A role for calcium/calmodulin kinase(s) in the regulation of GABA exocytosis

The Role of Calmodulin vs. Synaptotagmin in Exocytosis

Exocytosis is a Ca²⁺-regulated process that requires the participation of Ca²⁺ sensors. In the 1980s, two classes of Ca²⁺-binding proteins were proposed as putative Ca²⁺ sensors: EF-hand protein calmodulin, and the C2 domain protein synaptotagmin. In the next few decades, numerous studies determined that in the final stage of membrane fusion triggered by a micromolar boost in the level of Ca²⁺, the low affinity Ca²⁺-binding protein synaptotagmin, especially synaptotagmin 1 and 2, acts as the primary Ca²⁺ sensor, whereas calmodulin is unlikely to be functional due to its high Ca²⁺ affinity. However, in the meantime emerging evidence has revealed that calmodulin is involved in the earlier exocytotic steps prior to fusion, such as vesicle trafficking, docking and priming by acting as a high affinity Ca²⁺ sensor activated at submicromolar level of Ca²⁺. Calmodulin directly interacts with multiple regulatory proteins involved in the regulation of exocytosis, including VAMP, myosin V, Munc13, synapsin, GAP43 and Rab3, and switches on key kinases, such as type II Ca²⁺/calmodulin-dependent protein kinase, to phosphorylate a series of exocytosis regulators, including syntaxin, synapsin, RIM and Ca²⁺ channels. Moreover, calmodulin interacts with synaptotagmin through either direct binding or indirect phosphorylation. In summary, calmodulin and synaptotagmin are Ca²⁺ sensors that play complementary roles throughout the process of exocytosis. In this review, we discuss the complementary roles that calmodulin and synaptotagmin play as Ca²⁺ sensors during exocytosis.

VPAC1 and VPAC2 receptor activation on GABA release from hippocampal nerve terminals involve several different signalling pathways

BACKGROUND AND PURPOSE

Vasoactive intestinal peptide (VIP) is an important modulator of hippocampal synaptic transmission that influences both GABAergic synaptic transmission and glutamatergic cell excitability through activation of VPAC₁ and VPAC₂ receptors. Presynaptic enhancement of GABA release contributes to VIP modulation of hippocampal synaptic transmission.

EXPERIMENTAL APPROACH

We investigated which VIP receptors and coupled transduction pathways were involved in VIP enhancement of K⁺ -evoked [³ H]-GABA release from isolated nerve terminals of rat hippocampus.

KEY RESULTS

VIP enhancement of [³ H]-GABA release was potentiated in the presence of the VPAC₁ receptor antagonist PG 97-269 but converted into an inhibition in the presence of the VPAC₂ receptor antagonist PG 99-465, suggesting that activation of VPAC₁ receptors inhibits and activation of VPAC₂ receptors enhances, GABA release. A VPAC₁ receptor agonist inhibited exocytotic voltage-gated calcium channel (VGCC)-dependent [³ H]-GABA release through activation of protein G_i/o , an effect also dependent on PKC activity. A VPAC₂ receptor agonist enhanced both exocytotic VGCC-dependent release through protein G_s -dependent, PKA-dependent and PKC-dependent mechanisms and GABA transporter 1-mediated [³ H]-GABA release through a G_s protein-dependent and PKC-dependent mechanism.

CONCLUSIONS AND IMPLICATIONS

Our results show that VPAC₁ and VPAC₂ VIP receptors have opposing actions on GABA release from hippocampal nerve terminals through activation of different transduction pathways. As VPAC₁ and VPAC₂ receptors are located in different layers of Ammon's horn, our results suggest that these VIP receptors underlie different modulation of synaptic transmission to pyramidal cell dendrites and cell bodies, with important consequences for their possible therapeutic application in the treatment of epilepsy.

P2Y1 receptor inhibits GABA transport through a calcium signalling-dependent mechanism in rat cortical astrocytes

Astrocytes express a variety of purinergic (P2) receptors, involved in astrocytic communication through fast increases in [Ca(2+) ]i . Of these, the metabotropic ATP receptors (P2Y) regulate cytoplasmic Ca(2+) levels through the PLC-PKC pathway. GABA transporters are a substrate for a number of Ca(2+) -related kinases, raising the possibility that calcium signalling in astrocytes impact the control of extracellular levels of the major inhibitory transmitter in the brain. To access this possibility we tested the influence of P2Y receptors upon GABA transport into astrocytes. Mature primary cortical astroglial-enriched cultures expressed functional P2Y receptors, as evaluated through Ca(2+) imaging, being P2Y1 the predominant P2Y receptor subtype. ATP (100 μM, for 1 min) caused an inhibition of GABA transport through either GAT-1 or GAT-3 transporters, decreasing the Vmax kinetic constant. ATP-induced inhibition of GATs activity was still evident in the presence of adenosine deaminase, precluding an adenosine-mediated effect. This, was mimicked by a specific agonist for the P2Y1,12,13 receptor (2-MeSADP). The effect of 2-MeSADP on GABA transport was blocked by the P2 (PPADS) and P2Y1 selective (MRS2179) receptor antagonists, as well as by the PLC inhibitor (U73122). 2-MeSADP failed to inhibit GABA transport in astrocytes where intracellular calcium had been chelated (BAPTA-AM) or where calcium stores were depleted (α-cyclopiazonic acid, CPA). In conclusion, P2Y1 receptors in astrocytes inhibit GABA transport through a mechanism dependent of P2Y1 -mediated calcium signalling, suggesting that astrocytic calcium signalling, which occurs as a consequence of neuronal firing, may operate a negative feedback loop to enhance extracellular levels of GABA.

Centrally injected angiotensin II trans-synaptically activates angiotensin II-sensitive neurons in the anterior hypothalamic area of rats

Previously, we have demonstrated that pressure-ejected application of angiotensin II onto some neurons in the anterior hypothalamic area (AHA) of rats increases their firing rate. In contrast, pressure application of the angiotensin AT1 receptor antagonist losartan onto AHA neurons blocked the basal firing of the neurons. To investigate possible participation of these AHA neurons in the brain angiotensin system, we examined whether intracerebroventricular injection of angiotensin II results in an activation of angiotensin II-sensitive neurons in the AHA of rats. Intracerebroventricular injection of angiotensin II increased the firing rate of AHA angiotensin II-sensitive neurons. The angiotensin II-induced increase of unit firing in AHA neurons was abolished by pressure application of losartan onto the same neurons. In addition, the angiotensin II-induced increase of firing in AHA neurons was abolished by pressure application of N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W7), a calmodulin inhibitor, onto the same neurons. Pressure application of W7 onto AHA neurons affected neither the basal firing rate nor the increase in unit firing induced by pressure application of angiotensin II onto the same neurons. Intracerebroventricular injection of the cholinergic agonist carbachol did not affect the firing rate of angiotensin II-sensitive neurons in the AHA. These findings suggest that intracerebroventricular injection of angiotensin II activates AHA angiotensin II-sensitive neurons via angiotensinergic inputs to the neurons.

Evidence suggesting that angiotensins released not via synaptic inputs are involved in the basal activity of anterior hypothalamic neurons in rats

Previously, we have demonstrated that angiotensin II-sensitive neurons exist in the anterior hypothalamic area (AHA) and that these neurons are tonically activated by endogenous angiotensins in rats. Chemical stimulation of the lateral septal area (LSV) and medial amygdaloid nucleus (MeA), and intracerebroventricular injection of hypertonic saline, activated AHA angiotensin II-sensitive neurons. To investigate mechanisms of the basal activity of AHA angiotensin II-sensitive neurons, we examined the effect of N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W7), a calmodulin inhibitor, applied onto AHA neurons on the basal activity and the stimulus-evoked activation of these neurons. Male Wistar rats were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. Microinjections of carbachol into the LSV and corticotropin-releasing factor into the MeA, and intracerebroventricular injection of hypertonic saline, activated AHA angiotensin II-sensitive neurons. These three kinds of injection-induced activations of AHA neurons were abolished by pressure application of W7 onto the same neurons, while the calmodulin inhibitor did not affect the increase in firing of AHA neurons induced by pressure application of angiotensin II onto the same neurons. The pressure application of W7 did not affect the basal activity of AHA angiotensin II-sensitive neurons, whereas the angiotensin AT1 receptor antagonist losartan similarly applied inhibited it. These findings suggest that the basal activity of AHA angiotensin II-sensitive neurons is mediated by angiotensins released not via synaptic inputs.

Differential modulation of glutamatergic and cholinergic synapses by calcineurin in hippocampal CA1 fast-spiking interneurons

How signaling molecules in inhibitory interneurons modulate and coordinate the integration of synaptic inputs remains largely unknown. We investigated the kinetics and modulation of glutamatergic and cholinergic synapses on CA1 fast-spiking interneurons in hippocampal slices by using whole-cell clamp recording. Spontaneous synaptic currents mediated by either AMPA-type glutamate or nicotinic acetylcholine receptors on the interneurons can be classified into fast, slow and fast-slow based on their duration and decay phase. Effects of calcineurin, calmodulin-dependent protein phosphatase, on these two groups of synapses were examined by infusing an autoinhibitory peptide of calcineurin (CaN-AIP) into the recording neurons. CaN-AIP enhanced the amplitude of glutamatergic fast-EPSCs, as well as both amplitude and frequency of cholinergic fast-EPSCs. No significant changes in slow-EPSCs were observed during the infusion of CaN-AIP. Our results indicate that signal transmission at synapses, which are mediated by either AMPA-type glutamate or nicotinic acetylcholine receptors, appears different in the kinetics. The selective influence of calcineurin on different synapses in fast-spiking interneurons may play an important role in coordinating thousands of synaptic inputs in order to set neuronal excitability at proper levels.

Protein kinase A and calcium/calmodulin-dependent protein kinase II regulate glycine and GABA release in auditory brain stem nuclei

We reported previously that unilateral cochlear ablation (UCA) in young adult guinea pigs induced protein kinase C (PKC)-dependent plastic changes in the electrically evoked release of exogenous [14C]glycine ([14C]Gly) or [14C]-gamma-aminobutyric acid ([14C]GABA) in several brain stem auditory nuclei. The present study assessed whether such changes depended on protein kinase A (PKA) and calcium/calmodulin-dependent protein kinase II (CaMKII). In the major subdivisions of the cochlear nucleus (CN) and the main nuclei of the superior olivary complex (SOC) dissected from intact animals, dibutyryl-cyclic adenosine monophosphate (DBcAMP) (0.2 mM), a PKA activator, elevated release by 1.6-2.3-fold. The PKA inhibitor, H-89 (2 microM), did not alter the release but blocked the stimulatory effects of DBcAMP. These findings suggested that PKA could positively regulate glycinergic and GABAergic release. After UCA, PKA regulation declined and failed in the ventral CN but persisted in the SOC nuclei. After 145 postablation days, H-89 reversed elevations of [14C]GABA release in the medial nucleus of the trapezoid body (MNTB). A CaMKII inhibitor, KN-93, reversed depressions of [14C]Gly release in the DCN. Thus, the postablation plasticities in these nuclei probably depended on PKA or CaMKII. Both H-89 and KN-93 depressed [14C]Gly release in the lateral superior olive (LSO) and ipsilateral medial superior olive (MSO), suggesting that either kinase was used by endogenous mechanisms in these nuclei to upregulate glycinergic release. In contrast, KN-93 elevated [14C]GABA release in the contralateral MNTB, suggesting a downregulatory action of CaMKII, an action opposite to that of PKA.

Ca(2+)-evoked synaptic transmission and neurotransmitter receptor levels are impaired in the forebrain of trkb (-/-) mice

To determine the in vivo targets of long-lasting actions of TrkB signaling on synaptic function we analyze synaptic components of excitatory and inhibitory circuits in the cerebral cortex of trkB (-/-) mice. First, we show that K(+)-evoked glutamate and GABA release from forebrain mutant synaptosomes was decreased. Moreover, the dependence of regulated exocytosis on the SNARE SNAP-25 and the Ca(2+)-dependent neurotransmitter release were also impaired in trkB (-/-) mice. We also analyzed postsynaptic glutamate and GABA(A) ionotropic receptors in cortical areas of trkB mutant mice. By using Western blot we observed decreased levels of the AMPA receptor subunits GluR2/3 and GluR4 in trkB (-/-) forebrains. In contrast, the forebrain of mutant mice exhibited increased levels of the GABA(A) receptor subunit alpha3 and alpha5 and a reduction of the gamma2 subunit. Immunocytochemical analysis showed that the hippocampus and neocortex of mutant mice exhibited decreased numbers of interneurons positive for distinct AMPA and GABA(A) receptor subunits. Furthermore, alteration of inhibitory circuits in trkB (-/-) mice was also shown by the low expression of the GABA-synthesizing enzyme glutamic acid decarboxylase in mutant cortical areas. The present results indicate that long-lasting TrkB signaling is required for the precise adjustment of neurotransmitter release and for the correct composition of the fast glutamatergic and GABAergic receptor subunits in vivo.

Use-dependent decline of paired-pulse facilitation at Aplysia sensory neuron synapses suggests a distinct vesicle pool or release mechanism

We have characterized paired-pulse facilitation at Aplysia sensory neuron-to-motoneuron synapses. This simple form of very short-term synaptic plasticity displayed an unusual feature: it decreased dramatically with repeated testing. Synaptic depression at these synapses and this use-dependent decrease in paired-pulse facilitation occurred independently of each other. Paired-pulse facilitation was inversely correlated with the size of the initial synaptic connection and was absent at stronger synapses. The use-dependent decrease in paired-pulse facilitation occurred at the same rate at large synapses as at small synapses, although the initial paired-pulse facilitation at large synapses was substantially smaller. Rates of synaptic depression were also independent of initial synaptic strength. Paired-pulse facilitation was blocked by presynaptic EGTA injection, but not by postsynaptic EGTA or BAPTA injection. These results indicate that presynaptic Ca2+ influx plays a critical role in paired-pulse facilitation. However, the persistence of the decrease in paired-pulse facilitation for longer than 15 min suggests that Ca2+ from the first paired action potential produces facilitation via a modulatory mechanism rather than by summating with Ca2+ influx during the second paired action potential in activating the Ca2+ binding sites that initiate exocytosis. This modulatory mechanism may not involve protein phosphorylation because paired-pulse facilitation was unaffected by the protein kinase inhibitors H7 and KN-62. These findings further suggest that release by the second paired action potential occurs at sites distinct from those that mediate release by the first action potential.

Regulation of the gamma-aminobutyric acid transporter activity by protein phosphatases in synaptic plasma membranes

The influence of the phosphorylation dephosphorylation states on the gamma-aminobutyric acid (GABA) transporter activity of synaptic plasma membranes (SPM) was studied by using either specific phosphatase inhibitors or activators. Calyculin A and okadaic acid (phosphatase 1 and phosphatase 2A inhibitors) inhibited the GABA uptake by isolated SPM vesicles, whereas cyclosporin A (phosphatase 2B inhibitor) had a stimulatory effect (approximately 10%) which was higher (approximately 38%) when all these drugs were present in the reaction medium. On the other hand, intravesicular Ca2+, up to about 10 microM, inhibited the GABA uptake (approximately 50%) in a manner which appeared to be facilitated in the presence of PP1 and PP2A inhibitors and this inhibition was relieved by the calmodulin antagonist W-7. We also observed that isolated SPM vesicles contain both Ca(2+)-independent phosphatase activity that is significantly inhibited by PP1 and PP2A inhibitors, and Ca(2+)-dependent phosphatase activity that is abolished in the presence of the PP2B inhibitor, cyclosporin A. These results indicate that regulation of the SPM GABA transporter is determined by the internally localized Ca-calmodulin-dependent phosphatase activity (calcineurin), and that other phosphorylated sites, sensitive to PP1 and PP2A inhibitors, potentiate either the positive or negative effects exerted by those internal sites when they are in their phosphorylated or dephosphorylated states, respectively.

Effects of external pH variations on brain presynaptic sodium and calcium channels; repercussion on the evoked release of amino acid neurotransmitters

The effects of external pH (pHout) variations on the Na+ and on the Ca2+ dependent fractions of the evoked amino acid neurotransmitter release were separately investigated, using GABA as a model transmitter. In [3H]GABA loaded mouse brain synaptosomes, the external acidification (pHout 6.0) markedly decreased the Na+ dependent fraction of [3H]GABA release evoked by veratridine (10 microM) in the absence of external Ca2+, as well as the Ca2+ dependent fraction of [3H]GABA release evoked by high (20 mM) K+ in the absence of external Na+. The depolarization-induced elevation of [Na(i)] (monitored in synaptosomes loaded with the Na+ indicator dye, SBFI) and the depolarization-induced elevation of [Ca(i)] (monitored in synaptosomes loaded with the Ca2+ indicator dye fura-2) were also markedly decreased at pHout 6. On the contrary, the external alkalinization (pHout 8) facilitated all the above responses. A slight increase of the baseline release of the [3H]GABA was observed when pHout was changed from 7.4 to 8. This effect was only observed in the presence of Ca2+. pHout changes from 7.4 to 6 or to 7 did not modify the baseline release of the transmitter. All the effects of pHout variations on [3H]GABA release were independent on the presence of HCO3-. It is concluded that external H+ regulate amino acid neurotransmitter release by their actions on presynaptic Na+ channels, as well as on presynaptic Ca2+ channels.

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.

Characterization of melittin effects in synaptosomes

The effects of melittin at increasing concentrations on: [3H]GABA release from mouse brain synaptosomes; on the radioactivity released from [3H]arachidonic acid labeled synaptosomal membranes; on synaptosomes ultrastructure and on the leakage of the cytoplasmic marker, lactate-dehydrogenase (LDH) was investigated. Melittin 0.3, 1, 3, 7, and 10 microM progressively increases [3H]GABA release, but the efficacy of melittin is decreased when the amount of tissue exposed to a constant concentration of the toxin increases. The release of [3H]GABA induced by melittin below 3 microM is Ca2+ dependent, but not that induced by the higher concentrations. The Ca2+ dependent fraction of the [3H]GABA released by 0.3 microM melittin is selectively inhibited by 10 microM quinacrine and 1 microM nordihydroguaiaretic acid (NDGA) and facilitated by 3 microM indomethacin, whereas the Ca2+ independent fraction of the [3H]GABA released by melittin is not. In the presence of Ca2+, melittin 0.3, 1 and 10 microM progressively increases [3H]arachidonic acid release over control release, but the effectiveness of melittin is also decreased as the amount of tissue increases. No apparent changes in synaptosomes ultrastructure are observed in 0.3 microM treated synaptosomes, but a noticeable disorganization is produced in 10 microM melittin-treated synaptosomes, independently on the presence of external Ca2+. LDH activity only increases over control activity in the supernatant solutions of 10 microM melittin treated synaptosomes, also in a Ca2+ independent manner. Our interpretation of these results is that the Ca2+-dependent, pharmacologic sensitive component of melittin-induced release of [3H]GABA, unmasked when 0.3 microM melittin was used, involves the activation of a Ca2+-dependent type of membrane PLA2. The Ca2+-independent release of [3H]GABA is in contrast, highly probable to be due to the membrane perturbation produced by complex melittin/lipid interactions.

Characterization of the type of calcium channel primarily regulating GABA exocytosis from brain nerve endings

In an attempt to further characterize the type of Ca2+ channels primarily regulating GABA exocytosis, the effects of increasing concentrations of omega CTx MVIIC,-omega-Aga IVA and other Ca2+ channel blockers (nitrendipine, Cd2+ and Ni2+), commonly used for pharmacologically discerning among the various types of Ca2+ channels, were tested on the dissected Ca2+ dependent fraction of the depolarization evoked release of GABA from mouse brain synaptosomes. Our results show that omega-CTx MVIIC inhibits GABA exocytosis with a calculated IC50 of 3 microM and omega-Aga IVA with a calculated IC50 of 50 nM. The divalent cation Cd2+ only diminishes GABA exocytosis at 70 microM, but does not modify this response at lower concentrations (i.e. 1 and 10 microM). Neither nitrendipine (10 microM) nor Ni2+ (100 microM and 500 microM) modified GABA exocytosis. The failure of nitrendipine at a high concentration to inhibit GABA exocytosis discards L-type Ca2+ channels as the main regulators of this response; likewise that of Ni2+ discards Ca2+ channels of the N-type, and the failure of nM concentrations of omega-CTx MVIIC or 500 microM Ni2+, also discards alpha 1A/Q-type Ca2+ channels as the main regulators of the GABA response. On the basis of these results and in particular of the higher potency of omega-Aga IVA than omega-CTx MVIIC, it is concluded that the type of Ca2+ channels that primarily determine the exocytosis of GABA belong to a P-like type of Ca2+ channels.

omega-Aga IVA selectively inhibits the calcium-dependent fraction of the evoked release of [3H]GABA from synaptosomes

The effect of omega-Aga IVA, a P-type Ca2+ channel blocker, on the release of the inhibitory neurotransmitter GABA and on the elevation of Cai induced by depolarization was investigated in [3H]GABA and fura-2 preloaded mouse brain synaptosomes, respectively. Two strategies (i.e. 20 mM external K+ and veratridine) that depolarize by different mechanisms the preparation were used. High K+ elevates Cai and induces [3H]GABA release in the absence of external Na+ and in the presence of TTX, conditions that abolish veratridine induced responses. The effect of omega-Aga IVA on the Ca2+ and Na+ dependent fractions of the depolarization evoked release of [3H]GABA were separately investigated in synaptosomes depolarized with high K+ in the absence of external Na+ and with veratridine in the absence of external Ca2+, respectively. The Ca2+ dependent fraction of the evoked release of [3H]GABA and the elevation of Ca2+ induced by high K+ are markedly inhibited (about 50%) in synaptosomes exposed to omega-Aga IVA (300 nM) for 3 min before depolarization, whereas the Na+ dependent, Ca2+ independent carrier mediated release of [3H]GABA induced by veratridine, which is sensitive to verapamil and amiloride, is not modified by omega-Aga IVA. Our results indicate that an omega-Aga IVA sensitive type of Ca2+ channel is highly involved in GABA exocytosis.