Multiple voltage-sensitive calcium channels are probably involved in endogenous GABA release from striatal neurones differentiated in primary culture

Functional significance of nitric oxide in ionomycin-evoked [3H]GABA release from mouse cerebral cortical neurons

We investigated a role of nitric oxide (NO) on ionomycin-evoked [3H]GABA release using mouse cerebral cortical neurons. lonomycin dose-dependently released [3H]GABA up to 1 microM. The extent of the release by 0.1 microM ionomycin was in a range similar to that by 30 mM KCl. The ionomycin (0.1 microM)-evoked [3H]GABA release was dose-dependently inhibited by NO synthase inhibitors and hemoglobin, indicating that the ionomycin-evoked [3H]GABA release is mediated through NO formation. The inhibition of cGMP formation by 1H-[1,2,4] oxodizao [4,3-a] quinoxalin-1-one (ODQ), a selective inhibitor for NO-sensitive guanylate cyclase, showed no affects on the ionomycin-evoked [3H]GABA release. Tetrodotoxin and dibucaine significantly suppressed the ionomycin-evoked [3H]GABA release and ionomycin increased fluorescence intensity of bis-oxonol, suggesting the involvement of membrane depolarization in this release. The ionomycin-evoked [3H]GABA release was maximally reduced by about 50% by GABA uptake inhibitors. The concomitant presence of nifedipine and omega-agatoxin VIA (omega-ATX), inhibitors for L- and P/Q-type voltage-dependent calcium channels, respectively, caused the reduction in the ionomycin-evoked release by about 50%. The simultaneous addition of nifedipine, omega-ATX and nipecotic acid completely abolished the release. Although ionomycin released glutamate, (+)-5-methyl-1-,11-dihydro-5H-dibenzo-[a,d]cycloheptan-5,10-imine (MK-801) and 6,7-dinitroquinoxaline-2,3-dione (DNQX) showed no effects on the ionomycin-induced [3H]GABA release. Based on these results, it is concluded that NO formed by ionomycin plays a critical role in ionomycin-evoked [3H]GABA release from the neurons.

Suppression of Ca2+ influx through L-type voltage-dependent calcium channels by hydroxyl radical in mouse cerebral cortical neurons

In the present study, we investigated the effect of hydroxyl radical (.OH) produced by the Fenton reaction with FeSO(4) to H(2)O(2) on Ca2+ influx by measuring [(45)Ca2+] influx into mouse cerebral cortical neurons in primary culture.OH formed from 3 microM FeSO(4) and 0.01 microM H(2)O(2) significantly reduced 30 mM KCl-induced [(45)Ca2+] influx and this reduction was abolished by .OH scavengers such as N,N'-dimethylthiourea and mannitol. Nifedipine (1 microM), an inhibitor for L-type voltage-dependent Ca2+ channels (VDCCs) showed no additive effect on the reduction of the 30 mM KCl-induced [(45)Ca2+] influx, while the inhibitors for P/Q- and N-type VDCCs showed further suppression of the KCl-induced [(45)Ca2+] influx even in the presence of .OH. Bay k 8644, an activator of L-type VDCCs, dose-dependently stimulated [(45)Ca2+] influx, and this stimulation disappeared in the presence of nifedipine. Similarly, .OH also suppressed significantly [(45)Ca2+] influx induced by Bay k 8644. These inhibitory actions of .OH on the KCl- and Bay k 8644-induced [(45)Ca2+] influx were completely abolished by .OH scavengers. These results indicate that .OH has the activity to suppress Ca2+ influx through L-type VDCCs.

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.

Multiple pathways for regulation of the KCl-induced [3H]-GABA release by metabotropic glutamate receptors, in primary rat cortical cultures

In rat cortical primary cultures, group II- and III-metabotropic glutamate receptor-selective agonists concentration-dependently reduced KCl-induced [3H]GABA release, with IC50 values of 11 nM for LY354740, 80 nM for L(+)-2-amino-4-phosphonobutyric acid (L-AP4), 180 nM for DCG-IV, and 330 nM for L-SOP. The group II antagonists, LY341495 and EGLU, reversed the effect of LY354740, and the group III antagonist MTPG reversed the effect of L-AP4. In the presence of omega-conotoxin GVIA, LY354740 inhibited the remaining [3H]GABA release, whereas L-AP4 was inactive. In contrast, in the presence of nifedipine, L-AP4 inhibited the remaining [3H]GABA release, but LY354740 was no longer active. The PKA inhibitor, H89, blocked the effects of both L-AP4 and LY354740, whereas the PKC inhibitor Ro 31-8220 blocked only the effect of LY354740. Both Ro 31-8220 and H89 reduced the [3H]GABA release to 60% of control. In whole-cell, voltage-clamp experiments, LY354740 and L-AP4 inhibited voltage-gated calcium channel currents with IC50 values of 28 nM and 22 microM, respectively. The results suggest that, in these cells, KCl-induced [3H]GABA release is modulated by two different mechanisms, one involving group II receptors and a direct control of the Ca2+ channel activity, and the other mediated by group III receptors and possibly involving a regulation located downstream of the Ca2+ channel activation.

Regulation of [gamma-3H]aminobutyric acid transport by Ca2+ in isolated synaptic plasma membrane vesicles

We studied the effect of Ca2+ on the transport of the gamma-aminobutyric acid (GABA) by synaptic plasma membrane (SPM) vesicles isolated from sheep brain cortex and observed that intravesicular Ca2+ inhibits the [3H]GABA accumulation in a concentration-dependent manner. This inhibitory effect of Ca2+ exhibited two distinct components: one in the micromolar range of Ca2+ concentration, and the other in the millimolar range. Previous EGTA washing of the membranes, or incorporation of trifluoperazine into the vesicular space reduced the inhibitory action of Ca2+, particularly at low Ca2+ (1-5 microM). Okadaic acid (1 microM) also relieved the Ca2+ inhibition at low, but not at high Ca2+ concentrations (1 mM), whereas the calpain inhibitor I did not alter the effect of the low Ca2+, but it partially reduced (approximately 28%) the effect of Ca2+ in the millimolar range. The results indicate that the GABA transporter is regulated by low Ca2+ concentration (microM) and probably its effect is mediated by the (Ca2+ x calmodulin)-stimulated phosphatase 2B (calcineurin). In contrast, the GABA uptake inhibition observed at high Ca2+ concentrations (1 mM) is less specific, and probably it is partially related to the proteolytic activity of membrane bound calpain II.

ATP increases extracellular dopamine level through stimulation of P2Y purinoceptors in the rat striatum

The effect of ATP on release of dopamine (DA) from rat striatum was studied using in vivo microdialysis. ATP increased the striatal extracellular levels of DA dose-dependently. These analogs produced an increase in DA according to this order of potency: 2-methylthio ATP > ATP > or = alpha,beta-methylene ATP > ADP > AMP > adenosine. Adenosine 5'-[beta, gamma imido]-triphosphate had a more prolonged effect on the increase in DA level than ATP. The ATP-induced increase in DA was inhibited by adding suramin, a nonselective P2 purinoceptor antagonist, and reactive blue 2, a P2Y purinoceptor antagonist, but not inhibited by xanthine amine congener, an adenosine receptor antagonist. Pertussis toxin reduced the increase in DA produced by ATP, which suggests that the P2 purinoceptor may be coupled with a G-protein in the rat striatum. Results suggest that P2Y purinoceptors may involve an ATP-induced increase in DA. The ATP-induced release of DA was tetrodotoxin-sensitive, Ca(2+)-dependent and was abolished by omega-conotoxin GVIA, indicating that the opening of voltage-sensitive Na+ channel and the Ca2+ influx through the N-type voltage-dependent calcium channel are both required for the ATP-induced increase in DA. The ATP-induced increase in DA is presumably due to the release of DA via the stimulation of P2Y purinoceptors in the rat striatum.

Characterization of the carrier-mediated [3H]GABA release from isolated synaptic plasma membrane vesicles

Synaptic plasma membrane (SPM) vesicles were isolated under conditions which preserve most of their biochemical properties. Therefore, they appeared particularly useful to study the cytoplasmic GABA release mechanism through its neuronal transporter without interference of the exocytotic mechanism. In this work, we utilized SPM vesicles isolated from sheep brain cortex to investigate the process of [3H]GABA release induced by ouabain, veratridine and Na+ substitution by other monovalent cations (K+, Rb+, Li+, and choline). We observed that ouabain is unable to release [3H]GABA previously accumulated in the vesicles and, in our experimental conditions, it does not act as a depolarizing agent. In contrast, synaptic plasma membrane vesicles release [3H]GABA when veratridine is present in the external medium, and this process is sensitive to extravesicular Na+ and it is inhibited by extravesicular Ca2+ (1mM) under conditions which appear to permit its entry. However, veratridine-induced [3H]GABA release does not require membrane depolarization, since this drug does not induce any significant alteration in the membrane potential, which is determined by the magnitude of the ionic gradients artificially imposed to the vesicles. The substitution of Na+ by other monovalent cations promotes [3H]GABA release by altering the Na+ concentration gradient and the membrane potential of SPM vesicles. In the case of choline and Li+, we observed that the fraction of [3H]GABA released relatively to the total amount of neurotransmitter released by K+ or Rb+ is about 28% and 68%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of diverse omega-conopeptides on the in vivo release of glutamic and gamma-aminobutyric acids

omega-Conopeptides are antagonists of subtypes of neuronal calcium channels. Two omega-conopeptides, SVIB and MVIIC, have recently been identified which have a novel specificity for these ionophores. We have tested the actions these peptides, as well as the more selective MVIIA, on the release of glutamic acid and gamma-aminobutyric acid (GABA) in the hippocampus in vivo. For the assay of peptide effects on release, we used microdialysis to deliver multiple pulses of elevated potassium to the brain extracellular fluid. Peptide effects were quantitated from the decrement of the release with peptide perfused through the probes, in comparison to that in control experiments. Synthetic MVIIC caused a 40-50% decrement in the release of both glutamate and GABA at a probe concentration of about 200 nM. Synthetic SVI-B caused a 50% block at about 20-40 microM, while about 200 microM of MVIIA was required for 50% block. Chromatographic experiments showed that differences in potency between MVIIC and MVIIA were not explained by differential degradation. Blockade of release was also observed in the thalamus. MVIIC provides a tool for investigating the role of calcium mediated release of glutamate and GABA in physiological and pathological processes in the mammalian brain in vivo.

Effects of calcium channel agonist and antagonists on calcium-dependent events in CA1 hippocampal neurons

The effects of a variety of calcium channel modulators on different calcium-dependent events in CA1 pyramidal hippocampal neurons were analysed using intracellular recordings in an in vitro slice preparation. The following substances were tested: the dihydropyridine calcium agonist BAY K 8644, the dihydropyridine calcium antagonist nimodipine, the phenylalkylamine verapamil and the snail toxin omega-conotoxin GVIA (omega-CgTx). BAY K 8644 increased the repolarization time of the after hyperpolarization (AHP) following a spike burst. This effect was antagonized by nimodipine. BAY K 8644 also prolonged the calcium spike and, in some cases, increased the size of the synaptic events resulting from activation of the Schaffer collateral/commissural system. Nimodipine decreased the size of the AHP in some neurons but had no consistent effect on synaptic events. Verapamil at low concentrations (1-10 microM) had no significant effects on the calcium-dependent events in the hippocampus. Increasing the concentration (up to 100 microM) led to a progressive suppression of the AHP and of the slow inhibitory postsynaptic potential (IPSP), probably via an action on potassium conductances. In addition, the baclofen-induced hyperpolarization was blocked by verapamil. Interestingly, at this higher concentration, verapamil could suppress the AHP without depressing the calcium spike. omega-CgTx selectively blocked the synaptic events (especially the IPSPs) but had no effect on non-synaptic events. This last compound exhibits a high degree of selectivity, acting on N-type calcium channels which are involved in neurotransmitter release. Our results provide evidence that different classes of agents which act on calcium channels can be used to discriminate between different calcium-dependent responses in CA1 hippocampal neurons.

An in vivo microdialysis characterization of extracellular dopamine and GABA in dorsolateral striatum of awake freely moving and halothane anaesthetised rats

This study describes the results of a systematic characterization of extracellular dopamine (DA) and gamma-aminobutyric acid (GABA) recovered from dorsolateral striatum using in vivo microdialysis in rats following acute (2.5 h) and chronic (1 day, 2 day and 4 day) implantation of the probe. The voltage and calcium dependence of DA and GABA overflow was characterised by perfusion with the sodium channel blocker tetrodotoxin (TTX 10-6M) and with Ca2(+)-free Ringers perfusion medium. In addition, the effect of halothane anaesthesia on the responsiveness of these neurotransmitter substances to TTX and Ca2(+)-free perfusion medium was investigated. Perfusion with TTX decreased basal DA levels by at least 60% in all groups. The TTX-induced decrease was most profound in halothane-anaesthetised rats, 24 h after implantation of the probe. Responsiveness of GABA to TTX infusion was different between the groups. In acutely implanted halothane-anaesthetised rats basal GABA levels were unaltered by perfusion with TTX while in the remaining groups at least a 35% reduction was observed. In awake rats 2 days following implantation of the probe removal and replacement of the Ca2+ from the perfusion medium resulted in a reversible reduction of basal DA by 87%. In addition, basal GABA levels were decreased by 52%. This decrease was delayed and was not reversed 1.5 h after the Ca2(+)-free perfusion medium was replaced with normal perfusion medium although basal GABA levels returned to pre-experimental levels by the following day.(ABSTRACT TRUNCATED AT 250 WORDS)

Establishment of a long-term primary culture of striatal neurons

A new method of obtaining long-term primary cultures (lasting more than 8 weeks) of striatal neurons is described in this paper. The originality of the method consists of: (1) starting the culture for 3 days in a serum-free medium which allows attachment and neurite proliferation of neurons as well as the death of non-neuronal cells (mainly consisting of astrocytes); (2) introducing a limited amount of fetal calf serum (FCS) (2-5%) after 3 days in vitro (3 DIV), which likely provides optimal neuronal survival and attachment factors, and a limited amount of astrocyte proliferating factors. The period of introduction of serum, as well as the amount of serum introduced are critical factors. By phase contrast and transmission electron microscopy, we observed that neurons continued to develop neurite extensions, synaptic vesicles and synapse formations up to 50 DIV. Neuronal membranes, and synaptic contacts were particularly healthy up to 50 DIV. Interestingly, the number of astrocytes was constant between 30-50 DIV and limited to about 10%. We therefore obtained an equilibrium between neuronal and astrocyte differentiation and proliferation. It is likely that the small population of astrocytes, plus the low percentage of FCS added, provide essential factors for neuronal survival and differentiation, whereas a high density of differentiated neurons inhibited astrocyte cell proliferation. The clear-cut stability of these neuronal cultures goes in parallel with the stability of the pharmacological responses studied here: the coupling of carbachol and quisqualate receptors with the inositol phosphate production system. The culture method described here could be of particular interest to pursue biochemical, pharmacological and biological studies on neurons as well as on reciprocal interactions between neurons and astrocytes.

Omega-conotoxin GVIA and dihydropyridines discriminate two types of Ca2+ channels involved in GABA release from striatal neurons in culture

We previously reported that the opening of the L-type Ca2+ channel was only partly involved in the K(+)-evoked Ca2(+)-dependent gamma-aminobutyric acid (GABA) release from striatal neurons, suggesting that probably different types of voltage-sensitive Ca2+ channels were implicated in this physiological process. Here we demonstrate that omega-conotoxin GVIA, which has been reported to block L- and N-type neuronal Ca2+ channels, also partly inhibits the Ca2(+)-dependent GABA release. The maximal effects of omega-conotoxin GVIA and nifedipine, a highly specific antagonist of the L-type channels, were additive, a total inhibition of the Ca2(+)-dependent GABA release being obtained in the presence of both drugs. We therefore propose that omega-conotoxin GVIA and nifedipine block two different types of Ca2+ channels, both involved in the GABA release process.

A comparison of the effect of calcium channel ligands and GABAB agonists and antagonists on transmitter release and somatic calcium channel currents in cultured neurons

Glutamate release has been examined from cultured cerebellar granule neurons in the rat using the technique of prelabelling the releasable pool of glutamate with [3H]glutamine. Glutamate release was stimulated in control neurons by 2-min incubation with 50 mM K+, or in neurons continuously depolarized in Ca2(+)-free 50 mM K+ medium, by 2-min incubation with medium containing 5 mM Ca2+. The ability of the Ca2(+)-channel agonist (+)-202-791 to increase the stimulated release of [3H]glutamate was approximately doubled in the depolarized condition. The antagonist enantiomer (-)-202-791 produced a small inhibition of K(+)-stimulated release, whereas (-)-202-791 completely inhibited Ca2(+)-stimulated release from depolarized neurons at concentrations greater than 10 nM. (-)-Baclofen (100 microM) inhibited transmitter release similarly (25-30%) under the two conditions. Calcium-channel currents were recorded from cultured dorsal root ganglion neurons under control conditions at a holding potential of -80 mV, or in neurons depolarized to -30 mV. (-)-202-791 produced a greater effect at -30 than at -80 mV although even at -30 mV the inhibition was slow in onset and incomplete. (-)-Baclofen (100 microM) inhibited the amplitude of the calcium-channel current at both holding potentials by 30-50%, although it did not clearly slow activation of the current at the depolarized holding potential. The GABAB receptors associated with inhibition of glutamate release and of calcium-channel currents were both markedly blocked by phaclofen but not by 2-OH-saclofen. These findings suggest that the GABAB receptor associated with inhibitory modulation of transmitter release, and that associated with inhibition of calcium-channel currents show pharmacological similarities, and are able to exert their action even at levels of steady depolarization at which most N-type channels should be inactivated.

Activators and inactivators of calcium channels: effects in the central nervous system

The interactions of calcium antagonists or channel activators with the different classes of calcium channel are reviewed with particular emphasis on interactions with neuronal tissue; reasons for the failure of calcium antagonists to inhibit neurotransmitter release under normal circumstances are outlined. Calcium antagonists may be protective in several pathological situations and the possibilities of protection against ischaemic damage in the central nervous system are evaluated.

Ultrastructural localization of the GTP-binding protein Go in neurons

The ultrastructural localization of Go, a GTP-binding protein (G protein) highly expressed in nervous tissues, was performed in cultured fetal and adult murine neurons, using affinity-purified polyclonal antibodies against the alpha subunit of the Go protein (Go alpha). These antibodies recognized denatured Go alpha and both the native Go alpha-subunit and the Go alpha beta gamma heterotrimer. At the ultrastructural level, the positive immunoreactivity detected in cultured cells as well as in thin frozen sections, showed that Go was largely distributed in cell bodies and neuritic cytoplasm. Labelling was principally noted on the cytoplasmic face of the plasma membrane lining the cell body and the neurites, especially in 'cell-cell' contacts, but also in the cytoplasmic matrix, between endoplasmic reticulum and Golgi cisternae. No immunoreactivity was observed on the inner face of the pre- or postsynaptic membranes in both adult brain and in cultured neurons. This last finding strongly suggests that the Go protein is not involved in transducing chemical signals at the level of synapses, but more probably modulates the synaptic functions by controlling the activity of effectors localized outside of the synaptic densities.

Antagonism by tubocurarine and verapamil of the regenerative acetylcholine release from mouse motor nerve

The role of presynaptic acetylcholine receptors and Ca2+ channels in the regenerative acetylcholine release was studied in the cut muscle preparation of mouse phrenic nerve hemidiaphragm. The regenerative release shown as a prolonged endplate depolarization was evoked by stimulation of the nerve with a train of pulse at 75-300 Hz when acetylcholinesterase activity was depressed with neostigmine or by lowering temperature. Tubocurarine, cobratoxin, verapamil, diltiazem and nifedipine at low concentrations, which had a negligible effect on the endplate potential, shortened the duration of regenerative depolarization while leaving the amplitude unaffected. In contrast, Mn2+ at concentrations that markedly reduced the amplitude of single endplate potentials caused little suppression of the regenerative depolarization though intensive stimulation was needed to trigger the response. On the other hand, atropine inhibited the regenerative depolarization only at high concentrations which also depressed endplate potentials. These results indicate that the mechanism for evoking the regenerative release involves the activation of acetylcholine receptors and Ca2+ channels which are sensitive to tubocurarine and Ca2+ channel blockers. The Ca2+ channel concerned, however, appears to differ from that involved in the normal quantal release of acetylcholine.

Excitatory amino acid-evoked release of [3H]GABA from hippocampal neurons in primary culture

We investigated the release of gamma-[2,3-3H(N)]aminobutyric acid ([3H]GABA) from hippocampal neurons in primary cell culture. [3H]GABA release was stimulated by the excitatory amino acid neurotransmitter glutamate as well as by N-methyl-D-aspartate (NMDA) and kainate. Cell depolarization induced by raising [K+]o or by veratridine also stimulated [3H]GABA release. NMDA-induced release was completely blocked by 3-((+/-)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP+), Mg2+ and Zn2+ whereas the release induced by glutamate and kainate was much less susceptible to inhibition by these substances. Furthermore, removal of external Ca2+ inhibited NMDA-induced release, but not that induced by glutamate, kainate, veratridine or 50 mM K+. Removal of external Na+ reduced [3H]GABA release evoked by all stimuli, but to different extents. All of the excitatory amino acids tested increased [Ca2+]i within hippocampal neurons as assessed by fura-2 based microspectrofluorimetry. This increase in [Ca2+]i was completely dependent on the presence of external Ca2+. These results suggest that Ca2+-dependent and -independent forms of GABA release from hippocampal interneurons may occur. [3H]GABA release evoked by glutamate, kainate, veratridine or 50 mM K+, appeared to be mediated by the reversal of electrogenic, Na+-coupled GABA uptake. Release was inhibited by nipecotic acid, an inhibitor of the Na+-coupled GABA uptake system. However, release induced by NMDA may also include a Ca2+-dependent component.

Complex interaction between quisqualate and kainate receptors as revealed by measurement of GABA release from striatal neurons in primary culture

The effects of non-NMDA receptor agonists were tested on endogenous GABA and [3H]GABA release from highly purified striatal neurons differentiated in primary culture. Kainate (KA), glutamate (Glu) and quisqualate (QA) stimulated [3H]GABA release with EC50S = 85 +/- 20 (n = 6), 6.21 +/- 1.42 (n = 3) and 0.135 +/- 0.035 (n = 3) microM, respectively. KA was the most potent (in term of efficacy) agonist (maximal response at 10 mM: 935 +/- 51% (n = 6) increase over basal release) followed by Glu (at 100 microM: 404 +/- 34% (n = 5) increase) and QA (at 10 microM: 91 +/- 6% (n = 6) increase). Phencyclidine (PCP), which was without effect on QA- and KA-evoked GABA release, inhibited the Glu response by about 50%. QA totally inhibited KA (50 microM)-evoked GABA release with an IC50 = 0.39 +/- 0.11 (n = 4) in a competitive manner (Ki = 0.39 +/- 0.07 microM (n = 3]. Competitive inhibition of the KA response was also observed with the other agonists of the quisqualate receptor, Glu and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), suggesting that Glu, QA and AMPA act as partial agonists at the KA receptor. gamma-D-Glutamylaminomethylsulfonic acid (GAMS) also inhibited (IC50 = 2.1 mM) the KA response competitively. However the inhibition by GAMS and QA was not additive. The response to QA was rapidly inactivated (no response after 3 min stimulation) in contrast to the KA-evoked GABA release which remained maximal for at least 3 min. When neurons were first exposed to concanavalin A (con A), a lectin known to inhibit Glu receptor desensitisation on insect muscles, the QA response remained maximal for at least 6 min. Con A greatly enhanced the maximal responses to QA and AMPA and decreased their apparent affinities. The KA-evoked GABA release (but not the veratridine and NMDA effects) was also augmented (no change in the EC50 value) by con A. It is proposed that QA, AMPA and KA act at the same receptor-channel complex (termed G2 receptor) which is desensitised more rapidly when stimulated by QA or AMPA than when stimulated by KA.

Characterization of gamma-aminobutyric acid and dopamine overflow following acute implantation of a microdialysis probe

The present study characterized the voltage and calcium dependence of gamma-aminobutyric acid and dopamine overflow after the acute implantation of a microdialysis probe. Probes were implanted in dorsolateral striatum and globus pallidus. Experiments were performed under light halothane anesthesia. Basal, extracellular levels of GABA were not affected by tetrodotoxin (TTX) and were increased to 140 percent of basal values by calcium free Ringer. Basal, extracellular levels of dopamine were reduced to 14 percent of basal values by the addition of TTX and to 30 percent of basal values by the removal of calcium from the Ringer solution. The results suggest that in this in vivo preparation basal extracellular dopamine is largely of vesicular origin while GABA is not.

Involvement of DHP voltage-sensitive calcium channels and protein kinase C in thyroliberin (TRH) release by developing hypothalamic neurons in culture

The intracellular mechanisms regulating the process of thyroliberin (TRH) release were studied using fetal hypothalamic neurons grown in serum-free medium. In particular, we compared the effects of dihydropyridine (DHP) derivatives, omega-conotoxin and phorbol esters on basal and K+-evoked TRH release from 12 days in vitro (DIV) neurons. BAY K 8644, a DHP calcium channel agonist increased in a dose-related manner basal and K+-evoked TRH release. PN 200-110, an antagonist of DHP-sensitive calcium channels, completely suppressed the effect of BAY K 8644, whatever the extracellular K+ concentration, but did not modify basal or K+-evoked TRH release. In contrast, omega-conotoxin partially inhibited the two latter processes. The active phorbol ester 12-O-tetradecanoyl-phorbol-beta-acetate (TPA), and to a lesser extent Sn-1,2-dioctanoylglycerol (DAG), triggered TRH release. This effect was specific, time and dose dependent and only partly dependent on extracellular calcium. Simultaneous addition of BAY K 8644 and TPA to the cells displayed a synergistic effect. The same compounds were studied on younger neurons (6-DIV cultures): BAY K 8644 stimulated TRH release whereas neither 60 mM K+ nor TPA did. These results suggest that TRH release can be mediated at least by two intracellular routes: (i) increase of intracellular calcium mediated by the opening of different types of voltage sensitive calcium channels, and (ii) activation of protein kinase C (PKC). The asynchrony in the maturation of the intracellular mechanisms underlying TRH release may be explained by different subcellular localizations of these mechanisms in neurons and is discussed in relation to synapse differentiation.

Neurotransmitter-induced inositol phosphate formation in neurons in primary culture

Inositol-1,4,5-trisphosphate, produced in cells as a breakdown product of phosphatidylinositol-4,5-bisphosphate, induces, in many cell types, release of calcium from intracellular stores. In murine striatal neurons, differentiated in primary culture, carbachol, norepinephrine, glutamate, and neurotensin stimulate 3H-labeled inositol phosphate (3H-IP) production. The glutamate response was recently characterized as being mediated primarily by receptors of the quisqualate subtype. In the present study, we found that major differences exist between glutamate-stimulated 3H-IP formation and those stimulated by the other neuromediators. The maximal response to glutamate occurred before and during synaptogenesis and declined thereafter, whereas the maximal response to either carbachol or norepinephrine required complete neuronal differentiation. Although the glutamate response appears to be mediated exclusively by direct interaction with the neurotransmitter receptors, responses to carbachol, norepinephrine, and neurotensin were partially or completely blocked by tetrodotoxin.

Maitotoxin-evoked gamma-aminobutyric acid release is due not only to the opening of calcium channels

The effects of maitotoxin (MTX) on endogenous amino acid release were tested on highly purified striatal neurons differentiated in primary culture. MTX induced a large and concentration-dependent release of gamma-aminobutyric acid (GABA). This effect was abolished when experiments were performed in the absence of external Ca2+, and restored when Ca2+ ions were added after removing the MTX-containing Ca2+-free solution. MTX-induced amino acid release was not affected by 1 microM nifedipine and only slightly inhibited by 1 mM Co2+. MTX also induced a massive accumulation of 45Ca2+ in the neurons which, in contrast to the MTX-evoked GABA release, was totally blocked in the presence of 1 mM Co2+. Whereas 500 nM tetrodotoxin was without significant effect, MTX-evoked GABA release was dependent on the presence of external Na+ and sensitive to nipecotic acid, a GABA uptake inhibitor. It is concluded that, on striatal neurons, MTX induced Na+ influx only in the presence of external Ca2+. The increase in cytoplasmic Na+ ions then triggers the release of GABA.