Facilitation of calcium influx by propylene glycol through the voltage-dependent calcium channels in PC12 cells

Propylene glycol (PG) raises an intracellular calcium concentration ([Ca2+]i) in PC12 cells. The present study has been undertaken to examine whether or not the voltage-dependent Ca2+ channels are involved in the PG-induced rise in [Ca2+]i and, if so, to determine which types participate in it. CdCl2 (50 micro M) and the Ca2+ -free saline depressed the action of PG (0.5 - 10 %v/v)-induced [Ca2+]i rise. Although NiCl2 (50 micro M) at the same concentration as CdCl2, and omega-agatoxin (50 and 300 nM) had no effect on the PG-induced [Ca2+]i rise, each of omega-conotoxin (1 micro M), nifedipine (10 micro M), nicardipine (10 micro M), varapamil (10 micro M) and diltiazem (10 micro M) significantly decreased it. Electrical stimulation and Bay K 8644 (1 micro M) enhanced the PG-induced [Ca2+]i rise. The second phase of the [Ca2]i rise was fallen fast by nicardipine (10 micro M), but not by omega-conotoxin (1 micro M). The results obtained suggested that the Ca2+ influx through the L- and N-type Ca2+ channels are involved in the PG-induced [Ca2+]i rise.

The calcium receptor modulates the hyperpolarization-activated current in subfornical organ neurons

Here we report that neurons of the subfornical organ (SFO), a circumventricular structure devoid of a blood-brain barrier, show time-dependent, inward rectification indicative of the presence of a subthreshold, hyperpolarization-activated inward current (Ih). In whole-cell patch clamp experiments of isolated SFO neurons, we observed a Cs+-sensitive Ih in 47% of cells tested. Furthermore, we show that Ih is involved in the generation of evoked bursts in SFO neurons. An allosteric agonist of the extracellular calcium-sensing receptor (CaR) was found to potentiate Ih consistent with our previous observations of CaR-mediated bursting in SFO neurons. These studies indicate that a proportion of SFO neurons express Ih, and this may be one ionic mechanism through which bursting is regulated by various extracellular messengers.

Ca2+ oscillations in response to methamphetamine in dopamine neurons of the ventral tegmental area in rats subchronically treated with this drug

Mesolimbic dopamine neurons in the ventral tegmental area (VTA), which project to the nucleus accumbens and prefrontal cortex, play an important role in the regulation of emotion, rewarding, and cognition. The dopamine neurons in the VTA have also been implicated in schizophrenia and drug abuse. Methamphetamine (METH) can induce a schizophrenia-like psychosis. Thus, the VTA is a likely effector site for the action of METH. However, effects of METH on the mesolimbic dopamine neurons are largely unknown. We treated adult SD rats with METH (5 mg/kg/day) or saline for 7 days, isolated single VTA neurons from these treated rats, and monitored the neuronal activities by measuring cytosolic Ca2+ concentration ([Ca2+]i), which was followed by immunocytochemical identification of dopamine neurons. Acute administration of METH under superfusion conditions concentration-dependently increased [Ca2+]i in VTA dopamine neurons isolated from METH- and saline-treated rats. Furthermore, acutely administered METH induced oscillations of [Ca2+]i only in the dopamine neurons of the METH-treated group. The METH-induced [Ca2+]i oscillations were inhibited by Ca2+-free conditions and by Ca2+ channel blockers. In conclusion, subchronic METH treatment sensitizes VTA dopamine neurons to this drug, resulting in induction of [Ca2+]i oscillations. This sensitization of VTA dopamine neurons may account, at least in part, for the psycho-stimulant effects of METH, such as the dependence on and sensitization to METH.

Methylmercury affects multiple subtypes of calcium channels in rat cerebellar granule cells

We tested the ability of methylmercury (MeHg) to block calcium channel current in cultures of neonatal cerebellar granule cells using whole-cell patch clamp techniques and Ba(2+) as charge carrier. Low micromolar concentrations of MeHg (0.25-1 microM) reduced the amplitude of whole cell Ba(2+) current in a concentration- and time-dependent fashion; however, this effect was not voltage-dependent and the current-voltage relationship was not altered. Increasing the stimulation frequency hastened the onset and increased the magnitude of block at both 0.25 and 0.5 microM MeHg but not at 1 microM. In the absence of stimulation, all concentrations of MeHg were able to decrease current amplitude. The ability of several Ca(2+) channel antagonists (omega-conotoxin GVIA, omega-conotoxin MVIIC, omega-agatoxin IVA, calcicludine, and nimodipine) to alter the MeHg-induced effect was tested in an effort to determine if MeHg targets a specific subtype of Ca(2+) channel. Each of the antagonists tested was able to decrease a portion of whole cell Ba(2+) current under control conditions. However, none were able to attenuate the MeHg-induced block of whole cell Ba(2+) current, suggesting either that the mechanism of MeHg-induced block involves sites other than those influenced specifically by Ca(2+) channel antagonists or that MeHg was able to "outcompete" these toxins for their binding sites. These results show that acute exposure to submicromolar concentrations of MeHg can block Ba(2+) currents carried through multiple Ca(2+) channel subtypes in primary cultures of cerebellar granule cells. However, it is unlikely that the presence of a specific Ca(2+) channel subtype is able to render granule cells more susceptible to the neurotoxicologic actions of MeHg.

Cellular mechanisms involved in iso-osmotic high K+ solutions-induced contraction of the estrogen-primed rat myometrium

The aim of the present study was to investigate the mechanisms involved in the contraction evoked by iso-osmotic high K+ solutions in the estrogen-primed rat uterus. In Ca2+-containing solution, iso-osmotic addition of KCl (30, 60 or 90 mM K+) induced a rapid, phasic contraction followed by a prolonged sustained plateau (tonic component) of smaller amplitude. The KCl (60 mM)-induced contraction was unaffected by tetrodotoxin (3 microM), omega-conotoxin MVIIC (1 microM), GF 109203X (1 microM) or calphostin C (3 microM) but was markedly reduced by tissue treatment with neomycin (1 mM), mepacrine (10 microM) or U-73122 (10 microM). Nifedipine (0.01-0.1 microM) was significantly more effective as an inhibitor of the tonic component than of the phasic component. After 60 min incubation in Ca2+-free solution containing 3 mM EGTA, iso-osmotic KCl did not cause any increase in tension but potentiated contractions evoked by oxytocin (1 microM), sodium orthovanadate (160 micrM) or okadaic acid (20 microM) in these experimental conditions. In freshly dispersed myometrial cells maintained in Ca2+-containing solution and loaded with indo 1, iso-osmotic KCl (60 mM) caused a biphasic increase in the intracellular Ca2+ concentration ([Ca2+]i). In cells superfused for 60 min in Ca2+-free solution containing EGTA (1 mM), KCl did not increase [Ca2+]i. In Ca2+-containing solution, KCl (60 mM) produced a 76.0 +/- 16.2% increase in total [3H]inositol phosphates above basal levels and increased the intracellular levels of free arachidonic acid. These results suggest that, in the estrogen-primed rat uterus, iso-osmotic high K+ solutions, in addition to their well known effect on Ca2+ influx, activate other cellular processes leading to an increase in the Ca2+ sensitivity of the contractile machinery by a mechanism independent of extracellular Ca2+.

S-nitroso-l-cysteine releases norepinephrine in rat spinal synaptosomes

Although nitric oxide (NO) participates in development of hypersensitivity states in the spinal cord thought to underlie chronic pain, it also participates in analgesia produced by various drugs. In rats with a hypersensitivity state following peripheral nerve injury, spinal administration of an NO donor or l-cysteine alone produced no effect, whereas their combination, which yields s-nitroso-l-cysteine (SNC) powerfully reduced hypersensitivity. In the current study, we examined the ability of SNC to stimulate release of a known spinal analgesic neurotransmitter, norepinephrine (NE), as a possible mechanism of analgesic action of NO in the spinal cord. SNC (but not the NO donor alone or decomposed SNC) produced a concentration-dependent release of NE from rat spinal cord synaptosomes. The d-isomer of SNC was less potent than the l-isomer, and the effect of SNC was partially blocked by l-, but not d-leucine, implicating an interaction with the l-amino acid transporter. SNC-induced NE release was partially Na(+) dependent, but largely Ca(2+) independent. NE uptake inhibitors partially antagonized the effect of SNC, but guanylate cyclase inhibitors were without effect. These data are therefore consistent with NO stimulating NE release in the spinal cord via reaction with thiol containing compounds, such as cysteine, entry into NE terminals via active transport, and production of both exocytotic and carrier mediated release.

Overlapping selectivity of neurotoxin and dihydropyridine calcium channel blockers in cerebellar granule neurones

Calcium (Ca(2+)) currents have been studied extensively in cerebellar granule neurones, but much of the whole-cell pharmacology is inconsistent. Ca(2+) channel currents were recorded from granule neurones to investigate whether the commonly used Ca(2+) channel blockers show overlapping selectivity. Using combinations of toxin channel blockers, 45% of the total current was shown to be carried by Ca(2+) channels susceptible to block by the combined, or cumulative application of, omega-agatoxin IVA, omega-conotoxin GVIA and omega-conotoxin MVIIC, thus representing P/Q- and N-type channel currents. However, sequential application of these toxins showed that substantial overlap occurred in the proportions of current sensitive to individual toxins. Application of the 1, 4-dihydropyridine nicardipine at 1 microM, a concentration reported to be selective for L-type channels, blocked 16% of the total current, without reducing the current sensitive to the toxins used. However, greater concentrations of nicardipine (>10 microM) blocked a proportion of the total current that could not be accounted for by L-type channels alone. These results demonstrate that a pharmacological approach based on the L, N, P/Q, and R classification does not adequately describe the Ca(2+) channel subtypes found in cerebellar granule neurones due to substantial cross-selectivity to the drugs and toxins used.

Structure-activity relationship at the leucine side chain in a series of N,N-dialkyldipeptidyl-amines as N-type calcium channel blockers

Exploration of the SAR around the leucine side chain in a series of N,N-dialkyldipeptidylamines with potent functional activity at N-type VSCC is presented. A novel analog is disclosed which possesses improved aqueous solubility, in vivo activity in an audiogenic seizure model, and reversible blockade in electrophysiological assays.

All classes of calcium channel couple with equal efficiency to exocytosis in rat melanotropes, inducing linear stimulus-secretion coupling

1. The contribution of low voltage-activated (LVA) T-type Ca2+ channels and four different types of high voltage-activated (HVA) Ca2+ channel to exocytosis, and the relationship between calcium influx and exocytosis during action potentials (APs) were studied in pituitary melanotropes. 2. Selective HVA Ca2+ channel blockers reduced exocytosis, monitored by membrane capacitance measurements, proportional to the reduction in Ca2+ influx. The efficacy of Ca2+ in stimulating exocytosis did not change in the presence of the Ca2+ channel blockers, indicating that all HVA Ca2+ channels act together in stimulating exocytosis. 3. The relationship between Ca2+ influx and exocytosis during the AP was examined using APs recorded from spontaneously active melanotropes as command templates under voltage clamp. Under voltage clamp, multiphasic Ca2+ currents were activated over the entire duration of the APs, i.e. during the rising phase as well as the plateau phase. The maximum amplitude of the Ca2+ current coincided with the peak of the AP. 4. The relationship between Ca2+ entry and exocytosis was linear for the different phases of the AP. Also, the influx of Ca2+ through LVA T-type channels stimulated exocytosis with the same efficacy as through the HVA channels. 5. APs of increasing duration ( approximately 50 to approximately 300 ms) evoked increasing amounts of exocytosis. The number of entering Ca2+ ions and the capacitance change were linearly related to AP duration, resulting in a fixed relationship between Ca2+ entry and exocytosis. 6. The results show that Ca2+ ions, entering a melanotrope, couple with equal strength to exocytosis regardless of the channel type involved. We suggest that the linear relationship between Ca2+ entry and secretion observed under physiological conditions (during APs), results from the equal strength with which LVA and HVA channels in melanotropes couple to exocytosis. This guarantees that secretion takes place over the entire duration of the AP.

CaV2.2 and CaV2.3 (N- and R-type) Ca2+ channels in depolarization-evoked entry of Ca2+ into mouse sperm

As sperm prepare for fertilization, surface Ca(2+) channels must open to initiate required, Ca(2+)-mediated events. However, the molecular identity and functional properties of sperm Ca(2+) channels remain uncertain. Here, we use rapid local perfusion and single-cell photometry to examine the kinetics of calcium responses of mouse sperm to depolarizing stimuli. The linear rise of intracellular [Ca(2+)] evoked by approximately 10-s applications of an alkaline high [K(+)] medium directly reports activity of voltage-gated Ca(2+) channels. Little response occurs if external Ca(2+) is removed or if external or internal pH is elevated without depolarization. Responses are inhibited 30-40% by 30-100 micrometer Ni(2+) and more completely by 100-300 micrometer Cd(2+). They resist the dihydropyridines nitrendipine and PN200-110, but 1-10 micrometer mibefradil inhibits reversibly. They also resist the venom toxins calciseptine, omega-conotoxin MVIIC, and kurtoxin, but omega-conotoxin GVIA (5 micrometer) inhibits approximately 50%. GVIA also partially blocks transient, low voltage activated Ca(2+) currents of patch-clamped spermatids. Differential sensitivity of sperm responses to Ni(2+) and Cd(2+) and partial blockade by GVIA indicate that depolarization opens at least two types of voltage-gated Ca(2+) channels in epididymal sperm examined prior to capacitation. Involvement of a previously undetected Ca(V)2.2 (N-type) channel, suggested by the action of GVIA, is substantiated by immunodetection of Ca(2+) channel alpha(1B) subunits in sperm and sperm extracts. Resistance to dihydropyridines, calciseptine, MVIIC, and kurtoxin indicates that Ca(V)1, Ca(V)2.1, and Ca(V)3 (L-, P/Q-, and T-type) channels contribute little to this evoked response. Partial sensitivity to 1 micrometer mibefradil and an enhanced sensitivity of the GVIA-resistant component of response to Ni(2+) suggest participation of a Ca(V)2.3 (R-type) channel specified by previously found alpha(1E) subunits. Our examination of depolarization-evoked Ca(2+) entry indicates that mature sperm possess a larger palette of voltage-gated Ca(2+) channels than previously thought. Such diversity may permit specific responses to multiple cues encountered on the path to fertilization.

Voltage-clamp analysis of the potentiation of the slow Ca2+-activated K+ current in hippocampal pyramidal neurons

Exploring the principles that govern activity-dependent changes in excitability is an essential step to understand the function of the nervous system, because they act as a general postsynaptic control mechanism that modulates the flow of synaptic signals. We show an activity-dependent potentiation of the slow Ca2+-activated K+ current (sl(AHP)) which induces sustained decreases in the excitability in CA1 pyramidal neurons. We analyzed the sl(AHP) using the slice technique and voltage-clamp recordings with sharp or patch-electrodes. Using sharp electrodes-repeated activation with depolarizing pulses evoked a prolonged (8-min) potentiation of the amplitude (171%) and duration (208%) of the sl(AHP). Using patch electrodes, early after entering the whole-cell configuration (<20 min), responses were as those reported above. However, although the sl(AHP) remained unchanged, its potentiation was markedly reduced in later recordings, suggesting that the underlying mechanisms were rapidly eliminated by intracellular dialysis. Inhibition of L-type Ca2+ current by nifedipine (20 microM) markedly reduced the sl(AHP) (79%) and its potentiation (55%). Ryanodine (20 microM) that blocks the release of intracellular Ca2+ also reduced sl(AHP) (29%) and its potentiation (25%). The potentiation of the sl(AHP) induced a marked and prolonged (>50%; approximately equals 8 min) decrease in excitability. The results suggest that sl(AHP) is potentiated as a result of an increased intracellular Ca2+ concentration ([Ca2+]i) following activation of voltage-gated L-type Ca2+ channels, aided by the subsequent release of Ca2+ from intracellular stores. Another possibility is that repeated activation increases the Ca2+-binding capacity of the channels mediating the sl(AHP). This potentiation of the sl(AHP) could be relevant in hippocampal physiology, because the changes in excitability it causes may regulate the induction threshold of the long-term potentiation of synaptic efficacy. Moreover, the potentiation would act as a protective mechanism by reducing excitability and preventing the accumulation of intracellular Ca2+ to toxic levels when intense synaptic activation occurs.

A blocker-resistant, fast-decaying, intermediate-threshold calcium current in palaeocortical pyramidal neurons

The whole-cell patch-clamp technique was used to record Ca2+ currents in acutely dissociated neurons from layer II of guinea-pig piriform cortex (PC). Ba2+ (5 mM) was used as charge carrier. In a subpopulation of layer II cells ( approximately 22%) total Ba2+ currents (IBas) displayed a high degree (> 70%) of inactivation after 300 ms of steady depolarization. The application of L-, N- and P/Q-type Ca2+-channel blockers to these high-decay IBas left their fast inactivating component largely unaffected. The inactivation phase of the blocker-resistant, fast-decaying IBa thus isolated had a bi-exponential time course, with a fast time constant of approximately 20 ms and a slower time constant of approximately 100 ms at voltage levels positive to -10 mV. The voltage dependence of activation of the blocker-resistant, fast-decaying IBa was shifted by approximately 7-9 mV in the negative direction in comparison with those of other pharmacologically and/or kinetically different high-voltage-activated Ca2+ currents. We named this blocker-resistant, fast-decaying, intermediate-threshold current IRfi. The amplitude of IRfi decreased only slightly (by approximately 9%) when extracellular Ca2+ was substituted for Ba2+, in contrast with that of slowly decaying, high-voltage-activated currents, which was reduced by approximately 41% on average. Moreover, IRfi was substantially inhibited by low concentrations of Ni2+ (50 microM). We conclude that IRfi, because of its fast inactivation kinetics, intermediate threshold of activation and resistance to organic blockers, represents a definite, identifiable Ca2+ current different from classical high-voltage-activated currents and clearly distinguishable from classical IT. The striking similarity found between IRfi and Ca2+ currents resulting from heterologous expression of alpha1E-type channel subunits is discussed.

Ca(2+) influx stimulated phospholipase C activity in bovine adrenal chromaffin cells: responses to K(+) depolarization and histamine

The role of Ca(2+) influx in activating phospholipase C in bovine adrenal chromaffin cells has been investigated. Phospholipase C activity in response to K(+) depolarization (56 mM) was blocked by the L-type Ca(2+) channel antagonist nifedipine and partially inhibited by the omega-conotoxins GVIA and MVIIC. In contrast, phospholipase C activity in response to histamine receptor activation was unaffected by omega-conotoxin GVIA and partially inhibited by omega-conotoxin MVIIC or nifedipine. This response was however markedly inhibited by the non-selective Ca(2+) channel antagonists La(3+) or 1-[beta-[3-(4-Methoxyphenyl)propoxy]-4-methoyphenethyl]-H-imidazol e (SKF-96365). Despite this Ca(2+) dependence phospholipase C activity was not increased during periods of "capacitative" Ca(2+) inflow generated by histamine-, caffeine- or thapsigargin-mediated depletion of internal Ca(2+) stores. Thus, while Ca(2+) influx in response to K(+) depolarization or G-protein receptor activation can increase phospholipase C activity in these cells, in the latter case it appears to be ineffective unless there is concurrent agonist occupation of the receptor.

Pharmacological properties of Ca(V)3.2, a low voltage-activated Ca2+ channel cloned from human heart

Three genes encoding T-type Ca2+ channels have been described but their correspondence to the various native T-type Ca2+ currents remains uncertain. In particular, Ca(V)3.2 (or alpha1H) was cloned from a human heart library, its message was found abundantly in cardiac tissue, and expressed Ca(V)3.2 was shown to conduct low voltage-activated currents, which inactivate rapidly and are sensitive to Ni2+ and mibefradil. These observations suggested that Ca(V)3.2 might encode native cardiac T-type Ca2+ channels but more information on the pharmacology of Ca(V)3.2 was needed to confirm this hypothesis. In the present study, we compare the pharmacology of Ca(V)3.2 expressed in HEK293 cells and of native T-type Ca2+ channels in guinea pig atrial myocytes ("native-T"). (1) Ca(V)3.2 and native-T are insensitive to TTX and to toxins selective for N-, P-, or Q-type Ca2+ channels (omega-CTx-GVIA, omega-Aga-IVA, omega-CTx-MVIIC). (2) The half-maximal blocking concentration (IC50) of mibefradil on Ca(V)3.2 is near that on native-T and the block is similarly voltage-dependent. (3) Ca(V)3.2 is five- to sixfold less sensitive than native-T to the 1,4-dihydropyridine (DHP) amlodipine, suggesting a difference in the DHP binding site. (4) Both channels display similar (but not identical) sensitivities to the inorganic blockers Ni2+ and Cd2+ and the IC50s are in the range of values found for T-type Ca2+ currents in other cell types. (5) Ni2+ shifts the voltage dependence of Ca(V)3.2 activation but not that of native-T. The many similarities between the two channels support the contention that Ca(V)3.2 encodes cardiac T-type Ca2+ channels. The slight differences may be due to species variations and/or to the choice of splice variant.

A(2A) adenosine receptor facilitation of neuromuscular transmission: influence of stimulus paradigm on calcium mobilization

The influence of stimulus pulse duration on calcium mobilization triggering facilitation of evoked [(3)H]acetylcholine ([(3)H]ACh) release by the A(2A) adenosine receptor agonist CGS 21680C was studied in the rat phrenic nerve-hemidiaphragm. The P-type calcium channel blocker omega-agatoxin IVA (100 nM) decreased [(3)H]ACh release evoked with pulses of 0.04-ms duration, whereas nifedipine (1 microM) inhibited transmitter release with pulses of 1-ms duration. Depletion of intracellular calcium stores by thapsigargin (2 microM) decreased [(3)H]ACh release evoked by pulses of 1 ms, an effect observed even in the absence of extracellular calcium. With short (0.04-ms) stimulation pulses, when P-type calcium influx triggered transmitter release, facilitation of [(3)H]ACh release by CGS 21680C (3 nM) was attenuated by both thapsigargin (2 microM) and nifedipine (1 microM). With longer stimuli (1 ms), a situation in which both thapsigargin-sensitive internal stores and L-type channels are involved in ACh release, pretreatment with either omega-agatoxin IVA (100 nM) or nifedipine (1 microM) reduced the facilitatory effect of CGS 21680C (3 nM). The results suggest that A(2A) receptor activation facilitates ACh release from motor nerve endings through alternatively mobilizing the available calcium pools (thapsigargin-sensitive internal stores and/or P- or L-type channels) that are not committed to the release process in each stimulation condition.

Effects of intrathecal L- and N-type calcium channel blockers on the antinociception evoked by opioid agonists in the rat tail flick test

The effects of intrathecal administration of nimodipine or omega-conotoxin GVIA (L- and N-type calcium channel blockers, respectively) alone or followed by DAMGO, DADLE or bremazocine (mu-, delta- and kappa-opioid agonists, respectively) were studied on the rat tail flick test. The N- (16 to 64 pmoles), but not the L-type blocker (60 to 240 pmoles) produced a dose and time-dependent increase in the latency for the tail-flick reflex. DAMGO (30 to 120 pmoles) or bremazocine (190 to 560 pmoles), but not DADLE (50 to 200 pmoles), produced a dose-dependent increase in the latency for the tail-flick reflex. The effect of the highest dose of DAMGO was smaller, while the effects of DADLE and bremazocine were not changed after nimodipine (60 pmoles). The effects of DADLE were significantly potentiated, while the effects of DAMGO and bremazocine were not changed after omega-conotoxin GVIA (16 pmoles). The intrathecal administration of an N-type calcium channel blocker with a delta-opioid agonist seems to be the most effective combination to produce antinociception in the rat tail flick test.

Histamine H(3) receptors mediate inhibition of noradrenaline release from intestinal sympathetic nerves

1. The present study investigates whether presynaptic histamine receptors regulate noradrenaline release from intestinal sympathetic nerves. The experiments were performed on longitudinal muscle-myenteric plexus preparations of guinea-pig ileum, preincubated with [(3)H]-noradrenaline. 2. In the presence of rauwolscine, electrically-induced [(3)H]-noradrenaline release was inhibited by histamine or R-alpha-methylhistamine, whereas it was unaffected by pyridylethylamine, impromidine, pyrilamine, cimetidine, thioperamide or clobenpropit. The inhibitory effects of histamine or R-alpha-methylhistamine were antagonized by thioperamide or clobenpropit, but not by pyrilamine or cimetidine. In the absence of rauwolscine, none of these drugs modified the release of [(3)H]-noradrenaline. 3. The modulatory action of histamine was attenuated by pertussis toxin and abolished by N-ethylmaleimide. Tetraethylammonium or 4-aminopyridine enhanced the evoked tritium outflow and counteracted the inhibitory effect of histamine. However, the blocking effects of tetraethylammonium and 4-aminopyridine were no longer evident when their enhancing actions were compensated by reduction of Ca(2+) concentration in the superfusion medium. 4. Histamine-induced inhibition of tritium output was enhanced by omega-conotoxin or low Ca(2+) concentration, whereas it was not modified by nifedipine, forskolin, rolipram, phorbol myristate acetate, H7 or lavendustin A. 5. The present results indicate that presynaptic H(3) receptors, located on sympathetic nerve endings, mediate an inhibitory control on intestinal noradrenergic neurotransmission. It is suggested that these receptors are coupled to G(i)/G(o) proteins which modulate the activity of N-type Ca(2+) channels through a direct link, thus reducing the availability of extracellular Ca(2+) at the level of noradrenergic nerve terminals.

Synaptic transmission induces transient Ca2+ concentration changes in cultured myenteric neurones

The enteric nervous system controls most of the gastrointestinal functions. We applied confocal microscopy and the Ca2+ indicator Fluo-3 as an optical approach to study synaptic activation in cultures of myenteric neurones. The optical recording of [Ca2+]i (the intracellular Ca2+ concentration) was used to monitor activation, since [Ca2+]i is crucial in the coupling between neuronal excitation and the activation of several intracellular events. Extracellular fibre tract stimulation (2 s, 30 Hz) caused a transient [Ca2+]i rise in a subset of neurones (50%). These transients lasted for 5.2 s (n=36), with an average amplitude of 3.4 +/- 1.3 times the basal concentration. The removal of extracellular Ca2+ (n=15) or the application of 10-6 M tetrodotoxin (n=16) blocked this response. The N-type Ca2+-channel blocker omega-conotoxin (5 x 10 -7M) abolished the [Ca2+]i increase, while blockade of L-type and P/Q type Ca2+ channels had no effect. Single stimuli evoked a [Ca2+]i rise in the processes. omega-conotoxin-sensitive postsynaptic events required repetitive stimulation. Cholinergic blockade did not inhibit the [Ca2+]i rise in all neurones, suggesting that, besides acetylcholine, other neurotransmitters are involved. Optical imaging of [Ca2+]i can be used to study synaptic spread of activation in enteric neuronal circuits expressed in culture.

Calcium currents in retrogradely labeled pyramidal cells from rat sensorimotor cortex

Our previous studies of calcium (Ca(2+)) currents in cortical pyramidal cells revealed that the percentage contribution of each Ca(2+) current type to the whole cell Ca(2+) current varies from cell to cell. The extent to which these currents are modulated by neurotransmitters is also variable. This study was directed at testing the hypothesis that a major source of this variability is recording from multiple populations of pyramidal cells. We used the whole cell patch-clamp technique to record from dissociated corticocortical, corticostriatal, and corticotectal projecting pyramidal cells. There were significant differences between the three pyramidal cell types in the mean percentage of L-, P-, and N-type Ca(2+) currents. For both N- and P-type currents, the range of percentages expressed was small for corticostriatal and corticotectal cells as compared with cells which project to the corpus callosum or to the general population. The variance was significantly different between cell types for N- and P-type currents. These results suggest that an important source of the variability in the proportions of Ca(2+) current types present in neocortical pyramidal neurons is recording from multiple populations of pyramidal cells.

Histamine H(2) receptor activated chloride conductance in myenteric neurons from guinea pig small intestine

Whole cell perforated patch-clamp methods were used to investigate ionic mechanisms underlying histamine-evoked excitatory responses in small intestinal AH-type myenteric neurons. When physiological concentrations of Na(+), Ca(2+), and Cl(-) were in the bathing medium, application of histamine significantly increased total conductance as determined by stepping to 50 mV from a holding potential of -30 mV. The current reversed at a membrane potential of -30 +/- 5 (SE) mV and current-voltage relations exhibited outward rectification. The reversal potential for the histamine-activated current was unchanged by removal of Na(+) and Ca(2+) from the bathing medium. Reduction of Cl(-) from 155 mM to 55 mM suppressed the current when the neurons were in solutions with depleted Na(+) and Ca(2+). Current-voltage curves in solutions with reduced Cl(-) were linear and the reversal potential was changed from -30 +/- 5 mV to 7 +/- 4 mV. Niflumic acid, but not anthracene-9-carboxylic acid (9-AC) nor 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), suppressed the histamine-activated current. A membrane permeable analogue of cAMP evoked currents similar to those activated by histamine. A selective histamine H(2) receptor agonist (dimaprit) mimicked the action of histamine and a selective histamine H(2) receptor antagonist (cimetidine) blocked the conductance increase evoked by histamine. A selective adenosine A(1) receptor agonist (CCPA) reduced the histamine-activated current and a selective adenosine A(1) receptor antagonist (CPT) reversed the inhibitory action. The results suggest that histamine acts at histamine H(2) receptors to increase Cl(-) conductance in AH-type enteric neurons. Cyclic AMP appears to be a second messenger in the signal transduction process. Results with a selective adenosine A(1) receptor agonist and antagonist add to existing evidence for co-coupling of inhibitory adenosine A(1) receptors and histamine H(2) receptors to adenylate cyclase in AH-type enteric neurons.

Pharmacological characterization of a novel AVP(4-9) binding site in rat hippocampus

pGlu-Asn-Cys (Cys)-Pro-Arg-Gly-NH(2) (AVP(4-9)), a major metabolite C-terminal fragment of Arginine(8)-vasopressin (AVP), improves the disruption of the learning and memory, and is a far more potent in the mnemonic function than AVP. In this study, we pharmacologically characterized its putative binding site and mechanism of intracellular signaling. Radioligand binding assay showed that [35S]AVP(4-9) could detect specific binding sites in the rat hippocampus membrane preparations, and the binding site was specifically displaced by AVP(4-9) but not by either V(1) or V(2) antagonists. Furthermore, [35S]AVP(4-9) could not detect the cloned rat V(1a), V(1b) and V(2) vasopressin receptors. Even at a low doses (10-100 pM), AVP(4-9) caused an increase in both inositol(1,4, 5)-trisphosphate (Ins(1,4,5)P(3)) and intracellular calcium concentrations ([Ca(2+)](i)) in rat hippocampal cells. The AVP(4-9)-induced [Ca(2+)](i) increase was partially inhibited by the absence of Ca(2+) or by Ca(2+)-channel blocker, suggesting that AVP(4-9) caused the [Ca(2+)](i) increase via release from intracellular calcium store as well as influx from extracellular calcium. For the first time, this study provides evidence to show that AVP(4-9) activates Ins(1,4,5)P(3)/[Ca(2+)](i) pathway through a novel type of receptor in rat hippocampus, which might be potentially important in improving the mnemonic function.

Effect of streptozotocin-induced diabetes on the activity of calcium channels in rat dorsal horn neurons

We have previously found that spinal dorsal horn neurons from streptozotocin-diabetic rats, an animal model for diabetes mellitus, show the prominent changes in the mechanisms responsible for [Ca2+]i regulation. The present study aimed to further characterize the effects of streptozotocin-induced diabetes on neuronal calcium homeostasis. The cytoplasmic Ca2+ concentration ([Ca2+]i) was measured in Fura-2AM-loaded dorsal horn neurons from acutely isolated spinal cord slices using fluorescence technique. We studied Ca2+ entry through plasmalemmal Ca2+ channels during potassium (50 mM KCl)-induced depolarization. The K+-induced [Ca2+]i elevation was inhibited to a different extent by nickel ions, nifedipine and omega-conotoxin suggesting the co-expression of different subtypes of plasmalemmal voltage-gated Ca2+ channels. The suppression of [Ca2+]i transients by Ni2+ (50 microM) was the same in control and diabetic neurons. On the other hand, inhibition of [Ca2+]i transients by nifedipine (50 microM) and omega-conotoxin (1 microM) was much greater in diabetic neurons compared with normal animals. These data suggest that under diabetic conditions the activity of N- and L- but not T-type voltage-gated Ca2+ channels substantially increased in dorsal horn neurons.

Calcium currents in hair cells isolated from semicircular canals of the frog

L-type and R-type Ca(2+) currents were detected in frog semicircular canal hair cells. The former was noninactivating and nifedipine-sensitive (5 microM); the latter, partially inactivated, was resistant to omega-conotoxin GVIA (5 microM), omega-conotoxin MVIIC (5 microM), and omega-agatoxin IVA (0.4 microM), but was sensitive to mibefradil (10 microM). Both currents were sensitive to Ni(2+) and Cd(2+) (>10 microM). In some cells the L-type current amplitude increased almost twofold upon repetitive stimulation, whereas the R-type current remained unaffected. Eventually, run-down occurred for both currents, but was prevented by the protease inhibitor calpastatin. The R-type current peak component ran down first, without changing its plateau, suggesting that two channel types generate the R-type current. This peak component appeared at -40 mV, reached a maximal value at -30 mV, and became undetectable for voltages > or =0 mV, suggestive of a novel transient current: its inactivation was indeed reversibly removed when Ba(2+) was the charge carrier. The L-type current and the R-type current plateau were appreciable at -60 mV and peaked at -20 mV: the former current did not reverse for voltages up to +60 mV, the latter reversed between +30 and +60 mV due to an outward Cs(+) current flowing through the same Ca(2+) channel. The physiological role of these currents on hair cell function is discussed.

Multiple calcium pathways induce the expression of SNAP-25 protein in chromaffin cells

Incubation of bovine adrenal chromaffin cells in high K+ (38 mM) during 24-48 h enhanced 2.5 to five times the expression of SNAP-25 protein and mRNA, respectively. This increase was reduced 86% by furnidipine (an L-type Ca2+ channel blocker) but was unaffected by either omega-conotoxin GVIA (an N-type Ca2+ channel blocker) or -agatoxin IVA (a P/Q-type Ca2+ channel blocker). Combined blockade of N and P/Q channels with omega-conotoxin MVIIC did, however, block by 76% the protein expression. The inhibitory effects of fumidipine were partially reversed when the external Ca2+ concentration was raised from 1.6 to 5 mM. These findings, together with the fact that nicotinic receptor activation or Ca2+ release from internal stores also enhanced SNAP-25 protein expression, suggest that an increment of cytosolic Ca2+ concentration ([Ca2+]), rather than its source or Ca2+ entry pathway, is the critical signal to induce the protein expression. The greater coupling between L-type Ca2+ channels and protein expression might be due to two facts: (a) L channels contributed 50% to the global [Ca2+]i rise induced by 38 mM K+ in indo-1-loaded chromaffin cells and (b) L channels undergo less inactivation than N or P/Q channels on sustained stimulation of these cells.