Intracellular Action of Spermine on Neuronal Ca2+ and K+ Currents

Intra-and extracellular effects of the polyamine spermine on electrical activity and membrane currents of identified neurons in the abdominal ganglion of Aplysia californica were studied under current-and voltage-clamp conditions. Lonophoretic injection of spermine reduced the amplitude of action potentials and altered their time course as well as spontaneous discharge activity. Investigation of membrane currents showed that intracellular spermine suppressed the total outward current but increased the inward rectifier current. After separation of ion currents it was found that the voltage-activated, delayed K⁺ outward current and the Ca²⁺ inward current were reduced by intracellular spermine in a dose- and voltage-dependent manner. The block of the K⁺ current can be described by a voltage-dependent reaction, where one spermine molecule binds to one channel. The binding constant K_b, at zero voltage, and the effective valency, zδ, had values of 176/M and 0.41 for cell R-15, 223/M and 0.64 for cell L-11, and 137/M and 0.42 for cell L-3. Apparently, more than one spermine cation is needed to block one Ca²⁺ channel, since the coefficient n, which absorbs the molecularity and cooperativity of the reaction, had non-integral values between 1.34 and 2.22. The binding constant K_b and the effective valency zδ had values of 265/M and 0.64 for cell R-15, 821M and 0.56 for cell L-4, and 263/M and 0.51 for cell L-6. Intracellular spermine also blocked the Ca²⁺-activated K⁺ current induced by ionophoretic Ca²⁺-injections, but increased the current at prolonged times after spermine injection. Extracellular spermine had no effect on electrical activity or on membrane currents. The results indicate that intracellular spermine affects the electrical discharge activity of neurons by acting as a blocker and/or modulator at voltage-dependent membrane conductances.     

Ca2+ Channels and Ca2+-Activated K+ Channels in Adult Rat Gonadotrophin-Releasing Hormone Neurones

Gonadotrophin-releasing hormone (GnRH) neurones represent the final output neurones in the neuroendocrine system for the control of reproduction. To understand the reproductive neuroendocrine system, an investigation of the intrinsic and extrinsic properties of GnRH neurones is essential. In this review, we focus on the intrinsic properties and summarise our recent findings of ion channels expressed in rat GnRH neurones. Rat GnRH neurones express all four types of high voltage-activated Ca²⁺channel (L, N, P/Q, R) and the low voltage-activated Ca²⁺ channel (T). GnRH neurones also express two types of Ca²⁺-activated K⁺ [K(Ca)] channel: the small conductance Ca²⁺-activated K⁺ (SK) channel and the large conductance Ca²⁺- and voltage-activated K⁺ (BK) channel. The activities of these Ca²⁺ and K(Ca) channels regulate cell excitability and cellular calcium homeostasis.     

Modulation of big K+ channel activity by ryanodine receptors and L‐type Ca2+ channels in neurons

As metabotropic glutamate receptor type 1 (mGluR1) is known to couple L-type Ca²⁺ channels and ryanodine receptors (RyR, 1 ) in cerebellar granule cells, we examined if such a coupling could activate a Ca²⁺-sensitive K⁺ channel, the big K⁺ (BK) channel, in cultured cerebellar granule cells. We observed that (±)-1-amino-cyclopentane-trans-1,3-dicarboxylic acid (t-ACPD) and quisqualate (QA) stimulated the activity of BK channels. On the other hand, (2S, 3S, 4S)-α-carboxycyclopropyl-glycine (L-CCG-I) and l -(+)-2-amino-4-phosphonobutyrate (L-AP4) had no effect on BK channels, indicating a specific activation by group I mGluRs. Group I mGluRs stimulation of the basal BK channel activity was mimicked by caffeine and both effects were blocked by ryanodine and nifedipine. Interestingly, carbachol stimulated BK channel activity but through a pertussis toxin (PTX)-sensitive pathway that was independent of L-type Ca²⁺ channel activity. Our report indicates that unlike the muscarinic receptors, group I mGluRs activate BK channels by mobilizing an additional pathway involving RyR and L-type Ca²⁺ channels.     

Role of Voltage-gated Ca2+ Channels and Intracellular Ca2+ in Rat Sympathetic Neuron Survival and Function Promoted by High K+ and Cyclic AMP in the Presence or Absence of NGF

We have examined how NGF-dependent rat sympathetic neurons maintain Ca²⁺ homeostasis when challenged with high K⁺ or 8-(4-chlorophenylthio)cyclic AMP (CPTcAMP), two survival factors. In the presence of NGF, high K⁺ (55 mM) caused a stable, 65% reduction in the density of cell soma voltage-sensitive Ca²⁺ channels within 2 days. Although resting [Ca²⁺]_i was elevated by 1.6-fold, this was 50% less than the rise in [Ca²⁺]_i measured before down-regulation occurred, suggesting that down-regulation may help prevent the toxic effects of persistently elevated [Ca²⁺]_i. Inhibition of protein synthesis by cycloheximide blocked recovery from down-regulation. Moreover, treatment with cycloheximide or actinomycin-D caused a 2-fold rise in the peak Ca²⁺ current, suggesting that voltage-sensitive Ca²⁺ channel activity may be tonically attenuated during normal growth. In the absence of NGF, neurons survived for several days in high K⁺ medium with no significant rise in resting [Ca²⁺]_i, although neurites did not grow. Neither Ca²⁺ channel density nor resting [Ca²⁺]_i were altered in neurons surviving with CPTcAMP. Moreover, CPTcAMP lowered the dependence on extracellular Ca²⁺. However, the dihydropyridine antagonist nitrendipine blocked both high K⁺- and CPTcAMP-dependent survival although it had no effect in the presence of NGF. Thus, in the absence of NGF, sympathetic neurons do not require elevation of [Ca²⁺]_i above resting levels to survive with either high K⁺ or CPTcAMP, but dihydropyridine-sensitive Ca²⁺ channel activity may be essential for their survival promoting actions.     

Seletracetam (ucb 44212) inhibits high-voltage–activated Ca2+ currents and intracellular Ca2+ increase in rat cortical neurons in vitro

Purpose: We analyzed the effects of seletracetam (ucb 44212; SEL), a new antiepileptic drug candidate, in an in vitro model of epileptic activity. The activity of SEL was compared to the effects of levetiracetam (LEV; Keppra), in the same assays. Methods: Combined electrophysiologic and microfluorometric recordings were performed from layer V pyramidal neurons in rat cortical slices to study the effects of SEL on the paroxysmal depolarization shifts (PDSs), and the simultaneous elevations of intracellular Ca²⁺ concentration [Ca²⁺]_i. Moreover, the involvement of high‐voltage activated Ca²⁺ currents (HVACCs) was investigated by means of patch‐clamp recordings from acutely dissociated pyramidal neurons. Results: SEL significantly reduced both the duration of PDSs (IC₅₀ = 241.0 ± 21.7 nm ) as well as the number of action potentials per PDS (IC₅₀ = 82.7 ± 9.7 nm ). In addition, SEL largely decreased the [Ca²⁺]_i rise accompanying PDSs (up to 75% of control values, IC₅₀ = 345.0 ± 15.0 nm ). Furthermore, SEL significantly reduced HVACCs in pyramidal neurons. This effect was mimicked by ω‐conotoxin GVIA and, to a lesser extent, by ω‐conotoxin MVIIC, blockers of N‐ and Q‐type HVACC, respectively. The combination of these two toxins occluded the action of SEL, suggesting that N‐type Ca²⁺ channels, and partly Q‐type subtypes are preferentially targeted. Conclusions: These results demonstrate a powerful inhibitory effect of SEL on epileptiform events in vitro. SEL showed a higher potency than LEV. The effective limitation of [Ca²⁺]_i influx might be relevant for its antiepileptic efficacy and, more broadly, for pathologic processes involving neuronal [Ca²⁺]_i overload.     

Tuning the excitability of midbrain dopamine neurons by modulating the Ca2+ sensitivity of SK channels

Small conductance Ca²⁺ -activated K⁺ (SK) channels play a prominent role in modulating the spontaneous activity of dopamine (DA) neurons as well as their response to synaptically-released glutamate. SK channel gating is dependent on Ca²⁺ binding to constitutively bound calmodulin, which itself is subject to endogenous and exogenous modulation. In the present study, patch-clamp recording techniques were used to examine the relationship between the apparent Ca²⁺ affinity of cloned SK3 channels expressed in cultured human embryonic kidney 293 cells and the excitability of DA neurons in slices from rat substantia nigra using the positive and negative SK channel modulators, 6,7-dichloro-1 H -indole-2,3-dione-3-oxime and R- N -(benzimidazol-2-yl)-1,2,3,4-tetrohydro-1-naphtylamine. Increasing the apparent Ca²⁺ affinity of SK channels decreased the responsiveness of DA neurons to depolarizing current pulses, enhanced spike frequency adaptation and slowed spontaneous firing, effects attributable to an increase in the amplitude and duration of an apamin-sensitive afterhyperpolarization. In contrast, decreasing the apparent Ca²⁺ affinity of SK channels enhanced DA neuronal excitability and changed the firing pattern from a pacemaker to an irregular or bursting discharge. Both the reduction in apparent Ca²⁺ affinity and the bursting associated with negative SK channel modulation were gradually surmounted by co-application of the positive SK channel modulator. These results underscore the importance of SK channels in 'tuning' the excitability of DA neurons and demonstrate that gating modulation, in a manner analogous to physiological regulation of SK channels in vivo , represents a means of altering the response of DA neurons to membrane depolarization.     

Partial compensation for N-type Ca2+ channel loss by P/Q-type Ca2+ channels underlines the differential release properties supported by these channels at cerebrocortical nerve terminals

N-type and P/Q-type Ca²⁺ channels support glutamate release at central synapses. To determine whether the glutamate release mediated by these channels exhibits distinct properties, we have isolated each release component in cerebrocortical nerve terminals from wild-type mice by specifically blocking N-type Ca²⁺ channels with ω-conotoxin-GVIA and P/Q-type Ca²⁺ channels with ω-agatoxin-IVA. In addition, we have determined the release properties at terminals from mice lacking the α_1B subunit of N-type channels (Ca_v 2.2) to test the possibility that P/Q-type channels can compensate for the loss of N-type Ca²⁺ channels. We recently demonstrated that, while evoked glutamate release depends on P/Q- and N-type channels in wild-type nerve terminals, only P/Q-type channels participate in these knockout mice. Moreover, in nerve terminals expressing solely P/Q-type channels, metabotropic glutamate receptor 7 (mGluR7) fails to inhibit the evoked Ca²⁺ influx and glutamate release. Here, we show that the failure of mGluR7 to modulate evoked glutamate release is not due to a lack of receptors, as nerve terminals from mice lacking N-type Ca²⁺ channels express mGluR7. Indeed, we show that other receptor responses, such as the inhibition of forskolin-induced release, are preserved in these knockout mice. N-type channels are more loosely coupled to release than P/Q-type channels in nerve terminals from wild-type mice, as reflected by the tighter coupling of release in knockout nerve terminals. We conclude that the glutamate release supported by N- and P/Q-type channels exhibits distinct properties, and that P/Q-type channels cannot fully compensate for the loss of N-type channels.     

The Expression of Voltage-Gated Ca2+ Channels in Pituicytes and the Up-Regulation of L-Type Ca2+ Channels During Water Deprivation

The primary components of the neurohypophysis are the neuroendocrine terminals that release vasopressin and oxytocin, and pituicytes, which are astrocytes that normally surround and envelop these terminals. Pituicytes regulate neurohormone release by secreting the inhibitory modulator taurine in an osmotically‐regulated fashion and undergo a marked structural reorganisation in response to dehydration as well as during lactation and parturition. Because of these unique functions, and the possibility that Ca²⁺ influx could regulate their activity, we tested for the expression of voltage‐gated Ca²⁺ channel α1 subunits in pituicytes both in situ and in primary culture. Colocalisation studies in neurohypophysial slices show that pituicytes (identified by their expression of the glial marker S100β), are immunoreactive for antibodies directed against Ca²⁺ channel α1 subunits Ca_V2.2 and Ca_V2.3, which mediate N‐ and R‐type Ca²⁺ currents, respectively. Pituicytes in primary culture express immunoreactivity for Ca_V1.2, Ca_V2.1, Ca_V2.2, Ca_V2.3 and Ca_V3.1 (which mediate L‐, P/Q‐, N‐, R‐ and T‐type currents, respectively) and immunoblotting studies confirmed the expression of these Ca²⁺ channel α1 subunits. This increase in Ca²⁺ channel expression may occur only in pituicytes in culture, or may reflect an inherent capability of pituicytes to initiate the expression of multiple types of Ca²⁺ channels when stimulated to do so. We therefore performed immunohistochemistry studies on pituitaries obtained from rats that had been deprived of water for 24 h. Pituicytes in these preparations showed a significantly increased immunoreactivity to Ca_V1.2, suggesting that expression of these channels is up‐regulated during the adaptation to long‐lasting dehydration. Our results suggest that Ca²⁺ channels may play important roles in pituicyte function, including a contribution to the adaptation that occurs in pituicytes when the need for hormone release is elevated.     

Rises in [Ca2+]i mediate capsaicin- and proton-induced heat sensitization of rat primary nociceptive neurons

Capsaicin (CAPS) as well as acidic pH induces Ca2+ influx in a subset of rat dorsal root ganglion neurons. Here we show that CAPS as well as three different approaches to induce experimental tissue acidification (phosphate buffered solution pH 5.4, CO2-gassed solution pH 6.1 and NPE-caged protons) yielded a transient heat sensitization of peripheral nociceptive terminals in rat skin in vitro. The heat sensitization induced by CAPS (1 microM) could be prevented by preloading the neurons with the neuroprotective calcium chelator BAPTA-AM (1 mM). However, this pretreatment had no effect on the sensitization following exposure to acidic solutions (pH 5.4 and pH 6.1). Therefore, the membrane-permeant proton buffer SNARF-AM (200 microM) was used together with BAPTA-AM in order to prevent changes in intracellular pH. Under these conditions heat sensitization by low pH did not occur. To investigate the underlying membrane mechanisms, current recordings together with simultaneous calcium measurements using FURA-2 were performed in neurons isolated from rat dorsal root ganglia. In a subset of these neurons, an increase in [Ca2+]i and concomitant facilitation of heat-activated ionic currents was observed after application of CAPS as well as pH 5.6. Rises in [Ca2+]i thus appear to play an essential role in plastic changes not only of central neurons but also of peripheral nociceptive terminals which may account for heat hyperalgesia.     

Glutamate‐induced Co2+ uptake in rat auditory brainstem neurons reveals developmental changes in Ca2+ permeability of glutamate receptors

Ca²⁺ influx through glutamate receptors (GluRs) is thought to play a crucial part in developmental processes and neuronal plasticity. Here we have examined the spatiotemporal distribution of Ca²⁺-permeable GluRs in auditory brainstem neurons of the rat from birth to adulthood, using the cobalt-staining technique of Pruss and collaborators. In slices of young adult rats, 1 mm glutamate evoked intense cobalt uptake in subsets of neurons in the ventral cochlear nuclei, the medial nucleus of the trapezoid body, the lateral and the medial superior olive, and the lateral lemniscal nuclei. Neurones in the central nucleus of the inferior colliculus, and thalamic auditory nuclei appear to express few, if any, Ca²⁺-permeable GluRs. Thus, in adults, Ca²⁺-permeable GluRs are present in neurons of almost all main relay stations of the auditory brainstem. During development, cobalt-stained cells first appeared at about hearing onset (at postnatal day 12 [P12]). At P16, staining levels were highest and the pattern of distribution was already adult-like. The staining intensity slightly declined during the fourth postnatal week. In contrast, Ca²⁺-permeable receptors were detected in the external cortex of the inferior colliculus as early as P4. Our results show that auditory neurons, characterized by a high temporal precision in neuronal activity, display Ca²⁺-permeable GluRs. Because Ca²⁺ permeability appears at about the onset of hearing and is highest during the following 2 weeks, Ca²⁺ influx through GluRs is likely to be implicated in remodelling processes occurring during this ontogenetic period.     

Contact-dependent aggregation of functional Ca2+ channels, synaptic vesicles and postsynaptic receptors in active zones of a neuromuscular junction

To examine whether Ca2+ channels aggregate in a contact-dependent manner, we characterized the distribution of synaptic vesicles and postsynaptic receptors, and compared it to the location of Ca2+ entry sites, in a Xenopus laevis nerve-muscle coculture preparation using a localized Ca2+ detection method. The majority (75%) of Ca2+ entry sites at spontaneously formed nerve-muscle contacts were associated with enhanced immunofluorescence to the synaptic vesicle protein, SV2. In contrast, only 11% of recorded sites without Ca2+ transients exhibited significant SV2 immunofluorescence. When comparing the spatial distribution of synaptic markers with that of Ca2+ entry sites, we found that the majority of Ca2+ entry sites (61%) were associated with both enhanced SV2 immunofluorescence and R-BTX fluorescence, thereby identifying putative neurotransmitter release sites where Ca2+ channels, synaptic vesicles and postsynaptic receptors are colocalized. Using polystyrene beads coated with a heparin binding protein known to mediate in vitro postsynaptic receptor clustering, we show that the location of Ca2+ domains was associated with enhanced SV2 immunofluorescence at neurite-to-bead contacts. We conclude that the localization of functional Ca2+ channels to putative active zones follows a contact-dependent signalling mechanism similar to that known to mediate vesicle aggregation and AChR clustering.     

HIV-1 Envelope Proteins gp120 and gp160 Potentiate NMDA-induced [Ca2+]i Increase, Alter [Ca2+]i Homeostasis and Induce Neurotoxicity in Human Embryonic Neurons

The envelope glycoprotein gp120 of the human immunodeficiency virus HIV-1 has been proposed to cause neuron death in developing murine hippocampal cultures and rat retinal ganglion cells. In the present study, cultured human embryonic cerebral and spinal neurons from 8- to 10-week-old embryos were used to study the neurotoxic effect of gp120 and gp160. Electrophysiological properties as well as N -methyl- d -aspartate (NMDA)-induced currents were recorded from neurons maintained in culture for 10–30 days. Neither voltage-activated sodium or calcium currents nor NMDA-induced currents were affected by exposure of neurons to 250 pM gp120 or gp160. In contrast, when neurons were subjected to photometric measurements using the calcium dye indo-1 to monitor the intracellular free Ca²⁺ concentration ([Ca²⁺]_i), gp120 and gp160 (20–250 pM) potentiated the large rises in [Ca²⁺]_i induced by 50 μM NMDA. The potentiation of NMDA-induced Ca²⁺ responses required the presence of Ca²⁺ in the medium, and was abolished by the NMDA antagonist d -2-amino-5-phosphonovalerate (AP5) and the voltage-gated Ca²⁺ channel inhibitor nifedipine. Moreover, exposure of a subpopulation of spinal neurons (25% of the cells tested) to 20–250 pM gp120 or gp160 resulted in an increase in [Ca²⁺]_i that followed three patterns: fluctuations not affected by AP5, a single peak, and the progressive and irreversible rise of [Ca²⁺]_i. The neurotoxicity of picomolar doses of gp120 and gp160 cultures was estimated by immuno-fluorescence and colorimetric assay. Treatment of cultures with AP5 or nifedipine reduced gp120-induced toxicity by 70 and 100% respectively.     

Functional Ca2+ and Na+ channels on mouse Schwann cells cultured in serum‐free medium: regulation by a diffusible factor from neurons and by cAMP

Regulation of expression of functional voltage-gated ion channels for inward currents was studied in Schwann cells in organotypic cultures of dorsal root ganglia from E19 mouse embryos maintained in serum-free medium. Of the Schwann cells that did not contact axons, 46.5% expressed T-type Ca²⁺ conductances (I_CaT). Two days or more after excision of the ganglia, and consequent disappearance of neurites, I_CaT were detectable in only 10.9% of the cells, and the marker 04 disappeared. On Schwann cells deprived of neurons, T- (but not L-) type Ca²⁺ conductances were re-induced by weakly hydrolysable analogues of cAMP, and by forskolin (an activator of adenylyl cyclase) after long-term treatment (4 days). With CPT cAMP (0.1–2 mm ), 8Br cAMP, db cAMP or forskolin (0.01 or 0.1 mm ), the proportion of cells with I_CaT was not significantly different from the proportion in the cultures with neurons. These agents also induced expression in some cells of tetrodotoxin-resistant Na⁺ currents, which were rarely induced by neurons, but 04 was not re-induced by cAMP analogue treatments that re-induced I_CaT. Inward currents (Ba²⁺ or Na⁺) were partly restored (P < 0.05) on Schwann cells cultured for 6–7 days beneath a filter bearing cultured neurons. In contrast, addition of neuron-conditioned medium was ineffective. The results suggest that neurons activate, via diffusible and degradable factors, a subset of Schwann cell cAMP pathways leading to expression of I_CaT, and activate additional non-cAMP pathways that lead to expression of 04.     

Potassium depolarization of mammalian vestibular sensory cells increases [Ca2+]i through voltage‐sensitive calcium channels

The existence of voltage-sensitive Ca²⁺ channels in type I vestibular hair cells of mammals has not been conclusively proven. Furthermore, Ca²⁺ channels present in type II vestibular hair cells of mammals have not been pharmacologically identified. Fura-2 fluorescence was used to estimate, in both cell types, intracellular Ca²⁺ concentration ([Ca²⁺]_i) variations induced by K⁺ depolarization and modified by specific Ca²⁺ channel agonists and antagonists. At rest, [Ca²⁺]_i was 90 ± 20 nm in both cell types. Microperifusion of high-K⁺ solution (50 mm ) for 1 s increased [Ca²⁺]_i to 290 ± 50 nm in type I (n = 20) and to 440 ± 50 nm in type II cells (n = 10). In Ca²⁺-free medium, K⁺ did not alter [Ca²⁺]_i. The specific L-type Ca²⁺ channel agonist, Bay K, and antagonist, nitrendipine, modified in a dose-dependent manner the K⁺-induced [Ca²⁺]_i increase in both cell types with maximum effect at 2 μm and 400 nm , respectively. Ni²⁺, a T-type Ca²⁺ channel blocker, reduced K⁺-evoked Ca²⁺ responses in a dose-dependent manner. For elevated Ni²⁺ concentrations, the response was differently affected by Ni²⁺ alone, or combined to nitrendipine (500 nm ). In optimal conditions, nitrendipine and Ni²⁺ strongly depressed by 95% the [Ca²⁺]_i increases. By contrast, neither ω-agatoxin IVA (1 μm ), a specific P- and Q-type blocker, nor ω-conotoxin GVIA (1 μm ), a specific N-type blocker, affected K⁺-evoked Ca²⁺_i responses. These results provide the first direct evidence that L- and probably T-type channels control the K⁺-induced Ca²⁺ influx in both types of sensory cells.     

Axotomy-induced Changes in Ca2+ Homeostasis in Rat Sympathetic Ganglion Cells

Some of the marked biochemical and electrophysiological changes provoked by section of the axon in mature neurons suggest that the intracellular calcium concentration ([Ca²⁺]_i) may be increased. We have measured the [Ca²⁺]_i using the fluorescent indicator Indo-1 microinjected into rat superior cervical ganglion neurons. No differences in resting [Ca²⁺]_i levels were found between control neurons and cells which had been axotomized 7–10 days before. However, the rise in [Ca²⁺]_i evoked by orthodromic or antidromic stimulation and the recovery after the stimulating train were considerably slower in axotomized neurons than in control cells. We also found that the number of calbindin-D28k-immunopositive cells in the ganglion increases after axotomy, which could be related to the observed differences in calcium homeostasis.     

Multiple Types of Ca2+ Channels in Mouse Motor Nerve Terminals

We measured neurotransmitter release and motor nerve terminal currents in mouse phrenic nerve-diaphragm and triangularis sterni preparations, to evaluate the role of Ca²⁺-channel subtypes in regulating transmitter release. Saturated concentrations of either ωagatoxin IVA [ω-Aga-IVA (0.3 μM), a blocker of P-type Ca²⁺channels] or ω-conotoxin MVIIC [ω-CTx-MVIIC (2 μM), a P-and Q-type Ca²⁺-channel blocker], inhibited nerve-evoked muscle contractions and the amplitude of endplate potentials respectively. In contrast, combined treatment with nifedipine (50 μM, a blocker of L-type Ca²⁺ channels) plus ω-conotoxin GVIA [ω-CTx-GVIA (2 μM), a blocker of N-type Ca²⁺ channels] did not elicit inhibitory effects on nerve-evoked muscle contractions, endplate potentials or nerve terminal waveforms. Because of the non-linear relationship between endplate potentials and Ca²⁺ signals, a small decrease in presynaptic Ca²⁺ entry can significantly reduce the amplitude of the endplate potential. Thus, we applied 3, 4-diaminopyridine (3, 4-DAP, a k⁺-channel blocker) or high Ca²⁺(10 mM) to accelerate and amplify the endplate potentials and Ca²⁺ currents. The endplate potentials amplified by 3, 4-DAP or by high Ca²⁺ correspondingly proved to be quite resistant to both ω-Aga-IVA and ω-CTx-MVIIC; ωAga-IVA exerted only a partial inhibitory effect on endplate potentials, and the ω-Aga-IVA-resistant component was further inhibited by ω-CTx-MVIIC. The component that was resistant to the two toxins could be completely blocked by the non-selective Ca²⁺ channel blocker Cd²⁺ (300 μM). A combination of the two toxins had no significant effects on either spontaneous transmitter release or postsynaptic resting membrane potentials of the diaphragm preparation and the Na⁺ and K⁺ waveforms of the triangularis sterni preparations. This finding suggests a preferential inhibitory effect at a presynaptic site. Measuring the Ca²⁺ currents in the triangularis sterni also revealed partial inhibition by ω-CTx-MVIIC with further incomplete inhibition by ω-Aga-IVA. Cd²⁺ (300 μM) abolished the toxin-resistant component of the Ca²⁺ current. In contrast, a combination of nifedipine (50 μM) with ω-CTx-GVIA (2 μM) was without inhibitory effect. We conclude that multiple types of Ca²⁺channels, i.e. ω-Aga-IVA-sensitive, ω-CTx-MVIIC-sensitive and toxin-resistant Ca²⁺ channels, coexist in mouse motor nerve terminals.     

Transient Brain Ischaemia Provokes Ca2+, PIP2 and Calpain Responses Prior to Delayed Neuronal Death in Monkeys

To clarify the mechanism of postischaemic delayed cornu Ammonis (CA)-1 neuronal death, we studied correlations among calpain activation and its subcellular localization, the immunoreactivity of phosphatidylinositol 4,5-bisphosphate (PIP₂) and Ca²⁺ mobilization in the monkey hippocampus by two independent experimental approaches: in vivo transient brain ischaemia and in vitro hypoxia-hypoglycaemia of hippocampal acute slices. The CA-1 sector undergoing 20 min of ischaemia in vivo showed microscopically a small number of neuronal deaths on day 1 and almost global neuronal loss on day 5 after ischaemia. Immediately after ischaemia, CA-1 neurons ultrastructurally showed vacuolation and/or disruption of the lysosomes. Western blotting using antibodies against inactivated or activated μ-calpain demonstrated μ-calpain activation specifically in the CA-1 sector immediately after ischaemia. This finding was confirmed in the perikarya of CA-1 neurons by immunohistochemistry. CA-1 neurons on day 1 showed sustained activation of μ-calpain, and increased immunostaining for inactivated and activated forms of μ- and m-calpains and for PIP₂. Activated μ-calpain and PIP₂ were found to be localized at the vacuolated lysosomal membrane or endoplasmic reticulum and mitochondrial membrane respectively, by immunoelectron microscopy. Calcium imaging data using hippocampal acute slices showed that hypoxia-hypoglycaemia in vitro provoked intense Ca²⁺ mobilization with increased PIP₂ immunostaining specifically in CA-1 neurons. These data suggest that transient brain ischaemia increases intracellular Ca²⁺ and PIP₂ breakdown, which will activate calpain proteolytic activity. Therefore, we suggest that activated calpain at the lysosomal membrane, with the possible release of biodegrading enzyme, will cause postischaemic CA-1 neuronal death.     

Bradykinin and Angiotensin II-Induced [Ca2+]i Rise in Cultured Rat Pituitary Folliculo-Stellate Cells

Folliculo-stellate cells of the anterior pituitary are thought to modulate pituitary hormone secretion through a paracrine mechanism. Angiotensin II and pituitary adenylate cyclase-activating polypeptide (PACAP) have previously been shown to increase the intracellular Ca2+ concentration ([Ca2+]i) of these cells. In the present study, we examined the effects of various peptides such as bradykinin, angiotensin II, endothelin-1, PACAP, galanin and neurotensin by Ca2+-imaging of folliculo-stellate cells in primary culture. Bradykinin and angiotensin II increased [Ca2+]i in folliculo-stellate cells. Both responses were completely suppressed by thapsigargin and were significantly suppressed by the phospholipase C inhibitor, U-73122. Ryanodine did not significantly modify the responses. A B2 antagonist and angiotensin II receptor antagonist inhibited the response induced by bradykinin and angiotensin II, respectively. Endothelin-1 and PACAP increased [Ca2+]i in fewer than 50% of folliculo-stellate cells but galanin and neurotensin did not influence [Ca2+]i in any of the folliculo-stellate cells tested. These results indicate that bradykinin and angiotensin II increase [Ca2+]i in folliculo-stellate cells by activating phospholipase C through B2 receptor and AT1 receptor, respectively, and that endothelin-1 and PACAP also increase [Ca2+]i in some folliculo-stellate cells.