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

Auxiliary subunits operate as a molecular switch in determining gating behaviour of the unitary N-type Ca2+ channel current in Xenopus oocytes

1. We systematically examined the biophysical properties of omega-conotoxin GVIA-sensitive neuronal N-type channels composed of various combinations of the alpha1B, alpha2/delta and beta1b subunits in Xenopus oocytes. 2. Whole-cell recordings demonstrated that coexpression of the beta1b subunit decelerated inactivation, whereas the alpha2/delta accelerated both activation and inactivation, and cancelled the kinetic effects of the beta1b. The alpha2/delta and the beta1b controlled voltage dependence of activation differently: the beta1b significantly shifted the current-voltage relationship towards the hyperpolarizing direction; however, the alpha2/delta shifted the relationship only slightly in the depolarizing direction. The extent of voltage-dependent inactivation was modified solely by the beta1b. 3. Unitary currents measured using a cell-attached patch showed stable patterns of opening that were markedly different among subunit combinations in their kinetic parameters. The alpha2/delta and the beta1b subunits also acted antagonistically in regulating gating patterns of unitary N-type channels. Open time was shortened by the alpha2/delta, while the fraction of long opening was enhanced by the beta1b. The alpha2/delta decreased opening probability (Po), while the beta1b increased Po. alpha1Balpha2/deltabeta1b produced unitary activity with an open time distribution value in between those of alpha1Balpha2/delta and alpha1Bbeta1b. However, both the alpha2/delta and the beta1b subunits reduced the number of null traces. 4. These results suggest that the auxiliary subunits alone and in combination contribute differently in forming gating apparatuses in the N-type channel, raising the possibility that subunit interaction contributes to the generation of functional diversity of N-type channels in native neuronal preparations also.

Effects of erythropoietin on neuronal activity

Recently, erythropoietin (EPO) receptors and synthesis of EPO have been identified in the brain. To clarify the effects of EPO on neuronal cells, we investigated the effects of EPO on Ca2+ uptake, intracellular Ca2+ concentration, membrane potential, cell survival, release and biosynthesis of dopamine, and nitric oxide (NO) production in differentiated PC12 cells, which possess EPO receptors. EPO (10(-12)-10(-10) M) increased 45Ca2+ uptake and intracellular Ca2+ concentration in PC12 cells in a dose-related manner; these increases were inhibited by nicardipine (1 microM) or anti-EPO antibody (1:100 dilution). EPO induced membrane depolarization in PC12 cells. After a 5-day culture without serum and nerve growth factor (NGF), viable cell number decreased to 50% of that of the control cells cultured with serum and NGF. EPO (10(-13)-10(-10) M) increased the number of viable cells cultured without serum and NGF; this increase was blunted by nicardipine or anti-EPO antibody. Incubation with EPO (10(-13)-10(-10) M) stimulated mitogen-activated protein kinase activity in PC12 cells. EPO (10(-13)-10(-10) M) increased dopamine release from PC12 cells and tyrosine hydroxylase activity; these increases were sensitive to nicardipine or anti-EPO antibody. Following a 4-h incubation with EPO (10(-14)-10(-10) M), NO production was increased, which was blunted by nicardipine and anti-EPO antibody. In contrast, maximal NO synthase activity was not changed by EPO. These results suggest that EPO stimulates neuronal function and viability via activation of Ca2+ channels.

BK channel activation by brief depolarizations requires Ca2+ influx through L- and Q-type Ca2+ channels in rat chromaffin cells

BK channel activation by brief depolarizations requires Ca2+ influx through L- and Q-type Ca2+ channels in rat chromaffin cells. Ca2+- and voltage-dependent BK-type K+ channels contribute to action potential repolarization in rat adrenal chromaffin cells. Here we characterize the Ca2+ currents expressed in these cells and identify the Ca2+ channel subtypes that gate the activation of BK channels during Ca2+ influx. Selective Ca2+ channel antagonists indicate the presence of at least four types of high-voltage-gated Ca2+ channels: L-, N-, P, and Q type. Mean amplitudes of the L-, N-, P-, and Q-type Ca2+ currents were 33, 21, 12, and 24% of the total Ca2+ current, respectively. Five-millisecond Ca2+ influx steps to 0 mV were employed to assay the contribution of Ca2+ influx through these Ca2+ channels to the activation of BK current. Blockade of L-type Ca2+ channels by 5 microM nifedipine or Q-type Ca2+ channels by 2 microM Aga IVA reduced BK current activation by 77 and 42%, respectively. In contrast, blockade of N-type Ca2+ channels by brief applications of 1-2 microM CnTC MVIIC or P-type Ca2+ channels by 50-100 nM Aga IVA reduced BK current activation by only 11 and 12%, respectively. Selective blockade of L- and Q-type Ca2+ channels also eliminated activation of BK current during action potentials, whereas almost no effects were seen by the selective blockade of N- or P-type Ca2+ channels. Finally, the L-type Ca2+ channel agonist Bay K 8644 promoted activation of BK current by brief Ca2+ influx steps by more than twofold. These data show that, despite the presence of at least four types of Ca2+ channels in rat chromaffin cells, BK channel activation in rat chromaffin cells is predominantly coupled to Ca2+ influx through L- and Q-type Ca2+ channels.

alpha 1B N-type calcium channel isoforms with distinct biophysical properties

N-type calcium channels both generate the initial calcium signal to trigger neurotransmitter release and also interact with synaptic release proteins at many mammalian central nervous system synapses. Two isoforms of the alpha 1B N-type channel from rat brain (alpha 1B-I and alpha 1B-II) were found to differ in four regions: (1) a glutamate (Glu) to glycine (Gly) substitution in domain I S3; (2) a Gly to Glu substitution in the domain I-II linker; (3) the insertion or deletion of an alanine (Ala) in the domain I-II linker; and (4) the presence or absence of serine/phenylalanine/methionine/glycine (SFMG) in the linker between domain III S3-S4. Comparison of the electrophysiological properties of the alpha 1B-I and alpha 1B-II N-type channels shows that they exhibit distinct kinetics as well as altered current-voltage relations. Utilizing chimeric alpha 1B-I and alpha 1B-II cDNAs, we show that: (1) the Glu 177 to Gly substitution in domain I S3 increases the rate of activation by approximately 15-fold; (2) the presence or absence of Ala 415 in the domain I-II linker alters current-voltage relations by approximately 10 mV but does not affect channel kinetics; (3) the substitution of Gly 387 to Glu in the domain I-II linker also has no effect on kinetics; and (4) the presence or absence of SFMG (1236-1239) in domain III S3-S4 did not significantly affect channel current-voltage relations, kinetics, or steady state inactivation. We conclude that molecularly distinct alpha 1B isoforms are expressed in rat brain and may account for some of the functional diversity of N-type currents in native cells.

Interactions of presynaptic Ca2+ channels and snare proteins in neurotransmitter release

N- and P/Q-type Ca2+ channels are localized in high density in presynaptic nerve terminals and are crucial elements in neuronal excitation-secretion coupling. In addition to mediating Ca2+ entry to initiate transmitter release, they are thought to interact directly with proteins of the synaptic vesicle docking/fusion machinery. These Ca2+ channels can be purified from brain as a complex with SNARE proteins, which are involved in exocytosis. In addition, N-type and P/Q-type Ca2+ channels are colocalized with syntaxin in high-density clusters in nerve terminals. The synaptic protein interaction (synprint) sites in the intracellular loop II-III (LII-III) of both alpha 1B and alpha 1A subunits of N-type and P/Q-type Ca2+ channels bind to syntaxin, SNAP-25, and synaptotagmin. Ca2+ has a biphasic effect on the interactions of N-type Ca2+ channels with SNARE complexes, stimulating optimal binding in the range of 10-30 microM. PKC or CaM KII phosphorylation of the N-type synprint peptide inhibits interactions with SNARE complexes containing syntaxin and SNAP-25. Introduction of the synprint peptides into presynaptic superior cervical ganglion neurons reversibly inhibits EPSPs from synchronous transmitter release by 42%. At physiological Ca2+ concentrations, synprint peptides significantly reduce transmitter release in injected frog neuromuscular junctions in cell culture, consistent with detachment of 70% of the docked vesicles from Ca2+ channels as analyzed by a theoretical model. Together, these studies suggest that presynaptic Ca2+ channels not only provide the Ca2+ signal required by the exocytotic machinery, but also contain structural elements that are integral to vesicle docking, priming, and fusion processes.

The "psychic" neuron of the cerebral cortex

Remarkable advances in the identification, cloning, and localization of ion channels and receptors in the central nervous system have opened up unprecedented possibilities for relating structure to physiological function at the subcellular level of analysis. A singularly advanced property of select central nervous system neurons is their ability to exhibit increases in firing rate in relation to the mnemonic trace of a preceding event, a property that has been referred to as "working memory." Single-cell recordings from the prefrontal cortex of nonhuman primates have revealed neurons in the prefrontal cortex that possess "memory fields" analogous to the receptive field properties of sensory neurons. The integrity of these neurons has been shown to be essential for accurate performance in memory tasks performed by trained monkeys (and humans). We can now show that the excitability and/or tuning of these prefrontal neurons are subject to modulatory influences by dopamine, serotonin, GABA, and glutamate among other peptides and conventional neurotransmitters. I will describe the dopaminergic, serotonergic, and GABAergic innervation of pyramidal neurons engaged in working memory and the localization of neurotransmitter receptors through which they exert their actions. The findings reveal a remarkable degree of diversity in the subcellular localization and functionality of the five cloned dopamine receptors (D1, D2, D3, D4, and D5) and two serotonin (5HT2A and 5HT3) receptors that have been examined to date. The potential now exists for linking systems neurobiology with molecular biophysics to comprehend the highest functions of information processing that distinguish our species.

Dissection of the calcium channel domains responsible for modulation of neuronal voltage-dependent calcium channels by G proteins

The molecular determinants for G-protein regulation of neuronal calcium channels remain controversial. We have generated a series of alpha 1B/alpha 1E chimeric channels, since rat brain alpha 1E (rbEII), unlike human alpha 1E, showed no G-protein modulation. The study, carried out in parallel using D2 receptor modulation of calcium currents in Xenopus oocytes of G beta gamma modulation of calcium currents in COS-7 cells, consistently showed an essential role for domain I (from the N terminus to the end of the I-II loop) of the alpha 1B Ca2+ channel in G-protein regulation, with no additional effect of the C terminal of alpha 1B. The I-II loop alone of alpha 1B, or the I-II loop together with the C-terminal tail, was insufficient to confer G-protein modulation of alpha 1E (rbEII). We have further observed that the alpha 1E clone rbEII is truncated at the N-terminus compared to other alpha 1 subunits, and we isolated a PCR product from rat brain equivalent to a longer N-terminal isoform. The long N-terminal alpha 1E, unlike the short form, showed G-protein modulation. Furthermore, the equivalent truncation of alpha 1B (delta N1-55) abolished G-protein modulation of alpha 1B. Thus, we propose that the N terminus of alpha 1B and alpha 1E calcium channels contains essential molecular determinants for membrane-delimited G-protein inhibition, and that other regions, including the I-II loop and the C terminus, do not play a conclusive role alone.

Cellular sites for dynorphin activation of kappa-opioid receptors in the rat nucleus accumbens shell

The nucleus accumbens (Acb) is prominently involved in the aversive behavioral aspects of kappa-opioid receptor (KOR) agonists, including its endogenous ligand dynorphin (Dyn). We examined the ultrastructural immunoperoxidase localization of KOR and immunogold labeling of Dyn to determine the major cellular sites for KOR activation in this region. Of 851 KOR-labeled structures sampled from a total area of 10,457 microm2, 63% were small axons and morphologically heterogenous axon terminals, 31% of which apposed Dyn-labeled terminals or also contained Dyn. Sixty-eight percent of the KOR-containing axon terminals formed punctate-symmetric or appositional contacts with unlabeled dendrites and spines, many of which received convergent input from terminals that formed asymmetric synapses. Excitatory-type terminals that formed asymmetric synapses with dendritic spines comprised 21% of the KOR-immunoreactive profiles. Dendritic spines within the neuropil were the major nonaxonal structures that contained KOR immunoreactivity. These spines also received excitatory-type synapses from unlabeled terminals and were apposed by Dyn-containing terminals. These results provide ultrastructural evidence that in the Acb shell (AcbSh), KOR agonists play a primary role in regulating the presynaptic release of Dyn and other neuromodulators that influence the output of spiny neurons via changes in the presynaptic release of or the postsynaptic responses to excitatory amino acids. The cellular distribution of KOR complements those described previously for the reward-associated mu- and delta-opioid receptors in the Acb shell.

A neurocomputational theory of the dopaminergic modulation of working memory functions

The dopaminergic modulation of neural activity in the prefrontal cortex (PFC) is essential for working memory. Delay-activity in the PFC in working memory tasks persists even if interfering stimuli intervene between the presentation of the sample and the target stimulus. Here, the hypothesis is put forward that the functional role of dopamine in working memory processing is to stabilize active neural representations in the PFC network and thereby to protect goal-related delay-activity against interfering stimuli. To test this hypothesis, we examined the reported dopamine-induced changes in several biophysical properties of PFC neurons to determine whether they could fulfill this function. An attractor network model consisting of model neurons was devised in which the empirically observed effects of dopamine on synaptic and voltage-gated membrane conductances could be represented in a biophysically realistic manner. In the model, the dopamine-induced enhancement of the persistent Na+ and reduction of the slowly inactivating K+ current increased firing of the delay-active neurons, thereby increasing inhibitory feedback and thus reducing activity of the "background" neurons. Furthermore, the dopamine-induced reduction of EPSP sizes and a dendritic Ca2+ current diminished the impact of intervening stimuli on current network activity. In this manner, dopaminergic effects indeed acted to stabilize current delay-activity. Working memory deficits observed after supranormal D1-receptor stimulation could also be explained within this framework. Thus, the model offers a mechanistic explanation for the behavioral deficits observed after blockade or after supranormal stimulation of dopamine receptors in the PFC and, in addition, makes some specific empirical predictions.

Akt-dependent potentiation of L channels by insulin-like growth factor-1 is required for neuronal survival

The insulin-like growth factor-1 (IGF-1)/receptor tyrosine kinase recently has been shown to mediate neuronal survival and potentiate the activity of specific calcium channel subtypes; survival requires Akt, a serine/threonine kinase. We demonstrate here that Akt mediates the IGF-1-induced potentiation of L channel currents, but not that of N channels. Transient expression of wild-type, dominant-negative, and constitutively active forms of Akt in cerebellar granule neurons causes, respectively, no change in IGF-1/L channel potentiation, complete inhibition of potentiation, and a dramatic increase in basal L currents accompanied by the loss of ability to induce further increases. In no case is the IGF-1 potentiation of N currents affected. We additionally find that IGF-1 partially mediates granule neuron survival via L channel activity and that Akt-dependent L channel modulation is a necessary component. Interestingly, very brief exposure (1 min) to IGF-1 triggers nearly complete survival and requires L channel activity. These results strongly suggest that neuronal receptor tyrosine kinases can control long-term calcium-dependent processes via the rapid control of voltage-sensitive channels.

Differential plasma membrane targeting of voltage-dependent calcium channel subunits expressed in a polarized epithelial cell line

1. Voltage-dependent calcium channels (VDCCs) show a highly non-uniform distribution in many cell types, including neurons and other polarized secretory cells. We have examined whether this can be mimicked in a polarized epithelial cell line (Madin-Darby canine kidney), which has been used extensively to study the targeting of proteins. 2. We expressed the VDCC alpha1A, alpha1B or alpha1C subunits either alone or in combination with accessory subunits alpha2-delta and the different beta subunits, and examined their localization immunocytochemically. An alpha1 subunit was only targeted to the plasma membrane if co-expressed with the accessory subunits. 3. The combination alpha1C/alpha2-delta and all beta subunits was always localized predominantly to the basolateral membrane. It has been suggested that this is equivalent to somatodendritic targeting in neurons. 4. In contrast, the alpha1B subunit was expressed at the apical membrane with all the accessory subunit combinations, by 24 h after microinjection. This membrane destination shows some parallels with axonal targeting in neurons. 5. The alpha1A subunit was consistently observed at the apical membrane in the combinations alpha1A/alpha2-delta/beta1b or beta4. In contrast, when co-expressed with alpha2-delta/beta2a, alpha1A was clearly targeted to the basolateral membrane. 6. In conclusion, the VDCC alpha1 subunit appears to be the primary determinant for targeting the VDCC complex, but the beta subunit can modify this destination, particularly for alpha1A.

Identification of an integration center for cross-talk between protein kinase C and G protein modulation of N-type calcium channels

The modulation of presynaptic calcium channel activity by second messengers provides a fine tuning mechanism for neurotransmitter release. In neurons, the activation of certain G protein-coupled receptors reduces N-type channel activity by approximately 60%. In contrast, activation of protein kinase C (PKC) results in an approximately 50% increase in N-type channel activity, and subsequent G protein inhibition is antagonized. Here, we describe the molecular determinants that control the dual effects of PKC-dependent phosphorylation. The double substitution of two adjacent PKC consensus sites in the calcium channel domain I-II linker (Thr422, Ser425) to alanines abolished both PKC-dependent up-regulation and the PKC-G protein cross-talk. The single substitution of Ser425 to glutamic acid abolished PKC up-regulation but had no effect on G protein modulation. Replacement of Thr422 with glutamic acid eliminated PKC-dependent up-regulation and mimicked the effects of PKC phosphorylation on G protein inhibition. Our data suggest that Thr422 mediates the antagonistic effect of PKC on G protein modulation, while phosphorylation of either Thr422 or Ser425 are sufficient to increase N-type channel activity. Thus, Thr422 serves as a molecular switch by which PKC is able to simultaneously trigger the up-regulation of channel activity and antagonize G protein inhibition.

Voltage-gated Ca2+ conductances in acutely isolated guinea pig dorsal cochlear nucleus neurons

Although it is known that voltage-gated Ca2+ conductances (VGCCs) contribute to the responses of dorsal cochlear nucleus (DCN) neurons, little is known about the properties of VGCCs in the DCN. In this study, the whole cell voltage-clamp technique was used to examine the pharmacology and voltage dependence of VGCCs in unidentified DCN neurons acutely isolated from guinea pig brain stem. The majority of cells responded to depolarization with sustained inward currents that were enhanced when Ca2+ was replaced by Ba2+, were blocked partially by Ni2+ (100 microM), and were blocked almost completely by Cd2+ (50 microM). Experiments using nifedipine (10 microM), omegaAga IVA (100 nM) and omegaCTX GVIA (500 nM) demonstrated that a variety of VGCC subtypes contributed to the Ba2+ current in most cells, including the L, N, and P/Q types and antagonist-insensitive R type. Although a large depolarization from rest was required to activate VGCCs in DCN neurons, VGCC activation was rapid at depolarized levels, having time constants <1 ms at 22 degrees C. No fast low-threshold inactivation was observed, and a slow high-threshold inactivation was observed at voltages more positive than -20 mV, indicating that Ba2+ currents were carried by high-voltage activated VGCCs. The VGCC subtypes contributing to the overall Ba2+ current had similar voltage-dependent properties, with the exception of the antagonist-insensitive R-type component, which had a slower activation and a more pronounced inactivation than the other components. These data suggest that a variety of VGCCs is present in DCN neurons, and these conductances generate a rapid Ca2+ influx in response to depolarizing stimuli.

Interactions between proteins implicated in exocytosis and voltage-gated calcium channels

Neurotransmitter release from synaptic vesicles is triggered by voltage-gated calcium influx through P/Q-type or N-type calcium channels. Purification of N-type channels from rat brain synaptosomes initially suggested molecular interactions between calcium channels and two key proteins implicated in exocytosis: synaptotagmin I and syntaxin 1. Co-immunoprecipitation experiments were consistent with the hypothesis that both N- and P/Q-type calcium channels, but not L-type channels, are associated with the 7S complex containing syntaxin 1, SNAP-25, VAMP and synaptotagmin I or II. Immunofluorescence confocal microscopy at the frog neuromuscular junction confirmed that calcium channels, syntaxin 1 and SNAP-25 are co-localized at active zones of the presynaptic plasma membrane where transmitter release occurs. Experiments with recombinant proteins were performed to map synaptic protein interaction sites on the alpha 1A subunit, which forms the pore of the P/Q-type calcium channel. In vitro-translated 35S-synaptotagmin I bound to a site located on the cytoplasmic loop linking homologous domains II and III of the alpha 1A subunit. This direct link would target synaptotagmin, a putative calcium sensor for exocytosis, to a microdomain of calcium influx close to the channel mouth. Cysteine string proteins (CSPs) contain a J-domain characteristic of molecular chaperones that cooperate with Hsp70. They are located on synaptic vesicles and thought to be involved in modulating the activity of presynaptic calcium channels. CSPs were found to bind to the same domain of the calcium channel as synaptotagmin, and also to associate with VAMP. CSPs may act as molecular chaperones in association with Hsp70 to direct assembly or dissociation of multiprotein complexes at the calcium channel.

Calcium channel types with distinct presynaptic localization couple differentially to transmitter release in single calyx-type synapses

We studied how Ca2+ influx through different subtypes of Ca2+ channels couples to release at a calyx-type terminal in the rat medial nucleus of the trapezoid body by simultaneously measuring the presynaptic Ca2+ influx evoked by a single action potential and the EPSC. Application of subtype-specific toxins showed that Ca2+ channels of the P/Q-, N-, and R-type controlled glutamate release at a single terminal. The Ca2+ influx through the P/Q-type channels triggered release more effectively than Ca2+ influx through N- or R-type channels. We investigated mechanisms that contributed to these differences in effectiveness. Electrophysiological experiments suggested that individual release sites were controlled by all three subtypes of Ca2+ channels. Immunocytochemical staining indicated, however, that a substantial fraction of N- and R-type channels was located distant from release sites. Although these distant channels contributed to the Ca2+ influx into the terminal, they may not contribute to release. Taken together, the results suggest that the Ca2+ influx into the calyx via N- and R-type channels triggers release less effectively than that via P/Q-type because a substantial fraction of the N- and R-type channels in the calyx is localized distant from release sites.

Ca2+ channel antagonists and neuroprotection from cerebral ischemia

Stroke is the third leading cause of death and the main disabling neurologic disease. The finding in experimental studies that neuronal death does not occur immediately after ischemic injury has encouraged the development of neuroprotective agents. Various Ca2+ channel antagonists, that is, L-type-selective or non-selective derivatives from classical Ca2+ channel antagonists, have been examined for their ability of neuroprotection through improvement of cerebral blood circulation or inhibition of Ca2+ overload induced by excessive glutamate release. Although some of the antagonists showed efficient neuroprotection in animal models, systemic hypotension limited the utility of these drugs, and none of the compounds showed beneficial effects in treatments for acute ischemic stroke in clinical trials. Drugs other than Ca2+ channel antagonists developed on the basis of the glutamate-Ca2+ overload hypothesis were shown also to lack clinical benefit. Recently, some mechanisms have been proposed to interpret neuronal death in relation to hyperexcitability or apoptosis after ischemic insult. In these hypotheses, activation of the Ca2+ channel types selectively expressed in neuronal tissues is proposed as a critical step of the pathways toward neurodegeneration. Thus, it is increasingly recognized that developing highly selective compounds for neuronal Ca2+ channels is not only important for treatment of stroke but also for elucidation of mechanisms that underlie neurodegeneration.

Evidence for a voltage-dependent enhancement of neurotransmitter release mediated via the synaptic protein interaction site of N-type Ca2+ channels

Secretion of neurotransmitters is initiated by voltage-gated calcium influx through presynaptic, voltage-gated N-type calcium channels. These channels interact with the SNARE proteins, which are core components of the exocytosis process, via the synaptic protein interaction (synprint) site in the intracellular loop connecting domains II and III of their alpha1B subunit. Interruption of this interaction by competing synprint peptides inhibits fast, synchronous transmitter release. Here we identify a voltage-dependent, but calcium-independent, enhancement of transmitter release that is elicited by trains of action potentials in the presence of a hyperosmotic extracellular concentration of sucrose. This enhancement of transmitter release requires interaction of SNARE proteins with the synprint site. Our results provide evidence for a voltage-dependent signal that is transmitted by protein-protein interactions from the N-type calcium channel to the SNARE proteins and enhances neurotransmitter release by altering SNARE protein function.

L-Type Ca2+ channels mediate the slow Ca2+-dependent afterhyperpolarization current in rat CA3 pyramidal cells in vitro

Single-electrode voltage-clamp recordings were obtained from CA3 pyramidal cells in rat hippocampal organotypic slice cultures, and the slow Ca2+-dependent K+ current or afterhyperpolarization current (IAHP) was elicited with brief depolarizing voltage jumps. The slow IAHP was suppressed by the selective L-type Ca2+ channel antagonists isradipine (2 microM) or nifedipine (10 microM). In contrast, neither omega-conotoxin MVIIA (1 microM) nor omega-agatoxin IVA (200 nM), N-type and P/Q-type Ca2+ channel antagonists, respectively, attenuated this slow outward current. The slow IAHP was significantly reduced by thapsigargin (10 microM), a Ca2+ ATPase inhibitor that depletes intracellular Ca2+ stores, and by ryanodine (10-100 microM), which blocks Ca2+-induced Ca2+ release from intracellular compartments. At this concentration thapsigargin did not modify high-threshold Ca2+ current, which was, however, blocked by isradipine. Thus, in hippocampal CA3 pyramidal cells, Ca2+ influx through L-type Ca2+ channels is necessary to trigger the slow IAHP. Furthermore, intracellular Ca2+-activated Ca2+ stores represent a critical component in the transduction pathway leading to the generation of the slow IAHP.

Structure and function of neuronal Ca2+ channels and their role in neurotransmitter release

Electrophysiological studies of neurons reveal different Ca2+ currents designated L-, N-, P-, Q-, R-, and T-type. High-voltage-activated neuronal Ca2+ channels are complexes of a pore-forming alpha 1 subunit of about 190-250 kDa, a transmembrane, disulfide-linked complex of alpha 2 and delta subunits, and an intracellular beta subunit, similar to the alpha 1, alpha 2 delta, and beta subunits previously described for skeletal muscle Ca2+ channels. The primary structures of these subunits have all been determined by homology cDNA cloning using the corresponding subunits of skeletal muscle Ca2+ channels as probes. In most neurons, L-type channels contain alpha 1C or alpha 1D subunits, N-type contain alpha 1B subunits, P- and Q-types contain alternatively spliced forms of alpha 1A subunits, R-type contain alpha 1E subunits, and T-type contain alpha 1G or alpha 1H subunits. Association with different beta subunits also influences Ca2+ channel gating substantially, yielding a remarkable diversity of functionally distinct molecular species of Ca2+ channels in neurons.

Selective activation of Ca2+-activated K+ channels by co-localized Ca2+ channels in hippocampal neurons

Calcium entry through voltage-gated calcium channels can activate either large- (BK) or small- (SK) conductance calcium-activated potassium channels. In hippocampal neurons, activation of BK channels underlies the falling phase of an action potential and generation of the fast afterhyperpolarization (AHP). In contrast, SK channel activation underlies generation of the slow AHP after a burst of action potentials. The source of calcium for BK channel activation is unknown, but the slow AHP is blocked by dihydropyridine antagonists, indicating that L-type calcium channels provide the calcium for activation of SK channels. It is not understood how this specialized coupling between calcium and potassium channels is achieved. Here we study channel activity in cell-attached patches from hippocampal neurons and report a unique specificity of coupling. L-type channels activate SK channels only, without activating BK channels present in the same patch. The delay between the opening of L-type channels and SK channels indicates that these channels are 50-150 nm apart. In contrast, N-type calcium channels activate BK channels only, with opening of the two channel types being nearly coincident. This temporal association indicates that N and BK channels are very close. Finally, P/Q-type calcium channels do not couple to either SK or BK channels. These data indicate an absolute segregation of coupling between channels, and illustrate the functional importance of submembrane calcium microdomains.

Contrasting Ca2+ channel subtypes at cell bodies and synaptic terminals of rat anterioventral cochlear bushy neurones

1. Whole-cell patch clamp recordings were made from bushy cells of the anterioventral cochlear nucleus (aVCN) and their synaptic terminals (calyx of Held) in the medial nucleus of the trapezoid body (MNTB). 2. Both high voltage-activated (HVA) and low voltage-activated (LVA) calcium currents were present in acutely dissociated aVCN neurones and in identified bushy neurones from a cochlear nucleus slice. 3. The transient LVA calcium current activated rapidly on depolarization (half-activation, -59 mV) and inactivated during maintained depolarization (half-inactivation, -89 mV). This T-type current was observed in somatic recordings but was absent from presynaptic terminals. 4. On the basis of their pharmacological sensitivity, P/Q-type Ca2+ channels accounted for only 6 % of the somatic HVA, while L-, N- and R-type Ca2+ channels each accounted for around one-third of the somatic calcium current. 5. The divalent permeabilities of these native calcium channels were compared. The Ba2+/Ca2+ conductance ratios of the somatic HVA and LVA channels were 1.4 and 0.7, respectively. The conductance ratio of the presynaptic HVA current was 0.9, significantly lower that that of the somatic HVA current. 6. We conclude that LVA currents are expressed in the bushy cell body, but are not localized to the excitatory synaptic terminal. All of the HVA current subtypes are expressed in bushy cells, but there is a strong polarity to their localization; P-type contribute little to somatic currents but predominate at the synaptic terminal; L-, N- and R-types dominate at the soma, but contribute negligibly to calcium currents in the terminal.

N- and L-type calcium channel involvement in depolarization-induced suppression of inhibition in rat hippocampal CA1 cells

1. We investigated depolarization-induced suppression of inhibition (DSI) under whole-cell voltage clamp in CA1 pyramidal neurons of rat hippocampal slices. DSI, a transient reduction in monosynaptic evoked GABAAergic IPSCs lasting for approximately 1 min, was induced by depolarizing the pyramidal cell to -10 or 0 mV for 1 or 2 s. 2. Raising extracellular Ca2+ concentration increased DSI, and varying the DSI-inducing voltage step showed that the voltage dependence of DSI was like that of high-voltage-activated Ca2+ channels. 3. The P- and Q-type Ca2+ channel blocker omega-agatoxin TK (200 nM and 1 microM) and the R- and T-type Ca2+ channel blocker Ni2+ (100 microM) reduced IPSCs without reducing DSI. 4. The specific N-type Ca2+ channel antagonist omega-conotoxin GVIA (250 nM) reduced IPSC amplitudes and almost completely abolished DSI. 5. Blocking L-type Ca2+ channels with nifedipine (10 microM) had no effect on IPSCs or DSI induced by our standard protocol, but reduced DSI induced by the unclamped Na+- and Ca2+-dependent spikes that occurred when 2(triethylamino)-N-(2,6-dimethylphenyl)acetamide (QX-314) was omitted from the recording pipette solution. 6. Although intracellular Ca2+ stores were not measured, DSI was not affected by cyclopiazonic acid (CPA, 20-40 microM), a blocker of Ca2+ uptake into intracellular stores. 7. We conclude that DSI is initiated by Ca2+ influx through N- and, under certain conditions, L-type Ca2+ channels.

Intracellular pH buffering shapes activity-dependent Ca2+ dynamics in dendrites of CA1 interneurons

Voltage-gated calcium (Ca) channels are highly sensitive to cytosolic H+, and Ca2+ influx through these channels triggers an activity-dependent fall in intracellular pH (pHi). In principle, this acidosis could act as a negative feedback signal that restricts excessive Ca2+ influx. To examine this possibility, whole cell current-clamp recordings were taken from rat hippocampal interneurons, and dendritic Ca2+ transients were monitored fluorometrically during spike trains evoked by brief depolarizing pulses. In cells dialyzed with elevated internal pH buffering (high beta), trains of >15 action potentials (Aps) provoked a significantly larger Ca2+ transient. Voltage-clamp analysis of whole cell Ca currents revealed that differences in cytosolic pH buffering per se did not alter baseline Ca channel function, although deliberate internal acidification by 0.3 pH units blunted Ca currents by approximately 20%. APs always broadened during a spike train, yet this broadening was significantly greater in high beta cells during rapid but not slow firing rates. This effect of internal beta on spike repolarization could be blocked by cadmium. High beta also 1) enhanced the slow afterhyperpolarization (sAHP) seen after a spike train and 2) accelerated the decay of an early component of the sAHP that closely matched a sAHP conductance that could be blocked by apamin. Both of these effects on the sAHP could be detected at high but not low firing rates. These data suggest that activity-dependent pHi shifts can blunt voltage-gated Ca2+ influx and retard submembrane Ca2+ clearance, suggesting a novel feedback mechanism by which Ca2+ signals are shaped and coupled to the level of cell activity.

High safety factor for action potential conduction along axons but not dendrites of cultured hippocampal and cortical neurons

By using a combination of Ca2+ imaging and current-clamp recording, we previously reported that action potential (AP) conduction is reliably observed from the soma to axonal terminals in cultured cortical neurons. To extend these studies, we evaluated Ca2+ influx evoked by Na+ APs as a marker of AP conduction under conditions that are expected to lower the conduction safety factor to explore mechanisms of axonal and dendritic excitability. As expected, reducing the extracellular Na+ concentration from 150 to approximately 60 mM decreased the amplitude of APs recorded in the soma but surprisingly did not influence axonal conduction, as monitored by measuring Ca2+ transients. Furthermore, reliable axonal conduction was observed in dilute (20 nM) tetrodotoxin (TTX), despite a similar reduction in AP amplitude. In contrast, the Ca2+ transient measured along dendrites was markedly reduced in low Na+, although still mediated by TTX-sensitive Na+ channels. Dendritic action-potential evoked Ca2+ transients were also markedly reduced in 20 nM TTX. These data provide further evidence that strongly excitable axons are functionally compartmentalized from weakly excitable dendrites. We conclude that modulation of Na+ currents or membrane potential by neurotransmitters or repetitive firing is more likely to influence neuronal firing before AP generation than the propagation of signals to axonal terminals. In contrast, the relatively low safety factor for back-propagating APs in dendrites would suggest a stronger effect of Na+ current modulation.

Localization of Ca2+ channel subtypes on rat spinal motor neurons, interneurons, and nerve terminals

Ca2+ channels in distinct subcellular compartments of neurons mediate voltage-dependent Ca2+ influx, which integrates synaptic responses, regulates gene expression, and initiates synaptic transmission. Antibodies that specifically recognize the alpha1 subunits of class A, B, C, D, and E Ca2+ channels have been used to investigate the localization of these voltage-gated ion channels on spinal motor neurons, interneurons, and nerve terminals of the adult rat. Class A P/Q-type Ca2+ channels were present mainly in a punctate pattern in nerve terminals located along the cell bodies and dendrites of motor neurons. Both smooth and punctate staining patterns were observed over the surface of the cell bodies and dendrites with antibodies to class B N-type Ca2+ channels, indicating the presence of these channels in the cell surface membrane and in nerve terminals. Class C and D L-type and class E R-type Ca2+ channels were distributed mainly over the cell soma and proximal dendrites. Class A P/Q-type Ca2+ channels were present predominantly in the presynaptic terminals of motor neurons at the neuromuscular junction. Occasional nerve terminals innervating skeletal muscles from the hindlimb were labeled with antibodies against class B N-type Ca2+ channels. Staining of the dorsal laminae of the rat spinal cord revealed a complementary distribution of class A and class B Ca2+ channels in nerve terminals in the deeper versus the superficial laminae. Many of the nerve terminals immunoreactive for class B N-type Ca2+ channels also contained substance P, an important neuropeptide in pain pathways, suggesting that N-type Ca2+ channels are predominant at synapses that carry nociceptive information into the spinal cord.

Membrane traffic in polarized neurons

The plasma membrane of neurons can be divided into two domains, the soma-dendritic and the axonal. These domains perform different functions: the dendritic surface receives and processes information while the axonal surface is specialized for the rapid transmission of electrical impulses. This functional specialization is generated by sorting and anchoring mechanisms that guarantee the correct delivery and retention of specific membrane proteins. Our understanding of neuronal membrane protein sorting is primarily based on studies of protein overexpression in cultured neurons. These studies revealed that newly synthesized membrane proteins are segregated in the Golgi apparatus in the cell body from where they are transported to the axonal or dendritic surface. Such segregation presumably depends on sorting motifs in the proteins' primary structure. They appear to be located in the cytoplasmic tail for dendritic proteins and in the transmembrane-ectodomain for axonal proteins. Recent studies on neurotransmitter segregation suggest that anchoring in the correct subdomain of the plasma membrane also requires cytoplasmic tail information for binding to the cytoskeleton either directly or by linker proteins. Both mechanisms, sorting and retention, gradually mature during neural development. Young neurons appear to develop initial polarity by other mechanisms, presumably analogous to the mechanisms used by migrating cells.

Differential expression and association of calcium channel subunits in development and disease

Voltage-gated calcium channels (VDCC) are essential to neuronal maturation and differentiation. It is believed that important signaling information is encoded by VDCC-mediated calcium influx that has both spatial and temporal components. VDCC are multimeric complexes comprised of a pore-forming alpha1 subunit and auxiliary beta and alpha2/delta subunits. Changes in the fractional contribution of distinct calcium conductances to the total calcium current have been noted in developing and differentiating neurons. These changes are anticipated to reflect the differential expression and localization of the pore-forming alpha1 subunits. However, as in vitro studies have established that beta regulates the channel properties and targeting of alpha1, attention has been directed toward the developmental expression and assembly of beta isoforms. Recently, changes in the beta component of the omega-conotoxin GVIA (CTX)-sensitive N-type VDCC have indicated differential assembly of alpha1B with beta in postnatal rat brain. In addition, unique properties of beta4 have been noted with respect to its temporal pattern of expression and incorporation into N-type VDCC complexes. Therefore, the expression and assembly of specific alpha1/beta complexes may reflect an elaborate cellular strategy for regulating VDCC diversity. The importance of these developmental findings is bolstered by a recent study which identified mutations in the beta4 as the molecular defect in the mutant epileptic mouse (lethargic; lh/lh). As beta4 is normally expressed in both forebrain and cerebellum, one may consider the impact of the loss of beta4 upon VDCC assembly and activity. The importance of the beta1b and beta4 isoforms to calcium channel maturation and assembly is discussed.

Interactions between presynaptic calcium channels and proteins implicated in synaptic vesicle trafficking and exocytosis

Monoclonal antibodies were generated by immunizing mice with chick brain synaptic membranes and screening for immunoprecipitation of solubilized omega conotoxin GVIA receptors (N-type calcium channels). Antibodies against two synaptic proteins (p35--syntaxin 1 and p58--synaptotagmin) were produced and used to purify and characterize a ternary complex containing N-type channels associated with these two proteins. These results provided the first evidence for a specific interaction between presynaptic calcium channels and SNARE proteins involved in synaptic vesicle docking and calcium-dependent exocytosis. Immunoprecipitation experiments supported the conclusion that syntaxin 1/SNAP-25/VAMP/synaptotagmin I or II complexes associate with N-type, P/Q-type, but not L-type calcium channels from rat brain nerve terminals. Immunofluorescent confocal microscopy at the frog neuromuscular junction was consistent with the co-localization of syntaxin 1, SNAP-25, and calcium channels, all of which are predominantly expressed at active zones of the presynaptic plasma membrane facing post-synaptic folds rich in acetylcholine receptors. The interaction of proteins implicated in calcium-dependent exocytosis with presynaptic calcium channels may locate the sensor(s) that trigger vesicle fusion within a microdomain of calcium entry.

Metabolism and trafficking of N-type voltage-operated calcium channels in neurosecretory cells

The N-type voltage-operated calcium channel has been characterized over the years as a high-threshold channel, with variable inactivation kinetics, and a unique ability to bind with high affinity and specificity omega-conotoxin GVIA and related toxins. This channel is particularly expressed in some neurons and endocrine cells, where it participates in several calcium-dependent processes, including secretion. Omega-conotoxin GVIA was instrumental not only for the biophysical and pharmacological characterization of N-type channels but also for the development of in vitro assays for studying N-type VOCC subcellular localization, biosynthesis, turnover, as well as short-and long-term regulation of its expression. We here summarize our studies on N-type VOCC expression in neurosecretory cells, with a major emphasis on recent data demonstrating the presence of N-type channels in intracellular secretory organelles and their recruitment to the cell surface during regulated exocytosis.

Differential expression and association of calcium channel alpha1B and beta subunits during rat brain ontogeny

Calcium functions as an essential second messenger during neuronal development and synapse acquisition. Voltage-dependent calcium channels (VDCC), which are critical to these processes, are heteromultimeric complexes composed of alpha1, alpha2/delta, and beta subunits. beta subunits function to direct the VDCC complex to the plasma membrane as well as regulate its channel properties. The importance of beta to neuronal functioning was recently underscored by the identification of a truncated beta4 isoform in the epileptic mouse lethargic (lh) (Burgess, D. L., Jones, J. M., Meisler, M. H., and Noebels, J. L. (1997) Cell 88, 385-392). The goal of our study was to investigate the role of individual beta isoforms (beta1b, beta2, beta3, and beta4) in the assembly of N-type VDCC during rat brain development. By using quantitative Western blot analysis with anti-alpha1B-directed antibodies and [125I-Tyr22]omega-conotoxin GVIA (125I-CTX) radioligand binding assays, we observed that only a small fraction of the total alpha1B protein present in embryonic and early postnatal brain expressed high affinity 125I-CTX-binding sites. These results suggested that subsequent maturation of alpha1B or its assembly with auxiliary subunits was required to exhibit high affinity 125I-CTX binding. The temporal pattern of expression of beta subunits and their assembly with alpha1B indicated a developmental pattern of expression of beta isoforms: beta1b increased 3-fold from P0 to adult, beta4 increased 10-fold, and both beta2 and beta3 expression remained unchanged. As the beta component of N-type VDCC changed during postnatal development, we were able to identify both immature and mature forms of N-type VDCC. At P2, the relative contribution of beta is beta1b > beta3 >> beta2, whereas at P14 and adult the distribution is beta3 > beta1b = beta4. Although we observed no beta4 associated with the alpha1B at P2, beta4 accounted for 14 and 25% of total alpha1B/beta subunit complexes in P14 and adult, respectively. Thus, of the beta isoforms analyzed, only the beta4 was assembled with the rat alpha1B to form N-type VDCC with a time course that paralleled its level of expression during rat brain development. These results suggest a role for the beta4 isoform in the assembly and maturation of the N-type VDCC.

Presynaptic receptors

Activation of different types of G-protein-linked and ionotropic presynaptic receptors has been shown to regulate neurotransmitter release throughout the central and peripheral nervous systems. In the case of G-protein-linked receptors, three major mechanisms have been suggested: (a) inhibition of Ca channels in the nerve terminal; (b) the activation of presynaptic K channels, resulting in a reduction in the effectiveness of the action potential; and (c) direct modulation of one or more components of the neurotransmitter vesicle release apparatus. In the case of ionotropic presynaptic receptors, inhibition of release may be achieved through depolarization of the terminal and inactivation of Na and Ca channels. Activation of presynaptic ionotropic receptors that are appreciably Ca permeable can also enhance the release of transmitters as a result of their ability to raise [Ca]i in the terminal directly. Many transmitters employ several of these mechanisms, thus allowing considerable flexibility in the presynaptic regulation of transmitter release.

Substance P attenuates and DAMGO potentiates amygdala glutamatergic neurotransmission within the ventral pallidum

The amygdala (AMG), nucleus accumbens (NA) and ventral pallidum (VP) influence goal-oriented behaviors. However, the nature of the interactions among these regions has not been well characterized. Anatomical studies indicate that excitatory amino acids are contained in VP inputs from the AMG, and the NA is a primary source of VP substance P (SP) and opioids. The present study was designed to functionally characterize the NA and AMG projections to the VP, and to assess if opioids and SP can modulate AMG-mediated excitatory neurotransmission within the VP. To do so, extracellularly recorded electrophysiological responses of single VP neurons to electrical activation of VP afferents were monitored during microiontophoretic application of treatment ligands in chloral hydrate-anesthetized rats. The anatomically described glutamatergic inputs from the AMG, and SP inputs from the NA, were pharmacologically verified. It also was determined that even though iontophoretically applied SP increased the spontaneous activity of VP neurons, at ejection current levels that were below those necessary to produce this effect (termed sub-threshold), the tachykinin attenuated AMG stimulation-evoked glutamatergic neurotransmission. SP failed to modulate the excitations induced by iontophoretically applied glutamate suggesting that SP modulation of AMG-evoked excitations were mediated via a decrease in the pre-synaptic release of glutamate. Like SP, the effects of sub-threshold ejection currents of micro opioid agonist DAMGO on AMG-evoked responses were not predicted by the opioid's effects on spontaneous VP neuronal activity; DAMGO inhibited spontaneous firing but potentiated AMG-evoked glutamatergic neurotransmission. The opioid also potentiated effects of exogenous glutamate implying an interaction at a post-synaptic site. These results indicate that tachykinin and opioid neuropeptides contained in NA projection neurons can differentially modulate AMG glutamatergic inputs to the VP.

Specificity in the interaction of HVA Ca2+ channel types with Ca2+-dependent AHPs and firing behavior in neocortical pyramidal neurons

Intracellular recordings and organic and inorganic Ca2+ channel blockers were used in a neocortical brain slice preparation to test whether high-voltage-activated (HVA) Ca2+ channels are differentially coupled to Ca2+-dependent afterhyperpolarizations (AHPs) in sensorimotor neocortical pyramidal neurons. For the most part, spike repolarization was not Ca2+ dependent in these cells, although the final phase of repolarization (after the fast AHP) was sensitive to block of N-type current. Between 30 and 60% of the medium afterhyperpolarization (mAHP) and between approximately 80 and 90% of the slow AHP (sAHP) were Ca2+ dependent. Based on the effects of specific organic Ca2+ channel blockers (dihydropyridines, omega-conotoxin GVIA, omega-agatoxin IVA, and omega-conotoxin MVIIC), the sAHP is coupled to N-, P-, and Q-type currents. P-type currents were coupled to the mAHP. L-type current was not involved in the generation of either AHP but (with other HVA currents) contributes to the inward currents that regulate interspike intervals during repetitive firing. These data suggest different functional consequences for modulation of Ca2+ current subtypes.

Contact-dependent regulation of N-type calcium channel subunits during synaptogenesis

The developmental regulation of the N-type calcium channel during synaptogenesis was studied using cultured rat hippocampal neurons to elucidate the roles of extrinsic versus intrinsic cues in the expression and distribution of this channel. Prior to synapse formation, alpha1B and beta3 subunits of the N-type calcium channel were distributed diffusely throughout neurites, growth cones, and somata. As synaptogenesis proceeded, the subunit distributions became punctate and colocalized with the synaptic vesicle protein synaptotagmin. Isolated neurons were also examined to test for the requirement of extrinsic cues that control N-type calcium channel expression and distribution. These neurons expressed N-type calcium channel subunits, but their distributions remained diffuse. Functional omega-conotoxin GVIA-sensitive channels were expressed in isolated neurons, although the distribution of alpha1B subunits was diffuse. The distribution of the alpha1B subunit and synaptotagmin only became punctate when neuron-neuron contact was allowed. Thus, the expression of functional N-type calcium channels is the result of an intrinsic program while extrinsic regulatory cues mediated by neuron-neuron contact are required to control their distribution during synaptogenesis.

Electrical and calcium signaling in dendrites of hippocampal pyramidal neurons

This review discusses recent data regarding the different types of voltage-gated Na+, Ca2+, and K+ channels in dendrites of CA1 pyramidal neurons and their function for synaptic integration and plasticity. Na+ and Ca2+ channels are uniformly distributed throughout the dendrites, although Na+ channels in the soma and proximal dendrites differ in their inactivation properties from Na+ channels in more distal regions. Also, different regions of the neuron express different subtypes of Ca2+ channels. K+ channels are unevenly distributed, with the distal dendrites expressing a more than fivefold greater density of a transient A-type K+ channel than proximal regions. These K+ channels exert profound control over the excitability of the pyramidal neurons and the spread of synaptic potentials throughout the dendrites. The ways in which the active properties of dendrites may contribute toward the induction and maintenance of long-term synaptic plasticity are discussed.

Differential roles of two types of voltage-gated Ca2+ channels in the dendrites of rat cerebellar Purkinje neurons

The distribution and function of voltage-gated Ca2+ channels in Purkinje neurons in rat cerebellar slices were studied using simultaneous Ca2+ imaging and whole-cell patch clamp recording techniques. Voltage-gated Ca2+ channels were activated by applying depolarizing voltage steps through the pipette attached at the soma in a voltage-clamp mode in the presence of tetrodotoxin. Poor space clamp due to extensive arborization of the dendrites allowed the dendrites to fire Ca2+ spikes. Ca2+ imaging with Fura-2 injected through the pipette, showed a steady [Ca2+]i increase at the soma and transient, spike-linked [Ca2+]i jumps in the dendrites. omega-Agatoxin-IVA (200 nM) abolished the depolarization-induced Ca2+ spikes, the spike-linked [Ca2+]i increase in the dendrites, and the steady [Ca2+]i increase at the soma. omega-Conotoxin-GVIA (5 microM) and nifedipine (3 microM) had no significant effect on the depolarization-induced responses. In the presence of 4-aminopyridine(2 mM) and omega-Agatoxin-IVA, transient [Ca2+]i increases remained in the dendrites. Low concentrations of Ni2+(100 microM) reversibly suppressed this [Ca2+]i increase. The voltage for half-maximal activation and inactivation of this component were lower than -50 mV and -31 mV, respectively. In normal conditions, low concentration of Ni2+ slowed the onset of the Ca2+ spike without changing the time course of the spikes or the amplitude of the accompanying [Ca2+]i increase. These results show that omega-Agatoxin-IVA-sensitive Ca2+ channels are distributed both in the soma and the dendrites, and are responsible for dendritic Ca2+ spikes, whereas low-voltage activated, Ni2+-sensitive Ca2+ channels are distributed in the whole dendrites including both thick and fine branches, and provide boosting current for spike generation.

Upregulation of L-type Ca2+ channels in reactive astrocytes after brain injury, hypomyelination, and ischemia

Anti-peptide antibodies that specifically recognize the alpha1 subunit of class A-D voltage-gated Ca2+ channels and a monoclonal antibody (MANC-1) to the alpha2 subunit of L-type Ca2+ channels were used to investigate the distribution of these Ca2+ channel subtypes in neurons and glia in models of brain injury, including kainic acid-induced epilepsy in the hippocampus, mechanical and thermal lesions in the forebrain, hypomyelination in white matter, and ischemia. Immunostaining of the alpha2 subunit of L-type Ca2+ channels by the MANC-1 antibody was increased in reactive astrocytes in each of these forms of brain injury. The alpha1C subunits of class C L-type Ca2+ channels were upregulated in reactive astrocytes located in the affected regions in each of these models of brain injury, although staining for the alpha1 subunits of class D L-type, class A P/Q-type, and class B N-type Ca2+ channels did not change from patterns normally observed in control animals. In all of these models of brain injury, there was no apparent redistribution or upregulation of the voltage-gated Ca2+ channels in neurons. The upregulation of L-type Ca2+ channels in reactive astrocytes may contribute to the maintenance of ionic homeostasis in injured brain regions, enhance the release of neurotrophic agents to promote neuronal survival and differentiation, and/or enhance signaling in astrocytic networks in response to injury.

Pharmacologically and functionally distinct calcium currents of stomatogastric neurons

Previous studies have suggested the presence of different types of calcium channels in different regions of stomatogastric neurons. We sought to pharmacologically separate these calcium channel types. We used two different preparations from different regions of stomatogastric neurons to screen a range of selective calcium channel blockers. The two preparations were isolated cell bodies in culture, in which calcium current was measured directly, and isolated neuromuscular junction, in which synaptic transmission was the indirect assay for presynaptic calcium influx. The selective blockers were two different dihydropyridines, omega-Agatoxin IVA, and omega-Conotoxin GVIA. Cultured cell bodies possessed both high-threshold calcium current and calcium-activated outward current, similar to intact neurons. The calcium current had transient and maintained components, but both components had the same voltage dependence of activation and inactivation. Dihydropyridines at >/=10 microM blocked both high-threshold calcium current and calcium-activated outward current. Nanomolar doses of omega-Agatoxin IVA did not block calcium current, but micromolar doses did. omega-Conotoxin GVIA did not block either current. In contrast, at the neuromuscular junction, dihydropyridines reduced the amplitude of postsynaptic potentials by only a modest amount, whereas omega-Agatoxin IVA at doses as low as 64 nM reduced the amplitude of postsynaptic potentials almost entirely. These effects were presynaptic. omega-Conotoxin GVIA did not change the amplitude of postsynaptic potentials. The different pharmacological profiles of the two isolated preparations suggest that there are at least two different types of calcium channel in stomatogastric neurons and that omega-Agatoxin IVA and dihydropridines can be used to pharmacologically distinguish them.

Expression patterns of voltage-dependent calcium channel alpha 1 subunits (alpha 1A-alpha 1E) mRNA in rat retina

The transcript levels of the genes encoding for the different alpha1 (alpha1A-alpha1E) subunits of voltage-dependent calcium channels (VDCCs) were studied in the retina of the rat using RT-PCR, Northern blotting, and in situ hybridization. Abundant expression of alpha1A and alpha1B was found with RT-PCR and on Northern blots of total retina RNA, corresponding with high expression levels in all nuclear layers (outer and inner nuclear layers and the ganglion cell layer) of the retina. VDCC alpha1D mRNA was also present in all nuclear layers of the retina but at less abundant levels than alpha1A or alpha1B. Expression level of alpha1C in the retina was low as deduced from a faint Northern blot signal and a moderate yield after PCR amplification. VDCC alpha1E specific amplification of retinal cDNA yielded a longer product (designated alpha1E-L) than obtained from the hippocampus. Nucleotide sequencing of this PCR product revealed a 129 bp insert which is largely homologous (97%) with a previously described insert in the same position in human alpha1E cDNA. In situ hybridization in rat brain showed a differential expression pattern of the long and short variants of alpha1E mRNA. Northern blotting of retinal RNA confirmed the absence of the short variant (alpha1E-S), while alpha1E-L was present at low levels. In situ hybridization detected a significant level of expression of alpha1E-L in the inner nuclear layer. The prevalent expression of alpha1A and alpha1B, and to a lesser extent, of alpha1D, indicates that P/Q-, N-, and L-type calcium currents play a prominent role in the various cell types involved in the retinal signal-transduction pathway. The absence of alpha1C transcript in the retina suggests that the slowly inactivating L-type calcium currents involved in neurotransmitter release from the terminals of photoreceptors and bipolar cells may be encoded by the alpha1D isoform.

Olfactory reciprocal synapses: dendritic signaling in the CNS

Synaptic transmission between dendrites in the olfactory bulb is thought to play a major role in the processing of olfactory information. Glutamate released from mitral cell dendrites excites the dendrites of granule cells, which in turn mediate GABAergic dendrodendritic inhibition back onto mitral dendrites. We examined the mechanisms governing reciprocal dendritic transmission in rat olfactory bulb slices. We find that NMDA receptors play a critical role in this dendrodendritic inhibition. As with axonic synapses, the dendritic release of fast neurotransmitters relies on N- and P/Q-type calcium channels. The magnitude of dendrodendritic transmission is directly proportional to dendritic calcium influx. Furthermore, recordings from pairs of mitral cells show that dendrodendritic synapses can mediate lateral inhibition independently of axonal action potentials.

Decay of prepulse facilitation of N type calcium channels during G protein inhibition is consistent with binding of a single Gbeta subunit

We have examined the modulation of cloned and stably expressed rat brain N type calcium channels (alpha1B + beta1b + alpha2delta subunits) by exogenously applied purified G protein betagamma subunits. In the absence of Gbetagamma, barium currents through N type channels are unaffected by application of strong depolarizing prepulses. In contrast, inclusion of purified Gbetagamma in the patch pipette results in N type currents that initially facilitated upon application of positive prepulses followed by rapid reinhibition. Examination of the kinetics of Gbetagamma-dependent reinhibition showed that as the duration between the test pulse and the prepulse was increased, the degree of facilitation was attenuated in a monoexponential fashion. The time constant tau for the recovery from facilitation was sensitive to exogenous Gbetagamma, so that the inverse of tau linearly depended on the Gbetagamma concentration. Overall, the data are consistent with a model whereby a single Gbetagamma molecule dissociates from the channel during the prepulse, and that reassociation of Gbetagamma with the channel after the prepulse occurs as a bimolecular reaction.

Expression of Ca2+ channels from rat brain with model phenylketonuria in Xenopus oocytes

Ca2+ channels expressed in Xenopus oocytes using mRNA purified from the brain of the rats subjected to chronic treatment with l-phenylalanine in order to model conditions typical for the congenital disease called phenylketonuria (PKU) were studied using double microelectrode technique. The amplitude of Ca2+ channel currents (IBa, 40 mM Ba2+ as a charge carrier) directed in the oocytes by mRNA from the brain of the animals with model PKU was significantly smaller compared to the control animals (145+/-23 nA vs. 270+/-38 nA, p<0.025) while the voltage-dependence of both currents was similar and typical for that of high voltage-activated (HVA) Ca2+ channels. No evidence for the expression of low voltage-activated Ca2+ channels were found. The decrease of the overall HVA Ba2+ current under model PKU occurred primarily at the expense of the decaying, omega-conotoxin-sensitive component which accounted for about 64% of the total current amplitude in control, and apparently was associated with the activity of the expressed N-type Ca2+ channels. omega-Aga-IVA-sensitive, P/Q component of IBa that contributed not more than 10% to the total current in control showed no change under PKU conditions. In addition to the decreased amplitude, Ba2+ current from model PKU animals showed accelerated run-down during prolonged recording (50%/h compared to 15%/h in control). Our data suggest that hyperphenylalaninemic conditions affect the expression of preferentially N-type Ca2+ channels via the reduction of their specific mRNA content as well as influence the type and manner of channels regulation. The underexpression of N-type Ca2+ channels is consistent with the decrease in the overall number of synaptic contacts during PKU and may be one of the factors contributing to the severe damage of the brain function.

Distribution of L-type calcium channels in rat thalamic neurones

One major pathway for calcium entry into neurones is through voltage-activated calcium channels. The distribution of calcium channels over the membrane surface is important for their contribution to neuronal function. Electrophysiological recordings from thalamic cells in situ and after acute isolation demonstrated the presence of high-voltage activated calcium currents. The use of specific L-type calcium channel agonists and antagonists of the dihydropyridine type revealed an about 40% contribution of L-type channels to the total high-voltage-activated calcium current. In order to localize L-type calcium channels in thalamic neurones, fluorescent dihydropyridines were used. They were combined with the fluorescent dye RH414, which allowed the use of a ratio technique and thereby the determination of channel density. The distribution of L-type channels was analysed in the three main thalamic cell types: thalamocortical relay cells, local interneurones and reticular thalamic neurones. While channel density was highest in the soma and decreased significantly in the dendritic region, channels appeared to be clustered differentially in the three types of cells. In thalamocortical cells, L-type channels were clustered in high density around the base of dendrites, while they were more evenly distributed on the soma of interneurones. Reticular thalamic neurones exhibited high density of L-type channels in more central somatic regions. The differential localization of L-type calcium channels found in this study implies their predominate involvement in the regulation of somatic and proximal dendritic calcium-dependent processes, which may be of importance for specific thalamic functions, such as those mediating the transition from rhythmic burst activity during sleep to single spike activity during wakefulness or regulating the relay of visual information.

Ca2+-dependent and -independent interactions of the isoforms of the alpha1A subunit of brain Ca2+ channels with presynaptic SNARE proteins

Fast neurotransmission requires that docked synaptic vesicles be located near the presynaptic N-type or P/Q-type calcium channels. Specific protein-protein interactions between a synaptic protein interaction (synprint) site on N-type and P/Q-type channels and the presynaptic SNARE proteins syntaxin, SNAP-25, and synaptotagmin are required for efficient, synchronous neurotransmitter release. Interaction of the synprint site of N-type calcium channels with syntaxin and SNAP-25 has a biphasic calcium dependence with maximal binding at 10-20 microM. We report here that the synprint sites of the BI and rbA isoforms of the alpha1A subunit of P/Q-type Ca2+ channels have different patterns of interactions with synaptic proteins. The BI isoform of alpha1A specifically interacts with syntaxin, SNAP-25, and synaptotagmin independent of Ca2+ concentration and binds with high affinity to the C2B domain of synaptotagmin but not the C2A domain. The rbA isoform of alpha1A interacts specifically with synaptotagmin and SNAP-25 but not with syntaxin. Binding of synaptotagmin to the rbA isoform of alpha1A is Ca2+-dependent, with maximum affinity at 10-20 microM Ca2+. Although the rbA isoform of alpha1A binds well to both the C2A and C2B domains of synaptotagmin, only the interaction with the C2A domain is Ca2+-dependent. These differential, Ca2+-dependent interactions of Ca2+ channel synprint sites with SNARE proteins may modulate the efficiency of transmitter release triggered by Ca2+ influx through these channels.