Effects of fluoxetine on wild and mutant neuronal alpha 7 nicotinic receptors

Fluoksetinin sıçan torasik aort düz kasındaki vazoaktif etkileri

Amaç: Literatürdeki çalışmaların çoğu fluoksetinin kardiyo/serebrovasküler sistemler üzerindeki etkilerine odaklanmış olsa da, vazomotor etkisi hakkında bilinenler hala sınırlıdır. Bu çalışma, fluoksetinin sıçan torasik aort halkalarında düz kas üzerindeki vazoaktif etkilerini deneysel bir düzende araştırmak için planlanmıştır. Gereç ve Yöntem: 24 adet yetişkin Wistar albino rat iki gruba ayrıldı. Grup1-Endotel sağlam grup, Grup2-Endotel hasarlı grup. Servikal dislokasyon sonrası torasik aort izole edildi. Aort halkaları hemen Krebs solüsyonu içeren organ banyosu haznelerine yerleştirildi. Aort halkalarının izometrik gerimindeki değişiklikler kaydedildi. Fenilefrin 10-6M uygulandı ve kasılmalar kaydedildi. Daha sonra Grup 1'e kümülatif dozlarda (0.01, 0.1, 1, 2 mM) fluoksetin uygulandı. Grup 2'de endotel hasarı oluşturuldu. Asetilkolin 10-6M ile endotel hasarı kontrol edildikten sonra, halkalar bir saat yıkanarak ikinci doz fenilefrin hazneye eklendi. Ardından Grup 2'ye kümülatif olarak fluoksetin uygulanıp kasılmalar kaydedildi. Bulgular: Fluoksetinin doza bağımlı ana vazodilatör etkisi anlamlı olarak farklıyken [F (5.110) =72.740, p

Is the Antidepressant Activity of Selective Serotonin Reuptake Inhibitors Mediated by Nicotinic Acetylcholine Receptors?

It is generally assumed that selective serotonin reuptake inhibitors (SSRIs) induce antidepressant activity by inhibiting serotonin (5-HT) reuptake transporters, thus elevating synaptic 5-HT levels and, finally, ameliorates depression symptoms. New evidence indicates that SSRIs may also modulate other neurotransmitter systems by inhibiting neuronal nicotinic acetylcholine receptors (nAChRs), which are recognized as important in mood regulation. There is a clear and strong association between major depression and smoking, where depressed patients smoke twice as much as the normal population. However, SSRIs are not efficient for smoking cessation therapy. In patients with major depressive disorder, there is a lower availability of functional nAChRs, although their amount is not altered, which is possibly caused by higher endogenous ACh levels, which consequently induce nAChR desensitization. Other neurotransmitter systems have also emerged as possible targets for SSRIs. Studies on dorsal raphe nucleus serotoninergic neurons support the concept that SSRI-induced nAChR inhibition decreases the glutamatergic hyperstimulation observed in stress conditions, which compensates the excessive 5-HT overflow in these neurons and, consequently, ameliorates depression symptoms. At the molecular level, SSRIs inhibit different nAChR subtypes by noncompetitive mechanisms, including ion channel blockade and induction of receptor desensitization, whereas α9α10 nAChRs, which are peripherally expressed and not directly involved in depression, are inhibited by competitive mechanisms. According to the functional and structural results, SSRIs bind within the nAChR ion channel at high-affinity sites that are spread out between serine and valine rings. In conclusion, SSRI-induced inhibition of a variety of nAChRs expressed in different neurotransmitter systems widens the complexity by which these antidepressants may act clinically.

Different Classes of Antidepressants Inhibit the Rat α7 Nicotinic Acetylcholine Receptor by Interacting within the Ion Channel: A Functional and Structural Study

Several antidepressants inhibit nicotinic acetylcholine receptors (nAChRs) in a non-competitive and voltage-dependent fashion. Here, we asked whether antidepressants with a different structure and pharmacological profile modulate the rat α7 nAChR through a similar mechanism by interacting within the ion-channel. We applied electrophysiological (recording of the ion current elicited by choline, I_Ch, which activates α7 nAChRs from rat CA1 hippocampal interneurons) and in silico approaches (homology modeling of the rat α7 nAChR, molecular docking, molecular dynamics simulations, and binding free energy calculations). The antidepressants inhibited I_Ch with the order: norfluoxetine ~ mirtazapine ~ imipramine < bupropion ~ fluoxetine ~ venlafaxine ~ escitalopram. The constructed homology model of the rat α7 nAChR resulted in the extracellular vestibule and the channel pore is highly negatively charged, which facilitates the permeation of cations and the entrance of the protonated form of antidepressants. Molecular docking and molecular dynamics simulations were carried out within the ion−channel of the α7 nAChR, revealing that the antidepressants adopt poses along the receptor channel, with slightly different binding-free energy values. Furthermore, the inhibition of I_Ch and free energy values for each antidepressant-receptor complex were highly correlated. Thus, the α7 nAChR is negatively modulated by a variety of antidepressants interacting in the ion−channel.

Dual Antidepressant Duloxetine Blocks Nicotinic Receptor Currents, Calcium Signals and Exocytosis in Chromaffin Cells Stimulated with Acetylcholine

The inhibition of nicotinic acetylcholine receptors (nAChRs) has been proposed as a potential strategy to develop new antidepressant drugs. This is based on the observation that antidepressants that selectively block noradrenaline (NA) or serotonin (5-HT) reuptake also inhibit nAChRs. Dual antidepressants blocking both NA and 5-HT reuptake were proposed to shorten the delay in exerting their clinical effects; whether duloxetine, a prototype of dual antidepressants, also blocks nAChRs is unknown. Here we explored this question in bovine chromaffin cells (BCCs) that express native α₃, α₅, and α₇ nAChRs and in cell lines expressing human α₇, α₃β₄, or α₄β₂ nAChRs. We have found that duloxetine fully blocked the acetylcholine (ACh)-elicited nicotinic currents in BCCs with an IC₅₀ of 0.86 µM. Such blockade seemed to be noncompetitive, voltage dependent, and partially use dependent. The ACh-elicited membrane depolarization, the elevation of cytosolic calcium ([Ca²⁺]_c), and catecholamine release in BCCs were also blocked by duloxetine. This blockade developed slowly, and the recovery of secretion was also slow and gradual. Duloxetine did not affect Na⁺ or Ca²⁺ channel currents neither the high-K⁺-elicited [Ca²⁺]_c transients and secretion. Of interest was that in cell lines expressing human α₇, α₃β₄, and α₄β₂ nAChRs, duloxetine blocked nicotinic currents with IC₅₀ values of 0.1, 0.56, and 0.85 µM, respectively. Thus, in blocking α₇ receptors, which are abundantly expressed in the brain, duloxetine exhibited approximately 10-fold to 100- fold higher potency with respect to reported IC₅₀ values for various antidepressant drugs. This may contribute to the antidepressant effect of duloxetine.

Fluoxetine induces vasodilatation of cerebral arterioles by co-modulating NO/muscarinic signalling

Ischaemic stroke patients treated with Selective Serotonin Reuptake Inhibitors (SSRI) show improved motor, cognitive and executive functions, but the underlying mechanism(s) are incompletely understood. Here, we report that cerebral arterioles in the rat brain superfused with therapeutically effective doses of the SSRI fluoxetine showed consistent, dose-dependent vasodilatation (by 1.2 to 1.6-fold), suppressible by muscarinic and nitric oxide synthase (NOS) antagonists [atropine, NG-nitro-l-arginine methyl ester (l-NAME)] but resistant to nicotinic and serotoninergic antagonists (mecamylamine, methylsergide). Fluoxetine administered 10–30 min. following experimental vascular photo-thrombosis increased arterial diameter (1.3–1.6), inducing partial, but lasting reperfusion of the ischaemic brain. In brain endothelial b.End.3 cells, fluoxetine induced rapid muscarinic receptor-dependent increases in intracellular [Ca²⁺] and promoted albumin- and eNOS-dependent nitric oxide (NO) production and HSP90 interaction. In vitro, fluoxetine suppressed recombinant human acetylcholinesterase (rhAChE) activity only in the presence of albumin. That fluoxetine induces vasodilatation of cerebral arterioles suggests co-promotion of endothelial muscarinic and nitric oxide signalling, facilitated by albumin-dependent inhibition of serum AChE.

AChE and RACK1 promote the anti-inflammatory properties of fluoxetine

Selective serotonin reuptake inhibitors (SSRIs) show anti-inflammatory effects, suggesting a possible interaction with both Toll-like-receptor 4 (TLR4) responses and cholinergic signaling through as yet unclear molecular mechanism(s). Our results of structural modeling support the concept that the antidepressant fluoxetine physically interacts with the TLR4-myeloid differentiation factor-2 complex at the same site as bacterial lipopolysaccharide (LPS). We also demonstrate reduced LPS-induced pro-inflammatory interleukin-6 and tumor necrosis factor alpha in human peripheral blood mononuclear cells preincubated with fluoxetine. Furthermore, we show that fluoxetine intercepts the LPS-induced decreases in intracellular acetylcholinesterase (AChE-S) and that AChE-S interacts with the nuclear factor kappa B (NFκB)-activating intracellular receptor for activated C kinase 1 (RACK1). This interaction may prevent NFκB activation by residual RACK1 and its interacting protein kinase PKCβII. Our findings attribute the anti-inflammatory properties of SSRI to surface membrane interference with leukocyte TLR4 activation accompanied by intracellular limitation of pathogen-inducible changes in AChE-S, RACK1, and PKCβII.

Lack of behavioral and cognitive effects of chronic ethosuximide and gabapentin treatment in the Ts65Dn mouse model of Down syndrome

The Ts65Dn (TS) mouse model of Down syndrome (DS) displays a number of behavioral, neuromorphological and neurochemical phenotypes of the syndrome. Altered GABAergic transmission appears to contribute to the mechanisms responsible for the cognitive impairments in TS mice. Increased functional expression of the trisomic gene encoding an inwardly rectifying potassium channel, subfamily J, member 6 (KCNJ6) has been reported in DS and TS mice, along with the consequent impairment in GAB Aergic function. Partial display of DS phenotypes in mice harboring a single trisomy of Kcnj6 provides compelling evidence for a functional role of increased channel expression in some of the abnormal neurological phenotypes found in DS. Notably, the antiepileptic drug (AED) ethosuximide (ETH), but not other AEDs such as gabapentin (GAB), is known to inhibit KCNJ6 channels in mice. Here, we report the effect of chronic ETH and GAB on the behavioral and cognitive phenotypes of TS and disomic control (CO) mice. Neither drug significantly affected sensorimotor abilities, motor coordination or spontaneous activity in TS and CO mice. Also, ETH and GAB did not induce anxiety in the open field or plus maze tests, did not alter performance in the Morris water maze, and did not affect cued - or context - fear conditioning. Our results thus suggest that KCNJ6 may not be a promising drug target candidate in DS. As a corollary, they also show that long-term use of ETH and GAB is devoid of adverse behavioral and cognitive effects.

The nicotinic acetylcholine receptor as a target for antidepressant drug development

An important new area of antidepressant drug development involves targeting the nicotinic acetylcholine receptor (nAChR). This receptor, which is distributed widely in regions of the brain associated with depression, is also implicated in other important processes that are relevant to depression, such as stress and inflammation. The two classes of drugs that target nAChRs can be broadly divided into mecamylamine- and cytisine-based compounds. These drugs probably exert their effects via antagonism at α4β2 nAChRs, and strong preclinical data support the antidepressant efficacy of both classes when used in conjunction with other primary antidepressants (e.g., monoamine reuptake inhibitors). Although clinical data remain limited, preliminary results in this area constitute a compelling argument for further evaluation of the nAChR as a target for future antidepressant drug development.

Fluoxetine effects on sheepshead minnow (Cyprinodon variegatus) locomotor activity

Fluoxetine (FLX), a selective serotonin reuptake inhibitor, is among the top 100 drugs prescribed annually in the United States and the United Kingdom and is one of many pharmaceutical products that have been detected in global surface waters. Our study used sublethal concentrations to assess the impact of FLX exposure on sheepshead minnow (Cyprinodon variegatus) locomotor behavior. Exposures lasted for 56 hours, and fish were recorded for locomotor behavior assessment at six timepoints between 1-56 h post-dose. Behavior was recorded to quantify locomotor activity using line crossing counts. Animals treated with 300 μg L(-1) FLX exhibited reduced locomotor activity at 1, 25, 32, 49 and 56 h post-dose. An EC(25) value of 2 μg L(-1) (lower and upper 95 % confidence limits at 1.3, 43 μg L(-1), respectively) was determined for locomotor activity at 32 h of exposure. Changes in locomotor activity due to FLX exposure may have implications for the ecological response of populations to other natural and anthropogenic stressors.

Bimodal effects of fluoxetine on cerebral nitrergic neurogenic vasodilation in porcine large cerebral arteries

Fluoxetine-induced relaxation of the smooth muscle of small cerebral arteries is thought beneficial in treating mental disorders. The present study was designed to examine effect of fluoxetine on neurogenic nitrergic vasodilation in large cerebral arteries, using in vitro tissue myography, techniques of electrophysiology, calcium imaging and biochemistry. In isolated porcine endothelium-denuded basilar arteries in the presence of U-46619-induced active muscle tone, fluoxetine in low concentration (<0.03 μM) significantly enhanced nicotine- and choline-induced relaxations. The vasorelaxation, however, was blocked by higher concentration of fluoxetine (>0.3 μM) with maximum inhibition at 3 μM. At this concentration, fluoxetine did not affect the basal tone or vasorelaxations induced by transmural nerve stimulation, sodium nitroprusside, or isoproterenol. Furthermore, fluoxetine exclusively blocked nicotine-induced inward currents and calcium influx in cultured neurons of rat superior cervical ganglion and Xenopus oocytes expressing human α7-, α3β2-, or α4β2-nicotinic acetylcholine receptors (nAChRs). In addition, fluoxetine at 0.03 μM and 3 μM significantly enhanced and blocked, respectively, nicotine-induced norepinephrine (NE) release from cerebral perivascular sympathetic nerves. These results indicate that fluoxetine via axo-axonal interaction mechanism exhibits bimodal effects on nAChR-mediated neurogenic nitrergic dilation of basilar arteries. Fluoxetine in high concentrations decreases while in low concentrations it increases neurogenic vasodilation. These results from in vitro experimentation suggest that optimal concentrations of fluoxetine which increase or minimally affect neurogenic vasodilation indicative of regional cerebral blood flow may be important consideration in treating mental disorders.

Effect of glibenclamide on antinociceptive effects of antidepressants of different classes

OBJECTIVES

The purpose of this work was to determine whether the intraperitoneal administration of glibenclamide as a K ATP channel blocker could have an effect on the antinociceptive effects of antidepressants with different mechanisms of action.

METHODS

Three antidepressant drugs, amitriptyline as a dual-action, nonselective inhibitor of noradrenaline and a serotonin reuptake inhibitor, fluvoxamine as a selective serotonin reuptake inhibitor and maprotiline as a selective noradrenaline reuptake inhibitor, were selected, and the effect of glibenclamide on their antinociceptive activities was assessed in male Swiss mice (25-30 g) using a formalin test.

DISCUSSION

None of the drugs affected acute nociceptive responses during the first phase. Amitriptyline (5, 10 mg/ kg), maprotiline (10, 20 mg/kg) and fluvoxamine (20 and 30 mg/kg) effectively inhibited pain induction caused by the second phase of the formalin test. Glibenclamide (5 mg/kg) alone did not alter licking behaviors based on a comparison with the control group. However, the pretreatment of animals with glibenclamide (10 and 15 mg/kg) partially reversed the antinociceptive effects of fluvoxamine but not those of maprotiline. In addition, the highest dose of glibenclamide (15 mg/kg) partially prevented the analgesic effect of amitriptyline.

CONCLUSION

Therefore, it seems that adenosine triphosphate-dependent potassium channels have a major role in the analgesic activity of amitriptyline and fluvoxamine.

Interaction of selective serotonin reuptake inhibitors with neuronal nicotinic acetylcholine receptors

We compared the interaction of fluoxetine and paroxetine, two selective serotonin reuptake inhibitors (SSRIs), with the human (h) alpha4beta2, alpha3beta4, and alpha7 nicotinic acetylcholine receptors (AChRs) in different conformational states, using Ca(2+) influx, radioligand binding, and molecular docking approaches. The results established that (1) fluoxetine was more potent than paroxetine in inhibiting agonist-activated Ca(2+) influx on halpha4beta2 and halpha7 AChRs, whereas the potency of both SSRIs was practically the same in the halpha3beta4 AChR. [corrected] (2) SSRIs bind to the [(3)H]imipramine locus with a [corrected] higher affinity when the AChRs are in the desensitized states compared to the resting states. (3) The different receptor specificity for fluoxetine determined by their inhibitory potencies or binding affinities suggests different modes of interaction when the AChR is in the closed or activated state. (4) Neutral and protonated fluoxetine interacts with a binding domain located in the middle of the AChR ion channel. In conclusion, SSRIs inhibit the most important neuronal AChRs with potencies and affinities that are clinically relevant by binding to a luminal site that is shared with tricyclic antidepressants.

Nicotinic receptors: allosteric transitions and therapeutic targets in the nervous system

Nicotinic receptors - a family of ligand-gated ion channels that mediate the effects of the neurotransmitter acetylcholine - are among the most well understood allosteric membrane proteins from a structural and functional perspective. There is also considerable interest in modulating nicotinic receptors to treat nervous-system disorders such as Alzheimer's disease, schizophrenia, depression, attention deficit hyperactivity disorder and tobacco addiction. This article describes both recent advances in our understanding of the assembly, activity and conformational transitions of nicotinic receptors, as well as developments in the therapeutic application of nicotinic receptor ligands, with the aim of aiding novel drug discovery by bridging the gap between these two rapidly developing fields.

The effect of fluoxetine on bone regeneration in rat calvarial bone defects

OBJECTIVE

Fluoxetine is one of the most common medications used for the treatment of depression. Recent studies have demonstrated the possible effect of this drug on bone. The purpose of this study was to evaluate the effect of flouxetine on bone in Sprague-Dawley rats randomly divided into 3 groups (n = 7).

STUDY DESIGN

Two calvarial defects, 5 mm diameter, were prepared in parietal bone. The left defects were filled with natural bovine bone mineral (NBBM) and the right defects were left untreated. The first group did not receive fluoxetine. The second group and the third group received 7.5 mg and 15 mg flouxetine, respectively, daily by gastric gavage. The animals were killed 8 weeks after surgery, and the amount of bone regeneration was evaluated using histometric analysis.

RESULTS

The defects showed dose-dependent increase in bone formation. The bone fill length had statistical significant differences between NBBM/flouxetine (15 mg) group and other groups (P < .05). The presence of the NBBM had positive effect on the bone formation in all groups in so far as the maximum amount of the increasing effect was seen in those rats filled with NBBM that received 15 mg flouxetine (P < .05). The minimum bone length in fluoxetine-treated defects was seen in 7.5 mg defects (0.36 mm)

CONCLUSION

Fluoxetine may improve the amount of bone regeneration in the rat calvarial defects.

Behavioural and pharmacological mechanisms of bupropion's anti-smoking effects: recent preclinical and clinical insights

Ongoing studies continue to explore the behavioural and pharmacological effects of bupropion in smoking cessation studies and animal models of nicotine dependence. In the present review, the components of nicotine dependence that form the most likely targets of bupropion are identified within the context of an expanding preclinical and clinical literature regarding the anti-addictive properties of bupropion. Second, preclinical and clinical data that implicate specific pharmacological modes of action of bupropion in mediating the anti-smoking effects of the compound are discussed. Third, it is suggested that the unique mixed pharmacological profile of bupropion provides (1) attenuation of the multiple negative consequences of withdrawal via blockade of dopamine and noradrenaline reuptake; (2) replacement of the reward-facilitating and subjective effects of nicotine via blockade of dopaminergic reuptake; (3) attenuation of the rewarding effects of acute nicotine by nicotinic acetylcholine receptor blockade. The importance of species differences in bupropion metabolism in the interpretation of preclinical studies is highlighted. Finally, future studies are suggested to address identified gaps in the knowledge: most importantly, to provide stronger evidence for the role of noradrenaline reuptake inhibition in bupropion-induced attenuation of nicotine withdrawal. Future studies aimed at providing more evidence for the three-fold nature of the anti-smoking effects of bupropion are also suggested, along with the possibility of utilizing adjunct therapies to improve smoking cessation rates.

Direct inhibitory effect of fluoxetine on N-methyl-D-aspartate receptors in the central nervous system

BACKGROUND

Data accumulated in the last decade indicate that N-methyl-D-aspartate (NMDA) receptors might be involved in the pathophysiology of depression and the mechanism of action of antidepressants, although a direct inhibitory effect has been reported only in connection with tricyclic compounds, which interact with a wide range of receptors.

METHODS

Using whole-cell patch-clamp recording in rat cortical cell cultures, we investigated whether the selective serotonin reuptake inhibitor fluoxetine, which has a much better adverse effect profile, has a direct effect on NMDA receptors, and we compared its action to that of the tricyclic desipramine.

RESULTS

Both desipramine (concentration that causes 50% inhibition (IC(50)) = 3.13 microM) and fluoxetine (IC(50) = 10.51 microM) inhibited NMDA-evoked currents with similar efficacy in the clinically relevant low micromolar concentration range. However, in contrast to desipramine, the inhibition by fluoxetine was not voltage-dependent, and fluoxetine partially preserved its ability to associate with NMDA receptor in the presence of Mg(2+), suggesting different binding sites for the two drugs.

CONCLUSIONS

The fact that different classes of antidepressants were found to be low-affinity NMDA antagonists suggests that direct inhibition of NMDA receptors may contribute to the clinical effects of antidepressants.

Fluoxetine treatment increases trabecular bone formation in mice

Mounting evidence exists for the operation of a functional serotonin (5-HT) system in osteoclasts and osteoblasts, which involves both receptor activation and 5-HT reuptake. In previous work we showed that the serotonin transporter (5-HTT) is expressed in osteoclasts and that its activity is required by for osteoclast differentiation in vitro. The purpose of the current study was to determine the effect of treatment with fluoxetine, a specific serotonin reuptake inhibitor, on bone metabolism in vivo. Systemic administration of fluoxetine to Swiss-Webster mice for 6 weeks resulted in increased trabecular BV and BV/TV in femurs and vertebrae as determined by micro-computed tomography (microCT). This correlated with an increase in trabecular number, connectivity, and decreased trabecular spacing. Fluoxetine treatment also resulted in increased volume in vertebral trabecular bone. However, fluoxetine-treated mice were not protected against bone loss after ovariectomy, suggesting that its anabolic effect requires the presence of estrogen. The effect of blocking the 5-HTT on bone loss following an LPS-mediated inflammatory challenge was also investigated. Subcutaneous injections of LPS over the calvariae of Swiss-Webster mice for 5 days resulted in increased numbers of osteoclasts and net bone loss, whereas new bone formation and a net gain in bone mass was seen when LPS was given together with fluoxetine. We conclude that fluoxetine treatment in vivo leads to increased bone mass under normal physiologic or inflammatory conditions, but does not prevent bone loss associated with estrogen deficiency. These data suggest that commonly used anti-depressive agents may affect bone mass.

Nicotinic acetylcholine receptor antagonistic property of the selective dopamine uptake inhibitor, GBR-12909 in rat hippocampal slices

Previously we found that inhibitors of noradrenaline (NA) and/or 5-HT reuptake are able to inhibit neuronal nicotinic acetylcholine receptors (nAChRs) in the CNS most probably by a channel blocker-type mechanism. The aim of our study was to clarify whether selective dopamine uptake inhibitors also possess this property, therefore we investigated the effect of GBR-12909 on the nicotine-evoked release of [3H]NA from rat hippocampal slices. GBR-12909, similar to selective NA and 5-HT uptake blockers, inhibited the nicotine-evoked release with an IC50 of 2.32 microM. The ability of monoamine uptake blockers to inhibit nicotine-evoked [3H]NA release (IC50) and NA reuptake (Ki) showed no correlation, indicating that the NA uptake system is not involved in the inhibition of the response to nicotine. Previously we have shown in whole cell patch clamp experiments, that GBR-12909, depending on the stimulation pattern, inhibits Na+-currents with an IC50 in the 6-35 microM concentration range [Mike A, Karoly R, Vizi ES, Kiss JP (2003) Inhibitory effect of the DA uptake blocker GBR-12909 on sodium channels of hippocampal neurons. Neuroreport 14:1945-1949]. To study whether the inhibition of Na+-channels is involved in the action of GBR-12909 on the nicotine-evoked [(3)H]NA release, we compared the effect of GBR-12909 and the Na(+)-channel blocker tetrodotoxin (TTX) on the electrical stimulation- and nicotine-evoked response. TTX prevented the release of [3H]NA induced by both types of stimulation, whereas GBR-12909 inhibited only the nicotine-induced response, indicating that under our experimental conditions the target of GBR-12909 is not the Na+-channel. These data indicate that the selective DA uptake inhibitor GBR-12909 is able to inhibit nAChRs, that is, the nAChR antagonistic property of monoamine uptake inhibitors is independent of their selectivity. The fact that monoamine uptake inhibitors with different chemical structure and selectivity are able to inhibit nAChRs may reveal some common properties of nicotinic receptors and monoamine uptake carriers.

Fluoxetine as a potential pharmacotherapy for methamphetamine dependence: studies in mice

The monoamine transporters are the main targets of psychostimulant drugs, including methamphetamine (METH) and cocaine. Interestingly, the rewarding effects of cocaine are retained in dopamine transporter (DAT) knockout (KO) mice, while serotonin transporter (SERT) and DAT double KO mice do not exhibit conditioned place preference (CPP) to cocaine. These data suggest that SERT inhibition decreases the rewarding effects of psychostimulants. To further test this hypothesis, in the present study, we investigated the effects of intraperitoneal (i.p.) injections of 20 mg/kg fluoxetine, a selective serotonin reuptake inhibitor (SSRI), on 2 mg/kg METH (i.p.) CPP and locomotor sensitization to 1 mg/kg METH (i.p.) in C57BL/6J mice. Fluoxetine treatment before both the conditioning and preference tests abolished METH CPP. A two-way analysis of variance (ANOVA) revealed that METH CPP tended to be lower in mice pretreated with fluoxetine before the preference test than in control mice pretreated with saline before the preference test. Furthermore, pretreatment with fluoxetine had inhibitory effects on METH-induced locomotor sensitization. These results suggest that fluoxetine, a widely used medication for depression, may be also a useful tool for treating METH dependence.

Monoamine uptake inhibitors block α7-nAChR-mediated cerebral nitrergic neurogenic vasodilation

We have proposed that activation of cerebral perivascular sympathetic α7-nicotinic acetylcholine receptors (α7-nAChRs) by nicotinic agonists releases norepinephrine, which then acts on parasympathetic nitrergic nerves, resulting in release of nitric oxide and vasodilation. Using patch-clamp electrophysiology, immunohistochemistry, and in vitro tissue bath myography, we tested this axo-axonal interaction hypothesis further by examining whether blocking norepinephrine reuptake enhanced α7-nAChR-mediated cerebral nitrergic neurogenic vasodilation. The results indicated that choline- and nicotine-induced α7-nAChR-mediated nitrergic neurogenic relaxation in endothelium-denuded isolated porcine basilar artery rings was enhanced by desipramine and imipramine at lower concentrations (0.03–0.1 μM) but inhibited at higher concentrations (0.3–10 μM). In cultured superior cervical ganglion (SCG) neurons of the pig and rat, choline (0.1–30 mM)-evoked inward currents were reversibly blocked by 1–30 μM mecamylamine, 1–30 μM methyllycaconitine, 10–300 nM α-bungarotoxin, and 0.1–10 μM desipramine and imipramine, providing electrophysiological evidence for the presence of similar functional α7-nAChRs in cerebral perivascular sympathetic neurons of pigs and rats. In α7-nAChR-expressing Xenopus oocytes, choline-elicited inward currents were attenuated by α-bungarotoxin, imipramine, and desipramine. These monoamine uptake inhibitors appeared to directly block the α7-nAChR, resulting in diminished nicotinic agonist-induced cerebral nitrergic vasodilation. The enhanced nitrergic vasodilation by lower concentrations of monoamine uptake inhibitors is likely due to a greater effect on monoamine uptake than on α7-nAChR blockade. These results further support the hypothesis of axo-axonal interaction in nitrergic regulation of cerebral vascular tone.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

Molecular mechanisms and binding site locations for noncompetitive antagonists of nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors are pentameric proteins that belong to the Cys-loop receptor superfamily. Their essential mechanism of functioning is to couple neurotransmitter binding, which occurs at the extracellular domain, to the opening of the membrane-spanning cation channel. The function of these receptors can be modulated by structurally different compounds called noncompetitive antagonists. Noncompetitive antagonists may act at least by two different mechanisms: a steric and/or an allosteric mechanism. The simplest idea representing a steric mechanism is that the antagonist molecule physically blocks the ion channel. On the other hand, there exist distinct allosteric mechanisms. For example, noncompetitive antagonists may bind to the receptor and stabilize a nonconducting conformational state (e.g., resting or desensitized state), and/or increase the receptor desensitization rate. Barbiturates, dissociative anesthetics, antidepressants, and neurosteroids have been shown to inhibit nicotinic receptors by allosteric mechanisms and/or by open- and closed-channel blockade. Receptor modulation has proved to be highly complex for most noncompetitive antagonists. Noncompetitive antagonists may act by more than one mechanism and at distinct sites in the same receptor subtype. The binding site location for one particular molecule depends on the conformational state of the receptor. The mechanisms of action and binding affinities of noncompetitive antagonists differ among nicotinic receptor subtypes. Knowledge of the structure of the nicotinic acetylcholine receptor, the location of its noncompetitive antagonist binding sites, and the mechanisms of inhibition will aid the design of new and more efficacious drugs for treatment of neurological diseases.

Paradoxical rate-dependent effect of fluoxetine on captivity-induced stereotypies in bank voles

Treatment of stereotypies in human and animal clinics is still empirical due to our incomplete understanding of underlying mechanisms. As a consequence, experimental studies in controlled laboratory conditions are necessary. Bank voles (Clethrionomys glareolus), housed in barren laboratory cages, often develop stereotyped up-and-down jumping even before the age of 1 month. The aim of this study was to investigate how captivity-induced stereotypies respond to fluoxetine (Prozac), a selective serotonin reuptake inhibitor. During a control period of 10 days saline was administered daily s.c. to 6-month-old bank voles (n=24). In the subsequent 30 days, 16 animals received a dose of 10-mg/kg s.c. daily while 8 other animals were injected a dose of 20-mg/kg fluoxetine. Stereotypies, general activity, food intake and body weight were measured. Dose-related and paradoxical rate-dependent effects were found on stereotypies. After 10 days of treatment, the stereotypy levels of the animals with high pre-drug stereotypy rates decreased in contrast with those of low stereotypers which increased. No effect was found on general activity and food intake. Weekly weighing revealed an increase of weight during the total drug period and a return to pre-drug levels during the week thereafter.

Combined actions of zinc and fluoxetine on nicotinic acetylcholine receptors

Zinc and nicotinic acetylcholine receptors (nAChRs) seem to be associated with major depression, and some antidepressants, including fluoxetine (Prozac), antagonize nAChRs. Therefore, a study was made of the modulation of neuronal alpha4beta4 and muscle alpha1beta1gammadelta nAChRs, expressing in oocytes, by the combined action of zinc and fluoxetine. At a holding potential of -60 mV, 200 microM zinc increased by 361% the currents elicited by acetylcholine (ACh currents) for alpha4beta4 and by 182% for alpha1beta1gammadelta nAChRs. In contrast, 5 microM fluoxetine reduced the ACh currents to 31% for alpha4beta4 and to 45% for alpha1beta1gammadelta nAChRs. Additionally, fluoxetine reduced more the ACh currents in the presence of zinc: to 17% for alpha4beta4 and to 19% for alpha1beta1gammadelta nAChRs, and after washing out the fluoxetine the ACh current did not recover its zinc-potentiated value. Moreover, when ACh-activated nAChRs were exposed first to fluoxetine and then zinc was added, the potentiating effect of zinc was very small for muscle nAChRs and was nil for neuronal receptors. Thus, the inhibiting effect of fluoxetine prevails over the potentiating action of zinc. Finally, the effects of both zinc and fluoxetine were voltage independent, indicating that these substances interact outside the ion channel. As fluoxetine nullifies the effects of zinc, it appears that both substances interact in the same site. These results should help understand better the roles played by zinc, antidepressants, nAChRs and their combination in brain functions and in the treatment of depression.

Coupling of human nicotinic acetylcholine receptors alpha 7 to calcium channels in GH3 cells

The neuronal nicotinic acetylcholine receptor alpha7 (nAChR alpha7) may be involved in cognitive deficits in Schizophrenia and Alzheimer's disease. A fast pharmacological characterization of homomeric alpha7 receptors is mostly hampered by their low functional expression levels in heterologous expression systems. In the present study expression of homomeric nAChR alpha7 was achieved in GH3 rat pituitary cells. Alpha7 subunits were heterologously expressed as components of [125I]-labeled alpha-bungarotoxin binding nAChRs (Bmax: 1.2 pmol/mg protein). Function of the expressed alpha7 ion channels was assessed by patch-clamp recording and calcium imaging. While acetylcholine-induced currents desensitized within much less than 1 s, calcium-sensitive fluorescence transients peaked after 5-10 s and returned to background levels within 30 s only. The fluorescence signal was blocked by isradipine and removal of extracellular sodium indicated that in these cells opening of rapidly desensitizing alpha7 nAChR triggers calcium influx via voltage-gated, DHP-sensitive calcium channels. In this cellular system, agonists revealed the following rank order of potency: epibatidine>anatoxin A>AAR17779>ABT-594>DMPP>nicotine>GTS-21>cytisine>ABT-418>acetylcholine>choline>ABT-089. All of the signals were inhibited by the alpha7 antagonists alpha-bungarotoxin (pIC50: 7.4) and methyllycaconitine (pIC50: 7.8). Further, marketed antidepressants showed antagonistic activity with the following rank order of potency: fluoxetine>imipramine>paroxetine>sertraline. These data illustrate that coupling to voltage-gated calcium channels allows a rapid and reliable functional examination of nAChR alpha7.

Inhibition of G protein-activated inwardly rectifying K+ channels by various antidepressant drugs

G protein-activated inwardly rectifying K+ channels (GIRK, also known as Kir3) are activated by various G protein-coupled receptors. GIRK channels play an important role in the inhibitory regulation of neuronal excitability in most brain regions and the heart rate. Modulation of GIRK channel activity may affect many brain functions. Here, we report the inhibitory effects of various antidepressants: imipramine, desipramine, amitriptyline, nortriptyline, clomipramine, maprotiline, and citalopram, on GIRK channels. In Xenopus oocytes injected with mRNAs for GIRK1/GIRK2, GIRK2 or GIRK1/GIRK4 subunits, the various antidepressants tested, except fluvoxamine, zimelidine, and bupropion, reversibly reduced inward currents through the basal GIRK activity at micromolar concentrations. The inhibitions were concentration-dependent with various degrees of potency and effectiveness, but voltage- and time-independent. In contrast, Kir1.1 and Kir2.1 channels in other Kir channel subfamilies were insensitive to all of the drugs. Furthermore, GIRK current responses activated by the cloned A1 adenosine receptor were similarly inhibited by the tricyclic antidepressant desipramine. The inhibitory effects of desipramine were not observed when desipramine was applied intracellularly, and were not affected by extracellular pH, which changed the proportion of the uncharged to protonated desipramine, suggesting its action from the extracellular side. The GIRK currents induced by ethanol were also attenuated in the presence of desipramine. Our results suggest that inhibition of GIRK channels by the tricyclic antidepressants and maprotiline may contribute to some of the therapeutic effects and adverse side effects, especially seizures and atrial arrhythmias in overdose, observed in clinical practice.

Inhibitory effect of the DA uptake blocker GBR 12909 on sodium channels of hippocampal neurons

The effect of the selective dopamine uptake inhibitor GBR 12909 on TTX-sensitive sodium channels of cultured hippocampal neurons was investigated using whole cell patch-clamp technique. GBR 12909 dose-dependently inhibited sodium currents evoked by trains of depolarizing pulses with an IC50 of 6.3 microM. A weaker inhibition (IC50 = 17-35 microM) could be observed when currents were evoked by either single pulse depolarization or from hyperpolarized holding membrane potential. These data indicate that the extent of inhibition caused by GBR 12909 depends on the physiological activity pattern of neurons. Our results suggest that caution is needed for the interpretation of data when GBR 12909 is used for the inhibition of dopamine uptake at concentrations above the submicromolar range.

Inhibition of G protein-activated inwardly rectifying K+ channels by fluoxetine (Prozac)

1. The effects of fluoxetine, a commonly used antidepressant drug, on G protein-activated inwardly rectifying K(+) channels (GIRK, Kir3) were investigated using Xenopus oocyte expression assays. 2. In oocytes injected with mRNAs for GIRK1/GIRK2, GIRK2 or GIRK1/GIRK4 subunits, fluoxetine reversibly reduced inward currents through the basal GIRK activity. The inhibition by fluoxetine showed a concentration-dependence, a weak voltage-dependence and a slight time-dependence with a predominant effect on the instantaneous current elicited by voltage pulses and followed by slight further inhibition. Furthermore, in oocytes expressing GIRK1/2 channels and the cloned Xenopus A(1) adenosine receptor, GIRK current responses activated by the receptor were inhibited by fluoxetine. In contrast, ROMK1 and IRK1 channels in other Kir channel subfamilies were insensitive to fluoxetine. 3. The inhibitory effect on GIRK channels was not obtained by intracellularly applied fluoxetine, and not affected by extracellular pH, which changed the proportion of the uncharged to protonated fluoxetine, suggesting that fluoxetine inhibits GIRK channels from the extracellular side. 4. The GIRK currents induced by ethanol were also attenuated in the presence of fluoxetine. 5. We demonstrate that fluoxetine, at low micromolar concentrations, inhibits GIRK channels that play an important role in the inhibitory regulation of neuronal excitability in most brain regions and the heart rate through activation of various G-protein-coupled receptors. The present results suggest that inhibition of GIRK channels by fluoxetine may contribute to some of its therapeutic effects and adverse side effects, particularly seizures in overdose, observed in clinical practice.

Nicotinic acetylcholine receptors as targets for antidepressants

While the monoamine deficiency hypothesis of depression is still most commonly used to explain the actions of antidepressant drugs, a growing body of evidence has accumulated that is not adequately explained by the hypothesis. This article draws attention to contributions from another apparently common pharmacological property of antidepressant medications--the inhibition of nicotinic acetylcholine receptors (nAChR). Evidence is presented suggesting the hypercholinergic neurotransmission, which is associated with depressed mood states, may be mediated through excessive neuronal nicotinic receptor activation and that the therapeutic actions of many antidepressants may be, in part, mediated through inhibition of these receptors. In support of this hypothesis, preliminary evidence is presented suggesting that the potent, centrally acting nAChR antagonist, mecamylamine, which is devoid of monoamine reuptake inhibition, may reduce symptoms of depression and mood instability in patients with comorbid depression and bipolar disorder. If this hypothesis is supported by further preclinical and clinical research, nicotinic acetylcholine receptor antagonists may represent a novel class of therapeutic agents for treating mood disorders.

Effects of clomipramine on neuronal nicotinic acetylcholine receptors

The action of the tricyclic antidepressant clomipramine on membrane currents elicited by acetylcholine was studied in Xenopus oocytes expressing neuronal alpha2beta4 nicotinic acetylcholine receptors. Clomipramine inhibited the acetylcholine responses rapidly and reversibly, with a similar IC(50) when the oocytes were preincubated with clomipramine (1.3+/-0.2 microM) or when they were exposed simultaneously with acetylcholine and clomipramine (1.5+/-0.3 microM). The EC(50) was 39.9+/-2.1 microM for acetylcholine alone and 65.7+/-3.6 microM for acetylcholine in the presence of 2 microM clomipramine. The inhibitory effect of clomipramine was weakly voltage-dependent, with an electric distance of approximately 0.14. Moreover, clomipramine increased the rate of decay of currents elicited by acetylcholine. From all of these, we conclude that clomipramine reversibly and noncompetitively regulates neuronal alpha2beta4 nicotinic acetylcholine receptors by blocking the open receptor-channel complex at a site close to the extracellular vestibule of the channel. The actions of clomipramine on neuronal nicotinic acetylcholine receptors may play an important role in the treatment of mental depression and other mood disorders.

Inhibition of neuronal nicotinic acetylcholine receptors by La(3+)

A study was made of the effects of La(3+) on neuronal alpha 2 beta 4 nicotinic acetylcholine receptors expressed in Xenopus oocytes. La(3+) by itself (up to 10 microM) did not elicit significant membrane currents. However, La(3+) reversibly inhibited the ionic currents induced by acetylcholine (IC(50)=13.5+/-4.3 microM). When La(3+) and acetylcholine were simultaneously applied onto an oocyte, the level of inhibition of the acetylcholine response was the same as when the oocyte was first preincubated with La(3+) and then exposed to acetylcholine plus La(3+). In the presence of La(3+), the EC(50) decreased from 43.8+/-6.4 to 26.5+/-5.1 microM, suggesting a small increase in the affinity of acetylcholine for the receptors through a noncompetitive mechanism. The inhibition of acetylcholine response was independent of the membrane potential. From these results we conclude that La(3+) regulates nicotinic receptors, reversibly and noncompetitively, presumably by inhibiting allosterically the receptor through interactions at an external domain of the receptor complex.

Emerging targets for the treatment of depressive disorder

New agents offering novel mechanisms of action are required in the treatment of depressive disorder. Established agents targeting monoamine systems are unsatisfactory because of full and partial treatment resistance, delay in the onset of their effect and the occurrence of side effects. The monoamine hypothesis of depression is now recognised to provide an incomplete explanation of the pathophysiology of depression. New theories have recently developed and new targets for treatment have emerged. We briefly review some important candidate systems and therapeutic targets in depression: the hypothalamic-pituitary-adrenal axis (HPA) and the glucocorticoid and corticotrophin-releasing factor receptors, synaptic plasticity and neurotrophins and the N-methyl-D-aspartate (NMDA) receptor. The putative role of the neuropeptides substance P and neuropeptide Y, the nicotinic system and the potential therapeutic benefits of cannabinoids are also reviewed. Vagal nerve stimulation (VNS) and transcranial magnetic stimulation, serendipitous advances in treatment, are discussed briefly.

Antagonism of nicotinic acetylcholine receptors by inhibitors of monoamine uptake

A study was made of the effects of several monoamine-uptake inhibitors on membrane currents elicited by acetylcholine (ACh-currents) generated by rat neuronal alpha2beta4 and mouse muscle nicotinic acetylcholine receptors (AChRs) expressed in Xenopus laevis oocytes. For the two types of receptors the monoamine-uptake inhibitors reduced the ACh-currents albeit to different degrees. The order of inhibitory potency was norfluoxetine > clomipramine > indatraline > fluoxetine > imipramine > zimelidine > 6-nitro-quipazine > trazodone for neuronal alpha2beta4 AChRs, and norfluoxetine > fluoxetine > imipramine > clomipramine > indatraline > zimelidine > trazodone > 6-nitro-quipazine for muscle AChRs. Thus, the most potent inhibitor was norfluoxetine, whilst the weakest ones were trazodone, 6-nitro-quipazine and zimelidine. Effects of the tricyclic antidepressant imipramine were studied in more detail. Imipramine inhibited reversibly and non-competitively the ACh-current with a similar inhibiting potency for both neuronal alpha2beta4 and muscle AChRs. The half-inhibitory concentrations of imipramine were 3.65 +/- 0.30 microM for neuronal alpha2beta4 and 5.57 +/- 0.19 microM for muscle receptors. The corresponding Hill coefficients were 0.73 and 1.2 respectively. The inhibition of imipramine was slightly voltage-dependent, with electric distances of approximately 0.10 and approximately 0.12 for neuronal alpha2beta4 and muscle AChRs respectively. Moreover, imipramine accelerated the rate of decay of ACh- currents of both muscle and neuronal AChRs. The ACh-current inhibition was stronger when oocytes, expressing neuronal alpha2beta4 or muscle receptors, were preincubated with imipramine alone than when it was applied after the ACh-current had been generated, suggesting that imipramine acts also on non-activated or closed AChRs. We conclude that monoamine-uptake inhibitors reduce ACh-currents and that imipramine regulates reversibly and non- competitively neuronal alpha2beta4 and muscle AChRs through similar mechanisms, perhaps by interacting externally on a non-conducting state of the AChR and by blocking the open receptor-channel complex close to the vestibule of the channel. These studies may be important for understanding the regulation of AChRs as well as for understanding antidepressant- and side-effects of monoamine-uptake inhibitors.

Human neuronal threonine-for-leucine-248 alpha 7 mutant nicotinic acetylcholine receptors are highly Ca2+ permeable

A cDNA coding for the human neuronal nicotinic alpha7 receptor subunit with Leu-248 mutated to threonine was expressed in Xenopus oocytes. When activated by acetylcholine (AcCho), the receptors expressed generated currents that had low desensitization, linear current-voltage relation, and high apparent affinity for both AcCho and nicotine. These characteristics are similar to those already described for the chick threonine-for-leucine-247 alpha7 nicotinic AcCho receptor (nAcChoR) mutant (L247Talpha7). These properties were all substantially maintained when the human L248Talpha7 mutant was transiently expressed in human Bosc 23 cells. Simultaneous whole-cell clamp and fluorescence measurements with the Ca(2+) indicator dye Fura-2 showed that nicotine induced a Ca(2+) influx in standard 2 mM Ca(2+) solution. The average fractional Ca(2+) current flowing through L248Talpha7 nAcChoRs was 6.7%, which is larger than that flowing through muscle alpha(beta)epsilon(delta) nAcChoRs (4.1%). The relative Ca(2+) permeability, determined in oocytes in the absence of Cl(-), was measured from the shift in reversal potential caused by increasing the external Ca(2+) concentration from 1 to 10 mM. The human wild-type alpha7 nAcChoR was found to be more permeable than the L248Talpha7 mutant to Ca(2+). Our findings indicate that the Ca(2+) permeability of the homomeric alpha7 nAcChoR is larger than that of the heteromeric neuronal nicotinic receptors studied to date and is possibly similar to that of the N-methyl-D-aspartate subtype of brain glutamate receptors.

Human neuronal threonine-for-leucine-248 7 mutant nicotinic acetylcholine receptors are highly Ca2+ permeable

A cDNA coding for the human neuronal nicotinic alpha7 receptor subunit with Leu-248 mutated to threonine was expressed in Xenopus oocytes. When activated by acetylcholine (AcCho), the receptors expressed generated currents that had low desensitization, linear current-voltage relation, and high apparent affinity for both AcCho and nicotine. These characteristics are similar to those already described for the chick threonine-for-leucine-247 alpha7 nicotinic AcCho receptor (nAcChoR) mutant (L247Talpha7). These properties were all substantially maintained when the human L248Talpha7 mutant was transiently expressed in human Bosc 23 cells. Simultaneous whole-cell clamp and fluorescence measurements with the Ca(2+) indicator dye Fura-2 showed that nicotine induced a Ca(2+) influx in standard 2 mM Ca(2+) solution. The average fractional Ca(2+) current flowing through L248Talpha7 nAcChoRs was 6.7%, which is larger than that flowing through muscle alpha(beta)epsilon(delta) nAcChoRs (4.1%). The relative Ca(2+) permeability, determined in oocytes in the absence of Cl(-), was measured from the shift in reversal potential caused by increasing the external Ca(2+) concentration from 1 to 10 mM. The human wild-type alpha7 nAcChoR was found to be more permeable than the L248Talpha7 mutant to Ca(2+). Our findings indicate that the Ca(2+) permeability of the homomeric alpha7 nAcChoR is larger than that of the heteromeric neuronal nicotinic receptors studied to date and is possibly similar to that of the N-methyl-D-aspartate subtype of brain glutamate receptors.

Strychnine activates neuronal alpha7 nicotinic receptors after mutations in the leucine ring and transmitter binding site domains

Recent work has shown that strychnine, the potent and selective antagonist of glycine receptors, is also an antagonist of nicotinic acetylcholine (AcCho) receptors including neuronal homomeric alpha7 receptors, and that mutating Leu-247 of the alpha7 nicotinic AcCho receptor-channel domain (L247Talpha7; mut1) converts some nicotinic antagonists into agonists. Therefore, a study was made of the effects of strychnine on Xenopus oocytes expressing the chick wild-type alpha7 or L247Talpha7 receptors. In these oocytes, strychnine itself did not elicit appreciable membrane currents but reduced the currents elicited by AcCho in a reversible and dose-dependent manner. In sharp contrast, in oocytes expressing L247Talpha(7) receptors with additional mutations at Cys-189 and Cys-190, in the extracellular N-terminal domain (L247T/C189-190Salpha7; mut2), micromolar concentrations of strychnine elicited inward currents that were reversibly inhibited by the nicotinic receptor blocker alpha-bungarotoxin. Single-channel recordings showed that strychnine gated mut2-channels with two conductance levels, 56 pS and 42 pS, and with kinetic properties similar to AcCho-activated channels. We conclude that strychnine is a modulator, as well as an activator, of some homomeric nicotinic alpha7 receptors. After injecting oocytes with mixtures of cDNAs encoding mut1 and mut2 subunits, the expressed hybrid receptors were activated by strychnine, similar to the mut2, and had a high affinity to AcCho like the mut1. A pentameric symmetrical model yields the striking conclusion that two identical alpha7 subunits may be sufficient to determine the functional properties of alpha7 receptors.

Nicotinic acetylcholine receptor antagonistic activity of monoamine uptake blockers in rat hippocampal slices

The aim of our study was to investigate the effect of different monoamine uptake blockers on the nicotine-evoked release of [3H]noradrenaline ([3H]NA) from rat hippocampal slices. We found that desipramine (DMI), nisoxetine, cocaine, citalopram, and nomifensine inhibit the nicotine-evoked release of [3H]NA with an IC50 of 0.36, 0.59, 0.81, 0.93, and 1.84 microM, respectively. These IC50 values showed no correlation with the inhibitory effect (Ki) of monoamine uptake blockers on the neuronal NA transporter (r = 0.17, slope = 0.02), indicating that the NA uptake system is not involved in the process. In whole-cell patch clamp experiments neither drug blocked Na+ currents at 1 microM in sympathetic neurons from rat superior cervical ganglia, and only DMI produced a pronounced inhibition (52% decrease) at 10 microM. Comparison of the effect of DMI and tetrodotoxin (TTX) on the electrical stimulation- and nicotine-evoked release of [3H]NA showed that DMI, in contrast to TTX, inhibits only the nicotine-induced response, indicating that the target of DMI is not the Na+ channel. Our data suggest that monoamine uptake blockers with different chemical structure and selectivity are able to inhibit the nicotinic acetylcholine receptors in the CNS. Because these compounds are widely used in the therapy of depressed patients, our findings may have great importance in the evaluation of their clinical effects.

Antidepressants noncompetitively inhibit nicotinic acetylcholine receptor function

Nicotinic acetylcholine receptors (nAChRs) are diverse members of the neurotransmitter-gated ion channel superfamily and play critical roles in chemical signaling throughout the nervous system. The present study establishes for the first time the acute functional effects of sertraline (Zoloft), paroxetine (Paxil), nefazodone (Serzone), and venlafaxine (Effexor) on two human and one chick nAChR subtype. This study also confirms previous findings of nAChR functional block by fluoxetine (Prozac). Function of human muscle-type nAChR (alpha1/beta gammadelta) in TE671/RD cells, human autonomic nAChR (alpha3/beta4alpha5 +/- beta2) in SH-SY5Y neuroblastoma cells, or chick V274T mutant alpha7-nAChR heterologously expressed in native nAChR-null SH-EP1 epithelial cells was measured using 86Rb+ efflux assays. Functional blockade of human muscle-type and autonomic nAChRs is produced by each of the drugs in the low to intermediate micromolar range, and functional blockade of chick V274T-alpha7-nAChR is produced in the intermediate to high micromolar range. Functional blockade is insurmountable by increasing agonist concentrations at each nAChR subtype tested for each of these drugs, suggesting noncompetitive inhibition of nAChR function. These studies open the possibilities that nAChR subtypes in the brain could be targets for therapeutic antidepressants and could play roles in clinical depression.