Electrically-evoked neurotransmitter release in relation to alcoholism

The present study used electrical stimulation to distinguish the vesicular from the cytoplasmic component of transmitter release in human autopsy synaptosomes. We demonstrate that the present electrical stimulation paradigm can elicit successive release pulses from synaptosome preparation.

Differential involvement of N-type calcium channels in transmitter release from vasoconstrictor and vasodilator neurons

1 The effects of calcium channel blockers on co-transmission from different populations of autonomic vasomotor neurons were studied on isolated segments of uterine artery and vena cava from guinea-pigs. 2 Sympathetic, noradrenergic contractions of the uterine artery (produced by 200 pulses at 1 or 10 Hz; 600 pulses at 20 Hz) were abolished by the N-type calcium channel blocker omega-conotoxin (CTX) GVIA at 1-10 nM. 3 Biphasic sympathetic contractions of the vena cava (600 pulses at 20 Hz) mediated by noradrenaline and neuropeptide Y were abolished by 10 nM CTX GVIA. 4 Neurogenic relaxations of the uterine artery (200 pulses at 10 Hz) mediated by neuronal nitric oxide and neuropeptides were reduced < 50% by CTX GVIA 10-100 nM. 5 Capsaicin (3 muM) did not affect the CTX GVIA-sensitive or CTX GVIA-resistant neurogenic relaxations of the uterine artery. 6 The novel N-type blocker CTX CVID (100-300 nM), P/Q-type blockers agatoxin IVA (10-100 nM) or CTX CVIB (100 nM), the L-type blocker nifedipine (10 muM) or the 'R-type' blocker SNX-482 (100 nM), all failed to reduce CTX GVIA-resistant relaxations. The T-type channel blocker NiCl2 (100-300 muM) reduced but did not abolish the remaining neurogenic dilations. 7 Release of different neurotransmitters from the same autonomic vasomotor axon depends on similar subtypes of calcium channels. N-type channels are responsible for transmitter release from vasoconstrictor neurons innervating a muscular artery and capacitance vein, but only partly mediate release of nitric oxide and neuropeptides from pelvic vasodilator neurons.

High throughput analysis of endogenous glutamate release using a fluorescence plate reader

Recent discoveries of different modes of exocytosis and a plethora of molecules involved in neurotransmitter release has resulted in demand for more rapid and efficient methods for monitoring endogenous glutamate release from various tissue sources. In this article, we describe a high throughput microplate version of the enzyme-linked fluorescence detection method for the measurement of released glutamate, which utilises glutamate dehydrogenase, and the reduction of NADP to NADPH. Previous versions of this method rely upon cuvette-based fluorimeters for detection that are limited by large sample volumes and small numbers of samples that can be measured simultaneously. Comparison between the two methods shows that the microplate assay has comparable performance to the cuvette-based assay but has the capacity to analyse many times more samples in a given run. This increased capacity provides improved experimental design opportunities, higher experimental throughput and better comparison between experimental conditions. (c) 2005 Elsevier B.V. All rights reserved.

Calcium channels controlling acetylcholine release from preganglionic nerve terminals in rat autonomic ganglia

Little is known about the nature of the calcium channels controlling neurotransmitter release from preganglionic parasympathetic nerve fibres. In the present study, the effects of selective calcium channel antagonists and amiloride were investigated on ganglionic neurotransmission. Conventional intracellular recording and focal extracellular recording techniques were used in rat submandibular and pelvic ganglia, respectively. Excitatory postsynaptic potentials and excitatory postsynaptic currents preceded by nerve terminal impulses were recorded as a measure of acetylcholine release from parasympathetic and sympathetic preganglionic fibres following nerve stimulation. The calcium channel antagonists omega-conotoxin GVIA (N type), nifedipine and nimodipine (L type), omega-conotoxin MVIIC and omega-agatoxin IVA (P/Q type), and Ni2+ (R type) had no functional inhibitory effects on synaptic transmission in both submandibular and pelvic ganglia. The potassium-sparing diuretic, amiloride, and its analogue, dimethyl amiloride, produced a reversible and concentration-dependent inhibition of excitatory postsynaptic potential amplitude in the rat submandibular ganglion. The amplitude and frequency of spontaneous excitatory postsynaptic potentials and the sensitivity of the postsynaptic membrane to acetylcholine were unaffected by amiloride. In the rat pelvic ganglion, amiloride produced a concentration-dependent inhibition of excitatory postsynaptic currents without causing any detectable effects on the amplitude or configuration of the nerve terminal impulse. These results indicate that neurotransmitter release from preganglionic parasympathetic and sympathetic nerve terminals is resistant to inhibition by specific calcium channel antagonists of N-, L-, P/Q- and R-type calcium channels. Amiloride acts presynaptically to inhibit evoked transmitter release, but does not prevent action potential propagation in the nerve terminals, suggesting that amiloride may block the pharmacologically distinct calcium channel type(s) on rat preganglionic nerve terminals. (C) 1999 IBRO. Published by Elsevier Science Ltd.

Morphine-3-glucuronide's neuro-excitatory effects are mediated via indirect activation of N-methyl-D-aspartic acid receptors: Mechanistic studies in embryonic cultured hippocampal neurones

Indirect evidence indicates that morphine-3-glucuronide (M3G) may contribute significantly to the neuro-excitatory side effects (myoclonus and allodynia) of large-dose systemic morphine. To gain insight into the mechanism underlying M3G' s excitatory behaviors, We used fluo-3 fluorescence digital imaging techniques to assess the acute effects of M3G (5-500 muM) on the cytosolic calcium concentration ([Ca2+](CYT)) in cultured embryonic hippocampal neurones. Acute (3 min) exposure of neurones to M3G evoked [Ca2+](CYT) transients that were typically either (a) transient oscillatory responses characterized by a rapid increase in [Ca2+](CYT) oscillation amplitude that was sustained for at least similar to30 s or (b) a sustained increase in [Ca2+](CYT) that slowly recovered to baseline. Naloxone-pretreatment decreased the proportion of M3G-responsive neurones by 10%-25%, implicating a predominantly non-opioidergic mechanism. Although the naloxone-insensitive M3G-induced increases in [Ca2+](CYT) were completely blocked by N-methyl-D-aspartic acid (NMDA) antagonists and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) (alphaamino-3-hydroxy-5-methyl-4-isoxazolepropiordc acid/ kainate antagonist), CNQX did not block the large increase in [Ca2+](CYT) evoked by NMDA (as expected), confirming that N13G indirectly activates the NMDA receptor. Additionally, tetrodotoxin (Na+ channel blocker), baclofen (gamma-aminobutyric acid, agonist), MVIIC (P/Q-type calcium channel blocker), and nifedipine (L-type calcium channel blocker) all abolished M3G-induced increases in [Ca2+](CYT), suggesting that M3G may produce its neuro-excitatory effects by modulating neurotransmitter release. However, additional characterization is required.

Phosphatidylinositol 3-kinase C2 alpha is essential for ATP-dependent priming of neurosecretory granule exocytosis

Neurotransmitter release and hormonal secretion are highly regulated processes culminating in the calcium-dependent fusion of secretory vesicles with the plasma membrane. Here, we have identified a role for phosphatidylinositol 3-kinase C2 alpha (PI3K-C2 alpha) and its main catalytic product, PtdIns3P, in regulated exocytosis. In neuroendocrine cells, PI3K-C2 alpha is present on a subpopulation of mature secretory granules. Impairment of PI3K-C2 alpha function specifically inhibits the ATP-dependent priming phase of exocytosis. Overexpression of wild-type PI3K-C2 alpha enhanced secretion, whereas transfection of PC12 cells with a catalytically inactive PI3K-C2 alpha mutant or a 2xFYVE domain sequestering PtdIns3P abolished secretion. Based on these results, we propose that production of PtdIns3P by PI3K-C2 alpha is required for acquisition of fusion competence in neurosecretion.

Ciguatoxins: Cyclic polyether modulators of voltage-gated Iion channel function

Ciguatoxins are cyclic polyether toxins, derived from marine dinoflagellates, which are responsible for the symptoms of ciguatera poisoning. Ingestion of tropical and subtropical fin fish contaminated by ciguatoxins results in an illness characterised by neurological, cardiovascular and gastrointestinal disorders. The pharmacology of ciguatoxins is characterised by their ability to cause persistent activation of voltage-gated sodium channels, to increase neuronal excitability and neurotransmitter release, to impair synaptic vesicle recycling, and to cause cell swelling. It is these effects, in combination with an action to block voltage-gated potassium channels at high doses, which are believed to underlie the complex of symptoms associated with ciguatera. This review examines the sources, structures and pharmacology of ciguatoxins. In particular, attention is placed on their cellular modes of actions to modulate voltage-gated ion channels and other Na+-dependent mechanisms in numerous cell types and to current approaches for detection and treatment of ciguatera.

Phosphoinositide regulation of neuroexocytosis: adding to the complexity

Exocytosis of neurotransmitter containing vesicles supports neuronal communication. The importance of molecular interactions involving specific lipids has become progressively more evident and the lipid composition of both the synaptic vesicle and the pre-synaptic plasma membrane at the active zone has significant functional consequences for neurotransmitter release. Several classes of lipids have been implicated in exocytosis including polyunsaturated fatty acids and phosphoinositides. This minireview will focus on recent developments regarding the role of phosphoinositides in neurosecretion.

Selective loss of synaptic proteins in Alzheimer's disease: Evidence for an increased severity with APOE epsilon 4

A pathological feature of Alzheimer's disease (AD) is an area-specific neuronal loss that may be caused by excitotoxicity-related synaptic dysfunction. Relative expression levels of synaptopbysin, dynamin I, complexins I and II, N-cadherin, and alpha CaMKII were analysed in human brain tissue from AD cases and controls in hippocampus, and inferior temporal and occipital cortices. Synaptophysin and dynamin I are presynaptic terminal proteins not specific to any neurotransmitter system whereas complexin II, N-cadherin, and alpha CaMKII are specific for excitatory synapses. Complexin I is a presynaptic protein localised to inhibitory synapses. There were no significant differences in synaptophysin, dynamin I, N-cadherin, or alpha CaMKII protein levels between AD cases and controls. The complexin proteins were both markedly lower in AD cases than in controls (P < 0.01). Cases were also categorised by APOE genotype. Averaged across areas there was a 36% lowering of presynaptic proteins in AD cases carrying at least one epsilon 4 allele compared with in AD cases lacking the epsilon 4 allele. We infer that synaptic protein level is not indicative of neuronal loss, but the synaptic dysfunction may result from the marked relative loss of the complexins in AD, and lower levels of presynaptic proteins in AD cases with the APOE epsilon 4 allele. (c) 2006 Elsevier Ltd. All rights reserved.