Choline induces opposite changes in pyramidal neuron excitability and synaptic transmission through a nicotinic receptor-independent process in hippocampal slices.

Choline is present at cholinergic synapses as a product of acetylcholine degradation. In addition, it is considered a selective agonist for α5 and α7 nicotinic acetylcholine receptors (nAChRs). In this study, we determined how choline affects action potentials and excitatory synaptic transmission using extracellular and intracellular recording techniques in CA1 area of hippocampal slices obtained from both mice and rats. Choline caused a reversible depression of evoked field excitatory postsynaptic potentials (fEPSPs) in a concentration-dependent manner that was not affected by α7 nAChR antagonists. Moreover, this choline-induced effect was not mimicked by either selective agonists or allosteric modulators of α7 nAChRs. Additionally, this choline-mediated effect was not prevented by either selective antagonists of GABA receptors or hemicholinium, a choline uptake inhibitor. The paired pulse facilitation paradigm, which detects whether a substance affects presynaptic release of glutamate, was not modified by choline. On the other hand, choline induced a robust increase of population spike evoked by orthodromic stimulation but did not modify that evoked by antidromic stimulation. We also found that choline impaired recurrent inhibition recorded in the pyramidal cell layer through a mechanism independent of α7 nAChR activation. These choline-mediated effects on fEPSP and population spike observed in rat slices were completely reproduced in slices obtained from α7 nAChR knockout mice, which reinforces our conclusion that choline modulates synaptic transmission and neuronal excitability by a mechanism independent of nicotinic receptor activation.

Allosteric modulation of alpha 7 nicotinic receptors selectively depolarizes hippocampal interneurons, enhancing spontaneous GABAergic transmission.

The role of postsynaptic nicotinic receptors for acetylcholine (nAChRs) in mediating fast neurotransmission processes in the CNS is controversial. Here we have studied the modulation of synaptic transmission by an agonist (choline) and an allosteric modulator (5-OH-indole) of alpha7 nAChRs in rat hippocampal neuronal cultures. Choline evoked a fast inactivating inward current, causing neuron depolarization and action potential discharge, thereby enhancing the spontaneous postsynaptic current activity (sPSCs). This effect was markedly enhanced when both choline and 5-OH-indole were applied together and was blocked by the selective alpha7 nAChR antagonist methyllycaconitine. This choline action was suppressed by the GABA(A) receptor antagonist bicuculline, while the glutamatergic receptor antagonist kynurenic acid had no effect. Frequency, but not amplitude or area, of both excitatory and inhibitory miniature postsynaptic currents (mEPSCs and mIPSCs) were drastically reduced when Ca(2+) influx was blocked by Cd(2+). Additionally, nAChR activation did not modify the mIPSCs. These data suggest that Ca(2+) influx through the highly Ca(2+)-permeablealpha7 nAChRs was insufficient to directly activate neurotransmitter release, suggesting that a tight colocalization of this receptor with secretory hot spots is unlikely. In a few cases, the activation of alpha7 AChRs led to a suppression of spontaneous synaptic transmission. This effect may be related to the potentiation of GABAergic interneurons that inhibit the spontaneous activity of neurons making synapses with the cell under study. We suggest that GABA release is modulated by alpha7 nAChRs. Thus, selective allosteric modulators of alpha7 nAChRs could have potential therapeutic applications in brain disorders such as epilepsy and schizophrenia and in alterations of cognition and sensory processing.

Neuroprotection afforded by nicotine against oxygen and glucose deprivation in hippocampal slices is lost in alpha7 nicotinic receptor knockout mice.

Although alpha7-receptors are considered the main target for neuroprotection, other receptor subtypes (alpha4beta2 or alpha3beta4) have also been implicated. Hence, we have used alpha7-transgenic mice, to study the hypothesis that alpha7-receptors play a dominant role in mediating neuroprotection in an in vitro model of ischemia. We have used rat and mouse hippocampal slices to establish the model of nicotinic neuroprotection against oxygen and glucose deprivation (OGD). Neuronal damage caused by OGD during 1 h plus 3 h re-oxygenation, was quantified by measuring lactate dehydrogenase (LDH) release from hippocampal slices. In rat hippocampal slices, OGD increased over twofold basal LDH release. Such increase was reduced when treated with 10-100 microM nicotine; maximal protection afforded by nicotine amounted to 46%. This neuroprotection was antagonized by the non-selective nicotinic receptor for acetylcholine (nAChR) blocker mecamylamine (10 microM). In hippocampal slices from wild-type control mice, nicotine (100 microM) decreased by 54.4% LDH release evoked by OGD plus re-oxygenation. In contrast, nicotine failed to exert neuroprotection in alpha7 knockout mice. This finding reinforces the view that the hippocampal neuroprotective effects of nicotine are predominantly linked to alpha7 receptors.

A comparison between acetylcholine-like action potentials and square depolarizing pulses in triggering calcium entry and exocytosis in bovine chromaffin cells.

Depending on experimental conditions, cell model, and pattern and type of depolarizing stimuli, the relationship between calcium entry ([Ca2+]c) and the release of neurotransmitters and hormones varies from exponential (power of 3-4) to near linear (power of 1.5) or linear function. Here, we present a study using the more physiological stimulation pattern based on acetylcholine (ACh)-like action potentials, in voltage-clamped bovine chromaffin cells, with the perforated-patch configuration of the patch-clamp technique and 2 mM extracellular calcium. Trains of ACh-like action potentials or square depolarizing pulses of increasing length were applied, and calcium currents (ICa), total calcium entry (QCa), and exocytosis (DeltaCm) measured.

[Anticholinesterases in the treatment of Alzheimer's disease]. / Anticolinesterásicos en el tratamiento de la enfermedad de Alzheimer.

INTRODUCTION: Among the numerous pathophysiological theories that attempt to explain the development of Alzheimer's disease (AD) there are two facts that stand out above the rest: on the one hand, the formation of neurofibrillary tangles inside cells and, on the other, the extra-cellular deposition of beta-amyloid protein. These two mechanisms lead to neurodegeneration and the death of cells by means of a process called 'apoptosis' or 'programmed cell death'. In the early stages of this neurodegenerative process it is more pronounced in cholinergic-type brain centres. This led to the formulation of the so-called cholinergic theory of Alzheimer, which provides the rationale behind the use of the drugs that are currently available to treat this disease, namely, acetylcholine esterase (AChE) inhibitors (rivastigmine, donepezil and galanthamine). DEVELOPMENT AND CONCLUSIONS: We review the possible pharmacological approaches that could help to prevent or delay cell death, and which act on the mechanisms involved in the production of neurofibrillary tangles or the deposition of beta-amyloid protein. We also review the main characteristics of cholinergic neurotransmission, which will help us to understand the therapeutic approaches that have been applied in an attempt to enhance deficient cholinergic neurotransmission. One of the most notable of these is the amount of attention recently being paid to the enzyme AChE, which increases the bioavailability of the neurotransmitter in the cholinergic synapses by preventing the hydrolysis of acetylcholine; these are the only drugs currently available for the symptomatic treatment of this disease.

Calcium-dependent inhibition of L, N, and P/Q Ca2+ channels in chromaffin cells: role of mitochondria.

The hypothesis that the buffering of Ca(2+) by mitochondria could affect the Ca(2+)-dependent inhibition of voltage-activated Ca(2+) channels, (I(Ca)), was tested in voltage-clamped bovine adrenal chromaffin cells. The protonophore carbonyl cyanide m-chlorophenyl-hydrazone (CCCP), the blocker of the Ca(2+) uniporter ruthenium red (RR), and a combination of oligomycin plus rotenone were used to interfere with mitochondrial Ca(2+) buffering. In cells dialyzed with an EGTA-free solution, peak I(Ca) generated by 20 msec pulses to 0 or +10 mV, applied at 15 sec intervals, from a holding potential of -80 mV, decayed rapidly after superfusion of cells with 2 microm CCCP (tau = 16.7 +/- 3 sec; n = 8). In cells dialyzed with 14 mm EGTA, CCCP did not provoke I(Ca) loss. Cell dialysis with 4 microm ruthenium red or cell superfusion with oligomycin (3 microm) plus rotenone (4 microm) also accelerated the decay of I(Ca). After treatment with CCCP, decay of N- and P/Q-type Ca(2+) channel currents occurred faster than that of L-type Ca(2+) channel currents. These data are compatible with the idea that the elevation of the bulk cytosolic Ca(2+) concentration, [Ca(2+)](c), causes the inhibition of L- and N- as well as P/Q-type Ca(2+) channels expressed by bovine chromaffin cells. This [Ca(2+)](c) signal appears to be tightly regulated by rapid Ca(2+) uptake into mitochondria. Thus, it is plausible that mitochondria might efficiently regulate the activity of L, N, and P/Q Ca(2+) channels under physiological stimulation conditions of the cell.

omega-Agatoxin-IVA-sensitive calcium channels in bovine chromaffin cells.

A large component of the whole-cell currents through Ca2+ channels in bovine adrenomedullary chromaffin cells has been shown to be insensitive to both L-type and N-type Ca2+ channel blockers, suggesting the existence of a third type of Ca2+ channel. In the present paper, omega-agatoxin-IVA (AgTx), a selective blocker of P-type Ca2+ channels in mammalian neurons, has been used to investigate the presence of this subtype of Ca2+ channel in bovine chromaffin cells. Barium currents (IBa) through Ca2+ channels were recorded in whole-cell patch-clamped bovine chromaffin cells. IBa was blocked by AgTx in a dose-dependent and irreversible manner. At the maximal concentration used (1 microM), AgTx inhibited IBa by 49.5 +/- 3%. Such a blockade was also present when bovine chromaffin cells were pretreated with 10 microM furnidipine, a novel 1,4-dihydropyridine L-type channel blocker, and after treatment with 1 microM of the N-type channel blocker, omega-conotoxin GVIA (CgTx). A combination of these three types of Ca2+ channel blockers suppressed the macroscopic Ba2+ currents by 88%. We conclude that bovine chromaffin cells, in addition to N- and L-type Ca2+ channels, possess a P-like component in their whole-cell currents through the Ca2+ channels.

Q- and L-type Ca2+ channels dominate the control of secretion in bovine chromaffin cells.

Potassium-stimulated catecholamine release from superfused bovine adrenal chromaffin cells (70 mM K+ in the presence of 2 mM Ca2+ for 10 s, applied at 5-min intervals) was inhibited by the dihydropyridine furnidipine (3 microM) by 50%. omega-Conotoxin MVIIC (CTx-MVIIC, 3 microM) also reduced the secretory response by about half. Combined CTx-MVIIC plus furnidipine blocked 100% catecholamine release. 45Ca2+ uptake and cytosolic Ca2+ concentrations ([Ca2+]i) in K(+)-depolarized cells were partially blocked by furnidipine or CTx-MVIIC, and completely inhibited by both agents. The whole cell current through Ca2+ channels carried by Ba2+ (IBa) was partially blocked by CTx-MVIIC. Although omega-conotoxin GVIA (CTx-GVIA, 1 microM) and omega-agatoxin IVA (Aga-IVA, 0.2 microM) partially inhibited 45Ca2+ entry, IBa and the increase in [Ca2+]i, the combination of both toxins did not affect the K(+)-evoked secretory response. The results are compatible with the presence in bovine chromaffin cells of a Q-like Ca2+ channel which has a prominent role in controlling exocytosis. They also suggest that Q- and L-type Ca2+ channels, but not N- or P-types are localized near exocytotic active sites in the plasmalemma.

(+)-isradipine but not (-)-Bay-K-8644 exhibits voltage-dependent effects on cat adrenal catecholamine release.

1. Catecholamine release from cat adrenal glands perfused at a high rate (4 ml min-1) at 37 degrees C with polarizing (1.2 or 5.9 mM K+) or depolarizing (17.7, 35, 59 or 118 mM K+) solutions, was triggered by 5 or 10 s pulses of Ca2+ (0.5 or 2.5 mM) in the presence of various concentrations of K+. 2. In polarized glands, secretion was greater the higher the K+ concentration present during the secretory K+/Ca2+ test pulse. Thus, in 17.7 mM K+, catecholamine released was 162 +/- 27 ng per pulse, while in 118 mM K+ secretion rose to 1839 +/- 98 ng per pulse. In depolarized glands, secretion reached a peak of around 1000 ng per pulse in 35-59 mM K+; in 118 mM K+, secretion did not increase further, suggesting that voltage changes are implicated in the control of the secretory process. 3. Blockade of secretion by increased concentrations of (+)-isradipine was much more manifest in polarized glands. The higher the degree of depolarization was (35, 59 or 118 mM K+), the lower the IC50 s were. So, the ratios between the IC50 s in polarized and depolarized glands rose from 3.92 in 35 mM K+ to 26.7 in 118 mM K+. 4. In contrast, the Ca2+ channel activator (-)-Bay K 8644 potentiated catecholamine release evoked by K+/Ca2+ pulses equally well in polarized or depolarized glands. The ratios between EC50 s in polarized or depolarized glands were, respectively, 0.30, 0.59 and 0.69 for 17.7, 35 and 118 mM K+. 5. In simultaneous experiments, the two enantiomers of Bay K 8644 exhibited opposite effects on secretion. (+)-Bay K 8644 (a Ca21 channel blocker) inhibited secretion better in depolarized than in polarized glands, whilst (-)-Bay K 8644 potentiated secretion in a voltage-independent manner. 6. Potentiation of secretion by (-)-Bay K 8644 was concentration-dependent from 10-8 to 10-6M. At 10- 5M, such potentiation largely disappeared in both polarized and depolarized glands. However, this dual effect of (-)Bay K 8644 was better seen in depolarizing conditions, suggesting that using the same enantiomer, the voltage-dependence is only seen when blockade of secretion dominates. 7. In the presence of increasing concentrations of (-)Bay K 8644 (3 x 10-9, 3 x 10-8 and 3 x 10-7M), the concentration-response curves for (+)isradipine to inhibit secretion were displaced to the right. However, a Schild plot of (dose ratio - 1) against (-)-Bay K 8644 concentrations gave a slope of 0.6, suggesting that the interactions between (+)-isradipine and (-)Bay K 8644 were non-competitive in nature. The pA2 for (-)-Bay K 8644 was 9.13. 8. Overall, the results suggest that potentiation of secretion by (-)Bay K 8644 (a voltage-independent phenomenon), and blockade by (+)-isradipine or (+-Bay K 8644 (a voltage-dependent phenomenon) might be exerted through binding of the dihydropyridines activators and blockers to separate sites on chromaffin cell L-type Ca2 + channels.

Separation of two pathways for calcium entry into chromaffin cells.

1. The effects of various drugs on 45Ca + 40Ca uptake into cultured bovine adrenal chromaffin cells evoked by 1,1-dimethyl-4-phenylpiperazinium (DMPP) or high K, were studied. In the presence of 1 mM external 40Ca, with 45Ca as a radiotracer, unstimulated cells took up an average of 0.13 fmol/cell 40Ca and 772 c.p.m./10(6) cells of 45Ca (n = 76). Upon stimulation with DMPP (100 microM for 60 s) or K (59 mM for 60 s), Ca uptake increased to 0.92 and 1 fmol/cell, respectively. 2. Flunarizine behaved as a potent blocker of both DMPP- and K-evoked Ca uptake (IC50 of 1.76 and 1.49 microM, respectively for DMPP and K). A similar picture emerged with Cd ions, though Cd exhibited an IC50 against K (1.86 microM) slightly lower than the IC50 against DMPP (8.14 microM). 3. Clear cut differences were observed with amiloride, guanethidine, nimodipine and nisoldipine which behaved as selective blockers of DMPP-mediated Ca uptake responses: IC50 values to block DMPP effects were 290, 27, 1.1 and 1.63 microM respectively for amiloride, guanethidine, nimodipine and nisoldipine. Amiloride blocked K-evoked Ca uptake by only 35% and guanethidine did not affect it. Nisoldipine inhibited K-evoked Ca uptake only partially at low concentrations (about 30%); a second blocking component was observed at the highest concentration used (10 microM). At 10 microM, nimodipine blocked K-evoked Ca uptake by 50%. 4. Thus, it seems that the nicotinic receptor mediated Ca uptake pathway can be pharmacologically separated from the K-activated pathway. The results are compatible with the hypothesis that in cultured bovine adrenal chromaffin cells, stimulation of nicotinic receptors recruits a single type of Ca channel which is sensitive to flunarizine, Cd, amiloride, guanethidine, nimodipine and nisoldipine. The results also suggest that K depolarization might be recruiting in addition to this channel, another Ca channel which is highly sensitive to Cd and flunarizine, resistant to nisoldipine, nimodipine and amiloride, and insensitive to guanethidine.

Dotarizine versus flunarizine as calcium antagonists in chromaffin cells.

1. Dotarizine is a novel piperazine derivative structurally related to flunarizine that is currently being evaluated in clinical trials for its antimigraine and antivertigo effects. This clinical profile may be related to its Ca2+ antagonist properties. Therefore, the actions of both compounds as calcium antagonists were compared in bovine chromaffin cells. 2. Dotarizine and flunarizine blocked 45Ca2+ uptake into K+ depolarized chromaffin cells (70 mM K+/0.5 mM Ca2+ for 60 s) in a concentration-dependent manner, with IC50s of 4.8 and 6.7 microM, respectively. 3. Dotarizine and flunarizine also inhibited the whole-cell Ca2+ and Ba2+ currents (ICa, IBa) in voltage-clamped chromaffin cells, induced by depolarizing test pulses to 0 mV, during 50 ms, from a holding potential of -80 mV. Blockade exhibited IC50s of 4 microM for dotarizine and 2.2 microM for flunarizine. Dotarizine increased the rate of inactivation of ICa and IBa; inhibition of whole-cell currents was use-dependent. 4. Transient increases of the cytosolic Ca2+ concentration, [Ca2+]i, produced by K+ stimulation (70 mM K+ for 5 s) of single fura-2-loaded chromaffin cells, were also inhibited by dotarizine and flunarizine with IC50s of 1.2 and 0.6 microM, respectively. Upon washout of dotarizine, the [Ca2+]i increases recovered fully after 5-10 min. In contrast, the responses remained largely inhibited 10 min after washing out flunarizine. 5. Catecholamine release induced by K+ stimulation (10-s pulses of 70 mM) was inhibited by dotarizine with an IC50 of 2.6 microM and by flunarizine with an IC50 of 1.2 microM. The blocking effects of both compounds developed slowly, and was fully established after 20-30 min of superfusion. While blockade by dotarizine quickly reversed upon its washout, that of flunarizine lasted even 25 min after washing out.6. Catecholamine release from electroporated chromaffin cells triggered by 10 micro M Ca2+ was not affected by 10 micro M dotarizine or flunarizine.7. Overall, the results suggest that dotarizine behaves as a Ca2+ antagonist in bovine chromaffin cells. It exhibits a potency similar to flunarizine in blocking Ca2+ entry, Ca2+ channels, Cai2+ signals and secretion. The dotarizine effects are readily reversible suggesting that in contrast to flunarizine, it does not accumulate in cells. Dotarizine is devoid of intracellular effects on the secretory machinery. All its blocking effects on Ca2+ entry, [Ca2+]i rise and secretion are probably due to blockade of various Ca2+channel subtypes in chromaffin cells. This blockade is use-dependent and seems to be due to the increase by dotarizine of the rate of Ca2+ channel inactivation.

Differential effects of the neuroprotectant lubeluzole on bovine and mouse chromaffin cell calcium channel subtypes.

1. The effects of lubeluzole (a neuroprotective benzothiazole derivative) and its (-) enantiomer R91154 on whole-cell currents through Ca2+ channels, with 10 mM Ba2+ as charge carrier (IBa), have been studied in bovine and mouse voltage-clamped adrenal chromaffin cells. Currents generated by applying 50 ms depolarizing test pulses to 0 mV, from a holding potential of -80 mV, at 10 s intervals had an average magnitude of 1 nA. 2. Lubeluzole and R91154 blocked the peak IBa of bovine chromaffin cells in a time and concentration-dependent manner; their IC50s were 1.94 microM for lubeluzole and 2.54 microM for R91154. In a current-voltage protocol, lubeluzole (3 microM) inhibited peak IBa at all test potentials. However, no obvious shifts of the I-V curve were detected. 3. After 10 min exposure to 3 microM lubeluzole, the late current (measured at the end of the pulse) was inhibited more than the peak current. Upon wash out of the drug, the inactivation reversed first and then the peak current recovered. 4. Blockade of peak current was greater at more depolarizing holding potentials (i.e. 35% at -110 mV and 87% at -50 mV, after 10 min superfusion with lubeluzole). Inactivation of the current was pronounced at -110 mV, decreased at -80 mV and did not occur at -50 mV. 5. Intracellular dialysis of bovine voltage-clamped chromaffin cells with 3 microM lubeluzole caused neither blockade nor inactivation of IBa. The external application of 3 microM lubeluzole to those dialysed cells produced inhibition as well as inactivation of IBa. 6. The effects of lubeluzole (3 microM) on IBa in mouse chromaffin cells were similar to those in bovine chromaffin cells. At -80 mV holding potential, a pronounced inactivation of the current led to greater blockade of the late IBa (66%) as compared with peak IBa (46% after 10 min superfusion with lubeluzole). 7. In mouse chromaffin cells approximately half of the whole-cell IBa was sensitive to 3 microM nifedipine (L-type Ca2+ channels) and the other half to 3 microM omega-conotoxin MVIIC (non-L-type Ca2+ channels). In omega-conotoxin MVIIC-treated cells, 3 microM lubeluzole caused little blockade and inactivation of IBa. However in nifedipine-treated cells, lubeluzole caused a pronounced blockade and inactivation of IBa that reversed upon wash out of the compound. 8. The results are compatible with the idea that lubeluzole preferentially blocks non-L-types of voltage-dependent Ca2+ channels expressed by bovine and mouse chromaffin cells. The higher concentrations of the compound also block L-type Ca2+ channels. The mechanism of inhibition involves the access of lubeluzole to the open channel from the outside of the cell and promotion of its inactivation. The differential blockade of Ca2+ channel subtypes might contribute to the neuroprotective actions of lubeluzole (which exhibit stereoselectivity). However, in view of the lack of stereoselectivity in blocking Ca2+ channels, this effect cannot be the only explanation for the protective activity of lubeluzole in stroke.

R56865 inhibits catecholamine release from bovine chromaffin cells by blocking calcium channels.

1. The effects of R56865 (a new class of cardioprotective agent which prevents Na+ and Ca2+ overload in cardiac myocytes) on catecholamine release, whole-cell current through Ca2+ channels (IBa) and cytosolic Ca2+ concentrations, [Ca2+]i, have been studied in bovine chromaffin cells. 2. R56865 caused a time- and concentration-dependent blockade of catecholamine release from superfused cells stimulated intermittently with 5 s pulses of 59 mM K+. After 5 min superfusion, a 3 microM concentration inhibited secretion by 20%; the blockade increased gradually with perfusion time, to reach 85% after 40 min. The IC50 to block secretion after 5 min periods of exposure to increasing concentrations of R56865 was around 3.1 microM. The blocking effects of R56865 were reversible after 5-15 min wash out. In high Ca2+ solution (10 mM Ca2+), the degree of blockade of secretion diminished by 20% in comparison with 1 mM Ca2+. 3. In electroporated cells, R56865 (10 microM) did not modify the secretory response induced by the application of 10 microM free Ca2+. 4. R56865 blocked the peak IBa current in a concentration- and time-dependent manner; its IC50 was very similar to that obtained for secretion (3 microM). The compound not only reduced the size of the peak current but also promoted its inactivation; when the effects of R56865 were measured at the most inactivated part of the current, its IC50 was 1 microM. Both the inactivation and the reduction of the peak currents were reversible upon washing out the drug. 5. In fura-2-loaded single chromaffin cells the basal [Ca2+]i of around 100 nM was elevated to a peak of1.5 microM by the application of a 5 s pulse of 59 mM K+. R56865 (10 microM) did not affect the basal [Ca2+]but blocked by 90% the K+-evoked increase. This effect was fully reversible after 5-10 min of wash out.6. The results are compatible with the idea that R56865 blocks Ca2+ entry into K+-depolarized chromaffin cells by promoting the inactivation of voltage-dependent Ca2+ channels; this would lead to the limitation of the rise in [Ca2+]i and of the release of catecholamines. The restriction of catecholamine release may favour indirectly the known direct beneficial cardioprotective actions of R56865.

Otilonium: a potent blocker of neuronal nicotinic ACh receptors in bovine chromaffin cells.

1. Otilonium, a clinically useful spasmolytic, behaves as a potent blocker of neuronal nicotinic acetylcholine receptors (AChR) as well as a mild wide-spectrum Ca2+ channel blocker in bovine adrenal chromaffin cells. 2. 45Ca2+ uptake into chromaffin cells stimulated with high K+ (70 mM, 1 min) was blocked by otilonium with an IC50 of 7.6 microM. The drug inhibited the 45Ca2+ uptake stimulated by the nicotinic AChR agonist, dimethylphenylpiperazinium (DMPP) with a 79 fold higher potency (IC50 = 0.096 microM). 3. Whole-cell Ba2+ currents (IBa) through Ca2+ channels of voltage-clamped chromaffin cells were blocked by otilonium with an IC50 of 6.4 microM, very close to that of K(+)-evoked 45Ca2+ uptake. Blockade developed in 10-20 s, almost as a single step and was rapidly and almost fully reversible. 4. Whole-cell nicotinic AChR-mediated currents (250 ms pulses of 100 microM DMPP) applied at 30 s intervals were blocked by otilonium in a concentration-dependent manner, showing an IC50 of 0.36 microM. Blockade was induced in a step-wise manner. Wash out of otilonium allowed a slow recovery of the current, also in discrete steps. 5. In experiments with recordings in the same cells of whole-cell IDMPP, Na+ currents (INa) and Ca2+ currents (ICa), 1 microM otilonium blocked 87% IDMPP, 7% INa and 13% ICa. 6. Otilonium inhibited the K(+)-evoked catecholamine secretory response of superfused bovine chromaffin cells with an IC50 of 10 microM, very close to the IC50 for blockade of K(+)-induced 45Ca2+ uptake and IBa. 7. Otilonium inhibited the secretory responses induced by 10 s pulses of 50 microM DMPP with an IC50 of 7.4 nM. Hexamethonium blocked the DMPP-evoked responses with an IC50 of 29.8 microM, 4,000 fold higher than that of otilonium. 8. In conclusion, otilonium is a potent blocker of nicotinic AChR-mediated responses. The drugs also blocked various subtypes of neuronal voltage-dependent Ca2+ channels at a considerably lower potency. Na+ channels were unaffected by otilonium. This extraordinary potency of otilonium in blocking nicotinic AChR, unrecognised until now, might account in part for its well known spasmolytic effects.

Inhibition of nicotinic receptor-mediated responses in bovine chromaffin cells by diltiazem.

1. The effects of diltiazem on various functional parameters were studied in bovine cultured adrenal chromaffin cells stimulated with the nicotinic receptor agonist dimethylphenylpiperazinium (DMPP) or with depolarizing Krebs-HEPES solutions containing high K+ concentrations. 2. The release of [3H]-noradrenaline induced by DMPP (100 microM for 5 min) was gradually and fully inhibited by increasing concentrations of diltiazem (IC50 = 1.3 microM). In contrast, the highest concentration of diltiazem used (10 microM) inhibited the response to high K+ (59 mM for 5 min) by only 25%. 3. 45Ca2+ uptake into cells stimulated with DMPP (100 microM for 1 min) was also blocked by diltiazem in a concentration-dependent manner (IC50 = 0.4 microM). Again, diltiazem blocked the K(+)-evoked 45Ca2+ uptake (70 mM K+ for 1 min) only by 20%. In contrast, the N-P-Q-type Ca2+ channel blocker omega-conotoxin MVIIC depressed the K+ signal by 70%. In the presence of this toxin, diltiazem exhibited an additional small inhibitory effect, indicating that the compound was acting on L-type Ca2+ channels. 4. Whole-cell Ba2+ currents through Ca2+ channels in voltage-clamped chromaffin cells were inhibited by 3-10 microM diltiazem by 20-25%. The inhibition was readily reversed upon washout of the drug. 5. The whole-cell currents elicited by 100 microM DMPP (IDMPP) were inhibited in a concentration-dependent and reversible manner by diltiazem. Maximal effects were found at 10 microM, which reduced the peak IDMPP by 70%. The area of each curve represented by total current (QDMPP) was reduced more than the peak current. At 10 microM, the inhibition amounted to 80%; the IC50 for QDMPP inhibition was 0.73 microM, a figure close to the IC50 for 45Ca2+ uptake (0.4 microM) and [3H]-noradrenaline release (1.3 microM). The blocking effects of diltiazem developed very quickly and did not exhibit use-dependence; thus the drug blocked the channel in its closed state. The blocking effects of 1 microM diltiazem on IDMPP were similar at different holding potentials (inhibition by around 30% at -100, -80 or -50 mV). Diltiazem did not affect the current flow through voltage-dependent Na+ channels. 6. These data are compatible with the idea that diltiazem has little effect on Ca2+ entry through voltage-dependent Ca2+ channels in bovine chromaffin cells. Neither, does diltiazem affect INa. Rather, diltiazem acts directly on the neuronal nicotinic receptor ion channel and blocks ion fluxes, cell depolarization and the subsequent Ca2+ entry and catecholamine release. This novel effect of diltiazem might have clinical relevance since it might reduce the sympathoadrenal drive to the heart and blood vessels, thus contributing to the well established antihypertensive and cardioprotective effects of the drug.

Effects of the neuroprotectant lubeluzole on the cytotoxic actions of veratridine, barium, ouabain and 6-hydroxydopamine in chromaffin cells.

1. Incubation of bovine adrenal chromaffin cells with veratridine (10-100 microM) during 24 h, caused a concentration-dependent release of the cytosolic lactate dehydrogenase (LDH) into the bathing medium, an indicator of cell death. Lubeluzole or its R(-) enantiomer, R91154, did not enhance LDH release. Both lubeluzole and R91154 (0.3-10 microM) decreased the veratridine-induced LDH release. 2. Penfluridol did not increase LDH release at concentrations 0.003-1 microM; 3-10 microM increased LDH release to 50-60%, after 24 h exposure. Penfluridol (0.03-0.3 microM) did not protect against the cytotoxic effects of veratridine; at 1 microM, 15% protection was produced. Higher concentrations (3-10 microM) enhanced the cytotoxic effects of veratridine. 3. Ba2+ ions caused a concentration-dependent increase of LDH release. This cytotoxic effect was partially prevented by 3 microM lubeluzole and fully counteracted by 1 microM penfluridol. R91154 was less potent than lubeluzole and only protected against the lesion induced by 0.5 mM Ba2+. 4. Ouabain (10 microM during 24 h) increased LDH release to about 30%. Both lubeluzole (0.3-10 microM) and the lower concentrations of penfluridol (0.003-0.3 microM) prevented the ouabain cytotoxic effects. At higher concentrations (3 microM), penfluridol increased drastically the ouabain cytotoxic effects. 5. 6-Hydroxydopamine (6-OHDA) caused significant cytotoxic effects at 30 and 100 microM. Lubeluzole (3-10 microM) or penfluridol (0.03-0.3 microM) had no cytoprotective effects against 6-OHDA. 6. Lubeluzole (3 microM), R91154 (3 microM) and penfluridol (1 microM) blocked the current through Na+ channels in voltage-clamped chromaffin cells (I(Na)) by around 20-30%. Ca2+ current through Ca2+ channels (I(Ca)) was inhibited 57% by lubeluzole and R91154 and 50% by penfluridol. The effects of penfluridol were not washed out, but those of lubeluzole and R91154 were readily reversible. 7. Lubeluzole (3 microM) induced reversible blockade of the oscillations of the cytosolic Ca2+, [Ca2+]i, in fura-2-loaded cells exposed to 30 or 100 microM veratridine. Penfluridol (1 microM) inhibited those oscillations in an irreversible manner. 8. The results suggest that lubeluzole and its R-isomer caused cytoprotection against veratridine cell damage, by blocking the veratridine stimulated Na+ and Ca2+ entry, as well as the [Ca2+]i oscillations. The Ba2+ and ouabain cytotoxic effects were prevented more efficiently by penfluridol, likely by blocking the plasmalemmal Na+/Ca2+ exchanger. It remains dubious whether these findings are relevant to the reported neuroprotective action of lubeluzole in stroke; the doubt rests in the stereoselective protecting effects of lubeluzole in in vivo stroke models, as opposed to its lack of stereoselectivity in the in vitro model reported here.

Density of apamin-sensitive Ca(2+)-dependent K+ channels in bovine chromaffin cells: relevance to secretion.

Three objectives were defined when planning this study: (i) to identify binding sites for [125I]-apamin in intact bovine adrenal medulla chromaffin cells and to estimate their density and selectivity; (ii) to determine whether apamin modified the release of catecholamines evoked by brief pulses of dimethylphenylpiperazinium (DMPP, 1 or 5 microM for 10 sec), histamine (10 microM for 10 sec) or high K+ (20, 35 or 70 mM for 10 sec) applied to superfused cells; and (iii) to test whether apamin affected the profiles of the changes in cytosolic Ca2+ concentrations [Ca2+]i obtained in suspensions of cells loaded with fura-2 and stimulated with DMPP or histamine. At equilibrium, increasing concentrations of [125I]-apamin gave a saturation curve whose Scatchard transformation produced a Kd of 132 pM and a Bmax of 0.72 fmol/10(6) cells. Quinine, tetraethylammonium, charybdotoxin or glibenclamide (blockers of various subtypes of K+ channels) did not inhibit [125I]apamin binding. Binding was blocked by apamin and by d-tubocurarine, two blockers of small-conductance Ca(2+)-activated K+ channels (SK channels). The number of binding sites for [125I]apamin amounted to approx. 900 per single chromaffin cell, 0.72 sites per micron 2 surface area. Apamin (1 microM) enhanced the secretory response to histamine (10 microM), DMPP (1 or 5 microM) and high K+ (20 or 35 mM) by 2-3-fold. The response to 70 mM K+, however, was unaffected. Apamin also enhanced the peak [Ca2+]i increase produced by DMPP or histamine by approx. 30%. Overall, these results strongly support the hypothesis that under physiological conditions, SK channels control some of the electrical activity of chromaffin cells and indirectly, the opening of voltage-dependent Ca2+ channels, the access of Ca2+ to the secretory machinery and the rate of catecholamine release to the circulation from the intact adrenal gland.

Different sensitivities to dihydropyridines of catecholamine release from cat and ox adrenals.

Catecholamine release evoked by quick pulses of Ca2+ (2.5 mM) given sequentially at 30 min intervals to cat and ox adrenal glands perfused continuously with Ca2+ free Krebs-Tris solutions containing 35 or 118 mM K+, was studied. In the feline, the secretory response was highly sensitive to various dihydropyridine (DHP) derivatives. For instance, secretion was completely blocked by nM concentrations of (+)isradipine (IC50 between 3 and 4 nM) and markedly potentiated by (-)Bay-K-8644. In contrast, the bovine secretory responses were resistant to blockade by nitrendipine or (+)isradipine, as well as to potentiation by Bay-K-8644, even at microM concentrations. From these experiments, it seems clear that distinct subtypes of Ca2+ channels might mediate a similar secretory response in ox and cat adrenal chromaffin cells, at least in the present experimental conditions.

Dihydropyridine chirality at the chromaffin cell calcium channel.

The racemic mixture of the dihydropyridine PN200-110 (Sandoz) inhibits K+-evoked catecholamine release from cat adrenal glands with an IC50 of 4.1 nM; IC50's for (+)- and (-)-PN200-110 were 0.84 and 45.8 nM, respectively. While the (+)-enantiomer of the dihydropyridine Sandoz 202-791 potentiated secretion (EC50 = 100 nM), the (-)-enantiomer behaved as a potent inhibitor (IC50 = 10 nM). Since K+-evoked 45Ca-uptake was also potently inhibited by (+)-PN200-110, it seems that the chromaffin cell dihydropyridine receptor is associated to the voltage-dependent Ca-channel and that it exhibits an exquisite stereoselectivity.

Differential effects of forskolin and 1,9-dideoxy-forskolin on nicotinic receptor- and K+-induced responses in chromaffin cells.

The diterpene forskolin inhibits nicotine-evoked chromaffin cell Ca2+ influx, scinderin redistribution, F-actin disassembly and catecholamine secretion in a concentration-dependent (10-50 microM) fashion. On the other hand, forskolin showed weak inhibitory effects when the same responses were elicited by K+-induced depolarization. Similar concentrations of 1,9-dideoxy-forskolin, a forskolin analog which does not activate adenylate cyclase, blocked very effectively the responses evoked by either of the two stimuli. Patch-clamp (whole-cell configuration) studies demonstrated that both diterpenes blocked fast and reversibly peak and total chromaffin cell nicotinic acetylcholine receptor currents, effects not mediated through adenylate cyclase activation. Moreover, both forskolin and 1,9-dideoxy-forskolin exhibited Ca2+ channel blocking properties. However, 1,9-dideoxy-forskolin was more potent than forskolin as a Ca2+ channel blocker. Furthermore, 1,9-dideoxy-forskolin was also more potent than forskolin as a nicotinic acetylcholine receptor and Ca2+ channel blocker and it was more potent as a nicotinic acetylcholine receptor blocker than Ca2+ channel blocker. The results showed powerful cAMP-independent effects of the diterpenes and suggest caution in interpretation of cAMP effects on chromaffin cells when its cellular levels are modified by forskolin.