Serotonergic effects of dotarizine in coronary artery and in oocytes expressing 5-HT2 receptors

5-HT/GABA interaction in epilepsy

Epilepsy is a neurological condition characterized by synchronous neuronal oscillations (seizures) in the electroencephalogram. Seizures are classified in focal or generalized (depending on the brain territory interested during seizures), and in convulsive and/or not convulsive (depending on the presence or not of involuntary movements). The current pharmacological treatments are mainly based on GABA modulation although different neurotransmitters are also involved in epilepsy, including serotonin. However despite much extensive progress in the understanding of epilepsy mechanisms, still, a percentage of people with epilepsy are pharmaco-resistant calling for the need for new therapeutic targets. Here we review preclinical and human evidence showing that serotonin modulates epilepsy that this likely happens via a major modulation/interaction with GABA.

Cross-talk between Native Plasmalemmal Na+/Ca2+ Exchanger and Inositol 1,4,5-Trisphosphate-sensitive Ca2+ Internal Store in Xenopus Oocytes

Because the presence of a native plasmalemmal Na+/Ca2+ exchange (NCX) activity in Xenopus laevis oocytes remains controversial, its possible functional role in these cells is poorly understood. Here, in experiments on control oocytes and oocytes overexpressing a cloned NCX1 cardiac protein, confocal microscopy combined with electrophysiological techniques reveal that these cells express an endogenous NCX protein forming a functional microdomain with inositol 1,4,5-trisphosphate receptors (InsP3R) that controls intracellular Ca2+ in a restricted subplasmalemmal space. The following data obtained in control denuded oocytes are consistent with this view: (i) reverse transcription-PCR revealed that the oocyte expresses two transcripts for the NCX1 and NCX3 isoforms; (ii) immunofluorescence experiments showed that native NCX1 and InsP3Rs are largely codistributed in discrete areas of the plasma membrane in close apposition to the cortical endoplasmic reticulum shell; (iii) when stimulated by rabbit serum, which elevates intracellular Ca2+ mediated by InsP3, voltage-clamped oocytes display a large and transient inward Ca2+ -activated chloride current, IClCa, as a result of the Ca2+ rise at the inner surface membrane; (iv) this current is significantly enhanced by KB-R7943 and by an extracellular sodium-depleted medium, two maneuvers that prevent "Ca2+ extrusion" via NCX; and (v) blocking NCX enhanced the IClCa elicited by InsP3 but not by Ca2+ photolysis in oocytes injected with the respective caged compounds. Moreover, overexpression of cardiac NCX1, confirmed by confocal microscopy, has functional consequences for the "Ca2+ influx" but not for the serum-elicited "Ca2+ efflux" mode of basal exchange activity and does not alter the number of endogenous NCX/InsP3Rs colocalization sites. Our results suggest that native NCX, because of its strategic position, may regulate InsP3-mediated Ca2+ signaling during the early phases of oocyte maturation and/or fertilization, and furthermore foreign cardiac protein is excluded from the Ca2+ microdomains surrounding the native NCX/InsP3Rs complex in the oocyte.

Functions of 5-HT2A receptor and its antagonists in the cardiovascular system

The serotonin (5-hydroxytryptamine, 5-HT) receptors have conventionally been divided into seven subfamilies, most of which have several subtypes. Among them, 5-HT(2A) receptor is associated with the contraction of vascular smooth muscle, platelet aggregation and thrombus formation and coronary artery spasms. Accordingly, selective 5-HT(2A) antagonists may have potential in the treatment of cardiovascular diseases. Sarpogrelate, a selective 5-HT(2A) antagonist, has been introduced clinically as a therapeutic agent for the treatment of ischemic diseases associated with thrombosis. Molecular modeling studies also suggest that sarpogrelate is a 5-HT(2A) selective antagonist and is likely to have pharmacological effects beneficial in the treatment of cardiovascular diseases. This review describes the above findings as well as the signaling linkages of the 5-HT(2A) receptors and the mode of agonist binding to 5-HT(2A) receptor using data derived from molecular modeling and site-directed mutagenesis.

Molecular Mechanisms of Migraine

Migraine is a common complex disorder that affects a large portion of the population and thus incurs a substantial economic burden on society. The disorder is characterized by recurrent headaches that are unilateral and usually accompanied by nausea, vomiting, photophobia, and phonophobia. The range of clinical characteristics is broad and there is evidence of comorbidity with other neurological diseases, complicating both the diagnosis and management of the disorder. Although the class of drugs known as the triptans (serotonin 5-HT(1B/1D) agonists) has been shown to be effective in treating a significant number of patients with migraine, treatment may in the future be further enhanced by identifying drugs that selectively target molecular mechanisms causing susceptibility to the disease.Genetically, migraine is a complex familial disorder in which the severity and susceptibility of individuals is most likely governed by several genes that may be different among families. Identification of the genomic variants involved in genetic predisposition to migraine should facilitate the development of more effective diagnostic and therapeutic applications. Genetic profiling, combined with our knowledge of therapeutic response to drugs, should enable the development of specific, individually-tailored treatment.

Modulatory mechanism of the endogenous peptide catestatin on neuronal nicotinic acetylcholine receptors and exocytosis

The catestatin fragment of chromogranin A is the first known endogenous compound able to inhibit catecholamine release elicited by the activation of neuronal nicotinic acetylcholine receptors (nAChRs) of different animal species and catecholaminergic cell types. However, how catestatin regulates the receptor activity, which subunit combination of the heteropentameric forms of receptor is better blocked by the peptide, or how it affects the different stages of the exocytotic process have not yet been evaluated. To address these questions, we have assayed the effects of catestatin: (first) on the inward currents elicited by ACh (I(ACh)) in voltage-clamped oocytes expressing different combinations of nAChR subunits; and (second) on the cytosolic Ca2+ concentration, [Ca2+]c, and quantal release of catecholamines simultaneously monitored in single adrenal chromaffin cells stimulated with ACh. Catestatin potently blocks all the subtypes of nAChRs studied. Furthermore, it inhibits the alpha3beta4 current in a reversible, noncompetitive, voltage-, and use-dependent manner, a behavior compatible with open-channel blockade. In fura-2-loaded single chromaffin cells, the peptide reduced the [Ca2+]c signal and the total release of catecholamines elicited by ACh; however, catestatin did not modify the kinetics or the last step of the exocytotic process. Our results suggest that catestatin might play an autocrine regulatory role in neuroendocrine secretion through its interaction with different native nAChR subtypes; the extent of receptor blockade by the peptide could be acutely regulated by the intensity and duration of the presynaptic stimulus.

Multiplicity of mechanisms of serotonin receptor signal transduction

The serotonin (5-hydroxytryptamine, 5-HT) receptors have been divided into 7 subfamilies by convention, 6 of which include 13 different genes for G-protein-coupled receptors. Those subfamilies have been characterized by overlapping pharmacological properties, amino acid sequences, gene organization, and second messenger coupling pathways. Post-genomic modifications, such as alternative mRNA splicing or mRNA editing, creates at least 20 more G-protein-coupled 5-HT receptors, such that there are at least 30 distinct 5-HT receptors that signal through G-proteins. This review will focus on what is known about the signaling linkages of the G-protein-linked 5-HT receptors, and will highlight some fascinating new insights into 5-HT receptor signaling.

Mechanisms of blockade by the novel migraine prophylactic agent, dotarizine, of various brain and peripheral vessel contractility

The novel antimigraineur, dotarizine, inhibited 5-HT (5 hydroxytryptamine)-evoked contractions of rabbit vertebral, aorta, femoral and mesenteric arteries, with IC(50)s of 1.35, 1.40, 0.52 and 1.09 microM, respectively. Flunarizine had little effect on these contractions, while ketanserin was more potent (IC(50)s of 0.17 microM for vertebral, 0.22 microM for aorta, 0.05 microM for femoral and 0.03 microM for mesenteric arteries). At 10 microM, dotarizine caused 40% blockade of K(+)-evoked contractions of rabbit aorta, and 70% inhibition of 5-HT-evoked responses; these values were 30% and 20% for 10 microM flunarizine. Contractions of rabbit aorta elicited by noradrenaline, angiotensin II or prostaglandin F(2alpha) were not affected by 10 microM dotarizine or flunarizine. Ketanserin shifted to the right, in parallel, the concentration-response curves for 5-HT in rabbit aorta; however, dotarizine caused a non-competitive type of blockade, increasing the maximum 5-HT contraction at 30 nM and decreasing it at 3 and 30 microM. K(+)-evoked contractions of rabbit aorta were halved by 3 microM dotarizine in a voltage-independent manner; flunarizine caused a delayed-type, non-reversible post-drug blockade, and exhibited some voltage-dependence. Blockade by nifedipine was voltage-dependent and fully reversible. Ca(2+)-evoked contractions of depolarised bovine middle cerebral arteries were blocked by 1--3 microM dotarizine in a non-surmountable manner. Contraction of these vessels evoked by electrical stimulation was blocked 50% and 70% by 1 and 3 microM dotarizine, respectively. Dotarizine (1--3 microM) also inhibited to a similar extent the K(+)-evoked [(3)H]noradrenaline release from cultured rat sympathetic neurones. These data suggest that the mechanism of blockade by dotarizine of cerebral vessels contractility has three components: (i) presynaptic inhibition of noradrenaline release; (ii) blockade of postsynaptic vascular 5-HT receptors; (iii) blockade of Ca(2+)entry into the vascular smooth muscle cell cytosol. The compound does not affect the vascular receptors for noradrenaline, angiotensin II or prostaglandin F(2alpha).

The novel Na(+)/Ca(2+) exchange inhibitor KB-R7943 also blocks native and expressed neuronal nicotinic receptors

We studied the effects of the novel Na(+)/Ca(2+) exchange inhibitor KB-R7943, 2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea methanesulphonate, on the native nicotinic receptors present at the bovine adrenal chromaffin cells, as well as on rat brain alpha(3)beta(4) and alpha(7) nicotinic acetylcholine receptors (AChRs) expressed in Xenopus oocytes. As expected, KB-R7943 blocked the Na(+)-gradient dependent (45)Ca(2+) uptake into chromaffin cells (IC(50) of 5.5 microM); but in addition, the compound also inhibited the (45)Ca(2+) entry and the increase of cytosolic Ca(2+) concentration, [Ca(2+)](c), stimulated by 5 s pulses of ACh (IC(50) of 6.5 and 1.7 microM, respectively). In oocytes expressing alpha(3)beta(4) and alpha(7) nicotinic AChRs, voltage-clamped at -60 mV, inward currents elicited by 1 s pulses of 100 microM ACh (I(ACh)) were blocked by KB-R7943 with an IC(50) of 0.4 microM and a Hill coefficient of 0.9. Blockade of alpha(3)beta(4) currents by KB-R7943 was noncompetitive; moreover, the blocker (0.3 microM) became more active as the ACh concentration increased (34 versus 66% blockade at 30 microM and 1 mM ACh, respectively). Inhibition of alpha(3)beta(4) currents by 0.3 microM KB-R7943 was more pronounced at hyperpolarized potentials. If given within the ACh pulse (10 microM), the inhibition amounted to 33, 64 and 80% in oocytes voltage-clamped at -40, -60 and -100 mV, respectively. The onset of blockade was faster and the recovery slower at -100 mV; the reverse was true at -40 mV. In conclusion, KB-R7943 is a potent blocker of nicotinic AChRs; moreover, it displays many features of an open-channel blocker at the rat brain alpha(3)beta(4) AChR. These results should be considered when KB-R7943 is to be used to study Ca(2+) homeostasis in cells expressing nicotinic AChRs and the Na(+)/Ca(2+) exchanger.

Novel antimigraineur dotarizine releases Ca2+ from caffeine-sensitive Ca2+ stores of chromaffin cells

1. The novel antimigraineur, dotarizine (30 microM), increased cytosolic Ca2+ concentration, [Ca2+]c, in fura-2-loaded bovine adrenal chromaffin cells. This increase was transient, reached a peak in about 2 - 5 min (0.53+/-0.07 microM; n=19) and then declined to basal levels over a further 5 min period. 2. This transient rise of [Ca2+]c was mimicked by 1 microM thapsigargin and by 30 microM cyclopiazonic acid (CPA), but not by 30 microM flunarizine. Both thapsigargin and CPA occluded the effects of dotarizine and vice versa. 3. All three compounds suppressed the transient [Ca2+]c rises induced by caffeine (10 mM, 10 s); blockade induced by thapsigargin was irreversible and that induced by CPA and dotarizine was reversible. 4. Of the three compounds, only dotarizine blocked reversibly the [Ca2+]c spikes induced by short pulses of high K+ (70 mM, 5 s), suggesting that dotarizine blocks voltage-dependent Ca2+ channels but CPA and thapsigargin do not. 5. Dotarizine caused a gradual and reversible depletion of endoplasmic reticulum (ER) Ca2+ in chromaffin cells transfected with ER-targeted aequorin. CPA had a similar effect. 6. These data show that dotarizine shares with thapsigargin and CPA the ability to deplete Ca2+ in the ER; this novel action of dotarizine could be relevant to its prophylactic effects in migraine. Unlike thapsigargin and CPA, however, dotarizine additionally and reversibly blocks Ca2+ entry through voltage-dependent Ca2+ channels.

Differential blockade of rat alpha3beta4 and alpha7 neuronal nicotinic receptors by omega-conotoxin MVIIC, omega-conotoxin GVIA and diltiazem

Rat alpha3beta4 or alpha7 neuronal nicotinic acetylcholine receptors (AChRs) were expressed in Xenopus laevis oocytes, and the effects of various toxins and non-toxin Ca2+ channel blockers studied. Nicotinic AChR currents were elicited by 1 s pulses of dimethylphenylpiperazinium (DMPP, 100 microM) applied at regular intervals. The N/P/Q-type Ca2+ channel blocker omega-conotoxin MVIIC inhibited alpha3beta4 currents with an IC50 of 1.3 microM; the blockade was non-competitive and reversible. The alpha7 currents were unaffected. At 1 microM, omega-conotoxin GVIA (N-type Ca2+ channel blocker) inhibited by 24 and 20% alpha3beta4 and alpha7 currents, respectively. At 1 microM, omega-agatoxin IVA (a P/Q-type Ca2+ channel blocker) did not affect alpha7 currents and inhibited alpha3beta4 currents by only 15%. L-type Ca2+ channel blockers furnidipine, verapamil and, particularly, diltiazem exhibited a preferential blocking activity on alpha3beta4 nicotinic AChRs. The mechanism of alpha3beta4 currents blockade by omega-conotoxins and diltiazem differed in the following aspects: (i) the onset and reversal of the blockade was faster for toxins; (ii) the blockade by the peptides was voltage-dependent, while that exerted by diltiazem was not; (iii) diltiazem promoted the inactivation of the current while omega-toxins did not. These data show that, at concentrations currently employed as Ca2+ channel blockers, some of these compounds also inhibit certain subtypes of nicotinic AChR currents. Our data calls for caution when interpreting many of the results obtained in neurons and other cell types, where nicotinic receptor and Ca2+ channels coexist.

Effects of dotarizine and flunarizine on chromaffin cell viability and cytosolic Ca2+

Dotarizine (a novel piperazine derivative with antimigraine properties) and flunarizine (a Ca2+ channel antagonist) were compared concerning: first, their ability to cause chromaffin cell damage in vitro; second, the possible correlation of their octanol/water partition coefficients and those of another 28 compounds (i.e., Ca2+ channel antagonists, blockers of histamine H1 receptors, antimycotics, beta-adrenoceptor antagonists, neuroleptics), with their ability to cause cell damage; third, their capacity to protect the cells against the damaging effects of veratridine; and fourth, their capabilities to enhance the basal cytosolic Ca2+ concentration in fura-2-loaded single chromaffin cells, or to modify the pattern of [Ca2+]i oscillations elicited by veratridine. After 24-h exposure to 1-30 microM dotarizine, the viability of bovine adrenal chromaffin cells (measured under phase contrast or as lactate dehydrogenase, released into the medium) was similar to that of control, untreated cells; at 100 microM, 80% lactate dehydrogenase release was produced. At 1-3 microM flunarizine caused no cell damage; however 10 microM caused 20% lactate dehydrogenase release and 30 and 100 microM over 90% lactate dehydrogenase release. The time course of cell damage was considerably faster for flunarizine, in comparison to dotarizine. Out of 30 molecules tested (at 10 microM), having different octanol/water partition coefficients (log P), dotarizine was among the molecules causing no cell damage; flunarizine caused 20% cell loss, lidoflazine and verapamil over 50% cell loss, and penfluridol, draflazine, astemizole or nifedipine over 80% cell loss. No correlation was found between log P and cytotoxicity. Both dotarizine (10-30 microM) and flunarizine (3-10 microM) provided protection against veratridine-induced cell death; however, at 30 microM dotarizine afforded a pronounced protection while flunarizine enhanced the cytotoxic effects of veratridine. Dotarizine (30 microM) (but not flunarizine) caused a prompt transient elevation of the basal [Ca2+]i. Both compounds abolished the K+-induced increases of [Ca2+]i as well as the oscillations of [Ca2+]i induced by veratridine. The blocking effects of dotarizine were readily reversed after washout, while those of flunarizine were long-lasting. These differences might be relevant to the clinical use of dotarizine as an antimigraine drug.

Unmasking the functions of the chromaffin cell alpha7 nicotinic receptor by using short pulses of acetylcholine and selective blockers

Methyllycaconitine (MLA), alpha-conotoxin ImI, and alpha-bungarotoxin inhibited the release of catecholamines triggered by brief pulses of acetylcholine (ACh) (100 microM, 5 s) applied to fast-superfused bovine adrenal chromaffin cells, with IC50s of 100 nM for MLA and 300 nM for alpha-conotoxin ImI and alpha-bungarotoxin. MLA (100 nM), alpha-conotoxin ImI (1 microM), and alpha-bungarotoxin (1 microM) halved the entry of 45Ca2+ stimulated by 5-s pulses of 300 microM ACh applied to incubated cells. These supramaximal concentrations of alpha7 nicotinic receptor blockers depressed by 30% (MLA), 25% (alpha-bungarotoxin), and 50% (alpha-conotoxin ImI) the inward current generated by 1-s pulses of 100 microM ACh, applied to voltage-clamped chromaffin cells. In Xenopus oocytes expressing rat brain alpha7 neuronal nicotinic receptor for acetylcholine nAChR, the current generated by 1-s pulses of ACh was blocked by MLA, alpha-conotoxin ImI, and alpha-bungarotoxin with IC50s of 0.1 nM, 100 nM, and 1.6 nM, respectively; the current through alpha3 beta4 nAChR was unaffected by alpha-conotoxin ImI and alpha-bungarotoxin, and weakly blocked by MLA (IC50 = 1 microM). The functions of controlling the electrical activity, the entry of Ca2+, and the ensuing exocytotic response of chromaffin cells were until now exclusively attributed to alpha3 beta4 nAChR; the present results constitute the first evidence to support a prominent role of alpha7 nAChR in controlling such functions, specially under the more physiological conditions used here to stimulate chromaffin cells with brief pulses of ACh.