Effect of various depolarizing agents on endogenous amino acid neurotransmitter release in rat cortical neurons in culture

Neuronal Trans-differentiation by Transcription Factors Ascl1 and Nurr1: Induction of a Dopaminergic Neurotransmitter Phenotype in Cortical GABAergic Neurons

Neurons with a desired neurotransmitter phenotype can be differentiated from induced pluripotent stem cells or from somatic cells only through tedious protocols with relatively low yield. Readily available cortical neurons isolated from embryonic rat brain, which have already undergone a complete neuronal differentiation process, might serve as alternative template source. These cultures consist of 85% glutamatergic and 15% GABAergic neurons, and we attempted to trans-differentiate them into dopaminergic neurons. Transcription factors Nurr1, Lmx1A and Pitx3, essential determinants of a dopaminergic cell fate during CNS development, were not sufficient to induce tyrosine hydroxylase expression in a significant number of cells. Combining Nurr1 with the generic neuronal differentiator and re-programming factor Ascl1, however, resulted in generation of neurons which express dopaminergic markers TH, AADC, VMAT2 and DAT. Only neurons of GABAergic phenotype could be trans-differentiated towards a dopaminergic neurotransmitter phenotype, while for glutamatergic neurons, this process proved to be neurotoxic. Intriguingly, GABAergic neurons isolated from embryonal midbrain could not be trans-differentiated into dopaminergic neurons by Ascl1 and Nurr1. Thus, in principle, post-mitotic embryonal neurons can serve as templates for neurons with a desired neurotransmitter phenotype. However, neurotransmitter phenotype plasticity critically depends on the differentiation history of the template neurons, which can result in relatively low yields of dopaminergic neurons.

Neurotoxicity screening of (illicit) drugs using novel methods for analysis of microelectrode array (MEA) recordings

Annual prevalence of the use of common illicit drugs and new psychoactive substances (NPS) is high, despite the often limited knowledge on the health risks of these substances. Recently, cortical cultures grown on multi-well microelectrode arrays (mwMEAs) have been used for neurotoxicity screening of chemicals, pharmaceuticals, and toxins with a high sensitivity and specificity. However, the use of mwMEAs to investigate the effects of illicit drugs on neuronal activity is largely unexplored. We therefore first characterised the cortical cultures using immunocytochemistry and show the presence of astrocytes, glutamatergic and GABAergic neurons. Neuronal activity is concentration-dependently affected following exposure to six neurotransmitters (glutamate, GABA, serotonin, dopamine, acetylcholine and nicotine). Most neurotransmitters inhibit neuronal activity, although glutamate and acetylcholine transiently increase activity at specific concentrations. These transient effects are not detected when activity is determined during the entire 30min exposure window, potentially resulting in false-negative results. As expected, exposure to the GABAA-receptor antagonist bicuculline increases neuronal activity. Exposure to a positive allosteric modulator of the GABAA-receptor (diazepam) or to glutamate receptor antagonists (CNQX and MK-801) reduces neuronal activity. Further, we demonstrate that exposure to common drugs (3,4-methylenedioxymethamphetamine (MDMA) and amphetamine) and NPS (1-(3-chlorophenyl)piperazine (mCPP), 4-fluoroamphetamine (4-FA) and methoxetamine (MXE)) decreases neuronal activity. MXE most potently inhibits neuronal activity with an IC50 of 0.5μM, whereas 4-FA is least potent with an IC50 of 113μM. Our data demonstrate the importance of analysing neuronal activity within different time windows during exposure to prevent false-negative results. We also show that cortical cultures grown on mwMEAs can successfully be applied to investigate the effects of different (illicit) drugs on neuronal activity. Compared to investigating multiple single endpoints for neurotoxicity or neuromodulation, such as receptor activation or calcium channel function, mwMEAs can provide information on integrated aspects of drug-induced neurotoxicity more rapidly. Therefore, this approach could contribute to a faster insight in possible health risks and shorten the regulation process.

Endogenous cholinergic tone modulates spontaneous network level neuronal activity in primary cortical cultures grown on multi-electrode arrays

BACKGROUND

Cortical cultures grown long-term on multi-electrode arrays (MEAs) are frequently and extensively used as models of cortical networks in studies of neuronal firing activity, neuropharmacology, toxicology and mechanisms underlying synaptic plasticity. However, in contrast to the predominantly asynchronous neuronal firing activity exhibited by intact cortex, electrophysiological activity of mature cortical cultures is dominated by spontaneous epileptiform-like global burst events which hinders their effective use in network-level studies, particularly for neurally-controlled animat ('artificial animal') applications. Thus, the identification of culture features that can be exploited to produce neuronal activity more representative of that seen in vivo could increase the utility and relevance of studies that employ these preparations. Acetylcholine has a recognised neuromodulatory role affecting excitability, rhythmicity, plasticity and information flow in vivo although its endogenous production by cortical cultures and subsequent functional influence upon neuronal excitability remains unknown.

RESULTS

Consequently, using MEA electrophysiological recording supported by immunohistochemical and RT-qPCR methods, we demonstrate for the first time, the presence of intrinsic cholinergic neurons and significant, endogenous cholinergic tone in cortical cultures with a characterisation of the muscarinic and nicotinic components that underlie modulation of spontaneous neuronal activity. We found that tonic muscarinic ACh receptor (mAChR) activation affects global excitability and burst event regularity in a culture age-dependent manner whilst, in contrast, tonic nicotinic ACh receptor (nAChR) activation can modulate burst duration and the proportion of spikes occurring within bursts in a spatio-temporal fashion.

CONCLUSIONS

We suggest that the presence of significant endogenous cholinergic tone in cortical cultures and the comparability of its modulatory effects to those seen in intact brain tissues support emerging, exploitable commonalities between in vivo and in vitro preparations. We conclude that experimental manipulation of endogenous cholinergic tone could offer a novel opportunity to improve the use of cortical cultures for studies of network-level mechanisms in a manner that remains largely consistent with its functional role.

Impact of STZ-induced hyperglycemia and insulin-induced hypoglycemia in plasma amino acids and cortical synaptosomal neurotransmitters

In this work, we evaluated the effects of streptozotocin (STZ)-induced hyperglycemia and an acute episode of insulin-induced hypoglycemia in plasma amino acids and cortical neurotransmitters. For that purpose, we used citrate (vehicle)-treated Wistar rats, STZ-treated rats [i.p., 50 mg/kg body weight], and STZ-treated rats injected with insulin [s.c., dose adjusted with blood glucose levels] 1 h prior to sacrifice to induce an acute episode of hypoglycemia. Plasma was collected for determination of amino acids levels. In addition, cortical synaptosomal preparations were obtained and the total levels of neurotransmitters, levels of aspartate, glutamate, taurine, and GABA released by the action of KCl, iodoacetic acid (IAA), ouabain, and veratridine, membrane potential and ATP levels were evaluated. Compared with control rats, plasma from hypoglycemic rats presented increased levels of aspartate, glutamate, glutamine, and taurine whereas GABA levels were decreased in STZ and hypoglycemic rats. Similarly, glutamate and taurine levels were increased in hypoglycemic synaptosomes while GABA decreased in hypoglycemic and STZ-diabetic synaptosomes. The depolarizing agent KCl promoted an increase in aspartate, glutamate, and taurine release from hypoglycemic synaptosomes. The highest release of neurotransmitters occurred in the presence of veratridine and ouabain, two other depolarizing agents, in all groups of experimental animals. However, a higher release of glutamate was observed in the diabetic and hypoglycemic synaptosomes. No alterations were observed in synaptosomal membrane potential and ATP levels. These results show that in the presence of a metabolic insult a higher release of excitatory amino acids occurs, which may underlay the neuronal injury observed in type 1 diabetic patients under insulin therapy.

Aspartate release and signalling in the hippocampus

The Ca(2+)-dependent release of aspartate from hippocampal preparations was first reported 35 years ago, but the functional significance of this process remains uncertain. Aspartate satisfies all the criteria normally required for identification of a CNS transmitter. It is synthesized in nerve terminals, is accumulated and stored in synaptic vesicles, is released by exocytosis upon nerve terminal depolarization, and activates postsynaptic NMDA receptors. Aspartate may be employed as a neuropeptide-like co-transmitter by pathways that release either glutamate or GABA as their principal transmitter. Aspartate mechanisms include vesicular transport by sialin, vesicular content sensitive to glucose concentration, release mainly outside the presynaptic active zones, and selective activation of extrasynaptic NR1-NR2B NMDA receptors. Possible neurobiological functions of aspartate in immature neurons include activation of cAMP-dependent gene transcription and in mature neurons inhibition of CREB function, reduced BDNF expression, and induction of excitotoxic neuronal death. Recent findings suggest new experimental approaches toward resolving the functional significance of aspartate release.

Postsynaptic response to stimulation of the Schaffer collaterals with properties similar to those of synaptosomal aspartate release

Aspartate satisfies all the criteria normally required for identification of a CNS neurotransmitter. Nevertheless, little electrophysiological evidence supports the existence of aspartate transmission. In studies with rat hippocampal synaptosomes, chemically evoked aspartate release differed from glutamate release in its relative sensitivity to increased Ca(2+) concentration outside the presynaptic active zones, inefficient coupling to P/Q-type Ca(2+) channels, sensitivity to KB-R7943, and resistance to native Clostridial toxins. We took advantage of these differences to search for a potential aspartate-mediated response at Schaffer collateral synapses in organotypic hippocampal slice cultures. The slice cultures were pretreated with botulinum neurotoxin C (BoNT/C) to eliminate most of the glutamate release so that an expectedly smaller aspartate-like component of the compound EPSC could be detected by whole cell patch clamp recording. In control cultures, NMDA receptor activation accounted for only 18% of the evoked EPSC and an NR2B-selective antagonist reduced the NMDA receptor-mediated component by only 20%. Block of P/Q-type Ca(2+) channels essentially eliminated the response and 0.1 muM KB-R7943 had no significant effect. In BoNT/C-pretreated cultures, however, NMDA receptor activation accounted for 77% of the evoked EPSC and an NR2B-selective antagonist reduced the NMDA receptor-mediated component by 57%. Block of P/Q-type Ca(2+) channels reduced the response by only 28%, but 0.1 muM KB-R7943 reduced it by 45%. These results suggest that part of the Schaffer collateral synaptic response has pharmacological properties similar to those of synaptosomal aspartate release and may therefore be mediated at least partly by released aspartate.

Reduced aspartate release from rat hippocampal synaptosomes loaded with Clostridial toxin light chain by electroporation: evidence for an exocytotic mechanism

Aspartate can be released from certain hippocampal pathways along with glutamate or GABA. Although aspartate immunoreactivity has been localized to synaptic vesicles and aspartate release is Ca(2+)-dependent, there has been no clear evidence favoring an exocytotic mechanism. In particular, pretreatment with Clostridial toxins has not consistently inhibited aspartate release, even when release of glutamate from the same tissue samples was markedly inhibited. To address this issue directly, rat hippocampal synaptosomes were permeabilized transiently by electroporation in the presence of active or inactivated Clostridial toxin light chains. Loading rat hippocampal synaptosomes with the active light chain of tetanus toxin or of botulinum neurotoxins A, B or C reduced the K(+)-evoked release of aspartate at least as much as that of glutamate. These results confirm that aspartate is released by exocytosis in rat hippocampus.

Molecular mechanisms of glutamate release by bovine chromaffin cells in primary culture

Previous work indicated that glutamate could be involved in the regulation of catecholamine secretion in bovine chromaffin cells. Thus, the question arises on the source of this putative regulatory glutamate. In this work we have examined the possibility that glutamate could be released from chromaffin cells. Data from this study indicate that chromaffin cells are able to release glutamate when they are stimulated by different depolarising agents such as 60 mM KCl, 1 mM 4-aminopyridine or 50 microM veratridine. The amount of glutamate released by these compounds was 0.32 nmol/10(6) cells (9.24% of cellular glutamate content), 0.275 (7.86%) and 0.158 (4.52%) for KCl, 4-AP and veratridine stimulation, respectively. All these catecholamine-secretagogues induced glutamate secretion by two mechanisms: 1) a Ca(2+)-dependent, probably exocytotic, mechanism and 2) a Ca(2+)-independent mechanism mediated by reversion of the electrogenic glutamate transporter. Analysis of Ca(2+)-dependent and independent releases for different compounds carried out by several experimental approaches, indicate that Ca(2+)-dependent release was the predominant mechanism for release induced by 4-aminopyridine (84% of total release) and high KCl (63%) whilst Ca(2+)-independent release was predominant for veratridine (67%). The Ca(2+)-dependent glutamate release evoked by depolarisation of chromaffin cells with high KCl and 4-AP could be split into both a fast and a slow kinetic component, which might correspond to the release of docked and mobilised chromaffin granules, respectively. On the other hand, depolarisation of cells with veratridine result in glutamate release with only the fast kinetic component. In the case of 60 mM KCl-evoked glutamate release, the fast component exhibited a decay time of <1 s and accounted for 0.63 nmol glu/6x10(6) cells (70% of total exocytotic release), whereas the slow component, which exhibited a decay time of 231 s, accounted for the release of 0.27 nmol glu/6x10(6) cells (30% of total exocytotic release). By contrast in the case of 4-aminopyridine the fast component of exocytosis only represents a 19% of total secretion and the slow a 81% with a decay time of 94 s. These data are very similar to those found in neurones and support the possible intracellular origin of glutamate having a role in the regulation of catecholamine secretion from chromaffin cells. In support of this, we have found that glutamate secretion could be evoked by stimulation of the nicotinic cholinergic receptors.

Calcium channel types involved in intrinsic amino acid neurotransmitters release evoked by depolarizing agents in cortical neurons

Although numerous biochemical and electrophysiological studies have already established many of the properties of the putative Ca2+ receptor for exocytosis at the synapse, the molecular mechanism that involves the influx of Ca2+ and the release of neurotransmitters has remained elusive. Several relationships have been established between neurotransmitter release and Ca2+ channel involved, but no work attempting to connect a particular neurotransmitter release, the effector which produces the release and the opening of a Ca2+ channel type has been performed. This work shows, data dealing with this subject. Based on our results, we have reached the following conclusions: (1) Ca2+ channel types P/Q, N and L mediate Ca2+ entry evoked by high KCl and veratridine, and P/Q and N but not L-type Ca2+ channels are involved when the effector is 4-aminopyridine (4-AP); (2) When we compare the relationship between the amino acid release and the Ca2+ channels which are opened by different depolarizing agents, we find that the release of a particular amino acid neurotransmitter not only depends on the opening of the voltage-dependent Ca2+ channel but also on the effector which produces the opening; and (3) the amount of amino acid release evoked by the different depolarizing agents is not correlated with the elevation of intracellular Ca2+ produced by them. From all of these results, we may conclude that calcium concentration in the active zone is not the only important factor in mediating amino acid release.

Synaptosomal and vesicular accumulation of L-glutamate, L-aspartate and D-aspartate

We examined the vesicular accumulation of the excitatory amino-acid (EAA) neurotransmitters, L-glutamate and L-aspartate, together with the non-metabolisable EAA analogue D-aspartate. Synaptosomes derived from whole brain were incubated in various concentrations of [3H]-amino acids under conditions to facilitate vesicular turnover. Synaptosomes were then lysed in hypotonic medium and vesicles immunoprecipitated with monoclonal anti-synaptophysin antibodies coupled to sepharose beads. Using this method, saturable vesicular accumulation was observed for [3H]-L-glutamate, [3H]-L-aspartate, and [3H]-D-aspartate but not for the excitatory amino acid receptor ligands [3H]-AMPA or [3H]-kainate. Vesicular accumulation (t(1/2)=7.45 min) was markedly slower than synaptosomal accumulation (t(1/2)=1.03 min) and was substantially reduced at 4 degrees C. Maximal accumulation of [3H]-L-glutamate, [3H]-L-aspartate, and [3H]-D-aspartate was estimated to be 98, 68, and 112 pmol/mg of synaptosomal protein, respectively, and uptake affinities 1.6, 3.4, and 2.1 mM, respectively. Maximal accumulation of [3H]-L-glutamate was non-competitively inhibited by both 100 microM unlabeled L-aspartate and 100 microM D-aspartate, suggesting that all are accumulated into a common vesicular pool by different transporters.

Release of D,L-threo-beta-hydroxyaspartate as a false transmitter from excitatory amino acid-releasing nerve terminals

This study examined whether preaccumulated D,L-threo-beta-hydroxyaspartate (tHA), a competitive substrate for the high-affinity excitatory amino acid (EAA) transporter, is released as a false transmitter from EAA-releasing nerve terminals. Potassium-stimulation (50 mM for 1 min) evoked significant release of the endogenous EAAs (aspartate and glutamate) from superfused neocortical minislices. Endogenous EAA release was largely calcium-dependent and was inhibited by tetanus toxin, a neurotoxin which specifically blocks vesicular exocytosis. In parallel experiments, minislices were pre-incubated with 500 microM tHA. Potassium (50 mM) evoked significant release of tHA and this release was also calcium-dependent and reduced by tetanus toxin. Pre-accumulation of tHA did not affect the release of endogenous glutamate whereas the release of endogenous aspartate was significantly attenuated. These data suggest that tHA selectively accumulates in a vesicular aspartate pool and is released upon depolarization as a false transmitter from EAA nerve terminals.

Action of 4-aminopyridine on extracellular amino acids in hippocampus and entorhinal cortex: a dual microdialysis and electroencehalographic study in awake rats

In order to study the role of amino acids in the hippocampus and the entorhinal cortex during the convulsive process induced by 4-aminopyridine (4-AP), we have used a device allowing the simultaneous microdialysis and the recording of their electrical activity of both regions in freely moving rats. We found that infusion of 4-AP into the entorhinal cortex resulted in a large increase in extracellular glutamate and glutamine and small increases in glycine and taurine levels. Likewise, infusion of 4-AP into the hippocampus resulted in a major increase in glutamate, as well as slight increases in taurine and glycine. In both infused regions the peak concentration of extracellular glutamate was observed 15 min after 4-AP administration. No significant changes were found in the non-infused hippocampus or entorhinal cortex of the same rats. Simultaneous electroencephalographic recordings showed intense epileptiform activity starting during 4-AP infusion and lasting for the rest of the experiment (1 h) in both the entorhinal cortex and the hippocampus. The discharges were characterized by poly-spikes and spike-wave complexes that propagated almost immediately to the other region studied. These findings suggest that increased glutamatergic synaptic function in the circuit that connects both regions is involved in the epileptic seizures induced by 4-AP.