Omega-conotoxin-sensitive and -resistant transmitter release from the chick ciliary presynaptic terminal

Optogenetic probing and manipulation of the calyx-type presynaptic terminal in the embryonic chick ciliary ganglion

The calyx-type synapse of chick ciliary ganglion (CG) has been intensively studied for decades as a model system for the synaptic development, morphology and physiology. Despite recent advances in optogenetics probing and/or manipulation of the elementary steps of the transmitter release such as membrane depolarization and Ca²⁺ elevation, the current gene-manipulating methods are not suitable for targeting specifically the calyx-type presynaptic terminals. Here, we evaluated a method for manipulating the molecular and functional organization of the presynaptic terminals of this model synapse. We transfected progenitors of the Edinger-Westphal (EW) nucleus neurons with an EGFP expression vector by in ovo electroporation at embryonic day 2 (E2) and examined the CG at E8–14. We found that dozens of the calyx-type presynaptic terminals and axons were selectively labeled with EGFP fluorescence. When a Brainbow construct containing the membrane-tethered fluorescent proteins m-CFP, m-YFP and m-RFP, was introduced together with a Cre expression construct, the color coding of each presynaptic axon facilitated discrimination among inter-tangled projections, particularly during the developmental re-organization period of synaptic connections. With the simultaneous expression of one of the chimeric variants of channelrhodopsins, channelrhodopsin-fast receiver (ChRFR), and R-GECO1, a red-shifted fluorescent Ca²⁺-sensor, the Ca²⁺ elevation was optically measured under direct photostimulation of the presynaptic terminal. Although this optically evoked Ca²⁺ elevation was mostly dependent on the action potential, a significant component remained even in the absence of extracellular Ca²⁺. It is suggested that the photo-activation of ChRFR facilitated the release of Ca²⁺ from intracellular Ca²⁺ stores directly or indirectly. The above system, by facilitating the molecular study of the calyx-type presynaptic terminal, would provide an experimental platform for unveiling the molecular mechanisms underlying the morphology, physiology and development of synapses.

Involvement of glutamate 97 in ion influx through photo-activated channelrhodopsin-2

The light absorption of a channelrhodopsin-2 (ChR2) is followed by conformational changes to the molecule, which allows the channel structure to become permeable to cations. Previously, a single point mutation in ChR2, which replaces glutamate residue 97 with a nonpolar alanine (E97A), was found to attenuate the photocurrent, suggesting that the E97 residue is involved in ion flux regulation. Here, the significance of E97 and its counterpart ChR1 (E136) were extensively studied by mutagenesis, whereby we replaced these glutamates with aspartate (D), glutamine (Q) or arginine (R). We found that the charge at this position strongly influences ion permeation and that the photocurrents were attenuated in the order of ChR2>E97D≈E97Q>E97R. We observed similar results with our chimeric/synthetic/artificial construct, ChR-wide receiver (ChRWR), which contains the first to fifth transmembrane helices of ChR1. The E-to-Q or E-to-R mutations, but not the E-to-D mutation, strongly retarded the sensitivity to the Gd(3+)-dependent blocking of the ChR1 or ChR2 channels. Our results suggest that the glutamate residue at this position lies in the outer pore, where it interacts with a cation to facilitate dehydration, and that this residue is the primary binding target of Gd(3+).

Nicotinic receptors concentrated in the subsynaptic membrane do not contribute significantly to synaptic currents at an embryonic synapse in the chicken ciliary ganglion

Rapid synaptic transmission at the calyciform synapse in the embryonic chicken ciliary ganglion is mediated by two classes of nicotinic receptors: those containing alpha3 subunits [alpha3-nicotinic ACh receptors (nAChRs)] and those containing alpha7 subunits (alpha7-nAChRs). alpha3-nAChRs and alpha7-nAChRs are differentially distributed on the cell surface; alpha3-nAChRs are concentrated at postsynaptic densities, whereas both alpha7-nAChRs and alpha3-nAChRs are found extrasynaptically on somatic spines. I explored the contribution of alpha3-nAChRs and alpha7-nAChRs to uniquantal responses, measured as mEPSCs, or as evoked responses under low release probability conditions. The contribution that each nAChR makes to uniquantal response shape was determined by blocking one nAChR type; pharmacologically isolated alpha7-nAChR responses were kinetically fast (rise time, 0.32 +/- 0.02 ms; decay time, 1.66 +/- 0.18 ms; mean +/- SD; n = 6 cells), whereas pharmacologically isolated alpha3-nAChR responses were slow (rise time, 1.28 +/- 0.35 ms; decay time, 6.71 +/- 1.46 ms; n = 8 cells). In the absence of antagonists, most cells (11 of 14) showed heterogeneity in the kinetics of uniquantal responses, with approximately 25% of events exhibiting fast, alpha7-nAChR-like kinetics and approximately 75% of events exhibiting the kinetics expected of coactivation of alpha7-nAChRs and alpha3-nAChRs. Cells rarely showed significant numbers of uniquantal responses with slow, alpha3-nAChR-like kinetics, which was unexpected given that alpha3-nAChRs alone are concentrated at postsynaptic densities. The only site where ACh quanta can activate both alpha3-nAChRs and alpha7-nAChRs readily is on the somatic spines, where alpha7-nAChRs and alpha3-nAChRs are present extrasynaptically. At the calyciform synapse, rapid synaptic transmission is mediated apparently without participation of ionotropic receptors concentrated at postsynaptic densities.

An improved method for patch clamp recording and calcium imaging of neurons in the intact dorsal root ganglion in rats

The properties of dorsal root ganglion (DRG) neurons have been mostly investigated in culture of dissociated cells, and it is uncertain whether these cells maintain the electrophysiological properties of the intact DRG neurons. Few attempts have been made to record from DRG neurons in the intact ganglion using the patch clamp technique. In this study, rat DRGs were dissected and incubated for at least 1h at 37 degrees C in collagenase (10mg/ml). We used oblique epi-illumination to visualize DRG neurons and perform patch clamp recordings. All DRG neurons exhibited strong delayed rectifier potassium current and a high threshold for spike generation (-15 mV) that rendered the cells very weakly excitable, generating only one action potential upon strong current injection (>300 pA). It is therefore possible that cultured DRG neurons, commonly used in studies of pain processing, may be hyperexcitable because they acquired "neuropathic" properties due to the injury induced by their dissociation. Electrical stimulation of the attached root produced an antidromic spike in the soma that could be blocked by intracellular hyperpolarization or high frequency stimulation. Imaging intracellular calcium concentration with Oregon Green BAPTA-1 indicates that antidromic stimulation caused a long-lasting increase in intracellular calcium concentration mostly near the cell membrane. This study describes a simple approach to examine the electrophysiological and pharmacological properties and intracellular calcium signaling in DRG neurons in the intact ganglion where the effects of somatic spike invasion can be studied as well.

The transmitter release-site CaV2.2 channel cluster is linked to an endocytosis coat protein complex

Synaptic vesicles (SVs) are triggered to fuse with the surface membrane at the presynaptic transmitter release site (TRSs) core by Ca2+ influx through nearby attached CaV2.2 channels [see accompanying paper: Khanna et al. (2007)Eur. J. Neurosci., 26, 547-559] and are then recovered by endocytosis. In this study we test the hypothesis that the TRS core is linked to an endocytosis-related protein complex. This was tested by immunostaining analysis of the chick ciliary ganglion calyx presynaptic terminal and biochemical analysis of synaptosome lysate, using CaV2.2 as a marker for the TRS. We noted that CaV2.2 clusters abut heavy-chain (H)-clathrin patches at the transmitter release face. Quantitative coimmunostaining analysis (ICA/ICQ method) demonstrated a strong covariance of release-face CaV2.2 staining with that for the AP180 and intersectin endocytosis adaptor proteins, and a moderate covariance with H- or light-chain (L)-clathrin and dynamin coat proteins, consistent with a multimolecular complex. This was supported by coprecipitation of these proteins with CaV2.2 from brain synaptosome lysate. Interestingly, the channel neither colocalized nor coprecipitated with the endocytosis cargo-capturing adaptor AP2, even though this protein both colocalized and coprecipitated with H-clathrin. Fractional recovery analysis of the immunoprecipitated CaV2.2 complex by exposure to high NaCl (approximately 1 m) indicated that AP180 and S-intersectin adaptors are tightly bound to CaV2.2 while L-intersectin, H- and L-clathrin and dynamin form a less tightly linked subcomplex. Our results are consistent with two distinct clathrin endocytosis complexes: an AP2-containing, remote, non-TRS complex and a specialised, AP2-lacking, TRS-associated subcomplex linked via a molecular bridge. The most probable role of this subcomplex is to facilitate SV recovery after transmitter release.

The presynaptic CaV2.2 channel-transmitter release site core complex

CaV2.2 channels play a key role in the gating of transmitter release sites (TRS) at presynaptic terminals. Physiological studies predict that the channels are linked directly to the TRS but the molecular composition of this complex remains poorly understood. We have used a high-affinity anti-CaV2.2 antibody, Ab571, to test a range of proteins known to contribute to TRS function for both an association in situ and a link in vitro. CaV2.2 clusters were isolated intact on immunoprecipitation beads and coprecipitated with a number of these proteins. Quantitative staining covariance analysis (ICA/ICQ method) was applied to the transmitter release face of the giant calyx terminal in the chick ciliary ganglion to test for TRS proteins with staining intensities that covary in situ with CaV2.2, resulting in a covariance sequence of NSF>RIM>spectrin>Munc18>VAMP>alpha-catenin, CASK>SV2>Na+-K+ approximately 0. A high-NaCl dissociation challenge applied to the immunoprecipitated complex, using the fractional recovery (FR) method [Khanna, R., Li, Q. & Stanley, E.F. (2006) PLoS.ONE., 1, e67], was used to test which proteins were most intimately associated with the channel, generating an FR sequence for CaV2.2 of: VAMP>or=actin>tubulin, NSF, Munc18, syntaxin 1>spectrin>CASK, SNAP25>RIM, Na+-K+ pump, v-ATPase, beta-catenin approximately 0. Proteins associated with endocytosis are considered in a companion paper [Khanna et al. (2007)Eur. J. Neurosci., 26, 560-574]. With the exception of VAMP and RIM, the ICQ and FR sequences were consistent, suggesting that proteins that covary the most strongly with CaV2.2 in situ are also the most intimately attached. Our findings suggest that the CaV2.2 cluster is an integral element of a multimolecular vesicle-fusion module that forms the core of a multifunctional TRS.

Long splice variant N type calcium channels are clustered at presynaptic transmitter release sites without modular adaptor proteins

The presynaptic N type Ca channel (CaV2.2) is associated with the transmitter release site apparatus and plays a critical role in the gating of transmitter release. It has been suggested that a distinct CaV2.2 long C terminal splice variant is targeted to the nerve terminal and is anchored at the release face by calcium/calmodulin-dependent serine protein kinase (CASK) and Munc-18-interacting protein (MINT), two modular adaptor proteins. We used the isolated chick ciliary ganglion calyx terminal together with two new antibodies (L4569, L4570) selective for CaV2.2 long C terminal splice variant to test these hypotheses. CaV2.2 long C terminal splice variant was present at the presynaptic transmitter release sites, as identified by Rab3a-interacting molecule (RIM) co-staining and quantitative immunocytochemistry. CASK was also present at the terminal both in conjunction with, and independent of its binding partner, MINT. Immunoprecipitation of CaV2.2 long C terminal splice variant from brain lysate coprecipitated CASK, confirming that these two proteins can form a complex. However, CASK was not colocalized either with CaV2.2 long C terminal splice variant or the transmitter release site marker RIM at the calyx terminal release face. Neither was MINT colocalized with CaV2.2 long C terminal splice variant. Our results show that native CaV2.2 long C terminal splice variant is targeted to the transmitter release sites at an intact presynaptic terminal. However, the lack of enrichment of CASK at the release site combined with the failure of this protein or its partner MINT to colocalize with CaV2.2 argues against the idea that these modular adaptor proteins anchor CaV2.2 at presynaptic nerve terminals.

N type Ca2+ channels and RIM scaffold protein covary at the presynaptic transmitter release face but are components of independent protein complexes

Fast neurotransmitter release at presynaptic terminals occurs at specialized transmitter release sites where docked secretory vesicles are triggered to fuse with the membrane by the influx of Ca2+ ions that enter through local N type (CaV2.2) calcium channels. Thus, neurosecretion involves two key processes: the docking of vesicles at the transmitter release site, a process that involves the scaffold protein RIM (Rab3A interacting molecule) and its binding partner Munc-13, and the subsequent gating of vesicle fusion by activation of the Ca2+ channels. It is not known, however, whether the vesicle fusion complex with its attached Ca2+ channels and the vesicle docking complex are parts of a single multifunctional entity. The Ca2+ channel itself and RIM were used as markers for these two elements to address this question. We carried out immunostaining at the giant calyx-type synapse of the chick ciliary ganglion to localize the proteins at a native, undisturbed presynaptic nerve terminal. Quantitative immunostaining (intensity correlation analysis/intensity correlation quotient method) was used to test the relationship between these two proteins at the nerve terminal transmitter release face. The staining intensities for CaV2.2 and RIM covary strongly, consistent with the expectation that they are both components of the transmitter release sites. We then used immunoprecipitation to test if these proteins are also parts of a common molecular complex. However, precipitation of CaV2.2 failed to capture either RIM or Munc-13, a RIM binding partner. These findings indicate that although the vesicle fusion and the vesicle docking mechanisms coexist at the transmitter release face they are not parts of a common stable complex.

The Synaptic Vesicle Protein CSPα Prevents Presynaptic Degeneration

Cysteine string protein alpha (CSPalpha)--an abundant synaptic vesicle protein that contains a DNA-J domain characteristic of Hsp40 chaperones--is thought to regulate Ca2+ channels and/or synaptic vesicle exocytosis. We now show that, in young mice, deletion of CSPalpha does not impair survival and causes no significant changes in presynaptic Ca2+ currents or synaptic vesicle exocytosis as measured in the Calyx of Held synapse. At 2-4 weeks of age, however, CSPalpha-deficient mice develop a progressive, fatal sensorimotor disorder. The neuromuscular junctions and Calyx synapses of CSPalpha-deficient mice exhibit increasing neurodegenerative changes, synaptic transmission becomes severely impaired, and the mutant mice die at approximately 2 months of age. Our data suggest that CSPalpha is not essential for the normal operation of Ca2+ channels or exocytosis but acts as a presynaptic chaperone that maintains continued synaptic function, raising the possibility that enhanced CSPalpha function could attenuate neurodegenerative diseases.

Repolarization of the presynaptic action potential and short-term synaptic plasticity in the chick ciliary ganglion

Stimulation-induced increases in synaptic efficacy have been described as being composed of multiple independent processes that arise from the activation of distinct mechanisms at the presynaptic terminal. In the chick ciliary ganglion, four components of short-term synaptic plasticity have been described: F1 and F2 components of facilitation, augmentation, and potentiation. In the present study, intracellular recording from the presynaptic calyciform nerve terminal of the chick ciliary ganglion revealed that the late repolarization and afterhypolarization (AHP) phases of the presynaptic action potential are affected by repetitive stimulation and that the time course of these effects parallel that of facilitation. The effects of these changes in the presynaptic action potential time course on calcium influx were tested by using the recorded action potential waveforms as voltage command stimuli during whole-cell patch-clamp recordings from acutely isolated chick ciliary ganglion neurons. The "facilitated" action potential waveform (slowed repolarization, decreased AHP amplitude) evoked calcium current with slightly but significantly greater total calcium influx. Taken together, these results are consistent with the hypothesis that activity-dependent changes in the presynaptic action potential are one of several mechanisms contributing to the facilitation phase of stimulation-induced increases in transmitter release in this preparation.

Enhancement of presynaptic calcium current by cysteine string protein

The isolated chick ciliary neuron calyx synapse preparation was used to test cysteine string protein (CSP) action on presynaptic N-type Ca(2+) channels. Endogenous CSP was localized primarily to secretory vesicle clusters in the presynaptic nerve terminal. Introduction of recombinant CSP into the voltage clamped terminal resulted in a prominent increase in Ca(2+) current amplitude. However, this increase could not be attributed to a change in Ca(2+) channel kinetics, voltage dependence, prepulse inactivation, or G protein inhibition but was attributed to the recruitment of dormant channels. Secretory vesicle associated endogenous CSP may play an important role in enhancing Ca(2+) channel activity at the transmitter release site.

Syntaxin modulation of slow inactivation of N-type calcium channels

Syntaxin, a membrane protein vital in triggering vesicle fusion, interacts with voltage-gated N- and P/Q-type Ca(2+) channels. This biochemical association is proposed to colocalize Ca(2+) channels and presynaptic release sites, thus supporting rapid and efficient initiation of neurotransmitter release. The syntaxin channel interaction may also support a novel signaling function, to modulate Ca(2+) channels according to the state of the associated release machinery (Bezprozvanny et al., 1995; Wiser et al., 1996; see also Mastrogiacomo et al., 1994). Here we report that syntaxin 1A (syn1A) coexpressed with N-type channels in Xenopus oocytes greatly promoted slow inactivation gating, but had little or no effect on the onset of and recovery from fast inactivation. Accordingly, the effectiveness of syntaxin depended strongly on voltage protocol. Slow inactivation was found for N-type channels even in the absence of syntaxin and could be distinguished from fast inactivation on the basis of its slow kinetics, distinct voltage dependence (voltage-independent at potentials higher than the level of half-inactivation), and temperature independence (Q(10), approximately 0.8). Trains of action potential-like stimuli were more effective than steady depolarizations in stabilizing the slowly inactivated condition. Agents that stimulate protein kinase C decreased the inhibitory effect of syntaxin on N-type channels. Application of BoNtC1 to cleave syntaxin sharply attenuated the modulatory effects on Ca(2+) channel gating, consistent with structural analysis of syntaxin modulation, supporting use of this toxin to test for the impact of syntaxin on Ca(2+) influx in nerve terminals.

Syntaxin modulation of slow inactivation of N-type calcium channels

Syntaxin, a membrane protein vital in triggering vesicle fusion, interacts with voltage-gated N- and P/Q-type Ca(2+) channels. This biochemical association is proposed to colocalize Ca(2+) channels and presynaptic release sites, thus supporting rapid and efficient initiation of neurotransmitter release. The syntaxin channel interaction may also support a novel signaling function, to modulate Ca(2+) channels according to the state of the associated release machinery (Bezprozvanny et al., 1995; Wiser et al., 1996; see also Mastrogiacomo et al., 1994). Here we report that syntaxin 1A (syn1A) coexpressed with N-type channels in Xenopus oocytes greatly promoted slow inactivation gating, but had little or no effect on the onset of and recovery from fast inactivation. Accordingly, the effectiveness of syntaxin depended strongly on voltage protocol. Slow inactivation was found for N-type channels even in the absence of syntaxin and could be distinguished from fast inactivation on the basis of its slow kinetics, distinct voltage dependence (voltage-independent at potentials higher than the level of half-inactivation), and temperature independence (Q(10), approximately 0.8). Trains of action potential-like stimuli were more effective than steady depolarizations in stabilizing the slowly inactivated condition. Agents that stimulate protein kinase C decreased the inhibitory effect of syntaxin on N-type channels. Application of BoNtC1 to cleave syntaxin sharply attenuated the modulatory effects on Ca(2+) channel gating, consistent with structural analysis of syntaxin modulation, supporting use of this toxin to test for the impact of syntaxin on Ca(2+) influx in nerve terminals.

mu-Opioid receptor inhibits N-type Ca2+ channels in the calyx presynaptic terminal of the embryonic chick ciliary ganglion

A study was made on the mechanisms by which enkephalins inhibit synaptic transmission at calyx-type presynaptic terminals in the ciliary ganglion of chick embryos at stages 39-40. Excitatory postsynaptic currents (EPSCs) were recorded by nystatin-perforated patch clamp at low [Ca2+]o and high [Mg2+]o. [Leu5]enkephalin (L-ENK, 1-10 microM) reduced the quantal content (m) without changing the quantal size (q). This effect was antagonized by naloxone (1 microM). Similar results were observed under conventional whole-cell clamp of the postsynaptic neuron. A specific agonist of the mu-opioid receptor, [D-Ala2, M-Me-Phe4,Gly5]enkephalin-ol (DAMGO) reduced m without changing q. A specific agonist of the delta-opioid receptor, [d-Pen2, d-Pen5]enkephalin (DPDPE) also reduced m without changing q. Both L-ENK and [Met5]enkephalin (M-ENK) reduced the stimulus-dependent increment of the intraterminal Ca2+ concentration (Delta[Ca2+]t) without affecting the decay time constant of the intraterminal Ca2+ concentration and basal Ca2+ level. This effect was antagonized by naloxone. DAMGO reduced Delta[Ca2+]t more effectively than DPDPE. When extracellular Ca2+ was replaced by Ba2+, the stimulus-dependent increment of the intraterminal Ba2+ concentration (Delta[Ba2+]t) was also reduced by L-ENK or DAMGO. L-ENK reduced Delta[Ca2+]t even in the presence of 4-aminopyridine (4-AP), which blocks the transient K+ conductance during the falling phase of the presynaptic action potential. When N-type Ca2+ channels were blocked by omega-conotoxin GVIA (omega-CgTxGVIA), the Delta[Ca2+]t was no longer sensitive to L-ENK and DAMGO. It is suggested that enkephalins reduce the transmitter release through presynaptic opioid receptors. The mu-opioid receptor may suppress presynaptic Ca2+ influx by selectively inhibiting N-type Ca2+ channels.

Presynaptic calcium channels and the depletion of synaptic cleft calcium ions

The entry of calcium ions (Ca(2+)) through voltage-gated calcium channels is an essential step in the release of neurotransmitter at the presynaptic nerve terminal. Because the calcium channels are clustered at the release sites, the flux of Ca(2+) into the terminal inevitably removes the ion from the adjacent extracellular space, the synaptic cleft. We have used the large calyx-type synapse of the chick ciliary ganglion to test for synaptic cleft Ca(2+) depletion. The terminal was voltage clamped at a holding potential (V(H)) of -80 mV and a depolarizing pulse was applied to a range of potentials (-60 to +60 mV). The voltage pulse activated a sustained inward calcium current and was followed, on return of the membrane potential to V(H), by an inward calcium tail current. The amplitude of the tail current reflects both the number of open calcium channels at the end of the voltage pulse and the Ca(2+) electrochemical gradient. External barium was substituted for calcium as the charge-carrying ion because initial experiments demonstrated calcium-dependent inactivation of the presynaptic calcium channels. Tail current recruitment was compared in calyx nerve terminals that remained attached to the postsynaptic neuron and therefore retained a synaptic cleft, with terminals that had been fully isolated. In isolated terminals, the tail currents exhibited recruitment curves that could be fit by a Boltzmann distribution with a mean V(1/2) of 0.4 mV and a slope factor of 5.4. However, in attached calyces tail current recruitment was skewed to depolarized potentials with a mean V(1/2) of 11.9 mV and a slope factor of 12.0. The degree of skew of the recruitment curve in the attached calyces correlated with the amplitude of the inward current evoked by the step depolarization. The simplest interpretation of these findings is that during the depolarizing pulse Ba(2+) is removed from the synaptic cleft faster than it is replenished, thus reducing the tail current by reducing the driving force for ion entry. Ca(2+) depletion during presynaptic calcium channel activation is likely to be a general property of chemical transmission at fast synapses that sets a functional limit to the duration of sustained secretion. The synapse may have evolved to minimized cleft depletion by developing a calcium-efficient mechanism to gate transmitter release that requires the concurrent opening of only a few low conductance calcium channels.

Involvement of cGMP-dependent protein kinase in adrenergic potentiation of transmitter release from the calyx-type presynaptic terminal

I have previously reported that norepinephrine (NE) induces a sustained potentiation of transmitter release in the chick ciliary ganglion through a mechanism pharmacologically distinct from any known adrenergic receptors. Here I report that the adrenergic potentiation of transmitter release was enhanced by a phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX) and by zaprinast, an inhibitor of cGMP-selective phosphodiesterase. Exogenous application of the membrane-permeable cGMP, 8-bromo-cGMP (8Br-cGMP), potentiated the quantal transmitter release, and after potentiation, the addition of NE was no longer effective. On the other hand, 8Br-cAMP neither potentiated the transmitter release nor occluded the NE-induced potentiation. The NE-induced potentiation was blocked by neither nitric oxide (NO) synthase inhibitor nor NO scavenger. The quantal transmitter release was not potentiated by NO donors, e.g., sodium nitroprusside. The NE-induced potentiation and its enhancement by IBMX was antagonized by two inhibitors of protein kinase G (PKG), Rp isomer of 8-(4-chlorophenylthio) guanosine-3', 5'-cyclic monophosphorothioate and KT5823. As with NE-induced potentiation, the effects of 8Br-cGMP on both the resting intraterminal [Ca2+] ([Ca2+]i) and the action potential-dependent increment of [Ca2+]i (DeltaCa) in the presynaptic terminal were negligible. The reduction of the paired pulse ratio of EPSC is consistent with the notion that the NE- and cGMP-dependent potentiation of transmitter release was attributable mainly to an increase of the exocytotic fusion probability. These results indicate that NE binds to a novel adrenergic receptor that activates guanylyl cyclase and that accumulation of cGMP activates PKG, which may phosphorylate a target protein involved in the exocytosis of synaptic vesicles.

Ion channels in presynaptic nerve terminals and control of transmitter release

The primary function of the presynaptic nerve terminal is to release transmitter quanta and thus activate the postsynaptic target cell. In almost every step leading to the release of transmitter quanta, there is a substantial involvement of ion channels. In this review, the multitude of ion channels in the presynaptic terminal are surveyed. There are at least 12 different major categories of ion channels representing several tens of different ion channel types; the number of different ion channel molecules at presynaptic nerve terminals is many hundreds. We describe the different ion channel molecules at the surface membrane and inside the nerve terminal in the context of their possible role in the process of transmitter release. Frequently, a number of different ion channel molecules, with the same basic function, are present at the same nerve terminal. This is especially evident in the cases of calcium channels and potassium channels. This abundance of ion channels allows for a physiological and pharmacological fine tuning of the process of transmitter release and thus of synaptic transmission.

Involvement of cGMP-dependent protein kinase in adrenergic potentiation of transmitter release from the calyx-type presynaptic terminal

I have previously reported that norepinephrine (NE) induces a sustained potentiation of transmitter release in the chick ciliary ganglion through a mechanism pharmacologically distinct from any known adrenergic receptors. Here I report that the adrenergic potentiation of transmitter release was enhanced by a phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX) and by zaprinast, an inhibitor of cGMP-selective phosphodiesterase. Exogenous application of the membrane-permeable cGMP, 8-bromo-cGMP (8Br-cGMP), potentiated the quantal transmitter release, and after potentiation, the addition of NE was no longer effective. On the other hand, 8Br-cAMP neither potentiated the transmitter release nor occluded the NE-induced potentiation. The NE-induced potentiation was blocked by neither nitric oxide (NO) synthase inhibitor nor NO scavenger. The quantal transmitter release was not potentiated by NO donors, e.g., sodium nitroprusside. The NE-induced potentiation and its enhancement by IBMX was antagonized by two inhibitors of protein kinase G (PKG), Rp isomer of 8-(4-chlorophenylthio) guanosine-3', 5'-cyclic monophosphorothioate and KT5823. As with NE-induced potentiation, the effects of 8Br-cGMP on both the resting intraterminal [Ca2+] ([Ca2+]i) and the action potential-dependent increment of [Ca2+]i (DeltaCa) in the presynaptic terminal were negligible. The reduction of the paired pulse ratio of EPSC is consistent with the notion that the NE- and cGMP-dependent potentiation of transmitter release was attributable mainly to an increase of the exocytotic fusion probability. These results indicate that NE binds to a novel adrenergic receptor that activates guanylyl cyclase and that accumulation of cGMP activates PKG, which may phosphorylate a target protein involved in the exocytosis of synaptic vesicles.

Nicotinic acetylcholine receptors containing alpha7 subunits are required for reliable synaptic transmission in situ

Nicotinic acetylcholine receptors containing alpha7 subunits are widely expressed in the nervous system. The receptors are cation-selective, relatively permeable to calcium, and avid binders of alpha-bungarotoxin. Although the receptors can act both pre- and postsynaptically, their physiological significance is unclear. Using whole-cell patch-clamp analysis of chick ciliary ganglion neurons in situ, we show that the receptors are required for reliable synaptic transmission early in development. Stimulation of the presynaptic nerve root elicited a biphasic synaptic current, including a large rapidly decaying component generated by alpha7-containing receptors. Selective blockade of alpha7-containing receptors by perfusing the ganglion with alpha-bungarotoxin induced failures in synaptic transmission. One-half of the ciliary neurons that were tested failed when stimulated synaptically at 1 Hz, and two-thirds failed at 25 Hz. Failing cells missed, on average, 80% of the trials during a test train of stimuli. The ability to fire synaptically evoked action potentials after toxin treatment was correlated positively with the amplitude of the remaining synaptic current, suggesting that alpha7-containing receptors were needed to augment synaptic responses. Consistent with patch-clamp analysis, toxin blockade reduced the amplitude of the synaptically evoked compound action potential in the postganglionic nerve; it also desynchronized the firing of the remaining units. Methyllycaconitine, another antagonist of alpha7-containing receptors, mimicked alpha-bungarotoxin blockade. Toxin blockade had less impact on transmission in ganglia at the end of embryogenesis. The ability of the receptors to synchronize and sustain population firing, together with their ability to deliver calcium, may influence early developmental events such as target innervation and neuronal survival.

Nicotinic acetylcholine receptors containing alpha7 subunits are required for reliable synaptic transmission in situ

Nicotinic acetylcholine receptors containing alpha7 subunits are widely expressed in the nervous system. The receptors are cation-selective, relatively permeable to calcium, and avid binders of alpha-bungarotoxin. Although the receptors can act both pre- and postsynaptically, their physiological significance is unclear. Using whole-cell patch-clamp analysis of chick ciliary ganglion neurons in situ, we show that the receptors are required for reliable synaptic transmission early in development. Stimulation of the presynaptic nerve root elicited a biphasic synaptic current, including a large rapidly decaying component generated by alpha7-containing receptors. Selective blockade of alpha7-containing receptors by perfusing the ganglion with alpha-bungarotoxin induced failures in synaptic transmission. One-half of the ciliary neurons that were tested failed when stimulated synaptically at 1 Hz, and two-thirds failed at 25 Hz. Failing cells missed, on average, 80% of the trials during a test train of stimuli. The ability to fire synaptically evoked action potentials after toxin treatment was correlated positively with the amplitude of the remaining synaptic current, suggesting that alpha7-containing receptors were needed to augment synaptic responses. Consistent with patch-clamp analysis, toxin blockade reduced the amplitude of the synaptically evoked compound action potential in the postganglionic nerve; it also desynchronized the firing of the remaining units. Methyllycaconitine, another antagonist of alpha7-containing receptors, mimicked alpha-bungarotoxin blockade. Toxin blockade had less impact on transmission in ganglia at the end of embryogenesis. The ability of the receptors to synchronize and sustain population firing, together with their ability to deliver calcium, may influence early developmental events such as target innervation and neuronal survival.

Two components of transmitter release from the chick ciliary presynaptic terminal and their regulation by protein kinase C

1. A study was made of the effects of phorbol ester (phorbol 12-myristate 13-acetate, PMA, 0.1 microM) on the two components of evoked transmitter release, namely the fast synchronous and the slow asynchronous components, from the giant presynaptic terminal of the chick ciliary ganglion. The excitatory postsynaptic currents (EPSCs) were recorded under whole-cell voltage clamp of the postsynaptic neuron. 2. The decay time constant of the slow component was prolonged by replacing Ca2+ with Sr2+. In 5 mM [Sr2+]o the fast component decayed with a time constant of 2.6 +/- 1.4 ms whereas the slow component decayed with a time constant of 19 +/- 7 ms. 3. When stimulated with twin pulses with a short interpulse interval, the fast component of the second EPSC was often depressed whereas the slow component was usually facilitated. Both components were positively dependent on [Sr2+]o in a saturable manner, but the fast component approached its maximum at a lower [Sr2+]o than the slow component. 4. PMA potentiated both the fast and slow components to a similar extent and with a similar time course. For each component, the effect of PMA was less potent at high [Sr2+]o than at low [Sr2+]o. For either the fast or the slow component the PMA-induced potentiation was accompanied by a reduction in the paired-pulse ratio (PPR). 5. Despite the different dissociation constant for dextran-conjugated fura-2, the fluorescent ratio for intraterminal [Sr2+] ([Sr2+]i) decayed to the baseline after the nerve-evoked increment with a time course similar to that for [Ca2+]i, suggesting that intraterminal Sr2+ is buffered less efficiently than Ca2+. PMA did not increase the [Sr2+]i transients produced by stimulation of the presynaptic oculomotor nerve. 6. It is suggested that protein kinase C (PKC) modulates both the fast and slow components through common molecular mechanisms that upregulate the Sr2+ sensitivity of the vesicle fusion probability.

Protein kinase C potentiates transmitter release from the chick ciliary presynaptic terminal by increasing the exocytotic fusion probability

1. The giant presynaptic terminal of chick ciliary ganglion was used to examine how protein kinase C (PKC) modulates neurotransmitter release. Cholinergic excitatory postsynaptic currents (EPSCs) were recorded under whole-cell voltage clamp. 2. Although the EPSC was potentiated by phorbol ester (phorbol 12-myristate 13-acetate, PMA; 0.1 microM) in a sustained manner, the nicotine-induced current was unaffected. PMA increased the quantal content to 2.4 +/- 0.4 (n = 9) of control without changing the quantal size. 3. The inactive isoform of PMA, 4alpha-PMA, showed no significant effect on EPSCs. The PMA-induced potentiation was antagonized by two PKC inhibitors with different modes of action, sphingosine (20 microM) and bisindolylmaleimide I (10 microM). 4. When stimulated by twin pulses of short interval, the second EPSC was on average larger than the first EPSC (paired-pulse facilitation; PPF). PMA significantly decreased the PPF ratio with a time course similar to that of the potentiation of the first EPSC. 5. PMA did not affect resting [Ca2+]i or the action potential-induced [Ca2+]i increment in the giant presynaptic terminals. 6. The effect of PMA was less at 10 mM [Ca2+]o than at 1 mM [Ca2+]o. 7. When a train of action potentials was generated with a short interval, the EPSC was eventually depressed and reached a steady-state level. The recovery process followed a simple exponential relation with a rate constant of 0.132 +/- 0.029 s-1. PMA did not affect the recovery rate constant of EPSCs from tetanic depression. In addition, PMA did not affect the steady-state EPSC which should be proportional to the refilling rate of the readily releasable pool of vesicles. 8. These results conflict with the hypothesis that PKC upregulates the size of the readily releasable pool or the number of release sites. PKC appears to upregulate the Ca2+ sensitivity of the process that controls the exocytotic fusion probability.

Calcium channel subtypes differ at two types of cholinergic synapse in lumbar sympathetic neurones of guinea-pigs

1. The involvement of different presynaptic Ca2+ channels in transmission at 'weak' (subthreshold) and 'strong' (suprathreshold) synapses was investigated in guinea-pig paravertebral ganglia isolated in vitro. Selective Ca2+ channel antagonists were used to block excitatory synaptic currents evoked by stimulating single preganglionic axons. 2. The N-type Ca2+ channel blocker, omega-conotoxin GVIA (100 nM), reduced peak synaptic conductance by similar amounts at weak synapses (by 39 +/- 6 %) and strong synapses (34 +/- 6 %). 3. The P-type Ca2+ channel blocker, omega-agatoxin IVA (40 nM), significantly reduced transmitter release at weak synapses (by 42 +/- 6 %) but had only a small effect at strong synapses (reduced by 6 +/- 2 %). 4. Blockers of Q-, L- or T-type Ca2+ channels had no significant effects on peak synaptic conductance at either type of synapse. 5. We conclude that the two functionally distinct types of preganglionic terminal in sympathetic ganglia which synapse on the same neurone differ in their expression of particular types of voltage-dependent Ca2+ channels. Both types utilize N-type channels and channels resistant to blockade by specific antagonists, but Ca2+ entry through P-type channels makes a substantial contribution to acetylcholine release only at weak synapses.

Inhibition of a hyperpolarization-activated current by clonidine in rat dorsal root ganglion neurons

Whole cell voltage- and current-clamp recordings were carried out to investigate the effects of clonidine, an alpha 2-adrenoceptor agonist, in L4 and L5 dorsal root ganglion (DRG) neurons of the rat. In voltage-clamp mode, application of 20 microM clonidine reversibly reduced the inward current evoked by hyperpolarizing voltage steps. The "clonidine-sensitive current" was obtained by subtracting the current during clonidine application from the control current, and its properties were as follows. 1) It was a slowly activating inward current evoked by hyperpolarization. 2) The reversal potential in the standard extracellular solution ([K+]o = 5 mM, [Na+]o = 151 mM) was -38.3 mV, and reduction of [Na+]o shifted it to a more negative potential, whereas an increase of [K+]o shifted it to a more positive potential, indicating that the current was carried by Na+ and K+ (PNa/PK = 0.22). 3) The relationship between the chord conductance underlying the clonidine-sensitive current and voltage could be fitted by a Boltzmann equation. These results indicate that the clonidine-sensitive current corresponds to a hyperpolarization-activated current (Ih), i.e., clonidine inhibits Ih in rat DRG neurons. DRG neurons were classified as small (15.9-32.9 microns diam), medium-sized (33-42.9 microns), and large (43-63.6 microns), and 7 of 19, 24 of 25, and 22 of 22 of these types exhibited Ih with mean +/- SE clonidine-induced inhibition values of 36.1 +/- 3.5% (n = 7), 43.1 +/- 3.7% (n = 24), and 35.1 +/- 2.7% (n = 22), respectively. Clonidine application to L4 and L5 DRG neurons excised from rats the sciatic nerves of which had been transected 14-35 days previously (transected DRG neurons) also reduced Ih. In current-clamp mode, 9 of 13 intact and 4 of 6 transected medium-sized DRG neurons that exhibited Ih responded to clonidine with hyperpolarization (> 2 mV). Some medium-sized DRG neurons exhibited repetitive action potentials in response to a depolarizing current pulse, and clonidine reduced the firing discharge frequencies in 8 of 11 intact and 3 of 4 transected neurons tested. Injection of a hyperpolarizing current pulse produced time-dependent rectification in DRG neurons that exhibited Ih, and clonidine blocked this rectification in all intact and transected neurons tested. These results suggest that inhibition of Ih due to alpha 2-adrenoceptor activation contributes to modulation of DRG neuronal activity in rats. On the basis of our findings, we discuss the possible mechanisms whereby sympathetically released norepinephrine modulates the abnormal activity of DRG neuronal cell bodies after nerve injury.

Direct recording of nicotinic responses in presynaptic nerve terminals

Nicotinic acetylcholine receptors are widely expressed in the nervous system, but their functions remain poorly understood. One attractive hypothesis is that the receptors act presynaptically to modulate synaptic transmission. We provide a direct demonstration of presynaptic nicotinic receptors in situ by using whole-cell patch-clamp techniques to record currents in large presynaptic calyces that midbrain neurons form on ciliary neurons. Bath application of nicotine induced inward currents in the calyces capable of generating action potentials that overrode the limited space clamp achievable. The inward currents reversed near 0 mV and showed inward rectification common for neuronal nicotinic receptors. Tetrodotoxin (TTX) blocked the action potentials but not the inward currents. alpha-Bungarotoxin blocked both, consistent with the presynaptic receptors containing alpha7 subunits. Recording from the postsynaptic ciliary neurons during nicotine exposure revealed EPSCs that TTX blocked, presumably by blocking presynaptic action potentials. The postsynaptic cells also displayed bimodal inward currents caused by their own nicotinic receptors; the bimodal currents were not blocked by TTX but were blocked partially by alpha-bungarotoxin and completely by D-tubocurarine. Dye-filling with Lucifer yellow from the recording pipette confirmed the identity of patched structures and showed no dye transfer between calyx and ciliary neuron. When calyces or ciliary neurons were labeled en mass with neurobiotin and biocytin through nerve roots, dye transfer was rarely observed. Thus, electrical synapses were infrequent and unlikely to influence calyx responses. Immunochemical analysis of preganglionic nerve extracts identified receptors that bind alpha-bungarotoxin and contain alpha7 subunits. The results unambiguously document the existence of functional presynaptic nicotinic receptors.

Stimulation-induced changes in [Ca2+] in lizard motor nerve terminals

1. Motor axons were injected ionophoretically with one of five Ca(2+)-sensitive dyes (fluo-3, Calcium Green-2, Calcium Green-5N, fluo-3FF and Oregon Green BAPTA-5N). Changes in fluorescence (delta F/Frest) within motor terminal boutons following a single action potential and brief stimulus trains were monitored with high temporal resolution using a confocal microscope. 2. Stimulation-induced increases in delta F/Frest were confined primarily to boutons, with roughly uniform increases in all the boutons of a terminal. The increase in delta F/Frest began prior to, and decayed more slowly than, the endplate potential (EPP) recorded in the underlying muscle fibre. delta F/Frest was graded with bath [Ca2+]. Both delta F/Frest and the EPP were reduced, but not eliminated, by omega-conotoxin GVIA (5-10 microM). 3. For dyes with lower affinity for Ca2+ (e.g. Oregon Green BAPTA-5N, Kd approximately 60 microM) stimulation-induced increases in delta F/Frest were measured in the presence of the K+ channel blocker 3,4-diaminopyridine (3,4-DAP, 100 microM). During brief stimulus trains (4 at 50 Hz) in 3,4-DAP, the EPP exhibited profound depression, but the fluorescence increase associated with each stimulus showed little decrement, suggesting that depression was not mediated by a reduction in Ca2+ entry. 4. For dyes with a higher affinity for Ca2+ (e.g. fluo-3, Kd approximately 0.5-1 microM) stimulation-induced increases in delta F/Frest could also be measured in normal physiological saline. Increases in delta F/Frest were much greater with 3,4-DAP present, but the amplitude decreased with successive stimuli due to partial dye saturation. 5. Calculations suggested that following a single action potential the average [Ca2+] within a bouton increased by up to 150 nM in normal saline and 940 nM in 3,4-DAP. With low affinity dyes the delta F/Frest measured near the membrane had a higher peak amplitude and a faster early decay than that measured in the centre of the bouton, suggesting that substantial spatial [Ca2+] gradients exist within boutons for at least 15 ms following stimulation.

Calcium in sympathetic varicosities of mouse vas deferens during facilitation, augmentation and autoinhibition

1. The sympathetic nerve terminals of the mouse vas deferens were loaded with the calcium indicator Oregon Green 488 BAPTA-1 by orthograde transport along the postganglionic nerves. Changes in the calcium concentration in the varicosity (delta [Ca2+]v) were determined following single impulses, and short (5-impulse) and long (200-impulse) trains at 5 Hz. 2. All varicosities showed a significant delta [Ca2+]v in response to every single impulse. The elevated delta [Ca2+]v declined in two phases with similar kinetics for all varicosities: a fast phase (time constant, 0.42 +/- 0.05 s) and a moderate phase (3.6 +/- 0.4 s). 3. Line scanning confocal microscopy revealed that the delta [Ca2+] of a single terminal following single impulses was smaller for the intervaricose regions than for the varicosities. 4. Blockade of the voltage-sensitive calcium channels with Cd2+ (in calcium-free solution) completely blocked the delta [Ca2+]v on stimulation. The addition of either nifedipine (10 microM), omega-conotoxin GVIA (100 nM) or omega-agatoxin TK (100 nM) showed that 47 +/- 6% of the evoked response was mediated by N-type calcium channels. 5. Ryanodine (10 microM) did not significantly change the amplitude of delta [Ca2+]v in response to short trains. 6. Spontaneous increases in delta [Ca2+]v were observed in individual varicosities, with coupling in the increase of delta [Ca2+]v between varicosities. 7. The presynaptic alpha 2-receptor antagonist yohimbine (10 microM) increased the amplitude of delta [Ca2+]v in response to five impulses (5 Hz) by 54 +/- 14%, while the alpha 2-receptor agonist clonidine (1 microM) decreased the delta [Ca2+]v by 55 +/- 4%. 8. These results are discussed in terms of the hypotheses that the increased probability for secretion at sympathetic nerve terminals which accompanies facilitation and augmentation is due to the residual delta [Ca2+]v remaining after the calcium influx following impulses and that noradrenaline acts presynaptically to decrease the probability of secretion by modifying calcium influx.

Direct recording of nicotinic responses in presynaptic nerve terminals

Nicotinic acetylcholine receptors are widely expressed in the nervous system, but their functions remain poorly understood. One attractive hypothesis is that the receptors act presynaptically to modulate synaptic transmission. We provide a direct demonstration of presynaptic nicotinic receptors in situ by using whole-cell patch-clamp techniques to record currents in large presynaptic calyces that midbrain neurons form on ciliary neurons. Bath application of nicotine induced inward currents in the calyces capable of generating action potentials that overrode the limited space clamp achievable. The inward currents reversed near 0 mV and showed inward rectification common for neuronal nicotinic receptors. Tetrodotoxin (TTX) blocked the action potentials but not the inward currents. alpha-Bungarotoxin blocked both, consistent with the presynaptic receptors containing alpha7 subunits. Recording from the postsynaptic ciliary neurons during nicotine exposure revealed EPSCs that TTX blocked, presumably by blocking presynaptic action potentials. The postsynaptic cells also displayed bimodal inward currents caused by their own nicotinic receptors; the bimodal currents were not blocked by TTX but were blocked partially by alpha-bungarotoxin and completely by D-tubocurarine. Dye-filling with Lucifer yellow from the recording pipette confirmed the identity of patched structures and showed no dye transfer between calyx and ciliary neuron. When calyces or ciliary neurons were labeled en mass with neurobiotin and biocytin through nerve roots, dye transfer was rarely observed. Thus, electrical synapses were infrequent and unlikely to influence calyx responses. Immunochemical analysis of preganglionic nerve extracts identified receptors that bind alpha-bungarotoxin and contain alpha7 subunits. The results unambiguously document the existence of functional presynaptic nicotinic receptors.

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.

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.

The role of nitric oxide in inhibitory neurotransmission in the middle cerebral artery of the sheep

1. The involvement of nitric oxide (NO) as a mediator of inhibitory neurotransmission and its potential release mechanism in sheep isolated middle cerebral artery rings was investigated using NO synthase inhibitors, haemolysate, superoxide dismutase (SOD) and omega-conotoxin GVIA. In the presence of guanethidine (5 microM) and atropine (2 microM), transmural nerve stimulation of precontracted artery rings elicited an endothelium-independent vasodilator response that could be abolished by tetrodotoxin. 2. The magnitude of the vasodilator response was virtually abolished by NG-nitro-L-arginine-p-nitroanilide (L-NAPNA; 100-500 microM) and significantly reduced by NG-nitro-L-arginine (50 microM) or haemolysate (1 microliter ml-1). NG-nitro-D-arginine (50 microM) had no effect. In the presence of the NO synthase inhibitors, addition of L-arginine (300 microM) produced either no effect or a partial, transient restoration of inhibitor responses following electrical field stimulation (EFS). L-NAPNA (100 microM) did not affect the relaxant response to the NO donor SIN-1. These results suggest that NO is involved in the relaxation elicited by transmural nerve stimulation. 3. Superoxide dismutase (SOD; 150 Uml-1) did not produce any significant changes in the magnitude of the EFS-induced vasodilation. Thus, superoxide anions appear not to be a limiting factor for NO-mediated neurogenic vasodilation in sheep MCA. 4. omega-Conotoxin GVIA (100 nM) caused an almost immediate abolition of the EFS-induced vasoconstrictor response at resting tension, but had no effect on the vasodilator response at all frequencies of stimulation (0.5-8 Hz) tested. Thus, the neurotransmission process mediating this vasodilator response does not appear to involve Ca2+ entry via N-type Ca2+ channels.

Presynaptic calcium dynamics and transmitter release evoked by single action potentials at mammalian central synapses

The relationship between presynaptic calcium transients ([Ca2+]t) and transmitter release evoked by a single stimulus was both investigated experimentally and modeled at a mammalian central synapse, the CA3 to CA1 pyramidal cell synapse in guinea pig hippocampal slices. In the present study, we compared the low-affinity calcium indicator furaptra with the higher-affinity indicator fura-2. The 10-90% rise time of the furaptra transient was 2.4 ms compared to 7.8 ms with fura-2; the half-decay time (tau 1/2) was 30 ms for furaptra, compared to 238 ms for fura-2. The half-width of the calcium influx was 1.8 ms with furaptra, which provides an upper limit to the duration of the calcium current (ICa) evoked by an action potential. Modeling the decay time course of the furaptra transients led to the conclusion that the predominant endogenous calcium buffer in these terminals must have relatively slow kinetics (kon < 10(5)/M.s), although the presence of small amounts of fast buffers cannot be excluded. The relationship between the [Ca2+]t measured with furaptra and the postsynaptic response was the same as previously observed with fura-2: the postsynaptic response was proportional to about the fourth power (m approximately 4) of the amplitude of either [Ca2+]t or calcium influx. Thus, although fura-2 may be locally saturated by the high local [Ca2+] responsible for transmitter release, the volume-averaged fura-2 signal accurately reflects changes in this local concentration. The result that both indicators gave similar values for the power m constrains the amplitude of calcium influx in our model: Ica < 1 pA for 1 ms.

Enhanced fast synaptic transmission and a delayed depolarization induced by transient potassium current blockade in rat hippocampal slice as studied by optical recording

In hippocampal neurons, a slowly inactivating aminopyridine-sensitive transient potassium current, D-current, influences the time course of action potential repolarization and therefore activity-dependent Ca2+ entry. We used high-speed optical recording techniques to study the effects of selectively inhibiting D-current with 4-AP (40 microM) on transmission at the Schaffer collateral (CA3)-CA1 synapse in rat hippocampal slices stained with the voltage-sensitive dye RH-155. We observed that addition of 4-AP to the bathing solution resulted in (1) augmentation of a fast component of the optical signal corresponding to the postsynaptic EPSP and action potential, and (2) the appearance of a delayed depolarization of CA1 neurons and other adjacent cells. 4-AP appeared to alter the presynaptic action potential and the dynamics of synaptic transmission to both reduce the sensitivity of the postsynaptic EPSP and action potential to omega-toxin calcium channel blockers (omega-conotoxin GVIA and omega-agatoxin IVA) and the Ca(2+)-dependent potassium channel blocker charybdotoxin, and to increase sensitivity to the dihydropyridine nifedipine, the NMDA receptor blocker aminophosphonopentanoic acid, and the intracellular Ca2+ release inhibitor thapsigargin. The delayed depolarization induced by 4-AP was inhibited in hyperosmotic extracellular solution, suggesting that enhanced transmitter release resulted in increased accumulation of K+ in the extracellular space. Because 4-AP is a convulsant at concentrations similar to those used here, we suggest that the 4-AP-targeted channel(s) carrying D-current may contribute to the hyperexcitability associated with epilepsy.

Synaptic currents generated by neuronal acetylcholine receptors sensitive to alpha-bungarotoxin

Nicotinic acetylcholine receptors are widely distributed throughout the nervous system, but their functions remain largely unknown. One of the most abundant is a class of receptors that contains the alpha 7 gene product, has a high relative permeability to calcium, and binds alpha-bungarotoxin. Here, we report that receptors sensitive to alpha-bungarotoxin, though concentrated in perisynaptic clusters on neurons, can generate a large amount of the synaptic current. Residual currents through other nicotinic receptors are sufficient to elicit action potentials, but with slower rise times. This demonstrates a postsynaptic response for alpha-bungarotoxin-sensitive receptors on neurons and suggests that the functional domain of the postsynaptic membrane is broader than previously recognized.

Enhanced fast synaptic transmission and a delayed depolarization induced by transient potassium current blockade in rat hippocampal slice as studied by optical recording

In hippocampal neurons, a slowly inactivating aminopyridine-sensitive transient potassium current, D-current, influences the time course of action potential repolarization and therefore activity-dependent Ca2+ entry. We used high-speed optical recording techniques to study the effects of selectively inhibiting D-current with 4-AP (40 microM) on transmission at the Schaffer collateral (CA3)-CA1 synapse in rat hippocampal slices stained with the voltage-sensitive dye RH-155. We observed that addition of 4-AP to the bathing solution resulted in (1) augmentation of a fast component of the optical signal corresponding to the postsynaptic EPSP and action potential, and (2) the appearance of a delayed depolarization of CA1 neurons and other adjacent cells. 4-AP appeared to alter the presynaptic action potential and the dynamics of synaptic transmission to both reduce the sensitivity of the postsynaptic EPSP and action potential to omega-toxin calcium channel blockers (omega-conotoxin GVIA and omega-agatoxin IVA) and the Ca(2+)-dependent potassium channel blocker charybdotoxin, and to increase sensitivity to the dihydropyridine nifedipine, the NMDA receptor blocker aminophosphonopentanoic acid, and the intracellular Ca2+ release inhibitor thapsigargin. The delayed depolarization induced by 4-AP was inhibited in hyperosmotic extracellular solution, suggesting that enhanced transmitter release resulted in increased accumulation of K+ in the extracellular space. Because 4-AP is a convulsant at concentrations similar to those used here, we suggest that the 4-AP-targeted channel(s) carrying D-current may contribute to the hyperexcitability associated with epilepsy.

Calcium channel types contributing to excitatory and inhibitory synaptic transmission between individual hypothalamic neurons

The contribution of L-, N-, P- and Q-type Ca2+ channels to excitatory and inhibitory synaptic transmission and to whole-cell Ba2+ currents through Ca2+ channels (Ba2+ currents) was investigated in rat hypothalamic neurons grown in dissociated cell culture. Excitatory and inhibitory postsynaptic currents (EPSCs and IPSCs) were evoked by stimulating individual neurons under whole-cell patch-clamp conditions. The different types of high-voltage-activated (HVA) Ca2+ channels were identified using nifedipine, omega-Conus geographus toxin VIA (omega-CTx GVIA), omega-Agelenopsis aperta toxin IVA (omega-Aga IVA), and omega-Conus magus toxin VIIC (omega-CTx MVIIC). N-, but not P- or Q-type Ca2+ channels contributed to excitatory as well as inhibitory synaptic transmission together with Ca2+ channels resistant to the aforementioned Ca2+ channel blockers (resistant Ca2+ channels). Reduction of postsynaptic current (PSC) amplitudes by N-type Ca2+ channel blockers was significantly stronger for IPSCs than for EPSCs. In most neurons whole-cell Ba2+ currents were carried by L-type Ca2+ channels and by at least two other Ca2+ channel types, one of which is probably of the Q-type and the others are resistant Ca2+ channels. These results indicate a different contribution of the various Ca2+ channel types to excitatory and inhibitory synaptic transmission and to whole-cell currents in these neurons and suggest different functional roles for the distinct Ca2+ channel types.

Noradrenaline modulates transmitter release by enhancing the Ca2+ sensitivity of exocytosis in the chick ciliary presynaptic terminal

1. The giant presynaptic terminal of chick ciliary ganglion was used to examine how noradrenaline (NA) modulates neurotransmitter release. The cholinergic excitatory postsynaptic currents (EPSCs) were recorded under whole-cell voltage clamp of the postsynaptic neuron. 2. Although the EPSC was potentiated by NA, the current directly activated by acetylcholine (IACh) was unaffected. NA also increased the quantal contents without changing the quantal size. 3. The NA-dependent potentiation was antagonized by neither phentolamine nor propranolol. The EPSC was also potentiated by adrenaline and dopamine but not by normetanephrine, phenylephrine or isoprenaline. The EPSC was attenuated by clonidine. Therefore, NA potentiated the transmitter release through a receptor pharmacologically different from both alpha- and beta-adrenergic receptors. 4. The Ca2+ increment produced by an action potential (delta[Ca2+]pre) was reduced by NA through an alpha 2-adrenergic receptor. However, when alpha 2-adrenergic receptors were blocked, neither delta[Ca2+]pre nor resting Ca2+ were changed by NA. 5. The [Ca2+]o-EPSC relation was shifted by NA, decreasing the half-saturating [Ca2+]o, without changing the maximum. 6. It is concluded that NA-dependent potentiation of transmitter release was due to an increase in the Ca2+ sensitivity of the exocytotic process. The enhancement of the fusion probability is suggested.

Presynaptic excitability

Based on functional characterizations with electrophysiological techniques, the channels in nerve terminals appear to be as diverse as channels in nerve cell bodies (Table I). While most presynaptic Ca2+ channels superficially resemble either N-type or L-type channels, variations in detail have necessitated the use of subscripts and other notations to indicate a nerve terminal-specific subtype (e.g., Wang et al., 1993). Variations such as these pose a serious obstacle to the identification of presynaptic channels based solely on the effects of channel blockers on synaptic transmission. Pharmacological sensitivity alone is not likely to help in determining functional properties. Crucial details, such as voltage sensitivity and inactivation, require direct examination. It goes without saying that every nerve terminal membrane contains Ca2+ channels as an entry pathway so that Ca2+ can trigger secretion. However, there appears to be no general specification of channel type, other than the exclusion of T-type Ca2+ channels. T-type Ca2+ channels are defined functionally by strong inactivation and low threshold. Some presynaptic Ca2+ channels inactivate (posterior pituitary and Xenopus nerve terminals), and others have a somewhat reduced voltage threshold (retinal bipolar neurons and squid giant synapse). Perhaps it is just a matter of time before a nerve terminal Ca2+ channel is found with both of these properties. The high threshold and strong inactivation of T-type Ca2+ channels are thought to be adaptations for oscillations and the regulation of bursting activity in nerve cell bodies. The nerve terminals thus far examined have no endogenous electrical activity, but rather are driven by the cell body. On functional grounds, it is then reasonable to anticipate finding T-type Ca2+ channels in nerve terminals that can generate electrical activity on their own. The rarity of such behavior in nerve terminals may be associated with the rarity of presynaptic T-type Ca2+ channels. In four of the five preparations reviewed in this chapter--motor nerve, squid giant synapse, ciliary ganglion, and retina bipolar neurons--evidence was presented that supports a location for Ca2+ channels that is very close to active zones of secretion. All of these synapses secrete from clear vesicles, and the speed and specificity of transduction provided by proximity may be a common feature of these rapid synapses. In contrast, the posterior pituitary secretion apparatus may be triggered by higher-affinity Ca2+ receptors and lower concentrations of Ca2+ (Lindau et al., 1992). This would correspond with the slower performance of peptidergic secretion, but because of the large stimuli needed to evoke release from neurosecretosomes, the possibility remains that the threshold for secretion is higher than that reported. While the role of Ca2+ as a trigger of secretion dictates a requirement for voltage-activated Ca2+ channels as universal components of the presynaptic membrane, the presence of other channels is more difficult to predict. Depolarizations caused by voltage-activated Na+ channels activate the presynaptic Ca2+ channels, but whether this depolarization requires Na+ channels in the presynaptic membrane itself may depend on the electrotonic length of the nerve terminal. Variations in density between motor nerve terminals may reflect species differences in geometry. The high Na+ channel density in the posterior pituitary reflects the great electrotonic length of this terminal arbor. Whether Na+ channels are abundant or not in a presynaptic membrane, K+ channels provide the most robust mechanism for limiting depolarization-induced Ca2+ entry. K+ channel blockers enhance transmission at most synapses. In general, K+ channels are abundant in nerve terminals, although their apparent lower priority compared to Ca2+ channels in the eyes of many investigators leaves us with fewer detailed investigations in some preparations. Most nerve terminals have more than