Mepacrine-induced inhibition of the inward current mediated by 5-HT3 receptors in rat nodose ganglion neurones

Leite L, da S. Tavares-Henriques M, Lopes LAF, Oliveira LGR, Silva-Filho SE, da Silveira APS, Cuman RKN, de S. Silva-Comar FM, Comar JF, do A. Brasileiro L, dos Santos JN, de Freitas WR, Leão KV, da Silva JG, Klein RC, Klein MHF, da S. Ramos BH, Fernandes CKC, de L. Ribas DG, Oesterreich SA. Modulation of the Serotonergic Receptosome in the Treatment of Anxiety and Depression: A Narrative Review of the Experimental Evidence

Serotonin (5-HT) receptors are found throughout central and peripheral nervous systems, mainly in brain regions involved in the neurobiology of anxiety and depression. 5-HT receptors are currently promising targets for discovering new drugs for treating disorders ranging from migraine to neuropsychiatric upsets, such as anxiety and depression. It is well described in the current literature that the brain expresses seven types of 5-HT receptors comprising eighteen distinct subtypes. In this article, we comprehensively reviewed 5-HT1-7 receptors. Of the eighteen 5-HT receptors known today, thirteen are G protein-coupled receptors (GPCRs) and represent targets for approximately 40% of drugs used in humans. Signaling pathways related to these receptors play a crucial role in neurodevelopment and can be modulated to develop effective therapies to treat anxiety and depression. This review presents the experimental evidence of the modulation of the “serotonergic receptosome” in the treatment of anxiety and depression, as well as demonstrating state-of-the-art research related to phytochemicals and these disorders. In addition, detailed aspects of the pharmacological mechanism of action of all currently known 5-HT receptor families were reviewed. From this review, it will be possible to direct the rational design of drugs towards new therapies that involve signaling via 5-HT receptors.

Quinacrine blocks PrP (106-126)-formed channels

We investigated the action of the acridine derivative, quinacrine (QC), which has been shown to act as a noncompetitive channel inhibitor. The main effects of QC are voltage- and concentration-dependent changes in the kinetics of the prion protein fragment (PrP[106-126])-formed cation channels. The current-voltage relationships show that the maximal current (I) was not affected whereas the physiologically important mean current (I') was reduced as a result of changes in channel kinetics. These findings suggest that QC acts on the open state of the channels. The half-inhibitory concentration (IC50) for the dose-dependent effects of [QC]cis on the kinetic parameters of the PrP(106-126)-formed cation channel shows a reduction in the ratios Po(QC)/Po, Fo(QC)/Fo, and To(QC)/To, whereas Tc(QC)/Tc increases. Of these ratios, Po(QC)/Po was more sensitive than the others. The corresponding IC50 for these ratios were 51, 94, 86, and 250 microM QC, respectively. The QC-induced changes in the kinetic parameters were more apparent at positive voltages. IC50 values for Po were 95, 75, and 51 microM at +20, +80, and +140 mV, respectively. The fact that QC induced changes in the kinetics of this channel, although the conductance of the channel remained unchanged, indicates that QC may bind at the mouth of the channel via a mechanism known as fast channel block. The QC-induced changes in the kinetic parameters of this channel suggest that they are pathophysiologically significant because these channels could be the mechanisms by which amyloids induce membrane damage in vivo.

Binding characteristics of GYKI-46 903, a non-competitive ligand at 5-HT3 receptors

GYKI-46903 [(+)-(5S,6R)-4-(4-fluorophenyl)-6-propionyloxy-1-aza-bicyclo[3.3.1]-non-3-ene-hydrochloride], a cognition enhancer identified as a non-competitive antagonist of 5-HT3receptors in isolated guinea-pig ileum, was investigated for allosteric action at 5-HT3 receptors in rat cortical membranes by using [3H]granisetron. Equilibrium and kinetic protocols were applied and the competitive antagonist granisetron was included as a negative control. In competition studies, both granisetron and GYKI-46 903 displaced the radioligand with K(i) values of 0.20 +/- 0.02 and 79.84 +/- 0.28 nM, respectively. The inhibition curve for GYKI-46 903 resulted in a Hill slope significantly greater than unity ( 1.37 +/- 0.11), whereas the slope for granisetron was 0.88 +/- 0.08, not different from unity. These results indicate non-competitive and competitive interactions, respectively. Scatchard analysis yielded a linear plot, suggesting a single population of binding sites with a Kd of 0.13 +/- 0.01 nM and a Bmax) of 13.15 +/- 0.34 fmol per mg of protein. Scatchard plots obtained in the absence and presence of granisetron (0.1-3 nM) or GYKI-46 903 (30-1000 nM) revealed a concentration-dependent increase in Kd values by either of these compounds. Granisetron left the Bmax unchanged, but there was a significant increase in the Bmax by GYKI-46 903, which could point to an atypical allosteric interaction. The Schild plot derived from the Kd shifts induced by granisetron was linear with a slope of 1.02, not different from unity, as expected from a competitive interaction. The Schild regression for GYKI-46 903 was linear with a slope of 1.20, deviating significantly from unity, which may also indicate an allosteric interaction. Both the association and dissociation curves of [3H]granisetron were monoexponential. The dissociation rate constant (K(-1)) and the association rate constant (K(+1)) were 0.32 +/- 0.01 min(-1) and 1.15 min(-1) x nM(-1), respectively. The dissociation driven by an excess concentration of ondansetron ( 1 microM) in the absence and presence of granisetron (0.1-3 nM) or GYKI-46 903 (30-10 000 nM) was not influenced by the compounds under study, as compared with the control, indicating the lack of an allosteric effect on the dissociation. Summing up, the binding profile of GYKI-46 903 may reflect a mixed type of action, including a negative allosteric interaction.

Membrane impermeant antioestrogens discriminate between ligand- and voltage-gated cation channels in NG108-15 cells

Native 5-HT(3) and AChR ligand-gated cation channels can be inhibited (blocked) by the non-steroidal antioestrogen tamoxifen. However, the exact site and mechanism of inhibition by tamoxifen on these channels remain unclear. We have investigated the action of the membrane impermeant quaternary derivative, ethylbromide tamoxifen (EBT), on native ligand-gated 5-HT(3) receptor channels and voltage-gated K(+) channels in NG108-15 cells using whole cell patch clamp. Extracellular EBT inhibited whole cell cationic currents of 5-HT(3) receptors with IC(50) of 0.22+/-0.4 microM (n(H)=1.05+/-0.2). The channel block was characterised by voltage independent and use independent behaviour (similar to that of tamoxifen). EBT was unable to inhibit voltage-gated K(+) currents in NG108-15 cells. This was in contrast to the inhibition by tamoxifen which, at similar concentrations, accelerated the apparent inactivation of these outward K(+) currents. The inhibition of 5-HT(3) receptors by a membrane impermeant derivative of tamoxifen supports the view that the binding site for antioestrogens is extracellular and the inhibition is not mediated through genomic/transcriptional activity.

Clozapine's antipsychotic effects do not depend on blockade of 5-HT3 receptors

Sixteen known 5-HT3 receptor blockers, including clozapine, fully or partially reverse the inhibitory effect of 1 microM GABA on [35S]TBPS binding, indicating that they are also GABA(A) antagonists, some of them selective for subsets of GABA(A) receptors. The 5-HT3 receptor blocker, ondansetron, has been reported to produce some antipsychotic and anxiolytic effects. However, no antipsychotic effects have been reported for a large number of highly potent 5-HT3 receptor blockers. Like clozapine, ondansetron partially reverses the inhibitory effect of GABA on [35S]TBPS binding. Additivity experiments suggest that ten 5-HT3 receptor blockers tested at low concentrations preferentially block subtypes of GABA(A) receptors that are among those blocked by clozapine. Wiley and Porter (29) reported that MDL-72222, the most potent GABA(A) antagonist described here, partially generalizes (71%) with clozapine in rats trained to discriminate an interoceptive clozapine stimulus, but only at a dose that severely decreases responding. Tropisetron (ICS-205,930) exhibits both GABA-positive and GABA-negative effects. R-(+)-zacopride is 6-fold more potent than S-(-)-zacopride as a GABA(A) antagonist. We conclude that the observed antipsychotic and, possibly, anxiolytic effects of some 5-HT3 receptor blockers are due to selective antagonism of certain GABA(A) receptors, and not to blockade of 5-HT3 receptors. We speculate that the anxiolytic and sedative effects of clozapine and several other antipsychotic drugs may be due to selective blockade of alpha1beta2gamma2 GABA(A) receptors which are preferentially located on certain types of GABAergic interneurons (probably parvalbumin positive). Blockade of these receptors will increase the inhibitory output of these interneurons. So far, no highly potent GABA(A) antagonists with clozapine-like selectivity have been identified. Such compounds may exhibit improved clozapine-like antipsychotic activity.

Agonist and antagonist effects of histamine H3 receptor ligands on 5-HT3 receptor-mediated ion currents in NG108-15 cells

The ability of histamine H3 receptor ligands to interact with 5-HT3 receptors in NG108-15 cells was studied using the whole cell patch clamp recording technique. Imetit, a histamine H3 receptor agonist, generated inward currents and exhibited weak partial agonist activity at the 5-HT3 receptor (EC50 = 11.8 microM). Imetit-induced currents were slow to desensitize and at a high concentration reduced in size. The histamine H3 receptor antagonists iodophenpropit and thioperamide did not generate inward currents but were able to inhibit 5-hydroxytryptamine (5-HT) responses with an IC50 of 1.57+/-0.3 microM and 13.7+/-3.5 microM, respectively. Thioperamide is probably a non-competitive antagonist which may have more than one binding site on the receptor.

Inhibition of ligand-gated cation-selective channels by tamoxifen

The nonsteroidal antioestrogen tamoxifen has been shown to block a number of voltage-gated cation-selective channels but its effect on ligand-gated cation-selective channels has not been studied. We have investigated the action of tamoxifen and the related derivative toremifene on ligand-gated cationic nicotinic acetylcholine and 5-HT3 receptor channels. Tamoxifen and toremifene both inhibited cationic currents of adult-type human muscle nicotinic acetylcholine receptors expressed in Xenopus oocytes with similar IC50 values of 1.2 +/- 0.03 microM (nH = 0.84 +/- 0.02) and 1.2 +/- 0.1 microM (nH = 1.1 +/- 0.1), respectively. Tamoxifen could also block native 5-HT3 receptors in NG108-15 neuroblastoma/glioma hybrid cells with IC50 = 0.81 +/- 0.15 microM and nH of 1.3 +/- 0.3. The characteristics of block by tamoxifen at the 5-HT3 receptor were voltage- and use-independent. The inhibition of the 5-HT-evoked currents were not overcome by increasing concentrations of 5-HT consistent with a noncompetitive mechanism of block.

Novel blockade of Ca2+ current by quinacrine in smooth muscle cells of the guinea pig

Effects of quinacrine on voltage-dependent Ca2+ channel current (ICa) were examined using whole cell voltage clamp in single smooth muscle cells isolated from vas deferens and urinary bladder and single cardiac myocytes from ventricle of the guinea pig. When ICa was elicited by depolarization from a holding potential of -60 to 0 mV for 150 msec every 15 sec in vas deferens myocytes, external application of quinacrine reduced the amplitude of ICa in a concentration-dependent manner in a range of 0.1 approximately 30 microM, and the IC50 of quinacrine was 1.3 microM. The block was at least partly removed by washout. The block of ICa by 1 microM quinacrine in vas deferens myocytes greatly depended upon the activation potentials but only slightly on the holding potentials. Use-dependent development of the block was also observed. Addition of 300 microM quinacrine to the pipette-filling solution did not significantly affect ICa. The IC50 of quinacrine for ICa block in urinary bladder myocytes was 1.1 microM and comparable to that in vas deferens. On the other hand, IC50 for the block of ICa elicited by depolarization from -45 to 0 mV in cardiac ventricular myocytes was 5.6 microM. It is concluded that quinacrine is a potent blocker of L-type Ca2+ channels in two types of smooth muscle myocytes and that the potency appeared to be approximately five times higher than that in cardiac myocytes. The action of quinacrine may be due to the direct block of Ca2+ channels from outside of the cell membrane.