Developmental switch in the pharmacology of Ca2+ channels coupled to acetylcholine release

Experimentally monitored calcium dynamics at synaptic active zones during neurotransmitter release in neuron-muscle cell cultures

Ca2+-dependent K+ (BK) channels at varicosities in Xenopus nerve-muscle cell cultures were used to quantify experimentally the instantaneous active zone [Ca2+]AZ resulting from different rates and durations of Ca2+ entry in the absence of extrinsic buffers and correlate this with neurotransmitter release. Ca2+ tail currents produce mean peak [Ca2+]AZ ~ 30 μM; with continued influx, [Ca2+]AZ reaches ~45-60 μM at different rates depending on Ca2+ driving force and duration of influx. Both IBK and release are dependent on Ca2+ microdomains composed of both N- and L-type Ca channels. Domains collapse with a time constant of ~0.6 ms. We have constructed an active zone (AZ) model that approximately fits this data, and depends on incorporation of the high-capacity, low-affinity fixed buffer represented by phospholipid charges in the plasma membrane. Our observations suggest that in this preparation, (1) some BK channels, but few if any of the Ca2+ sensors that trigger release, are located within Ca2+ nanodomains while a large fraction of both are located far enough from Ca channels to be blockable by EGTA, (2) the IBK is more sensitive than the excitatory postsynaptic current (EPSC) to [Ca2+]AZ (K1/2-26 μM vs. ~36 μM [Ca2+]AZ); (3) with increasing [Ca2+]AZ, the IBK grows with a Hill coefficient of 2.5, the EPSC with a coefficient of 3.9; (4) release is dependent on the highest [Ca2+] achieved, independent of the time to reach it; (5) the varicosity synapses differ from mature frog nmjs in significant ways; and (6) BK channels are useful reporters of local [Ca2+]AZ.

Calcium channel agonists protect against neuromuscular dysfunction in a genetic model of TDP-43 mutation in ALS

TAR DNA binding protein (TDP-43, encoded by the TARDBP gene) has recently been shown to be associated with amyotrophic lateral sclerosis (ALS), but the early pathophysiological deficits causing impairment in motor function are unknown. Here we expressed the wild-type human gene (wtTARDBP) or the ALS mutation G348C (mutTARDBP) in zebrafish larvae and characterized their motor (swimming) activity and the structure and function of their neuromuscular junctions (NMJs). Of these groups only mutTARDBP larvae showed impaired swimming and increased motoneuron vulnerability with reduced synaptic fidelity, reduced quantal transmission, and more orphaned presynaptic and postsynaptic structures at the NMJ. Remarkably, all behavioral and cellular features were stabilized by chronic treatment with either of the L-type calcium channel agonists FPL 64176 or Bay K 8644. These results indicate that expression of mutTARDBP results in defective NMJs and that calcium channel agonists could be novel therapeutics for ALS.

Acetylcholine release at neuromuscular junctions of adult tottering mice is controlled by N-(cav2.2) and R-type (cav2.3) but not L-type (cav1.2) Ca2+ channels

The mutation in the alpha(1A) subunit gene of the P/Q-type (Ca(v)2.1) Ca(2+) channel present in tottering (tg) mice causes ataxia and motor seizures that resemble absence epilepsy in humans. P/Q-type Ca(2+)channels are primarily involved in acetylcholine (ACh) release at mammalian neuromuscular junctions. Unmasking of L-type (Ca(v)1.1-1.2) Ca(2+) channels occurs in cerebellar Purkinje cells of tg mice. However, whether L-type Ca(2+) channels are also up-regulated at neuromuscular junctions of tg mice is unknown. We characterized thoroughly the pharmacological sensitivity of the Ca(2+) channels, which control ACh release at adult tg neuromuscular junctions. Block of N- and R-type (Ca(v)2.2-2.3), but not L-type Ca(2+) channels, significantly reduced quantal content of end-plate potentials in tg preparations. Neither resting nor KCl-evoked miniature end-plate potential frequency differed significantly between tg and wild type (WT). Immunolabeling of Ca(2+) channel subunits alpha(1A), alpha(1B), alpha(1C), and alpha(1E) revealed an apparent increase of alpha(1B), and alpha(1E) staining, at tg but not WT neuromuscular junctions. This presumably compensates for the deficit of P/Q-type Ca(2+)channels, which localized presynaptically at WT neuromuscular junctions. No alpha(1C) subunits juxtaposed with pre- or postsynaptic markers at either WT or tg neuromuscular junctions. Thus, in adult tg mice, immunocytochemical and electrophysiological data indicate that N- and R-type channels both assume control of ACh release at motor nerve terminals. Recruitment of alternate subtypes of Ca(2+) channels to control transmitter release seems to represent a commonly occurring method of neuronal plasticity. However, it is unclear which conditions underlie recruitment of Ca(v)2 as opposed to Ca(v)1-type Ca(2+) channels.

Ion Channel Development, Spontaneous Activity, and Activity-Dependent Development in Nerve and Muscle Cells

At specific stages of development, nerve and muscle cells generate spontaneous electrical activity that is required for normal maturation of intrinsic excitability and synaptic connectivity. The patterns of this spontaneous activity are not simply immature versions of the mature activity, but rather are highly specialized to initiate and control many aspects of neuronal development. The configuration of voltage- and ligand-gated ion channels that are expressed early in development regulate the timing and waveform of this activity. They also regulate Ca2+influx during spontaneous activity, which is the first step in triggering activity-dependent developmental programs. For these reasons, the properties of voltage- and ligand-gated ion channels expressed by developing neurons and muscle cells often differ markedly from those of adult cells. When viewed from this perspective, the reasons for complex patterns of ion channel emergence and regression during development become much clearer.

Tetanic depression is overcome by tonic adenosine A(2A) receptor facilitation of L-type Ca(2+) influx into rat motor nerve terminals

Motor nerve terminals possess multiple voltage-sensitive calcium channels operating acetylcholine (ACh) release. In this study, we investigated whether facilitation of neuromuscular transmission by adenosine generated during neuronal firing was operated by Ca(2+) influx via 'prevalent' P-type or via the recruitment of 'silent' L-type channels. The release of [(3)H]ACh from rat phrenic nerve endings decreased upon increasing the stimulation frequency of the trains (750 pulses) from 5 Hz (83 +/- 4 x 10(3) disintegrations per minute per gram (d.p.m. g(-1)); n = 11) to 50 Hz (30 +/- 3 x 10(3) d.p.m. g(-1); n = 5). The P-type Ca(2+) channel blocker, omega-agatoxin IVA (100 nm) reduced (by 40 +/- 10%; n = 6) the release of [(3)H]ACh evoked by 50-Hz trains, while nifedipine (1 microM, an L-type blocker) was inactive. Tetanic depression was overcome (88 +/- 6 x 10(3) d.p.m. g(-1); n = 12) by stimulating the phrenic nerve with 50-Hz bursts (five bursts of 150 pulses, 20 s interburst interval). In these conditions, omega-agatoxin IVA (100 nM) failed to affect transmitter release, but nifedipine (1 microM) decreased [(3)H]ACh release by 21 +/- 7% (n = 4). Inactivation of endogenous adenosine with adenosine deaminase (ADA, 0.5 U ml(-1)) reduced (by 54 +/- 8%, n = 5) the release of [(3)H]ACh evoked with 50-Hz bursts. This effect was opposite to the excitatory actions of adenosine (0.5 mm), S-(p-nitrobenzyl)-6-thioinosine (5 microM, an adenosine uptake blocker) and CGS 21680C (3 nM, a selective A(2A) receptor agonist); as the A(1) receptor agonist R-N(6)-phenylisopropyl adenosine (R-PIA, 300 nM) failed to affect the release of [(3)H]ACh, the results indicate that adenosine generated during 50-Hz bursts exerts an A(2A)-receptor-mediated tonus. The effects of ADA (0.5 U ml(-1)) and CGS 21680C (3 nm) were prevented by nifedipine (1 microM). Blocking tonic A(2A) receptor activation, with ADA (0.5 U ml(-1)) or 3,7-dimethyl-1-propargyl xanthine (10 microM, an A(2A) antagonist), recovered omega-agatoxin IVA (100 nM) inhibition and caused the loss of function of nifedipine (1 microM). Data indicate that, in addition to the predominant P-type Ca(2+) current triggering ACh release during brief tetanic trains, motoneurones possess L-type channels that may be recruited to facilitate transmitter release during high-frequency bursts. The fine-tuning control of Ca(2+) influx through P- or L-type channels is likely to be mediated by endogenous adenosine. Therefore, tonic activation of presynaptic A(2A) receptors operating Ca(2+) influx via L-type channels may contribute to overcome tetanic depression during neuronal firing.

A syntaxin 1, Galpha(o), and N-type calcium channel complex at a presynaptic nerve terminal: analysis by quantitative immunocolocalization

Presynaptic Ca(V)2.2 (N-type) calcium channels are subject to modulation by interaction with syntaxin 1 and by a syntaxin 1-sensitive Galpha(O) G-protein pathway. We used biochemical analysis of neuronal tissue lysates and a new quantitative test of colocalization by intensity correlation analysis at the giant calyx-type presynaptic terminal of the chick ciliary ganglion to explore the association of Ca(V)2.2 with syntaxin 1 and Galpha(O). Ca(V)2.2 could be localized by immunocytochemistry (antibody Ab571) in puncta on the release site aspect of the presynaptic terminal and close to synaptic vesicle clouds. Syntaxin 1 coimmunoprecipitated with Ca(V)2.2 from chick brain and chick ciliary ganglia and was widely distributed on the presynaptic terminal membrane. A fraction of the total syntaxin 1 colocalized with the Ca(V)2.2 puncta, whereas the bulk colocalized with MUNC18-1. Galpha(O,) whether in its trimeric or monomeric state, did not coimmunoprecipitate with Ca(V)2.2, MUNC18-1, or syntaxin 1. However, the G-protein exhibited a punctate staining on the calyx membrane with an intensity that varied in synchrony with that for both Ca channels and syntaxin 1 but only weakly with MUNC18-1. Thus, syntaxin 1 appears to be a component of two separate complexes at the presynaptic terminal, a minor one at the transmitter release site with Ca(V)2.2 and Galpha(O), as well as in large clusters remote from the release site with MUNC18-1. These syntaxin 1 protein complexes may play distinct roles in presynaptic biology.

Voltage-gated calcium channels in adult rat inferior colliculus neurons

The inferior colliculus (IC) plays a key role in the processing of auditory information and is thought to be an important site for genesis of wild running seizures that evolve into tonic-clonic seizures. IC neurons are known to have Ca(2+) channels but neither their types nor their pharmacological properties have been as yet characterized. Here, we report on biophysical and pharmacological properties of Ca(2+) channel currents in acutely dissociated neurons of adult rat IC, using electrophysiological and molecular techniques. Ca(2+) channels were activated by depolarizing pulses from a holding potential of -90 mV in 10 mV increments using 5 mM barium (Ba(2+)) as the charge carrier. Both low (T-type, VA) and high (HVA) threshold Ca(2+) channel currents that could be blocked by 50 microM cadmium, were recorded. Pharmacological dissection of HVA currents showed that nifedipine (10 microM, L-type channel blocker), omega-conotoxin GVIA (1 microM, N-type channel blocker), and omega-agatoxin TK (30 nM, P-type channel blocker) partially suppressed the current by 21%, 29% and 22%, respectively. Since at higher concentration (200 nM) omega-agatoxin TK also blocks Q-type channels, the data suggest that Q-type Ca(2+) channels carry approximately 16% of HVA current. The fraction of current (approximately 12%) resistant to the above blockers, which was blocked by 30 microM nickel and inactivated with tau of 15-50 ms, was considered as R-type Ca(2+) channel current. Consistent with the pharmacological evidences, Western blot analysis using selective Ca(2+) channel antibodies showed that IC neurons express Ca(2+) channel alpha(1A), alpha(1B), alpha(1C), alpha(1D), and alpha(1E) subunits. We conclude that IC neurons express functionally all members of HVA Ca(2+) channels, but only a subset of these neurons appear to have developed functional LVA channels.

Somatostatin receptors

In 1972, Brazeau et al. isolated somatostatin (somatotropin release-inhibiting factor, SRIF), a cyclic polypeptide with two biologically active isoforms (SRIF-14 and SRIF-28). This event prompted the successful quest for SRIF receptors. Then, nearly a quarter of a century later, it was announced that a neuropeptide, to be named cortistatin (CST), had been cloned, bearing strong resemblance to SRIF. Evidence of special CST receptors never emerged, however. CST rather competed with both SRIF isoforms for specific receptor binding. And binding to the known subtypes with affinities in the nanomolar range, it has therefore been acknowledged to be a third endogenous ligand at SRIF receptors. This review goes through mechanisms of signal transduction, pharmacology, and anatomical distribution of SRIF receptors. Structurally, SRIF receptors belong to the superfamily of G protein-coupled (GPC) receptors, sharing the characteristic seven-transmembrane-segment (STMS) topography. Years of intensive research have resulted in cloning of five receptor subtypes (sst(1)-sst(5)), one of which is represented by two splice variants (sst(2A) and sst(2B)). The individual subtypes, functionally coupled to the effectors of signal transduction, are differentially expressed throughout the mammalian organism, with corresponding differences in physiological impact. It is evident that receptor function, from a physiological point of view, cannot simply be reduced to the accumulated operations of individual receptors. Far from being isolated functional units, receptors co-operate. The total receptor apparatus of individual cell types is composed of different-ligand receptors (e.g. SRIF and non-SRIF receptors) and co-expressed receptor subtypes (e.g. sst(2) and sst(5) receptors) in characteristic proportions. In other words, levels of individual receptor subtypes are highly cell-specific and vary with the co-expression of different-ligand receptors. However, the question is how to quantify the relative contributions of individual receptor subtypes to the integration of transduced signals, ultimately the result of collective receptor activity. The generation of knock-out (KO) mice, intended as a means to define the contributions made by individual receptor subtypes, necessarily marks but an approximation. Furthermore, we must now take into account the stunning complexity of receptor co-operation indicated by the observation of receptor homo- and heterodimerisation, let alone oligomerisation. Theoretically, this phenomenon adds a novel series of functional megareceptors/super-receptors, with varied pharmacological profiles, to the catalogue of monomeric receptor subtypes isolated and cloned in the past. SRIF analogues include both peptides and non-peptides, receptor agonists and antagonists. Relatively long half lives, as compared to those of the endogenous ligands, have been paramount from the outset. Motivated by theoretical puzzles or the shortcomings of present-day diagnostics and therapy, investigators have also aimed to produce subtype-selective analogues. Several have become available.

Calcium channel subtypes contributing to acetylcholine release from normal, 4-aminopyridine-treated and myasthenic syndrome auto-antibodies-affected neuromuscular junctions

1 Acetylcholine release at the neuromuscular junction relies on rapid, local and transient calcium increase at presynaptic active zones, triggered by the ion influx through voltage-dependent calcium channels (VDCCs) clustered on the presynaptic membrane. Pharmacological investigation of the role of different VDCC subtypes (L-, N-, P/Q- and R-type) in spontaneous and evoked acetylcholine (ACh) release was carried out in adult mouse neuromuscular junctions (NMJs) under normal and pathological conditions. 2 omega-Agatoxin IVA (500 nM), a specific P/Q-type VDCC blocker, abolished end plate potentials (EPPs) in normal NMJs. However, when neurotransmitter release was potentiated by the presence of the K(+) channel blocker 4-aminopyridine (4-AP), an omega-agatoxin IVA- and omega-conotoxin MVIIC-resistant component was detected. This resistant component was only partially sensitive to 1 micro M omega-conotoxin GVIA (N-type VDCC blocker), but insensitive to any other known VDCC blockers. Spontaneous release was dependent only on P/Q-type VDCC in normal NMJs. However, in the presence of 4-AP, it relied on L-type VDCCs too. 3 ACh release from normal NMJs was compared with that of NMJs of mice passively injected with IgGs obtained from patients with Lambert-Eaton myasthenic syndrome (LEMS), a disorder characterized by a compromised neurotransmitter release. Differently from normal NMJs, in LEMS IgGs-treated NMJs an omega-agatoxin IVA-resistant EPP component was detected, which was only partially blocked by calciseptine (1 micro M), a specific L-type VDCC blocker. 4 Altogether, these data demonstrate that multiple VDCC subtypes are present at the mouse NMJ and that a resistant component can be identified under 'pharmacological' and/or 'pathological' conditions.

Contribution of L-type Ca(2+) channels to evoked transmitter release in cultured Xenopus nerve-muscle synapses

1. Simultaneous pre- and postsynaptic patch recordings were obtained from the varicosity synapses formed by Xenopus motoneurons on muscle cells in embryonic cultures, in order to elucidate the contribution of N- and L-type Ca(2+) channels to the varicosity Ca(2+) current (I(Ca)) and evoked transmitter release. 2. Although N-type channels are predominant in the varicosities and generally thought to be responsible for all evoked release, in most synapses a fraction of I(Ca) and release could be reversibly blocked by the L-type channel antagonist nifedipine, and enhanced by the agonist Bay K8644. Up to 50 % (mean, 21 %) of the I(Ca) evoked by a voltage clamp waveform mimicking a normal presynaptic action potential (APWF) is composed of L-type current. 3. Surprisingly, the nifedipine-sensitive (L) channels activated more rapidly (time-constant, 0.46 ms at +30 mV) than the nifedipine-insensitive (N) channels (time constant, 1.42 ms). Thus the L-type current would play a disproportionate role in the I(Ca) linked to a normal action potential. 4. The relationship between I(Ca) and release was the same for nifedipine-sensitive and -resistant components. The N- and L-components of I(Ca) are thus equally potent in evoking release. This may represent an immature stage before N-type channels become predominant. 5. Replacing Ca(2+) in the medium with Ba(2+) strongly enhanced the L-type component, suggesting that L-type channels may be inactivated at Ca(2+) levels close to those at rest. 6. We speculate that populations of L-type channels in different parts of the neuron may be recruited or inactivated by fluctuations of the cytosolic Ca(2+) concentration within the physiological range.

Calcium channels coupled to neurotransmitter release at dually innervated neuromuscular junctions in the newborn rat

We studied the effect of several calcium channel blockers (omega-Conotoxin-GVIA, 1 and 3microM; omega-Agatoxin-IVA, 100nM; Nitrendipine, 1 and 10microM) on evoked transmitter release at singly and dually innervated endplates of the levator auris longus muscle from three- to six-day-old rats. In dually innervated fibers, a second endplate potential may appear after the first one when we increase the stimulation intensity. The lowest and highest endplate potential amplitudes are designated "small endplate potential" and "large endplate potential", respectively. The percentage of doubly innervated junctions remains almost constant throughout the age range examined. Nevertheless, the percentage of junctions innervated by three or more terminal axons drops, whereas the singly innervated junctions increase. Therefore, between postnatal days 3 and 6, roughly half the neuromuscular junctions may experience the final process of axonal elimination. The synaptic efficacy of the large endplate potential in dual junctions, measured as the mean amplitude of the synaptic potential and mean quantal content, was the same as in the junctions that had become recently mono-innervated in the same postnatal period. In singly innervated fibers, the endplate potential size was strongly reduced by both the P/Q-type voltage-dependent calcium channel blocker omega-Agatoxin-IVA (79.17+/-4.02%; P < 0.05) and the N-type voltage-dependent calcium channel blocker omega-Conotoxin-GVIA (56.31+/-7.80%; P < 0.05), whereas endplate potential amplitude was not significantly changed by the L-type voltage-dependent calcium channel blocker Nitrendipine. In dually innervated fibers, the P/Q-type voltage-dependent calcium channel blocker omega-Agatoxin-IVA and L-type voltage-dependent calcium channel blocker Nitrendipine increased the size of the small endplate potential (161.29+/-47.87% and 109.32+/-11.03%, respectively; P < 0.05 in both cases) and reduced the large endplate potential (74.42+/-15.32% and 70.91+/-10.04%, respectively; P < 0.05 in both cases). The N-type voltage-dependent calcium channel blocker omega-Conotoxin-GVIA significantly increased the small endplate potential in the first few minutes after toxin application (at 10min: 90.23+/-17.38%; P < 0.05). This increase was not maintained, while the large endplate potential was strongly inhibited (69.25+/-7.5%; P < 0.05). In conclusion, in the dually innervated endplates of the newborn rat, presynaptic calcium channel types can have different roles in transmitter release from each of the two inputs, which suggests that nerve terminal voltage-dependent calcium channels are involved in neonatal synaptic maturation.

Calcium channel isoforms underlying synaptic transmission at embryonic Xenopus neuromuscular junctions

Studies on the amphibian neuromuscular junction have indicated that N-type calcium channels are the sole mediators of stimulus-evoked neurotransmitter release. We show, via both presynaptic and postsynaptic voltage-clamp measurements, that dihydropyridine (DHP)-sensitive calcium channels also contribute to stimulus-evoked release at developing Xenopus neuromuscular junctions. Whereas inhibition of postsynaptic responses by omega-conotoxin (omega-Ctx) GVIA has been taken previously as evidence that only N-type channels mediate transmitter release, we find that both N-type and DHP-sensitive calcium currents are sensitive to this toxin. The unusual sensitivity of DHP-sensitive calcium channels to omega-Ctx GVIA in presynaptic terminals raises the possibility that this channel type may have escaped detection in previous physiological studies on adult frog neuromuscular junctions. Alternatively, the additional channel isoforms may be present only during early development, when they may serve to strengthen collectively presynaptic release during critical periods of synaptogenesis.

Multiple types of calcium channels mediate transmitter release during functional recovery of botulinum toxin type A-poisoned mouse motor nerve terminals

The involvement of different types of voltage-dependent calcium channels in nerve-evoked release of neurotransmitter was studied during recovery from neuromuscular paralysis produced by botulinum toxin type A intoxication. For this purpose, a single subcutaneous injection of botulinum toxin (1 IU; DL50) on to the surface of the mouse levator auris longus muscle was performed. The muscles were removed at several time-points after injection (i.e. at one, two, three, four, five, six and 12 weeks). Using electrophysiological techniques, we studied the effect of different types of calcium channel blockers (nitrendipine, omega-conotoxin-GVIA and omega-agatoxin-IVA) on the quantal content of synaptic transmission elicited by nerve stimulation. Morphological analysis using the conventional silver impregnation technique was also made. During the first four weeks after intoxication, sprouts were found at 80% of motor nerve terminals, while at 12 weeks their number was decreased and the nerve terminals were enlarged. The L-type channel blocker nitrendipine (1 microM) inhibited neurotransmitter release by 80% and 30% at two and five weeks, respectively, while no effects were found at later times. The N-type channel blocker omega-conotoxin-GVIA (1 microM) inhibited neurotransmitter release by 50-70% in muscles studied at two to six weeks, respectively, and had no effect 12 weeks after intoxication. The P-type channel blocker omega-agatoxin-IVA (100 nM) strongly reduced nerve-evoked transmitter release (>90%) at all the time-points studied. Identified motor nerve terminals were also sensitive to both nitrendipine and omega-conotoxin-GVIA. This study shows that multiple voltage-dependent calcium channels were coupled to transmitter release during the period of sprouting and consolidation, suggesting that they may be involved in the nerve ending functional recovery process.

Preferential formation of strong synapses during re-innervation of guinea-pig sympathetic ganglia

1. Re-innervation of partially denervated sympathetic ganglion cells was investigated using intracellular recording in guinea-pig lumbar paravertebral ganglia in vitro. The question addressed was whether the pattern of innervation by strong (suprathreshold) and weak (subthreshold) inputs seen normally was restored during re-innervation. 2. L5 ganglion cells each normally received 3.9 +/- 0.2 preganglionic inputs of which 1.2 +/- 0.1 were strong. Only 0.9 +/- 0.1 inputs arose from the L4 segment, the last of the thoracolumbar outflow, and only 11 % of these were strong. 3. Three to five weeks after cutting the sympathetic chain above the L4 white ramus, each neurone received 2.1 +/- 0.1 inputs after sprouting of surviving axons. Nearly 60 % of neurones received a strong input and the normal ratio of weakstrong synapses was restored. 4. Total charge transfer evoked by L4 inputs increased from 8.8 +/- 1.4 to 27. 6 +/- 2.4 pC per neurone after re-innervation, reaching 77 % of that in normal ganglia. This was primarily due to the formation of new strong inputs of normal size. 5. The synaptic events at the new strong synapses and the types of Ca2+ channel mediating transmitter release (N-type and channels resistant to specific antagonists) were the same as those at control strong synapses. 6. The data indicate that, following partial denervation, sprouting of surviving preganglionic axons results in the preferential formation of strong synapses with the same characteristics as those in normal ganglia. Thus the pattern of functional transmission by a single strong input to each cell was restored rather than the recovery of the number of synaptic connections.

kappa- and mu-opioids reverse the somatostatin inhibition of Ca2+ currents in ciliary and dorsal root ganglion neurons

Neuromodulators, including transmitters and peptides, modify neuronal excitability. In most neurons, multiple neuromodulator receptors are present on a single cell. Previous work has demonstrated either occlusive or additive effects when two neuromodulators that target the same ion channel are applied together. In this study, we characterize the modulation of Ca2+ and K+ channels in embryonic chick ciliary ganglion neurons by somatostatin (Som) and opioids, including the effects of these neuromodulators when applied in combination. We report a modulation of calcium current by kappa- or mu-opioids that can prevent Som effects when applied before Som and can replace Som effects when applied after Som. We term these effects demodulation because they do not have the characteristics of simple occlusion but rather represent a dominant effect of opioid-mediated modulation of calcium channels over Som-mediated modulation. These opioid effects persist in the presence of kinase and phosphatase inhibitors, as well as after alteration of the intracellular Ca2+ concentration. Furthermore, they are present in both whole-cell and perforated-patch recording configurations. These effects of opioids on Som-mediated modulation do not seem to be mediated by a general uncoupling of Som receptors from G-protein-coupled signaling systems because K+ current modulation by Som can persist in the presence of opioids. Demodulation by opioids was also observed in dorsal root ganglion neurons on the modulation of calcium current by GABA and norepinephrine (NE). In both preparations, this demodulatory interaction occurred between voltage-independent (opioids) and voltage-dependent (Som, GABA, and NE) modulatory pathways.

Calcium channels coupled to neurotransmitter release at neonatal rat neuromuscular junctions

1. The effects of different calcium channel blockers (omega-agatoxin IVA (omega-Aga IVA), omega-conotoxin GVIA (omega-CgTx GVIA) and dihydropyridines) were tested on spontaneous and evoked transmitter release at embryonic and newborn rat neuromuscular junctions (NMJs). 2. The nerve-evoked transmitter release quantal content (m) was strongly reduced by the P/Q-type voltage-dependent calcium channel (VDCC) blocker omega-Aga IVA (100 nM) at newly formed endplates of embryos and 0- to 11-day-old rats, in agreement with the effect of this blocker on transmitter release at mature and reinnervating muscles. 3. omega-CgTx GVIA (1-5 microM), the N-type VDCC blocker, also caused a significant reduction in m at newly formed NMJs early in development (embryos and 0- to 4-day-old rats), while it was ineffective in more mature animals (5- to 11-day-old rats). 4. L-type channel blockers, nitrendipine (1 microM) and nifedipine (1 microM), did not significantly affect neurally evoked release at developing NMJs. However, nifedipine (10 microM) was able to increase m significantly at 0- to 4-day-old rat NMJs. 5. At developing NMJs, K+-evoked transmitter release was dependent on Ca2+ entry through VDCCs of the P/Q-type family (100 nM omega-Aga IVA reduced 70 % of the K+-evoked miniature endplate potential frequency). N- and L-type VDCC blockers did not affect this type of release. 6. We conclude that at rat neuromuscular junctions the presynaptic calcium channel types involved in transmitter release undergo developmental changes during the early postnatal period.

Effects of Cd2+, Pb2+ and CH3Hg+ on high voltage-activated calcium currents in pheochromocytoma (PC12) cells: potency, reversibility, interactions with extracellular Ca2+ and mechanisms of block

Effects of the neurotoxic heavy metals Cd2+, Pb2+ and CH3Hg+ on current carried by Ca2+ ions (I(Ca)) through high-voltage activated Ca2+ channels in nerve growth factor (NGF)-differentiated pheochromocytoma (PC12) cells were examined to characterize possible differences in the mechanism of action of these metals on Ca2+ channel function. Specifically, the potency and reversibility of effect on I(Ca) by each metal was examined, as well as the relationship between extracellular [Ca2+] and potency of block of I(Ca) by Cd2+ and Pb2+. In addition, the effect of each of these metals on Ca2+ channels when applied to the intracellular side of the membrane was also examined. When extracellular solution contained 20, 10 or 5 mM Ca2+, the estimated IC50 values (total metal concentration) for block of I(Ca) were 15, 10, and 6.5 microM for Cd2+ and 7.5, 2.0 and 1.1 microM for Pb2+, respectively. CH3Hg+ (1-10 microM) blocked I(Ca) (20 mM Ca2+) in a time- and concentration-dependent manner. When cells were washed with metal-free solutions, block of I(Ca) by Cd2+ was reversed rapidly, whereas block by Pb2+ was reversed only partially, and block of I(Ca) by CH3Hg+ was not reversed. When Pb2+ and CH3Hg+ treated cells were washed in metal-free solutions containing 50 microM D-penicillamine (DPEN), block of I(Ca) by 10 microM Pb2+ was rapidly and completely reversed, whereas, block of I(Ca) by 5 microM CH3Hg+ was not reversed. Higher concentrations (500 microM) of 2,3-dimercapto-1-propane sulfonic acid (DMPS) did reverse partially the block of I(Ca) by 5 and 10 microM CH3Hg+. When Cd2+, Pb2+ or CH3Hg+ was present in the intracellular solution, Ca2+ channel currents were significantly reduced. These results characterize effects of Cd2+ on Ca2+ channels and demonstrate that Cd2+, Pb2+ and CH3Hg+ differ in their actions on Ca2+ channels.

Role of cell-cell interactions in the developmental regulation of Ca2+-activated K+ currents in vertebrate neurons

The functional expression of the Ca2+-activated K+ current (IK[Ca]) is dependent on cell-cell interactions in developing chick autonomic neurons. In chick ciliary ganglion (CG) neurons, expression of macroscopic IK[Ca] coincides with the formation of synapses with target tissues. CG neurons that develop in vivo in the absence of normal target tissues fail to express functional IK[Ca], although voltage-activated Ca2+ currents and most other ionic currents are expressed at normal amplitudes and densities. CG neurons placed in cell culture prior to formation of synapses with target tissues also fail to express macroscopic IK[Ca]. However, CG neurons cultured in the presence of a heat- and trypsin-sensitive extract of target tissues express IK[Ca] at normal levels. Similarly, interactions with target tissue appear to regulate the expression of whole-cell IK[Ca] in developing chick sympathetic ganglion neurons, although the relevant trophic factors appear to be different from those required by CG neurons. In addition to target tissue interactions, an intact preganglionic innervation is required for the normal in vivo development of IK[Ca] in chick CG neurons. The trophic effects of the afferent innervation do not require synaptic activation of the CG neurons, indicating secretion of a trophic factor, possibly an isoform of beta-neuregulin. The results are consistent with the hypothesis that target- and nerve terminal-derived trophic factors interact at a posttranslational level in the regulation of a functional IK[Ca]. Together, this body of data demonstrates an essential role for cell-cell interactions in the differentiation of neuronal excitability.

Developmental changes in calcium channel types mediating synaptic transmission in rat auditory brainstem

1. Calcium channel blockers were tested on excitatory postsynaptic currents (EPSCs) at the synapse formed by the calyx of Held on the principal cells in the medial nucleus of trapezoid body (MNTB) in brainstem slices of 4- to 14-day-old rats. 2. At postnatal day 4-9 (P4-9), EPSCs were irreversibly suppressed by the P/Q-type Ca2+ channel blocker omega-agatoxin-IVA (omega-Aga-IVA, 200 nM) and also by the N-type Ca2+ channel blocker omega-conotoxin GVIA (omega-CgTx, 2 microM). A small fraction of EPSCs was resistant to both toxins but abolished by Cd2+ (100 microM). 3. After P7, the omega-CgTx-sensitive EPSC fraction diminished and eventually disappeared after P10. Concomitantly the fraction insensitive to both toxins decreased and became undetectable after P10. 4. In contrast, the omega-Aga-IVA-sensitive EPSC fraction increased with development and became predominant after P10. All through the developmental period examined, the L-type Ca2+ channel blocker nicardipine (10 microM) had no effect. 5. We conclude that presynaptic Ca2+ channel types triggering transmitter release undergo developmental switching during the early postnatal period.

Ontogeny of the L-type voltage sensitive calcium channels in chick embryo retinospheroids

The L-type voltage sensitive calcium channels (VSCC) of chick embryo retinospheroids were characterized during the development in vitro. Functionally, the activity of VSCC was characterized by continuously monitoring the changes in the intracellular free Ca2+ concentration (delta[Ca2+]i) with indo-1, in response to 30 mM KCl. The contribution of the L-type VSCC was evaluated using the L-type VSCC antagonist, nitrendipine. We also characterized the binding of [3H]nitrendipine to retinospheroid membranes during development, and determined the Kd and Bmax values. We observed that the changes in [Ca2+]i in response to 30 mM KCl increased from 159.46 +/- 6.62 nM at 0 days in vitro (DIV) retinospheroids to 704.4 +/- 59.9 nM at 14 DIV retinospheroids. Nitrendipine (2 microM) blocked the delta[Ca2+]i response by approximately 67% in all ages tested. No significant difference in the Kd values for the nitrendipine binding was observed during in vitro development of the retinospheroids. However, the Bmax increased from 27.99 +/- 1.95 fmol/mg protein in 0 DIV retinospheroids to 131.09 +/- 14.24 fmol/mg protein in 14 DIV retinospheroids, supporting the delta[Ca2+]i results. The results presented suggest that the increase in [Ca2+]i during development was due to an increase in the number of L-type channels. Therefore, the expression of L-type VSCC is developmentally regulated during retinogenesis in vitro and accompanies neuronal maturation, probably regulating the Ca2+ input crucial to the onset of important intracellular Ca2+-dependent functions.

Developmental changes in calcium current pharmacology and somatostatin inhibition in chick parasympathetic neurons

Voltage-dependent calcium (Ca2+) currents were characterized and modulatory effects of somatostatin were measured in acutely dissociated chick ciliary ganglion neurons at embryonic stages 34, 37, and 40. This developmental time period coincides with the period of synapse formation between ciliary ganglion neurons and peripheral eye muscles. At all three developmental stages Ca2+ current could be blocked almost completely by combined application of omega-CgTX GVIA and nitrendipine. At young embryonic ages there was significant overlap in sensitivity, with approximately 75% of the current sensitive to either blocker applied independently. By stage 40, there was very little or no overlap in sensitivity, with approximately 75% of the current blocked by omega-CgTX GVIA (N-type) and 30% blocked by nitrendipine (L-type). These data are consistent with earlier findings that the pharmacology of acetylcholine release from ciliary ganglion nerve terminals changes during development from sensitivity to both dihydropyridines and omega-CgTX GVIA to selective sensitivity to omega-CgTX GVIA (Gray et al., 1992). Somatostatin reduced Ca2+ current by 50-60% at all three developmental stages. At early developmental stages somatostatin receptors coupled predominantly to the current that was sensitive to both omega-CgTX GVIA and nitrendipine. By stage 40, somatostatin primarily inhibited classically defined N-type current (selectively sensitive to omega-CgTX GVIA). Thus, somatostatin receptor coupling to Ca2+ channels persisted throughout development as Ca2+ current pharmacology changed.

Novel modulatory effect of L-type calcium channels at newly formed neuromuscular junctions

This study aimed to examine changes of presynaptic voltage-sensitive calcium channel (VSCC) subtypes during synapse formation and regeneration in relation to transmitter release at the neuromuscular junction (NMJ). Synaptic potentials were recorded from developing rat NMJs and from regenerating mouse and frog NMJs. As in normal adult NMJs, evoked transmitter release was reduced by an N-type VSCC blocker in the frog and by a P/Q-type VSCC blocker in the mammal at immature NMJs; however, various L-type VSCC blockers, both dihydropyridine and nondihydropyridine antagonists, increased evoked but not spontaneous release in a dose-dependent manner at newly formed NMJs. This presynaptic potentiation disappeared as NMJs matured. A rapid intracellular Ca2+ buffer, bis(O-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid-AM, prevented the potentiation effect of nifedipine, but a slow Ca2+ buffer, EGTA-AM, did not. Thus, the potentiation effect of L-type blockers requires Ca2+ transients. Pretreatment with Ca2(+)-activated K+ channel blockers, iberiotoxin or charybdotoxin, did not prevent potentiation by nifedipine at regenerating frog NMJs. Thus, Ca(2+)-activated K+ channels were not likely involved in this potentiation. In contrast, no additional potentiation by nifedipine was seen in muscles pretreated with pertussis toxin (PTX), a G-protein blocker, which by itself enhances evoked transmitter release at regenerating frog NMJs. These results suggest the existence of multiple subtypes of VSCCs at newly formed motor nerve terminals. In addition to the normal N- or P/Q-type VSCCs that mediate transmitter release, L-type VSCCs may play a novel modulatory role in evoked transmitter release by activating a mechanism linked to PTX-sensitive G-proteins during synapse maturation.

Differential expression of somatostatin receptors in the rat eye: SSTR4 is intensely expressed in the iris/ciliary body

Distribution of somatostatin receptors (SSTR1-5) was determined in the rat eye. Reverse transcriptase-polymerase chain reaction (RT-PCR) analyses revealed that SSTR4 and SSTR2 are major subtypes expressed predominantly in the iris/ciliary body and retina, respectively, and that SSTR1, SSTR3 and SSTR5 are minor subtypes expressed preferentially in the posterior eye segments including the retina. In situ hybridization showed predominant SSTR4 expression in the posterior iris epithelium and ciliary body, suggesting functional roles of somatostatin in the autonomic nervous system in the anterior segments of the eye. The differential expression of SSTR1-5 may be related to distinct roles of somatostatin in the physiology of different ocular tissues.

A somatostatin receptor inhibits noradrenaline release from chick sympathetic neurons through pertussis toxin-sensitive mechanisms: comparison with the action of alpha 2-adrenoceptors

The effects of somatostatin and analogues were investigated in cultures of chick sympathetic neurons. Electrically evoked tritium overflow from cultures labelled with [3H]noradrenaline was reduced by somatostatin-14 in a concentration-dependent manner, with half maximal effects at 0.3 nM and a maximum of 45% inhibition. Somatostatin-28 was equipotent to somatostatin-14 (half maximal concentration at 0.5 nM), and seglitide was less potent, the effects being half maximal at 4.2 nM. The inhibitory action of somatostatin-14 on stimulation-evoked overflow desensitized within minutes at 100 nM, but not at 10 nM, and was abolished by a pretreatment of neurons with pertussis toxin. All somatostatin analogues reduced voltage-activated Ca2+ currents recorded in the whole-cell configuration of the patch-clamp technique, with somatostatin-14 being equipotent to somatostatin-28, but more potent than seglitide. However, the inhibition of Ca2+ currents occurred at concentrations more than ten-fold higher than those required for the reduction of stimulation evoked 3H overflow. The action of somatostatin upon Ca2+ currents was also abolished by pertussis toxin and desensitized within minutes. In preceding experiments, alpha 2-adrenoceptor activation had been found to reduce transmitter release and Ca2+ currents of chick sympathetic neurons through a pertussis toxin-sensitive mechanism. In the present study, the alpha 2-adrenergic agonist UK 14,304 completely occluded the inhibition of Ca2+ currents and of electrically evoked overflow by somatostatin-14. Neither UK 14,304 nor somatostatin affected the resting membrane potential or voltage-dependent K+ currents. These results demonstrate that chick sympathetic neurons possess SRIF1 type somatostatin receptors which control transmitter release. This effect is mediated by pertussis toxin-sensitive GTP binding proteins and apparently involves an inhibition of voltage-activated Ca2+ channels, but not a modulation of K+ channels. Since alpha 2-adrenergic agonists share all of these actions and occlude the effects of somatostatin, alpha 2-adrenoceptors and SRIF1 receptors seem to regulate sympathetic transmitter release via common signalling mechanisms.

Membrane delimited and intracellular soluble pathways in the somatostatin modulation of ACh release

The signal transduction cascade between the activation of the somatostatin (SOM) receptor and modulation of transmitter release was study using Acetylcholine (Ach) release measurements and patch clamp recordings of Ca2+ current from acutely dissociated St 40 ciliary ganglion neurons. As in intact synapses, somal ACh release was blocked by 100 nM SOM or 100 microM dibutyril cGMP, and the SOM-mediated inhibition could be reversed by 10 microM 1-NAME (a selective inhibitor of nitric oxide synthase, NOS) or 100 microM Rp-8p-CPT-cGMPs (a selective inhibitor of a cGMP protein dependent kinase, PKG). In whole cell recordings, SOM inhibition of Ca2+ current rapidly relaxes to control levels but is sustained in perforated patch recordings which decreases cell dialysis. Inhibition of NOS or PKG in perforated patch recordings, however caused SOM effects to become transient again. We hypothesize that PKG alters the characteristics of the membrane-delimited G protein inhibition of Ca2+ current. Therefore SOM receptors trigger a membrane-delimited signal transduction cascade that is modulated by soluble messengers, converging on voltage activated Ca2+ channels. When both pathways are active together, SOM causes a sustained inhibition of neuronal Ca2+ current leading to a decrease in transmitter release.

The expression of neuronal voltage-dependent calcium channels in human cerebellum

Little is known about the comparative distribution of voltage-dependent calcium channel subtypes in normal human brain. Previous studies in experimental animals have predominantly focused on the regional expression of single alpha 1 genes. We describe the preparation of riboprobes and antisera specific for human alpha 1A, alpha 1B and alpha 1E subunits and their application in comprehensive mapping studies of the human cerebellum. Within the cerebellar cortex, these pore forming proteins were found to have differential localisations when examined in adjacent sections. The alpha 1A and alpha 1B subunits broadly colocalised and were both present, though at apparently different levels, in the molecular, Purkinje and granule cell layers whilst alpha 1E was predominantly expressed in Purkinje cells. In the dentate nucleus, an area which has received little attention in previous studies, alpha 1A was highly expressed in regions in which Purkinje cell nerve terminals form synapses with deep cerebellar neurones.

Impairment of syntaxin by botulinum neurotoxin C1 or antibodies inhibits acetylcholine release but not Ca2+ channel activity

The involvement of syntaxin, an omega-conotoxin-sensitive Ca2+ channel-associated protein, in acetylcholine release was studied at synapses formed between rat sympathetic neurons in culture. Transmission at these synapses involved omega-conotoxin-sensitive Ca2+ channels because a dose-dependent inhibition was observed when omega-conotoxin was bath-applied. Confocal microscope examination of immunofluorescent staining showed that syntaxin had a similar distribution to synaptic vesicle-associated membrane proteins, synaptophysin and vesicle-associated membrane protein/synaptobrevin-2, indicating that syntaxin molecules are concentrated in the presynaptic terminals. Botulinum neurotoxin C1 applied extracellularly or intracellularly into presynaptic neurons blocked synaptic transmission. Introduction of a monoclonal antibody, or polyclonal antibodies, to syntaxin into the presynaptic neuron depressed the evoked release of acetylcholine without affecting Ca2+ influx through Ca2+ channels. These results suggest that syntaxin plays an important role in release of neurotransmitter by a nerve impulse and that this mechanism is downstream of Ca2+ influx.

Neuronal differentiation in cultures of murine neural crest. I. Neurotransmitter expression

This study examines the properties of neurons differentiating in cultures of mammalian neural crest cells. The neurons fall into two categories: (1) a population of early differentiating (ED) neurons generated from precursors that are postmitotic at the time of plating; and (2) a late differentiating (LD) population of neurons arising from dividing precursor cells. The ED population of neurons survive for only 2-3 days while the LD neurons survive for many weeks. Both groups of neurons express the neuronal marker, neurofilament, as well as adrenergic and cholinergic characteristics. The latter two traits are evident as immunoreactivity for tyrosine hydroxylase (TH)/dopamine-beta-hydroxylase (D beta H) and choline acetyltransferase (ChAT), respectively. The LD neurons also contain immunoreactivity for a number of neuropeptides including, substance P (SP), neuropeptide Y (NPY), vasoactive intestinal polypeptide (VIP), calcitonin gene related polypeptide (CGRP), and somatostatin (SOM). Immunoreactivity for SP, CGRP, and VIP are found in virtually all of the LD neurons while SOM and NPY are found in a smaller percentage of the neurons.

Functional dependence of Ca(2+)-activated K+ current on L- and N-type Ca2+ channels: differences between chicken sympathetic and parasympathetic neurons suggest different regulatory mechanisms

The influx of Ca2+ ions controls many important processes in excitable cells, including the regulation of the gating of Ca(2+)-activated K+ channels (the current IK[Ca]). Various IK[Ca] channels contribute to the regulation of the action-potential waveform, the repetitive discharge of spikes, and the secretion of neurotransmitters. It is thought that large-conductance IK[Ca] channels must be closely colocalized with Ca2+ channels (ICa) to be gated by Ca2+ influx. We now report that IK[Ca] channels can be preferentially colocalized with pharmacologically distinct subtypes of voltage-activated Ca2+ channel and that this occurs differently in embryonic chicken sympathetic and parasympathetic neurons. The effects of various dihydropyridines and omega-conotoxin on voltage-activated Ca2+ currents (ICa) and Ca(2+)-activated K+ currents (IK[Ca]) were examined by using perforated-patch whole-cell recordings from embryonic chicken ciliary and sympathetic ganglion neurons. Application of nifedipine or omega-conotoxin each caused a 40-60% reduction in ICa, whereas application of S-(-)-BAY K 8644 potentiated ICa in ciliary ganglion neurons. But application of omega-conotoxin had little or no effect on IK[Ca], whereas nifedipine and S-(-)-BAY K 8644 inhibited and potentiated IK[Ca], respectively. These results indicate that IK[Ca] channels are preferentially coupled to L-type, but not to N-type, Ca2+ channels on chicken ciliary ganglion neurons. Chicken sympathetic neurons also express dihydropyridine-sensitive and omega-conotoxin-sensitive components of ICa. However, in those cells, application of omega-conotoxin caused a 40-60% reduction in IK[Ca], whereas nifedipine reduced IK[Ca] but only in a subpopulation of cells. Therefore, IK[Ca] in sympathetic neurons is either coupled to N-type Ca2+ channels or is not selectively coupled to a single Ca(2+)-channel subtype. The preferential coupling of IK[Ca] channels with distinct ICa subtypes may be part of a mechanism to allow for selective modulation of neurotransmitter release. Preferential coupling may also be important for the differentiation and development of vertebrate neurons.

L-type Ca2+ channel is involved in the regulation of spontaneous transmitter release at developing neuromuscular synapses

Involvement of an L-type Ca2+ channel in the regulation of spontaneous transmitter release was studied in Xenopus nerve-muscle cultures. The frequency of spontaneous synaptic currents, which reflects impulse-independent acetylcholine release from the nerve terminals, showed a marked increase in high-K+ medium or after treatment with a phorbol ester, 12-O-tetradecanoyl-phorbol 13-acetate, a drug that activates protein kinase C and depolarizes the presynaptic neuron. The potentiation effect of high K+ and 12-O-tetradecanoyl-phorbol 13-acetate requires Ca2+ influx through the L-type Ca2+ channel in the plasma membrane, since it was significantly reduced by the presence of nifedipine, verapamil or diltiazem and enhanced by Bay K 8644, an L-type Ca2+ channel agonist. It was shown recently that adenosine 5'-triphosphate markedly potentiates the spontaneous acetylcholine release at these synapses through the binding of P2-purinoceptors and the activation of protein kinase C. We found in the present study that potentiation effects of adenosine 5'-triphosphate are inhibited by L-type Ca2+ channel blockers, suggesting that the L-type Ca2+ channel is responsible for the positive regulation of spontaneous acetylcholine secretion by adenosine 5'-triphosphate at the developing neuromuscular synapses. Our data suggest that modulation of the L-type Ca2+ channel in embryonic motor nerve terminals is important for the regulation of spontaneous transmitter release.