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.

Functional and pharmacological properties of GABArho1delta51 receptors

Gamma-aminobutyrate is the main inhibitory neurotransmitter in the vertebrate brain, and the gamma-aminobutyric acid (GABA) receptor subunit GABArho1delta51 is an alternatively spliced form of the GABArho1 receptor that was recently isolated from human retina cDNA libraries. The rho1delta51 receptor subunit lacks 17 amino acids in the extracellular N-terminal domain and, when expressed in Xenopus oocytes, forms functional homomeric GABA receptors. Unexpectedly, even after a such a big deletion, the fundamental properties of the deleted variant receptors are very similar to those of the complete GABArho1 receptors. For example, both types of receptors are bicuculline resistant, desensitize very little, and are negatively modulated by Zn2+ and positively modulated by La3+. In spite of such similarities, the GABArho1delta51 receptors are more sensitive to GABA, to the specific GABA(C) antagonist (1,2,5,6-tetrahydropyridine-4-yl)methylphosphinic acid and to Zn2+, than the complete GABArho1 receptors. The GABArho1delta51 receptors extend the variety of inhibitory receptors in the retina. Their functional significance still remains to be determined.

Muscarinic receptor heterogeneity in follicle-enclosed Xenopus oocytes

1. Ionic current responses elicited by acetylcholine (ACh) in follicle-enclosed Xenopus oocytes (follicles) were studied using the two-electrode voltage-clamp technique. ACh generated a fast chloride current (Fin) and inhibited K+ currents gated by cAMP (IK,cAMP) following receptor activation by adenosine, follicle-stimulating hormone or noradrenaline. These previously described cholinergic responses were confirmed to be of the muscarinic type, and were independently generated among follicles from different frogs. 2. Inhibition of IK,cAMP was about 100 times more sensitive to ACh than Fin activation; the half-maximal effective concentrations (EC50) were 6.6 +/- 0.4 and 784 +/- 4 nM, respectively. 3. Both responses were blocked by several muscarinic receptor antagonists. Using the respective EC50 concentrations of ACh as standard, the antagonist 4-diphenylacetoxy-N-methylpiperidine methiodide blocked the two effects with very different potencies. Fin was blocked with a half-maximal inhibitory concentration (IC50) of 2.4 +/- 0.07 nM, whilst the IC50 for IK,cAMP inhibition was 5.9 +/- 0.2 microM. 4. Oxotremorine, a muscarinic agonist, preferentially stimulated IK, cAMP inhibition (EC50 = 15.8 +/- 1.4 microM), whilst Fin was only weakly activated. In contrast, oxotremorine inhibited Fin generated by ACh with an IC50 of 2.3 +/- 0.7 microM. 5. Fin elicited via purinergic receptor stimulation was not affected by oxotremorine, indicating that the inhibition produced was specific to the muscarinic receptor, and suggesting that muscarinic actions do not exert a strong effect on follicular cell-oocyte coupling. 6. Using reverse transcription-PCR, transcripts of a previously cloned muscarinic receptor from Xenopus (XlmR) were amplified from the RNA of both the isolated follicular cells and the oocyte. The pharmacological and molecular characteristics suggest that XlmR is involved in IK,cAMP inhibition. 7. In conclusion, follicular cells possess two different muscarinic receptors, one resembling the M2 (or M4) subtype and the other the M3 subtype. These receptors are coupled to distinct membrane mechanisms leading to independent regulation of two membrane conductances.

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.

Blockage of mouse muscle nicotinic receptors by serotonergic compounds

Xenopus laevis oocytes were used to analyse the effects of serotonin (5-hydroxytryptamine, 5-HT) and serotonergic agents on ionic currents elicited by the activation of mammalian muscle nicotinic acetylcholine receptors (AChRs). 5-HT as well as other serotonergic agents, such as ketanserin, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), methysergide, spiperone, or fluoxetine alone (up to 1 mM), did not elicit membrane currents in Xenopus oocytes expressing AChRs, but they reversibly reduced the current elicited by acetylcholine (ACh-current). Serotonin was applied before, together with or after ACh application, and its effects were examined on desensitizing and non-desensitizing ACh-currents. 5-HT reduced the amplitude and accelerated the desensitization of the desensitizing currents. In contrast, non-desensitizing currents were reduced in amplitude but their time course was not significantly affected. With the same concentration of 5-HT the inhibition was stronger on desensitizing than on non-desensitizing ACh-currents. For example, 100 microM 5-HT reduced the peak of a desensitizing ACh-current to 0. 48 +/- 0.06 (peak current ratio) and after 40 s the current was reduced to a ratio of 0.25 +/- 0.04, whereas a non-desensitizing ACh-current was reduced to a ratio of 0.66 +/- 0.01. All the serotonergic agents tested inhibited the ACh-currents rapidly and reversibly, suggesting that they are acting directly on the AChRs. The half-inhibitory concentration, IC50, of 5-HT acting on non-desensitizing currents elicited by 250 nM ACh was 247 +/- 26 microM and the Hill coefficient was 0.88, suggesting a single site for the interaction of 5-HT with the receptor. It appears that 5-HT inhibits AChRs non-competitively because neither the half-effective concentration of ACh, EC50, for ACh-current nor the Hill coefficient were affected by 5-HT. Furthermore, the extent of inhibition of 5-HT on AChRs did not depend on the nicotinic agonist (suberyldicholine, ACh or nicotine). The inhibition of AChRs by serotonergic agents was voltage-dependent. The electrical distance of the binding site for 5-HT was 0.75, whereas for the other serotonergic agents tested it was 0.22, suggesting that ketanserin, 8-OH-DPAT, methysergide, spiperone and fluoxetine act within the ion channel, but at a site more external than that for 5-HT. These substances inhibited the ACh-current more potently than 5-HT. We conclude that 5-HT and serotonergic agents inhibit, in a non-competitive manner, the ACh-current in muscle AChRs by blocking the open receptor-channel complex. Moreover, 5-HT appears to promote the desensitized state of the receptor when the current is elicited by high ACh concentrations.

Subunit-selective modulation of GABAA receptors by the non-steroidal anti-inflammatory agent, mefenamic acid

Mefenamic acid (MFA) has anti-convulsant and pro-convulsant effects in vivo, and has been shown to potentiate and inhibit GABAA (gamma-aminobutyric acid) receptors in vitro. In this study, whole-cell currents were recorded from Xenopus oocytes and human embryonic kidney (HEK) cells expressing human recombinant GABAA receptors to resolve the molecular mechanisms by which MFA modulates GABAA receptor function. We demonstrate that MFA potentiated GABA-activated currents for alpha1beta2 gamma2S (EC50 = 3.2 +/- 0.5 microM), but not for alpha1beta1 gamma2S receptors. MFA also enhanced GABA-activated responses and directly activated alpha1beta2/beta3 GABAA receptors, but inhibited responses to GABA on alpha1beta1 constructs (IC50 = 40 +/- 7.2 microM). A comparison of beta1, beta2 and beta3 subunits suggested that the positive modulatory action of MFA involved asparagine (N) 290 in the second transmembrane domain (TM2) of the beta2 and beta3 subunits. Mutation of N290 to serine (S) markedly reduced modulation by MFA in alpha1beta2(N290S)gamma2S receptors, whereas alpha1beta1(S290N)gamma2S constructs revealed potentiated responses to GABA (EC50 = 7.8 +/- 1.7 microM) and direct activation by MFA. The potentiation by MFA displayed voltage sensitivity. The direct activation, potentiation and inhibitory aspects of MFA action were predominantly conferred by the beta subunits as the spontaneously active homomeric beta1 and beta3 receptors were susceptible to modulation by MFA. Molecular comparisons of MFA, loreclezole and etomidate, agents which exhibit similar selectivity for GABAA receptors, revealed their ability to adopt similar structural conformations. This study indicates that N290 in TM2 of beta2 and beta3 subunits is important for the regulation of GABAA receptor function by MFA. Our data provide a potential molecular mechanism for the complex central effects of MFA in vivo.

Antagonistic action of pitrazepin on human and rat GABA(A) receptors

Pitrazepin, 3-(piperazinyl-1)-9H-dibenz(c,f) triazolo(4,5-a)azepin is a piperazine antagonist of GABA in a variety of electrophysiological and in vitro binding studies involving GABA and glycine receptors. In the present study we have investigated the effects of pitrazepin, and the GABA(A) antagonist bicuculline, on membrane currents elicited by GABA in Xenopus oocytes injected with rat cerebral cortex mRNA or cDNAs encoding alpha1beta2 or alpha1beta2gamma2s human GABA(A) receptor subunits. The three types of GABA(A) receptors expressed were reversibly antagonized by bicuculline and pitrazepin in a concentration-dependent manner. GABA dose-current response curves for the three types of receptors were shifted to the right, in a parallel manner, by increasing concentrations of pitrazepin. Schild analyses gave pA2 values of 6.42+/-0.62, n = 4, 6.41+/-1.2, n = 5 and 6.21+/-1.24, n = 6, in oocytes expressing rat cerebral cortex, alpha1beta2 or alpha1beta2gamma2s human GABA(A) receptors respectively (values are given as means +/- s.e. mean), and the Hill coefficients were all close to unity. All this is consistent with the notion that pitrazepin acts as a competitive antagonist of these GABA(A) receptors; and that their antagonism by pitrazepin is not strongly dependent on the subunit composition of the receptors here studied. Since pitrazepin has been reported to act also at the benzodiazepine binding site, we studied the effect of the benzodiazepine antagonist Ro 15-1788 (flumazenil) on the inhibition of alpha1beta2gamma2s receptors by pitrazepin. Co-application of Ro 15-1788 did not alter the inhibiting effect of pitrazepin. Moreover, pitrazepin did not antagonize the potentiation of GABA-currents by flunitrazepam. All this suggests that pitrazepin does not affect the GABA receptor-chloride channel by interacting with the benzodiazepine receptor site.

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.

Cloning and expression of a P2y purinoceptor from the adult bovine corpus callosum

We have isolated an ATP receptor clone by screening a bovine corpus callosum cDNA library. The clone includes one open reading frame encoding for a protein of 373 amino acid residues (42 kDa) which belongs to the G-protein-coupled receptor superfamily. In Xenopus oocytes, this clone expressed an ATP receptor that triggered an oscillatory current in response to ATP (EC50 approximately 20 microM). This current may have resulted from the activation of phospholipase C, the formation of inositol trisphosphate, and the release of Ca2+, which then opens Cl- channels. The order of potency for ATP receptor agonists was 2-MeSATP approximately ATP >> alpha, beta-MeATP > adenosine, and UTP was ineffective, a pharmacological profile consistent with that of a P2y purinoceptor. Northern blot analysis of mRNAs from various bovine brain tissues showed that the gene is expressed in the cerebellum, medulla, corpus callosum, hippocampus, superior colliculus, frontal cortex, and retina. In situ RT-PCR showed transcripts of the gene in many glial cells and endothelial cells of the corpus callosum. The cloned receptor may play an important role in neuron-glial signaling under normal and pathological conditions.

Effects of Zn2+ on wild and mutant neuronal alpha7 nicotinic receptors

Zn2+ is a key structural/functional component of many proteins and is present at high concentrations in the brain and retina, where it modulates ligand-gated receptors. Therefore, a study was made of the effects of zinc on homomeric neuronal nicotinic receptors expressed in Xenopus oocytes after injection of cDNAs encoding the chicken wild or mutant alpha7 subunits. In oocytes expressing wild-type receptors, Zn2+ alone did not elicit appreciable membrane currents. Acetylcholine (AcCho) elicited large currents (IAcCho) that were reduced by Zn2+ in a reversible and dose-dependent manner, with an IC50 of 27 microM and a Hill coefficient of 0.4. The inhibition of IAcCho by Zn2+ was competitive and voltage-independent, a behavior incompatible with a channel blockade mechanism. In sharp contrast, in oocytes expressing a receptor mutant, with a threonine-for-leucine 247 substitution (L247Talpha7), subnanomolar concentrations of Zn2+ elicited membrane currents (IZn) that were reversibly inhibited by the nicotinic receptor blockers methyllycaconitine and alpha-bungarotoxin. Cell-attached single-channel recordings showed that Zn2+ opened channels that had a mean open time of 5 ms and a conductance of 48 pS. At millimolar concentrations Zn2+ reduced IAcCho and the block became stronger with cell hyperpolarization. Thus, Zn2+ is a reversible blocker of wild-type alpha7 receptors, but becomes an agonist, as well as an antagonist, following mutation of the highly conserved leucine residue 247 located in the M2 channel domain. We conclude that Zn2+ is a modulator as well as an activator of homomeric nicotinic alpha7 receptors.

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

Fluoxetine is used in the treatment of a variety of clinical disorders including depression and obesity, and of cocaine detoxification or alcoholism. It is generally believed that fluoxetine exerts its clinical effects because it selectively blocks 5-hydroxytryptamine (5HT) reuptake into nerve terminals. In here we describe that fluoxetine antagonized the neuronal homomeric alpha 7 nicotinic acetylcholine receptors (nAChR) expressed in Xenopus oocytes, with an IC50 of 43 microM, when fluoxetine was coapplied with ACh, and of 1.6 microM when the oocytes were pretreated briefly with fluoxetine. A similar block occurred in oocytes expressing L247T alpha 7 mutant nAChR. Furthermore, blockage of mutant alpha 7 receptors appeared non-competitive and was stronger with cell membrane hyperpolarization. Cell-attached single channel recordings in oocytes expressing L247T alpha 7 mutant nAChR showed that the voltage-dependence of the blockage by fluoxetine could be due to a drastic decrease in channel opening frequency accompanied by marked channel flickering and reduced channel conductance. We conclude that fluoxetine behaves as a reversible blocker of both wild and mutant alpha 7 receptors; and that the Leu-247T mutation in the channel domain renders the blockage of alpha 7 nAChR by fluoxetine voltage-dependent. These effects of fluoxetine on alpha 7 receptors may be clinically important.

Cloning and functional expression of alternative spliced variants of the rho1 gamma-aminobutyrate receptor

The rho1 gamma-aminobutyrate receptor (GABArho1) is expressed predominantly in the retina and forms homomeric GABA-gated Cl- channels that are clearly different from the multisubunit GABAA receptors. In contrast to these, GABArho1 receptors desensitize very little and are not blocked by bicuculline. In addition to GABArho1, two new variants were identified in human retina cDNA libraries. Cloning and sequence analysis showed that both variants contain large deletions in the putative extracellular domain of the receptor. These deletions extend from a common 5' site to different 3' sites. The cDNA with the largest deletion, named GABArho1Delta450, contains a complete ORF identical to that of GABArho1 but missing 450 nt. This cDNA encodes a protein of 323 aa, identical to the GABArho1, but has a deletion of 150 aa in the amino-terminal extracellular domain. GABArho1Delta450 mRNA injected into Xenopus oocytes did not produce functional GABA receptors. The second GABArho1 variant (GABArho1Delta51) contains a 51-nt deletion. In Xenopus oocytes, GABArho1Delta51 led to the expression of GABA receptors that had the essential GABArho1 characteristics of low desensitization and bicuculline resistance. Therefore, alternative splicing increases the coding potential of this gene family expressed in the human retina, but the functional diversity created by the alternative spliced forms is still not understood.

mRNAs coding for neurotransmitter receptors and voltage-gated sodium channels in the adult rabbit visual cortex after monocular deafferentiation

It has been postulated that, in the adult visual cortex, visual inputs modulate levels of mRNAs coding for neurotransmitter receptors in an activity-dependent manner. To investigate this possibility, we performed a monocular enucleation in adult rabbits and, 15 days later, collected their left and right visual cortices. Levels of mRNAs coding for voltage-activated sodium channels, and for receptors for kainate/alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), N-methyl-D-aspartate (NMDA), gamma-aminobutyric acid (GABA), and glycine were semiquantitatively estimated in the visual cortices ipsilateral and contralateral to the lesion by the Xenopus oocyte/voltage-clamp expression system. This technique also allowed us to study some of the pharmacological and physiological properties of the channels and receptors expressed in the oocytes. In cells injected with mRNA from left or right cortices of monocularly enucleated and control animals, the amplitudes of currents elicited by kainate or AMPA, which reflect the abundance of mRNAs coding for kainate and AMPA receptors, were similar. There was no difference in the sensitivity to kainate and in the voltage dependence of the kainate response. Responses mediated by NMDA, GABA, and glycine were unaffected by monocular enucleation. Sodium channel peak currents, activation, steady-state inactivation, and sensitivity to tetrodotoxin also remained unchanged after the enucleation. Our data show that mRNAs for major neurotransmitter receptors and ion channels in the adult rabbit visual cortex are not obviously modified by monocular deafferentiation. Thus, our results do not support the idea of a widespread dynamic modulation of mRNAs coding for receptors and ion channels by visual activity in the rabbit visual system.

Cl- currents activated via purinergic receptors in Xenopus follicles

Ionic currents elicited via purinergic receptors located in the membrane of Xenopus follicles were studied using electrophysiological techniques. Follicles responded to ATP-activating inward currents with a fast time course (F(in)). In Ringer solution, reversal potential (Erev) of F(in) was -22 mV, which did not change with external substitutions of Na- or K+, whereas solutions containing 50 or 5% of normal Cl- concentration shifted Erev to about +4 and +60 mV, respectively, and decreased F(in) amplitude, indicating that F(in) was carried by Cl-.F(in) had an onset delay of approximately 400 ms, measured by application of a brief jet of ATP from a micropipette positioned near the follicle (50 microns). F(in) was inhibited by 50% in follicles pretreated with pertussis toxin. This suggests a G protein-mediated receptor channel pathway. F(in) was mimicked by 2-MeSATP and UTP, the potency order (half-maximal effective concentration) was 2-MeSATP (194 nM) > UTP (454 nM) > ATP (1,086 nM). All agonists generated Cl- currents and displayed cross-inhibition on the others. F(in) activation by acetylcholine also cross-inhibited F(in)-ATP responses, suggesting that all act on a common channel-activation pathway.

Efficient coupling of 5-HT1a receptors to the phospholipase C pathway in Xenopus oocytes

To investigate the receptor-channel coupling pathway, the coding region of the 5-HT1a receptor was subcloned into two plasmid vectors pSP64(polyA+) and pSP64T. Compared to the original 5-HT1a receptor construct G-21, both new constructs increased greatly the expression of functional 5-HT1a receptors in Xenopus oocytes, which developed large inward current responses to 5-HT. These responses were dose-dependent (EC50 approximately 150 nM), and could be elicited also by 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). The 5-HT1a receptor mediated current had an oscillatory time course, and a reversal potential close to the equilibrium potential for Cl- (ca. -25 mV). Moreover, during and for some minutes following the application of 5-HT, these oocytes acquired the property of generating a transient inward current when their membrane was hyperpolarized. These features are characteristic of responses mediated by other receptors (e.g. muscarinic, angiotensin, serum receptors, etc.) that are known to couple to the endogenous PLC/PI second messenger pathway in Xenopus oocytes. In particular, the 5-HT1a receptor mediated current was very similar to the current induced by 5-HT-stimulation of heterogenic 5-HT2c receptors. Our results show further that the 5-HT1a receptor couples to the endogenous PLC/PI pathway much less efficiently than the 5-HT2c receptor. These results demonstrate clearly that the human 5-HT1a receptor can couple efficiently to the Xenopus oocyte endogenous PLC/PI pathway, and provide additional evidence for cell-specific signal transduction.

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.

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.

Glutamate receptor-mediated toxicity in optic nerve oligodendrocytes

In cultured oligodendrocytes isolated from perinatal rat optic nerves, we have analyzed the expression of ionotropic glutamate receptor subunits as well as the effect of the activation of these receptors on oligodendrocyte viability. Reverse transcription-PCR, in combination with immunocytochemistry, demonstrated that most oligodendrocytes differentiated in vitro express the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunits GluR3 and GluR4 and the kainate receptor subunits GluR6, GluR7, KA1 and KA2. Acute and chronic exposure to kainate caused extensive oligodendrocyte death in culture. This effect was partially prevented by the AMPA receptor antagonist GYKI 52466 and was completely abolished by the non-N-methyl-D-aspartate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), suggesting that both AMPA and kainate receptors mediate the observed kainate toxicity. Furthermore, chronic application of kainate to optic nerves in vivo resulted in massive oligodendrocyte death which, as in vitro, could be prevented by coinfusion of the toxin with CNQX. These findings suggest that excessive activation of the ionotropic glutamate receptors expressed by oligodendrocytes may act as a negative regulator of the size of this cell population.

Irreversible antagonism of 5HT2c receptors by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ)

To determine if N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), a carboxyl group activating agent, can inactivate 5HT2c receptors, we have examined the effects of EEDQ on 5HT2c receptor-mediated responses to 5-hydroxytryptamine (5HT) in Xenopus oocytes, and on the binding of [3H]5HT to 5HT2c receptors in transfected HeLa cells. In oocytes expressing rat 5HT2c receptors, EEDQ inhibited the 5HT2c receptor-mediated Cl- currents; and the response did not recover more than 24 h after removal of the EEDQ. To see if this effect of EEDQ was on the receptor itself, the binding of 5HT to 5HT2c receptors was studied in transfected HeLa cells. EEDQ decreased the specific binding of [3H]5HT to 5HT2c receptors. At approximately 22 degrees C, incubating the membranes with 2 x 10(-4) M EEDQ for 1 h caused a 40% decrease in the Bmax, without changing the K(d). At 37 degrees C, the same treatment with EEDQ blocked [3H]5HT binding completely. Half-maximal inhibition occurred at 5 microM EEDQ at both temperatures, and washing for 1.5 h did not restore the binding, suggesting that the inactivation of 5HT2c receptor binding was practically irreversible. Results from both systems showed clearly that EEDQ is an irreversible antagonist of 5HT2c receptors and therefore can be used for many studies of this receptor.

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.

Blockage of 5HT2C serotonin receptors by fluoxetine (Prozac)

Fluoxetine (Prozac) inhibited the membrane currents elicited by serotonin (5-hydroxytryptamine; 5HT) in Xenopus oocytes expressing either cloned 5HT2C receptors or 5HT receptors encoded by rat cortex mRNA. Responses of 5HT2C receptors, elicited by nM concentrations of 5HT, were rapidly and reversibly blocked by micromolar concentrations of fluoxetine. For responses elicited by 1 microM 5HT, the IC50 of fluoxetine inhibition was approximately 20 microM. In accord with the electrophysiological results, fluoxetine inhibited the binding of [3H]5HT to 5HT2C receptors expressed in HeLa cells (Ki approximately 65-97 nM), and the binding to 5HT receptors in rat cortex membranes was also inhibited but less efficiently (Ki approximately 56 microM). Our results show that fluoxetine is a competitive and reversible antagonist of 5HT2C receptors and suggest that some therapeutic effects of fluoxetine may involve blockage of 5HT receptors, in addition to its known blockage of 5HT transporters. Similar work may help to design more selective compounds for use in the treatment of brain disorders.

Co-expression of the neuronal alpha7 and L247T alpha7 mutant subunits yields hybrid nicotinic receptors with properties of both wild-type alpha7 and alpha7 mutant homomeric receptors

Injection of cDNA encoding the neuronal alpha7 subunit into Xenopus oocytes yields homomeric receptors showing responses to AcCho that have low affinity, fast desensitization, nonlinear current-voltage (I-V) relation, and sensitivity to alpha-bungarotoxin (alpha-BTX) and 5-hydroxytryptamine (5HT), both substances acting as antagonists. Mutation of the Leu-247, located in the channel domain, changes 5HT from an antagonist to an agonist, slows the rate of desensitization, renders the I-V relation linear, and increases the affinity for acetylcholine (AcCho). A study was made of receptors expressed after injecting Xenopus oocytes with mixtures of cDNAs encoding the wild-type alpha7 (WT alpha7) and the L247T alpha7 mutated nicotinic AcCho receptors (nAcChoRs). The receptors expressed were again blocked by alpha-bungarotoxin (100 nM) but exhibited both WT alpha7 and alpha7 mutant functional characteristics. Out of eight different types of hybrid receptors identified, most were inhibited by 5HT (1 mM) and showed low sensitivity to AcCho, like the WT alpha7 receptors, but exhibited a slow rate of desensitization and an I-V relation similar to those of alpha7 mutant receptors. Together, these findings indicate that the increased nAcChoR affinity and the decreased nAc-ChoR desensitization after Leu-247 mutation are uncoupled events. We propose that receptor diversity is predicted by permutations of WT alpha7 and L247T alpha7 subunits in a pentameric symmetrical model and that even partial replacement of Leu-247 with a polar residue within the leucine ring in the channel domain considerably influences the properties of neuronal alpha7 nAcChoRs.

Local anesthetics inhibit receptors coupled to phosphoinositide signaling in Xenopus oocytes

Effects and the mechanism of action of quaternary amine local anesthetics on ligand- and voltage-activated ion currents were studied using voltage-clamped ovarian follicles and oocytes from Xenopus laevis. The fast inward and slow outward currents in response to acetylcholine were unaltered by procaine, whereas the oscillatory and smooth inward chloride currents (ICl) were abolished. Potassium currents (IK) elicited by norepinephrine and oscillatory ICl elicited by lysophosphatidic acid were blocked. Procaine caused a noncompetitive inhibition of oscillatory ICl mediated by heterologously expressed neurotransmitter receptors from the rat brain. Threefold differences were found in the procaine sensitivity of the 5-HT2a and 5-HT2c receptors. The rank order of intrinsic inhibitory activity of local anesthetics was: procaine > lidocaine > dibucaine > tetracaine. Extra- or intracellular application of procaine did not alter the Ca2+-activated Cl- current, indicating that neither the endogenous voltage-gated Ca2+ nor the Ca2+-activated Cl- channels account for the inhibition. Procaine caused only a slight reduction in ICl elicited by photolysis of caged inositol 1,4,5-trisphosphate (InsP3) and did not abolish ICl triggered by GTP[gamma-S]-induced direct activation of G proteins. For receptors coupling to the phosphoinositide/Ca2+ signal transduction pathway, the primary and physiologically relevant site of procaine action appears to be on the extracellular surface, upstream from the G protein, presumably on the receptor.

Thrombin-induced membrane currents in native Xenopus follicles

The presence and properties of thrombin receptors have been investigated in Xenopus Laevis follicles. In follicles, voltage-clamped at -60 mV or -80 mV, sub-micromolar concentrations of thrombin (Thr) induce smooth inward currents carried by chloride ions, which desensitize over tens of minutes. This desensitization is prevented by PKC inhibitors (staurosporine and H7). Responses are inhibited by hirudin. Inconsistent responses can be observed in defolliculated oocytes. Thr responses, readily recorded in hypo-osmotic solution are abolished by perfusion of normal frog "Ringer" or by the addition of 50 mM sucrose. Thus Thr acts on receptors present in the follicular cells triggering the opening of chloride channels by acting on receptors, likely to be located on follicular cells, via a PKC-regulated pathway.

Design and in vitro pharmacology of a selective gamma-aminobutyric acidC receptor antagonist

In mammals, receptors for the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) are divided into three pharmacological classes, which are denoted GABAA, GABAB, and GABAC. GABAC receptors are defined by their insensitivity to the GABAA receptor antagonist bicuculline and the GABAB receptor agonist (-)-baclofen. GABAC receptors probably are a heterogeneous group of proteins. The most extensively studied mammalian GABAC receptors are those found in neurons of the outer retina. These receptors are GABA-gated Cl- channels comprised of p subunits, of which there are two subtypes. The physiological functions served by GABAC receptors are largely unknown; to determine the functions, it would be useful to have GABAC-selective ligands. In a previous study, we found that isoguvacine, a GABAA-selective agonist, and 3-aminopropyl-(methyl)phosphinic acid (3-APMPA), a GABAB-selective agonist, show affinity for retinal GABAC receptors. In particular, 3-APMPA is an antagonist with low micromolar potency (Kb approximately 1 microM). Here, we report the synthesis and pharmacological characterization of (1,2,5,6-tetrahydropyridine-4-yl)methylphosphinic acid (TPMPA), a hybrid of isoguvacine and 3-APMPA designed to retain affinity for GABAC receptors but not to interact with GABAA or GABAB receptors. Electrical assays show that TPMPA is a competitive antagonist of cloned human mu 1 GABAC receptors expressed in Xenopus laevis oocytes (Kb approximately 2 microM). TPMPA is > 100-fold weaker as an inhibitor of rat brain GABAA receptors expressed in oocytes (Kb approximately 320 microM) and has only weak agonist activity on GABAB receptors assayed in rat hippocampal slices (EC50 approximately 500 microM). TPMPA should be a useful pharmacological probe with which to investigate GABAC receptor function in the outer retina and in any other areas of the nervous system in which these types of receptor are present.