Effects of serotonergic agents on neuronal nicotinic acetylcholine receptors

LSD and ketanserin and their impact on the human autonomic nervous system

The interest in lysergic acid diethylamide (LSD) has sparked again due to its supposed positive effects on psychopathological conditions. Yet, most research focuses on the actions of LSD on the central nervous system. The interaction with the autonomic nervous system (ANS) has been neglected so far. Therefore, the aim was to assess the effects of LSD and the serotonin 2A receptor antagonist ketanserin on the ANS as assessed by heart rate variability (HRV) measures and their correlation with subjective drug-induced effects in a randomized, placebo-controlled crossover trial. Thus, ANS activity was derived from electrocardiogram recordings after intake of placebo, LSD or ketanserin, and LSD by calculating R-peak-based measures of sympathetic and parasympathetic activity. Repeated measure ANOVA and partial correlation for HRV measures and subjective experience questionnaires were performed. LSD predominantly increased sympathetic activity, while ketanserin counteracted this effect on the ANS via an increase of parasympathetic tone. Sympathetic activity was positively and parasympathetic activity negatively associated with psychedelic effects of LSD. Furthermore, Placebo HRV measures predicted subjective experiences after LSD intake. The association between trait ANS activity and LSD-induced subjective experiences may serve as a candidate biomarker set for the effectiveness of LSD in the treatment of psychopathological conditions.

Antagonistic effect of dopamine structural analogues on human GABAρ1 receptor

GABAergic and dopaminergic pathways are co-localized in several areas of the central nervous system and recently several reports have shown co-release of both neurotransmitters. The GABA-A receptor (β and ρ1 subunits) is modulated by dopamine (DA) and, interestingly, GABAρ1 can be modulated by several biogenic amines. Here we explored the effects of the metabolites of the dopaminergic pathway and other structural analogues of DA on GABAρ1 and the DA gated ion channel (LGC-53) from Caenorhabditis elegans expressed in Xenopus laevis oocytes. Our findings show an antagonistic effect of the metabolite 3-Methoxytyramine (3-MT, IC₅₀ = 285 ± 30 µM) with similar potency compared to DA on induced GABA currents; however, it was inactive on LGC-53. The structural DA analogues and metabolites, 3, 4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), 2-phenylethylamine (β-PEA) and 4-amino-1-butanol (4-AM-1-OH), antagonized GABAρ1 currents, whereas β-PEA acted as partial agonists on LGC-53, indicating that the putative binding sites of both receptors may share structural characteristics. These results suggest that the DA metabolites 3-MT, DOPAC and HVA modulate GABAρ1 and possibly affect the activity of the receptors that include this subunit in vivo.

Effects of the antidepressant mirtazapine and zinc on nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) and zinc are associated with regulation of mood and related disorders. In addition, several antidepressants inhibit muscle and neuronal nAChRs and zinc potentiates inhibitory actions of them. Moreover, mirtazapine (a noradrenergic, serotonergic and histaminergic antidepressant) inhibits muscarinic AChRs and its effects on nAChRs are unknown. Therefore, we studied the modulation of muscle α1β1γd nAChRs expressed in oocytes and native α7-containing nAChRs in hippocampal interneurons by mirtazapine and/or zinc, using voltage-clamp techniques. The currents elicited by ACh in oocytes (at -60 mV) were similarly inhibited by mirtazapine in the absence and presence of 100 μM zinc (IC₅₀ ∼15 μM); however, the ACh-induced currents were stronger inhibited with 20 and 50 μM mirtazapine in the presence of zinc. Furthermore, the potentiation of ACh-induced current by zinc in the presence of 5 μM mirtazapine was 1.48 ± 0.06, and with 50 μM mirtazapine zinc potentiation did not occur. Interestingly, in stratum radiatum interneurons (at -70 mV), 20 μM mirtazapine showed less inhibition of the current elicited by choline (Ch) than at 10 μM (0.81 ± 0.02 and 0.74 ± 0.02 of the Ch-induced current, respectively). Finally, the inhibitory effects of mirtazapine depended on membrane potential: 0.81 ± 0.02 and 0.56 ± 0.05 of the control Ch-induced current at -70 and -20 mV, respectively. These results indicate that mirtazapine interacts with muscle and neuronal nAChRs, possibly into the ion channel; that zinc may increase the sensitivity of nAChRs to mirtazapine; and that mirtazapine decreases the sensitivity of nAChRs to zinc.

Neuromuscular blocking effect of fluoxetine and its interaction with rocuronium

As selective serotonin reuptake inhibitors have an inhibitory effect on nicotinic acetylcholine receptors, they may affect the neuromuscular transmission and interact with neuromuscular blockers. This study was designed to observe the effect of fluoxetine on neuromuscular transmission and its interaction with rocuronium using the rat phrenic nerve hemidiaphragm and rabbit head drop methods. Rat phrenic nerve hemidiaphragms were mounted and stimulated using a train of four pulses (TOF). The effect of fluoxetine was studied on both indirectly and directly stimulated basal twitch responses by plotting cumulative dose response curves (DRCs). DRCs of rocuronium were obtained in the absence, and presence of 5 μm and 20 μm fluoxetine to study its interaction. ED5 , ED50 and ED95 values of rocuronium DRCs in absence and presence of fluoxetine were calculated. Fluoxetine significantly inhibited twitch responses in both indirect and directly stimulated preparations. Fluoxetine (20 μm) caused an increase in the potency of rocuronium such that the ED50 and ED95 values of rocuronium DRCs were significantly decreased. Partially inhibited twitch responses by fluoxetine (100 μm) were not reversed by neostigmine (3.3 μm) or 3,4 diaminopyridine (0.25 μm). Rabbits were given fluoxetine 0.25 mg kg(-1) and 1 mg kg(-1) orally for 15 days, and on 15th day, rocuronium infusion was given, and time for head drop was recorded. The time of head drop was significantly reduced in fluoxetine pretreated as compared to control group. Fluoxetine blocks the neuromuscular transmission and increases the potency of rocuronium-induced neuromuscular block.

Structural basis of ligand recognition in 5-HT3 receptors

The crystal structures of a binding protein engineered to recognize serotonin (5-HT) and the anti-emetic granisetron with affinities comparable to the 5-HT₃ receptor reveal important structural details of ligand recognition in the 5-HT₃ receptor.

The 5-HT₃ receptor is a pentameric serotonin-gated ion channel, which mediates rapid excitatory neurotransmission and is the target of a therapeutically important class of anti-emetic drugs, such as granisetron. We report crystal structures of a binding protein engineered to recognize the agonist serotonin and the antagonist granisetron with affinities comparable to the 5-HT₃ receptor. In the serotonin-bound structure, we observe hydrophilic interactions with loop E-binding site residues, which might enable transitions to channel opening. In the granisetron-bound structure, we observe a critical cation–π interaction between the indazole moiety of the ligand and a cationic centre in loop D, which is uniquely present in the 5-HT₃ receptor. We use a series of chemically tuned granisetron analogues to demonstrate the energetic contribution of this electrostatic interaction to high-affinity ligand binding in the human 5-HT₃ receptor. Our study offers the first structural perspective on recognition of serotonin and antagonism by anti-emetics in the 5-HT₃ receptor.

Dopamine and serotonin modulate human GABAρ1 receptors expressed in Xenopus laevis oocytes

GABAρ1 receptors are highly expressed in bipolar neurons of the retina and to a lesser extent in several areas of the central nervous system (CNS), and dopamine and serotonin are also involved in the modulation of retinal neural transmission. Whether these biogenic amines have a direct effect on ionotropic GABA receptors was not known. Here, we report that GABAρ1 receptors, expressed in X. laevis oocytes, were negatively modulated by dopamine and serotonin and less so by octopamine and tyramine. Interestingly, these molecules did not have effects on GABA(A) receptors. 5-Carboxamido-tryptamine and apomorphine did not exert evident effects on any of the receptors. Schild plot analyses of the inhibitory actions of dopamine and serotonin on currents elicited by GABA showed slopes of 2.7 ± 0.3 and 6.1 ± 1.8, respectively, indicating a noncompetitive mechanism of inhibition. The inhibition of GABAρ1 currents was independent of the membrane potential and was insensitive to picrotoxin, a GABA receptor channel blocker and to the GABAρ-specific antagonist (1,2,5,6-tetrahydropyridine-4-yl)methyl phosphinic acid (TPMPA). Dopamine and serotonin changed the sensitivity of GABAρ1 receptors to the inhibitory actions of Zn(2+). In contrast, La(3+) potentiated the amplitude of the GABA currents generated during negative modulation by dopamine (EC(50) 146 μM) and serotonin (EC(50) 196 μM). The functional role of the direct modulation of GABAρ receptors by dopamine and serotonin remains to be elucidated; however, it may represent an important modulatory pathway in the retina, where GABAρ receptors are highly expressed and where these biogenic amines are abundant.

Quercetin Inhibits α3β4 Nicotinic Acetylcholine Receptor-Mediated Ion Currents Expressed in Xenopus Oocytes

Quercetin mainly exists in the skin of colored fruits and vegetables as one of flavonoids. Recent studies show that quercetin, like other flavonoids, has diverse pharmacological actions. However, relatively little is known about quercetin effects in the regulations of ligand-gated ion channels. In the previous reports, we have shown that quercetin regulates subsets of homomeric ligand-gated ion channels such as glycine, 5-HT(3A) and α7 nicotinic acetylcholine receptors. In the present study, we examined quercetin effects on heteromeric neuronal α3β4 nicotinic acetylcholine receptor channel activity expressed in Xenopus oocytes after injection of cRNA encoding bovine neuronal α3 and β4 subunits. Treatment with acetylcholine elicited an inward peak current (I(ACh)) in oocytes expressing α3β4 nicotinic acetylcholine receptor. Co-treatment with quercetin and acetylcholine inhibited I(ACh) in oocytes expressing α3β4 nicotinic acetylcholine receptors. The inhibition of I(ACh) by quercetin was reversible and concentration-dependent. The half-inhibitory concentration (IC(50)) of quercetin was 14.9±0.8 µM in oocytes expressing α3β4 nicotinic acetylcholine receptor. The inhibition of I(ACh) by quercetin was voltage-independent and non-competitive. These results indicate that quercetin might regulate α3β4 nicotinic acetylcholine receptor and this regulation might be one of the pharmacological actions of quercetin in nervous systems.

Vestibular-mediated increase in central serotonin plays an important role in hypergravity-induced hypophagia in rats

Exposure to a hypergravity environment induces acute transient hypophagia, which is partially restored by a vestibular lesion (VL), suggesting that the vestibular system is involved in the afferent pathway of hypergravity-induced hypophagia. When rats were placed in a 3-G environment for 14 days, Fos-containing cells increased in the paraventricular hypothalamic nucleus, the central nucleus of the amygdala, the medial vestibular nucleus, the raphe nucleus, the nucleus of the solitary tract, and the area postrema. The increase in Fos expression was completely abolished or significantly suppressed by VL. Therefore, these regions may be critical for the initiation and integration of hypophagia. Because the vestibular nucleus contains serotonergic neurons and because serotonin (5-HT) is a key neurotransmitter in hypophagia, with possible involvement in motion sickness, we hypothesized that central 5-HT increases during hypergravity and induces hypophagia. To examine this proposition, the 5-HT concentrations in the cerebrospinal fluid were measured when rats were reared in a 3-G environment for 14 days. The 5-HT concentrations increased in the hypergravity environment, and these increases were completely abolished in rats with VL. Furthermore, a 5-HT(2A) antagonist (ketanserin) significantly reduced 3-G (120 min) load-induced Fos expression in the medial vestibular nucleus, and chronically administered ketanserin ameliorated hypergravity-induced hypophagia. These results indicate that hypergravity induces an increase in central 5-HT via the vestibular input and that this increase plays a significant role in hypergravity-induced hypophagia. The 5-HT(2A) receptor is involved in the signal transduction of hypergravity stress in the vestibular nucleus.

Functional interactions between nicotinic and P2X receptors in celiac ganglia neurons

Here we characterized the cross-inhibitory interactions between nicotinic and P2X receptors of celiac neurons from the guinea pig by recording whole-cell currents induced by 1mM ACh (I(ACh)), 1mM ATP (I(ATP)) and by the simultaneous application of both agonists (I(ACh)(+ATP)). I(ACh) and I(ATP) were inhibited by hexamethonium (nicotinic channel blocker) and PPADS (P2X receptor antagonist), respectively. The amplitude of I(ACh)(+ATP) was equal to the current induced by the most effective agonist, indicating a current occlusion. Various observations indicate that I(ACh)(+ATP) is carried out through both nicotinic (nACh) and P2X channels: i) I(ACh)(+ATP) desensitisation kinetics were in between that of I(ACh) and I(ATP); ii) application of ATP+ACh, decreased I(ACh) and I(ATP), whereas no cross-desensitisation was observed between nACh and P2X receptors; iii) ATP did not affect I(ACh) in the presence of PPADS or after P2X receptor desensitisation; and iv) ACh did not affect I(ATP) when nACh channels were blocked with hexamethonium or after nACh receptor desensitisation. Current occlusion is not mediated by activation of metabotropic receptors as it is: i) voltage dependent (was not observed at + 5 mV); ii) present at low temperature (10 degrees C) and after inhibition of protein kinase activity (with staurosporine); and iii) absent at 30 microM ATP and 30 microM ACh (concentrations that should activate metabotropic receptors). In conclusion, current occlusion described here is similar to the previously reported myenteric neurons. This occlusion is likely the result of allosteric interactions between these receptors.

Mammalian nicotinic acetylcholine receptors: from structure to function

The classical studies of nicotine by Langley at the turn of the 20th century introduced the concept of a "receptive substance," from which the idea of a "receptor" came to light. Subsequent studies aided by the Torpedo electric organ, a rich source of muscle-type nicotinic receptors (nAChRs), and the discovery of alpha-bungarotoxin, a snake toxin that binds pseudo-irreversibly to the muscle nAChR, resulted in the muscle nAChR being the best characterized ligand-gated ion channel hitherto. With the advancement of functional and genetic studies in the late 1980s, the existence of nAChRs in the mammalian brain was confirmed and the realization that the numerous nAChR subtypes contribute to the psychoactive properties of nicotine and other drugs of abuse and to the neuropathology of various diseases, including Alzheimer's, Parkinson's, and schizophrenia, has since emerged. This review provides a comprehensive overview of these findings and the more recent revelations of the impact that the rich diversity in function and expression of this receptor family has on neuronal and nonneuronal cells throughout the body. Despite these numerous developments, our understanding of the contributions of specific neuronal nAChR subtypes to the many facets of physiology throughout the body remains in its infancy.

5-Hydroxytryptamine directly inhibits neuronal nicotinic acetylcholine receptors in rat trigeminal ganglion neurons

In the present study, whole-cell patch clamp recording technique was used to investigate the action of 5-hydroxytryptamine (5-HT) on the function of native neuronal nicotinic acetylcholine receptors expressed in the rat trigeminal ganglion neurons. Inward currents (I(nic)) caused by externally-applied nicotine were observed in majority of the examined neurons, which were mediated by alpha-bungarotoxin-insensitive nicotinic acetylcholine receptors. We found that 5-HT could reversibly inhibit I(nic) in a concentration-dependent manner, and the inhibition did not involve 5-HT receptors. Other serotonergic agents, such as 2-methyl-5-HT, alpha-methyl-5-HT, sumatriptan and ICS-205,930, also had similar inhibitory effects on I(nic). 5-HT inhibited nicotinic acetylcholine receptors in a non-competitive manner, as 5-HT decreased the maximal current response to nicotine but had no effect on the threshold and EC(50). The inhibition of I(nic) by 5-HT was voltage-dependent and became stronger at hyperpolarized potentials. These results indicated that 5-HT directly inhibited nicotinic acetylcholine receptors in the trigeminal ganglion neurons. As a local modulator of the nicotinic acetylcholine receptor, 5-HT might play a role in the modulation of sensory information.

Chronologically overlapping occurrences of nicotine-induced anxiety- and depression-related behavioral symptoms: effects of anxiolytic and cannabinoid drugs

BACKGROUND

Anxiety and depression are among the most frequently-observed psychiatric symptoms associated with nicotine (NC). In addition to the similarity to other addictive drugs, these NC-induced symptoms are characteristic in that the opposite behavioral effects, i.e. anxiolytic and antidepressant effects, which may reinforce the habitual use of NC, have also been reported. In the present study, the time course of anxiety- and depression-related behavioral alterations was examined in mice. Furthermore, based on the reported similarity in the mechanisms responsible for NC-induced anxiety- and depression-related symptoms, as well as the contribution of brain cannabinoid (CB) receptors to these behavioral symptoms, the effects of anxiolytics and CB receptor ligands (CBs) against these behavioral symptoms were investigated.

RESULTS

Repeated subcutaneous NC treatments (0.3 mg/kg, 4 days), compared with a single treatment (0.5 mg/kg), caused both prolonged anxiogenic effects in the elevated plus-maze test, and prolonged depressive effects in the forced swimming test, even at 120 min time point after the last NC treatment. A transient anxiolytic preference for open arms was also observed in the elevated plus-maze test. Among the anxiolytics and CBs, the serotonin 1A (5-HT1A) antagonist WAY 100135 and the endogenous mixed CB agonist/antagonist virodhamine (VD), when administered intraperitoneally before each NC treatment, provided the strongest antagonistic effects against the anxiety-related symptoms. However, against the depression-related symptoms, only VD provided significant antagonistic effects in both single and repeated treatment groups.

CONCLUSION

The present results support the presence of a chronological overlap of NC-induced anxiety- and depression-related behavioral symptoms, and the contribution of brain CB receptors to these behavioral symptoms. The repeated NC-induced prolongation of these behavioral symptoms and the early transient anxiolytic behavioral alterations support an increased possibility of the habitual use of NC. Furthermore, based on the antagonistic effects of VD, one can predict that the characteristic effects on brain CB receptors as a mixed CB agonist/antagonist contributed to its therapeutic effects as both an anxiolytic and an antidepressant.

Cross-inhibitory interactions between GABAA and P2X channels in myenteric neurones

Inhibitory interactions between GABA(A)[induced by gamma-aminobutyric acid (GABA)] and P2X [activated by adenosine 5'-triphosphate (ATP)] receptors of myenteric neurones from the guinea pig small intestine were characterized using whole-cell recordings. Currents induced by GABA (I(GABA)) or ATP (I(ATP)) were inhibited by picrotoxin or pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid, respectively. Currents induced by GABA + ATP (I(GABA+ATP)) were only as large as the current induced by the most effective transmitter, revealing current occlusion. This occlusion requires maximal activation of at least one of these receptors. Sequential applications of neurotransmitters, and kinetic and pharmacological properties of I(GABA+ATP) indicate that they are carried through both GABA(A) and P2X channels. ATP did not affect I(GABA) in neurones: (i) in which P2X channels were not present; (ii) after inhibiting P2X channels with Ca2+ (iii) in the presence of pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid, a P2X receptor antagonist; (iv) after P2X receptor desensitization or (v) at I(ATP) reversal potential. Similarly, GABA did not affect P2X-mediated currents in neurones: (i) in which GABA(A) channels were not present; (ii) in the presence of picrotoxin, a GABA(A) channel blocker; (iii) after GABA(A) receptor desensitization or (iv) at the I(GABA) reversal potential. Current occlusion occurred as fast as current activation and it was still present in the absence of Ca2+, at 11 degrees C, after adding to the pipette solution a cocktail of protein kinase inhibitors (staurosporine + genistein + K-252a), after substituting the GTP in the pipette with GDP-beta-S and after treating the cells with N-ethylmaleimide. Taken together, all of these results are consistent with a model of cross-inhibition between GABA(A) and P2X.

Quantification of nicotinic acetylcholine receptors in Parkinson's disease with (123)I-5IA SPECT

We quantified in vivo brain nicotinic acetylcholine receptor (nAChR) distributions in patients with Parkinson's disease (PD) and evaluated correlations between nAChR distributions and clinical variables of the patients, especially dopaminergic medications. Ten patients with PD without dementia underwent 5-(123)I-iodo-3-(2(S)-azetidinylmethoxy)pyridine ((123)I-5IA) single photon emission computed tomography (SPECT) and the data were compared with those of 10 age-matched healthy volunteers. Correlation analyses between (123)I-5IA distribution volumes (DVs) in each brain region and clinical variables of the patients were also performed. The PD group showed a statistically significant decrease (20-25%) in the brainstem and frontal cortex as compared with the control group. Although age, duration of disease, daily dose of levodopa, duration of PD medication use, and scores on the motor section of Unified Parkinson's Disease Rating Scale were not significantly correlated with DV values in any brain regions, high daily doses of dopamine agonist showed a significant negative correlation with DVs in the cerebellum, and temporal, parietal and occipital cortices. These findings suggest that patients with PD without dementia can show reductions especially in the brainstem and frontal cortex. They also suggest that dopamine agonists can have a negative influence on the distribution of nAChRs.

The effect of the 5-HT1A receptor agonist, 8-OH-DPAT, on motion-induced emesis in Suncus murinus

In the present study we evaluated the role of 5-HT(1A) receptors in mediating the inhibitory action of 8-OH-DPAT, a 5-HT(1A) receptor agonist, in motion sickness in Suncus murinus. 8-OH-DPAT (0.1 mg/kg, i. p) attenuated motion-induced emesis which was associated with an increase in the latency of the onset to the first emetic episode. Pre-treatment with methysergide (a 5-HT(1/2/7) receptor antagonist, 1.0 mg/kg, i. p.), WAY-100635 (a 5-HT(1A) receptor antagonist, 1.0 mg/kg, i. p.), SB269970A (a 5-HT(7) receptor antagonist, 1.0 and 5.0 mg/kg, i. p.), ondansetron (a 5-HT(3) receptor antagonist, 1.0 mg/kg, i. p) or GR13808 (a 5-HT(4) receptor antagonist, 0.5 mg/kg, i. p) failed to modify the inhibitory action of 8-OH-DPAT on motion sickness. Furthermore, the application of either methysergide, WAY-100635, SB269970A, ondansetron or GR13808 alone had no effect on motion sickness in its own right. These data indicate that neither 5-HT(1A) nor any 5-HT(2) receptor subtypes, 5-HT(3), 5-HT(4) and 5-HT(7) receptors are likely to be involved in the inhibition of motion-induced emesis mediated by 8-OH-DPAT.

Excitatory effect of ATP on rat area postrema neurons

ATP-induced inward currents and increases in the cytosolic Ca(2+) concentration ([Ca](in)) were investigated in neurons acutely dissociated from rat area postrema using whole-cell patch-clamp recordings and fura-2 microfluorometry, respectively. The ATP-induced current (I (ATP)) and [Ca](in) increases were mimicked by 2-methylthio-ATP and ATP-gammaS, and were inhibited by P2X receptor (P2XR) antagonists. The current-voltage relationship of the I (ATP) exhibited a strong inward rectification, and the amplitude of the I (ATP) was concentration-dependent. The I (ATP) was markedly reduced in the absence of external Na(+), and the addition of Ca(2+) to Na(+)-free saline increased the I (ATP). ATP did not increase [Ca](in) in the absence of external Ca(2+), and Ca(2+) channel antagonists partially inhibited the ATP-induced [Ca](in) increase, indicating that ATP increases [Ca](in) by Ca(2+) influx through both P2XR channels and voltage-dependent Ca(2+) channels. There was a negative interaction between P2XR- and nicotinic ACh receptor (nAChR)-channels, which depended on the amplitude and direction of current flow through either channel. Current occlusion was observed at V (h)s between -70 and -10 mV when the I (ATP) and ACh-induced current (I (ACh)) were inward, but no occlusion was observed when these currents were outward at a V (h) of +40 mV. The I (ATP) was not inhibited by co-application of ACh when the I (ACh) was markedly decreased either by removal of permeant cations, by setting V (h) close to the equilibrium potential of I (ACh), or by the addition of d-tubocurarine or serotonin. These results suggest that the inhibitory interaction is attributable to inward current flow of cations through the activated P2XR- and nAChR-channels.

Antiemetics of the 5-hydroxytryptamine 3A antagonist class inhibit muscle nicotinic acetylcholine receptors

Antagonists of the serotonergic 5-hydroxytryptamine 3A receptor (5-HT(3A)R) and muscle nicotinic acetylcholine receptors (nAChR) are widely used in anesthesia practice. Both 5-HT(3A)R and nAChR are ligand-gated ion channels with known pharmacological overlap between some of their agonists and antagonists. We studied the actions of clinically used 5-HT(3A)R antagonist antiemetics and nondepolarizing muscle blockers on ionic currents elicited by the activation of mammalian 5-HT(3A)R and muscle nAChR, expressed in Xenopus laevis oocytes. Currents were recorded using a whole-cell two-electrode voltage clamp technique. Dolasetron, ondansetron, and granisetron reversibly inhibited 5-HT(3A)R function at nanomolar concentrations with 50% inhibitory concentrations (IC(50)) of 11.8, 6.4, and 0.2 nM; the rank order of inhibition correlated well with their clinical antiemetic potencies. The principal metabolite of dolasetron, hydrodolasetron, was 40 times more potent than the parent compound on 5-HT(3A)R (IC(50) = 0.29 nM). The potency of the nondepolarizing muscle blocker d-tubocurarine in blocking 5-HT(3A)R was similar to that of the antiemetics and significantly more than vecuronium and rapacuronium (IC(50) = 11.4 nM, 18.9 microM, 60.5 microM). Conversely, ondansetron, dolasetron, and granisetron also reversibly inhibited nAChR currents in a dose-dependent manner with IC(50)s of 14.2, 7.8, and 4.4 microM for the adult nAChR and 16.0, 18.6, and 13.9 microM for the embryonic nAChR. Again, hydrodolasetron showed significantly (10 times) more inhibitory potency on the adult nAChR than the parent compound dolasetron. These results indicate that drugs that target specific ligand-gated ion channels may also affect other ion channel types.

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.

Inhibitory interactions between 5-HT3 and P2X channels in submucosal neurons

Inhibitory interactions between 5-HT subtype 3 (5-HT(3)) and P2X receptors were characterized using whole cell recording techniques. Currents induced by 5-HT (I(5-HT)) and ATP (I(ATP)) were blocked by tropisetron (or ondansetron) and pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid, respectively. Currents induced by 5-HT + ATP (I(5-HT+ATP)) were only as large as the current induced by the most effective transmitter, revealing current occlusion. Occlusion was observed at membrane potentials of -60 and 0 mV (for inward currents), but it was not present at +40 mV (for outward currents). Kinetic and pharmacological properties of I(5-HT+ATP) indicate that they are carried through 5-HT(3) and P2X channels. Current occlusion occurred as fast as activation of I(5-HT) and I(ATP), was still present in the absence of Ca(2+) or Mg(2+), after adding staurosporine, genistein, K-252a, or N-ethylmaleimide to the pipette solution, after substituting ATP with proportional to, beta-methylene ATP or GTP with GTP-gamma-S in the pipette, and was observed at 35 degrees C, 23 degrees C, and 8 degrees C. These results are in agreement with a model that considers that 5-HT(3) and P2X channels are in functional clusters and that these channels might directly inhibit each other.

Serotonin antagonizes the human neuronal alpha7 nicotinic acetylcholine receptor and becomes an agonist after L248T alpha7 mutation

The effects of serotonin (5-hydroxytryptamine or 5HT) on chick alpha7 nicotinic receptors have already been described. However similar studies on human alpha7 receptors have been lacking. To begin to fill this deficiency, studies were made on wild-type and mutant human alpha7 (halpha7) receptors expressed in Xenopus oocytes or human BOSC 23 cells. In oocytes wild-type halpha7 receptors were blocked by 5HT, and this block was voltage-dependent. In contrast, 5HT acted as an agonist on halpha7-mutant receptors (L248T). Outside-out membrane-patches from BOSC 23 cells expressing halpha7-mutant receptors exhibited spontaneous channel openings of two conductance levels (59 pS and 76 pS) and short mean open time (0.9 ms). halpha7-Mutant channels activated by nicotine or 5HT displayed similar conductances and high Ca(2+) permeability; but longer duration (2.7 ms) than the spontaneous openings. Mutations at Cys190 and Cys191, in the extracellular N-terminus of the human alpha7 gene, did not prevent receptor expression and incorporation in the oocyte membrane (determined by alpha-bungarotoxin binding). However, both 5HT and nicotine were incapable of gating the channels, indicating that the mutated Cys residues are in, or near, the 5HT- and nicotine-binding site. This is the first report that alpha7 receptors have spontaneous openings; and that 5HT is an agonist of halpha7-mutant receptors, and an antagonist of halpha7-wild-type receptors, through interactions at, or near the acetylcholine-binding sites.

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.

Effects of ginsenosides, active components of ginseng, on nicotinic acetylcholine receptors expressed in Xenopus oocytes

We investigated the effects of ginsenosides, the active ingredient of ginseng, on neuronal or muscle-type nicotinic acetylcholine receptor channel activity expressed in Xenopus oocytes after injection of cRNA encoding bovine neuronal alpha3beta4, alpha7 or human muscle alphabetadeltavarepsilon subunits. Treatment with acetylcholine elicited an inward peak current (I(ACh)) in oocytes expressing nicotinic acetylcholine receptor subtypes. Cotreatment with ginsenoside Rg2 and acetylcholine inhibited I(ACh) in oocytes expressing with alpha3beta4 or alphabetadeltavarepsilon but not in oocytes expressing alpha7 nicotinic acetylcholine receptors. The inhibition of I(ACh) by ginsenoside Rg2 was reversible and dose-dependent. The half-inhibitory concentrations (IC50) of ginsenoside Rg2 were 60.2+/-14.1 and 15.7+/-3.5 microM in oocytes expressing alpha3beta4 and alphabetadeltavarepsilon nicotinic acetylcholine receptors, respectively. The inhibition of I(ACh) by ginsenoside Rg2 was voltage-independent and noncompetitive. Other ginsenosides besides ginsenoside Rg2 also inhibited I(ACh) in oocytes expressing alpha3beta4 or alphabetadeltavarepsilon nicotinic acetylcholine receptors. The order of potency for the inhibition of I(ACh) was ginsenoside Rg2>Rf>Re>Rg1>Rc>Rb2>Rb1 in oocytes expressing alpha3beta4 nicotinic acetylcholine receptors and was ginsenoside Rg2>Rf>Rg1>Re>Rb1>Rc>Rb2 in oocytes expressing alphabetadeltavarepsilon nicotinic acetylcholine receptors. These results indicate that ginsenosides might regulate nicotinic acetylcholine receptors in a differential manner and this regulation might be one of the pharmacological actions of Panax ginseng.

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.

Inhibition of nicotinic acetylcholine receptors by bicuculline

A study was made on the effects of bicuculline, the classical gamma-aminobutyric acid-A receptor antagonist, on heteromeric mouse muscle alphabetagammadelta, heteromeric neuronal rat alpha2beta4 and alpha4beta2 and homomeric human alpha7 nicotinic acetylcholine receptors (nAChRs), expressed in Xenopus oocytes. Bicuculline reduced the ACh-induced currents in a rapid and reversible way, with IC50 values of 34+/-1.5 microM for mouse muscle alphabetagammadelta and 12.4+/-0.7 and 18+/-1 microM for rat neuronal alpha2beta4 and alpha4beta2 nAChRs, respectively. Therefore, the three types of heteromeric receptors are inhibited by bicuculline but the neuronal alpha2beta4 and alpha4beta2 receptors were more sensitive than the muscle alphabetagammadelta receptor. The Hill coefficients for ACh-current inhibition were close to one for all types of receptors, suggesting a single site of action for bicuculline inhibition of nAChRs. Bicuculline shifted the ACh-dose-current response curve to the right and the maximal current was reduced, a reduction that for the heteromeric receptors was not overcome by high concentrations of ACh. The effect of bicuculline was examined at different membrane potentials, and the ACh-current-membrane potential relationships obtained indicate that the inhibition by bicuculline is voltage-dependent for muscle alphabetagammadelta and neuronal alpha2beta4 and alpha4beta2 nAChRs. All these results are consistent with the notion that bicuculline blocks the heteromeric muscle and neuronal nAChRs in a non-competitive way. Studies were also made on the wild type (wt alpha7) and mutant leu-to-threo (L248T) homomeric human neuronal alpha7-nAChRs. In sharp contrast to the heteromeric ACh receptors examined, bicuculline blocked in a competitive way the homomeric wt alpha7-nAChRs, as evidenced by a parallel shift of the bicuculline dose-ACh-current inhibition on raising the ACh concentration. Moreover, similar to the effects of serotonin on wt and mutant alpha7 ACh receptors, the mutation converted bicuculline from an antagonist into a competitive agonist. All this suggests that bicuculline may serve as a lead molecule to design new anticholinergic substances.

Nicotine attenuates DOI-induced head-twitch response in mice: implications for Tourette syndrome

Tourette syndrome (TS), a chronic neuropsychiatric disorder, is characterized by motor and vocal tics. Preliminary clinical studies indicate possible therapeutic benefits of nicotine in the treatment of Tourette's syndrome (TS). It has been proposed that twitches of the head in mice or twitches of head and shoulders in rats following administration of the selective 5HT(2A/C) agonist DOI (1-)2,5-dimethoxy-4-iodophenyl-2-aminopropane, can serve as an animal model of tics in TS. In this study, the effects of acute and chronic administration of nicotine on DOI-induced head twitch response (HTR) in male albino ICR mice were evaluated. Both acute and chronic nicotine (daily injections for 10 days) reduced the DOI-induced HTR. Moreover, chronic administration of DOI (1 mg/kg/day for 10 days) resulted in 65% increase in [125I]alpha-bungarotoxin binding in cerebellum and 41% increase in striatal [3H]cytisine binding. However, the acute inhibitory effects of nicotine were not blocked by pretreatment with the nicotinic antagonist, mecamylamine. Indeed, at higher doses, mecamylamine also reduced the DOI-induced HTR. The data suggest that both nicotine and mecamylamine may be of therapeutic potential in the treatment of some symptoms of TS.

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.

A motif present in the main cytoplasmic loop of nicotinic acetylcholine receptors and catalases

A motif containing five conserved amino acids (RXPXTH(X)14P) was detected in 111 proteins, including 82 nicotinic acetylcholine receptor (nAChR) subunits and 20 catalases. To explore possible functional roles of this motif in nAChRs two approaches were used: first, the motif sequences in nAChR subunits and catalases were analysed and compared; and, second, deletions in the rat alpha2 and beta4 nAChR subunits expressed in Xenopus oocytes were analysed. Compared to the three-dimensional structure of bovine hepatic catalase, structural coincidences were found in the motif of catalases and nAChRs. On the other hand, partial deletions of the motif in the alpha2 or beta4 subunits and injection of the mutants into oocytes was followed by a very weak expression of functional nAChRs; oocytes injected with alpha2 and beta4 subunits in which the entire motif had been deleted failed to elicit any acetylcholine currents. The results suggest that the motif may play a role in the activation of nAChRs.

5-Hydroxytryptamine and atropine inhibit nicotinic receptors in submucosal neurons

The whole-cell recording technique was used to investigate the pharmacological properties of acetylcholine-activated ion channels of cultured submucosal neurons from guinea-pig small intestine. Acetylcholine induced whole-cell membrane currents (I(ACh)) in a concentration-dependent manner (EC(50)=79 microM). I(ACh) exhibited strong inward rectification, had a reversal potential of +19+/-2 mV (Na(+) outside, Cs(+) inside), was reversibly inhibited in a concentration-dependent manner by hexamethonium (EC(50)=5 microM) and atropine (EC(50)=1.6 microM), and was unaffected by alpha-bungarotoxin (30 nM). Atropine was less potent in inhibiting the currents induced by 30 microM acetylcholine than those induced by 1 mM acetylcholine. I(ACh) was mimicked by the current induced by nicotine (I(Nic); EC(50)=52 microM). I(Nic) was also blocked by atropine (EC(50)=1.7 microM) and hexamethonium (EC(50)=3.6 microM). 5-Hydroxytryptamine (5-HT) also inhibited I(ACh) in a concentration-dependent manner (EC(50)=180 microM) in the experiments carried out in the presence of a 5-HT(3) receptor antagonist. 5-HT had a similar inhibitory effect after the desensitization of 5-HT(3) receptors or in neurons with relative small 5-HT(3)-mediated currents. The inhibitory actions of hexamethonium, atropine, and 5-HT on I(ACh) were voltage-dependent. Thus, inhibition was significantly smaller for outward currents (recorded at +40 mV) than for inward currents (recorded at -60 mV). Our observations indicate that the I(ACh) of submucosal neurons are mediated by activation of nicotinic channels, which are blocked by atropine, 5-HT, and hexamethonium. The possibility that one of the 5-HT roles in the gastrointestinal tract might be to directly modulate nicotinic channels is discussed.

Modulation of alpha2beta4 neuronal nicotinic acetylcholine receptors by zinc

A study was made of the modulation of nicotinic acetylcholine receptors by the divalent cation zinc. Rat neuronal nicotinic receptors (alpha2beta4) were expressed in Xenopus oocytes and membrane currents evoked by acetylcholine (ACh currents) were recorded using a two microelectrode voltage clamp. In non-injected oocytes, or in oocytes expressing alpha2beta4 receptors, Zn2+ by itself (1 microM-4 mM) generated only very small membrane currents. In contrast, in oocytes expressing alpha2beta4 receptors, Zn2+ greatly and reversibly increased the ACh current, without affecting considerably its time course. The ACh current potentiation by Zn2+ was weakly dependent on the membrane potential (2.33+/-0.10 times the control current at -100 mV vs 2.04+/-0.06 at -60 mV, suggesting that Zn2+ interacts with the receptor in the vestibule of the ion channel or at an external domain of the protein. The inward rectification of control and Zn2+-potentiated ACh-currents was similar. We conclude that Zn2+ positively and reversibly modulates neuronal nicotinic receptors in a practically voltage-independent manner and without affecting their rate of desensitization. These results will help to understand better the roles played by Zn2+ in brain functions.

Nicotine, brain nicotinic receptors, and neuropsychiatric disorders

Neuronal nicotinic acetylcholine receptors (nAChRs) represent a large family of ligand-gated cation channels with diverse structures and properties. In contrast to the muscular nAChRs, the physiological functions of neuronal nAChRs are not well defined to date. Behavioral studies indicate that brain nAChRs participate in complex functions such as attention, memory, and cognition, whereas clinical data suggest their involvement in the pathogenesis of certain neuropsychiatric disorders (Alzheimer's and Parkinson's diseases, Tourette's syndrome, schizophrenia, depression, etc.). For the majority of these disorders, the use of nAChRs' agonists may represent either a prophylactic (especially for Alzheimer's and Parkinson's diseases) or a symptomatic treatment. The possible mechanisms underlying these beneficial effects as well as the characteristics and potential therapeutic use of new, subtype-selective nAChRs agonists are presented.

Inherited and experimentally induced changes in gating kinetics of muscle nicotinic acetylcholine receptor

Ligand-gated ion channels (LGIC) allow rapid responses in the nervous system. The nicotonic acetylcholine receptor (AChR) has been the model for structure-function relationship studies on this superfamily. The AChR undergoes the following functional events: 1. Binding of the neurotransmitter. 2. Opening of the ion channel. 3. Conduction of ions across the pore. 4. Desensitization. The equilibrium among these processes can be perturbed by alteration in the primary structure of the AChR or by the presence of pharmacological agents. Changes in the primary sequence leading to modifications in gating kinetics may occur in association with physiological or pathological processes. Such changes can also be genetically engineered to gain insights into structure-function relationships.

Nicotinic acetylcholine receptors assembled from the alpha7 and beta3 subunits

Intracellular recordings were performed in voltage-clamped Xenopus oocytes upon injection with a mixture of cDNAs encoding the beta3 and mutant alpha7 (L247Talpha7) neuronal nicotinic acetylcholine receptor (nAChR) subunits. The expressed receptors maintained sensitivity to methyllycaconitine and to alpha-bungarotoxin but exhibited a functional profile strikingly different from that of the homomeric L247Talpha7 receptor. The heteromeric L247Talpha7beta3 nAChR had a lower apparent affinity and a faster rate of desensitization than L247Talpha7 nAChR, exhibited nonlinearity in the I-V relationship, and was inhibited by 5-hydroxytryptamine, much like wild type alpha7 (WTalpha7) nAChR. Single channel recordings in cell-attached mode revealed unitary events with a slope conductance of 19 picosiemens and a lifetime of 5 ms, both values being much smaller than those of the homomeric receptor channel. Upon injection with a mixture of WTalpha7 and beta3 cDNAs, clear evidence was obtained for the plasma membrane assembly of heteromeric nAChRs, although ACh could not activate these receptors. It is concluded that beta3, long believed to be an orphan subunit, readily co-assembles with other subunits to form heteromeric receptors, some of which may be negative regulators of cholinergic function.

Block by 5-hydroxytryptamine and apomorphine of recombinant human neuronal nicotinic receptors

The effects of 5-hydroxytryptamine and apomorphine on human neuronal nicotinic acetylcholine receptor/channels were examined by expressing these channels in Xenopus oocytes. Functional channels were expressed by combining one type of alpha subunits (alpha3 or alpha4) and one type of beta subunits (beta2 or beta4). 5-Hydroxytryptamine (100 microM to 1 mM) and apomorphine (10 to 100 microM) inhibited an inward current activated by acetylcholine in the oocytes expressing the channels. The sensitivity to 5-hydroxytryptamine or apomorphine depended on subunit combinations. When concentration-response relationship was obtained for the acetylcholine-activated current, the maximal response was reduced by these compounds. The inhibition by these compounds exhibited voltage-dependence: the inhibition was augmented at negative potentials. The results suggest that 5-hydroxytryptamine and apomorphine noncompetitively inhibits human recombinant nicotinic acetylcholine receptor/channels, presumably by acting on channel pores.

Effects of choline and other nicotinic agonists on the tectum of juvenile and adult Xenopus frogs: a patch-clamp study

We have used anatomical methods and whole-cell patch-clamp recording to assess the distribution of nicotinic receptors in the tectum of Xenopus frogs and to measure effects of nicotinic ligands (carbachol, cytisine and nicotine) on glutamatergic spontaneous miniature excitatory postsynaptic currents. Our results confirm that retinotectal axons account for the majority of nicotinic receptors in the tectum and that nicotinic agonists exert presynaptic effects that increase the rate of transmitter release on to tectal cells. The nicotinic blockers mecamylamine and methyllycaconitine reduced responses to carbachol and cytisine. A small percentage of cells also showed postsynaptic responses. We have assessed whether there are developmental changes in the frequency of occurrence of spontaneous miniature excitatory postsynaptic currents. The first three months post-metamorphosis fall within the critical period for the dramatic plasticity displayed by binocular inputs during development in Xenopus. During this period, visual activity governs the formation of orderly maps relayed from the ipsilateral eye via the cholinergic projection from the nucleus isthmi to the tectum. In this study, we have found that critical-period tecta (two to 12 weeks postmetamorphosis) tend to have higher spontaneous activity than do older tecta (two to 69 weeks postmetamorphosis), and that nicotinic agonists increase that activity in both groups, with the result that the peak rates in response to nicotinic agonists are higher during the critical period than later. We also investigated the possible role of choline as an agonist of nicotinic receptors in the tectum. We have found that choline, as well as carbachol and cytisine, can cause a reversible increase in the rate of miniature excitatory postsynaptic currents. This result may help to explain how the isthmotectal projection, which accounts for the overwhelming majority of cholinergic input to the tectum, can exert effects on retinotectal terminals even though there are no morphologically identifiable synapses between the two populations. We have examined the morphology of cells filled with biocytin during the patch-clamp experiments, and we find that cells with dendrites in the stratum zonale, a layer with particularly dense input from the contralateral nucleus isthmi, have higher spontaneous activity than cells with dendrites that do not extend into that layer. Nicotinic agonists increased the activity recorded in both classes of cells. In addition, four pretectal cells were identified. Nicotinic agonists increased the rate of spontaneous activity recorded in that population. The results indicate that retinotectal transmission in the superior colliculus can be increased presynaptically by activity of the cholinergic projections of the nucleus isthmi. This modulation may be the basis for observations that blocking of cholinergic input disrupts the formation of topographic retinotectal projections. Moreover, the ability of choline to activate these receptors suggests that this metabolite of acetylcholine may permit paracrine activation of presynaptic receptors even though the tectum contains high acetylcholinesterase activity.

Modulation of nicotinic acetylcholine receptors by strychnine

Strychnine, a potent and selective antagonist at glycine receptors, was found to inhibit muscle (alpha1beta1gammadelta, alpha1beta1gamma, and alpha1beta1delta) and neuronal (alpha2beta2 and alpha2beta4) nicotinic acetylcholine receptors (AcChoRs) expressed in Xenopus oocytes. Strychnine alone (up to 500 microM) did not elicit membrane currents in oocytes expressing AcChoRs, but, when applied before, concomitantly, or during superfusion of acetylcholine (AcCho), it rapidly and reversibly inhibited the current elicited by AcCho (AcCho-current). Although in the three cases the AcCho-current was reduced to the same level, its recovery was slower when the oocytes were preincubated with strychnine. The amount of AcCho-current inhibition depended on the receptor subtype, and the order of blocking potency by strychnine was alpha1beta1gammadelta > alpha2beta4 > alpha2beta2. With the three forms of drug application, the Hill coefficient was close to one, suggesting a single site for the receptor interaction with strychnine, and this interaction appears to be noncompetitive. The inhibitory effects on muscle AcChoRs were voltage-independent, and the apparent dissociation constant for AcCho was not appreciably changed by strychnine. In contrast, the inhibitory effects on neuronal AcChoRs were voltage-dependent, with an electrical distance of approximately 0.35. We conclude that strychnine regulates reversibly and noncompetitively the embryonic type of muscle AcChoR and some forms of neuronal AcChoRs. In the former case, strychnine presumably inhibits allosterically the receptor by binding at an external domain whereas, in the latter case, it blocks the open receptor-channel complex.

The alpha9 nicotinic acetylcholine receptor shares pharmacological properties with type A gamma-aminobutyric acid, glycine, and type 3 serotonin receptors

In the present study, we provide evidence that the alpha9 nicotinic acetylcholine receptor (nAChR) shares pharmacological properties with members of the Cys-loop family of receptors. Thus, the type A gamma-aminobutyric acid receptor antagonist bicuculline, the glycinergic antagonist strychnine, and the type 3 serotonin receptor antagonist ICS-205,930 block ACh-evoked currents in alpha9-injected Xenopus laevis oocytes with the following rank order of potency: strychnine > ICS-205,930 > bicuculline. Block by antagonists was reflected in an increase in the acetylcholine (ACh) EC50 value, with no changes in agonist maximal response or Hill coefficient, which suggests a competitive type of block. Moreover, whereas neither gamma-aminobutyric acid nor glycine modified ACh-evoked currents, serotonin blocked responses to ACh in a concentration-dependent manner. The present results suggest that the alpha9 nAChR must conserve in its primary structure some residues responsible for ligand binding common to other Cys-loop receptors. In addition, it adds further evidence that the alpha9 nAChR and the cholinergic receptor present at the base of cochlear outer hair cells have similar pharmacological properties.

Binding sites for exogenous and endogenous non-competitive inhibitors of the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor (AChR) is the paradigm of the neurotransmitter-gated ion channel superfamily. The pharmacological behavior of the AChR can be described as three basic processes that progress sequentially. First, the neurotransmitter acetylcholine (ACh) binds the receptor. Next, the intrinsically coupled ion channel opens upon ACh binding with subsequent ion flux activity. Finally, the AChR becomes desensitized, a process where the ion channel becomes closed in the prolonged presence of ACh. The existing equilibrium among these physiologically relevant processes can be perturbed by the pharmacological action of different drugs. In particular, non-competitive inhibitors (NCIs) inhibit the ion flux and enhance the desensitization rate of the AChR. The action of NCIs was studied using several drugs of exogenous origin. These include compounds such as chlorpromazine (CPZ), triphenylmethylphosphonium (TPMP+), the local anesthetics QX-222 and meproadifen, trifluoromethyl-iodophenyldiazirine (TID), phencyclidine (PCP), histrionicotoxin (HTX), quinacrine, and ethidium. In order to understand the mechanism by which NCIs exert their pharmacological properties several laboratories have studied the structural characteristics of their binding sites, including their respective locations on the receptor. One of the main objectives of this review is to discuss all available experimental evidence regarding the specific localization of the binding sites for exogenous NCIs. For example, it is known that the so-called luminal NCIs bind to a series of ring-forming amino acids in the ion channel. Particularly CPZ, TPMP+, QX-222, cembranoids, and PCP bind to the serine, the threonine, and the leucine ring, whereas TID and meproadifen bind to the valine and extracellular rings, respectively. On the other hand, quinacrine and ethidium, termed non-luminal NCIs, bind to sites outside the channel lumen. Specifically, quinacrine binds to a non-annular lipid domain located approximately 7 A from the lipid-water interface and ethidium binds to the vestibule of the AChR in a site located approximately 46 A away from the membrane surface and equidistant from both ACh binding sites. The non-annular lipid domain has been suggested to be located at the intermolecular interfaces of the five AChR subunits and/or at the interstices of the four (M1-M4) transmembrane domains. One of the most important concepts in neurochemistry is that receptor proteins can be modulated by endogenous substances other than their specific agonists. Among membrane-embedded receptors, the AChR is one of the best examples of this behavior. In this regard, the AChR is non-competitively modulated by diverse molecules such as lipids (fatty acids and steroids), the neuropeptide substance P, and the neurotransmitter 5-hydroxytryptamine (5-HT). It is important to take into account that the above mentioned modulation is produced through a direct binding of these endogenous molecules to the AChR. Since this is a physiologically relevant issue, it is useful to elucidate the structural components of the binding site for each endogenous NCI. In this regard, another important aim of this work is to review all available information related to the specific localization of the binding sites for endogenous NCIs. For example, it is known that both neurotransmitters substance P and 5-HT bind to the lumen of the ion channel. Particularly, the locus for substance P is found in the deltaM2 domain, whereas the binding site for 5-HT and related compounds is putatively located on both the serine and the threonine ring. Instead, fatty acid and steroid molecules bind to non-luminal sites. More specifically, fatty acids may bind to the belt surrounding the intramembranous perimeter of the AChR, namely the annular lipid domain, and/or to the high-affinity quinacrine site which is located at a non-annular lipid domain. Additionally, steroids may bind to a site located on the extracellular hydrophi

Potentiation and inhibition of neuronal nicotinic receptors by atropine: competitive and noncompetitive effects

Atropine, the classic muscarinic receptor antagonist, inhibits ion currents mediated by neuronal nicotinic acetylcholine receptors expressed in Xenopus laevis oocytes. At the holding potential of -80 mV, 1 microM atropine inhibits 1 mM acetylcholine-induced inward currents mediated by rat alpha2beta2, alpha2beta4, alpha3beta2, alpha3beta4, alpha4beta2, alpha4beta4, and alpha7 nicotinic receptors by 12-56%. Inward currents induced with a low agonist concentration are equally inhibited (alpha3beta2, alpha3beta4), less inhibited (alpha2beta4, alpha7), or potentiated (alpha4beta2, alpha4beta4) by 1 microM atropine. Effects on the more sensitive alpha4beta4 nicotinic receptors were investigated in detail by systematic variation of acetylcholine and atropine concentrations and of membrane potential. At high agonist concentration, atropine inhibits alpha4beta4 nicotinic receptor-mediated ion current in a noncompetitive, voltage-dependent way with IC50 values of 655 nM at -80 mV and of 4.5 microM at -40 mV. At low agonist concentration, 1 microM atropine potentiates alpha4beta4 nicotinic receptor-mediated ion current. This potentiating effect is surmounted by high concentrations of acetylcholine, indicating a competitive interaction of atropine with the nicotinic receptor, and potentiation is also reversed at high atropine concentrations. Steady state effects of acetylcholine and atropine are accounted for by a model for combined receptor occupation and channel block, in which atropine acts on two distinct sites. The first site is associated with noncompetitive ion channel block. The second site is associated with competitive potentiation, which appears to occur when the agonist recognition sites of the receptor are occupied by acetylcholine and atropine. The apparent affinity of atropine for the agonist recognition sites of the alpha4beta4 nicotinic acetylcholine receptor is estimated to be 29.9 microM.

Opposite effects of lanthanum on different types of nicotinic acetylcholine receptors

The effects of lanthanum (La3+) were studied on muscle and neuronal nicotinic acetylcholine receptors (AChRs) expressed in Xenopus oocytes. La3+ exerts a dose-dependent positive modulation on alpha1 beta1 gamma8 muscle AChRs, whereas it modulates negatively either alpha2 beta2, alpha2 beta4 or alpha3 beta4 neuronal AChRs. Moreover, La3+ appears to accelerate the desensitization of neuronal receptors. In both muscle and neuronal AChRs, the respective potentiating or inhibiting effects of La3+ on the ACh-currents are voltage-independent, suggesting that La3+ is acting at a site located in the external domain of the receptor.

Serotonin-1A antagonists attenuate the effects of nicotine withdrawal on the auditory startle response

Withdrawal from the chronic administration of nicotine has previously been shown to lead to an enhanced auditory startle response in rats. In order to explore the neuropharmacology and neurophysiology underlying this phenomenon, we examined the effects of various 5-hydroxytryptamine (5-HT)-1A antagonists and agonists on the nicotine-withdrawal-enhanced auditory startle response in male rats. Animals were treated with nicotine (6 mg/kg/day nicotine base, via subcutaneously implanted osmotic minipumps) for 12 days. After 12 days the pumps were removed and the animals allowed to undergo spontaneous withdrawal for several days. In agreement with previous results, nicotine withdrawal led to a significant elevation of the auditory startle response. Pretreatment with the 5-HT-1A agonists (+)8-OH-DPAT (0.001-0.1 mg/kg) and LY274600 (0.3-3.0 mg/kg) either had no affect or exacerbated the nicotine-withdrawal-enhanced startle response. Pretreatment with the 5-HT-1A antagonists NAN-190 (1-3 mg/kg), LY206130 (1-10 mg/kg), or WAY-100635 (0.1-1.0 mg/kg) blocked the increase in the startle response caused by nicotine withdrawal at doses that had no effect on baseline startle responses. These data indicate that 5-HT-1A receptors play a role in the neurophysiology of nicotine withdrawal. In addition, 5-HT-1A antagonists may be able to relieve some nicotine withdrawal symptoms in man and may represent a novel pharmacotherapy for smoking cessation.

Topology of ligand binding sites on the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor (AChR) presents two very well differentiated domains for ligand binding that account for different cholinergic properties. In the hydrophilic extracellular region of both alpha subunits there exist the binding sites for agonists such as the neurotransmitter acetylcholine (ACh) and for competitive antagonists such as d-tubocurarine. Agonists trigger the channel opening upon binding while competitive antagonists compete for the former ones and inhibit its pharmacological action. Identification of all residues involved in recognition and binding of agonist and competitive antagonists is a primary objective in order to understand which structural components are related to the physiological function of the AChR. The picture for the localisation of the agonist/competitive antagonist binding sites is now clearer in the light of newer and better experimental evidence. These sites are mainly located on both alpha subunits in a pocket approximately 30-35 A above the surface membrane. Since both alpha subunits are sequentially identical, the observed high and low affinity for agonists on the receptor is conditioned by the interaction of the alpha subunit with the delta or the gamma chain, respectively. This relationship is opposite for curare-related drugs. This molecular interaction takes place probably at the interface formed by the different subunits. The principal component for the agonist/competitive antagonist binding sites involves several aromatic residues, in addition to the cysteine pair at 192-193, in three loops-forming binding domains (loops A-C). Other residues such as the negatively changed aspartates and glutamates (loop D), Thr or Tyr (loop E), and Trp (loop F) from non-alpha subunits were also found to form the complementary component of the agonist/competitive antagonist binding sites. Neurotoxins such as alpha-, kappa-bungarotoxin and several alpha-conotoxins seem to partially overlap with the agonist/competitive antagonist binding sites at multiple point of contacts. The alpha subunits also carry the binding site for certain acetylcholinesterase inhibitors such as eserine and for the neurotransmitter 5-hydroxytryptamine which activate the receptor without interacting with the classical agonist binding sites. The link between specific subunits by means of the binding of ACh molecules might play a pivotal role in the relative shift among receptor subunits. This conformational change would allow for the opening of the intrinsic receptor cation channel transducting the external chemical signal elicited by the agonist into membrane depolarisation. The ion flux activity can be inhibited by non-competitive inhibitors (NCIs). For this kind of drugs, a population of low-affinity binding sites has been found at the lipid-protein interface of the AChR. In addition, several high-affinity binding sites have been found to be located at different rings on the M2 transmembrane domain, namely luminal binding sites. In this regard, the serine ring is the locus for exogenous NCIs such as chlorpromazine, triphenylmethylphosphonium, the local anaesthetic QX-222, phencyclidine, and trifluoromethyliodophenyldiazirine. Trifluoromethyliodophenyldiazirine also binds to the valine ring, which is the postulated site for cembranoids. Additionally, the local anaesthetic meproadifen binding site seems to be located at the outer or extracellular ring. Interestingly, the M2 domain is also the locus for endogenous NCIs such as the neuropeptide substance P and the neurotransmitter 5-hydroxytryptamine. In contrast with this fact, experimental evidence supports the hypothesis for the existence of other NCI high-affinity binding sites located not at the channel lumen but at non-luminal binding domains. (ABSTRACT TRUNCATED)

Neuronal nicotinic threonine-for-leucine 247 alpha7 mutant receptors show different gating kinetics when activated by acetylcholine or by the noncompetitive agonist 5-hydroxytryptamine

Mutation of the highly conserved leucine residue (Leu-247) converts 5-hydroxytryptamine (5HT) from an antagonist into an agonist of neuronal homomeric alpha7 nicotinic acetylcholine receptor expressed in Xenopus oocytes. We show here that acetylcholine (AcCho) activates two classes of single channels with conductances of 44 pS and 58 pS, similar to those activated by 5HT. However, the mean open time of AcCho-gated ion channels (11 ms) is briefer than that of 5HT-gated ion channels (18 ms). Furthermore, whereas the open time of AcCho channels lengthens with hyperpolarization, that of 5HT channels is decreased. In voltage-clamped oocytes, the apparent affinity of the alpha7 mutant receptor for 5HT is not modified by the presence of dihydro-beta-erythroidine, which acts on the AcCho binding site in a competitive manner. This indicates a noncompetitive action of 5HT on nicotinic acetylcholine receptors. Considered together, our findings show that AcCho gates alpha7 mutant channels with similar conductance but with different kinetic profile than the channels gated by 5HT, suggesting that the two agonists act on different docking sites. These results will help to understand the crosstalk between cholinergic and serotonergic systems in the central nervous system.

Blockage of muscle and neuronal nicotinic acetylcholine receptors by fluoxetine (Prozac)

Fluoxetine (Prozac), a widely used antidepressant, is said to exert its medicinal effects almost exclusively by blocking the serotonin uptake systems. The present study shows that both muscle and neuronal nicotinic acetylcholine receptors are blocked, in a noncompetitive and voltage-dependent way, by fluoxetine, which also increases the rate of desensitization of the nicotinic receptors. Because these receptors are very widely distributed in the both central and peripheral nervous systems, the blocking action of fluoxetine on nicotinic receptors may play an important role in its antidepressant and other therapeutical effects. Our findings will help to understand the mode of action of fluoxetine, and they may also help to develop more specific medicinal drugs.

Nicotinic acetylcholine receptors on hippocampal neurons: distribution on the neuronal surface and modulation of receptor activity

The recent development of a technique that uses infrared microscopy for the visualization of well-defined areas on the surface of neurons, and a computerized system of micromanipulators led to the discovery that functional nicotinic acetylcholine receptors (nAChRs) are expressed at higher density on the dendrites than on the soma of rat hippocampal neurons. The finding that the expression of alpha-bungarotoxin-sensitive, alpha 7-bearing, nAChRs and dihydro-beta-erythroidine-sensitive, alpha 4 beta 2 nAChRs tends to increase along the dendritic length suggests that these receptors may be highly involved in the integration of synaptic functions in hippocampal neurons. The present report also discusses the finding that ligands such as the anticholinesterase galanthamine can modulate the nAChR activity by binding to a novel receptor site, and that 5-hydroxytryptamine (5-HT) may serve as an endogenous ligand for this site. The ability of 5-HT to modulate the nAChR function in vivo supports the concept that the overall CNS function is determined not only by the neuronal network established by the neuronal wiring, but also by a chemical network established by the ability of a single substance to act as the primary neurotransmitter in one system and as a co-transmitter in another system.

Threonine-for-leucine mutation within domain M2 of the neuronal alpha(7) nicotinic receptor converts 5-hydroxytryptamine from antagonist to agonist

A study was made of the effects of 5-hydroxytryptamine (5HT) on homomeric neuronal nicotinic receptors (nAcChoR) expressed in Xenopus oocytes after injection of cDNA encoding the wild-type chicken alpha(7) subunit. Acetylcholine (AcCho) elicited large currents (IAcCho) that were reduced by 5HT in a reversible and dose-dependent manner, with a half-inhibitory concentration (IC50) of 56 microM and a Hill coefficient (nH) of 1.2. The inhibition of IAcCho by 5HT was noncompetitive and voltage independent, a behavior incompatible with a channel blockade mechanism. 5HT alone did not elicit membrane currents in oocytes injected with the wild-type alpha(7) subunit cDNA. In contrast, 5HT elicited membrane currents (I5HT) in oocytes injected with cDNA encoding an alpha(7) mutant subunit with a threonine-for-leucine-247 substitution (L247T alpha(7)). I5HT was inhibited by the potent nicotinic receptor blockers alpha-bungarotoxin (100 nM) and methyllycaconitine (1 microM). Furthermore, the characteristics of I5HT, including its voltage dependence, were similar to those of IAcCho. The 5HT dose-I5HT response gave an apparent dissociation constant EC50 of 23.5 microM and a Hill coefficient nH of 1.7, which were not modified by the presence of AcCho. Similarly, the apparent affinity of L247T alpha(7) for AcCho as well as its cooperativity were not influenced by 5HT, indicating a lack of mutual interactions between 5HT and AcCho. These results show that 5HT is a potent noncompetitive antagonist of neuronal alpha(7) nAcChoR, but it becomes a noncompetitive agonist following mutation of the highly conserved leucine residue 247 located in the channel domain M2.

Serotonergic modulation of muscle acetylcholine receptors of different subunit composition

Modulation of muscle acetylcholine (AcCho) receptors (AcChoRs) by serotonin [5-hydroxytryptamine (5HT)] and other serotonergic compounds was studied in Xenopus laevis oocytes. Various combinations of alpha, beta, gamma, and delta subunit RNAs were injected into oocytes, and membrane currents elicited by AcCho were recorded under voltage clamp. Judging by the amplitudes of AcCho currents generated, the levels of functional receptor expression were: alpha beta gamma delta > alpha beta delta > alpha beta gamma > alpha gamma delta. The alpha beta gamma delta and alpha beta delta AcChoR Subtypes were strongly blocked by 5HT, whereas the alpha beta gamma receptor was blocked only slightly. The order of blocking potency of AcChoRs by 5HT was: alpha beta delta > alpha beta gamma delta > alpha beta gamma. 5HT receptor antagonists, such as methysergide and spiperone, were even more potent blockers of AcChoRs than 5HT but did not show much subunit selectivity. Blockage of alpha beta gamma delta and alpha beta delta receptors by 5HT was voltage-dependent, and the voltage dependence was abolished when the delta subunit was omitted. These findings may need to be taken into consideration when trying to elucidate the mode of action of many clinically important serotonergic compounds.

Transgenic expression of human acetylcholinesterase induces progressive cognitive deterioration in mice

BACKGROUND

Cognitive deterioration is a characteristic symptom of Alzheimer's disease. This deterioration is notably associated with structural changes and subsequent cell death which occur, primarily, in acetylcholine-producing neurons, progressively damaging cholinergic neurotransmission. We have reported previously that excess acetylcholinesterase (AChE) alters structural features of neuromuscular junctions in transgenic Xenopus tadpoles. However, the potential of cholinergic imbalance to induce progressive decline of memory and learning in mammals has not been explored.

RESULTS

To approach the molecular mechanisms underlying the progressive memory deficiencies associated with impaired cholinergic neurotransmission, we created transgenic mice that express human AChE in brain neurons. With enzyme levels up to two-fold higher than in control mice, transgenic mice displayed an age-independent resistance to the hypothermic effects of the AChE inhibitor, paraoxon. In addition to this improved scavenging capacity for anti-AChEs, however, these transgenic mice also resisted muscarinic, nicotinic and serotonergic agonists, indicating that secondary pharmacological changes had occurred. The transgenic mice also developed progressive learning and memory impairments, although their locomotor activities and open-field behaviour remained similar to those of matched control mice. By six months of age, transgenic mice lost their ability to respond to training in a spatial learning water maze test, whereas they performed normally in this test at the age of four weeks. This animal model is therefore suitable for investigating the transcriptional changes associated with cognitive deterioration and for testing drugs that may attenuate progressive damage.

CONCLUSION

We conclude that upsetting cholinergic balance may by itself cause progressive memory decline in mammals, suggesting that congenital and/or acquired changes in this vulnerable balance may contribute to the physiopathology of Alzheimer's disease.