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.

Lysophosphatidic acid-induced neurite retraction in PC12 cells: control by phosphoinositide-Ca2+ signaling and Rho

The endogenous phospholipid mediator lysophosphatidic acid (LPA) caused growth cone collapse, neurite retraction, and cell flattening in differentiated PC12 cells. Neurite retraction was blocked by cytochalasin B and ADP-ribosylation of the small-molecular-weight G protein Rho by the Clostridium botulinum C-3 toxin. LPA induced a transient rise in the level of inositol 1,4,5-trisphosphate, and retraction was blocked by inhibitors of phospholipase beta. Repeated application of LPA elicited homologous desensitization of the Ca2+ mobilization response. The activation of the phosphoinositide (PIP)-Ca2+ second messenger system played a permissive role in the morphoregulatory response. Blockers of protein kinase C--chelerythrine, a myristoylated pseudosubstrate peptide, staurosporine, and depletion of protein kinase C from the cells by long-term phorbol ester treatment--all diminished neurite retraction by interfering with LPA-induced Ca2+ mobilization, which was required for the withdrawal of neurites. A brief 15-min treatment with 4 beta-phorbol 12-myristate 13-acetate also blocked retraction and Ca2+ mobilization, by inactivating the LPA receptor. Inhibition of protein tyrosine phosphorylation by herbimycin diminished retraction. Although activation of the PIP-Ca2+ second messenger system appears necessary for the Rho-mediated rearrangements of the actin cytoskeleton, bradykinin, which activates similar signaling events, failed to cause retraction, indicating that a yet unidentified novel mechanism is also involved in the LPA-induced morphoregulatory response.

Lysophosphatidic acid-induced neurite retraction in PC12 cells: neurite-protective effects of cyclic AMP signaling

Effects of the cyclic AMP second messenger system were studied on the retraction of neurites elicited by the phospholipid mediator lysophosphatidic acid (LPA) in PC12 cells. LPA stimulation inhibited adenylyl cyclase, indicating that the LPA receptor couples to the heterotrimeric Gi proteins. However, pertussis toxin or expression of dominant negative Ras did not prevent neurite retraction. In contrast, cholera toxin, forskolin, and application of dibutyryl-cyclic AMP prevented neurite retraction. The neurite-protective effect of forskolin was blocked by Rp-adenosine 3',5'-phosphorothioate. Forskolin and dibutyryl-cyclic AMP both failed to protect neurites in A126-1B2 and 123.7 cells, which lack cyclic AMP-activated protein kinase. Data indicate that elevation of cyclic AMP levels triggers a cyclic AMP-activated protein kinase-dependent mechanism that opposes the functioning of the morphoregulatory signaling activated by LPA. ADP-ribosylation of Rho by the Clostridium botulinum C-3 toxin in 123.7 cells caused neuronal differentiation, indicated by neurite extension, and blocked LPA-induced neurite retraction. LPA activates Gq- and Gi-linked signaling in parallel; therefore, a morphoregulatory signaling network hypothesis is proposed versus the simplistic approach of a signaling pathway. The signaling network integrates the receptor-activated individual, sequential, and parallel signaling events into an interactive network whose individual components may fulfill required and permissive functions encoding the cellular response.

Functional role of follicular cells in the generation of osmolarity-dependent Cl- currents in Xenopus follicles

1. Osmolarity-dependent (osmo-dependent) ionic currents from follicle-enclosed Xenopus oocytes (follicles) were studied using the two-microelectrode voltage-clamp technique, combined with intra-oocyte pressure injection of sucrose or polyethylene glycols (PEGs). 2. Intra-oocyte injections of sucrose or PEG (3-25 nmol) generated inward membrane currents (follicles held at -60 mV) associated with an increase in membrane conductance. These currents were carried mainly by chloride ions (ICl(osm)), and were strongly attenuated by increasing the tonicity of the external medium, or by external application of La3+ (0.1-1 mM). 3. The ability to generate ICl(osm) depended on the molecular weight of the injected PEG. Injections of PEG 200 or 300 generated ICl(osm) in 95% of the follicles tested, PEG 600 generated comparable currents in only 20% of the follicles, while similar injections of PEG 1000 did not elicit ICl(osm). 4. Octanol (1-1.5 mM), a gap junction channel blocker, reversibly inhibited 50-90% of the ICl(osm) generated by injections of sucrose or PEG 300. Moreover, sucrose or PEG injections did not elicit ICl(osm) in defolliculated oocytes. 5. It is concluded that an increase in the internal osmolarity of the follicular cells activates a mechanism, probably involving cellular swelling, which leads to the opening of ICl(osm) channels most probably located in the follicular cell membrane.

A Windows software package to record from voltage-clamped Xenopus oocytes

We have written a software package to record, display and analyze membrane currents elicited by neurotransmitter receptors or voltage-activated channels in voltage-clamped Xenopus oocytes. This suite, which consists of 4 applications, runs on IBM-PC compatible microcomputers under Windows 3.1. The recording programs use Direct Memory Access (DMA) to access the analog-digital board. The first program, NicPulse, is aimed at studying voltage-activated channels. It delivers voltage steps to the voltage-clamp and records the resulting membrane current. The second program, NicScope, emulates a dual-trace digital oscilloscope. It operates either in continuous or triggered mode, and is used chiefly to display neurotransmitter-induced responses in oocytes. The third recording program, VRamp, automatically determines the voltage-current relationship of drug-activated responses (I/V curve), by applying a voltage ramp and recording the subsequent clamping current. The last program, NicView, is designed to analyze records taken with NicScope and NicPulse. The present paper will discuss several issues regarding the design of these programs, and will give a brief description of each application.

Incorporation of reconstituted acetylcholine receptors from Torpedo into the Xenopus oocyte membrane

Xenopus oocytes are a valuable aid for studying the molecular structure and function of ionic channels and neurotransmitter receptors. Their use has recently been extended by the demonstration that oocytes can incorporate foreign membranes carrying preassembled receptors and channels. Here we show that when reconstituted in an artificial lipid matrix and injected into Xenopus oocytes, purified nicotinic acetylcholine receptors are efficiently inserted into the plasma membrane, where they form "clusters" of receptors that retain their native properties. This constitutes an innovative approach that, besides allowing the analyses of membrane fusion processes, is also a powerful technique for studying the characteristics and regulation of many membrane proteins (with their native stoichiometry and configuration) upon reinsertion into the membrane of a very convenient host cell system.

Activation of GABA rho 1 receptors by glycine and beta-alanine

Neurons of the vertebrate retina possess receptors for many neurotransmitters. Particularly interesting is a new type of GABA receptor (GABA rho) that, in contrast to GABAA and GABAB receptors, shows very little desensitization, is not blocked by bicuculline, and is not activated by baclofen. Homomeric human GABA rho 1 receptors were expressed in Xenopus oocytes. In addition to GABA, and related agonists, GABA rho 1 receptors respond to glycine (Gly) and beta-alanine (beta-Ala) by generating Cl- currents that do not desensitize and are resistant to bicuculline. The half-maximal concentrations for Gly and beta-Ala currents were 14.2 +/- 1.3 mM and 0.66 +/- 0.11 mM respectively. The current responses to Gly and beta-Ala were blocked by picrotoxin and TBPS. The cross-sensitivity of GABA rho 1 receptors to Gly and beta-Ala may play a role in retinal physiology.

Incorporation of acetylcholine receptors and Cl- channels in Xenopus oocytes injected with Torpedo electroplaque membranes

A method was developed to transplant assembled nicotinic acetylcholine receptors (AcChoRs) and Cl- channels from the electric organ of Torpedo to the membrane of Xenopus oocytes. Membrane vesicles from Torpedo electroplaques were injected into the oocytes and, within a few hours, the oocyte membrane acquired AcChoRs and Cl- channels. The mechanism of expression of these receptors and channels is very different from that which follows the injection of mRNA, since the appearance of receptors after membrane injection does not require de novo protein synthesis or N-glycosylation. This, and other controls, indicate that the foreign receptor-bearing membranes fuse with the oocyte membrane and cause the appearance of functional receptors and channels. All this makes the Xenopus oocyte an even more powerful tool for studies of the structure and function of membrane proteins.

A monovalent cationic conductance that is blocked by extracellular divalent cations in Xenopus oocytes

1. Native Xenopus oocytes were voltage clamped and exposed to Ringer solutions containing low concentrations of divalent cations. Oocytes, held at -60 mV, developed a reversible non-inactivating smooth inward current (Ic) associated with an increase in membrane conductance. 2. Ic was selectively carried by cations (Na+, K+), indicating that the current was not the result of a non-specific membrane breakdown, but was due instead to removal of a blocking effect of divalent cations on a specific population of endogenous ionic channels located in the oocyte membrane. 3. The blocking effects of Ca2+ and Mg2+ were voltage dependent, implying action at a binding site within the pore of the cationic channel. For example, the half-maximal inhibition (IC50) of Ic by Ca2+ was 61 microM in oocytes held at -60 mV and 212 microM in oocytes held at 0 mV. 4. The Ic channels could be unblocked by depolarization of the membrane even in the presence of physiological concentrations of Ca2+ or Mg2+. The unblocking of the channels was observed as a slowly developing outward current. 5. The novel cationic current was substantially reduced following in vitro maturation of oocytes by treatment with progesterone (10 microM, 4-5 h). 6. The physiological role of Ic channels remains to be elucidated. Nonetheless, their characteristics explain the ionic basis of the sensitivity of oocytes to reductions in extracellular divalent cations and raise the possibility that the channels play a role in calcium homeostasis.

Two forms of acetylcholine receptor gamma subunit in mouse muscle

Nicotinic acetylcholine receptors (nAcChoRs) of skeletal muscle are heterosubunit ligand-gated channels that mediate signal transmission from motor nerves to muscle. While cloning murine nAcChoR subunits, to gain an insight into the receptor diversity across species, we detected two forms of gamma subunits in the myogenic C2C12 cell line. Both forms are functional when expressed in Xenopus oocytes. One gamma subunit [long gamma (gamma 1)] was almost identical to that previously cloned in the murine BC3H-1 tumor cell line. The second form of gamma subunit [short gamma (gamma s)] lacked 156 bp (52 amino acids) in the extracellular N terminus, adjoining the hydrophobic segment M1, which corresponds to the fifth exon of the gamma-subunit gene. The two forms of gamma subunit coexist during myogenesis in vitro and in 17-day embryonic and denervated adult muscle fibers in vivo. However, the gamma s variant was the only form of gamma subunit in newborn muscle. In dissociated muscle fibers of newborn mice, AcCho-evoked channel openings were more prolonged when compared with C2C12 myotubes or denervated adult muscle fibers. The gamma s subunit may, thus, contribute to the structural and functional diversity of nAcChoRs in muscle cells.

Effects of serotonergic agents on neuronal nicotinic acetylcholine receptors

In Xenopus oocytes expressing neuronal nicotinic acetylcholine receptors (nAcChoRs), made up of alpha 2 and beta 4 subunits, acetylcholine (AcCho) elicited ionic membrane currents (AcCho currents) that were modulated by serotonergic agents. Both agonists and antagonists specific for various serotonin (5-hydroxytryptamine, 5HT) receptor subtypes interacted directly with alpha 2 beta 4 nAcChoRs: 5HT, (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin, methysergide, spiperone, and ketanserin reversibly reduced the amplitude of AcCho currents and accelerated their decay. The AcCho-current time course decayed with two exponential functions. In the presence of 5HT, the fast time constant of current decay (tau f) was not greatly modified, but the slow time constant (tau s) was reduced. With AcCho and 5HT both at 100 microM, tau s was reduced from 140 s to 85 s. The order of potency for inhibition of AcCho current amplitudes was (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin > methysergide > spiperone > ketanserin > 5HT. The inhibition was voltage-dependent but the magnitude of the voltage dependence for the different blockers did not correspond to their blocking potency: e.g., the block with spiperone was stronger than with 5HT, but it was less voltage-dependent. Our results suggest that serotonergic agents block neuronal nAcChoRs in a noncompetitive manner, similar to the block of muscle nAcChoR by curare and other substances. These results show that neuronal nAcChoR channels that have been activated by their specific neurotransmitter may be modulated by nonspecific neurotransmitters and their antagonists. These effects may help to better understand brain functions as well as the mode of action of the many serotonergic agents that are used in medical practice.

Xenopus Gq alpha subunit activates the phosphatidylinositol pathway in Xenopus oocytes but does not consistently induce oocyte maturation

We cloned the Xenopus laevis form of Gq alpha subunit to study its effects on oocyte maturation. Injection of Xenopus Gq alpha mRNA into stage 6 oocytes activated the phospholipase C/phosphatidylinositol pathway. The oocyte membrane became permeable to calcium ions and was able to generate transient inward currents (T(in)), due to the opening of Ca(2+)-dependent Cl- channels. The T(in) amplitude developed over several hours and disappeared by 24 hr. Diacylglycerol levels were found to parallel the appearance and disappearance of the T(in). The concurrent decline of T(in) values and diacylglycerol was not due to a failure in the synthesis of Gq alpha protein, which was produced continuously for > 24 hr. After Xenopus Gq alpha mRNA injection, germinal vesicle breakdown (GVBD) was variable (0-100%) in stage 6 oocytes, whereas none of the stage 4 oocytes underwent GVBD. In contrast, stage 6 oocytes injected with mRNA encoding the Go alpha G protein consistently underwent GVBD but did not acquire T(in). Our results show that activation of phospholipase C is not an absolute requisite for the induction of maturation, although in oocytes of some frogs phospholipase C activation can trigger a pathway to GVBD.

Osmo-dependent Cl- currents activated by cyclic AMP in follicle-enclosed Xenopus oocytes

The role of adenosine 3',5'-cyclic monophosphate (cAMP) in generating the osmo-dependent slow inward membrane currents (S(in)) elicited by activation of follicle stimulating hormone (FSH) or acetylcholine (ACh) receptors was studied in voltage-clamped, follicle-enclosed oocytes of Xenopus laevis (follicles). Forskolin (FSK) also generated S(in) currents, and in low concentrations it potentiated the S(in) currents elicited by FSH but not those elicited by ACh. Moreover, intra-oocyte injections of cAMP elicited similar slow inward currents (cAMP-S(in)) that: (i) were carried mainly by chloride ions; (ii) were abolished by defolliculating the oocytes; and (iii) were dependent on the osmolarity of the external medium. Compared with the Ca(2+)-dependent chloride channels that are located in the oocyte membrane; the cAMP-activated S(in) channels were less permeable to I- and Br-, and their current-voltage relation did not rectify strongly at negative potentials. Generation of cAMP-S(in) desensitized the FSH-S(in) currents, but did not have effects on both the S(in) and the fast chloride current (F(in)) specifically elicited by ACh. Furthermore, follicular phospholipase C activation through stimulation of angiotensin II (AII) receptors failed to generate the current responses elicited by ACh. We conclude that cAMP acts as a potent second messenger in generating the osmo-dependent Cl- currents elicited by FSH but not those elicited by ACh. The mechanisms underlying the ACh responses remain unknown. The osmo-dependent chloride channels activated by cAMP may play a role in the control of volume of the follicular cells-oocyte complex.

Cationic modulation of rho 1-type gamma-aminobutyrate receptors expressed in Xenopus oocytes

A study was made of the effects of di- and trivalent cations on homomeric rho 1-type gamma-aminobutyrate (GABA rho 1) receptors expressed in Xenopus oocytes after injection of mRNA coding for the GABA rho 1 subunit. GABA elicited large currents with a Kd approximately 1 microM. The properties of these GABA rho 1 receptors were similar to those of native bicuculline-resistant GABA receptors expressed by retinal mRNA. GABA rho 1 currents showed very little desensitization, were blocked by picrotoxin but not by bicuculline, and were not modulated by barbiturates, benzodiazepines, or beta-carbolines. Zn2+ reversibly decreased GABA rho 1 responses (IC50 = 22 microM). Other divalent cations were also tested and their rank order of potency was: Zn2+ approximately Ni2+ approximately Cu2+ >> Cd2+, whereas Ba2+, Co2+, Sr2+, Mn2+, Mg2+, and Ca2+ showed little or no effect. In contrast, La3+ reversibly potentiated the GABA currents mediated by homomeric GABA rho 1 receptors, with an EC50 = 135 microM and a maximal potentiation of about 100% (GABA, 1 microM; La3+, 1 mM). Other lanthanides showed similar effects (Lu3+ > Eu3+ > Tb3+ > Gd3+ > Er3% > Nd3+ > La3+ > Ce3+). Thus, GABA rho 1 receptors contain sites for cationic recognition, and in particular, Zn2+ may play a role during synaptic transmission in the retina.

Expression of neurotransmitter receptors and Ca2+ channels in the adult fornix and optic nerve

Using Xenopus oocytes, we have investigated the presence of mRNA coding for neurotransmitter receptors and channels in the adult bovine fornix and optic nerve. Oocytes injected with mRNA from either area, expressed comparable levels of functional receptors to several neurotransmitters, including glutamate and angiotensin II, as well as voltage-activated Ca2+ channels. Glutamate receptors were mainly of the AMPA/kainate type and were expressed in astrocyte cultures from the regions studied, suggesting that the receptors are present at least in this type of glial cells in vivo. These results show that the white matter areas in the adult brain express receptors and channels. These molecules may play an important role in the transfer of information between axons and glial cells.

Glutamate receptors in astrocytic end-feet

Glial cells in the mammalian central nervous system, like neurons, possess glutamate receptors suggesting that neuronal-glial communication via neurotransmitters is an important component of brain function. In the bovine corpus callosum, GLUR-1, an alpha-amino-3-hydroxy-5-methyl-4 isoxazolepropionate (AMPA)/kainate receptor subtype, is a major type of glutamate receptor. It is distributed along astrocytic processes and it is particularly abundant in the end-feet and the glial fibers surrounding the capillaries. This close spatial relationship between GLUR-1 receptors in astrocytes and endothelial cells suggests that these receptors may be activated by glutamate transported across the blood-brain barrier and thus regulate the barrier itself by ensuing astrocyte-endothelial cell interactions.

Properties of angiotensin II receptors in glial cells from the adult corpus callosum

The existence and the properties of angiotensin II receptors in the adult bovine and human corpus callosum (CC) were investigated by using Xenopus oocytes and primary glial cell cultures. In oocytes injected with CC mRNA, angiotensin II elicited oscillatory Cl- currents due to activation of the inositol phosphate/Ca(2+)-receptor-channel coupling system. The receptors expressed in oocytes and in CC cultures were pharmacologically similar to the AT1 receptor type as assayed by binding. Northern blot analysis and in situ hybridization studies in sections from CC and in glial cultures revealed that the receptors were molecularly related to the AT1 receptor and that they were present in astrocytes. In these cells, activation of the receptors with angiotensin II increased de novo DNA synthesis, promoted the release of aldosterone, and induced c-Fos expression. These findings indicate that CC astrocytes possess functional AT1 receptors that participate in various physiological processes.

Electrophysiological properties of newborn and adult rat spinal cord glycine receptors expressed in Xenopus oocytes

The properties of glycine receptors (GlyRs) from newborn and adult rat spinal cord were studied in Xenopus oocytes injected with whole mRNA or the heavy (H) or light (L) mRNA fractions encoding their respective GlyRs. Mean open times and conductances of channels gated by H- or L-GlyRs were determined by noise analysis or voltage jumps. We found that adult H- and L-GlyRs opened channels that differed in their mean open time but had the same channel conductance. Both H- and L-GlyRs gated Cl- currents that displayed a similarly strong outward rectification. Nevertheless, single channels of adult H- and L-GlyRs did not rectify and their mean open times were only slightly altered by voltage. It follows that the outward rectification of adult GlyRs is due mainly to a reduction in the number of open channels. In contrast to H-GlyRs, whose characteristics seem to remain essentially unchanged with age, L-GlyRs from newborn and adult rats have different properties. Channels of newborn L-GlyRs have a higher conductance, longer open time, and greater voltage dependency than those from the adult. Interestingly, properties of newborn GlyRs expressed by whole mRNA were markedly different from those encoded by newborn or adult L or H mRNA. These results demonstrate that the functional heterogeneity of GlyRs is developmentally regulated.

Lysophosphatidic acid possesses dual action in cell proliferation

Lysophosphatidic acid (LPA) induces mitogenic responses in cultured fibroblasts through a pertussis toxin-sensitive signaling pathway. In contrast, we have shown that LPA inhibits the proliferation of Sp2/0-Ag14 myeloma cells. To resolve this apparent controversy, LPA-elicited responses in cell proliferation and the underlying second messenger mechanisms were compared in Sp2/0-Ag14 myeloma and NIH 3T3 fibroblast cells. The antimitogenic response was not elicited by micromolar concentrations of phosphatidic acid, phosphatidylglycerol, or diacylglycerol. In NIH 3T3 and Sp2 cells, LPA elicited an increase in inositol trisphosphate and a subsequent transient increase in free cytoplasmic Ca2+. Unlike the mitogenic response in NIH 3T3 cells, the antimitogenic effect was not affected by pertussis toxin; on the contrary, it was accompanied by an increase in cAMP. In Sp2 cells, cAMP analogs, forskolin, and isobutylmethylxanthine inhibited cell proliferation and enhanced LPA action in an additive manner, suggesting that an LPA-elicited increase in cAMP-mediated signaling was responsible for the antimitogenic response. In addition to the mitogenic response in fibroblasts and the antimitogenic response in tumor cell lines, there are some cell types (Jurkat T-cell lymphoma and primary astrocytes) in which LPA is ineffective in altering cell proliferation. The cell-type-specific dual action of LPA suggests that this endogenous lipid mediator when released from activated cells might play an important role as a regulator, rather than a ubiquitous inducer, of cell proliferation.

Effects of fenamates and other nonsteroidal anti-inflammatory drugs on rat brain GABAA receptors expressed in Xenopus oocytes

Xenopus oocytes injected with poly(A)+ RNA from rat cerebral cortex express functional gamma-aminobutyric acid (GABA)A receptors with electrical properties and pharmacology similar to those of receptors studied in situ. Fenamates, a class of nonsteroidal anti-inflammatory drugs (NSAIDs), had a dual effect on GABA-activated membrane current responses. Currents elicited by low concentrations of GABA were potentiated, whereas currents elicited by high concentrations of GABA were inhibited. The levels of the two effects varied among fenamates. For example, 10 microM mefanamic acid potentiated 10 microM GABA responses by approximately 300% (EC50 approximately 5 microM) and inhibited maximal responses by 30% (IC50 approximately 30 microM). In contrast, 10 microM niflumic acid potentiated 10 microM GABA responses by only 30% (EC50 approximately 10 microM) and inhibited maximal responses by 60% (IC50 approximately 7 microM). Preliminary structure-activity studies suggested that modulatory activity is dependent on the preferred conformations of fenamate molecules and on specific phenyl-ring substitutions. Thirteen other NSAIDs (all prostaglandin synthesis inhibitors) were likewise assayed for effects on GABA-activated currents. Of these, only the salicyclic acid diflunisal induced comparable potentiation and inhibition. Our experiments raise two interesting possibilities: that fenamates could serve as lead structures in the development of novel GABAA receptor modulators and that fenamates might affect GABAA receptor function at a normal clinical dosage.

Novel Cl- currents elicited by follicle stimulating hormone and acetylcholine in follicle-enclosed Xenopus oocytes

Voltage-clamp techniques were used to study the membrane currents elicited by follicle stimulating hormone (FSH) and acetylcholine (ACh) in follicle-enclosed oocytes of Xenopus laevis (follicles). Both agonists caused complex responses that were more evident when the follicles were in hypotonic Ringer solution (HR; 190.4 mosM). In this medium, currents activated by FSH regularly showed three phases whereas currents activated by ACh displayed three to six phases. At a holding potential of -60 mV, FSH, and ACh responses involved combinations of inward and outward currents. Both FSH and ACh responses included a slow smooth inward component that was associated with an increase in membrane conductance, mainly to Cl- (S(in)). This current was strongly dependent on the osmolarity of the external solution: an increase in osmolarity of the HR solution of 18-20 mosM caused a 50% decrease in S(in). In contrast, a fast and transient Cl- current (F(in)) specifically elicited by ACh was not dependent on osmolarity. Both, F(in) and S(in) currents required the presence of follicular cells, since defolliculation using three different methods abolished all the response to FSH and at least four components of the ACh responses. The membrane channels carrying F(in) and oscillatory Cl- currents elicited by stimulation of ACh or serum receptors, were much more permeable to I- and Br- than Cl-, whereas S(in) channels were equally permeable to these anions. Unlike the oscillatory Cl- currents generated in the oocyte itself, S(in) and F(in) currents in follicle-enclosed oocytes were not abolished by chelation of intracellular Ca2+, either with EGTA or BAPTA, which suggests that intracellular Ca2+ does not play a critical role in the activation of these currents. Our experiments show that S(in) and F(in) currents are quite distinct from the previously characterized oscillatory Cl- responses of oocytes. Moreover, the results strongly suggest that the FSH and ACh receptors, the Cl- channels mediating the F(in) and S(in) currents, together with the necessary elements for their activation, are all located in the follicular cells and not in the oocyte. Many aspects of follicular cell physiology in Xenopus laevis, and other species, are regulated by hormones and neurotransmitters, including FSH and ACh. The follicular Cl- currents described in this paper may play an important role in the follicular cell-oocyte development.

mRNAs coding for neurotransmitter receptors in rabbit and rat visual areas

Levels of mRNAs encoding neurotransmitter receptors in the visual cortex, lateral geniculate nucleus, and superior colliculus of the rabbit and rat, and properties of the receptors expressed, were studied using Xenopus laevis oocytes. mRNA extracted from these areas was injected into the oocytes, which then acquired functional receptors. Electrical recordings of neurotransmitter-induced membrane currents reflect the relative amounts of mRNAs encoding the corresponding receptors. Receptors to gamma aminobutyric acid (GABA), kainate, glutamate, and serotonin exhibited uniformly high levels of expression, whereas expression of receptors to glycine and N-methyl-D-aspartate was uniformly low. In contrast, the expression of receptors to acetylcholine and substance P was highly non-uniform. Expression of acetylcholine receptors was high in oocytes injected with mRNA from the visual cortex, low for the lateral geniculate nucleus, and very low or absent for the superior colliculus. Conversely, the currents elicited by substance P were large in oocytes injected with superior colliculus mRNA, but were small or absent in oocytes injected with mRNAs from the other regions. Immunohistochemical analysis, at the light and electron microscopic levels, was used to localize choline acetyltransferase, the acetylcholine-synthesizing enzyme, and substance P-containing synaptic boutons in the three visual areas. Their presence closely paralleled the potency of mRNAs coding for acetylcholine and substance P receptors. The ability of rat mRNA, from each visual area, to induce neurotransmitter receptors was similar to that observed in the corresponding rabbit mRNAs. In addition to the marked differential distribution of mRNA encoding neurotransmitter receptors in the visual system, our findings reveal the probable existence of as yet uncharacterized receptors, whose new molecular forms may be revealed by further study. Our results also provide the basic information required for subsequent studies on the effect of monocular deprivation on the expression of neurotransmitter receptors in the visual system.

Neurotransmitter receptors and voltage-dependent Ca2+ channels encoded by mRNA from the adult corpus callosum

The presence of mRNAs encoding neurotransmitter receptors and voltage-gated channels in the adult human and bovine corpus callosum was investigated using Xenopus oocytes. Oocytes injected with mRNA extracted from the corpus callosum expressed functional receptors to glutamate, acetylcholine, and serotonin, and also voltage-operated Ca2+ channels, all with similar properties in the two species studied. Acetylcholine and serotonin elicited oscillatory Cl- currents due to activation of the inositol phosphate-Ca2+ receptor-channel coupling system. Glutamate and its analogs N-methyl-D-aspartate (NMDA), kainate, quisqualate, and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) induced smooth currents. The non-NMDA responses showed a strong inward rectification at positive potentials and were potently blocked by 6,7-dinitroquinoxaline-2,3-dione, as observed for the AMPA/kainate glutamate receptors GLUR1 and GLUR3. Furthermore, in situ hybridization experiments showed that GLUR1 and GLUR3 mRNAs are present in corpus callosum cells that were labeled with antiserum to glial fibrillary acid protein and that, in primary cell cultures, had the morphology of type 2 astrocytes. These results indicate that glial cells in the adult corpus callosum possess mRNA encoding functional neurotransmitter receptors and Ca2+ channels. These molecules may provide a mechanism for glial-neuronal interactions.