Relative potencies of metal ions on transmitter release at mouse motor nerve terminals

Continuous infusion of manganese improves contrast and reduces side effects in manganese-enhanced magnetic resonance imaging studies

The ability to administer systemically high doses of manganese as contrast agent while circumventing its toxicity is of particular interest for exploratory MRI studies of the brain. Administering low doses either repeatedly or continuously over time has been shown to enable the acquisition of satisfactory MRI images of the mouse brain without apparent side effects. Here we have systematically compared the obtained MRI contrast and recorded potential systemic side effects such as stress response and muscle strength impairment in relation to the achieved contrast. We show in mice that administering MnCl₂ via osmotic infusion pumps allows for a side-effect free delivery of a high cumulative dose of manganese chloride (480mg/kg bodyweight in 8 days). High contrast in MRI was achieved while we did not observe the weight loss or distress seen in other studies where mice received manganese via fractionated intraperitoneal injections of lower doses of manganese. As the normal daily conduct of the mice was not affected, this new manganese delivery method might be of particular use to study brain activity over several days. This may facilitate the phenotyping of new transgenic mouse models, the study of chronic disease models and the monitoring of changes in brain activity in long-term behavioral studies.

Purine P2Y receptors in ATP-mediated regulation of non-quantal acetylcholine release from motor nerve endings of rat diaphragm

We established the effect of ATP, which is released together with acetylcholine (ACh), on the non-quantal ACh release (NQR) in rat diaphragm endplates and checked what kind of purine receptors are involved. NQR was estimated by the amplitude of endplate hyperpolarization (the H-effect) following the blockade of postsynaptic nicotinic receptors and cholinesterase. 100 μM ATP reduced the H-effect to 66% of the control. The action of ATP remained unchanged after the inhibition of ionotropic P2X receptors by Evans blue and PPADS, but disappeared after the application of the broad spectrum P2 receptor antagonist suramin, metabotropic P2Y receptor blocker reactive blue 2 and U73122, an inhibitor of phospholipase C. P2Y-mediated regulation is not coupled to presynaptic voltage-dependent Ca(2+) channels. During the simultaneous application of ATP and glutamate (which is another ACh cotransmitter reducing non-quantal release), the additive depressant effect led to a disappearance of the H-effect. This can be explained by the independence of the action of ATP and glutamate. Unlike the effects of purines on the spontaneous quantal secretion of ACh, its non-quantal release is regulated via P2Y receptors coupled to G(q/11) and PLC. ATP thus regulates the neuromuscular synapse by two different pathways.

L-type Ca2+ channels in the enteric nervous system mediate oscillatory Cl- secretion in guinea pig colon

The enteric nervous system regulates epithelial ion and fluid secretion. Our previous study has shown that the low (0.2-1 mM) concentrations of Ba2+, a K+ channel inhibitor, evoke Ca2+-dependent oscillatory Cl- secretion via activation of submucosal cholinergic neurons in guinea pig distal colon. However, it is still unclear which types of Ca2+ channels are involved in the oscillation at the neuroepithelial junction. We investigated the inhibitory effects of organic and inorganic Ca2+ channel antagonists on the short circuit current (I(sc)) of colonic epithelia (mucosa-submucosa sheets) mounted in Ussing chambers. The amplitude (412 +/- 37 microA cm(-2)) and frequency (2.6 +/- 0.1 cycles min(-1)) of the Ba2+-induced I(sc) in normal (1.8 mM) Ca2+ solution (n = 26) significantly decreased by 37.6% and 38.5%, respectively, in the low (0.1 mM) Ca2+ solution (n = 14). The I(sc) amplitude was reversibly inhibited by either verapamil (an L-type Ca2+ channel antagonist) or divalent cations (Cd2+, Mn2+, Ni2+) in a concentration-dependent manner. The concentration of verapamil for half-maximum inhibition (IC50) was 4 and 2 microM in normal and low Ca2+ solution, respectively. The relative blocking potencies of metal ions were Cd2+ > Mn2+, Ni2+ in normal Ca2+ solution. In contrast, the frequency of I(sc) was unchanged over the range of concentrations of the Ca2+ channel antagonists used. Our results show that the oscillatory I(sc) evoked by Ba2+ involves L-type voltage-gated Ca2+ channels. We conclude that L-type Ca2+ channels play a key role in the oscillation at the neuroepithelial junctions of guinea pig colon.

Ventilatory effects of gap junction blockade in the NTS in awake rats

We tested the hypothesis that focally perfusing carbenoxolone, which blocks gap junctions, into the nucleus tractus solitarius (NTS) would reduce the ventilatory response to CO(2). We measured minute ventilation (V(E)), tidal volume (V(T)) and respiratory frequency (F(R)) responses to increasing concentrations of inspired CO(2) (F(I)(CO(2) = 0-8%) in rats during wakefulness. Focal perfusion of acetazolamide (10 microM) into the NTS increased V(E) and V(T) during exposure to room air. Carbenoxolone (300 microM) decreased the V(E) and V(T) response to CO(2) when perfused within, but not adjacent to the NTS in animals less than 10 weeks of age. F(R) was decreased at F(I)(CO(2) = 4% in these animals. Carbenoxolone did not decrease V(E), V(T) or F(R) in animals 10 weeks of age and older. Carbenoxolone did not decrease V(E), V(T) or F(R) when focally perfused outside the NTS at any age tested. The NTS is an important CO(2) chemosensory site at all ages, and gap junctions amplify the ventilatory response to CO(2) in animals less than 10 weeks of age.

Adenosine decreases both presynaptic calcium currents and neurotransmitter release at the mouse neuromuscular junction

A controversy currently exists as to the mechanism of action by which adenosine, an endogenous mediator of neurotransmitter depression, reduces the evoked release of the neurotransmitter acetylcholine (ACh) at the skeletal neuromuscular junction. Specifically, it is uncertain whether adenosine inhibits ACh release from mammalian motor nerve endings by reducing Ca(2+) calcium entry through voltage-gated calcium channels or, as is the case at amphibian motor nerve endings, by an effect downstream of Ca(2+) entry. In an attempt to address this controversy, the effects of adenosine on membrane ionic currents and neurotransmitter release were studied at neuromuscular junctions in adult mouse phrenic nerve hemidiaphragm preparations. In wild-type mice, adenosine (500 microm-1 mm) reduced prejunctional Ca(2+) currents simultaneously with a reduction in evoked ACh release. In Rab3A knockout mice, which have been shown to have an increased sensitivity to adenosine, the simultaneous reduction in Ca(2+) currents and ACh secretion occurred at significantly lower adenosine concentrations (< or = 50 microM). Measurements of nerve terminal Na(+) and K(+) currents made simultaneously with evoked ACh release demonstrated that the decreases in Ca(2+) currents were not attributable to changes in cation entry through voltage-gated Na(+) or K(+) channels. Furthermore, no effects of adenosine on residual ionic currents were observed when P/Q-type calcium channels were blocked by Cd(2+) or omega-agatoxin-IVA. The results demonstrate that inhibition of evoked neurotransmitter secretion by adenosine is associated with a reduction in Ca(2+) calcium entry through voltage-gated P/Q Ca(2+) channels at the mouse neuromuscular junction. Whilst it may be that adenosine inhibits ACh release by different mechanisms at amphibia and mammalian neuromuscular junctions, it is also possible that the secretory apparatus is more intimately coupled to the Ca(2+) channels in the mouse such that an effect on the secretory machinery is reflected as changes in Ca(2+) currents.

Sensitivity of the neuronal network biosensor to environmental threats

It is widely acknowledged that there is a critical need for broad-spectrum environmental threat detection. While cells/tissue-based biosensors have been discussed for many years as a means of meeting this critical need, these kinds of systems have met with logistic concerns, in particular with regard to stability. Our group has been working with cultured neuronal networks, which have the capacity to respond to a wide range of neuroactive compounds and are sufficiently robust to be shipped to end users. The basis of operation involves extracellular recording using thin-film microelectrode arrays where spontaneous bioelectrical activity, that is, spike firing, can be monitored in a noninvasive manner conducive for potentially long-term measurements. This work describes the current status of our efforts for the fabrication of a portable biosensor that incorporates cultured neuronal networks grown over standardized microelectrode arrays. Based on our protocol for aqueous phase sample introduction under constant flow conditions, minimal variation in mean spike rate is observed, consistent with temporal stability, such that changes of > 10% are readily distinguished. To demonstrate the capability of this system, changes are reported in mean spike rate and network synchronization resulting from exposure to different model environmental threats, cadmium and strychnine. The sensitivity of this assay approach and implications of the experimental findings for environmental threat detection are discussed.

Differential expression of α1 and β subunits of voltage dependent Ca2+ channel at the neuromuscular junction of normal and p/q Ca2+ channel knockout mouse

Voltage-dependent calcium channels (VDCC) have a key role in neuronal function transforming the voltage signals into intracellular calcium signals. They are composed of the pore-forming alpha(1) and the regulatory alpha(2)delta, gamma and beta subunits. Molecular and functional studies have revealed which alpha(1) subunit gene product is the molecular constituent of each class of native calcium channel (L, N, P/Q, R and T type). Electrophysiological and immunocytochemical studies have suggested that at adult mouse motor nerve terminal (MNT) only P/Q type channels, formed by alpha(1A) subunit, mediate evoked transmitter release. The generation of alpha(1A)-null mutant mice offers an opportunity to study the expression and localization of calcium channels at a synapse with complete loss of P/Q calcium channel. We have investigated the expression and localization of VDCCs alpha(1) and beta subunits at the wild type (WT) and knockout (KO) mouse neuromuscular junction (NMJ) using fluorescence immunocytochemistry. The alpha(1A) subunit was observed only at WT NMJ and was absent at denervated muscles and at KO NMJ. The subunits alpha(1B), alpha(1D) and alpha(1E) were also present at WT NMJ and they were over- expressed at KO NMJ suggesting a compensatory expression due to the lack of the alpha(1A). On the other hand, the beta(1b), beta(2a) and beta(4) were present at the same levels in both genotypes. The presence of other types of VDCC at WT NMJ indicate that they may play other roles in the signaling process which have not been elucidated and also shows that other types of VDCC are able to substitute the alpha(1A) subunit, P/Q channel under certain pathological conditions.

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.

Involvement of Hg2+-sensitive sulfhydryl groups in regulating noradrenaline release induced by S-nitrosocysteine in rat brain slices

Nitric oxide has been shown to regulate neurotransmitter release. Previously, we reported that S-nitrosothiols such as S-nitrosocysteine (SNC) stimulate noradrenaline (NA) release in rat hippocampus in vivo and in vitro. To examine the role of sulfhydryl groups in SNC-induced NA release, the effects of metal ions such as Hg2+ and N-ethylmaleimide (NEM, a sulfhydryl alkylating agent) on [3H]NA release from labeled rat brain slices (hippocampus and cerebral cortex) were studied and compared with the effects of SNC. The addition of 200 microM HgCl2, but not Pb2+, Zn2+, or Cd2+, stimulated [3H]NA release from both types of slices in the presence of extracellular CaCl2. p-Chloromercuribenzoic acid (p-CMBA) also stimulated [3H]NA release. NEM stimulated [3H]NA release from both types of slices in the presence and absence of extracellular CaCl2. The effect of 200 microM NEM was enhanced, but the effect of 200 microM SNC was inhibited by co-addition of 200 microM p-CMBA in the absence of extracellular CaCl2. The concentration-response curve of SNC shifted to the right after co-addition of 200 microM p-CMBA or 100 microM HgCl2, although the effect of 200 microM NEM was additive to the effect of SNC. These findings demonstrate that SNC acts as a sulfhydryl agent on proteins that regulate NA release, and that SNC may share the same sulfhydryl groups with Hg compounds. The effect of T-588 ¿(R)-(-)-(benzo[b]thiophen-5-yl)-2-[2-(N,N-diethylamino)ethoxy]eth anol hydrochloride¿, a novel cognitive enhancer and a stimulator of NA release, was compared with the effects of sulfhydryl reagents.

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.

Differential localization of voltage-dependent calcium channel alpha1 subunits at the human and rat neuromuscular junction

Neurotransmitter release is regulated by voltage-dependent calcium channels (VDCCs) at synapses throughout the nervous system. At the neuromuscular junction (NMJ) electrophysiological and pharmacological studies have identified a major role for P- and/or Q-type VDCCs in controlling acetylcholine release from the nerve terminal. Additional studies have suggested that N-type channels may be involved in neuromuscular transmission. VDCCs consist of pore-forming alpha1 and regulatory beta subunits. In this report, using fluorescence immunocytochemistry, we provide evidence that immunoreactivity to alpha1A, alpha1B, and alpha1E subunits is present at both rat and human adult NMJs. Using control and denervated rat preparations, we have been able to establish that the subunit thought to correspond to P/Q-type channels, alpha1A, is localized presynaptically in discrete puncta that may represent motor nerve terminals. We also demonstrate for the first time that alpha1A and alpha1B (which corresponds to N-type channels) may be localized in axon-associated Schwann cells and, further, that the alpha1B subunit may be present in perisynaptic Schwann cells. In addition, the alpha1E subunit (which may correspond to R/T-type channels) seems to be localized postsynaptically in the muscle fiber membrane and concentrated at the NMJ. The possibility that all three VDCCs at the NMJ are potential targets for circulating autoantibodies in amyotrophic lateral sclerosis is discussed.

Differential localization of voltage-dependent calcium channel alpha1 subunits at the human and rat neuromuscular junction

Neurotransmitter release is regulated by voltage-dependent calcium channels (VDCCs) at synapses throughout the nervous system. At the neuromuscular junction (NMJ) electrophysiological and pharmacological studies have identified a major role for P- and/or Q-type VDCCs in controlling acetylcholine release from the nerve terminal. Additional studies have suggested that N-type channels may be involved in neuromuscular transmission. VDCCs consist of pore-forming alpha1 and regulatory beta subunits. In this report, using fluorescence immunocytochemistry, we provide evidence that immunoreactivity to alpha1A, alpha1B, and alpha1E subunits is present at both rat and human adult NMJs. Using control and denervated rat preparations, we have been able to establish that the subunit thought to correspond to P/Q-type channels, alpha1A, is localized presynaptically in discrete puncta that may represent motor nerve terminals. We also demonstrate for the first time that alpha1A and alpha1B (which corresponds to N-type channels) may be localized in axon-associated Schwann cells and, further, that the alpha1B subunit may be present in perisynaptic Schwann cells. In addition, the alpha1E subunit (which may correspond to R/T-type channels) seems to be localized postsynaptically in the muscle fiber membrane and concentrated at the NMJ. The possibility that all three VDCCs at the NMJ are potential targets for circulating autoantibodies in amyotrophic lateral sclerosis is discussed.