Presynaptic calcium current modulation by a metabotropic glutamate receptor

Postnatal development of phase-locked high-fidelity synaptic transmission in the medial nucleus of the trapezoid body of the rat

Synaptic transmission in the medial nucleus of the trapezoid body of rats was analyzed in postnatal days 4-13 (P4-P13) by applying the whole-cell patch-recording technique to brain slices. In P4-P6 animals, evoked EPSCs fluctuated extensively in amplitude and occurred in marked asynchrony, followed by spontaneous EPSCs. With development of animals, the evoked EPSCs increased in amplitude, and the rise time became faster. In addition, the synaptic transmission became phase-locked. The coefficient of variation (CV) of EPSC amplitude decreased with development (0.32 +/- 0.03 for P4-P5 and 0. 05 +/- 0.01 for P9-P11). The amplitude of miniature EPSCs did not change throughout the postnatal days investigated (-30.2 +/- 0.3 pA at -70 mV). The CV was dependent on extracellular Ca2+ concentration ([Ca2+]o) and was reduced with the increase of [Ca2+]o, and this [Ca2+]o dependence was shifted toward lower [Ca2+]o with development. Direct patch recording of the presynaptic terminals demonstrated an increase in Ca2+ currents during these postnatal days. The phase-locked high-fidelity transmission in this synapse is achieved with development likely through the increase of Ca2+ currents and Ca2+ sensitivity of transmitter release mechanisms in the presynaptic terminal.

Calcium current during a single action potential in a large presynaptic terminal of the rat brainstem

1. The calcium current of a 'giant' synaptic terminal (the calyx of Held) was studied using two-electrode voltage clamp in slices of the rat brainstem. 2. In terminals with a long axon (length > 100 microns), the passive current transient decayed biexponentially following voltage steps. In terminals with a short axon (length < 30 microns), the slow component was reduced or absent. These terminals also had small slow calcium tail currents following long depolarizing voltage steps, suggesting that these are largely due to axonal calcium channels. 3. Terminals were voltage clamped with action potential waveform commands. At both 24 and 36 degrees C the calcium current began shortly after the peak of the action potential and ended before the terminal was fully repolarized. 4. The calcium current during the repolarization phase was 69 +/- 1% (n = 3) of maximal, judged from the increase in this current when a plateau phase was added to the action potential waveform. 5. A Hodgkin-Huxley m2 model, based on the measured activation and deactivation of the calcium current, reproduced both the time course and the amplitude increase of the calcium currents during the different action potential waveforms well. 6. The fast gating of the calcium channels in the terminal ensures that they are effectively opened during the repolarization phase of an action potential. This implies that the distance between open calcium channels is minimized, which is in agreement with the view that multiple calcium channels are needed to release a vesicle in this synapse.

Group II and III metabotropic glutamate receptors modulate paired pulse depression in the rat dentate gyrus in vitro

We have investigated the effect of a number of group I, II and III metabotropic glutamate (mGlu) receptor agonists and antagonists on paired pulse depression in the medial perforant path of the rat dentate gyrus in vitro. A triphasic pattern of a large depression at short intervals (10-50 ms), a reduction of this depression at intermediate intervals (50-200 ms) and again a large depression at late intervals (> 200 ms) was observed. The group I mGlu receptor agonist, (S)-3,5-dihydroxy phenylglycine ((S)-DHPG; 20 microM) had no significant effect on paired pulse depression at any interstimulus intervals. The mGlu receptor group II and III agonists, L-CCG-1 ((2S,3S,4S)-alpha-(carboxy-cyclopropyl)-glycine), DCG-IV ((2S,1'R,2'R,3'R)-2-2',3'-dicarboxy cyclopropylglycine), 1S,3R-ACPD (1S,3R-1-aminocyclopentate-1,3-dicarboxylic acid) and L-AP4 (L-2-amino-4-phosphono butyric acid) reduced paired pulse depression at interstimulus intervals of 200 ms or less. Application of the non specific mGlu receptor antagonist, MCPG (alpha-methyl carboxy-phenylglycine; 200 microM) completely inhibited the 1S,3R ACPD-induced reduction in paired pulse depression but was without effect on the L-AP4 response. The relatively specific group II antagonist MCCG ((2S,3S,4S)-2-methyl-2-carboxy cycloproprylglycine) at 200 microM and 500 microM, attenuated but did not completely inhibit the DCG-IV induced reduction of paired pulse depression. The putative group III pre-synaptic mGlu receptor antagonist alpha-methyl-L-AP4 and MSOP ((RS)-alpha-methylserine-O-phosphate) both at 200 microM inhibited the L-AP4-induced reduction in paired pulse depression at intermediate phase interstimulus intervals but not at early interstimulus intervals. These results specifically demonstrate the involvement of group III and III mGlu receptor ligands in the modulation of paired pulse depression in the medial perforant pathway.

Inhibition of synaptic transmission by neuropeptide Y in rat hippocampal area CA1: modulation of presynaptic Ca2+ entry

Neuropeptide Y (NPY) agonists inhibit glutamate release by a presynaptic action at the CA3-CA1 synapse of rat hippocampus. We have examined the relationship between [Capre]t via presynaptic, voltage-dependent calcium channels (VDCCs), measured optically by using the fluorescent calcium indicator fura-2, and transmitter release, measured electrophysiologically. Activation of presynaptic NPY Y2 receptors reduced [Capre]t and thereby inhibited synaptic transmission. Multiple calcium channels are involved in synaptic transmission at this synapse. Activation of Y2 receptors inhibits N-type, P/Q-type, and unidentified presynaptic VDCCs. The inhibition of each of these calcium channel types contributes to the reduction of [Capre]t by Y2 receptors. Activation of adenosine receptors fully occluded the inhibition of presynaptic calcium influx by Y2 receptors but not the inhibition by GABAB receptors, suggesting a convergent action for Y2 and adenosine receptors, probably by coupling to the same G-protein.

Presynaptic depression at a calyx synapse: the small contribution of metabotropic glutamate receptors

Synaptic depression of evoked EPSCs was quantified with stimulation frequencies ranging from 0.2 to 100 Hz at the single CNS synapse formed by the calyx of Held in the rat brainstem. Half-maximal depression occurred at approximately 1 Hz, with 10 and 100 Hz stimulation frequencies reducing EPSC amplitudes to approximately 30% and approximately 10% of their initial magnitude, respectively. The time constant of recovery from depression elicited by 10 Hz afferent fiber stimulation was 4.2 sec. AMPA and NMDA receptor-mediated EPSCs depressed in parallel at 1-5 Hz stimulation frequencies, suggesting that depression was induced by presynaptic mechanism(s) that reduced glutamate release. To determine the contribution of autoreceptors to depression, we studied the inhibitory effects of the metabotropic glutamate receptor (mGluR) agonists (1S, 3S)-ACPD and L-AP4 and found them to be reversed in a dose-dependent manner by (RS)-alpha-cyclopropyl-4-phosphonophenylglycine (CPPG), a novel and potent competitive antagonist of mGluRs. At 300 microM, CPPG completely reversed the effects of L-AP4 and (1S, 3S)-ACPD, but reduced 5-10 Hz elicited depression by only approximately 6%. CPPG-sensitive mGluRs, presumably activated by glutamate spillover during physiological synaptic transmission, thus contribute on the order of only 10% to short-term synaptic depression. We therefore suggest that the main mechanism contributing to the robust depression elicited by 5-10 Hz afferent fiber stimulation of the calyx of Held synapse is synaptic vesicle pool depletion.

Differential presynaptic localization of metabotropic glutamate receptor subtypes in the rat hippocampus

Neurotransmission in the hippocampus is modulated variously through presynaptic metabotropic glutamate receptors (mGluRs). To establish the precise localization of presynaptic mGluRs in the rat hippocampus, we used subtype-specific antibodies for eight mGluRs (mGluR1-mGluR8) for immunohistochemistry combined with lesioning of the three major hippocampal pathways: the perforant path, mossy fiber, and Schaffer collateral. Immunoreactivity for group II (mGluR2) and group III (mGluR4a, mGluR7a, mGluR7b, and mGluR8) mGluRs was predominantly localized to presynaptic elements, whereas that for group I mGluRs (mGluR1 and mGluR5) was localized to postsynaptic elements. The medial perforant path was strongly immunoreactive for mGluR2 and mGluR7a throughout the hippocampus, and the lateral perforant path was prominently immunoreactive for mGluR8 in the dentate gyrus and CA3 area. The mossy fiber was labeled for mGluR2, mGluR7a, and mGluR7b, whereas the Schaffer collateral was labeled only for mGluR7a. Electron microscopy further revealed the spatial segregation of group II and group III mGluRs within presynaptic elements. Immunolabeling for the group III receptors was predominantly observed in presynaptic active zones of asymmetrical and symmetrical synapses, whereas that for the group II receptor (mGluR2) was found in preterminal rather than terminal portions of axons. Target cell-specific segregation of receptors, first reported for mGluR7a (Shigemoto et al,., 1996), was also apparent for the other group III mGluRs, suggesting that transmitter release is differentially regulated by 2-amino-4-phosphonobutyrate-sensitive mGluRs in individual synapses on single axons according to the identity of postsynaptic neurons.

Presynaptic modulation of glutamate release targets different calcium channels in rat cerebrocortical nerve terminals

We have studied which type/s of Ca2+-channel/s support glutamate exocytosis and its modulation by presynaptic receptors in cerebrocortical nerve terminals. Depolarization of nerve terminals with 30 mM KCl induced a Ca2+-dependent release of 3.64 +/- 0.25 nmol/mg of protein. The addition of either 2 microM omega-conotoxin-GVIA or 200 nM omega-agatoxin-IVA reduced the KCl-evoked release by 47.7 +/- 3.5% and 70.4 +/- 8.9% respectively, and by 85.7 +/- 4.1% when both toxins were co-applied. The activation of adenosine A1 receptors with N6-cyclohexyladenosine or the activation of metabotropic glutamate receptors with L(+)-2-amino-4-phosphonobutyrate inhibited the KCl-evoked release by 41.0 +/- 5.9 and 54.3 +/- 10% respectively. The extent of these inhibitions was not altered by the prior addition of 2 microM omega-conotoxin-GVIA but they were significantly enhanced when omega-agatoxin-IVA was added together with the adenosine A1 receptor agonist or the metabotropic glutamate receptor agonist, suggesting that omega-conotoxin-GVIA-sensitive and not omega-agatoxin-IVA-sensitive Ca2+-channels are involved in the action of these inhibitory receptors. By contrast, the facilitation of glutamate release that follows the activation of the protein kinase C, either with phorbol esters or with the stimulation of phospholipase C-linked metabotropic receptors, was expressed by both omega-conotoxin-GVIA-sensitive and omega-agatoxin-sensitive Ca2+-channels. It is concluded that different Ca2+-channels support the modulation of glutamate release by presynaptic receptors.

Physiology and pharmacology of native glycine receptors in developing rat auditory brainstem neurons

Glycinergic neurotransmission is mediated via inhibitory glycine receptors (GlyRs) which are heterogeneous during development. Electrophysiological studies performed on recombinant GlyRs have identified different pharmacological properties and attributed them to differences in their subunit composition. Here, we report on age-related changes in the response properties of native GlyRs in the mammalian brain. Whole-cell patch-clamp recordings were obtained from neurons of the medial nucleus of the trapezoid body (MNTB), a major relay station in the mammalian auditory brainstem. Experiments were performed in acute medullary slices of rats between postnatal day (P) 1 and P15, a period during which synapse maturation occurs. Glycine-induced currents were present throughout the period under investigation and displayed age-related modifications in their amplitude, kinetic characteristics, and sensitivity to drugs. Current amplitudes and GlyR desensitization behavior increased with age. The alpha 1 subunit-specific GlyR antagonist cyanotriphenylborate (CTB) was barely effective in reducing glycine-induced currents during the first few postnatal days, yet a significant increase of the inhibitory effect occurred after the first postnatal week. This finding indicates that alpha 1 subunit-containing GlyRs become expressed only postnatally in the MNTB. Picrotoxin, which most effectively blocks recombinant alpha 2-homooligomers, reduced glycine-induced currents in neonatal MNTB neurons, suggesting that alpha 2-homooligomers may form native GlyR isoforms. Our results show that the physiology and pharmacology of GlyRs in the auditory brainstem underlie age-related changes which are most probably produced through a replacement of "neonatal" alpha 2 subunits with "adult" alpha 1 subunits.

Properties of ATP receptor-mediated synaptic transmission in the rat medial habenula

The properties of central ATP-mediated synaptic currents were studied using whole-cell patch-clamp recording in rat medial habenula slices. Release was shown to be calcium dependent with a Hill coefficient of approximately 2. The voltage dependence of synaptic current amplitudes was approximately linear. Some reduction of the synaptic current amplitudes was observed at 10 mM extracellular calcium, suggesting calcium block/permeability of the channels. This was confirmed by observation of current-voltage reversal potentials in different calcium concentrations. We estimate that the channels underlying half the synapses showed a negligible calcium permeability. In the other four out of eight synapses the results suggest a very high calcium permeability with an estimated PCa/PCs of > 10. Thus, at -70 mV, in 1 mM calcium, more than 15% of the ATP-mediated synaptic current is estimated to be carried by calcium, but only at synapses with calcium-permeable channels. Net current through these synaptic channels is also controlled by the voltage dependence of synaptic current decay time constants (increasing e-fold for 158 mV depolarization) and by a strong dependence of transmitter release on the frequency of stimulation of the presynaptic neurone, with failure rates increasing 3-fold as stimulation rates were increased from 1 to 10 Hz.

Modulation of excitatory synaptic transmission in locus coeruleus by multiple presynaptic metabotropic glutamate receptors

Metabotropic glutamate receptors have been implicated in modulation of synaptic transmission in many different systems. This study reports the effects of selective activation of metabotropic glutamate receptors on synaptic transmission in intracellularly recorded locus coeruleus neurons in brain slice preparations. Perfusion of either L-2-amino-4-phosphonobutyric acid (L-AP4; 0.1-500 microM) or (+/-)-1-aminocyclopentane-trans-1,3,dicarboxylic acid (t-ACPD; 0.1-500 microM) caused a depression of excitatory postsynaptic potentials in a dose-dependent fashion to about 70% inhibition. Both agonists exerted their effects at relatively low concentrations with estimated EC50s of 2.6 microM and 11.5 microM for L-AP4 and t-ACPD, respectively. This inhibition was not observed with the potent group I metabotropic glutamate receptor agonist (RS)-3,5-dihydroxyphenylglycine (DHPG; 100 microM). Conversely, (R)-4-carboxy-3-hydroxyphenyl-glycine (4C-3H-PG), a group I antagonist/group II agonist, and 2R,4R-4-aminopyrrolidine-2,4-dicarboxylate (APDC), a novel and specific group II agonist, also caused an inhibition of excitatory postsynaptic potentials. Both t-ACPD and L-AP4 produced an increase in paired-pulse facilitation, and failed to change the locus coeruleus response to focally applied glutamate, indicating a presynaptic locus of action. The L-AP4 inhibition was antagonized by (S)-amino-2-methyl-4-phosphonobutanoic acid (MAP4: group III antagonist) but not by (RS)-alpha-methyl-4-carboxyphenylglycine [(RS)-MCPG; mixed antagonist], suggesting that this agonist acts through a type 4 metabotropic glutamate receptor. Conversely, t-ACPD was antagonized by MCPG and by ethyl glutamate (group II antagonist), but not by aminoindan dicarboxylic acid (AIDA; group I antagonist) or MAP4, suggesting that this agonist acts on a type 2 or 3 metabotropic glutamate receptor. Taken together, these results suggest that two pharmacologically distinct presynaptic metabotropic glutamate receptors function in an additive fashion to inhibit excitatory synaptic transmission in locus coeruleus neurons. These receptors may be involved in a feedback mechanism and as such may function as autoreceptors for excitatory amino acids.

The calcium channel and the organization of the presynaptic transmitter release face

Calcium influx through ion channels located on the release face of the presynaptic nerve terminal gates the release of neurotransmitters by the fusion of the secretory vesicle and the discharge of its contents. Recently, several lines of research have indicated that the relationship between the Ca2+ channel and the release site might be more complex than dictated simply by its role as an ion conduit. The evidence suggests that the channel and the transmitter-release mechanism exist as a multimolecular entity and that this interaction has functional consequences, not only on the mechanisms and properties of transmitter release, but also on the behavior of the presynaptic Ca2+ channel itself.

G protein-coupled inwardly rectifying K+ channels (GIRKs) mediate postsynaptic but not presynaptic transmitter actions in hippocampal neurons

To study the role of G protein-coupled, inwardly rectifying K+ (GIRK) channels in mediating neurotransmitter actions in hippocampal neurons, we have examined slices from transgenic mice lacking the GIRK2 gene. The outward currents evoked by agonists for GABA(B) receptors, 5HT1A receptors, and adenosine A1 receptors were essentially absent in mutant mice, while the inward current evoked by muscarinic receptor activation was unaltered. In contrast, the presynaptic inhibitory action of a number of presynaptic receptors on excitatory and inhibitory terminals was unaltered in mutant mice. These included GABA(B), adenosine, muscarinic, metabotropic glutamate, and NPY receptors on excitatory synapses and GABA(B) and opioid receptors on inhibitory synapses. These findings suggest that a number of G protein-coupled receptors activate the same class of postsynaptic K+ channel, which contains GIRK2. In addition, the GIRK2 channels play no role in the inhibition mediated by presynaptic G protein-coupled receptors, suggesting that the same receptor can couple to different effector systems according to its subcellular location in the neuron.

Cellular mechanisms for preservation of timing in central auditory pathways

The faithful preservation of acoustic timing information, as signals are passed from one synaptic level to another, requires a convergence of morphological, biophysical, and biochemical specializations in auditory neurons. Recent studies have focused on the adaptive membrane properties of neurons in the auditory brainstem. These include analyses of neurotransmitter receptors and voltage-gated channels, as well as the mechanisms of transmitter release and its modulation. The molecular composition of the relevant proteins are now being demonstrated, including the glutamate receptor Dflop (GluR-Dflop) subunit of AMPA receptors and members of the Kv1 and Kv3 families of potassium channels.

Electrophysiological properties of the human N-type Ca2+ channel: I. Channel gating in Ca2+, Ba2+ and Sr2+ containing solutions

We have characterized the properties of the human N-type Ca2+ channel produced by the stable co-expression of the alpha(1B-1), alpha(2b)delta and beta(1b) subunits. The channel displayed the expected pharmacology with respect to the toxins omega-CTx-GVIA and omega-CTx-MVIIC, which depressed currents in a voltage-independent fashion. We characterized a variety of biophysical properties of the channel under conditions in which either Ca2+, Ba2+ or Sr2+ was the sole extracellular divalent ion. In all three ions, current-voltage relationships revealed that the channel was clearly high-voltage activated. Current activation was significantly slower in Ca2+ than either Sr2+ or Ba2+. Construction of conductance-voltage relationships from tail current measurements indicated that the channel was more high-voltage activated in Ca2+ than in either Sr2+ or Ba2+. The rank order of current amplitude at +4 mV was Ba2+ > Sr2+ > or = Ca2+. Elevation of the extracellular concentration of Ba2+ increased maximal current amplitude and shifted the current-voltage relationship to the right. In all three ions channel inactivation was complex consisting of three distinct exponentials. Recovery from inactivation was slow taking several seconds to reach completion. Steady-state inactivation curves revealed that channel inactivation became detectable at holding potentials of between -101 and -91 mV depending on the permeating species. The rank order of mid-points of steady state inactivation was (most negative) Sr2+ > Ca2+ > Ba2+ (most positive). Deactivation of the N-type Ca2+ channel was voltage-dependent and very fast in all three ions. The deactivation rate in Ba2+ was significantly slower than that in both Ca2+ and Sr2+, however the voltage-dependence of deactivation rate was indistinguishable in all three ions.

L-AP4 inhibition of depolarization-evoked cGMP formation in rat cerebellum

The effects of the group III mGluR agonist, L-2-amino-4-phosphonobutyrate (L-AP4), on depolarization-stimulated cGMP levels in adult rat cerebellar slices were determined. L-AP4 elicited a concentration-dependent, complete inhibition of cGMP formation stimulated by 4-aminopyridine (4-AP; 1 mM), yielding an IC50 value of 4.2 +/- 1.6 microM (n = 3). The 4-AP response was also reduced by the P-type Ca2+ channel toxins omega-conotoxin MVIIC (3 microM; 39 +/- 7% inhibition) and omega-Agatoxin IVA (30 nM; 53 +/- 4%), and was abolished in the absence of Ca2+ or in the presence of Co2+. The inhibitions of the 4-AP cGMP response by 10 microM L-AP4 and 30 nM omega-Agatoxin IVA were not additive, indicating that part of the actions of L-AP4 in the cerebellum involves the modulation of P-type Ca2+ channels.

Somatostatin inhibits excitatory transmission at rat hippocampal synapses via presynaptic receptors

Somatostatin is one of the major peptides in interneurons of the hippocampus. It is believed to play a role in memory formation and to reduce the susceptibility of the hippocampus to seizure-like activity. However, at the cellular level, the actions of somatostatin on hippocampal neurons are still controversial, ranging from inhibition to excitation. In the present study, we measured autaptic currents of hippocampal neurons isolated in single-neuron microcultures. Somatostatin and the analogous peptides seglitide and octreotide reduced glutamatergic, but not GABAergic, autaptic currents via pertussis toxin-sensitive G-proteins. This effect was observed whether autaptic currents were mediated by NMDA or non-NMDA glutamate receptors. Furthermore, somatostatin did not affect currents evoked by the direct application of glutamate, but reduced the frequency of spontaneously occurring excitatory autaptic currents. These results show that presynaptic somatostatin receptors of the SRIF1 family inhibit glutamate release at hippocampal synapses. Somatostatin, seglitide, and octreotide also reduced the frequency of miniature excitatory postsynaptic currents in mass cultures without affecting their amplitudes. In addition, all three agonists inhibited voltage-activated Ca2+ currents at neuronal somata, but failed to alter K+ currents, effects that were also abolished by pertussis toxin. Thus, presynaptic somatostatin receptors in the hippocampus selectively inhibit excitatory transmission via G-proteins of the Gi/Go family and through at least two separate mechanisms, the modulation of Ca2+ channels and an effect downstream of Ca2+ entry. This presynaptic inhibition by somatostatin may provide a basis for its reportedly anticonvulsive action.

Heterogeneity of release probability, facilitation, and depletion at central synapses

Previous studies of short-term plasticity in central nervous systems synapses have largely focused on average synaptic properties. In this study, we use recordings from putative single synaptic release sites in hippocampal slices to show that significant heterogeneity exists in facilitation and depletion among synapses. In particular, the amount of paired-pulse facilitation is inversely related to the initial release probability of the synapse. We also examined depletion at individual synapses using high frequency stimulation, and estimated the size of the readily releasable vesicle pool, which averaged 5.0 +/- 3.0 quanta (n = 13 synapses). In addition, these experiments demonstrate that the release probability at a synapse is directly correlated with the size of its readily releasable vesicle pool.

Synaptic transmission: well-placed modulators

Metabotropic glutamate receptors are involved in the modulation of synaptic transmission; their localization in perisynaptic areas would appear to limit their activation by endogenous glutamate, but recent reports suggest that this strategic placement allows use-dependent activation of these synaptic modulators.

Prolonged anticonvulsant action of glutamate metabotropic receptor agonists in inferior colliculus of genetically epilepsy-prone rats

The anticonvulsant activity of (S)-4-carboxy-3-hydroxyphenylglycine ((S)-4C3HPG) (an antagonist of Group I and an agonist of Group II metabotropic glutamate (mGlu) receptors), of (1S,3S)-1-aminocyclopentane-1,3-dicarboxylic acid ((1S,3S)-ACPD) (an agonist of Group II mGlu receptors), and of L-serine-O-phosphate (an agonist of Group III mGlu receptors) was studied against sound-induced seizures in genetically epilepsy-prone (GEP) rats following bilateral microinjection into the inferior colliculus. All 3 drugs produce dose-dependent suppression of all phases of sound-induced seizures (wild running, clonic and tonic). (S)-4C3HPG produces an immediate and short-lasting (< 2 h) protection against sound-induced seizures with an ED50 value of 4.3 (3.2-5.7) nmol, at 5 min. The preferential agonists of Group II and Group III mGlu receptors produce an immediate, transient (< 10 min) proconvulsant effect followed by a prolonged (> 1 day) anticonvulsant effect against sound-induced seizures. The anticonvulsant ED50 value for (1S,3S)-ACPD is 9 (5-18) nmol at 2 h, and for L-serine-O-phosphate is 36 (6.5-199) nmol at 2 days. It is concluded that mGlu receptor activation potently modifies seizure threshold.

Direct measurements of presynaptic calcium and calcium-activated potassium currents regulating neurotransmitter release at cultured Xenopus nerve-muscle synapses

The understanding of neurotransmitter release at vertebrate synapses has been hampered by the paucity of preparations in which presynaptic ionic currents and postsynaptic responses can be monitored directly. We used cultured embryonic Xenopus neuromuscular junctions and simultaneous pre- and postsynaptic patch-clamp current-recording procedures to identify the major presynaptic conductances underlying the initiation of neurotransmitter release. Step depolarizations and action potential waveforms elicited Na and K currents along with Ca and Ca-activated K (KCa) currents. The onset of KCa current preceded the peak of the action potential. The predominantly omega-CgTX GVIA-sensitive Ca current occurred primarily during the falling phase, but there was also significant Ca2+ entry during the rising phase of the action potential. The postsynaptic current began a mean of 0.7 msec after the time of maximum rate of rise of the Ca current. omega-CgTX also blocked KCa currents and transmitter release during an action potential, suggesting that Ca and KCa channels are colocalized at presynaptic active zones. In double-ramp voltage-clamp experiments, KCa channel activation is enhanced during the second ramp. The 1 msec time constant of decay of enhancement with increasing interpulse interval may reflect the time course of either the deactivation of KCa channels or the diffusion/removal of Ca2+ from sites of neurotransmitter release after an action potential.

Presynaptic inhibition of elicited neurotransmitter release

Activation of presynaptic receptors for a variety of neurotransmitters and neuromodulators inhibits transmitter release at many synapses. Such presynaptic inhibition might serve as a means of adjusting synaptic strength or preventing excessive transmitter release, or both. Previous evidence showed that presynaptic modulators inhibit Ca2+ channels and activate K+ channels at neuronal somata. These modulators also inhibit spontaneous transmitter release by mechanisms downstream of Ca2+ entry. The relative contribution of the above mechanisms to the inhibition of elicited release has been debated for a long time. Recent evidence at synapses where the relationship between transmitter release and presynaptic Ca2+ influx has been well characterized suggests that inhibition of presynaptic voltage-dependent Ca2+ channels plays the major role in presynaptic inhibition of elicited neurotransmitter release. In addition, modulation of the release machinery might contribute to inhibition of elicited release.

Evidence for functional metabotropic glutamate receptors in the dorsal cochlear nucleus

The parallel fibers (PFs) of the dorsal cochlear nucleus (DCN) molecular layer use glutamate as a neurotransmitter. Although metabotropic glutamate receptors (mGluRs) have been identified on cells postsynaptic to the PFs, little is known about the effects of mGluR activation in PF synaptic transmission in the DCN. To investigate these effects, PF-evoked field potentials were recorded from the DCN in guinea pig brain stem slice preparations. The alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated components of the field response were reversibly depressed by bathing the slice in the mGluR agonists (+/-)-aminocyclopentane-1,3-dicarboxylic acid (trans-ACPD) or (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD]. A similar depression was produced by the mGluR1/5 agonist (RS)-3,5-dihydroxyphenylglycine, but not by the mGluR2/3 agonist (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine or by the mGluR4/6/7/8 agonist L(+)-2-amino-4-phosphonobutyric acid. In addition to the AMPA component, an N-methyl-D-aspartate (NMDA) receptor-dependent component of the field potentials could be identified when the slices were bathed in a low magnesium solution. Under these conditions, the ACPD-induced depression of the AMPA component did not completely recover, whereas the depression of the NMDA component usually recovered and potentiated in some slices. Intracellular recordings of PF-evoked responses were obtained to ascertain which neuronal populations were affected by mGluR activation. Activation of mGluRs produced a reversible depression of PF-evoked responses in cartwheel cells that was not accompanied by any changes in paired-pulse facilitation. The PF-evoked responses recorded from pyramidal cells were unaffected by mGluR activation. Both cell types exhibited a reversible depolarization during (1S,3R)-ACPD application. Subsequent experiments explored the involvement of protein kinases in mediating the effects of mGluRs. The protein kinase C (PKC) activator phorbol-12,13-diacetate partially inhibited the mGluR-mediated depression of the field response; however, the PKC inhibitor 2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl)-maleimide or the protein kinase A inhibitor N-[2-((p-bromocinnamyl)amino)ethyl]-5-isoquinolinesulfonamide had little effect on the actions of (1S,3R)-ACPD. These results demonstrate that functional mGluRs are present at PF synapses and are capable of modulating PF synaptic transmission in the DCN.

Bursts of action potential waveforms relieve G-protein inhibition of recombinant P/Q-type Ca2+ channels in HEK 293 cells

1. A variety of neurotransmitters act through G-protein-coupled receptors to decrease synaptic transmission, largely by inhibiting the voltage-gated calcium channels that trigger neurotransmitter release. However, these presynaptic calcium channels are typically inaccessible to electrophysiological characterization. We have reconstituted a part of this inhibition using recombinant P/Q-type calcium channels and M2 acetylcholine receptors in HEK 293 cells. 2. One of the most interesting features of G-protein inhibition of calcium channels is that strong step depolarization transiently relieves the inhibition. We have found that short bursts of action potential voltage waveforms can also relieve the inhibition, increasing calcium current through G-protein-inhibited channels but not through uninhibited channels. 3. The extent of this relief increased linearly with the duration of the action potential waveforms. 4. This result provides the strongest evidence to date favouring the possibility that relief of G-protein inhibition can occur during high frequency trains of action potentials. This effect may constitute a novel form of short-term synaptic plasticity that is sensitive to action potential timing and duration.

Either N- or P-type calcium channels mediate GABA release at distinct hippocampal inhibitory synapses

Transmitter release at most central synapses depends on multiple types of calcium channels. Identification of the channels mediating GABA release in hippocampus is complicated by the heterogeneity of interneurons. Unitary IPSPs were recorded from pairs of inhibitory and pyramidal cells in hippocampal slice cultures. The N-type channel antagonist omega-conotoxin MVIIA abolished IPSPs generated by interneurons in st. radiatum, whereas the P/Q-type antagonist omega-agatoxin IVA had no effect. In contrast, omega-agatoxin IVA abolished IPSPs generated by st. lucidum and st. oriens interneurons, but omega-conotoxin MVIIA had no effect. After unitary IPSPs were blocked by toxin, transmission could not be restored by increasing presynaptic calcium entry. The axons of the two types of interneurons terminated within distinct strata of area CA3. Thus, GABA release onto pyramidal cells, unlike glutamate release, is mediated entirely by either N- or P-type calcium channels, depending on the presynaptic cell and the postsynaptic location of the synapse.