Dihydropyridines modulate K+-evoked amino acid and adenosine release from cerebellar neuronal cultures

Auto-inhibition of rat parallel fibre-Purkinje cell synapses by activity-dependent adenosine release

Adenosine is an important signalling molecule involved in a large number of physiological functions. In the brain these processes are as diverse as sleep, memory, locomotion and neuroprotection during episodes of ischaemia and hypoxia. Although the actions of adenosine, through cell surface G-protein-coupled receptors, are well characterized, in many cases the sources of adenosine and mechanisms of release have not been defined. Here we demonstrate the activity-dependent release of adenosine in the cerebellum using a combination of electrophysiology and biosensors. Short trains of electrical stimuli delivered to the molecular layer in vitro, release adenosine via a process that is both TTX and Ca²⁺ sensitive. As ATP release cannot be detected, adenosine must either be released directly or rapidly produced by highly localized and efficient extracellular ATP breakdown. Since adenosine release can be modulated by receptors that act on parallel fibre–Purkinje cell synapses, we suggest that the parallel fibres release adenosine. This activity-dependent adenosine release exerts feedback inhibition of parallel fibre–Purkinje cell transmission. Spike-mediated adenosine release from parallel fibres will thus powerfully regulate cerebellar circuit output.

Exocytotic release of alanine from cultured cerebellar neurons

Many neurons release a variety of amino acids in response to depolarizing stimuli. Although some of these amino acids, namely, glutamate, aspartate, and gamma-aminobutyric acid (GABA), have been qualified as neurotransmitters, functional roles of the other amino acids including alanine remain obscure. We investigated the mechanism and the origin of alanine release from cultured rat cerebellar cells. High-K(+)-induced depolarization produced a considerable amount (139+/-8 pmol/2 min/dish) of alanine release, comparable to that of glutamate (103+/-7 pmol/2 min/dish). Other depolarizing agents including veratridine or 4-aminopyridine also induced alanine release, suggesting that the major source is excitable neurons, rather than non-excitable glial cells. Depolarization-evoked alanine release was suppressed in the absence of extracellular Ca(2+), and was almost abolished by treating the cells with botulinum type B neurotoxin (BoNT/B), indicating that alanine is released by Ca(2+)-dependent exocytosis of vesicle-associated membrane protein-2 (VAMP-2)-containing vesicles. The properties of alanine release were different from those of glutamate and GABA in several aspects: (a) Depolarization-dependent alanine release appeared as early as 7 days in vitro, much earlier than that of GABA. (b) Fifty microM kainate, which causes selective cell death of GABAergic neurons in the culture, only partially reduced alanine release, whereas it had no effect on glutamate release. (c) Alanine release was not affected by phorbol ester, which enhanced glutamate and GABA release in a kinase-dependent manner. We therefore conclude that alanine release occurs via exocytosis of a pool of synaptic vesicles distinct from those containing glutamate or GABA.

Adenosine in the central nervous system: release mechanisms and extracellular concentrations

Adenosine has several functions within the CNS that involve an inhibitory tone of neurotransmission and neuroprotective actions in pathological conditions. The understanding of adenosine production and release in the brain is therefore of fundamental importance and has been extensively studied. Conflicting results are often obtained regarding the cellular source of adenosine, the stimulus that induces release and the mechanism for release, in relation to different experimental approaches used to study adenosine production and release. A neuronal origin of adenosine has been demonstrated through electrophysiological approaches showing that neurones can release significant quantities of adenosine, sufficient to activate adenosine receptors and to modulate synaptic functions. Specific actions of adenosine are mediated by different receptor subtypes (A(1), A(2A), A(2B) and A(3)), which are activated by various ranges of adenosine concentrations. Another important issue is the measurement of adenosine concentrations in the extracellular fluid under different conditions in order to know the degree of receptor stimulation and understand adenosine central actions. For this purpose, several experimental approaches have been used both in vivo and in vitro, which provide an estimation of basal adenosine levels in the range of 50-200 nM. The purpose of this review is to describe pathways of adenosine production and metabolism, and to summarize characteristics of adenosine release in the brain in response to different stimuli. Finally, studies performed to evaluate adenosine concentrations under physiological and hypoxic/ischemic conditions will be described to evaluate the degree of adenosine receptor activation.

Clonidine modulates BAY K 8644-induced rat behavior and neurotransmitter changes in the brain

BAY K 8644 (methyl-1,4-dihydro-2, 6-dimethyl-3-nitro-4[2-trifluoromethyl-phenyl]-pyridine-5-carboxylate), an activator of dihydropyridine-sensitive Ca(2+) channels, injected in rats [2 mg/kg intraperitoneally (i.p.)], induces behavioral changes including ataxia, increased sensitivity to auditory stimulation, stiff tail, arched back, limb tonus and clonus, and rolling over. Neurochemical changes in the brain 45 min after application of 2 mg/kg were characterized by a significant decrease of noradrenaline in the amygdala (-27.8%, P<0.02) and piriform cortex (-16.3%, P<0.02). No significant changes of catecholamines were found in the hippocampal subregions CA1, CA3 and dentate gyrus or in the septum as compared to controls. The dopamine metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), in the amygdala were elevated by 60% (P<0.02) and 66.7% (P<0.02), respectively. In the septum, a 52.6% (P<0.02) increase of HVA was observed. Analysis of amino acids revealed a marked increase of gamma-aminobutyric acid (GABA) content (+50.4%, P<0.001) in the septum. Pretreatment of the rats with the alpha(2)-adrenoceptor agonist, clonidine (0.1 mg/kg i.p.), 30 min before BAY K 8644 (2 mg/kg i.p.) injection completely abolished the behavioral and neurochemical changes. The data suggest that the Ca(2+)-dependent neurotransmitter release provoked by BAY K 8644 can be modulated by stimulation of presynaptic alpha(2)-adrenoceptors. The effect of clonidine on the GABAergic system may represent an important mechanism involved in the prevention of BAY K 8644-induced behavior.

Presynaptic calcium channels and field-evoked transmitter exocytosis from cultured cerebellar granule cells

Regulated exocytosis from cultured rat cerebellar granule cells can be localized by the vesicle specific marker FM2-10 to specific sites, the highest density of which are at visible varicosities coinciding with neurite-neurite contacts. Exocytosis can be evoked by uniform electrical field pulses, which initiate tetrodotoxin-sensitive action potentials, or by elevated KCl. [3H]D-Aspartate is an authentic false transmitter in this preparation, judged by sensitivity of release to bafilomycin A1 and tetanus toxin. The coupling of presynaptic voltage-activated Ca2+ channels to [3H]D-aspartate exocytosis was determined during field stimulation. The peak cytoplasmic free Ca2+ concentration achieved in the varicosities was proportional to Ca2+ entry during a 10 strain of pulses. L-type Ca2+ channels did not contribute to either Ca2+ entry or [3H]D-aspartate exocytosis. The P-type Ca2+ channel antagonist omega-agatoxin-IVA (30 nM) only inhibited at 75% of the varicosities, although a mean 15% inhibition of Ca2+ entry caused a 39% inhibition of exocytosis. In contrast the N-type Ca2+ channel inhibitor omega-conotoxin-GVIA (1 microM), which inhibited at virtually all varicosities, caused mean inhibitions of Ca2+ entry and exocytosis of 26% and 24% respectively. The toxin omega-conotoxin-MVIIC (5 microM), which inhibits N-, P- and Q-type Ca2+ channels, was effective at all varicosities. The Q-type component of Ca2+ entry was calculated to be only 5-10%; however, the additional inhibition of exocytosis was 30%. Thus P-type and particularly Q-type channels appear to be more closely coupled to exocytosis than N-type Ca2+ channels. The residual Ca2+ entry following 5 microM omega-conotoxin-MVIIC is scarcely coupled to release. The omega-agatoxin-IVA and omega-conotoxin-GVIA inhibitions of both Ca2+ entry and exocytosis were additive and varied stochastically between individual varicosities. These results demonstrate that both Q- and P-type Ca2+ channels are highly efficient in their coupling to amino acid exocytosis, with N-type less efficient, and L-type channels not at all. The Ca2+ channel types coupled to exocytosis are also able to support exocytosis when evoked by either brief field-evoked action potentials or prolonged depolarization with KCl, indicating that these presynaptic channels, in contrast to those on the somata of the cells, can respond to widely different patterns of activation.

Glutamate exocytosis from cerebellar granule cells: the mechanism of a transition to an L-type Ca2+ channel coupling

When cerebellar granule cells in the presence of 1.3 mM calcium chloride (Ca2+) are depolarized by high potassium chloride (KCl), the release of endogenous glutamate is coupled to a high threshold Ca2+ channel blocked by the spider toxin omega Agatoxin-glutamate-release-inhibitor (Aga-GI) and insensitive to the L-type voltage-dependent Ca2+ channel-inhibitor nifedipine. A prolonged KCl depolarization in the absence of Ca2+ followed by addition of 5 mM Ca2+ results in an enhanced nifedipine-sensitive Ca2+ entry; glutamate exocytosis retains sensitivity to tetanus toxin and bafilomycin A1, is now totally inhibited by nifedipine and shows greatly reduced sensitivity to AGA-GI. Single cell Ca2+ imaging indicates that the L-type channel modulating release is preferentially located at somatic regions rather than neurites. A different pattern of vesicle endocytosis monitored with the fluorescent indicator FM1-43 is seen in response to the two depolarization protocols. Furthermore, vesicles loaded during depolarization with high KCl in the presence of 5 mM Ca2+ extensively exocytose dye in a nifedipine-insensitive manner in response to a second similar stimulation but release little dye in response to stimulus with high KCl in the absence of Ca2+ followed by the addition of 5 mM Ca2+. In contrast, vesicles loaded by stimulating with KCl in the absence of Ca2+ followed by the addition of 5 mM Ca2+ can be released by a second similar stimulus and this release is sensitive to nifedipine. Nifedipine sensitivity is not induced in cerebellar synaptosomes subjected to stimulation with high KCl in the absence of Ca2+ followed by the re-addition of 5 mM Ca2+. The results indicate that different populations of channels and vesicles may be functional during two depolarization protocols.

The calcium channel coupled to the exocytosis of L-glutamate from cerebellar granule cells is inhibited by the spider toxin, Aga-GI

The increase in cytosolic calcium, [Ca2+]c, evoked with 50 mM KCl in cerebellar granule cells consists of four components; (1) a rapidly inactivating transient or spike; (2) a nifedipine-sensitive non-inactivating plateau; (3) an Aga-GI (spider toxin) sensitive non-inactivating plateau; (4) a residual non-inactivating plateau insensitive to nifedipine and Aga-GI. None of these components is blocked by synthetic arginine polyamine toxin, spermine, (+)-MK-801 hydrogen maleate, D(-)-2-amino-5-phosphonopentanoic acid or omega-conotoxin-GVIA. The proposed P-type channel antagonist, omega-agatoxin-IVA, has a limited but non-significant effect on the elevated plateau [CA2+]c.L-type Ca2+ channels are located primarily on the soma whereas the component of the plateau which is blocked specifically by Aga-GI is localized primarily on the cell neurites. The latter component is coupled to the exocytosis of endogenous glutamate evoked with 50 mM KCl.

Involvement of calcium channels in depolarization-evoked release of adenosine from spinal cord synaptosomes

The potential involvement of L- and N-type voltage-sensitive calcium (Ca2+) channels and a voltage-independent receptor-operated Ca2+ channel in the release of adenosine from dorsal spinal cord synaptosomes induced by depolarization with K+ and capsaicin was examined. Bay K 8644 (10 nM) augmented release of adenosine in the presence of a partial depolarization with K+ (addition of 6 mM) but not capsaicin (1 and 10 microM). This augmentation was dose dependent from 1 to 10 nM and was followed by inhibition of release from 30 to 100 nM. Nifedipine and nitrendipine inhibited the augmenting effect of Bay K 8644 in a dose-dependent manner, but neither antagonist had any effect on release of adenosine produced by K+ (24 mM) or capsaicin (1 and 10 microM). omega-Conotoxin inhibited K(+)-evoked release of adenosine in a dose-dependent manner but had no effect on capsaicin-evoked release. Ruthenium red blocked capsaicin-induced release of adenosine but had no effect on K(+)-evoked release. Although L-type voltage-sensitive Ca2+ channels can modulate release of adenosine when synaptosomes are partially depolarized with K+, N-type voltage-sensitive Ca2+ channels are primarily involved in K(+)-evoked release of adenosine. Capsaicin-evoked release of adenosine does not involve either L- or N-type Ca2+ channels, but is dependent on a mechanism that is sensitive to ruthenium red.

Spontaneous and evoked release of [3H]taurine from a P2 subcellular fraction of the rat retina

The effects of spontaneous and evoked [3H]taurine release from a P2 fraction prepared from rat retinas were studied. The P2 fraction was preloaded with [3H]taurine under conditions of high-affinity uptake and then examined for [3H]taurine efflux utilizing superfusion techniques. Exposure of the P2 fraction to high K+ (56 mM) evoked a Ca(2+)-independent release of [3H]taurine. Li+ (56 mM) and veratridine (100 microM) had significantly less effect (8-15% and 15-30%, respectively) on releasing [3H]taurine compared to the K(+)-evoked release. 4-Aminopyridine (1 mM) had no effect on the release of [3H]taurine. The spontaneous release of [3H]taurine was also Ca(2+)-independent. When Na+ was omitted from the incubation medium K(+)-evoked [3H]taurine release was inhibited by approximately 40% at the first 5 minute depolarization period but was not affected at a second subsequent 5 minute depolarization period. The spontaneous release of [3H]taurine was inhibited by 60% in the absence of Na+. Substitution of Br- for Cl- had no effect on the release of either spontaneous or K(+)-evoked [3H]taurine release. However, substitution of the Cl- with acetate, isethionate, or gluconate decreased K(+)-evoked [3H]taurine release. Addition of taurine to the superfusion medium (homoexchange) resulted in no significant increase in [3H]taurine efflux. The taurine-transport inhibitor guanidinoethanesulfonic acid increased the spontaneous release of [3H]taurine by approximately 40%. These results suggest that the taurine release of [3H]taurine is not simply a reversal of the carrier-mediated uptake system. It also appears that taurine is not released from vesicles within the synaptosomes but does not rule out the possibility that taurine is a neurotransmitter. The data involving chloride substitution with permeant and impermeant anions support the concept that the major portion of [3H]taurine release is due to an osmoregulatory action of taurine while depolarization accounts for only a small portion of [3H]taurine release.

35 mM K(+)-stimulated 45Ca2+ uptake in cerebellar granule cell cultures mainly results from NMDA receptor activation

In primary cultures of cerebellar granule cells, the Ca2+ influx resulting from K+ depolarization (35 mM) was equal to one-third of that observed with 100 microM N-methyl-D-aspartate (NMDA) and was reduced in a major part (90%) by NMDA receptor antagonists. The rank order of potency of these competitive and non-competitive NMDA receptor antagonists was very close to their affinity for the NMDA and phencyclidine sites respectively. Granular cell depolarization with 35 mM K+ also induced a large increase in the extracellular glutamate concentration. Repeated washes of the culture wells, addition of glutamate pyruvate transaminase (+2 mM pyruvate), or pretreatment of the cells with tetanus toxin resulted in a parallel reduction of the extracellular glutamate concentration and 45Ca2+ uptake measured after a 35 mM K+ stimulation. Dihydropyridine (BAY K-8644) stimulated the release of glutamate in a nifedipine-sensitive manner in the presence of 15 mM K+. However, nifedipine (1 microM), which decreased by 60% the K(+)-induced 45Ca2+ uptake, did not reduce the 35 mM K(+)-evoked glutamate release. Taken together, these results demonstrated that in cerebellar granule cell cultures, 90% of the 35 mM K(+)-stimulated 45Ca2+ influx resulted from the release of glutamate and the consecutive activation of NMDA receptors. Activation of these glutamate receptors then allows Ca2+ influx to occur through L-type voltage-operated Ca2+ channels.

Astrocyte taurine

The evidence presented, together with the lack of solid evidence for a specific receptor site, strongly suggests that taurine does not act as a traditional neurotransmitter in the CNS. In fact, the properties seen to be governing its efflux from both glial cells and neurons argue strongly in favor of a primary role in volume regulation. However, subsequent to its release into the extracellular space, it is possible that the inherent neuroactive properties (e.g., inhibitory neuromodulation and Ca(2+)-level modulation) may be important at the synapse, at the cell plasma membrane, and intracellularly in further directing the level of neuronal activity. Whether or not the levels of released taurine are great enough to sustain these effects has still to be determined.

K(+)-stimulated amino acid release from cultured cerebellar neurons: comparison of static and dynamic stimulation paradigms

The release of several endogenous amino acids and adenosine from rat cerebellar neuronal cultures following elevated K+ exposure in the presence and absence of added Ca2+ was studied. The amino acids aspartate (ASP), glutamate (GLU) and GABA were released from the cultures in a dose- and Ca(2+)-dependent manner. Taurine (TAU) and the nucleoside adenosine (ADN) efflux rates were dose-dependent but Ca(2+)-independent, and basal levels increased in the absence of Ca2+. The K+ depolarization induced release of serine (SER), alanine (ALA) and proline (PRO), was not dose-dependent and in the absence of extracellular Ca2+ (with added Mg2+) higher basal release of SER and ALA, but not PRO, was noted. These findings demonstrate that in addition to known cerebellar neurotransmitters, other neuroactive and neutral amino acids are released from cultured cerebellar neurons in response to K+ depolarization. Their observed efflux suggests they may have as yet unidentified roles in neuronal function with different classes of efflux corresponding to: neurotransmitter-type release (ASP, GLU, GABA), an osmoregulatory, possibly neuromodulatory-type release (TAU), a Ca(2+)-insensitive, possibly neuromodulatory-type release (ADN), and a depolarization-sensitive release (SER, ALA, PRO) of which SER and ALA are partially Ca(2+)-sensitive.

Nifedipine has paradoxical effects on the development of kindling but not on kindled seizures in amygdala-kindled rats

The effects of nifedipine, an antagonist of voltage-operated calcium channels, on the development of amygdala kindling and on the production of fully kindled seizures, stimulated from the amygdala, were investigated. Rats were treated daily with two doses (5 and 50 mg/kg, i.p.) of nifedipine during the development of kindling. Both doses of nifedipine retarded the development of kindled seizures and 50 mg/kg of nifedipine prolonged the latency to the occurrence of bilateral forelimb clonus. In contrast to these antiepileptogenic effects, however, both doses also increased the duration of afterdischarge. This resulted in a striking increase in the cumulative duration of afterdischarge, required to reach stage 4 and 5 seizures. Contrary to the results of a previous study, 50 mg/kg of nifedipine did not produce any significant effect on fully kindled seizures, regardless of the interval (5 min-24 hr) between injection and stimulation of kindling. These results suggested that although nifedipine inhibited the propagation processes of seizures during development of kindling, it appeared to increase the duration of epileptic activity at the kindling focus.

Analysis of adenosine immunoreactivity, uptake, and release in purified cultures of developing chick embryo retinal neurons and photoreceptors

We have investigated the presence of endogenous adenosine and of mechanisms for adenosine uptake and release in chick embryo retinal neurons and photoreceptors grown in purified cultures in the absence of glial cells. Simultaneous autoradiographic and immunocytochemical analysis showed that endogenous adenosine and the uptake mechanism for this nucleoside colocalize in practically all the photoreceptors, but only in approximately 20% of the neurons. Approximately 25% of the neurons showed either immunocytochemical labeling or autoradiographic labeling, while greater than 50% of the neurons were unlabeled with both techniques. [3H]Adenosine uptake was saturable and could be inhibited by nitrobenzylthioinosine and dipyridamole and by pretreatment of the [3H]adenosine with adenosine deaminase. Although these observations indicate that the uptake is specific for adenosine, only 35% of accumulated radioactivity was associated with adenosine, with the remaining 65% representing inosine, hypoxanthine, and nucleotides plus uric acid. Adenosine as well as several of its metabolites were released by the cells under basal as well as K(+)-stimulated conditions. Potassium-enhanced release was blocked by 10 mM CoCl2 or in Ca2(+)-free, Mg2(+)-rich solutions. The results indicate that retinal cells that synthesize, store, and release adenosine differentiate early during embryogenesis and are therefore consistent with a hypothetical role for adenosine in retinal development.

K(+)- and temperature-evoked taurine efflux from hypothalamic astrocytes

Hypothalamic astrocytes in culture released taurine, a suspected inhibitory amino acid neurotransmitter/neuromodulator/osmoregulator, in response to isoosmotically increasing extracellular K+ in a dose-dependent fashion. In the absence of added Ca2+, basal release levels rose to approach those obtained after exposure to 60 mM K+ in the presence of 2.5 mM Ca2+, and were only partially lowered by the addition of 10 mM Mg2+. Stimulation with K+ (60 mM) did not further increase taurine efflux above the high basal levels seen in the absence of Ca2+. Under standard conditions complete replacement of Na+ with choline Cl had little effect on basal taurine release, but reduced K(+)-evoked (60 mM) efflux by 60%. The temperature dependence of the basal levels of taurine released from hypothalamic astrocytes was similar to that seen for cultured cerebellar astrocytes and neurons over the range 5-50 degrees C. Taurine release increased from 5 to 15 degrees C, remained constant between 15 and 33 degrees C, decreased between 33 and 37 degrees C and increased thereafter. The infection point of increased basal taurine release seen around 37 degrees C (most prominent in astrocytes), may be of physiological significance. Results presented also show that the ion (Na+, Ca2+ and K+) sensitivities of taurine efflux for cultured hypothalamic astrocytes are similar to those previously reported for cultured astrocytes from the cerebellum.