Biochemical properties and subcellular distribution of an N-type calcium channel alpha 1 subunit

Depolarization-induced ERK phosphorylation depends on the cytosolic Ca2+ level rather than on the Ca2+ channel subtype of chromaffin cells

The contribution of Ca2+ entry through different voltage-activated Ca2+ channel (VACC) subtypes to the phosphorylation of extracellular signal regulated kinase (ERK) was examined in bovine adrenal-medullary chromaffin cells. High K+ depolarization (40 mM, 3 min) induced ERK phosphorylation, an effect that was inhibited by specific mitogen-activated protein kinase kinase inhibitors. By using selective inhibitors, we observed that depolarization-induced ERK phosphorylation completely depended on protein kinase C-alpha (PKC-alpha), but not on Ca2+/calmodulin-dependent protein kinase nor cyclic AMP-dependent protein kinase. Blockade of L-type Ca2+ channels by 3 microm furnidipine, or blockade of N channels by 1 micromomega-conotoxin GVIA reduced ERK phosphorylation by 70%, while the inhibition of P/Q channels by 1 micromomega-agatoxin IVA only caused a 40% reduction. The simultaneous blockade of L and N, or P/Q and N channels completely abolished this response, yet 23% ERK phosphorylation remained when L and P/Q channels were simultaneously blocked. Confocal imaging of cytosolic Ca2+ elevations elicited by 40 mm K+, showed that Ca2+ levels increased throughout the entire cytosol, both in the presence and the absence of Ca2+ channel blockers. Fifty-eight percent of the fluorescence rise depended on Ca2+ entering through N channels. Thus, ERK phosphorylation seems to depend on a critical level of Ca2+ in the cytosol rather than on activation of a given Ca2+ channel subtype.

Expression and modulation of an invertebrate presynaptic calcium channel alpha1 subunit homolog

Here we report the first assessment of the expression and modulation of an invertebrate alpha1 subunit homolog of mammalian presynaptic Cav2 calcium channels (N-type and P/Q-type) in mammalian cells. Our data show that molluscan channel (LCav2a) isolated from Lymnaea stagnalis is effectively membrane-targeted and electrophysiologically recordable in tsA-201 cells only when the first 44 amino acids of LCav2a are substituted for the corresponding region of rat Cav2.1. When coexpressed with rat accessory subunits, the biophysical properties of LCav2a-5'rbA resemble those of mammalian N-type calcium channels with respect to activation and inactivation, lack of pronounced calcium dependent inactivation, preferential permeation of barium ions, and cadmium block. Consistent with reports of native Lymnaea calcium currents, the LCav2a-5'rbA channel is insensitive to micromolar concentrations of omega-conotoxin GVIA and is not affected by nifedipine, thus confirming that it is not of the L-type. Interestingly, the LCav2a-5'rbA channel is almost completely and irreversibly inhibited by guanosine 5'-3-O-(thio)triphosphate but not regulated by syntaxin1, suggesting that invertebrate presynaptic calcium channels are differently modulated from their vertebrate counterparts.

Antigen selectivity characteristic of polyclonal antibodies against omega-conotoxin GVIA and N-type voltage-dependent calcium channels

The antibodies against omega-conotoxin GVIA (omega-CTX GVIA; N-type voltage-dependent calcium channel [VDCC] blocker) and B1Nt (N-terminal segment [residues 1-13] of BI alpha1 subunits of VDCCs) were prepared, and the selectivity for each antigen omega-CTX GVIA and B1Nt was investigated. For the antigen selectivity of anti-omega-CTX GVIA antibody against omega-CTX GVIA, ELISA, and immunoprecipitation were used. The reactions for ELISA and immunoprecipitation were observed except when antibody IgG purified by Protein A-Sepharose CL-4B from nonimmunized serum (purified NI-Ab) was used. The specific reactions were inhibited by 10 nM omega-CTX GVIA, but not by omega-CTX SVIB (N-type VDCC blocker), omega-CTX MVIIC (N- and P-type VDCC blocker), or omega-Aga IVA (P-type VDCC blocker). For the antigen selectivity of the anti-B1Nt antibody, analyses by ELISA, immunoprecipitation, and Western blotting were conducted. The reactions were observed except when NI-Ab was used. The ELISA and immunoprecipitation reactions were inhibited by the antigen peptide B1Nt, and the IC50 values were about 1.2 x 10(-8) and 1.3 x 10(-8) M, respectively. The bands of 210 and 190 kD by Western blotting of crude membranes from chick brain were also inhibited by 1 microM B1Nt. These results suggest that the antibodies prepared against omega-CTX GVIA and B1Nt in this work have high selectivity for their antigen. Therefore we assume that the antibodies against omega-CTX GVIA and B1Nt are useful tools for the analyses of the function and distribution of N-type VDCCs. The anti omega-CTX GVIA antibody must also be useful for the radioimmunoassay of omega-CTX GVIA.

Functional interactions between presynaptic calcium channels and the neurotransmitter release machinery

In vertebrates, the physical coupling between presynaptic calcium channels and synaptic vesicle release proteins enhances the efficiency of neurotransmission. Recent evidence indicates that these synaptic proteins may feedback directly on synaptic release by negatively regulating calcium entry, and indirectly through pathways involving second messenger molecules. Studies of individual neurons from both vertebrates and invertebrates have provided novel insights into the roles of scaffolding proteins in calcium channel targeting and neurotransmitter release. These studies require us to expand current models of synaptic transmission.

Nature and expression of dihydropyridine-sensitive and -insensitive calcium currents in hair cells of frog semicircular canals

Ca(2+) currents in hair cells of the frog crista ampullaris were studied using the whole-cell patch-clamp technique. Currents were recorded in situ from hair cells in peripheral, intermediate and central regions of the sensory epithelium. Two types of Ca(2+) currents were found: a partially inactivating current that was expressed by nearly all central cells and by about 65% of intermediate and peripheral cells, and a sustained current expressed by the remaining cell population. The mean Ca(2+) current amplitude was larger in intermediate cells than in central or peripheral cells. The two types of Ca(2+) currents were composed of two components: a large, nifedipine-sensitive (NS) current and a small, nifedipine-insensitive (NI) current. The latter was resistant to SNX-482, omega-conotoxin MVIIC and omega-agatoxin IVA and to omega-conotoxin GVIA, antagonists of R, P/Q and N-type Ca(2+) channels. The amplitude of NS and NI currents varied among peripheral cells, where the current density gradually increased from the beginning of the region toward its end. No significant variation of Ca(2+) current density was detected in hair cells of either intermediate or central regions. These results demonstrate the presence of regional and intraregional variations in the expression of L and non-L Ca(2+) channels in the frog crista ampullaris. Finally, immunocytochemical investigations revealed the presence of Ca(2+) channel subunits of the alpha(1D) type and the unexpected expression of alpha(1B)-subunits.

A confirmation of 125I-omega-conotoxin labeled sites in a crude membrane fraction from chick brain as the alpha1 subunit of N-type calcium channels

Omega-conotoxin GVIA (omega-CTX), as a selective blocker for an N-type Ca2+ channel, has been conveniently used in many molecular biochemical and pharmacological experiments. There has been little elucidation of 125I-omega-CTX binding sites (mainly the 135-kDa band) in the crude membranes from chick brain, although the characteristics of specific 125I-omega-CTX binding and labeling sites in chick brain membranes have been investigated in our previous research. In this work, our goal is to further identify 125I-omega-CTX labeling sites in chick brain membranes by using anti-B1Nt antibodies (against the N-terminal segment B1Nt of N- or P-type Ca2+ channel alpha1-subunits). The 25I-omega-CTX-labeled sites in chick brain membranes could be solubilized and immunoprecipitated by using an anti B1Nt antibody. The molecular weight of the immunoprecipitated protein was determined as 135 kDa, which is inconsistent with that of the specific 125I-omega-CTX binding protein reported previously. Moreover, the 125-omega-CTX-labeled protein could be purified by the method of preparative SDS-PAGE and recognized by anti-B1Nt antibodies in Western blotting analysis. These results indicated that anti-B1Nt antibodies could truly recognize 125I-omega-CTX-labeled sites as the main band of 135 kDa from chick brain membranes, and the omega-CTX-labeled site (mainly the 135-kDa band) should be N-type Ca2+ channel alpha1-subunits.

Temporal alterations in voltage gated Ca2+ channel immunoreactivities in the gerbil hippocampus following ischemic insults

In the present study, temporal changes of voltage-gated Ca(2+) channel (VGCC) immunoreactivities were evaluated in the gerbil hippocampus following ischemia. P/Q-type VGCC immunoreactivity was elevated in the hippocampus in the 3 h post-ischemic group. In the 30 min post-ischemic group, N-type VGCC immunoreactivity began to increase only in the CA1 region. L-type (alpha1C) VGCC immunoreactivity was significantly increased in the 12 h post-ischemic group. L-type (alpha1D) VGCC immunoreactivity began to increase in the CA1 region in the 30 min post-ischemic group and peaked in the 12 h post-ischemic group. These findings suggest that the altered VGCC immunoreactivities following ischemia may play an important role in the ischemic neuronal injury.

Subtype-selective reconstitution of synaptic transmission in sympathetic ganglion neurons by expression of exogenous calcium channels

Fast cholinergic neurotransmission between superior cervical ganglion neurons (SCGNs) in cell culture is initiated by N-type Ca(2+) currents through Ca(v)2.2 channels. To test the ability of different Ca(2+)-channel subtypes to initiate synaptic transmission in these cells, SCGNs were injected with cDNAs encoding Ca(v)1.2 channels, which conduct L-type currents, Ca(v)2.1 channels, which conduct P/Q-type Ca(2+) currents, and Ca(v)2.3 channels, which conduct R-type Ca(2+) currents. Exogenously expressed Ca(v)2.1 channels were localized in nerve terminals, as assessed by immunocytochemistry with subtype-specific antibodies, and these channels effectively initiated synaptic transmission. Injection with cDNA encoding Ca(v)2.3 channels yielded a lower level of presynaptic labeling and synaptic transmission, whereas injection with cDNA encoding Ca(v)1.2 channels resulted in no presynaptic labeling and no synaptic transmission. Our results show that exogenously expressed Ca(2+) channels can mediate synaptic transmission in SCGNs and that the specificity of reconstitution of neurotransmission (Ca(v)2.1 > Ca(v)2.3 >> Ca(v)1.2) follows the same order as in neurons in vivo. The specificity of reconstitution of neurotransmission parallels the specificity of trafficking of these Ca(v) channels to nerve terminals.

Action potential afterdepolarization mediated by a Ca2+-activated cation conductance in myenteric AH neurons

We investigated the nature of afterdepolarizing potentials in AH neurons from the guinea-pig duodenum using whole-cell patch-clamp recordings in intact myenteric ganglia. Afterdepolarizing potentials were minimally activated following action-potential firing under normal conditions, but after application of charybdotoxin (40 nM) or tetraethyl ammonium (TEA; 10-20 mM) to the bathing solution, prominent afterdepolarizing potentials followed action potentials. The whole-cell current underlying afterdepolarizing potentials (I(ADP)) in the presence of TEA (10-20 mM) reversed at -38 mV and was not voltage-dependent. Reduction of NaCl in the bathing (Krebs) solution to 58 mM shifted the reversal potential of the I(ADP) to -58 mV, suggesting that the current underlying the afterdepolarizing potential was carried by a mixture of cations. The relative contributions of Na(+) and K(+) to this current were estimated to be about 1:5. Substitution of external Na(+) with N-methyl D-glucamine blocked the current while replacement of internal Cl(-) with gluconate did not block the I(ADP). The I(ADP) was also inhibited when CsCl-filled patch pipettes were used. The I(ADP) was blocked or substantially decreased in amplitude in the presence of N-type Ca(2+) channel antagonists, omega-conotoxin GVIA and omega-conotoxin MVIIC, respectively, and was eliminated by external Cd(2+), indicating that it was dependent on Ca(2+) entry. The I(ADP) was also inhibited by ryanodine (10-20 microM), indicating that Ca(2+)-induced Ca(2+) release was involved in its activation. Niflumic acid consistently inhibited the I(ADP) with an IC(50) of 63 microM. Using antibodies against the pore-forming subunits of L-, N- and P/Q-type voltage-gated Ca(2+) channels, we have demonstrated that myenteric AH neurons express N- and P/Q, but not L-type voltage-gated Ca(2+) channels. We conclude that the ADP in myenteric AH neurons, in the presence of an L-type Ca(2+)-channel blocker, is generated by the opening of Ca(2+)-activated non-selective cation channels following action potential-mediated Ca(2+) entry mainly through N-type Ca(2+) channels. Ca(2+) release from ryanodine-sensitive stores triggered by Ca(2+) entry contributes significantly to the activation of this current.

Active contribution of dendrites to the tonic and trimodal patterns of activity in cerebellar Purkinje neurons

The cerebellum is responsible for coordination of movement and maintenance of balance. Cerebellar architecture is based on repeats of an anatomically well defined circuit. At the center of these functional circuits are Purkinje neurons, which form the sole output of the cerebellar cortex. It is proposed that coordination of movement is achieved by encoding timing signals in the rate of firing and pattern of activity of Purkinje cells. An understanding of cerebellar timing requires an appreciation of the intrinsic firing behavior of Purkinje cells and the extent to which their activity is regulated within the functional circuits. We have examined the spontaneous firing of Purkinje neurons in isolation from the rest of the cerebellar circuitry by blocking fast synaptic transmission in acutely prepared cerebellar slices. We find that, intrinsically, mature Purkinje cells show a complex pattern of activity in which they continuously cycle among tonically firing, bursting, and silent modes. This trimodal pattern of activity emerges as the cerebellum matures anatomically and functionally. Concurrent with the transformation of the immature tonically firing cells to those with the trimodal pattern of activity, the dendrites assume a prominent role in regulating the excitability of Purkinje cells. Thus, alterations in the rate and pattern of activity of Purkinje neurons are not solely the result of synaptic input but also arise as a consequence of the intrinsic properties of the cells.

Functional identification of a cloned squid presynaptic voltage-dependent calcium channel

We previously cloned a voltage-dependent Ca2+ channel alpha1 subunit LoCa(v)2 cDNA from the squid optic lobe. LoCa(v)2 is designated as a non-L-type voltage-dependent Ca2+ channel based on its amino acid sequence. We performed functional expression experiments of LoCa(v)2 in oocytes and characterized the expressed currents electrophysiologically and pharmacologically. The LoCa(v)2 current was high voltage-activated and the peak current was maximal at +20 mV and lasted for long during activation. The LoCa(v)2 current was not inhibited by the drugs and toxins examined except for omega-agatoxin IVA and PLTX-II. Omega-agatoxin IVA, which is a P-type channel blocker, moderately inhibited the LoCa(v)2 current at higher concentration. PLTX-II, which blocks insect presynaptic Ca2+ channel, inhibited the LoCa(v)2 current at lower concentration. Immunohistochemical investigation showed that the LoCa(v)2 protein may exist at presynaptic terminals in the squid optic lobe. These results suggest that LoCa(v)2 is an omega-agatoxin IVA and PLTX-II-sensitive presynaptic Ca2+ channel in the squid nervous system.

The making of a complex spike: ionic composition and plasticity

Climbing fiber (CF) activation evokes a large all-or-nothing electrical response in Purkinje cells (PCs), the complex spike. It has been suggested that the role of CFs (and thus complex spikes) is that of a "teacher" in simple learning paradigms such as associative eyeblink conditioning. An alternative hypothesis describes the olivocerebellar system as part of a timing device and denies a role of the CF input in learning. To date, neither of these hypotheses nor others can definitively be verified or discounted. Similarly, the complex spike evades a clear understanding when it comes to the cellular events underlying complex spike generation. What is known, however, is that complex spikes are associated with large dendritic calcium signals that are required for the induction of long-term depression (LTD) at the parallel fiber (PF)-PC synapse. PF-LTD is a form of long-term synaptic plasticity that has been suggested to underlie certain forms of cerebellar motor learning. In contrast to the PF input, the CF input has been considered invariant. Our recent discovery of LTD at the CF input shows that complex spikes are less static than previously assumed. In addition to depression of CF-evoked excitatory postsynaptic currents, long-lasting, selective reduction of slow complex spike components could be observed after brief CF tetanization. To understand the functional implications of CF-LTD, it is crucial to know the types of currents constituting the specific complex spike components. Here we review the "anatomy" of the complex spike as well as our observations of activity-dependent complex spike waveform modifications. In addition, we discuss which properties CF-LTD might add to the circuitry of the cerebellar cortex.

Ultrastructural localization of enkephalin and mu-opioid receptors in the rat ventral tegmental area

Enkephalins are endogenous ligands for opioid receptors whose activation potently modulates the output of mesocorticolimbic dopaminergic neurons within the ventral tegmental area. Many of the reinforcing effects of enkephalins in the mesocorticolimbic system are mediated by mu-opioid receptors. To determine the sites for Leu(5)-enkephalin activation of mu-opioid receptors in the ventral tegmental area, we examined the dual electron microscopic immunocytochemical localization of their respective antigens in this region of rat brain. Enkephalin immunoperoxidase reaction product and mu-opioid receptor immunogold-silver labeling showed similar cellular and subcellular distribution in both the paranigral and parabrachial subdivisions of the ventral tegmental area. Enkephalin immunoreactivity was mainly localized in small unmyelinated axons (50.4%) and in axon terminals (40.4%). The majority of these terminals formed symmetric, inhibitory-type synapses, many of which were on dendrites expressing plasmalemmal mu-opioid receptors. Appositional contacts were also often seen between axons or terminals that were differentially labeled for the two antigens. In addition, some of the enkephalin-labeled terminals and a few somatodendritic profiles showed a plasmalemmal or vesicular localization of mu-opioid receptors. Our results indicate that dendritic targets of inhibitory terminals, as well as nearby axon terminals, are potential sites for enkephalin activation of mu-opioid receptors throughout the ventral tegmental area. Moreover, co-localization of enkephalin and mu-opioid receptors in selective neuronal profiles may indicate an autoregulatory role for these receptors or their internalization along with the bound ligand in this brain region.

Ca2+ channels and epilepsy

The epilepsies encompass diverse seizure disorders afflicting as many as 50 million people worldwide. Many forms of epilepsy are intractable to current therapies and there is a pressing need to develop agents and strategies to not only suppress seizures, but also cure epilepsy. Recent insights from molecular genetics and pharmacology now point to an important role for voltage-dependent calcium channels in epilepsy. In this article, I first provide an introduction to the classification of the epilepsies and an overview of neuronal Ca(2+) channels. Next, I attempt to review the evidence for a role of Ca(2+) channels in epilepsy and the insights gained from genetics and pharmacology. Lastly, I describe new avenues for how such information might be exploited in the development of therapeutic reagents.

Melanin-concentrating hormone depresses L-, N-, and P/Q-type voltage-dependent calcium channels in rat lateral hypothalamic neurons

Melanin-concentrating hormone (MCH), a cyclic 19-amino-acid peptide, is synthesized exclusively by neurons in the lateral hypothalamic (LH) area. It is involved in a number of brain functions and recently has raised interest because of its role in energy homeostasis. MCH axons and receptors are found throughout the brain. Previous reports set the foundation for understanding the cellular actions of MCH by using non-neuronal cells transfected with the MCH receptor gene; these cells exhibited an increase in cytoplasmic calcium in response to MCH, suggesting an excitatory action for the peptide. In the study presented here, we have used whole-cell recording in 117 neurons from LH cultures and brain slices to examine the actions of MCH. MCH decreased the amplitude of voltage-dependent calcium currents in almost all tested neurons. The inhibition desensitized rapidly (18 s to half maximum at 100 nM concentration) and was dose-dependent (IC(50) = 7.8 nM) when activated with a pulse from -80 mV to 0 mV. A priori activation of G-proteins with GTPgammaS completely eliminated the MCH-induced effect at low MCH concentrations and reduced the MCH-induced effect at high MCH concentrations. Inhibition of G-proteins with pertussis toxin (PTX) blocked the MCH-induced inhibitory effect at high MCH concentrations. Pre-pulse depolarization resulted in an attenuation of the MCH-induced inhibition of calcium currents in most neurons. These data suggest that MCH exerts an inhibitory effect on calcium currents via PTX-sensitive G-protein pathways, probably the G(i)/G(o) pathway, in LH neurons. L-, N- and P/Q-type calcium channels were identified in LH neurons, with L- and N-type channels accounting for most of the voltage-activated current (about 40 % each); MCH attenuated each of the three types (mean 50 % depression), with the greatest inhibition found for N-type currents. In contrast to previous data on non-neuronal cells showing an MHC-evoked increase in calcium, our data suggest that the reverse occurs in LH neurons. The attenuation of calcium currents is consistent with an inhibitory action for the peptide in neurons.

RIM binding proteins (RBPs) couple Rab3-interacting molecules (RIMs) to voltage-gated Ca(2+) channels

Ca(2+) influx through voltage-gated channels initiates the exocytotic fusion of synaptic vesicles to the plasma membrane. Here we show that RIM binding proteins (RBPs), which associate with Ca(2+) channels in hair cells, photoreceptors, and neurons, interact with alpha(1D) (L type) and alpha(1B) (N type) Ca(2+) channel subunits. RBPs contain three Src homology 3 domains that bind to proline-rich motifs in alpha(1) subunits and Rab3-interacting molecules (RIMs). Overexpression in PC12 cells of fusion proteins that suppress the interactions of RBPs with RIMs and alpha(1) augments the exocytosis triggered by depolarization. RBPs may regulate the strength of synaptic transmission by creating a functional link between the synaptic-vesicle tethering apparatus, which includes RIMs and Rab3, and the fusion machinery, which includes Ca(2+) channels and the SNARE complex.

Cdk5/p35 regulates neurotransmitter release through phosphorylation and downregulation of P/Q-type voltage-dependent calcium channel activity

Cyclin-dependent kinase 5 (Cdk5) is a proline-directed serine/threonine kinase with close structural homology to the mitotic Cdks. The complex of Cdk5 and p35, the neuron-specific regulatory subunit of Cdk5, plays important roles in brain development, such as neuronal migration and neurite outgrowth. Moreover, Cdk5 is thought to be involved in the promotion of neurodegeneration in Alzheimer's disease. Cdk5 is abundant in mature neurons; however, its physiological functions in the adult brain are unknown. Here we show that Cdk5/p35 regulates neurotransmitter release in the presynaptic terminal. Both Cdk5 and p35 were abundant in the synaptosomes. Roscovitine, a specific inhibitor of Cdk5 in neurons, induced neurotransmitter release from the synaptosomes in response to membrane depolarization and enhanced the EPSP slopes in rat hippocampal slices. The electrophysiological study using each specific inhibitor of the voltage-dependent calcium channels (VDCCs) and calcium imaging revealed that roscovitine enhanced Ca2+ influx from the P/Q-type VDCC. Moreover, Cdk5/p25 phosphorylated the intracellular loop connecting domains II and III (L(II-III)) between amino acid residues 724 and 981 of isoforms cloned from rat brain of the alpha1A subunit of P/Q-type Ca2+ channels. The phosphorylation inhibited the interaction of L(II-III) with SNAP-25 and synaptotagmin I, which were plasma membrane soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) receptor (SNARE) proteins and were required for efficient neurotransmitter release. These results strongly suggest that Cdk5/p35 inhibits neurotransmitter release through the phosphorylation of P/Q-type VDCC and downregulation of the channel activity.

Mechanisms of the release of anterogradely transported neurotrophin-3 from axon terminals

Neurotrophins have profound effects on synaptic function and structure. They can be derived from presynaptic, as well as postsynaptic, sites. To date, it has not been possible to measure the release of neurotrophins from axon terminals in intact tissue. We implemented a novel, extremely sensitive assay for the release and transfer of anterogradely transported neurotrophin-3 (NT-3) from a presynaptic to a postsynaptic location that uses synaptosomal fractionation after introduction of radiolabeled NT-3 into the retinotectal projection of chick embryos. Release of the anterogradely transported NT-3 in intact tissue was assessed by measuring the amount remaining in synaptosomal preparations after treatment of whole tecta with pharmacological agents. Use of this assay reveals that release of NT-3 from axon terminals is increased by depolarization, calcium influx via N-type calcium channels, and cAMP analogs, and release is most profoundly increased by excitation with kainic acid or mobilization of calcium from intracellular stores. NT-3 release depends on extracellular sodium, CaM kinase II activity, and requires intact microtubules and microfilaments. Dantrolene inhibits the high potassium-induced release of NT-3, indicating that release of calcium from intracellular stores is required. Tetanus toxin also inhibits NT-3 release, suggesting that intact synaptobrevin or synaptobrevin-like molecules are required for exocytosis. Ultrastructural autoradiography and immunolabel indicate that NT-3 is packaged in presumptive large dense-core vesicles. These data show that release of NT-3 from axon terminals depends on multiple regulatory proteins and ions, including the mobilization of local calcium. The data provide insight in the mechanisms of anterograde neurotrophins as synaptic modulators.

mu-Opioid receptor gene expression: the role of NCAM

We have investigated the mechanisms regulating the expression of the mu-opioid receptor, using P19 mouse embryonal carcinoma cells, which normally lack this receptor, but which can be induced to express it in aggregated cells by retinoic acid treatment. The expression level of mu-opioid receptor mRNA was found to be closely correlated with aggregation status, and more specifically by cell to cell interaction requiring neural cell adhesion molecules (NCAM). We showed that NCAM activates the mu-opioid receptor gene through a pathway involving phospholipase C-arachidonic acid-calcium channel-calcium/calmodulin kinase II. A similar pathway was previously shown to promote neurite outgrowth, however, with distinct specificity, including the role of calcium channels. Activation of L-type calcium channels elevated mu-opioid receptor expression, while N-type-channel activity had the opposite effect. The effect of anti-NCAM-antibody treatment was not due to retardation of general neural development and was specific to the mu-opioid receptor gene. Our results indicate that the P19 system is an useful model to study the expression of the mu-opioid receptor gene.

Identification and characterization of novel human Ca(v)2.2 (alpha 1B) calcium channel variants lacking the synaptic protein interaction site

The physical interaction between the presynaptic vesicle release complex and the large cytoplasmic region linking domains II and III of N-type (Ca(v)2.2) calcium channel alpha(1)B subunits is considered to be of fundamental importance for efficient neurotransmission. By PCR analysis of human brain cDNA libraries and IMR32 cell mRNA, we have isolated novel N-type channel variants, termed Ca(v)2.2-Delta1 and Delta2, which lack large parts of the domain II-III linker region, including the synaptic protein interaction site. They appear to be widely expressed across the human CNS as indicated by RNase protection assays. When expressed in tsA-201 cells, both novel variants formed barium-permeable channels with voltage dependences and kinetics for activation that were similar to those observed with the full-length channel. All three channel types exhibited the hallmarks of prepulse facilitation, which interestingly occurred independently of G-protein betagamma subunits. By contrast, the voltage dependence of steady-state inactivation seen with both Delta1 and Delta2 channels was shifted toward more depolarized potentials, and recovery from inactivation of Delta1 and Delta2 channels occurred more rapidly than that of the full-length channel. Moreover, the Delta1 channel was dramatically less sensitive to both omega-conotoxin MVIIA and GVIA than either the Delta2 variant or the full-length construct. Finally, the domain II-III linker region of neither variant was able to effectively bind syntaxin in vitro. These results suggest that the structure of the II-III linker region is an important determinant of N-type channel function and pharmacology. The lack of syntaxin binding hints at a unique physiological function of these channels.

Neurotransmitter release from tottering mice nerve terminals with reduced expression of mutated P- and Q-type Ca2+-channels

Neurotransmitter release is triggered by Ca2+-influx through multiple sub-types of high voltage-activated Ca2+-channels. Tottering mice have a mutation in the alpha1A pore-forming subunit of P- and Q-type Ca2+-channels, two prominent sub-types that regulate transmitter release from central nerve terminals. Immunoblotting analysis of purified forebrain terminals from tottering mice revealed an 85% reduction in the protein expression level of the mutated alpha1A subunit compared to expression of the alpha1A subunit in wild-type terminals. In contrast, the expression of the alpha1B subunit of the N-type Ca2+-channels was unchanged. Release of the amino acids glutamate and GABA and of the neuropeptide cholecystokinin (CCK) induced by a short (100 ms) depolarization pulse was unchanged in the terminals of tottering mice. Studies using specific blockers of Ca2+-channels however, revealed a reduced contribution of P- and Q-type Ca2+-channels to glutamate and cholecystokinin release, whereas a greater reliance on N-type Ca2+-channels for release of these transmitters was observed. In contrast, the contribution of the P-, Q- and N-type Ca2+-channels to the release of GABA was not altered in tottering mice. These results indicate that the expression of the alpha1A subunit was decreased in terminals from tottering mice, and that a decreased contribution of P- and Q-type Ca2+-channels to the release of glutamate and cholecystokinin was functionally compensated by an increased contribution of N-type Ca2+-channels.

Calcineurin enhances L-type Ca(2+) channel activity in hippocampal neurons: increased effect with age in culture

The Ca(2+)/calmodulin-dependent protein phosphatase, calcineurin, modulates a number of key Ca(2+) signaling pathways in neurons, and has been implicated in Ca(2+)-dependent negative feedback inactivation of N-methyl-D-aspartate receptors and voltage-sensitive Ca(2+) channels. In contrast, we report here that three mechanistically disparate calcineurin inhibitors, FK-506, cyclosporin A, and the calcineurin autoinhibitory peptide, inhibited high-voltage-activated Ca(2+) channel currents by up to 40% in cultured hippocampal neurons, suggesting that calcineurin acts to enhance Ca(2+) currents. This effect occurred with Ba(2+) or Ca(2+) as charge carrier, and with or without intracellular Ca(2+) buffered by EGTA. Ca(2+)-dependent inactivation of Ca(2+) channels was not affected by FK-506. The immunosuppressant, rapamycin, and the protein phosphatase 1/2A inhibitor, okadaic acid, did not decrease Ca(2+) channel current, showing specificity for effects on calcineurin. Blockade of L-type Ca(2+) channels with nimodipine fully negated the effect of FK-506 on Ca(2+) channel current, while blockade of N-, and P-/Q-type Ca(2+) channels enhanced FK-506-mediated inhibition of the remaining L-type-enriched current. FK-506 also inhibited substantially more Ca(2+) channel current in 4-week-old vs. 2-week-old cultures, an effect paralleled by an increase in calcineurin A mRNA levels. These studies provide the first evidence that calcineurin selectively enhances L-type Ca(2+) channel activity in neurons. Moreover, this action appears to be increased concomitantly with the well-characterized increase in L-type Ca(2+) channel availability in hippocampal neurons with age-in-culture.

Distribution of high-voltage-activated calcium channels in cultured gamma-aminobutyric acidergic neurons from mouse cerebral cortex

The localization of voltage-gated calcium channel (VGCC) alpha(1) subunits in cultured GABAergic mouse cortical neurons was examined by immunocytochemical methods. Ca(v)1.2 and Ca(v)1.3 subunits of L-type VGCCs were found in cell bodies and dendrites of GABA-immunopositive neurons. Likewise, the Ca(v)2.3 subunit of R-type VGCCs was expressed in a somatodendritic pattern. Ca(v)2.2 subunits of N-type channels were found exclusively in small varicosities that were identified as presynaptic nerve terminals based on their expression of synaptic marker proteins. Two splice variants of the Ca(v)2.1 subunit of P/Q-type VGCCs showed widely differing expression patterns. The rbA isoform displayed a purely somatodendritic staining pattern, whereas the BI isoform was confined to axon-like fibers and nerve terminals. The nerve terminals of these cultured GABAergic neurons express Ca(v)2.2 either alone or in combination with Ca(v)2.1 (BI isoform) but never express Ca(v)2.1 alone. The functional association between VGCCs and the neurotransmitter release machinery was probed using the FM1-43 dye-labeling technique. N-type VGCCs were found to be tightly coupled to exocytosis in these cultured cortical neurons, and P-type VGCCs were also important in a fraction of the cells. The predominant role of N-type VGCCs in neurotransmitter release and the specific localization of the BI isoform of Ca(v)2.1 in the nerve terminals of these neurons distinguish them from previously studied central neurons. The complementary localization patterns observed for two different isoforms of the Ca(v)2.1 subunits provide direct evidence for alternative splicing as a means of generating functional diversity among neuronal calcium channels.

A ratiometric imaging method for mapping ion flux densities

A heterogeneous distribution of ion channels on the cell surface is a prerequisite for several cellular functions. Thus, there has been considerable interest in methods allowing the mapping of ion channel distributions. Here we report on a novel ratiometric imaging technique appropriate to measure spatially resolved ion flux signals by using ion sensitive dyes. However, given that certain relevant cell properties like the surface to volume ratio may exhibit significant spatial heterogeneities, the local influx signal cannot be interpreted as a measure of the local open channel concentration or flux density. To overcome this problem, we suggest an internal normalization procedure, which, in analogy to, but clearly distinct from, well-established ratioing techniques, eliminates effects which would otherwise obscure the desired result. Ratioing is performed on flux signals from a given cell, triggered by two different, subsequent stimuli. If the two stimuli address different ion channels, the flux density distribution caused by two channel types can be determined relative to each other. In cases where one of the stimuli triggers a spatially homogeneous flux signal, ratioing yields an ion flux density map for a given channel type. Thus distribution patterns of ion channels active during a given stimulus may be derived.

Biochemical and anatomical evidence for specialized voltage-dependent calcium channel gamma isoform expression in the epileptic and ataxic mouse, stargazer

Inherited forms of ataxia and absence seizures in mice have been linked to defects in voltage-dependent calcium channel subunits. However, a correlation between the sites of neuronal dysfunction and the impact of the primary lesion upon calcium channel subunit expression or function has not been clearly established. For example, the mutation in stargazer mice has pleiotropic consequences including synaptic alterations in cerebellar granule cells, hippocampal CA3/mossy fibers, and cortical neurons in layer V that, presumably, lead to ataxia and seizures. Genetic analysis of stargazer mice determined that the defective gene encodes a protein expressed in brain (gamma2) with limited homology to the skeletal muscle L-type calcium channel gamma1 subunit. Although additional gamma isoforms have been subsequently identified primarily in neural tissue, little was known about the proteins they encode. Therefore, this study explored the distribution and biochemical properties of gamma2 and other gamma isoforms in wild-type and stargazer brain. We cloned human gamma2, gamma3, and gamma4 isoforms, produced specific anti-peptide antibodies to gamma isoforms and characterized both heterologously expressed and endogenous gamma. We identified regional specificity in the expression of gamma isoforms by western analysis and immunohistochemistry. We report for the first time that the mutation in the stargazer gene resulted in the loss of gamma2 protein. Furthermore, no compensatory changes in the expression of gamma3 or gamma4 protein were evident in stargazer brain. In contrast to other voltage-dependent calcium channel subunits, gamma immunostaining was striking in that it was primarily detected in regions highly enriched in excitatory glutamatergic synapses and faintly detected in cell bodies, suggesting a role for gamma in synaptic functions. Sites of known synaptic dysfunction in stargazer (the hippocampal CA3 region, dentate gyrus, and cerebellar molecular layer) were revealed as relying primarily upon gamma2, as total gamma isoform expression was dramatically decreased in these regions. Electron microscopy localized anti-gamma antibody immunostaining to dendritic structures of hippocampal mossy fiber synapses, with enrichment at postsynaptic densities. To assess the association of native gamma with voltage-dependent calcium channel or alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor subunits, gamma isoforms (gamma2, gamma3 and gamma4) were detergent solubilized from mouse forebrain. Antibodies against a highly conserved C-terminal epitope present in gamma2, gamma3 and gamma4 immunoprecipitated voltage-dependent calcium channel subunits (alpha1B), providing the first in vivo evidence that gamma and voltage-dependent calcium channels form stable complexes. Furthermore, both anti-gamma2 antibodies and anti-alpha1B antibodies independently immunoprecipitated the AMPA receptor subunit, GluR1, from mouse forebrain homogenates. In summary, loss of gamma2 immunoreactivity in stargazer is precisely localized so as to contribute to previously characterized synaptic defects. The data in this paper provide compelling evidence that gamma isoforms form complexes in vivo with voltage-dependent calcium channels as well as AMPA receptors, are selectively and differentially expressed in neuronal processes, and localize primarily to dendritic structures in the hippocampal mossy fiber region.

Interactions between presynaptic Ca2+ channels, cytoplasmic messengers and proteins of the synaptic vesicle release complex

Influx of Ca(2+) through presynaptic voltage-gated Ca(2+) channels is a key step in rapid neurotransmitter release. The amount of Ca(2+) entering through these channels is modulated by a plethora of intracellular messenger molecules, including betagamma-subunits of G proteins, and protein kinases. In addition, Ca(2+) channels bind physically to proteins of the vesicle-release machinery in a Ca(2+)-dependent manner, which can, in turn, regulate the activity of Ca(2+) channels. Recent evidence suggests that second messengers and presynaptic vesicle-release proteins do not regulate Ca(2+) channel activity as independent entities, but that there is extensive crosstalk between these two mechanisms. The complex interactions between second messengers, vesicle-release proteins and voltage-gated Ca(2+) channels might provide multiple avenues for fine-tuning Ca(2+) entry into the presynaptic terminal and, consequently, neurotransmission.

Calcium regulation of neuronal gene expression

Plasticity is a remarkable feature of the brain, allowing neuronal structure and function to accommodate to patterns of electrical activity. One component of these long-term changes is the activity-driven induction of new gene expression, which is required for both the long-lasting long-term potentiation of synaptic transmission associated with learning and memory, and the activity dependent survival events that help to shape and wire the brain during development. We have characterized molecular mechanisms by which neuronal membrane depolarization and subsequent calcium influx into the cytoplasm lead to the induction of new gene transcription. We have identified three points within this cascade of events where the specificity of genes induced by different types of stimuli can be regulated. By using the induction of the gene that encodes brain-derived neurotrophic factor (BDNF) as a model, we have found that the ability of a calcium influx to induce transcription of this gene is regulated by the route of calcium entry into the cell, by the pattern of phosphorylation induced on the transcription factor cAMP-response element (CRE) binding protein (CREB), and by the complement of active transcription factors recruited to the BDNF promoter. These results refine and expand the working model of activity-induced gene induction in the brain, and help to explain how different types of neuronal stimuli can activate distinct transcriptional responses.

A cyclic AMP-regulated negative feedforward system for neuritogenesis revealed in a neuroblastomaxglioma hybrid cell line

We examined the role of second messengers during the neuritogenesis that accompanies neuronal differentiation in a neuroblastomaxglioma hybrid cell line (NG108-15). NG108-15 cells extended neurites after treatment with dibutyryl cyclic AMP. This dibutyryl cyclic AMP treatment evoked the synthesis of voltage-dependent Ca(2+) channel proteins in the cells. The number of neurites was decreased by Ca(2+) influx under condition of high K(+). Interestingly, the increase of neurites stimulated by dibutyryl cyclic AMP and the decrease of neurites caused by high K(+) were both reversible. This is the first study to demonstrate that cyclic AMP regulates a negative feedforward system for neuritogenesis, which links with Ca(2+) signaling. Such a dual role of cyclic AMP may play an important part in precise neurite targeting.

Cerebellar long-term depression: characterization, signal transduction, and functional roles

Cerebellar Purkinje cells exhibit a unique type of synaptic plasticity, namely, long-term depression (LTD). When two inputs to a Purkinje cell, one from a climbing fiber and the other from a set of granule cell axons, are repeatedly associated, the input efficacy of the granule cell axons in exciting the Purkinje cell is persistently depressed. Section I of this review briefly describes the history of research around LTD, and section II specifies physiological characteristics of LTD. Sections III and IV then review the massive data accumulated during the past two decades, which have revealed complex networks of signal transduction underlying LTD. Section III deals with a variety of first messengers, receptors, ion channels, transporters, G proteins, and phospholipases. Section IV covers second messengers, protein kinases, phosphatases and other elements, eventually leading to inactivation of DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazolone-propionate-selective glutamate receptors that mediate granule cell-to-Purkinje cell transmission. Section V defines roles of LTD in the light of the microcomplex concept of the cerebellum as functionally eliminating those synaptic connections associated with errors during repeated exercises, while preserving other connections leading to the successful execution of movements. Section VI examines the validity of this microcomplex concept based on the data collected from recent numerous studies of various forms of motor learning in ocular reflexes, eye-blink conditioning, posture, locomotion, and hand/arm movements. Section VII emphasizes the importance of integrating studies on LTD and learning and raises future possibilities of extending cerebellar research to reveal memory mechanisms of implicit learning in general.

High voltage-activated Ca2+ currents in rat supraoptic neurones: biophysical properties and expression of the various channel alpha1 subunits

The diversity of Ca2+ currents was studied in voltage-clamped acutely dissociated neurones from the rat supraoptic nucleus (SON), and the expression of the various corresponding pore-forming alpha1 subunits determined by immunohistochemistry. We observed the presence of all high voltage-activated L-, N-, P/Q- and R-type currents. We did not observe low-voltage-activated T-type current. The multimodal current/voltage relationships of L- and R-type currents indicated further heterogeneity within these current types, each exhibiting two components that differed by a high (-20 mV) and a lower (-40 mV) threshold potential of activation. L- and R-type currents were fast activating and showed time-dependent inactivation, conversely to N- and P/Q-type currents, which activated more slowly and did not inactivate. The immunocytochemical staining indicated that the soma and proximal dendrites of SON neurones were immunoreactive for Cav1.2, Cav1.3 (forming L-type channels), Cav2.1 (P/Q-type), Cav2.2 (N-type) and Cav2.3 subunits (R-type). Each subunit exhibited further specificity in its distribution throughout the nucleus, and we particularly observed strong immunostaining of Cav1.3 and Cav2.3 subunits within the dendritic zone of the SON. These data show a high heterogeneity of Ca2+ channels in SON. neurones, both in their functional properties and cellular distribution. The lower threshold and rapidly activating L- and R-type currents should underlie major Ca2+ entry during action potentials, while the slower and higher threshold N- and P/Q-type currents should be preferentially recruited during burst activity. It will be of key interest to determine their respective role in the numerous Ca2+-dependent events that control the activity and physiology of SON neurones

Differential alterations in the distribution of voltage-gated calcium channels in aged rat cerebellum

In the present study, we used immunohistochemistry and Western blot analysis to determine region-specific changes in the distribution of voltage-gated calcium channels (VGCCs) in aged rat cerebellum. Age-dependent changes in the staining intensity of the alpha(1C) and alpha(1D) subunits were prominent in the Purkinje cells, whereas there was no change in the expression of the alpha(1A) and alpha(1B) subunits. In the aged rat, in particular, immunoreactivity for the alpha(1C) subunits were increased in the dendrites as well as in the cell bodies of Purkinje cells. On the other hand, decreases in immunoreactivity for alpha(1A) and alpha(1D) subunits were found in the molecular or granular layers. However, only alpha(1D) subunit immunoreactivity was decreased in the aged cerebellum membrane by Western blot analysis that, while not addressing regional specificity, further confirmed an age-related decrease in alpha(1D) subunit. These age-related changes in alpha(1D) subunit expression might reflect a gradual loss of regulation for L-type Ca(2+) channels in the senescent period. The first demonstration of age-related alterations in VGCC expression may provide useful data for future investigations on aging and neurodegenerative diseases.

Distribution of various calcium channel alpha(1) subunits in murine DRG neurons and antinociceptive effect of omega-conotoxin SVIB in mice

Immunohistological study revealed the differential localization of subtypes of voltage-dependent calcium channels in the dorsal root ganglion neurons. Intrathecal injection of omega-conotoxin SVIB, an analogue of omega-conotoxin GVIA, which acts on N-type voltage-dependent calcium channels, significantly shortened the licking time in the late phase of a formalin test.

Physiological patterns of electrical stimulation can induce neuronal gene expression by activating N-type calcium channels

Activity-dependent neuronal gene expression is thought to require activation of L-type calcium channels, a view based primarily on studies in which chronic potassium (K(+)) depolarization was used to mimic neuronal activity. However, N-type calcium channels are primarily inactivated during chronic depolarization, and their potential contribution to gene expression induced by physiological patterns of stimulation has not been defined. In the present study, electrical stimulation of dissociated primary sensory neurons at 5 Hz, or treatment with elevated K(+), produced a large increase in the percentage of neurons that express tyrosine hydroxylase (TH) mRNA and protein. However, blockade of L-type channels, which completely inhibited K(+)-induced expression, had no effect on TH expression induced by patterned stimulation. Conversely, blockade of N-type channels completely inhibited TH induction by patterned stimulation, whereas K(+)-induced expression was unaffected. Similar results were obtained for depolarization-induced expression of the immediate early genes Nurr1 and Nur77. In addition, TH induction by patterned stimulation was significantly reduced by inhibitors of PKA and PKC but was unaffected by inhibition of the mitogen-activated protein kinase (MAPK) pathway. On the other hand, K(+)-induced TH expression was significantly reduced by inhibition of the MAPK pathway but was unaffected by inhibitors of PKA or PKC. These results demonstrate that N-type calcium channels can directly link phasic membrane depolarization to gene expression, challenging the view that activation of L-type channels is required for nuclear responses to physiological patterns of activity. Moreover, our data show that phasic and chronic depolarizing stimuli act through distinct mechanisms to induce neuronal gene expression.

Distinct molecular determinants govern syntaxin 1A-mediated inactivation and G-protein inhibition of N-type calcium channels

We have reported recently that syntaxin 1A mediates two effects on N-type channels transiently expressed in tsA-201 cells: a hyperpolarizing shift in the steady-state inactivation curve as well as a tonic inhibition of the channel by G-protein betagamma subunits (Jarvis et al., 2000). Here we have examined some of the molecular determinants and factors that modulate the action of syntaxin 1A on N-type calcium channels. With the additional coexpression of SNAP25, the syntaxin 1A-induced G-protein modulation of the channel became reduced in magnitude by approximately 50% but nonetheless remained significantly higher than the low levels of background inhibition seen with N-type channels alone. In contrast, coexpression of nSec-1 did not reduce the syntaxin 1A-mediated G-protein inhibition; however, interestingly, nSec-1 was able to induce tonic G-protein inhibition even in the absence of syntaxin 1A. Both SNAP25 and nSec-1 blocked the negative shift in half-inactivation potential that was induced by syntaxin 1A. Activation of protein kinase C via phorbol esters or site-directed mutagenesis of three putative PKC consensus sites in the syntaxin 1A binding region of the channel (S802, S896, S898) to glutamic acid (to mimic a permanently phosphorylated state) did not affect the syntaxin 1A-mediated G-protein modulation of the channel. However, in the S896E and S898E mutants, or after PKC-dependent phosphorylation of the wild-type channels, the susceptibility of the channel to undergo shifts in half-inactivation potential was removed. Thus, separate molecular determinants govern the ability of syntaxin 1A to affect N-type channel gating and its modulation by G-proteins.

Spatial and temporal distribution of N-type Ca(2+) channels in gerbil global cerebral ischemia

In the present study, we have investigated the spatial and temporal distribution of voltage-gated calcium channels in the gerbil model of global cerebral ischemia using immunohistochemistry. Distinct localizations of P-type (alpha(1A)), N-type (alpha(1B)), and L-type (alpha(1C) and alpha(1D)) Ca(2+) channels were observed in the hippocampus at days 1-5 after ischemic injury. However, increased expression of N-type Ca(2+) channels was detectable in brain regions vulnerable to ischemia only at days 2 and 3 after ischemic injury. The pyramidal cell bodies of CA1-3 areas and the granule cell bodies of the dentate gyrus were intensely stained at days 2 and 3 following ischemic injury. Transient changes in N-type Ca(2+) channel expression were also observed in the affected cerebral cortex and striatum at days 2 and 3 after ischemic injury. Although the present study has not addressed the multiple mechanisms contributing to the intracellular free Ca(2+) concentration ([Ca(2+)](i)) increase in the ischemic brain, the first demonstration of the transient increase in N-type Ca(2+) channels may prove useful for future investigations.

Chronic high-dose glycine nutrition: effects on rat brain cell morphology

BACKGROUND

Facilitation of N-methyl-D-aspartate (NMDA) receptor-mediated neurotransmission via administration of glycine site agonists of the NMDA receptor (e.g., glycine, D-serine), and glycine transport inhibitors may represent an innovative pharmacologic strategy in schizophrenia; however, given the potential involvement of NMDA receptors in the neurotoxicity of excitatory amino acids, possible neurotoxic effects of glycinergic compounds need to be explored. Furthermore, studying brain adaptations to chronic administration of glycine site agonists may provide insights into the therapeutic mechanisms of these drugs.

METHODS

Adult rats were randomized to one of three nutritional regimens (no glycine supplementation, 1 g/kg/day, or 5 g/kg/day glycine supplementation) and to one of three treatment durations (1, 3, or 5 months). Serum glycine and serine levels at sacrifice and brain sections were examined using histologic markers of neurodegeneration (cresyl violet and silver impregnation staining) and immunohistochemical staining of glial fibrillary acidic protein, microtubule-associated protein, and neurofilament 200. To explore additional neural adaptations to high-dose glycine treatment, immunostaining was also performed for class B, N-type Ca(2+) channels.

RESULTS

Serum glycine levels increased dose dependently during glycine nutrition, whereas serine levels were not changed. In hippocampal dentate gyrus, the percentage of hypertrophied astrocytes transiently increased at 1 month. At 3 and 5 months of glycine treatment, the density of class B, N-type Ca(2+) channels was reduced in parietal cortex and hippocampus. No evidence of neuronal or glial cell excitotoxic damage or degeneration was registered at either of the treatment intervals studied.

CONCLUSIONS

These findings demonstrate for the first time that in vivo administration of high-dose glycine may induce brain morphological changes without causing neurotoxic effects. A reduction in density of class B, N-type Ca(2+) channels in specific brain regions may represent one general adaptation to long-term, high-dose glycine treatment.

Antinociceptive action of amlodipine blocking N-type Ca2+ channels at the primary afferent neurons in mice

We investigated the antinociceptive action of amlodipine, a dihydropyridine derivative, which acts on both L- and N-type voltage-dependent Ca2+ channels (VDCCs), in mice. Intrathecal injection of amlodipine (300 nmol/kg) significantly shortened the licking time in the late phase of a formalin test, while no effect was found with another dihydropyridine derivative, nicardipine (300 nmol/kg). Cilnidipine and omega-conotoxin GVIA also showed marked analgesic effects under the same experimental conditions. Transcripts of alpha1A, alpha1B, alpha1E, alpha1F, alpha1H, beta3, and beta4 subunits were detected by polymerase-chain reaction (PCR) in the dorsal root ganglion, suggesting the existence of a variety of voltage-dependent Ca2+ channels. Electrophysiological experiments showed that amlodipine and cilnidipine inhibit N-type currents in the dorsal root ganglion cells. These results suggest that amlodipine, cilnidipine, and omega-conotoxin GVIA exert their antinociceptive actions by blocking N-type Ca2+ channels in the primary nociceptive afferent fibers. Blocking of the Ca2+ channels results in attenuation of synaptic transmission of nociceptive neurons. Furthermore, it is suggested that some N-type Ca2+ channel blockers might have therapeutic potential as analgesics when applied directly into the subarachnoidal space.

Co-localization of vesicles and P/Q Ca2+-channels explains the preferential distribution of exocytotic active zones in neurites emitted by bovine chromaffin cells

We have taken advantage of the differences between the preferential localization of secretion in the terminals of neurite-emitting bovine chromaffin cells in contrast with the random distribution secretion in spherical cells to study the possible molecular factors determining such localization by using immunofluorescence and confocal microscopy techniques. By analyzing the distribution of dopamine beta-hydroxylase present in the membrane of chromaffin granules, we found that vesicles migrate and accumulate in dense packages in the terminals of neurite processes. Neither members of the fusion core complex such as SNAP-25, nor nicotinic receptors are preferentially located in the terminals as would be expected from elements defining sites of release, thereby suggesting the presence of additional factors. Interestingly, we observed a preferential distribution of the P/Q subtype of Ca2+ channels in these neurite terminals and co-localization with vesicles present in these structures, in sharp contrast with the overall distribution of the L subtype channels. Using the same immunofluorescence techniques we were unable to detect N-type calcium channels. In addition, omega-agatoxin IVA was able to block 70% of the exocytotic release occurring into the neurites, whereas L-type blockers had a weak effect. Taken together our results strongly indicate that the co-localization of vesicles and clusters of P/Q Ca2+ channels may explain the precise localization of exocytotic sites in the terminals of neurite-emitting chromaffin cells, whereas the distribution of secretory sites in round cells may arise from the random presence of these factors as indicated by their partial co-localization.

The Cav2.1/alpha1A (P/Q-type) voltage-dependent calcium channel mediates inhibitory neurotransmission onto mouse cerebellar Purkinje cells

The effects of voltage-dependent calcium channel (VDCC) antagonists on spontaneous inhibitory postsynaptic currents (sIPSCs) in mouse Purkinje cells were examined using in vitro cerebellar slices. The inorganic ion Cd2+ reduced sIPSC amplitude and frequency. No additional block was seen with the Na+ channel antagonist tetrodotoxin (TTX) suggesting that all action potential-evoked inhibitory GABA release was mediated by high-voltage-activated VDCCs. No evidence was found for involvement of Cav1/alpha1C and alpha1D (L-type), Cav2.2/alpha1B (N-type) or Cav2.3/alpha1E (R-type) high-voltage-activated VDCCs or low-voltage-activated Cav3/alpha1G, alpha1H and alpha1I (T-type) VDCCs in mediating presynaptic GABA release. Blockade of sIPSCs by 200 nM omega-agatoxin IVA implicated the Cav2.1/alpha1A (P/Q-type) subtype of high-voltage-activated VDCCs in mediating inhibitory transmission. Inhibition by omega-agatoxin IVA was similar to that seen with Cd2+ and TTX. Selective antibodies directed against the Cav2.1 subunit revealed staining in regions closely opposed to Purkinje cell somata. Cav2.1 staining was colocalized with staining for antibodies against glutamic acid decarboxylase and corresponded well with the pericellular network formed by GABAergic basket cell interneurons. Antibody labelling of Cav2.3 revealed a region-specific expression. In the cerebellar cortex anterior lobe, Cav2.3 staining was predominantly somatodendritic; whilst in the posterior lobe, perisomatic staining was seen primarily. However, electrophysiological data was not consistent with a role for the Cav2.3 subunit in mediating presynaptic GABA release. No consistent staining was seen for other Cav (alpha1) subunits. Electrophysiological and immunostaining data support a predominant role for Cav2.1 subunits in mediating action potential-evoked inhibitory GABA release onto mouse Purkinje cells.

Functional disorders of the sympathetic nervous system in mice lacking the alpha 1B subunit (Cav 2.2) of N-type calcium channels

N-type voltage-dependent Ca(2+) channels (VDCCs), predominantly localized in the nervous system, have been considered to play an essential role in a variety of neuronal functions, including neurotransmitter release at sympathetic nerve terminals. As a direct approach to elucidating the physiological significance of N-type VDCCs, we have generated mice genetically deficient in the alpha(1B) subunit (Ca(v) 2.2). The alpha(1B)-deficient null mice, surprisingly, have a normal life span and are free from apparent behavioral defects. A complete and selective elimination of N-type currents, sensitive to omega-conotoxin GVIA, was observed without significant changes in the activity of other VDCC types in neuronal preparations of mutant mice. The baroreflex response, mediated by the sympathetic nervous system, was markedly reduced after bilateral carotid occlusion. In isolated left atria prepared from N-type-deficient mice, the positive inotropic responses to electrical sympathetic neuronal stimulation were dramatically decreased compared with those of normal mice. In contrast, parasympathetic nervous activity in the mutant mice was nearly identical to that of wild-type mice. Interestingly, the mutant mice showed sustained elevation of heart rate and blood pressure. These results provide direct evidence that N-type VDCCs are indispensable for the function of the sympathetic nervous system in circulatory regulation and indicate that N-type VDCC-deficient mice will be a useful model for studying disorders attributable to sympathetic nerve dysfunction.

Allosteric modulation of Ca2+ channels by G proteins, voltage-dependent facilitation, protein kinase C, and Ca(v)beta subunits

N-type and P/Q-type Ca(2+) channels are inhibited by neurotransmitters acting through G protein-coupled receptors in a membrane-delimited pathway involving Gbetagamma subunits. Inhibition is caused by a shift from an easily activated "willing" (W) state to a more-difficult-to-activate "reluctant" (R) state. This inhibition can be reversed by strong depolarization, resulting in prepulse facilitation, or by protein kinase C (PKC) phosphorylation. Comparison of regulation of N-type Ca(2+) channels containing Cav2.2a alpha(1) subunits and P/Q-type Ca(2+) channels containing Ca(v)2.1 alpha(1) subunits revealed substantial differences. In the absence of G protein modulation, Ca(v)2.1 channels containing Ca(v)beta subunits were tonically in the W state, whereas Ca(v)2.1 channels without beta subunits and Ca(v)2.2a channels with beta subunits were tonically in the R state. Both Ca(v)2.1 and Ca(v)2.2a channels could be shifted back toward the W state by strong depolarization or PKC phosphorylation. Our results show that the R state and its modulation by prepulse facilitation, PKC phosphorylation, and Ca(v)beta subunits are intrinsic properties of the Ca(2+) channel itself in the absence of G protein modulation. A common allosteric model of G protein modulation of Ca(2+)-channel activity incorporating an intrinsic equilibrium between the W and R states of the alpha(1) subunits and modulation of that equilibrium by G proteins, Ca(v)beta subunits, membrane depolarization, and phosphorylation by PKC accommodates our findings. Such regulation will modulate transmission at synapses that use N-type and P/Q-type Ca(2+) channels to initiate neurotransmitter release.

Control of gating mode by a single amino acid residue in transmembrane segment IS3 of the N-type Ca2+ channel

N-type Ca(2+) channels can be inhibited by neurotransmitter-induced release of G protein betagamma subunits. Two isoforms of Ca(v)2.2 alpha1 subunits of N-type calcium channels from rat brain (Ca(v)2.2a and Ca(v)2.2b; initially termed rbB-I and rbB-II) have different functional properties. Unmodulated Ca(v)2.2b channels are in an easily activated "willing" (W) state with fast activation kinetics and no prepulse facilitation. Activating G proteins shifts Ca(v)2.2b channels to a difficult to activate "reluctant" (R) state with slow activation kinetics; they can be returned to the W state by strong depolarization resulting in prepulse facilitation. This contrasts with Ca(v)2.2a channels, which are tonically in the R state and exhibit strong prepulse facilitation. Activating or inhibiting G proteins has no effect. Thus, the R state of Ca(v)2.2a and its reversal by prepulse facilitation are intrinsic to the channel and independent of G protein modulation. Mutating G177 in segment IS3 of Ca(v)2.2b to E as in Ca(v)2.2a converts Ca(v)2.2b tonically to the R state, insensitive to further G protein modulation. The converse substitution in Ca(v)2.2a, E177G, converts it to the W state and restores G protein modulation. We propose that negatively charged E177 in IS3 interacts with a positive charge in the IS4 voltage sensor when the channel is closed and produces the R state of Ca(v)2.2a by a voltage sensor-trapping mechanism. G protein betagamma subunits may produce reluctant channels by a similar molecular mechanism.

L-Type calcium channels mediate calcium oscillations in early postnatal Purkinje neurons

Ca(2+) signaling is important in many fundamental neuronal processes including neurotransmission, synaptic plasticity, neuronal development, and gene expression. In cerebellar Purkinje neurons, Ca(2+) signaling has been studied primarily in the dendritic region where increases in local Ca(2+) have been shown to occur with both synaptic events and spontaneous electrical activity involving P-type voltage-gated Ca(2+) channels (VGCCs), the predominant VGCC expressed by Purkinje neurons. Here we show that Ca(2+) signaling is also a prominent feature of immature Purkinje neurons at developmental stages that precede expression of dendritic structure and involves L-type rather than P-type VGCCs. Immature Purkinje neurons acutely dissociated from postnatal day 4-7 rat pups exhibit spontaneous cytoplasmic Ca(2+) oscillations. The Ca(2+) oscillations require entry of extracellular Ca(2+), are blocked by tetrodotoxin, are communicated to the nucleus, and correlate closely with patterns of endogenously generated spontaneous and evoked electrical activity recorded in the neurons. Immunocytochemistry showed that L-, N-, and P/Q-types of VGCCs are present on the somata of the Purkinje neurons at this age. However, only the L-type VGCC antagonist nimodipine effectively antagonized the Ca(2+) oscillations; inhibitors of P/Q and N-type VGCCs were relatively ineffective. Release of Ca(2+) from intracellular Ca(2+) stores significantly amplified the Ca(2+) signals of external origin. These results show that a somatic signaling pathway that generates intracellular Ca(2+) oscillations and involves L-type VGCCs and intracellular Ca(2+) stores plays a prominent role in the Ca(2+) dynamics of early developing Purkinje neurons and may play an important role in communicating developmental cues to the nucleus.

Associative long-term depression in the hippocampus is dependent on postsynaptic N-type Ca2+ channels

Long-term depression (LTD) is a form of synaptic plasticity that can be induced either by low-frequency stimulation of presynaptic fibers or in an associative manner by asynchronous pairing of presynaptic and postsynaptic activity. We investigated the induction mechanisms of associative LTD in CA1 pyramidal neurons of the hippocampus using whole-cell patch-clamp recordings and Ca(2+) imaging in acute brain slices. Asynchronous pairing of postsynaptic action potentials with EPSPs evoked with a delay of 20 msec induced a robust, long-lasting depression of the EPSP amplitude to 43%. Unlike LTD induced by low-frequency stimulation, associative LTD was resistant to the application of d-AP-5, indicating that it is independent of NMDA receptors. In contrast, associative LTD was inhibited by (S)-alpha-methyl-4-carboxyphenyl-glycine, indicating the involvement of metabotropic glutamate receptors. Furthermore, associative LTD is dependent on the activation of voltage-gated Ca(2+) channels by postsynaptic action potentials. Both nifedipine, an L-type Ca(2+) channel antagonist, and omega-conotoxin GVIA, a selective N-type channel blocker, abolished the induction of associative LTD. 8-hydroxy-2-dipropylaminotetralin (OH-DPAT), a 5-HT(1A) receptor agonist, inhibited postsynaptic Ca(2+) influx through N-type Ca(2+) channels, without affecting presynaptic transmitter release. OH-DPAT also inhibited the induction of associative LTD, suggesting that the involvement of N-type channels makes synaptic plasticity accessible to modulation by neurotransmitters. Thus, the modulation of N-type Ca(2+) channels provides a gain control for synaptic depression in hippocampal pyramidal neurons.

Localization and pharmacological characterization of voltage dependent calcium channels in cultured neocortical neurons

The physiological significance and subcellular distribution of voltage dependent calcium channels was defined using calcium channel blockers to inhibit potassium induced rises in cytosolic calcium concentration in cultured mouse neocortical neurons. The cytosolic calcium concentration was measured using the fluorescent calcium chelator fura-2. The types of calcium channels present at the synaptic terminal were determined by the inhibitory action of calcium channel blockers on potassium-induced [3H]GABA release in the same cell preparation. L-, N-, P-, Q- and R-/T-type voltage dependent calcium channels were differentially distributed in somata, neurites and nerve terminals. omega-conotoxin MVIIC (omega-CgTx MVIIC) inhibited approximately 40% of the Ca(2+)-rise in both somata and neurites and 60% of the potassium induced [3H]GABA release, indicating that the Q-type channel is the quantitatively most important voltage dependent calcium channel in all parts of the neuron. After treatment with thapsigargin the increase in cytosolic calcium was halved, indicating that calcium release from thapsigargin sensitive intracellular calcium stores is an important component of the potassium induced rise in cytosolic calcium concentration. The results of this investigation demonstrate that pharmacologically distinct types of voltage dependent calcium channels are differentially localized in cell bodies, neurites and nerve terminals of mouse cortical neurons but that the Q-type calcium channel appears to predominate in all compartments.