Elevation of basal intracellular calcium as a central element in the activation of brain macrophages (microglia): suppression of receptor-evoked calcium signaling and control of release function

Microglia-brain macrophages are immune-competent cells of the CNS and respond to pathologic events. Using bacterial lipopolysaccharide (LPS) as a tool to activate cultured mouse microglia, we studied alterations in the intracellular calcium concentration ([Ca 2+]i) and in the receptor-evoked generation of transient calcium signals. LPS treatment led to a chronic elevation of basal [Ca 2+]i along with a suppression of evoked calcium signaling, as indicated by reduced [Ca 2+]i transients during stimulation with UTP and complement factor 5a. Presence of the calcium chelator BAPTA prevented the activation-associated changes in [Ca 2+]i and restored much of the signaling efficacy. We also evaluated downstream consequences of a basal [Ca 2+]i lifting during microglial activation and found BAPTA to strongly attenuate the LPS-induced release of nitric oxide (NO) and certain cytokines and chemokines. Furthermore, microglial treatment with ionomycin, an ionophore elevating basal [Ca 2+]i, mimicked the activation-induced calcium signal suppression but failed to induce release activity on its own. Our findings suggest that chronic elevation of basal [Ca 2+]i attenuates receptor-triggered calcium signaling. Moreover, increased [Ca 2+]i is required, but by itself is not sufficient, for release of NO and certain cytokines and chemokines. Elevation of basal [Ca 2+]i could thus prove a central element in the regulation of executive functions in activated microglia.

Calcium influx via L- and N-type calcium channels activates a transient large-conductance Ca2+-activated K+ current in mouse neocortical pyramidal neurons

Ca2+-activated K+ currents and their Ca2+ sources through high-threshold voltage-activated Ca2+ channels were studied using whole-cell patch-clamp recordings from freshly dissociated mouse neocortical pyramidal neurons. In the presence of 4-aminopyridine, depolarizing pulses evoked transient outward currents and several components of sustained currents in a subgroup of cells. The fast transient current and a component of the sustained currents were Ca2+ dependent and sensitive to charybdotoxin and iberiotoxin but not to apamin, suggesting that they were mediated by large-conductance Ca2+-activated K+ (BK) channels. Thus, mouse neocortical neurons contain both inactivating and noninactivating populations of BK channels. Blockade of either L-type Ca2+ channels by nifedipine or N-type Ca2+ channels by omega-conotoxin GVIA reduced the fast transient BK current. These data suggest that the transient BK current is activated by Ca2+ entry through both N- and L-type Ca2+ channels. The physiological role of the fast transient BK current was also examined using current-clamp techniques. Iberiotoxin broadened action potentials (APs), indicating a role of BK current in AP repolarization. Similarly, both the extracellular Ca2+ channel blocker Cd2+ and the intracellular Ca2+ chelator BAPTA blocked the transient component of the outward current and broadened APs in a subgroup of cells. Our results indicate that the outward current in pyramidal mouse neurons is composed of multiple components. A fast transient BK current is activated by Ca2+ entry through high-threshold voltage-activated Ca2+ channels (L- and N-type), and together with other voltage-gated K+ currents, this transient BK current plays a role in AP repolarization.

Inositol 1,4,5-trisphosphate (IP3)-mediated Ca2+ release evoked by metabotropic agonists and backpropagating action potentials in hippocampal CA1 pyramidal neurons

We examined the properties of [Ca(2+)](i) changes that were evoked by backpropagating action potentials in pyramidal neurons in hippocampal slices from the rat. In the presence of the metabotropic glutamate receptor (mGluR) agonists t-ACPD, DHPG, or CHPG, spikes caused Ca(2+) waves that initiated in the proximal apical dendrites and spread over this region and in the soma. Consistent with previously described synaptic responses (Nakamura et al., 1999a), pharmacological experiments established that the waves were attributable to Ca(2+) release from internal stores mediated by the synergistic effect of receptor-mobilized inositol 1,4, 5-trisphosphate (IP(3)) and spike-evoked Ca(2+). The amplitude of the changes reached several micromoles per liter when detected with the low-affinity indicators fura-6F, fura-2-FF, or furaptra. Repetitive brief spike trains at 30-60 sec intervals generated increases of constant amplitude. However, trains at intervals of 10-20 sec evoked smaller increases, suggesting that the stores take 20-30 sec to refill. Release evoked by mGluR agonists was blocked by MCPG, AIDA, 4-CPG, MPEP, and LY367385, a profile consistent with the primacy of group I receptors. At threshold agonist concentrations the release was evoked only in the dendrites; threshold antagonist concentrations were effective only in the soma. Carbachol and 5-HT evoked release with the same spatial distribution as t-ACPD, suggesting that the distribution of neurotransmitter receptors was not responsible for the restricted range of regenerative release. Intracellular BAPTA and EGTA were approximately equally effective in blocking release. Extracellular Cd(2+) blocked release, but no single selective Ca(2+) channel blocker prevented release. These results suggest that IP(3) receptors are not associated closely with specific Ca(2+) channels and are not close to each other.

The endogenous calcium buffer and the time course of transducer adaptation in auditory hair cells

Mechanoelectrical transducer currents in turtle auditory hair cells adapt to maintained stimuli via a Ca2+-dependent mechanism that is sensitive to the level of internal calcium buffer. We have used the properties of transducer adaptation to compare the effects of exogenous calcium buffers in the patch electrode solution with those of the endogenous buffer assayed with perforated-patch recording. The endogenous buffer of the hair bundle was equivalent to 0.1-0.4 mM BAPTA and, in a majority of cells, supported adaptation in an external Ca2+ concentration of 70 microM similar to that in turtle endolymph. The endogenous buffer had a higher effective concentration, and the adaptation time constant was faster in cells at the high-frequency end than at the low-frequency end of the cochlea. Experiments using buffers with different Ca2+-binding rates or dissociation constants indicated that the speed of adaptation and the resting open probability of the transducer channels could be differentially regulated and imply that the endogenous buffer must be a fast, high-affinity buffer. In some hair cells, the transducer current did not decay exponentially during a sustained stimulus but displayed damped oscillations at a frequency (58-230 Hz) that depended on external Ca2+ concentration. The gradient in adaptation time constant and the tuned transducer current at physiological levels of calcium buffer and external Ca2+ suggest that transducer adaptation may contribute to hair cell frequency selectivity. The results are discussed in terms of feedback regulation of transducer channels mediated by Ca2+ binding at two intracellular sites.

Evidence for opening of hair-cell transducer channels after tip-link loss

The mechanosensitive transducer channels of hair cells have long been proposed to be gated directly by tension in the tip links. These are thin, elastic extracellular elements connecting the tips of adjacent stereocilia located on the apical surface of the cell. If this hypothesis is true, the channels should close after destruction of tip links. The hypothesis was tested pharmacologically using receptor currents obtained in response to mechanical stimulation of the stereociliary bundle of outer hair cells isolated from the adult guinea pig cochlea. Application of elastase (20 U/ml) or 1,2-bis(2-aminophenoxy)ethane-N,N,N', N'-tetra-acetic acid (BAPTA; 5 mM), both of which are known to disrupt tip links in other hair-cell preparations, led to the expected irreversible loss of receptor currents. However, the cells then displayed a maintained inward current, implying that channels were left permanently open. This current was similar in magnitude to the receptor current before treatment and was reduced reversibly by known blockers of mechanosensitive channels, namely, dihydrostreptomycin (100 microM), amiloride (300 microM), and gadolinium ions (1 mM). These observations suggest that the maintained current flows through the mechanosensitive channels. Electron microscopical analysis of isolated hair cells, exposed to the same concentrations of elastase or BAPTA as in the electrophysiological experiments, demonstrated an almost total loss of tip links in hair bundles that showed no evidence of other mechanical damage. It is concluded that although the tip links are required for mechanoelectrical transduction, the channels are not gated directly by the tip links.

Membrane tension in swelling and shrinking molluscan neurons

When neurons undergo dramatic shape and volume changes, how is surface area adjusted appropriately? The membrane tension hypothesis-namely that high tensions favor recruitment of membrane to the surface whereas low tensions favor retrieval-provides a simple conceptual framework for surface area homeostasis. With membrane tension and area in a feedback loop, tension extremes may be averted even during excessive mechanical load variations. We tested this by measuring apparent membrane tension of swelling and shrinking Lymnaea neurons. With hypotonic medium (50%), tension that was calculated from membrane tether forces increased from 0.04 to as much as 0.4 mN/m, although at steady state, swollen-cell tension (0. 12 mN/m) exceeded controls only threefold. On reshrinking in isotonic medium, tension reduced to 0.02 mN/m, and at the substratum, membrane invaginated, creating transient vacuole-like dilations. Swelling increased membrane tension with or without BAPTA chelating cytoplasmic Ca2+, but with BAPTA, unmeasurably large (although not lytic) tension surges occurred in approximately two-thirds of neurons. Furthermore, in unarborized neurons voltage-clamped by perforated-patch in 50% medium, membrane capacitance increased 8%, which is indicative of increasing membrane area. The relatively damped swelling-tension responses of Lymnaea neurons (no BAPTA) were consistent with feedback regulation. BAPTA did not alter resting membrane tension, but the large surges during swelling of BAPTA-loaded neurons demonstrated that 50% medium was inherently treacherous and that tension regulation was impaired by subnormal cytoplasmic [Ca2+]. However, neurons did survive tension surges in the absence of Ca2+ signaling. The mechanism to avoid high-tension rupture may be the direct tension-driven recruitment of membrane stores.

The relation of exocytosis and rapid endocytosis to calcium entry evoked by short repetitive depolarizing pulses in rat melanotropic cells

Melanotropic cells release predocked, large, dense-cored vesicles containing alpha-melanocyte stimulating hormone in response to calcium entry through voltage-gated calcium channels. Our first objective was to study the relationship between exocytosis, rapid endocytosis, and calcium entry evoked by short step depolarizations in the order of duration of single action potentials (APs). Exocytosis and rapid endocytosis were monitored by capacitance measurements. We show that short step depolarizations (40 msec) evoke the fast release of only approximately 3% of the predocked release-ready vesicle pool. Second, we asked what the distance is between voltage-gated calcium channels and predocked vesicles in these cells by modulating the intracellular buffer capacity. Exocytosis and rapid endocytosis were differentially affected by low concentrations of the calcium chelator EGTA. EGTA slightly attenuated exocytosis at 100 microM relative to 50 microM, but exocytosis was strongly depressed at 400 microM, showing that calcium ions have to travel a large distance to stimulate exocytosis. Nevertheless, the efficacy of calcium ions to stimulate exocytosis was constant for pulse durations between 2 and 40 msec, indicating that in melanotropes, exocytosis is related linearly to the amount and duration of calcium entry during a single AP. Rapid endocytosis was already strongly depressed at 100 microM EGTA, which shows that the process of endocytosis itself is calcium dependent in melanotropic cells. Furthermore, rapid endocytosis proceeded with a time constant of approximately 116 msec at 33 degrees C, which is three times faster than at room temperature. There was a strong correlation between the amplitude of endocytosis and the amplitude of exocytosis immediately preceding endocytosis. Both this correlation and the fast time constant of endocytosis suggest that the exocytotic vesicle is retrieved rapidly.

Ca2+ influx amplifies protein kinase C potentiation of recombinant NMDA receptors

Protein kinase C (PKC) potentiates NMDA receptors in hippocampal, trigeminal, and spinal neurons. Although PKC phosphorylates the NMDA receptor subunit NR1 at four residues within the C terminal splice cassette C1, the molecular mechanisms underlying PKC potentiation of NMDA responses are not yet known. The present study examined the role of Ca2+ in PKC potentiation of recombinant NMDA receptors expressed in Xenopus oocytes. We found that Ca2+ influx through PKC-potentiated NMDA receptors can further increase the NMDA response ("Ca2+ amplification"). Ca2+ amplification required a rise in intracellular Ca2+ concentration at or near the intracellular end of the channel and was independent of Ca2+-activated Cl- current. Ca2+ amplification depended on extracellular Ca2+ concentration during NMDA application and not during PKC activation. Ca2+ amplification was reduced by the membrane-permeant Ca2+-chelating agent BAPTA-AM. Mutant receptors with greatly reduced Ca2+ permeability did not exhibit Ca2+ amplification. Receptors containing the NR1 N-terminal splice cassette showed more Ca2+ amplification, possibly because of their larger basal current and therefore greater Ca2+ influx. Contrary to expectation, splicing out the two C-terminal splice cassettes of NR1 enhanced PKC potentiation in a manner independent of extracellular Ca2+. This observation indicates that PKC potentiation does not require phosphorylation of the C1 cassette of the NR1 subunit. PKC potentiation of NMDA receptors in vivo is likely to be affected by Ca2+ amplification of the potentiated signal; the degree of amplification will depend in part on alternative splicing of the NR1 subunit, which is regulated developmentally and in a cell-specific manner.

Impact of cytoplasmic calcium buffering on the spatial and temporal characteristics of intercellular calcium signals in astrocytes

The impact of calcium buffering on the initiation and propagation of mechanically elicited intercellular Ca2+ waves was studied using astrocytes loaded with different exogenous, cell membrane-permeant Ca2+ chelators and a laser scanning confocal or video fluorescence microscope. Using an ELISA with a novel antibody to BAPTA, we showed that different cell-permeant chelators, when applied at the same concentrations, accumulate to the same degree inside the cells. Loading cultures with BAPTA, a high Ca2+ affinity chelator, almost completely blocked calcium wave occurrence. Chelators having lower Ca2+ affinities had lesser affects, as shown in their attenuation of both the radius of spread and propagation velocity of the Ca2+ wave. The chelators blocked the process of wave propagation, not initiation, because large [Ca2+]i increases elicited in the mechanically stimulated cell were insufficient to trigger the wave in the presence of high Ca2+ affinity buffers. Wave attenuation was a function of cytoplasmic Ca2+ buffering capacity; i.e., loading increasing concentrations of low Ca2+ affinity buffers mimicked the effects of lesser quantities of high-affinity chelators. In chelator-treated astrocytes, changes in calcium wave properties were independent of the Ca2+-binding rate constants of the chelators, of chelation of other ions such as Zn2+, and of effects on gap junction function. Slowing of the wave could be completely accounted for by the slowing of Ca2+ ion diffusion within the cytoplasm of individual astrocytes. The data obtained suggest that alterations in Ca2+ buffering may provide a potent mechanism by which the localized spread of astrocytic Ca2+ signals is controlled.

Low-frequency stimulation of afferent Adelta-fibers induces long-term depression at primary afferent synapses with substantia gelatinosa neurons in the rat

Impulses in primary afferent nerve fibers may produce short- or long-lasting modifications in spinal nociception. Here we have identified a robust long-term depression (LTD) of synaptic transmission in substantia gelatinosa neurons that can be induced by low-frequency stimulation of primary afferent Adelta-fibers. Synaptic transmission between dorsal root afferents and neurons in the substantia gelatinosa of the spinal cord dorsal horn was examined by intracellular recording in a transverse slice dorsal root preparation of rat spinal cord. Conditioning stimulation of dorsal roots with 900 pulses given at 1 Hz (10 V, 0.1 msec) produced LTD of EPSP amplitudes in substantia gelatinosa neurons to 41 +/- 10% of control that lasted for at least 2 hr. When A- and C-fibers were recruited, conditioning stimulation was as effective as A-fiber stimulation alone. After LTD, synaptic strength could be increased to its original level by applying a second, high-frequency tetanic stimulus to the dorsal root, indicating that LTD is reversible and not attributable to damage of individual synapses. Bath application of the GABAA receptor antagonist bicuculline and glycine receptor antagonist strychnine did not affect LTD. When NMDA receptors were blocked by bath application of D-2-amino-5-phosphonovaleric acid, LTD was abolished or strongly reduced. Loading substantia gelatinosa neurons with Ca2+ chelator BAPTA also blocked or reduced LTD. After incubation of slices with calyculin A, a selective and membrane permeable inhibitor of protein phosphatases 1 and 2A, LTD was not attenuated. We propose that this form of LTD may be relevant for long-lasting segmental antinociception after afferent stimulation.

Mechanisms and effects of intracellular calcium buffering on neuronal survival in organotypic hippocampal cultures exposed to anoxia/aglycemia or to excitotoxins

Neuronal calcium loading attributable to hypoxic/ischemic injury is believed to trigger neurotoxicity. We examined in organotypic hippocampal slice cultures whether artificially and reversibly enhancing the Ca2+ buffering capacity of neurons reduces the neurotoxic sequelae of oxygen-glucose deprivation (OGD), whether such manipulation has neurotoxic potential, and whether the mechanism underlying these effects is pre- or postsynaptic. Neurodegeneration caused over 24 hr by 60 min of OGD was triggered largely by NMDA receptor activation and was attenuated temporarily by pretreating the slices with cell-permeant Ca2+ buffers such as 1, 2 bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid acetoxymethyl ester (BAPTA-AM). This pretreatment produced a transient, reversible increase in intracellular buffer content as demonstrated autoradiographically using slices loaded with 14C-BAPTA-AM and by confocal imaging of slices loaded with the BAPTA-AM analog calcium green-acetoxymethyl ester (AM). The time courses of 14C-BAPTA retention and of neuronal survival after OGD were identical, indicating that increased buffer content is necessary for the observed protective effect. Protection by Ca2+ buffering originated presynaptically because BAPTA-AM was ineffective when endogenous transmitter release was bypassed by directly applying NMDA to the cultures, and because pretreatment with the low Ca2+ affinity buffer 2-aminophenol-N,N,O-triacetic acid acetoxymethyl ester, which attenuates excitatory transmitter release, attenuated neurodegeneration. Thus, in cultured hippocampal slices, enhancing neuronal Ca2+ buffering unequivocally attenuates or delays the onset of anoxic neurodegeneration, likely by attenuating the synaptic release of endogenous excitatory neurotransmitters (excitotoxicity).