-like molecule that is responsible for intoxication. Functional experiments in vivo and in vitro point towards the glutamatergic ionotropic molecular actions of caramboxin, which explains its convulsant and neurodegenerative properties.

Diverse levels of an inwardly rectifying potassium conductance generate heterogeneous neuronal behavior in a population of dorsal cochlear nucleus pyramidal neurons.

Homeostatic mechanisms maintain homogeneous neuronal behavior among neurons that exhibit substantial variability in the expression levels of their ionic conductances. In contrast, the mechanisms, which generate heterogeneous neuronal behavior across a neuronal population, remain poorly understood. We addressed this problem in the dorsal cochlear nucleus, where principal neurons exist in two qualitatively distinct states: spontaneously active or not spontaneously active. Our studies reveal that distinct activity states are generated by the differential levels of a Ba(2+)-sensitive, inwardly rectifying potassium conductance (K(ir)). Variability in K(ir) maximal conductance causes variations in the resting membrane potential (RMP). Low K(ir) conductance depolarizes RMP to voltages above the threshold for activating subthreshold-persistent sodium channels (Na(p)). Once Na(p) channels are activated, the RMP becomes unstable, and spontaneous firing is triggered. Our results provide a biophysical mechanism for generating neural heterogeneity, which may play a role in the encoding of sensory information.

Increased bursting glutamatergic neurotransmission in an auditory forebrain area of the zebra finch (Taenopygia guttata) induced by auditory stimulation.

The caudomedial nidopallium (NCM) is a telencephalic area involved in auditory processing and memorization in songbirds, but the synaptic mechanisms associated with auditory processing in NCM are largely unknown. To identify potential changes in synaptic transmission induced by auditory stimulation in NCM, we used a slice preparation for path-clamp recordings of synaptic currents in the NCM of adult zebra finches (Taenopygia guttata) sacrificed after sound isolation followed by exposure to conspecific song or silence. Although post-synaptic GABAergic and glutamatergic currents in the NCM of control and song-exposed birds did not present any differences regarding their frequency, amplitude and duration after song exposure, we observed a higher probability of generation of bursting glutamatergic currents after blockade of GABAergic transmission in song-exposed birds as compared to controls. Both song-exposed males and females presented an increase in the probability of the expression of bursting glutamatergic currents, however bursting was more commonly seen in males where they appeared even without blocking GABAergic transmission. Our data show that song exposure changes the excitability of the glutamatergic neuronal network, increasing the probability of the generation of bursts of glutamatergic currents, but does not affect basic parameters of glutamatergic and GABAergic synaptic currents.

Salicylate activates KATP channels and reduces spontaneous firing in glycinergic cartwheel neurons in the dorsal cochlear nucleus of rats.

High doses of salicylate induce tinnitus in humans and experimental animals. The Dorsal Cochlear Nucleus is implicated with the genesis of tinnitus, and increased activity in this nucleus is seen in animal models of tinnitus. Incubation of brainstem slices containing the DCN with millimolar salicylate reduces the spontaneous firing of glycinergic cartwheel neurons and glycinergic neurotransmission on fusiform neurons, the principal neuron of this nucleus. However, the mechanism of salicylate mediating this effect is not known. Recently, we have shown that KATP channels strongly modulate the spontaneous firing of cartwheel neurons. We tested if KATP channels could mediate the effects of salicylate on cartwheel neurons. Perfusion of 1.4 mM salicylate hyperpolarizes the membrane of cartwheel neurons and stops firing. Salicylate produces an outward current similar to the KATP current seen in quiet cartwheel neurons. Activation of this current is occluded by the KATP agonist diazoxide, which is produced by the opening of KATP channels. The antagonist of AMP-kinase (AMPK), dorsomorphim, inhibited salicylate effects, suggesting that they could be mediated by activation of this kinase. Still, the AMPK agonist, AICAR, did not reproduce salicylate effects but occluded them. Additionally, inhibiting mitochondrial ATP synthesis with the protonophore CCCP reproduced, albeit with less efficacy, and inhibited the effects of salicylate. We concluded that salicylate in millimolar concentrations opens KATP channels in DCN cartwheel neurons, inhibiting spontaneous firing of these neurons, probably by activating AMPK and reducing mitochondrial ATP synthesis.

Meningeal dendritic cells drive neuropathic pain through elevation of the kynurenine metabolic pathway in mice.

Neuropathic pain is one of the most important clinical consequences of injury to the somatosensory system. Nevertheless, the critical pathophysiological mechanisms involved in neuropathic pain development are poorly understood. In this study, we found that neuropathic pain is abrogated when the kynurenine metabolic pathway (KYNPATH) initiated by the enzyme indoleamine 2,3-dioxygenase 1 (IDO1) is ablated pharmacologically or genetically. Mechanistically, it was found that IDO1-expressing dendritic cells (DCs) accumulated in the dorsal root leptomeninges and led to an increase in kynurenine levels in the spinal cord. In the spinal cord, kynurenine was metabolized by kynurenine-3-monooxygenase-expressing astrocytes into the pronociceptive metabolite 3-hydroxykynurenine. Ultimately, 3-hydroxyanthranilate 3,4-dioxygenase-derived quinolinic acid formed in the final step of the canonical KYNPATH was also involved in neuropathic pain development through the activation of the glutamatergic N-methyl-D-aspartate receptor. In conclusion, these data revealed a role for DCs driving neuropathic pain development through elevation of the KYNPATH. This paradigm offers potential new targets for drug development against this type of chronic pain.

Inhaled molecular hydrogen attenuates intense acute exercise-induced hippocampal inflammation in sedentary rats.

Physical exercise-induced inflammation may be beneficial when exercise is regular but it may be harmful when exercise is intense and performed by unaccustomed individuals/rats. Molecular hydrogen (H2) has recently emerged as a powerful anti-inflammatory, antioxidant and anti-apoptotic molecule in a number of pathological conditions, but little is known about its putative role under physiological conditions such as physical exercise. Therefore, we tested the hypothesis that H2 decreases intense acute exercise-induced inflammation in the hippocampus, since it is a brain region particularly susceptible to inflammation. Moreover, we also assessed hippocampus oxidative status. Rats ran on a sealed treadmill inhaling either the H2 (2% H2, 21% O2, balanced with N2) or the control gas (0% H2, 21% O2, balanced with N2) and hippocampal samples were collected immediately or 3 h after exercise. We measured hippocampal levels of cytokines [tumor necrosis factor-α (TNF-α), interleukin (IL)-1ß, IL-6 and IL-10] and oxidative markers [superoxide dismutase (SOD), thiobarbituric acid reactive species (TBARS) and nitrite/nitrate (NOx)]. Exercise increased TNF-α, IL-6 and IL-10 immediately after the session, whereas no change in IL-1ß levels was observed. Conversely, exercise did not cause any change in SOD activity, TBARS and NOx levels. H2 inhibited the exercise-induced surges in TNF-α and IL-6, and potentiated the IL-10 surge, immediately after the exercise. Moreover, no change in IL1-ß levels of rats inhaling H2 was observed. Regarding the oxidative stress markers, H2 failed to cause any change in SOD activity, TBARS and NOx levels. No significant change was observed in any of the assessed parameters 3 h after the exercise bout. These data are consistent with the notion that H2 acts as a powerful anti-inflammatory agent not only down-modulating pro-inflammatory cytokines (TNF-α and IL-6) but also upregulating an anti-inflammatory cytokine (IL-10) production without affecting the local oxidative stress status. These data indicate that H2 effectively decreases exercise-induced inflammation in the hippocampus, despite the fact that this region is particularly prone to inflammatory insults.

Are L-glutamate and ATP cotransmitters of the peripheral chemoreflex in the rat nucleus tractus solitarius?

Peripheral chemoreflex activation in awake rats or in the working heart-brainstem preparation (WHBP) produces sympathoexcitation, bradycardia and an increase in the frequency of phrenic nerve activity. Our focus is the neurotransmission of the sympathoexcitatory component of the chemoreflex within the nucleus of the tractus solitarius (NTS), and recently we verified that the simultaneous antagonism of ionotropic glutamate and purinergic P(2) receptors in the NTS blocked the pressor response and increased thoracic sympathetic activity in awake rats and WHBP, respectively, in response to peripheral chemoreflex activation. These previous data suggested the involvement of ATP and L-glutamate in the NTS in the processing of the sympathoexcitatory component of the chemoreflex by unknown mechanisms. For a better understanding of these mechanisms, here we used a patch-clamp approach in brainstem slices to evaluate the characteristics of the synaptic transmission of NTS neurons sending projections to the ventral medulla, which include the premotor neurons involved in the generation of the sympathetic outflow. The NTS neurons sending projections to the ventral medulla were identified by previous microinjection of the membrane tracer dye, 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI), in the ventral medulla and the spontaneous (sEPSCs) and tractus solitarius (TS)-evoked excitatory postsynaptic current (TS-eEPSCs) were recorded using patch clamp. With this approach, we made the following observations on NTS neurons projecting to the ventral medulla: (i) the sEPSCs and TS-eEPSCs of DiI-labelled NTS neurons were completely abolished by 6,7-dinitroquinoxaline-2,3(1H,4H)-dione (DNQX), an antagonist of ionotropic non-NMDA glutamatergic receptors, showing that they are mediated by L-glutamate; (ii) application of ATP increased the frequency of appearance of spontaneous glutamatergic currents, reflecting an increased exocytosis of glutamatergic vesicles; and (iii) ATP decreased the peak of TS-evoked glutamatergic currents. We conclude that L-glutamate is the main neurotransmitter of spontaneous and TS-evoked synaptic activities in the NTS neurons projecting to the ventral medulla and that ATP has a dual modulatory role on this excitatory transmission, facilitating the spontaneous glutamatergic transmission and inhibiting the TS-evoked glutamatergic transmission. These data also suggest that ATP is not acting as a cotransmitter with L-glutamate, at least at the level of this subpopulation of NTS neurons studied.

Myosin Va phosphorylated on Ser1650 is found in nuclear speckles and redistributes to nucleoli upon inhibition of transcription.

Nuclear actin and nuclear myosins have been implicated in the regulation of gene expression in vertebrate cells. Myosin V is a class of actin-based motor proteins involved in cytoplasmic vesicle transport and anchorage, spindle-pole alignment and mRNA translocation. In this study, myosin-Va, phosphorylated on a conserved serine in the tail domain (phospho-ser(1650) MVa), was localized to subnuclear compartments. A monoclonal antibody, 9E6, raised against a peptide corresponding to phosphoserine(1650) and flanking regions of the murine myosin Va sequence, was immunoreactive to myosin Va heavy chain in cellular and nuclear extracts of HeLa cells, PC12 cells and B16-F10 melanocytes. Immunofluorescence microscopy with this antibody revealed discrete irregular spots within the nucleoplasm that colocalized with SC35, a splicing factor that earmarks nuclear speckles. Phospho-ser(1650) MVa was not detected in other nuclear compartments, such as condensed chromatin, Cajal bodies, gems and perinucleolar caps. Although nucleoli also were not labeled by 9E6 under normal conditions, inhibition of transcription in HeLa cells by actinomycin D caused the redistribution of phospho-ser(1650) MVa to nucleoli, as well as separating a fraction of phospho-ser(1650) MVa from SC35 into near-neighboring particles. These observations indicate a novel role for myosin Va in nuclear compartmentalization and offer a new lead towards the understanding of actomyosin-based gene regulation.

The ion channels and synapses responsible for the physiological diversity of mammalian lower brainstem auditory neurons.

The auditory part of the brainstem is composed of several nuclei specialized in the computation of the different spectral and temporal features of the sound before it reaches the higher auditory regions. There are a high diversity of neuronal types in these nuclei, many with remarkable electrophysiological and synaptic properties unique to these structures. This diversity reflects specializations necessary to process the different auditory signals in order to extract precisely the acoustic information necessary for the auditory perception by the animal. Low threshold Kv1 channels and HCN channels are expressed in neurons that use timing clues for auditory processing, like bushy and octopus cells, in order to restrict action potential firing and reduce input resistance and membrane time constant. Kv3 channels allow principal neurons of the MNTB and pyramidal DCN neurons to fire fast trains of action potentials. Calcium channels on cartwheel DCN neurons produce complex spikes characteristic of these neurons. Calyceal synapses compensate the low input resistance of bushy and principal neurons of the MNTB by releasing hundreds of glutamate vesicles resulting in large EPSCs acting in fast ionotropic glutamate receptors, in order to reduce temporal summation of synaptic potentials, allowing more precise correspondence of pre- and post-synaptic potentials, and phase-locking. Pre-synaptic calyceal sodium channels have fast recovery from inactivation allowing extremely fast trains of action potential firing, and persistent sodium channels produce spontaneous activity of fusiform neurons at rest, which expands the dynamic range of these neurons. The unique combinations of different ion channels, ionotropic receptors and synaptic structures create a unique functional diversity of neurons extremely adapted to their complex functions in the auditory processing.

Increased hippocampal GABAergic inhibition after long-term high-intensity sound exposure.

Exposure to loud sounds is related to harmful mental and systemic effects. The hippocampal function can be affected to either high-intensity sound exposure or long-term sound deprivation. We previously showed that hippocampal long-term potentiation (LTP) is inhibited after ten days of daily exposure to 2 minutes of high-intensity noise (110 dB), in the hippocampi of Wistar rats. Here we investigated how the glutamatergic and GABAergic neurotransmission mediated by ionotropic receptors is affected by the same protocol of high-intensity sound exposure. We found that while the glutamatergic transmission both by AMPA/kainate and NMDA receptors in the Schaffer-CA1 synapses is unaffected by long-term exposure to high-intensity sound, the amplitude of the inhibitory GABAergic currents is potentiated, but not the frequency of both spontaneous and miniature currents. We conclude that after prolonged exposure to short periods of high-intensity sound, GABAergic transmission is potentiated in the hippocampal CA1 pyramidal neurons. This effect could be an essential factor for the reduced LTP in the hippocampi of these animals after high-intensity sound exposure. We conclude that prolonged exposure to high- intensity sound could affect hippocampal inhibitory transmission and consequently, its function.

Optimizing synaptic architecture and efficiency for high-frequency transmission.

Bursts of neuronal activity are transmitted more effectively as synapses mature. However, the mechanisms that control synaptic efficiency during development are poorly understood. Here, we study postnatal changes in synaptic ultrastructure and exocytosis in a calyx-type nerve terminal. Vesicle pool size, exocytotic efficiency (amount of exocytosis per Ca influx), Ca current facilitation, and the number of active zones (AZs) increased with age, whereas AZ area, number of docked vesicles per AZ, and release probability decreased with age. These changes led to AZs that are less prone to multivesicular release, resulting in reduced AMPA receptor saturation and desensitization. A greater multiplicity of small AZs with few docked vesicles, a larger pool of releasable vesicles, and a higher efficiency of release thus promote prolonged high-frequency firing in mature synapses.

A novel role for MNTB neuron dendrites in regulating action potential amplitude and cell excitability during repetitive firing.

Principal cells of the medial nucleus of the trapezoid body (MNTB) are simple round neurons that receive a large excitatory synapse (the calyx of Held) and many small inhibitory synapses on the soma. Strangely, these neurons also possess one or two short tufted dendrites, whose function is unknown. Here we assess the role of these MNTB cell dendrites using patch-clamp recordings, imaging and immunohistochemistry techniques. Using outside-out patches and immunohistochemistry, we demonstrate the presence of dendritic Na+ channels. Current-clamp recordings show that tetrodotoxin applied onto dendrites impairs action potential (AP) firing. Using Na+ imaging, we show that the dendrite may serve to maintain AP amplitudes during high-frequency firing, as Na+ clearance indendritic compartments is faster than axonal compartments. Prolonged high-frequency firing can diminish Na+ gradients in the axon while the dendritic gradient remains closer to resting conditions; therefore, the dendrite can provide additional inward current during prolonged firing. Using electron microscopy, we demonstrate that there are small excitatory synaptic boutons on dendrites. Multi-compartment MNTB cell simulations show that, with an active dendrite, dendritic excitatory postsynaptic currents (EPSCs) elicit delayed APs compared with calyceal EPSCs. Together with high- and low-threshold voltage-gated K+ currents, we suggest that the function of the MNTB dendrite is to improve high-fidelity firing, and our modelling results indicate that an active dendrite could contribute to a 'dual' firing mode for MNTB cells (an instantaneous response to calyceal inputs and a delayed response to non-calyceal dendritic excitatory postsynaptic potentials).