Modulatory effects of noradrenergic and serotonergic signaling pathway on neurovascular coupling.

Dynamic changes in astrocyte Ca are recognized as contributors to functional hyperemia, a critical response to increased neuronal activity mediated by a process known as neurovascular coupling (NVC). Although the critical role of glutamatergic signaling in this process has been extensively investigated, the impact of behavioral state, and the release of behavior-associated neurotransmitters, such as norepinephrine and serotonin, on astrocyte Ca dynamics and functional hyperemia have received less attention. We used two-photon imaging of the barrel cortex in awake mice to examine the role of noradrenergic and serotonergic projections in NVC.

Basal lamina: A novel pH regulator at the neuromuscular junction.

Proton concentration can change within the cleft during synaptic activity due to vesicular release and Ca extrusion from cellular compartments. These changes within the synaptic cleft can impact neural activity by proton-dependent modulation of ion channel function. The pH transient differs in magnitude and direction between synapses, requiring different synapse types to be measured to generate a complete understanding of this mechanism and its impacts on physiology.

Mitochondrial phosphagen kinases support the volatile power demands of motor nerve terminals.

Motor neurons are the longest neurons in the body, with axon terminals separated from the soma by as much as a meter. These terminals are largely autonomous with regard to their bioenergetic metabolism and must burn energy at a high rate to sustain muscle contraction. Here, through computer simulation and drawing on previously published empirical data, we determined that motor neuron terminals in Drosophila larvae experience highly volatile power demands.

Modulatory Effects of Noradrenergic and Serotonergic Signaling Pathway on Neurovascular Coupling.

Dynamic changes in astrocyte Ca are recognized as contributors to functional hyperemia, a critical response to increased neuronal activity mediated by a process known as neurovascular coupling (NVC). Although the critical role of glutamatergic signaling in this process has been extensively investigated, the impact of behavioral state, and the release of behavior-associated neurotransmitters, such as norepinephrine and serotonin, on astrocyte Ca dynamics and functional hyperemia have received less attention. We used two-photon imaging of the barrel cortex in awake mice to examine the role of noradrenergic and serotonergic projections in NVC.

Preserving and enhancing mitochondrial function after stroke to protect and repair the neurovascular unit: novel opportunities for nanoparticle-based drug delivery.

The neurovascular unit (NVU) is composed of vascular cells, glia, and neurons that form the basic component of the blood brain barrier. This intricate structure rapidly adjusts cerebral blood flow to match the metabolic needs of brain activity. However, the NVU is exquisitely sensitive to damage and displays limited repair after a stroke.

Postsynaptic Calcium Extrusion at the Mouse Neuromuscular Junction Alkalinizes the Synaptic Cleft.

Neurotransmission is shaped by extracellular pH. Alkalization enhances pH-sensitive transmitter release and receptor activation, whereas acidification inhibits these processes and can activate acid-sensitive conductances in the synaptic cleft. Previous work has shown that the synaptic cleft can either acidify because of synaptic vesicular release and/or alkalize because of Ca extrusion by the plasma membrane ATPase (PMCA).

Developmental shift to mitochondrial respiration for energetic support of sustained transmission during maturation at the calyx of Held.

A considerable amount of energy is expended following presynaptic activity to regenerate electrical polarization and maintain efficient release and recycling of neurotransmitter. Mitochondria are the major suppliers of neuronal energy, generating ATP via oxidative phosphorylation. However, the specific utilization of energy from cytosolic glycolysis rather than mitochondrial respiration at the presynaptic terminal during synaptic activity remains unclear and controversial.

Neuronal Glutamatergic Synaptic Clefts Alkalinize Rather Than Acidify during Neurotransmission.

The dogma that the synaptic cleft acidifies during neurotransmission is based on the corelease of neurotransmitters and protons from synaptic vesicles, and is supported by direct data from sensory ribbon-type synapses. However, it is unclear whether acidification occurs at non-ribbon-type synapses. Here we used genetically encoded fluorescent pH indicators to examine cleft pH at conventional neuronal synapses.

Presynaptic loss of dynamin-related protein 1 impairs synaptic vesicle release and recycling at the mouse calyx of Held.

Key Points: This study characterizes the mechanisms underlying defects in synaptic transmission when dynamin-related protein 1 (DRP1) is genetically eliminated. Viral-mediated knockout of DRP1 from the presynaptic terminal at the mouse calyx of Held increased initial release probability, reduced the size of the synaptic vesicle recycling pool and impaired synaptic vesicle recycling. Transmission defects could be partially restored by increasing the intracellular calcium buffering capacity with EGTA-AM, implying close coupling of Ca channels to synaptic vesicles was compromised.

Presynaptic GCaMP expression decreases vesicle release probability at the calyx of Held.

Synaptic vesicle (SV) exocytosis is intimately dependent on free local Ca near active zones. Genetically encoded calcium indicators (GECIs) have become an indispensable tool to monitor calcium dynamics during physiological responses, and they are widely used as a proxy to monitor activity in neuronal ensembles and at synaptic terminals. However, GECIs' ability to bind Ca at physiologically relevant concentration makes them strong candidates to affect calcium homeostasis and alter synaptic transmission by exogenously increasing Ca buffering.

Age-related defects in short-term plasticity are reversed by acetyl-L-carnitine at the mouse calyx of Held.

Hearing acuity and sound localization are affected by aging and may contribute to cognitive dementias. Although loss of sensorineural conduction is well documented to occur with age, little is known regarding short-term synaptic plasticity in central auditory nuclei. Age-related changes in synaptic transmission properties were evaluated at the mouse calyx of Held, a sign-inverting relay synapse in the circuit for sound localization, in juvenile adults (1 month old) and late middle-aged (18-21 months old) mice.

Activity-induced Ca signaling in perisynaptic Schwann cells of the early postnatal mouse is mediated by P2Y receptors and regulates muscle fatigue.

Perisynaptic glial cells respond to neural activity by increasing cytosolic calcium, but the significance of this pathway is unclear. Terminal/perisynaptic Schwann cells (TPSCs) are a perisynaptic glial cell at the neuromuscular junction that respond to nerve-derived substances such as acetylcholine and purines. Here, we provide genetic evidence that activity-induced calcium accumulation in neonatal TPSCs is mediated exclusively by one subtype of metabotropic purinergic receptor.

Glycolysis selectively shapes the presynaptic action potential waveform.

Mitochondria are major suppliers of cellular energy in neurons; however, utilization of energy from glycolysis vs. mitochondrial oxidative phosphorylation (OxPhos) in the presynaptic compartment during neurotransmission is largely unknown. Using presynaptic and postsynaptic recordings from the mouse calyx of Held, we examined the effect of acute selective pharmacological inhibition of glycolysis or mitochondrial OxPhos on multiple mechanisms regulating presynaptic function.

A Well-Defined Readily Releasable Pool with Fixed Capacity for Storing Vesicles at Calyx of Held.

The readily releasable pool (RRP) of vesicles is a core concept in studies of presynaptic function. However, operating principles lack consensus definition and the utility for quantitative analysis has been questioned. Here we confirm that RRPs at calyces of Held from 14 to 21 day old mice have a fixed capacity for storing vesicles that is not modulated by Ca2+.

Overexpression of synapsin Ia in the rat calyx of Held accelerates short-term plasticity and decreases synaptic vesicle volume and active zone area.

Synapsins are synaptic vesicle (SV) proteins organizing a component of the reserve pool of vesicles at most central nervous system synapses. Alternative splicing of the three mammalian genes results in multiple isoforms that may differentially contribute to the organization and maintenance of the SV pools. To address this, we first characterized the expression pattern of synapsin isoforms in the rat calyx of Held.

Dysmyelination of auditory afferent axons increases the jitter of action potential timing during high-frequency firing.

Auditory neuropathies are linked to loss of temporal acuity of sound-evoked signals, which may be related to myelin loss. However, it is not known how myelin loss affects the waveform and temporal precision of action potentials (APs) in auditory CNS nerve terminals. Here we investigated the excitability of the calyx of Held nerve terminal in dysmyelinated auditory brainstems using the Long-Evans Shaker (LES) rat, a spontaneous mutant where compact myelin wrapping does not occur due to a genetic deletion of myelin basic protein.

Synaptic inhibition in the olfactory bulb accelerates odor discrimination in mice.

Local inhibitory circuits are thought to shape neuronal information processing in the central nervous system, but it remains unclear how specific properties of inhibitory neuronal interactions translate into behavioral performance. In the olfactory bulb, inhibition of mitral/tufted cells via granule cells may contribute to odor discrimination behavior by refining neuronal representations of odors. Here we show that selective deletion of the AMPA receptor subunit GluA2 in granule cells boosted synaptic Ca(2+) influx, increasing inhibition of mitral cells.

Synaptic vesicle endocytosis: fast and slow modes of membrane retrieval.

Several modes of synaptic vesicle release, retrieval and recycling have been identified. In a well-established mode of exocytosis, termed 'full-collapse fusion', vesicles empty their neurotransmitter content fully into the synaptic cleft by flattening out and becoming part of the presynaptic membrane. The fused vesicle membrane is then reinternalized via a slow and clathrin-dependent mode of compensatory endocytosis that takes several seconds.

Synaptic vesicle endocytosis at a CNS nerve terminal: faster kinetics at physiological temperatures and increased endocytotic capacity during maturation.

Synaptic vesicle membrane must be quickly retrieved and recycled after copious exocytosis to limit the depletion of vesicle pools. The rate of endocytosis at the calyx of Held nerve terminal has been measured directly using membrane capacitance measurements from immature postnatal day P7-P10 rat pups at room temperature (RT: 23-24 degrees C). This rate has an average time constant of tens of seconds and becomes slower when the amount of exocytosis (measured as capacitance jump) increases.

Physiological temperatures reduce the rate of vesicle pool depletion and short-term depression via an acceleration of vesicle recruitment.

The timing and strength of synaptic transmission is profoundly dependent on temperature. However, the temperature dependence of the multiple mechanisms that contribute to short-term synaptic plasticity is poorly understood. Here, we use voltage-clamp recordings to quantify the temperature dependence of exocytosis at the calyx of Held synapse.

Glutamate transporter studies reveal the pruning of metabotropic glutamate receptors and absence of AMPA receptor desensitization at mature calyx of Held synapses.

We examined the effect of glutamate transporter blockade at the calyx of Held synapse. In immature synapses [defined as postnatal day 8 (P8) to P10 rats], transporter blockade causes tonic activation of NMDA receptors and strong inhibition of the AMPA receptor-mediated EPSC amplitude. EPSC inhibition was blocked with a metabotropic glutamate receptor (mGluR) antagonist [1 microm LY341495 (2S-2-amino-2-(1S,2S-2-carboxycycloprop-1-yl)-3-(xanth-9-yl)propanoic acid)], suggesting that elevated resting glutamate concentration specifically activates group II and group III mGluRs.

Presynaptic Na+ channels: locus, development, and recovery from inactivation at a high-fidelity synapse.

Na+ channel recovery from inactivation limits the maximal rate of neuronal firing. However, the properties of presynaptic Na+ channels are not well established because of the small size of most CNS boutons. Here we study the Na+ currents of the rat calyx of Held terminal and compare them with those of postsynaptic cells.

Retroinhibition of presynaptic Ca2+ currents by endocannabinoids released via postsynaptic mGluR activation at a calyx synapse.

We investigated the mechanisms by which activation of group I metabotropic glutamate receptors (mGluRs) and CB1 cannabinoid receptors (CB1Rs) leads to inhibition of synaptic currents at the calyx of Held synapse in the medial nucleus of the trapezoid body (MNTB) of the rat auditory brainstem. In approximately 50% of the MNTB neurons tested, activation of group I mGluRs by the specific agonist (s)-3,5-dihydroxyphenylglycine (DHPG) reversibly inhibited AMPA receptor- and NMDA receptor-mediated EPSCs to a similar extent and reduced paired-pulse depression, suggestive of an inhibition of glutamate release. Presynaptic voltage-clamp experiments revealed a reversible reduction of Ca2+ currents by DHPG, with no significant modification of the presynaptic action potential waveform.

Mutation and activation of Galpha s similarly alters pre- and postsynaptic mechanisms modulating neurotransmission.

Constitutive activation of Galphas in the Drosophila brain abolishes associative learning, a behavioral disruption far worse than that observed in any single cAMP metabolic mutant, suggesting that Galphas is essential for synaptic plasticity. The intent of this study was to examine the role of Galphas in regulating synaptic function by targeting constitutively active Galphas to either pre- or postsynaptic cells and by examining loss-of-function Galphas mutants (dgs) at the glutamatergic neuromuscular junction (NMJ) model synapse. Surprisingly, both loss of Galphas and activation of Galphas in either pre- or postsynaptic compartment similarly increased basal neurotransmission, decreased short-term plasticity (facilitation and augmentation), and abolished posttetanic potentiation.