Spontaneous neurotransmitter release and Ca2+--how spontaneous is spontaneous neurotransmitter release?

Understanding of Spinal Wide Dynamic Range Neurons and Their Modulation on Pathological Pain

The spinal dorsal horn (SDH) transmits sensory information from the periphery to the brain. Wide dynamic range (WDR) neurons within this relay site play a critical role in modulating and integrating peripheral sensory inputs, as well as the process of central sensitization during pathological pain. This group of spinal multi-receptive neurons has attracted considerable attention in pain research due to their capabilities for encoding the location and intensity of nociception. Meanwhile, transmission, processing, and modulation of incoming afferent information in WDR neurons also establish the underlying basis for investigating the integration of acupuncture and pain signals. This review aims to provide a comprehensive examination of the distinctive features of WDR neurons and their involvement in pain. Specifically, we will examine the regulation of diverse supraspinal nuclei on these neurons and analyze their potential in elucidating the mechanisms of acupuncture analgesia.

Neuronal Ca2+ signalling at rest and during spontaneous neurotransmission

Action potential driven neuronal signalling drives several electrical and biochemical processes in the nervous system. However, neurons can maintain synaptic communication and signalling under resting conditions independently of activity. Importantly, these processes are regulated by Ca²⁺ signals that occur at rest. Several studies have suggested that opening of voltage-gated Ca²⁺ channels near resting membrane potentials, activation of NMDA receptors in the absence of depolarization or Ca²⁺ release from intracellular stores can drive neurotransmitter release as well as subsequent signalling in the absence of action potentials. Interestingly, recent studies have demonstrated that manipulation of resting neuronal Ca²⁺ signalling yielded pronounced homeostatic synaptic plasticity, suggesting a critical role for this resting form of signalling in regulation of synaptic efficacy and neuronal homeostasis. Given their robust impact on synaptic efficacy and neuronal signalling, neuronal resting Ca²⁺ signals warrant further mechanistic analysis that includes the potential role of store-operated Ca²⁺ entry in these processes.

Mechanisms, pools, and sites of spontaneous vesicle release at synapses of rod and cone photoreceptors

Photoreceptors have depolarized resting potentials that stimulate calcium-dependent release continuously from a large vesicle pool but neurons can also release vesicles without stimulation. We characterized the Ca(2+) dependence, vesicle pools, and release sites involved in spontaneous release at photoreceptor ribbon synapses. In whole-cell recordings from light-adapted horizontal cells (HCs) of tiger salamander retina, we detected miniature excitatory post-synaptic currents (mEPSCs) when no stimulation was applied to promote exocytosis. Blocking Ca(2+) influx by lowering extracellular Ca(2+) , by application of Cd(2+) and other agents reduced the frequency of mEPSCs but did not eliminate them, indicating that mEPSCs can occur independently of Ca(2+) . We also measured release presynaptically from rods and cones by examining quantal glutamate transporter anion currents. Presynaptic quantal event frequency was reduced by Cd(2+) or by increased intracellular Ca(2+) buffering in rods, but not in cones, that were voltage clamped at -70 mV. By inhibiting the vesicle cycle with bafilomycin, we found the frequency of mEPSCs declined more rapidly than the amplitude of evoked excitatory post-synaptic currents (EPSCs) suggesting a possible separation between vesicle pools in evoked and spontaneous exocytosis. We mapped sites of Ca(2+) -independent release using total internal reflectance fluorescence (TIRF) microscopy to visualize fusion of individual vesicles loaded with dextran-conjugated pHrodo. Spontaneous release in rods occurred more frequently at non-ribbon sites than evoked release events. The function of Ca(2+) -independent spontaneous release at continuously active photoreceptor synapses remains unclear, but the low frequency of spontaneous quanta limits their impact on noise.

Spinal cord thermosensitivity: An afferent phenomenon?

We review the evidence for thermoregulatory temperature sensors in the mammalian spinal cord and reach the following conclusions. 1) Spinal cord temperature contributes physiologically to temperature regulation. 2) Parallel anterolateral ascending pathways transmit signals from spinal cooling and spinal warming: they overlap with the respective axon pathways of the dorsal horn neurons that are driven by peripheral cold- and warm-sensitive afferents. 3) We hypothesize that these ‘cold’ and ‘warm’ ascending pathways transmit all extracranial thermosensory information to the brain. 4) Cutaneous cold afferents can be activated not only by cooling the skin but also by cooling sites along their axons: we consider that this is functionally insignificant in vivo. 5) By a presynaptic action on their central terminals, local spinal cooling enhances neurotransmission from incoming ‘cold’ afferent action potentials to second order neurons in the dorsal horn; this effect disappears when the spinal cord is warm. 6) Spinal warm sensitivity is due to warm-sensitive miniature vesicular transmitter release from afferent terminals in the dorsal horn: this effect is powerful enough to excite second order neurons in the ‘warm’ pathway independently of any incoming sensory traffic. 7) Distinct but related presynaptic mechanisms at cold- and warm-sensitive afferent terminals can thus account for the thermoregulatory actions of spinal cord temperature.

Molecular underpinnings of synaptic vesicle pool heterogeneity

Neuronal communication relies on chemical synaptic transmission for information transfer and processing. Chemical neurotransmission is initiated by synaptic vesicle fusion with the presynaptic active zone resulting in release of neurotransmitters. Classical models have assumed that all synaptic vesicles within a synapse have the same potential to fuse under different functional contexts. In this model, functional differences among synaptic vesicle populations are ascribed to their spatial distribution in the synapse with respect to the active zone. Emerging evidence suggests, however, that synaptic vesicles are not a homogenous population of organelles, and they possess intrinsic molecular differences and differential interaction partners. Recent studies have reported a diverse array of synaptic molecules that selectively regulate synaptic vesicles' ability to fuse synchronously and asynchronously in response to action potentials or spontaneously irrespective of action potentials. Here we discuss these molecular mediators of vesicle pool heterogeneity that are found on the synaptic vesicle membrane, on the presynaptic plasma membrane, or within the cytosol and consider some of the functional consequences of this diversity. This emerging molecular framework presents novel avenues to probe synaptic function and uncover how synaptic vesicle pools impact neuronal signaling.

Exocytosis and synaptic vesicle function

Synaptic vesicles release their vesicular contents to the extracellular space by Ca(2+)-triggered exocytosis. The Ca(2+)-triggered exocytotic process is regulated by synaptotagmin (Syt), a vesicular Ca(2+)-binding C2 domain protein. Synaptotagmin 1 (Syt1), the most studied major isoform among 16 Syt isoforms, mediates Ca(2+)-triggered synaptic vesicle exocytosis by interacting with the target membranes and SNARE/complexin complex. In synapses of the central nervous system, synaptobrevin 2, a major vesicular SNARE protein, forms a ternary SNARE complex with the plasma membrane SNARE proteins, syntaxin 1 and SNAP25. The affinities of Ca(2+)-dependent interactions between Syt1 and its targets (i.e., SNARE complexes and membranes) are well correlated with the efficacies of the corresponding exocytotic processes. Therefore, different SNARE protein isoforms and membrane lipids, which interact with Syt1 with various affinities, are capable of regulating the efficacy of Syt1-mediated exocytosis. Otoferlin, another type of vesicular C2 domain protein that binds to the membrane in a Ca(2+)-dependent manner, is also involved in the Ca(2+)-triggered synaptic vesicle exocytosis in auditory hair cells. However, the functions of otoferlin in the exocytotic process are not well understood. In addition, at least five different types of synaptic vesicle proteins such as synaptic vesicle protein 2, cysteine string protein α, rab3, synapsin, and a group of proteins containing four transmembrane regions, which includes synaptophysin, synaptogyrin, and secretory carrier membrane protein, are involved in modulating the exocytotic process by regulating the formation and trafficking of synaptic vesicles.

Catecholamine exocytosis during low frequency stimulation in mouse adrenal chromaffin cells is primarily asynchronous and controlled by the novel mechanism of Ca2+ syntilla suppression

Adrenal chromaffin cells (ACCs), stimulated by the splanchnic nerve, generate action potentials (APs) at a frequency near 0.5 Hz in the resting physiological state, at times described as 'rest and digest'. How such low frequency stimulation in turn elicits sufficient catecholamine exocytosis to set basal sympathetic tone is not readily explained by the classical mechanism of stimulus-secretion coupling, where exocytosis is synchronized to AP-induced Ca(2+) influx. By using simulated action potentials (sAPs) at 0.5 Hz in isolated patch-clamped mouse ACCs, we show here that less than 10% of all catecholaminergic exocytosis, measured by carbon fibre amperometry, is synchronized to an AP. The asynchronous phase, the dominant phase, of exocytosis does not require Ca(2+) influx. Furthermore, increased asynchronous exocytosis is accompanied by an AP-dependent decrease in frequency of Ca(2+) syntillas (i.e. transient, focal Ca(2+) release from internal stores) and is ryanodine sensitive. We propose a mechanism of disinhibition, wherein APs suppress Ca(2+) syntillas, which themselves inhibit exocytosis as they do in the case of spontaneous catecholaminergic exocytosis.

Physiological temperatures drive glutamate release onto trigeminal superficial dorsal horn neurons

Trigeminal sensory afferent fibers terminating in nucleus caudalis (Vc) relay sensory information from craniofacial regions to the brain and are known to express transient receptor potential (TRP) ion channels. TRP channels are activated by H(+), thermal, and chemical stimuli. The present study investigated the relationships among the spontaneous release of glutamate, temperature, and TRPV1 localization at synapses in the Vc. Spontaneous excitatory postsynaptic currents (sEPSCs) were recorded from Vc neurons (n = 151) in horizontal brain-stem slices obtained from Sprague-Dawley rats. Neurons had basal sEPSC rates that fell into two distinct frequency categories: High (≥10 Hz) or Low (<10 Hz) at 35°C. Of all recorded neurons, those with High basal release rates (67%) at near-physiological temperatures greatly reduced their sEPSC rate when cooled to 30°C without amplitude changes. Such responses persisted during blockade of action potentials indicating that the High rate of glutamate release arises from presynaptic thermal mechanisms. Neurons with Low basal frequencies (33%) showed minor thermal changes in sEPSC rate that were abolished after addition of TTX, suggesting these responses were indirect and required local circuits. Activation of TRPV1 with capsaicin (100 nM) increased miniature EPSC (mEPSC) frequency in 70% of neurons, but half of these neurons had Low basal mEPSC rates and no temperature sensitivity. Our evidence indicates that normal temperatures (35-37°C) drive spontaneous excitatory synaptic activity within superficial Vc by a mechanism independent of presynaptic action potentials. Thus thermally sensitive inputs on superficial Vc neurons may tonically activate these neurons without afferent stimulation.

New insights into molecular mechanism(s) underlying the presynaptic action of nitric oxide on GABA release

BACKGROUND

Nitric oxide (NO) is an important presynaptic modulator of synaptic transmission. Here, we aimed to correlate the release of the major inhibitory neurotransmitter GABA with intracellular events occurring in rat brain axon terminals during their exposure to NO in the range of nanomolar-low micromolar concentrations.

METHODS

Using [(3)H]GABA and fluorescent dyes (Fluo 4-AM, acridine orange and rhodamine 6G), the following parameters were evaluated: vesicular and cytosolic GABA pools, intracellular calcium concentration, synaptic vesicle acidification, and mitochondrial membrane potential. Diethylamine NONOate (DEA/NO) and S-nitroso-N-acetylpenicillamine (SNAP) were used as NO donors.

RESULTS

DEA/NO and SNAP (in the presence of dithiothreitol (DTT)) stimulated external Ca(2+)-independent [(3)H]GABA release, which was not attributed to a rise in intracellular calcium concentration. [(3)H]GABA release coincided with increasing GABA level in cytosol and decreasing the vesicular GABA content available for exocytotic release. There was a strong temporal correlation between NO-induced increase in cytosolic [GABA] and dissipation of both synaptic vesicle proton gradient and mitochondrial membrane potential. Dissipation was reversible, and recovery of both parameters correlated in time with re-accumulation of [(3)H]GABA into synaptic vesicles. The molar ratio of DTT to SNAP determined the rate and duration of the recovery processes.

CONCLUSIONS

We suggest that NO can stimulate GABA release via GABA transporter reversal resulting from increased GABA levels in cytosol. The latter is reversible and appears to be due to S-nitrosylation of key proteins, which affect the energy status of the pre-synapse.

GENERAL SIGNIFICANCE

Our findings provide new insight into molecular mechanism(s) underlying the presynaptic action of nitric oxide on inhibitory neurotransmission.

Exploring the central modulation hypothesis: do ancient memory mechanisms underlie the pathophysiology of trigger points?

A myofascial trigger point (TrP) is a point of focal tenderness, associated with a taut band of muscle fibers, that can develop in any skeletal muscle. TrPs are a common source of pain and motor dysfunction in humans and other vertebrates. There is no universally accepted pathophysiology to explain the etiology, symptomatology and treatment of TrPs. This article reviews and extends the author's previously published hypothesis for the pathophysiology of TrPs, "Trigger Points and Central Modulation-A New Hypothesis." The author proposes that central nervous system-maintained global changes in α-motoneuron function, resulting from sustained plateau depolarization, rather than a local dysfunction of the motor endplate, underlie the pathogenesis of TrPs.

Dopamine Release Suppression Dependent on an Increase of Intracellular Ca(2+) Contributed to Rotenone-induced Neurotoxicity in PC12 Cells

Rotenone is an inhibitor of mitochondrial complex I that produces a model of Parkinson's disease (PD), in which neurons undergo dopamine release dysfunction and other features. In neurons, exocytosis is one of the processes associated with dopamine release and is dependent on Ca(2+) dynamic changes of the cell. In the present study, we have investigated the exocytosis of dopamine and the involvement of Ca(2+) in dopamine release in PC12 cells administrated with rotenone. Results demonstrated that rotenone led to an elevation of intracellular Ca(2+) through Ca(2+) influx by opening of the voltage-gated Ca(2+) channel and influenced the soluble N-ethylmaleimide attachment protein receptor (SNARE) proteins expression (including syntaxin, vesicle-associated membrane protein 2 (VAMP2) and synaptosome-associated protein 25 (SNAP-25)); pretreatment with a blocker of L-type voltage-activated Ca(2+) channels (nifedipine) decreased the intracellular dopamine levels and ROS formation, increased the cell viability and enhanced the neurite outgrowth and exocytosis of synaptic vesicles. These results indicated that the involvement of intracellular Ca(2+) was one of the factors resulting in suppression of dopamine release suppression in PC12 cells intoxicated with rotenone, which was associated with the rotenone-induced dopamine neurotoxicity.

Roles of the calcium sensing receptor in the central nervous system

The calcium sensing receptor (CaSR) is expressed by subpopulations of neuronal and glial cells throughout the brain and is activated by extracellular calcium [Formula: see text] . During development, the CaSR regulates neuronal cell growth and migration as well as oligodendroglial maturation and function. Emerging evidence suggests that in nerve terminals, CaSR is implicated in synaptic plasticity and neurotransmission. In this review, we analyze the roles attributed to CaSR in regulating diverse brain functions, including central regulation of body fluid composition and blood pressure. We also discuss the potential relevance of Ca(2+)-sensing in brain by other family C G protein-coupled receptors. Finally, evidence that the CaSR contributes to the pathogenesis of various brain disorders raises the possibility that pharmacological modulators of the CaSR may have therapeutic benefit.

Coordinated development of voltage-gated Na+ and K+ currents regulates functional maturation of forebrain neurons derived from human induced pluripotent stem cells

Like embryonic stem (ES) cells, human induced pluripotent stem (hiPS) cells can differentiate into neuronal cells. However, it is unclear how their exquisite neuronal function is electrophysiologically coordinated during differentiation and whether they are functionally identical to human ES cell-derived neurons. In this study, we differentiated hiPS and ES cells into pyramidal-like neurons and conducted electrophysiological characterization over the 4-week terminal differentiation period. The human neuron-like cells express forebrain pyramidal cell markers NeuN, neurofilament, the microtubule-associated protein 2 (MAP2), the paired box protein Pax-6 (PAX6), Tuj1, and the forkhead box protein G1 (FoxG1). The size of developing neurons increased continuously during the 4-week culture, and cell-resting membrane potentials (RMPs) underwent a negative shift from -40 to -70 mV. Expression of the muscarinic receptor-modulated K(+) currents (IM) participated in the development of cell RMPs and controlled excitability. Immature neurons at week 1 could only fire abortive action potentials (APs) and the frequency of AP firing progressively increased with neuronal maturation. Interestingly, the developmental change of voltage-gated Na(+) current (INa) did not correlate with the change in the AP firing frequency. On the other hand, the transient outward K(+) current (IA), but not the delayed rectifier current (IK) contributed to the high frequency firing of APs. Synaptic activities were observed throughout the 4-week development. These morphological and electrophysiological features were almost identical between iPS and ES cell-derived neurons. This is the first systematic investigation showing functional evidence that hiPS cell-derived neurons possess similar neuronal activities as ES cell-derived neurons. These data support that iPS cell-derived neural progenitor cells have the potential for replacing lost neurons in cell-based therapy.

The unsilent majority-TRPV1 drives "spontaneous" transmission of unmyelinated primary afferents within cardiorespiratory NTS

Cranial primary afferent sensory neurons figure importantly in homeostatic control of visceral organ systems. Of the two broad classes of visceral afferents, the role of unmyelinated or C-type class remains poorly understood. This review contrasts key aspects of peripheral discharge properties of C-fiber afferents and their glutamate transmission mechanisms within the solitary tract nucleus (NTS). During normal prevailing conditions, most information arrives at the NTS through myelinated A-type nerves. However, most of visceral afferent axons (75-90%) in NTS are unmyelinated, C-type axons. Centrally, C-type solitary tract (ST) afferent terminals have presynaptic transient receptor potential vanilloid type 1 (TRPV1) receptors. Capsaicin activation of TRPV1 blocks phasic or synchronous release of glutamate but facilitates release of glutamate from a separate pool of vesicles. This TRPV1-operated pool of vesicles is active at normal temperatures and is responsible for actively driving a 10-fold higher release of glutamate at TRPV1 compared with TRPV1- terminals even in the absence of afferent action potentials. This novel TRPV1 mechanism is responsible for an additional asynchronous release of glutamate that is not present in myelinated terminals. The NTS is rich with presynaptic G protein-coupled receptors, and the implications of TRPV1-operated glutamate offer unique targets for signaling in C-type sensory afferent terminals from neuropeptides, inflammatory mediators, lipid metabolites, cytokines, and cannabinoids. From a homeostatic view, this combination could have broad implications for integration in chronic pathological disturbances in which the numeric dominance of C-type endings and TRPV1 would broadly disturb multisystem control mechanisms.

Type 1 inositol (1,4,5)-trisphosphate receptor activates ryanodine receptor 1 to mediate calcium spark signaling in adult mammalian skeletal muscle

Functional coupling between inositol (1,4,5)-trisphosphate receptor (IP(3)R) and ryanodine receptor (RyR) represents a critical component of intracellular Ca(2+) signaling in many excitable cells; however, the role of this mechanism in skeletal muscle remains elusive. In skeletal muscle, RyR-mediated Ca(2+) sparks are suppressed in resting conditions, whereas application of transient osmotic stress can trigger activation of Ca(2+) sparks that are restricted to the periphery of the fiber. Here we show that onset of these spatially confined Ca(2+) sparks involves interaction between activation of IP(3)R and RyR near the sarcolemmal membrane. Pharmacological prevention of IP(3) production or inhibition of IP(3)R channel activity abolishes stress-induced Ca(2+) sparks in skeletal muscle. Although genetic ablation of the type 2 IP(3)R does not appear to affect Ca(2+) sparks in skeletal muscle, specific silencing of the type 1 IP(3)R leads to ablation of stress-induced Ca(2+) sparks. Our data indicate that membrane-delimited signaling involving cross-talk between IP(3)R1 and RyR1 contributes to Ca(2+) spark activation in skeletal muscle.

Hypoxic intensity: a determinant for the contribution of ATP and adenosine to the genesis of carotid body chemosensory activity

Excitatory effects of adenosine and ATP on carotid body (CB) chemoreception have been previously described. Our hypothesis is that both ATP and adenosine are the key neurotransmitters responsible for the hypoxic chemotransmission in the CB sensory synapse, their relative contribution depending on the intensity of hypoxic challenge. To test this hypothesis we measured carotid sinus nerve (CSN) activity in response to moderate and intense hypoxic stimuli (7 and 0% O(2)) in the absence and in the presence of adenosine and ATP receptor antagonists. Additionally, we quantified the release of adenosine and ATP in normoxia (21% O(2)) and in response to hypoxias of different intensities (10, 5, and 2% O(2)) to study the release pathways. We found that ZM241385, an A(2) antagonist, decreased the CSN discharges evoked by 0 and 7% O(2) by 30.8 and 72.5%, respectively. Suramin, a P(2)X antagonist, decreased the CSN discharges evoked by 0 and 7% O(2) by 64.3 and 17.1%, respectively. Simultaneous application of both antagonists strongly inhibited CSN discharges elicited by both hypoxic intensities. ATP release by CB increased in parallel to hypoxia intensity while adenosine release increased preferably in response to mild hypoxia. We have also found that the lower the O(2) levels are, the higher is the percentage of adenosine produced from extracellular catabolism of ATP. Our results demonstrate that ATP and adenosine are key neurotransmitters involved in hypoxic CB chemotransduction, with a more relevant contribution of adenosine during mild hypoxia, while vesicular ATP release constitutes the preferential origin of extracellular adenosine in high-intensity hypoxia.

Calcium sensing in exocytosis

Neurotransmitters, neuropeptides and hormones are released through regulated exocytosis of synaptic vesicles and large dense core vesicles. This complex and highly regulated process is orchestrated by SNAREs and their associated proteins. The triggering signal for regulated exocytosis is usually an increase in intracellular calcium levels. Besides the triggering role, calcium signaling modulates the precise amount and kinetics of vesicle release. Thus, it is a central question to understand the molecular machineries responsible for calcium sensing in exocytosis. Here we provide an overview of our current understanding of calcium sensing in neurotransmitter release and hormone secretion.

Cyclic ADP-ribose requires CD38 to regulate the release of ATP in visceral smooth muscle

It is well established that the intracellular second messenger cADP-ribose (cADPR) activates Ca(2+) release from the sarcoplasmic reticulum through ryanodine receptors. CD38 is a multifunctional enzyme involved in the formation of cADPR in mammals. CD38 has also been reported to transport cADPR in several cell lines. Here, we demonstrate a role for extracellular cADPR and CD38 in modulating the spontaneous, but not the electrical field stimulation-evoked, release of ATP in visceral smooth muscle. Using a small-volume superfusion assay and an HPLC technique with fluorescence detection, we measured the spontaneous and evoked release of ATP in bladder detrusor smooth muscles isolated from CD38(+/+) and CD38(-/-) mice. cADPR (1 nM) enhanced the spontaneous overflow of ATP in bladders isolated from CD38(+/+) mice. This effect was abolished by the inhibitor of cADPR receptors on sarcoplasmic reticulum 8-bromo-cADPR (80 μM) and by ryanodine (50 μm), but not by the nonselective P2 purinergic receptor antagonist pyridoxal phosphate 6-azophenyl-2',4'-disulfonate (30 μM). cADPR failed to facilitate the spontaneous ATP overflow in bladders isolated from CD38(-/-) mice, indicating that CD38 is crucial for the enhancing effects of extracellular cADPR on spontaneous ATP release. Contractile responses to ATP were potentiated by cADPR, suggesting that the two adenine nucleotides may work in synergy to maintain the resting tone of the bladder. In conclusion, extracellular cADPR enhances the spontaneous release of ATP in the bladder by influx via CD38 and subsequent activation of intracellular cADPR receptors, probably causing an increase in intracellular Ca(2+) in neuronal cells.

Spontaneous neurotransmission: an independent pathway for neuronal signaling?

Recent findings suggest that spontaneous neurotransmission is a bona fide pathway for interneuronal signaling that operates independent of evoked transmission via distinct presynaptic as well as postsynaptic substrates. This article will examine the role of spontaneous release events in neuronal signaling by focusing on aspects that distinguish this process from evoked neurotransmission, and evaluate the mechanisms that may underlie this segregation.

Cannabinoid receptor agonists potentiate action potential-independent release of GABA in the dentate gyrus through a CB1 receptor-independent mechanism

We report a novel excitatory effect of cannabinoid agonists on action potential-independent GABAergic transmission in the rat dentate gyrus. Specifically, we find that both WIN55,212-2 and anandamide increase the frequency of miniature IPSCs (mIPSCs)recorded from hilar mossy cells without altering event amplitude, area, rise time, or decay. The effect of WIN55,212-2 on mIPSCs is insensitive to AM251 and preserved in CB1 −/− animals,indicating that it does not depend on activation of CB1 receptors. It is also insensitive to AM630 and unaffected by capsazepine suggesting that neither CB2 nor TRPV1 receptors are involved. Further, it is blocked by pre-incubation in suramin and by a selective protein kinase A inhibitor (H-89), and is mimicked (and occluded) by bath application of forskolin. Similar CB1 receptor-independent facilitation of exocytosis is not apparent when recording evoked IPSCs in the presence of AM251, suggesting that the exocytotic mechanism that produces WIN55,212-2 sensitive mIPSCs is distinct from that which produces CB1 sensitive and action potential-dependent release. Despite clear independence from action potentials, WIN55,212-2 mediated facilitation of mIPSCs requires calcium, and yet is insensitive to chelation of calcium in the postsynaptic cell. Finally, we demonstrate that both bath application of 2-arachidonoylglycerol(2-AG) and depolarization-induced release of endogenous cannabinoids have minimal effect on mIPSC frequency. Cumulatively, our results indicate that cannabinoid ligands can selectively facilitate action potential-independent exocytosis of GABA in the rat dentate gyrus, and further emphasize that this new cannabinoid sensitive signalling system is distinct from previously described CB1 receptor-dependent systems in numerous respects.

Differential regulation of spontaneous and evoked neurotransmitter release at central synapses

Recent studies have begun to scrutinize the presynaptic machinery and vesicle populations that give rise to action potential evoked and spontaneous forms of neurotransmitter release. In several cases this work produced unexpected results which lend support to the notion that regulation, mechanisms, postsynaptic targets and possibly presynaptic origins of evoked and spontaneous neurotransmitter release differ. Furthermore, the list of regulatory pathways that impact spontaneous and evoked release in a divergent manner is rapidly growing. These findings challenge our classical views on the relationship between evoked and spontaneous neurotransmission. In contrast to the well-characterized neuromodulatory pathways that equally suppress or augment all forms of neurotransmitter release, molecular substrates specifically controlling spontaneous release remain unclear. In this review, we outline possible mechanisms that may underlie the differential regulation of distinct forms of neurotransmission and help demultiplex complex neuronal signals and generate parallel signaling events at their postsynaptic targets.

Ethanol-enhanced GABA release: a focus on G protein-coupled receptors

While research on the actions of ethanol at the GABAergic synapse has focused on postsynaptic mechanisms, recent data have demonstrated that ethanol also facilitates GABA release from presynaptic terminals in many, but not all, brain regions. The ability of ethanol to increase GABA release can be regulated by different G protein-coupled receptors (GPCRs), such as the cannabinoid-1 receptor, corticotropin-releasing factor 1 receptor, GABA(B) receptor, and the 5-hydroxytryptamine 2C receptor. The intracellular messengers linked to these GPCRs, including the calcium that is released from internal stores, also play a role in ethanol-enhanced GABA release. Hypotheses are proposed to explain how ethanol interacts with the GPCR pathways to increase GABA release and how this interaction contributes to the brain region specificity of ethanol-enhanced GABA release. Defining the mechanism of ethanol-facilitated GABA release will further our understanding of the GABAergic profile of ethanol and increase our knowledge of how GABAergic neurotransmission may contribute to the intoxicating effects of alcohol and to alcohol dependence.

Eph/ephrin interactions modulate vascular sympathetic innervation

Ephs and ephrins are membrane-bound proteins that interact to modulate axon growth and neuronal function. We tested the hypothesis that eph/ephrin interactions affected the growth and function of vascular sympathetic innervation. Using RT-PCR analyses, we detected both classes of ephs (A and B) and both classes of ephrins (A and B) in sympathetic ganglia from neonatal and adult rats. Both classes of ephs (A and B) and both classes of ephrins (A and B) bound to the cell bodies and neurites of dissociated postganglionic sympathetic neurons. Messenger RNAs encoding for both classes of ephs (A and B) and both classes of ephrins (A and B) were also detected in sympathetically innervated arteries from neonatal and adult rats. These data suggest that ephrins/ephs on nerve fibers of postganglionic sympathetic neurons could interact with ephs/ephrins on cells in innervated arteries. We found that ephA4 reduced reinnervation of denervated femoral arteries. Reinnervation in the presence of ephA4-Fc (38.9±6.6%) was significantly less than that in the presence of IgG-Fc (62±10%; n=5; p<0.05; one-tailed unpaired t-test). These data indicate that eph/ephrin interactions modulated the growth of vascular sympathetic innervation. We also found that ephA4 increased basal release of norepinephrine from nerve terminals of isolated tail arteries. These data indicate that eph/ephrin interactions affect the growth and function of vascular sympathetic innervation.

Opposing functions of two sub-domains of the SNARE-complex in neurotransmission

The SNARE-complex consisting of synaptobrevin-2/VAMP-2, SNAP-25 and syntaxin-1 is essential for evoked neurotransmission and also involved in spontaneous release. Here, we used cultured autaptic hippocampal neurons from Snap-25 null mice rescued with mutants challenging the C-terminal, N-terminal and middle domains of the SNARE-bundle to dissect out the involvement of these domains in neurotransmission. We report that the stabilities of two different sub-domains of the SNARE-bundle have opposing functions in setting the probability for both spontaneous and evoked neurotransmission. Destabilizing the C-terminal end of the SNARE-bundle abolishes spontaneous neurotransmitter release and reduces evoked release probability, indicating that the C-terminal end promotes both modes of release. In contrast, destabilizing the middle or deleting the N-terminal end of the SNARE-bundle increases both spontaneous and evoked release probabilities. In both cases, spontaneous release was affected more than evoked neurotransmission. In addition, the N-terminal deletion delays vesicle priming after a high-frequency train. We propose that the stability of N-terminal two-thirds of the SNARE-bundle has a function for vesicle priming and limiting spontaneous release.

Chronic intermittent hypoxia affects integration of sensory input by neurons in the nucleus tractus solitarii

The autonomic nervous and respiratory systems, as well as their coupling, adapt over a wide range of conditions. Chronic intermittent hypoxia (CIH) is a model for recurrent apneas and induces alterations in breathing and increases in sympathetic nerve activity which may ultimately result in hypertension if left untreated. These alterations are believed to be due to increases in the carotid body chemoreflex pathway. Here we present evidence that the nucleus tractus solitarii (nTS), the central brainstem termination site of chemoreceptor afferents, expresses a form of synaptic plasticity that increases overall nTS activity following intermittent hypoxia. Following CIH, an increase in presynaptic spontaneous neurotransmitter release occurs under baseline conditions. Furthermore, during and following afferent stimulation there is an augmentation of spontaneous transmitter release that occurs out of synchrony with sensory stimulation. On the other hand, afferent evoked synchronous transmitter release is attenuated. Overall, this shift from synchronous to asynchronous transmitter release enhances nTS cellular discharge. The role of the neurotransmitter dopamine in CIH-induced plasticity is also discussed. Dopamine attenuates synaptic transmission in nTS cells by blockade of N-type calcium channels, and this mechanism occurs tonically following normoxia and CIH. This dopaminergic pathway, however, is not altered in CIH. Taken together, alterations in nTS synaptic activity may play a role in the changes of chemoreflex function and cardiorespiratory activity in the CIH apnea model.

Suppression of Ca2+ syntillas increases spontaneous exocytosis in mouse adrenal chromaffin cells

A central concept in the physiology of neurosecretion is that a rise in cytosolic [Ca²⁺] in the vicinity of plasmalemmal Ca²⁺ channels due to Ca²⁺ influx elicits exocytosis. Here, we examine the effect on spontaneous exocytosis of a rise in focal cytosolic [Ca²⁺] in the vicinity of ryanodine receptors (RYRs) due to release from internal stores in the form of Ca²⁺ syntillas. Ca²⁺ syntillas are focal cytosolic transients mediated by RYRs, which we first found in hypothalamic magnocellular neuronal terminals. (scintilla, Latin for spark; found in nerve terminals, normally synaptic structures.) We have also observed Ca²⁺ syntillas in mouse adrenal chromaffin cells. Here, we examine the effect of Ca²⁺ syntillas on exocytosis in chromaffin cells. In such a study on elicited exocytosis, there are two sources of Ca²⁺: one due to influx from the cell exterior through voltage-gated Ca²⁺ channels, and that due to release from intracellular stores. To eliminate complications arising from Ca²⁺ influx, we have examined spontaneous exocytosis where influx is not activated. We report here that decreasing syntillas leads to an increase in spontaneous exocytosis measured amperometrically. Two independent lines of experimentation each lead to this conclusion. In one case, release from stores was blocked by ryanodine; in another, stores were partially emptied using thapsigargin plus caffeine, after which syntillas were decreased. We conclude that Ca²⁺ syntillas act to inhibit spontaneous exocytosis, and we propose a simple model to account quantitatively for this action of syntillas.

Synaptotagmin-1 functions as a Ca2+ sensor for spontaneous release

Spontaneous ‘mini’ release occurs at all synapses, but its nature remains enigmatic. Here, we show that in murine cortical neurons, >95% of spontaneous release is induced by Ca²⁺-binding to synaptotagmin-1, the Ca²⁺-sensor for fast synchronous neurotransmitter release. Thus, spontaneous and evoked release use the same Ca²⁺-dependent release mechanism. As a consequence, synaptotagmin-1 mutations that alter its Ca²⁺-affinity alter spontaneous and evoked release correspondingly. Paradoxically, synaptotagmin-1 deletions (as opposed to point mutations) massively increase spontaneous release. This increased spontaneous release remains Ca²⁺-dependent, but is activated at lower Ca²⁺-concentrations, and with a lower Ca²⁺-cooperativity, than synaptotagmin-driven spontaneous release. Thus, in addition to serving as Ca²⁺-sensor for spontaneous and evoked release, synaptotagmin-1 clamps a second, more sensitive Ca²⁺-sensor for spontaneous release which resembles the Ca²⁺-sensor for evoked asynchronous release. Viewed together, these data suggest that synaptotagmin-1 controls both evoked and spontaneous release at a synapse as a simultaneous Ca²⁺-dependent activator and clamp of exocytosis.

Leaky synapses: regulation of spontaneous neurotransmission in central synapses

The mechanisms underlying spontaneous neurotransmitter release are not well understood. Under physiological as well as pathophysiological circumstances, spontaneous fusion events can set the concentration of ambient levels of neurotransmitter within the synaptic cleft and in the extracellular milieu. In the brain, unregulated release of excitatory neurotransmitters, exacerbated during pathological conditions such as stroke, can lead to neuronal damage and death. In addition, recent findings suggest that under physiological circumstances spontaneous release events can trigger postsynaptic signaling events independent of evoked neurotransmitter release. Therefore, elucidation of mechanisms underlying spontaneous neurotransmission may help us better understand the functional significance of this form of release and provide tools for its selective manipulation. For instance, our recent investigations indicate that the level of cholesterol in the synapse plays a critical role in limiting spontaneous synaptic vesicle fusion. Therefore, alterations in synaptic cholesterol metabolism can be a critical determinant of glutamatergic neurotransmission at rest. This article aims to provide a closer look into our current understanding of the mechanisms underlying spontaneous neurotransmission and the signaling triggered by these unitary release events.