Modulation of vertebrate neuronal calcium channels by transmitters

Actions of aluminum on voltage-activated calcium channel currents

1. Extracellular and intracellular effects of aluminum (Al) on voltage-activated calcium channel currents (VACCCs) of cultured rat dorsal root ganglion (DRG) neurons were investigated. Al (0.54 to 5.4 micrograms/ml = 20-200 microM) applied extracellularly reduces VACCCs in a concentration-dependent manner. The IC50 was calculated to be 2.3 micrograms/ml (83 microM). All types of VACCCs were similarly reduced by Al treatment. A slight shift of the current-voltage relation to depolarized potentials was observed for higher Al concentrations (> 2 micrograms/ml). The action of Al was found to be use dependent, with little recovery (max. 20%) after wash. 2. The effect of Al was highly pH dependent in the investigated range (pH 6.4 to 7.8). We observed a rightward shift of the concentration-response curve at pH 7.7 (IC50:3.1 micrograms/ml) and a leftward shift at pH 6.4 (IC50:0.56 microgram/ml) compared to the concentration-response curve at pH 7.3. 3. The VACCC declined when 2.7 micrograms/ml Al was added to the internal solution. A steady state was reached within a few minutes. Additional extracellular application of the same concentration lead to an additional decrease of the current. These observations strongly suggest the existence of both intracellular and extracellular accessible binding sites for Al on voltage-activated calcium channels (VACCs). 4. The special characteristics of the action of Al on VACCCs, i.e., the irreversibility, use dependence, and pH dependence, as well as the additional internal binding site may contribute to its neurotoxicity.

Modulation of ion-channel function by G-protein-coupled receptors

Neurotransmitters acting through G-protein-coupled receptors change the electrical excitability of neurons. Activation of receptors can affect the voltage dependence, the speed of gating, and the probability of opening of various ion channels, thus changing the computational state and outputs of a neuron. Each cell expresses many kinds of receptors, and uses several intracellular signaling pathways to modulate channel function in different ways. It has become possible to dissect these pathways by combining pharmacological and biophysical experiments. Recent patch-clamp work in sympathetic neurons will be summarized to illustrate the mechanisms underlying modulation and its significance.

Depolarization-induced suppression of GABAergic inhibition in rat hippocampal pyramidal cells: G protein involvement in a presynaptic mechanism

Following postsynaptic activation of a pyramidal cell, the degree of GABAergic synaptic inhibition that the cell receives is reduced dramatically for many seconds. Previously, we found that induction of depolarization-induced suppression of inhibition (DSI) required post-synaptic increases in intracellular [Ca2+], but absence of a decrease in responsiveness to iontophoretically applied GABA left the mechanism of DSI expression uncertain. We investigated DSI with whole-cell voltage-clamp recordings in rat hippocampal slices. Bath-applied carbachol was ordinarily used to increase the spontaneous action potential-induced IPSCs (sIPSCs) and enhance detectability of DSI; synaptically released ACh has the same effects. TTX-sensitive sIPSCs are markedly reduced by DSI, whereas TTX-insensitive miniature IPSC amplitudes do not change, suggesting that DSI represents a retrograde influence on presynaptic GABA release. A lag (approximately 1 s) prior to maximal DSI and prevention of DSI by pertussis toxin pointed to a G protein-linked second messenger that may be presynaptic, since perturbation of postsynaptic G protein function did not alter DSI.

Modification of cysteine residues within G(o) and other neuronal proteins by exposure to nitric oxide

Nitric oxide (NO), a free-radical gas produced endogenously by some neurons, functions as a diffusible intercellular messenger and appears to play a role in activity-dependent modification of synaptic efficacy in the mammalian CNS. The molecular targets and mechanisms of action of NO in neurons remain largely uncharacterized. Employing in vitro brain slices and isolated synaptosomes, we show here that exposure to exogenous or endogenously generated NO results in the modification of cysteine residues within neuronal proteins, as revealed by reduced binding of agents which react with cysteine sulfhydryls. In particular, exposure of synaptosomes to NO inhibits subsequent thiol-linked ADP-ribosylation of the heterotrimeric G-protein, G(o), by pertussis toxin. Our results demonstrate directly that NO may exert its neuronal effects through modification of protein cysteine thiols, and identify G(o) as a potential synaptic target of NO.

Muscarinic (M1) receptor-mediated inhibition of K(+)-evoked [3H]-noradrenaline release from human neuroblastoma (SH-SY5Y) cells via inhibition of L- and N-type Ca2+ channels

1. Human neuroblastoma (SH-SY5Y) cells were preincubated with [3H]-noradrenaline ([3H]-NA) in the presence of 0.2 mM pargyline to examine the modulation of K(+)-evoked [3H]-NA release by muscarinic agonists. 2. Release of [3H]-NA evoked by 4 min exposure to 100 mM K+ could be partially inhibited by 5 microM nifedipine and partially inhibited by 100 nM omega-conotoxin GVIA (omega-CgTx). When nifedipine and omega-CgTx were added together, evoked release was inhibited by approximately 93%. 3. K(+)-evoked [3H]-NA release was inhibited by > 90% by pretreatment of cells for 2 min with muscarine, carbachol or oxotremorine methiodide (each at 300 microM). For muscarine, inhibition of evoked release was both time- and concentration-dependent and was reversible. Muscarine also inhibited [3H]-NA release evoked by veratridine (28 microM) and replacement of extracellular Ca2+ with Ba2+, but not that evoked by the Ca2+ ionophore, A23187 (19 microM). 4. Residual K(+)-evoked [3H]-NA release measured in the presence of either nifedipine (5 microM) or omega-CgTx (100 nM) was inhibited by muscarine with a similar potency as release evoked in the absence of either Ca2+ channel blocker. Pretreatment of cells for 16-24 h with pertussis toxin (200 ng ml-1) did not affect K(+)-evoked release per se or the ability of muscarine to inhibit such release. 5. Muscarinic inhibition of K(+)-evoked [3H]-NA release was potently antagonized by pirenzepine (pA2 8.14) and by hexahydrosiladiphenidol (pA2 9.03), suggesting the involvement of an M1 receptor. 6. Our results demonstrate that 100 mM K+-evoked release of [3H]-NA from the human neuroblastoma is mediated by activation of both L- and N-type Ca2+ channels. Activation of muscarinic Ml receptors can inhibit release via a pertussis toxin-insensitive mechanism which involves non-selective inhibition of L- and N-type Ca2+ channels.

Roles of N-type and Q-type Ca2+ channels in supporting hippocampal synaptic transmission

Several types of calcium channels found in the central nervous system are possible participants in triggering neurotransmitter release. Synaptic transmission between hippocampal CA3 and CA1 neurons was mediated by N-type calcium channels, together with calcium channels whose pharmacology differs from that of L- and P-type channels but resembles that of the Q-type channel encoded by the alpha 1A subunit gene. Blockade of either population of channels strongly increased enhancement of synaptic transmission with repetitive stimuli. Even after complete blockade of N-type channels, transmission was strongly modulated by stimulation of neurotransmitter receptors or protein kinase C. These findings suggest a role for alpha 1A subunits in synaptic transmission and support the idea that neurotransmitter release may depend on multiple types of calcium channels under physiological conditions.

omega-Conotoxin and Cd2+ stimulate the recruitment to the plasmamembrane of an intracellular pool of voltage-operated Ca2+ channels

125I-omega-conotoxin binding to neuroblastoma cells at 37 degrees C continuously increased, reaching a plateau after 6-8 hr; this was up to 6 times higher than that observed at lower temperatures. The same effect was induced by short pulses with omega-conotoxin followed by a chase period at 37 degrees C in control medium. Cd2+ also induced up-regulation of surface 125I-omega-conotoxin-binding sites. Fura-2 and patch-clamp experiments showed that the recruited binding sites corresponded to functional voltage-operated Ca2+ channels. Permeabilization experiments revealed a large intracellular pool of 125I-omega-conotoxin-binding sites, whose recruitment to the plasmamembrane was prevented by brefeldin A and nocodazole. These data suggest that specific stimuli might induce voltage-operated Ca2+ channel translocation to plasmamembrane and, in this way, modulate presynaptic events.

The pharmacology and function of central GABAB receptors

In conclusion, GABAB receptors enable GABA to modulate neuronal function in a manner not possible through GABAA receptors alone. These receptors are present at both pre- and postsynaptic sites and can exert both inhibitory and disinhibitory effects. In particular, GABAB receptors are important in regulating NMDA receptor-mediated responses, including the induction of LTP. They also can regulate the filtering properties of neural networks, allowing peak transmission in the frequency range of theta rhythm. Finally, GABAB receptors are G protein-coupled to a variety of intracellular effector systems, and thereby have the potential to produce long-term changes in the state of neuronal activity, through actions such as protein phosphorylation. Although the majority of the effects of GABAB receptors have been reported in vitro, recent studies have also demonstrated that GABAB receptors exert electrophysiological actions in vivo. For example, GABAB receptor antagonists reduce the late IPSP in vivo and consequently can decrease inhibition of spontaneous neuronal firing following a stimulus (Lingenhöhl and Olpe, 1993). In addition, blockade of GABAB receptors can increase spontaneous activity of central neurons, suggesting the presence of GABAB receptor-mediated tonic inhibition (Andre et al., 1992; Lingenhöhl and Olpe, 1993). Despite these electrophysiological effects, antagonism of GABAB receptors has generally been reported to produce few behavioral actions. This lack of overt behavioral effects most likely reflects the modulatory nature of the receptor action. Nevertheless, two separate behavioral studies have recently reported an enhancement of cognitive performance in several different animal species following blockade of GABAB receptors (Mondadori et al., 1992; Carletti et al., 1993). Because of their small number of side effects, GABAB receptor antagonists may represent effective therapeutic tools for modulation of cognition. Alternatively, the lack of overt behavioral effects of GABAB receptors may indicate that these receptors are more important in pathologic rather than normal physiological states (Wojcik et al., 1989). For example, a change in receptor affinity or receptor number brought on by the pathology could enhance the effectiveness of GABAB receptors. Of significance, CGP 35348 has been shown to block absence seizures in genetically seizure prone animals, while inducing no seizures in control animals (Hosford et al., 1992; Liu et al., 1992). Thus, GABAB receptors may represent effective sites for pharmacological regulation of absence seizures. Perhaps further behavioral effects of these receptors will become apparent only after additional studies have been performed using the highly potent antagonists that have been recently introduced.(ABSTRACT TRUNCATED AT 400 WORDS)

Kinetics of GABAB receptor-mediated inhibition of calcium currents and excitatory synaptic transmission in hippocampal neurons in vitro

The time courses of the gamma-aminobutyric acid type B (GABAB) receptor-mediated inhibition of excitatory synaptic transmission and of action potential-evoked calcium currents were studied in hippocampal neurons in vitro with step-like changes of a saturating baclofen concentration. Inhibition mediated by postsynaptic GABAB receptors was excluded pharmacologically. Both presynaptic inhibition and reduction of calcium currents developed and declined exponentially with similar time constants of about 0.2 and 3 s, respectively. The close correlation of the time courses indicates that fast, G protein-mediated depression of voltage-gated calcium channels and thus direct reduction of the presynaptic calcium influx may contribute to the GABAB receptor-induced inhibition of excitatory synaptic transmission in hippocampal neurons in vitro.

Presynaptic inhibition in the hippocampus

Presynaptic receptors for virtually all transmitters have been identified throughout the nervous system. Recent studies in the hippocampus provide new insights into the mechanisms by which the activation of these receptors leads to presynaptic inhibition of transmitter release, and characterize the second messengers involved in coupling presynaptic receptors to their effectors. Presynaptic receptors also provide a tractable route via which the amount of transmitter release may be selectively regulated in therapeutically useful ways.

Amine modulation of electrical coupling in the pyloric network of the lobster stomatogastric ganglion

1. The neurons of the pyloric network of the lobster (Panulirus interruptus) stomatogastric ganglion organize their rhythmic motor output using both chemical and electrical synapses. The 6 electrical synapses within this network help set the firing phases of the pyloric neurons during each rhythmic cycle. We examined the modulatory effects of the amines dopamine (DA), serotonin (5HT) and octopamine (Oct) on coupling at all the electrical synapses of the pyloric network. 2. Electrical coupling within the pacemaker group [anterior burster (AB) to pyloric dilator (PD), and PD-PD] was non-rectifying, while coupling at the other electrical synapses [AB to ventral dilator (VD), PD-VD, lateral pyloric (LP) to pyloric (PY), and PY-PY] was rectifying. 3. Dopamine decreased or increased the coupling strength of all the pyloric electrical synapses: the sign of the effect depended upon which neuron was the target of current injection. For example, DA decreased AB-->PD coupling (i.e., when current was injected into the AB) but increased coupling in the other direction, PD-->AB. Dopamine decreased AB to VD coupling when current was injected into either neuron. Serotonin also had mixed effects; it enhanced PD-->AB coupling but decreased AB to VD and PD to VD coupling in both directions. Octopamine's only effect was to reduce PD-->VD coupling. 4. Dopamine increased the input resistance of the AB neuron but decreased the input resistance of the PD and VD neurons. Serotonin reduced the input resistance of the VD and PY neurons, while Oct did not significantly change the input resistance of any pyloric neuron. 5. The characteristic modulation of electrical coupling by each amine may contribute to the unique motor pattern that DA, 5HT and Oct each elicit from the pyloric motor network.

Molecular diversity of Ca2+ channel alpha 1 subunits from the marine ray Discopyge ommata

In many neurons, transmitter release from presynaptic terminals is triggered by Ca2+ entry via dihydropyridine-insensitive Ca2+ channels. We have looked for cDNAs for such channels in the nervous system of the marine ray Discopyge ommata. One cDNA (doe-2) is similar to dihydropyridine-sensitive L-type channels, and two cDNAs (doe-1 and doe-4) are similar to the subfamily of dihydropyridine-insensitive non-L-type channels. doe-4, which encodes a protein of 2326 aa, most closely resembles a previously cloned N-type channel. doe-1, which encodes a protein of 2223 aa, is a member of a separate branch of the non-L-type channels. Northern blot analysis reveals that doe-1 is abundant in the forebrain. doe-4 is more plentiful in the electric lobe and, therefore, may control neurotransmitter release in motor nerve terminals. These results show that the familial pattern of Ca(2+)-channel genes has been preserved from a stage in evolution before the divergence of higher and lower vertebrates > 400 million years ago. The cloning of these channels may be a useful starting point for elucidating the role of the Ca2+ channels in excitation-secretion coupling in nerve terminals.

GABAB receptor inhibition of P-type Ca2+ channels in central neurons

P-type Ca2+ channels in cerebellar Purkinje neurons were inhibited by GABA and the GABAB receptor agonist baclofen. Inhibition of P-type Ca2+ channel current involved changes in voltage dependence and kinetics. Baclofen induced a slow phase of activation and altered tail current kinetics, and inhibition could be partly overcome by large depolarizations. These effects were mimicked by internal application of GTP gamma S, which also made the action of baclofen irreversible. In spinal cord neurons, use of selective channel blockers showed that baclofen inhibited both P-type and N-type Ca2+ channels, but not L-type Ca2+ channels; a high threshold current resistant to blockers of P-type, N-type, and L-type channels was also modulated by baclofen. These results show that stimulation of GABAB receptors in central neurons can modulate P-type Ca2+ channels through a G protein-mediated mechanism similar to the one linked to N-type Ca2+ channels.

Inhibition of high voltage-activated calcium currents by L-glutamate receptor-mediated calcium influx

The modulation of high voltage-activated (HVA) Ca2+ currents by L-glutamate and its agonists was investigated in cultured rat hypothalamic neurons. L-Glutamate and agonists selective for NMDA or non-NMDA receptors reversibly inhibited HVA Ca2+ currents. The putative presynaptic glutamate receptor agonist L-2-amino-4-phosphonobutyric acid and the selective metabotropic agonist trans-ACPD were ineffective. Inhibition was dependent on the presence of extracellular Ca2+ and blocked by internal perfusion of the cells with BAPTA. The calmodulin antagonists trifluoperazine and calmidazolium completely prevented the inhibition. Increases in the intracellular Ca2+ concentration due to Ca2+ influx through non-NMDA receptor channels were visualized using fura-2. These results indicate that not only NMDA but also non-NMDA receptor channels in these neurons are permeable for Ca2+ and that Ca2+ influx through these channels activates a calmodulin-dependent mechanism, which leads to HVA Ca2+ current inhibition.

Modulation of calcium current, intracellular calcium levels and cell survival by glucose deprivation and growth factors in hippocampal neurons

Basic fibroblast growth factor (bFGF) and nerve growth factor (NGF) can protect CNS neurons against ischemic/excitotoxic insults, but the mechanism of action is unknown. Imaging of the calcium indicator dye fura-2 and whole-cell patch clamp recordings of calcium currents were used to examine the mechanisms whereby hypoglycemia damages and growth factors protect cultured rat hippocampal neurons. When cultures were deprived of glucose, massive neuronal death occurred 16-24 h following the onset of hypoglycemia. Early hypoglycemia-induced changes included calcium current inhibition and a reduction in intracellular free calcium levels ([Ca2+]i) without morphological signs of neuronal damage. Later changes included a large elevation of [Ca2+]i which was causally involved in neuronal damage. NGF and bFGF prevented or reduced both the early and later responses to hypoglycemia. The growth factors increased calcium (barium) current and [Ca2+]i to normal limits during the early stages of hypoglycemia and prevented the later elevation in [Ca2+]i and neuronal damage. Nifedipine, but not omega-conotoxin, blocked calcium currents. The increased calcium current caused by the growth factors was apparently not sufficient to protect neurons against hypoglycemic damage since K+ depolarization during the early stages of hypoglycemia did not prevent and, in fact exacerbated, the subsequent neuronal damage. In addition, exposure of neurons to K+, NGF or bFGF only during the first 1 h of hypoglycemia did not protect against hypoglycemic damage. Taken together, the data suggest that neurons initially respond to hypoglycemia with a reduction in calcium currents which may provide a means to maintain [Ca2+]i within a concentration range conducive to cell survival. Prolonged energy deprivation eventually results in a failure of calcium extrusion systems, glutamate receptor activation and a loss of neuronal calcium homeostasis. Taken together, the data indicate that the mechanism of growth factor protection against energy deprivation involves prevention of the late rise in [Ca2+]i.

Inhibitory effects of histamine and bradykinin on calcium current in smooth muscle cells isolated from guinea-pig ileum

1. Single smooth muscle cells were isolated from the longitudinal muscle layer of the guinea-pig ileum and within 10 h Ca(2+)-currents (ICa) were recorded using the whole-cell patch clamp technique. 2. Histamine (10 microMs) and bradykinin (BK, 1 microM) suppressed ICa; the effect had two phases: a rapid and transient suppression of ICa followed by a sustained suppression. Acetylcholine and substance P appeared to have similar effects but these were not investigated in detail. 3. The effects of histamine and BK on ICa were established by high intracellular concentrations of the Ca2+ buffer EGTA (30 mM) or 1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) (5 mM) in the absence of Ca2+ added to the pipette solution. When [Ca2+]i was strongly buffered to 125 or 190 nM by BAPTA-Ca2+ mixtures in the pipette the transient suppression of ICa was blocked but the sustained effect still occurred. This indicated that the transient effect was caused by a rise in [Ca2+]i. The sustained effect, in contrast, did not seem to be caused by a rise in [Ca2+]i but did show Ca2+ dependence because it did not occur if [Ca2+]i was abnormally low. 4. Application of caffeine (10 mM) to deplete stored Ca2+ or intracellular heparin (1 mM) to block the action of D-myo-inositol 1,4,5-trisphosphate (IP3) to release stored Ca2+ prevented the transient but not the sustained suppression of ICa. Heparin also blocked the transient Ca(2+)-activated K+ current in response to histamine or BK. Both transient and sustained suppressions of Ca2+ channel activity were observed in the absence of extracellular Ca2+ when current was carried mostly by Na+ ions. 5. Intracellular guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S; 10 or 100 microM) induced a gradual decline of ICa upon which transient decreases of current were superimposed. Histamine caused a larger than normal inhibition of ICa and no recovery occurred on wash-out. Intracellular guanosine 5'-O-(2-thiodiphosphate) (GDP-beta-S; 1 mM) abolished the effects of histamine and BK on ICa.(ABSTRACT TRUNCATED AT 400 WORDS)

Altered prevalence of gating modes in neurotransmitter inhibition of N-type calcium channels

G protein-mediated inhibition of voltage-activated calcium channels by neurotransmitters has important consequences for the control of synaptic strength. Single-channel recordings of N-type calcium channels in frog sympathetic neurons reveal at least three distinct patterns of gating, designated low-Po, medium-Po, and high-Po modes according to their probability of being open (Po) at -10 millivolts. The high-Po mode is responsible for the bulk of divalent cation entry in the absence of neurotransmitter. Norepinephrine greatly decreased the prevalence of high-Po gating and increased the proportion of time a channel exhibited low-Po behavior or no activity at all, which thereby reduced the overall current. Directly observed patterns of transition between the various modes suggest that activated G protein alters the balance between modal behaviors that freely interconvert even in the absence of modulatory signaling.

Regulation by protein kinase-C of putative P-type Ca channels expressed in Xenopus oocytes from cerebellar mRNA

Xenopus oocytes injected with rat cerebellar mRNA expressed functional voltage-dependent Ca channels detected as an inward Ba current (IBa). The pharmacological resistance to dihydropyridines and omega-conotoxin together with the blockade obtained with Agelenopsis aperta venom suggest that these channels could be somehow assimilated to P-type Ca channels. The precise nature of the transplanted Ca channels was assessed by hybrid-arrest experiments using a specific oligonucleotide antisense-derivated from the recently cloned alpha 1-subunit of P channels (BI-1 clone). In addition, we demonstrate that exogenous Ca channel activity was enhanced by two different PKC activators (a phorbol ester and a structural analog to diacylglycerol). The general electrophysiological and pharmacological properties of the stimulated Ca channels remain unchanged. This potentiation induced by PKC activators is antagonized by a PKC inhibitor (staurosporine) and by a monoclonal antibody directed against PKC. It is concluded that P-type Ca channels are potentially regulated by PKC phosphorylation and the functional relevance of this intracellular pathway is discussed.

Enhancement of N- and L-type calcium channel currents by protein kinase C in frog sympathetic neurons

The effect of protein kinase C (PKC) stimulation on Ca2+ channels was studied in frog sympathetic neurons. 12,13-Phorbol dibutyrate (PDBu) consistently augmented Ca2+ channel currents in whole-cell recordings. This enhancement was blocked by staurosporine and PKC(19-31), but not produced by 4 alpha-phorbol 12,13-didecanoate, indicating that PDBu acts via PKC. Both N- and L-type currents, as isolated pharmacologically, were increased. PKC enhancement was independent of the extent of G protein activation, indicating that it was not caused by removal of tonic G protein inhibition. In unitary recordings PDBu produced dramatic increases in single N- and L-type channel activity by sharply decreasing closed time intervals between adjacent openings, but did not alter the unitary current size or mean open time. This up-modulation by PKC may constitute a positive feedback mechanism in the regulation of neuronal Ca2+ channel activity.

Neurotransmitter modulation of calcium channels is dependent on the charge carrier used in the recording of currents

Currents through calcium channels were recorded using calcium, barium and strontium as charge carriers in NG108-15 cells. The mean normalised peak current amplitude at 0 mV was not significantly different between the charge carriers; however, the sustained component (measured at the end of the 500 ms command step) was ca. 3 times larger in barium and strontium. Further, the inhibition by acetylcholine or noradrenaline, although the same at the peak of the current envelope, was significantly greater on the sustained portion of the current for barium and strontium. Increasing internal calcium-buffering (to reduce calcium-dependent inactivation with calcium as the charge carrier) did not increase the amount of inhibition of the sustained portion of current. These results suggest a cautious approach to analysis of neurotransmitter modulation of calcium currents using other charge carriers than calcium.

Protein kinase C modulates glutamate receptor inhibition of Ca2+ channels and synaptic transmission

Fast synaptic transmission in the central nervous system can be modulated by neurotransmitters and second-messenger pathways. For example, transmission at glutamatergic synapses can be depressed by the metabotropic glutamate receptor, providing autoreceptor-mediated negative feedback. Metabotropic glutamate receptor inhibition of Ca2+ channels may contribute to this pathway. In contrast, stimulation of protein kinase C can enhance excitatory synaptic transmission, whereas both depression and enhancement of Ca2+ current have been reported. Here we show that in hippocampal CA3 and cortical pyramidal neurons, activation of protein kinase C enhances current through N-type Ca2+ channels and, in addition, dramatically reduces G protein-dependent inhibition of these same channels by the metabotropic glutamate receptor. In parallel experiments on fast excitatory transmission at corticostriatal synapses, kinase C activators were similarly found to reduce the inhibitory effect produced by stimulation of the metabotropic glutamate receptor. The results show that second-to-second control of Ca2+ channels by the metabotropic glutamate receptor can itself be modulated on a slower timescale by protein kinase C. These mechanisms may be used in the control of fast excitatory synaptic transmission.

Substance P and somatostatin inhibit calcium channels in rat sympathetic neurons via different G protein pathways

We studied inhibition of N-type Ca2+ channels in rat superior cervical ganglion neurons by substance P (SP) and somatostatin-14 (Som). In whole-cell clamp, 70 of 82 acutely dissociated neurons showed inhibition (mean 37%) by 500 nM SP, and 54 of 61 showed inhibition by 240 nM Som (mean 57%). Pertussis toxin (PTX) blocked Som but not SP inhibition; intracellular dialysis with 2 mM GDP-beta-S attenuated inhibition with either peptide. Inhibition was voltage dependent with Som but not with SP. Neurokinin A (1 microM) or B was without effect, implicating NK1 tachykinin receptors. In cell-attached patches with bath-applied drugs, to test for a diffusible messenger, inhibition by SP or Som was only 8%. Thus, SP signaling is voltage independent and PTX insensitive; Som inhibition is voltage dependent and PTX sensitive; and both are membrane delimited.

AMPA receptor activation potentiates zinc neurotoxicity

Extracellular Zn2+ attenuates NMDA receptor-mediated neurotoxicity and increases AMPA receptor-mediated toxicity. Known electrophysiological effects of Zn2+ predict only the former. We considered the possibility that the latter rather reflects AMPA potentiation of Zn2+ toxicity, perhaps mediated by neuronal depolarization and Zn2+ entry through voltage-gated Ca2+ channels. High K+ or kainate also potentiated Zn2+ toxicity, and AMPA plus Zn2+ toxicity was attenuated by raising extracellular Ca2+, or by Ca2+ channel blockers. AMPA plus Zn2+ exposure induced an increase in fluorescence from neurons loaded with the Zn(2+)-sensitive dye TS-Q and increased subsequent 45Ca2+ accumulation. The ability of AMPA receptor activation to potentiate Zn2+ toxicity may be relevant to neuronal death associated with intense activation of glutamatergic pathways.

[Release of acetylcholine and its regulation]

The mechanism of acetylcholine (ACh) release and its regulation is a widely studied subject still underdebated. Although the vesicular hypothesis for ACh release is at present largely accepted, alternative theories have been proposed. ACh release is triggered by calcium influx through specific presynaptic Ca2+ channels. The modulation of this calcium influx appears as the main mechanism through which ACh release is regulated. This can be achieved by direct modification of the presynaptic Ca2+ channel opening or indirectly by a change in the polarization level of the presynaptic membrane due to the opening or closing of other presynaptic channels (usually K+ channels). The increase in the intracellular Ca2+ concentration that triggers ACh release is also under the control of Ca2+ membrane exchanges and intracellular Ca2+ buffers. ACh synthesis that takes place in the cytoplasm of the terminal, can itself be modulated leading to changes in the quantity of ACh available for release. All these regulatory mechanisms can be initiated by the activation of presynaptic receptors to either ACh itself (autoreceptors) or to other transmitters (heteroreceptors). Most often, these presynaptic receptors seem to require the transducing role of G proteins and the involvement of various second messengers. Some illnesses concerning the cholinergic system can be related to a disfunction of one of these presynaptic regulatory mechanisms.