GTP-binding protein beta gamma subunits mediate presynaptic calcium current inhibition by GABA(B) receptor

Dynamic aspects of presynaptic calcium currents mediating synaptic transmission

Ca2+ entry through voltage-gated Ca2+ channels (VGCC) triggers transmitter release. Direct recording of Ca2+ currents from the calyx of Held nerve terminal revealed that presynaptic VGCCs undergo various modulations via presynaptic G protein-coupled receptors (GPCRs), Ca2+-binding proteins and a developmental switch of their alpha1 subunits. Dynamic changes of presynaptic VGCCs alter synaptic efficacy, thereby contributing to a variety of modulations of the CNS function.

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. Using mGluR subtype-specific agonists and antagonists, we determined that increased glutamate activates presynaptic mGluR2/3 and mGluR8 receptors but not mGluR4, although this receptor is present. Surprisingly, in older animals (P16-P18), transporter blockade had no effect on EPSC amplitude because of a developmental downregulation of group II/III mGluR activation in rats and mice. In contrast to other CNS synapses, we observed no effect of transporter blockade on EPSC decay kinetics, although expression of glutamate transporters was strong in nearby glial processes at both P9 and P17. Finally, using a low-affinity AMPA receptor antagonist (gamma-D-glutamylglycine), we show that desensitization occurs at P8-P10 but is absent at P16-P18, even during trains of high-frequency (100-300 Hz) stimulation. We suggest that diffusion and transporter activation are insufficient to clear synaptically released glutamate at immature calyces, resulting in significant desensitization. Thus, mGluRs may be expressed in the immature calyx to help limit glutamate release. In the more mature calyx, there is a far smaller diffusional barrier attributable to the highly fenestrated synaptic terminal morphology, so AMPA receptor desensitization is avoided and mGluR-mediated inhibition is not necessary.

Presynaptic N-type and P/Q-type Ca2+ channels mediating synaptic transmission at the calyx of Held of mice

At the nerve terminal, both N- and P/Q-type Ca2+ channels mediate synaptic transmission, with their relative contribution varying between synapses and with postnatal age. To clarify functional significance of different presynaptic Ca2+ channel subtypes, we recorded N-type and P/Q-type Ca2+ currents directly from calyces of Held nerve terminals in alpha1A-subunit-deficient mice and wild-type (WT) mice, respectively. The most prominent feature of P/Q-type Ca2+ currents was activity-dependent facilitation, which was absent for N-type Ca2+ currents. EPSCs mediated by P/Q-type Ca2+ currents showed less depression during high-frequency stimulation compared with those mediated by N-type Ca2+ currents. In addition, the maximal inhibition by the GABAB receptor agonist baclofen was greater for EPSCs mediated by N-type channels than for those mediated by P/Q-type channels. These results suggest that the developmental switch of presynaptic Ca2+ channels from N- to P/Q-type may serve to increase synaptic efficacy at high frequencies of activity, securing high-fidelity synaptic transmission.

G protein-dependent presynaptic inhibition mediated by AMPA receptors at the calyx of Held

The alpha-amino-3-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) is an ionotropic receptor mediating excitatory synaptic transmission, but it can also interact with intracellular messengers. Here we report that, at the calyx of Held in the rat auditory brainstem, activation of AMPARs induced inward currents in the nerve terminal and inhibited presynaptic Ca2+ currents (I(pCa)), thereby attenuating glutamatergic synaptic transmission. The AMPAR-mediated I(pCa) inhibition was disinhibited by a strong depolarizing pulse and occluded by the nonhydrolyzable GTP analog GTPgammaS loaded into the terminal. We conclude that functional AMPARs are expressed at the calyx of Held nerve terminal and that their activation inhibits voltage-gated Ca2+ channels by an interaction with heterotrimeric GTP-binding proteins (G proteins). Thus, at a central glutamatergic synapse, presynaptic AMPARs have a metabotropic nature and regulate transmitter release by means of G proteins.

Gbetagamma acts at the C terminus of SNAP-25 to mediate presynaptic inhibition

Presynaptic inhibition mediated by G protein-coupled receptors may involve a direct interaction between G proteins and the vesicle fusion machinery. The molecular target of this pathway is unknown. We demonstrate that Gbetagamma-mediated presynaptic inhibition in lamprey central synapses occurs downstream from voltage-gated Ca(2+) channels. Using presynaptic microinjections of botulinum toxins (BoNTs) during paired recordings, we find that cleavage of synaptobrevin in unprimed vesicles leads to an eventual exhaustion of synaptic transmission but does not prevent Gbetagamma-mediated inhibition. In contrast, cleavage of the C-terminal nine amino acids of the 25 kDa synaptosome-associated protein (SNAP-25) by BoNT A prevents Gbetagamma-mediated inhibition. Moreover, a peptide containing the region of SNAP-25 cleaved by BoNT A blocks the Gbetagamma inhibitory effect. Finally, removal of the last nine amino acids of the C-terminus of SNAP-25 weakens Gbetagamma interactions with soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes. Thus, the C terminus of SNAP-25, which links synaptotagmin I to the SNARE complex, may represent a target of Gbetagamma for presynaptic inhibition.

G protein {beta}{gamma} subunits mediate presynaptic inhibition of transmitter release from rat superior cervical ganglion neurones in culture

The activation of presynaptic G protein-coupled receptors (GPCRs) is widely reported to inhibit transmitter release; however, the lack of accessibility of many presynaptic terminals has limited direct analysis of signalling mediators. We studied GPCR-mediated inhibition of fast cholinergic transmission between superior cervical ganglion neurones (SCGNs) in culture. The adrenoceptor agonist noradrenaline (NA) caused a dose-related reduction in evoked excitatory postsynaptic potentials (EPSPs). NA-induced EPSP decrease was accompanied by effects on the presynaptic action potential (AP), reducing AP duration and amplitude of the after-hyperpolarization (AHP), without affecting the pre- and postsynaptic membrane potential. All effects of NA were blocked by yohimbine and synaptic transmission was reduced by clonidine, consistent with an action at presynaptic alpha2-adrenoceptors. NA-induced inhibition of transmission was sensitive to pre-incubation of SCGNs with pertussis toxin (PTX), implicating the involvement of Galpha(i/o)betagamma subunits. Expression of Galpha transducin, an agent which sequesters G protein betagamma (Gbetagamma) subunits, in the presynaptic neurone caused a time-dependent attenuation of NA-induced inhibition. Injection of purified Gbetagamma subunits into the presynaptic neurone inhibited transmission, and also reduced the AHP amplitude. Furthermore, NA-induced inhibition was occluded by pre-injection of Gbetagamma subunits. The Ca(2+) channel blocker Cd(2+) mimicked NA effects on transmitter release. Cd(2+), NA and Gbetagamma subunits also inhibited somatic Ca(2+) current. In contrast to effects on AP-evoked transmitter release, NA had no clear action on AP-independent EPSPs induced by hypertonic solutions. These results demonstrate that Gbetagamma subunits functionally mediate inhibition of transmitter release by alpha2-adrenoceptors and represent important regulators of synaptic transmission at mammalian presynaptic terminals.

Vesicle endocytosis requires dynamin-dependent GTP hydrolysis at a fast CNS synapse

Molecular dependence of vesicular endocytosis was investigated with capacitance measurements at the calyx of Held terminal in brainstem slices. Intraterminal loading of botulinum toxin E revealed that the rapid capacitance transient implicated as "kiss-and-run" was unrelated to transmitter release. The release-related capacitance change decayed with an endocytotic time constant of 10 to 25 seconds, depending on the magnitude of exocytosis. Presynaptic loading of the nonhydrolyzable guanosine 5'-triphosphate (GTP) analog GTPgS or dynamin-1 proline-rich domain peptide abolished endocytosis. These compounds had no immediate effect on exocytosis, but caused a use-dependent rundown of exocytosis. Thus, the guanosine triphosphatase dynamin-1 is indispensable for vesicle endocytosis at this fast central nervous system (CNS) synapse.

Opioid inhibition of GABAergic neurotransmission in mechanically isolated rat periaqueductal gray neurons

The descending pain control system is activated by opioid peptides mainly at the midbrain periaqueductal gray (PAG). Although activation of presynaptic opioid receptors has been reported to inhibit gamma-aminobutyric acid (GABA) release, the exact electrophysiological mechanisms are controversial. To elucidate the mechanisms involved in the opioid modulation of presynaptic GABA release, we isolated single PAG neurons with functionally intact synaptic terminals by a mechanical dissociation in the absence of enzyme. With the conventional whole-cell recording mode under the voltage-clamp conditions, the spontaneous miniature inhibitory postsynaptic currents (mIPSCs) were recorded. Bicuculline completely and reversibly blocked mIPSCs. A specific mu-opioid agonist, [d-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO), reversibly reduced the frequency of mIPSCs without any alteration of amplitude. The inhibitory effect of DAMGO was blocked by N-ethylmaleimide. Blockade of presynaptic Ca(2+) influx by cadmium or depletion of extracellular Ca(2+) did not alter the DAMGO inhibition. In addition, K(+) channels blockers, Ba(2+) or 4-aminopyridine, did not affect the DAMGO effect. The present study indicates that activation of presynaptic mu-opioid receptors coupled to G-proteins inhibits GABA release through unknown intracellular mechanisms downstream of Ca(2+) influx.

Exogenous nerve growth factor attenuates opioid-induced inhibition of voltage-activated Ba2+ currents in rat sensory neurons

Nerve growth factor (NGF) promotes the survival of embryonic sensory neurons and maintains the phenotypic characteristics of primary nociceptive neurons postnatally. NGF also contributes to nociceptor activation and hyperalgesia during inflammatory pain states. The purpose of this study was to determine whether NGF might have an additional pronociceptive action by interfering with opioid-mediated analgesia in primary nociceptive neurons. Sensory neurons were isolated from the dorsal root ganglia of weanling rats and kept in standard culture conditions either with or without exogenous NGF (50 ng/ml). Currents through voltage-gated calcium channels were recorded from individual neurons using the whole cell patch clamp technique with Ba(2+) as the charge carrier (I(Ba)). The micro-opioid agonist fentanyl (1 microM) and the GABA(B) agonist baclofen (50 microM) were used to test G protein-dependent inhibition of I(Ba). Fentanyl inhibited I(Ba) by an average of 38+/-4% in untreated cells vs. 25+/-2% in NGF-treated cells (P<0.01). NGF had no effect on I(Ba) current magnitude or kinetics. The NGF-induced attenuation of opioid action was observed as early as 4 h after exposure, but was not seen when NGF was applied by bath perfusion for up to 40 min, suggesting that the effect was not mediated by a rapid phosphorylation event. The effect of NGF was prevented by K-252a (100 nM), an inhibitor of TrkA autophosphorylation. Baclofen-induced inhibition of I(Ba), on the other hand, was not affected by NGF treatment, suggesting that NGF modulation of opioid-mediated inhibition occurred upstream from the G protein. This was supported by the finding that GTP-gamma-S, an agonist independent G protein activator, inhibited I(Ba) similarly in both untreated and NGF treated cells. The results show that NGF selectively attenuated opioid-mediated inhibition of I(Ba) via TrkA receptor activation, possibly by altering opioid receptor function.

Cannabinoid-induced presynaptic inhibition at the primary afferent trigeminal synapse of juvenile rat brainstem slices

Systemic or intraventricular administration of cannabinoids causes analgesic effects, but relatively little is known for their cellular mechanism. Using brainstem slices with the mandibular nerve attached, we examined the effect of cannabinoids on glutamatergic transmission in superficial trigeminal caudal nucleus of juvenile rats. The exogenous cannabinoid receptor agonist WIN 55,212-2 (WIN), as well as the endogenous agonist anandamide, hyperpolarized trigeminal caudal neurones and depressed the amplitude of excitatory postsynaptic potentials (EPSPs) or currents (EPSCs) monosynaptically evoked by stimulating mandibular nerves in a concentration-dependent manner. The inhibitory action of WIN was blocked or fully reversed by the CB1 receptor antagonist SR 141716A. WIN had no effect on the amplitude of miniature excitatory postsynaptic currents (mEPSCs) recorded in the presence of tetrodotoxin or cadmium. The inhibitory effect of WIN on EPSCs was greater for those with longer synaptic latency, suggesting that cannabinoids have a stronger effect on C-fibre EPSPs than on Adelta-fibre EPSPs. Ba2+ (100 microm) blocked the hyperpolarizing effect of cannabinoids, but did not affect their inhibitory effect on EPSPs. The N-type Ca2+ channel blocker omega-conotoxin GVIA (omega-CgTX) occluded the WIN-mediated presynaptic inhibition, whereas the P/Q-type Ca2+ channel blocker omega-agatoxin TK (omega-Aga) had no effect. These results suggest that cannabinoids preferentially activate CB1 receptors at the nerve terminal of small-diameter primary afferent fibres. Upon activation, CB1 receptors may selectively inhibit presynaptic N-type Ca2+ channels and exocytotic release machinery, thereby attenuating the transmitter release at the trigeminal nociceptive synapses.

Distinct roles of Kv1 and Kv3 potassium channels at the calyx of Held presynaptic terminal

Despite identification of >100 potassium channel subunits, relatively little is known about their roles in synaptic transmission. To address this issue we recorded presynaptic potassium currents (IPK) directly from the calyx of Held terminal in brainstem slices of rats. IPK was composed of a 4-aminopyridine (4-AP)-sensitive component and a smaller 4-AP-insensitive component composed of an iberiotoxin-sensitive current and an unidentified slowly activating potassium current. IPK could also be separated into a tetraethylammonium (TEA; 1 mm)-sensitive high-voltage-activated component and a margatoxin (10 nm)-sensitive low-voltage-activated component, which was also blocked by dendrotoxin-I (200 nm) and tityustoxin-Kalpha (100 nm). In outside-out patches excised from calyceal terminals, TEA (1 mm) consistently and to a large extent attenuated IPK, whereas margatoxin attenuated IPK only in a subset of patches (three of seven). Immunocytochemical examination using Kv subtype-specific antibodies indicated that multiple Kv1 and Kv3 subtypes were present at the calyceal terminal. In paired presynaptic and postsynaptic whole-cell recordings, TEA (1 mm) increased both the duration and peak amplitude of presynaptic action potentials and simultaneously potentiated EPSCs. Margatoxin alone had no such effect but reduced the amount of depolarization required for action potential generation, thereby inducing a burst of spikes when the nerve terminal was depolarized for a prolonged period. Thus, at the calyx of Held terminal, Kv3 channels directly regulate evoked transmitter release, whereas Kv1 channels reduce nerve terminal excitability, thereby preventing aberrant transmitter release. We conclude that both Kv3 and Kv1 channels contribute differentially to maintaining the fidelity of synaptic transmission at the calyx of Held.

Phosphoinositide 3-kinase cascade facilitates mu-opioid desensitization in sensory neurons by altering G-protein-effector interactions

Signaling via G-protein-coupled receptors undergoes desensitization after prolonged agonist exposure. Here we investigated the role of phosphoinositide 3-kinase (PI3K) and its downstream pathways in desensitization of micro-opioid inhibition of neuronal Ca2+ channels. In cultured mouse dorsal root ganglion neurons, two mechanistically different forms of desensitization were observed after acute or chronic treatment with the micro agonist [D-Ala2, N-MePhe4, Gly-ol5]-enkephalin (DAMGO). Chronic DAMGO desensitization was heterologous in nature and significantly attenuated by blocking the activity of PI3K or mitogen-activated protein kinase (MAPK). A combined application of PI3K and MAPK inhibitors showed no additive effect, suggesting that these two kinases act in a common pathway to facilitate chronic desensitization. Acute DAMGO desensitization, however, was not affected by the inhibitors. Furthermore, upregulation of the PI3K-Akt pathway in mutant mice lacking phosphatase and tensin homolog, a lipid phosphatase counteracting PI3K, selectively enhanced chronic desensitization in a PI3K- and MAPK-dependent manner. Using the prepulse facilitation (PPF) test, we further examined changes in the voltage-dependent component of DAMGO action that requires direct interactions between betagamma subunits of G-proteins and Ca2+ channels. DAMGO-induced PPF was diminished after chronic treatment, suggesting disruption of G-protein-channel interactions. Such disruption could occur at the postreceptor level, because chronic DAMGO also reduced GTPgammaS-induced PPF that was independent of receptor activation. Again, inhibition of PI3K or MAPK reduced desensitization of PPF. Our data suggest that the PI3Kcascade involving MAPK and Akt enhances micro-opioid desensitization via postreceptor modifications that interfere with G-protein-effector interactions.

Adenosine A(1) receptor-mediated presynaptic inhibition at the calyx of Held of immature rats

At the calyx of Held synapse in brainstem slices of 5- to 7-day-old (P5-7) rats, adenosine, or the type 1 adenosine (A1) receptor agonist N6-cyclopentyladenosine (CPA), inhibited excitatory postsynaptic currents (EPSCs) without affecting the amplitude of miniature EPSCs. The A1 receptor antagonist 8-cyclopentyltheophylline (CPT) had no effect on the amplitude of EPSCs evoked at a low frequency, but significantly reduced the magnitude of synaptic depression caused by repetitive stimulation at 10 Hz, suggesting that endogenous adenosine is involved in the regulation of transmitter release. Adenosine inhibited presynaptic Ca(2+) currents (IpCa) recorded directly from calyceal terminals, but had no effect on presynaptic K+ currents. When EPSCs were evoked by IpCa during simultaneous pre- and postsynaptic recordings, the magnitude of the adenosine-induced inhibition of IpCa fully explained that of EPSCs, suggesting that the presynaptic Ca(2+) channel is the main target of A1 receptors. Whereas the N-type Ca(2+) channel blocker omega-conotoxin attenuated EPSCs, it had no effect on the magnitude of adenosine-induced inhibition of EPSCs. During postnatal development, in parallel with a decrease in the A1 receptor immunoreactivity at the calyceal terminal, the inhibitory effect of adenosine became weaker. We conclude that presynaptic A1 receptors at the immature calyx of Held synapse play a regulatory role in transmitter release during high frequency transmission, by inhibiting multiple types of presynaptic Ca(2+) channels.

Distinct roles of Kv1 and Kv3 potassium channels at the calyx of Held presynaptic terminal

Despite identification of >100 potassium channel subunits, relatively little is known about their roles in synaptic transmission. To address this issue we recorded presynaptic potassium currents (IPK) directly from the calyx of Held terminal in brainstem slices of rats. IPK was composed of a 4-aminopyridine (4-AP)-sensitive component and a smaller 4-AP-insensitive component composed of an iberiotoxin-sensitive current and an unidentified slowly activating potassium current. IPK could also be separated into a tetraethylammonium (TEA; 1 mm)-sensitive high-voltage-activated component and a margatoxin (10 nm)-sensitive low-voltage-activated component, which was also blocked by dendrotoxin-I (200 nm) and tityustoxin-Kalpha (100 nm). In outside-out patches excised from calyceal terminals, TEA (1 mm) consistently and to a large extent attenuated IPK, whereas margatoxin attenuated IPK only in a subset of patches (three of seven). Immunocytochemical examination using Kv subtype-specific antibodies indicated that multiple Kv1 and Kv3 subtypes were present at the calyceal terminal. In paired presynaptic and postsynaptic whole-cell recordings, TEA (1 mm) increased both the duration and peak amplitude of presynaptic action potentials and simultaneously potentiated EPSCs. Margatoxin alone had no such effect but reduced the amount of depolarization required for action potential generation, thereby inducing a burst of spikes when the nerve terminal was depolarized for a prolonged period. Thus, at the calyx of Held terminal, Kv3 channels directly regulate evoked transmitter release, whereas Kv1 channels reduce nerve terminal excitability, thereby preventing aberrant transmitter release. We conclude that both Kv3 and Kv1 channels contribute differentially to maintaining the fidelity of synaptic transmission at the calyx of Held.

Phosphoinositide 3-kinase cascade facilitates mu-opioid desensitization in sensory neurons by altering G-protein-effector interactions

Signaling via G-protein-coupled receptors undergoes desensitization after prolonged agonist exposure. Here we investigated the role of phosphoinositide 3-kinase (PI3K) and its downstream pathways in desensitization of micro-opioid inhibition of neuronal Ca2+ channels. In cultured mouse dorsal root ganglion neurons, two mechanistically different forms of desensitization were observed after acute or chronic treatment with the micro agonist [D-Ala2, N-MePhe4, Gly-ol5]-enkephalin (DAMGO). Chronic DAMGO desensitization was heterologous in nature and significantly attenuated by blocking the activity of PI3K or mitogen-activated protein kinase (MAPK). A combined application of PI3K and MAPK inhibitors showed no additive effect, suggesting that these two kinases act in a common pathway to facilitate chronic desensitization. Acute DAMGO desensitization, however, was not affected by the inhibitors. Furthermore, upregulation of the PI3K-Akt pathway in mutant mice lacking phosphatase and tensin homolog, a lipid phosphatase counteracting PI3K, selectively enhanced chronic desensitization in a PI3K- and MAPK-dependent manner. Using the prepulse facilitation (PPF) test, we further examined changes in the voltage-dependent component of DAMGO action that requires direct interactions between betagamma subunits of G-proteins and Ca2+ channels. DAMGO-induced PPF was diminished after chronic treatment, suggesting disruption of G-protein-channel interactions. Such disruption could occur at the postreceptor level, because chronic DAMGO also reduced GTPgammaS-induced PPF that was independent of receptor activation. Again, inhibition of PI3K or MAPK reduced desensitization of PPF. Our data suggest that the PI3Kcascade involving MAPK and Akt enhances micro-opioid desensitization via postreceptor modifications that interfere with G-protein-effector interactions.

Direct modulation of synaptic vesicle priming by GABA(B) receptor activation at a glutamatergic synapse

Second messenger cascades involving G proteins and calcium are known to modulate neurotransmitter release. A prominent effect of such a cascade is the downmodulation of presynaptic calcium influx, which markedly reduces evoked neurotransmitter release. Here we show that G-protein-mediated signalling, such as through GABA (gamma-amino butyric acid) subtype B (GABA(B)) receptors, retards the recruitment of synaptic vesicles during sustained activity and after short-term depression. This retardation occurs through a lowering of cyclic AMP, which blocks the stimulatory effect of increased calcium concentration on vesicle recruitment. In this signalling pathway, cAMP (functioning through the cAMP-dependent guanine nucleotide exchange factor) and calcium/calmodulin cooperate to enhance vesicle priming. The differential modulation of the two forms of synaptic plasticity, presynaptic inhibition and calcium-dependent recovery from synaptic depression, is expected to have interesting consequences for the dynamic behaviour of neural networks.

Role for G protein Gbetagamma isoform specificity in synaptic signal processing: a computational study

Computational modeling is used to investigate the functional impact of G protein-mediated presynaptic autoinhibition on synaptic filtering properties. It is demonstrated that this form of autoinhibition, which is relieved by depolarization, acts as a high-pass filter. This contrasts with vesicle depletion, which acts as a low-pass filter. Model parameters are adjusted to reproduce kinetic slowing data from different Gbetagamma dimeric isoforms, which produce different degrees of slowing. With these sets of parameter values, we demonstrate that the range of frequencies filtered out by the autoinhibition varies greatly depending on the Gbetagamma isoform activated by the autoreceptors. It is shown that G protein autoinhibition can enhance the spatial contrast between a spatially distributed high-frequency signal and surrounding low-frequency noise, providing an alternate mechanism to lateral inhibition. It is also shown that autoinhibition can increase the fidelity of coincidence detection by increasing the signal-to-noise ratio in the postsynaptic cell. The filter cut, the input frequency below which signals are filtered, depends on several biophysical parameters in addition to those related to Gbetagamma binding and unbinding. By varying one such parameter, the rate at which transmitter unbinds from autoreceptors, we show that the filter cut can be adjusted up or down for several of the Gbetagamma isoforms. This allows for great synapse-to-synapse variability in the distinction between signal and noise.

Neuronal calcium sensor 1 and activity-dependent facilitation of P/Q-type calcium currents at presynaptic nerve terminals

P/Q-type presynaptic calcium currents (IpCa) undergo activity-dependent facilitation during repetitive activation at the calyx of the Held synapse. We investigated whether neuronal calcium sensor 1 (NCS-1) may underlie this phenomenon. Direct loading of NCS-1 into the nerve terminal mimicked activity-dependent IpCa facilitation by accelerating the activation time of IpCa in a Ca2+-dependent manner. A presynaptically loaded carboxyl-terminal peptide of NCS-1 abolished IpCa facilitation. These results suggest that residual Ca2+ activates endogenous NCS-1, thereby facilitating IpCa. Because both P/Q-type Ca2+ channels and NCS-1 are widely expressed in mammalian nerve terminals, NCS-1 may contribute to the activity-dependent synaptic facilitation at many synapses.

Short-term plasticity at the calyx of Held

Synapses show widely varying degrees of short-term facilitation and depression. Several mechanisms have been proposed to underlie short-term plasticity, but the contributions of presynaptic mechanisms have been particularly difficult to study because of the small size of synaptic boutons in the mammalian brain. Here we review the functional properties of the calyx of Held, a giant nerve terminal that has shed new light on the general mechanisms that control short-term plasticity. The calyx of Held has also provided fresh insights into the strategies used by synapses to extend their dynamic range of operation and preserve the timing of sensory stimuli.

Activation of the epsilon isoform of protein kinase C in the mammalian nerve terminal

Activation of protein kinase C (PKC) by phorbol ester facilitates hormonal secretion and transmitter release, and phorbol ester-induced synaptic potentiation (PESP) is a model for presynaptic facilitation. A variety of PKC isoforms are expressed in the central nervous system, but the isoform involved in the PESP has not been identified. To address this question, we have applied immunocytochemical and electrophysiological techniques to the calyx of Held synapse in the medial nucleus of the trapezoid body (MNTB) of rat auditory brainstem. Western blot analysis indicated that both the Ca(2+)-dependent "conventional" PKC and Ca(2+)-independent "novel" PKC isoforms are expressed in the MNTB. Denervation of afferent fibers followed by organotypic culture, however, selectively decreased "novel" epsilon PKC isoform expressed in this region. The afferent calyx terminal was clearly labeled with the epsilon PKC immunofluorescence. On stimulation with phorbol ester, presynaptic epsilon PKC underwent autophosphorylation and unidirectional translocation toward the synaptic side. Chelating presynaptic Ca(2+), by using membrane permeable EGTA analogue or high concentration of EGTA directly loaded into calyceal terminals, had only a minor attenuating effect on the PESP. We conclude that the Ca(2+)-independent epsilon PKC isoform mediates the PESP at this mammalian central nervous system synapse.