Responses to GABA, glycine and beta-alanine induced in Xenopus oocytes by messenger RNA from chick and rat brain

Glycine receptors support excitatory neurotransmitter release in developing mouse visual cortex

Glycine receptors (GlyRs) are found in most areas of the brain, and their dysfunction can cause severe neurological disorders. While traditionally thought of as inhibitory receptors, presynaptic-acting GlyRs (preGlyRs) can also facilitate glutamate release under certain circumstances, although the underlying molecular mechanisms are unknown. In the current study, we sought to better understand the role of GlyRs in the facilitation of excitatory neurotransmitter release in mouse visual cortex. Using whole-cell recordings, we found that preGlyRs facilitate glutamate release in developing, but not adult, visual cortex. The glycinergic enhancement of neurotransmitter release in early development depends on the high intracellular to extracellular Cl(-) gradient maintained by the Na(+)-K(+)-2Cl(-) cotransporter and requires Ca(2+) entry through voltage-gated Ca(2+) channels. The glycine transporter 1, localized to glial cells, regulates extracellular glycine concentration and the activation of these preGlyRs. Our findings demonstrate a developmentally regulated mechanism for controlling excitatory neurotransmitter release in the neocortex.

Discovery of potent CLC chloride channel inhibitors

Anion-transport proteins are central to all of physiology, for processes ranging from regulating bone-density, muscle excitability, and blood pressure, to facilitating extreme-acid survival of pathogenic bacteria. 4,4-Diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) has been used as an anion-transport inhibitor for decades. In this study, we demonstrate that polythiourea products derived from DIDS hydrolysis inhibit three different CLC chloride-transport proteins, ClC-ec1, ClC-0, and ClC-Ka, more effectively than DIDS itself. The structures of the five major products were determined by NMR spectroscopy, mass spectrometry, and chemical synthesis. These compounds bind directly to the CLC proteins, as evidenced by the fact that inhibition of ClC-0 occurs only from the intracellular side and inhibition of ClC-Ka is prevented by the point mutation N68D. These polythioureas are the highest affinity inhibitors known for the CLCs and provide a new class of chemical probes for dissecting the molecular mechanisms of chloride transport.

Expression of functional neurotransmitter receptors in Xenopus oocytes after injection of human brain membranes

The Xenopus oocyte is a very powerful tool for studies of the structure and function of membrane proteins, e.g., messenger RNA extracted from the brain and injected into oocytes leads to the synthesis and membrane incorporation of many types of functional receptors and ion channels, and membrane vesicles from Torpedo electroplaques injected into oocytes fuse with the oocyte membrane and cause the appearance of functional Torpedo acetylcholine receptors and Cl(-) channels. This approach was developed further to transplant already assembled neurotransmitter receptors from human brain cells to the plasma membrane of Xenopus oocytes. Membranes isolated from the temporal neocortex of a patient, operated for intractable epilepsy, were injected into oocytes and, within a few hours, the oocyte membrane acquired functional neurotransmitter receptors to gamma-aminobutyric acid, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, kainate, and glycine. These receptors were also expressed in the plasma membrane of oocytes injected with mRNA extracted from the temporal neocortex of the same patient. All of this makes the Xenopus oocyte a more useful model than it already is for studies of the structure and function of many human membrane proteins and opens the way to novel pathophysiological investigations of some human brain disorders.

A possible mechanism underlying corymine inhibition of glycine-induced Cl- current in Xenopus oocytes

We previously reported that corymine, an alkaloid extracted from the leaves of Hunteria zeylanica native to Thailand, inhibited glycine-induced chloride current using a receptor expression model of Xenopus oocytes. In this study, we investigated the mechanism underlying the inhibitory action of this alkaloid on glycine current using the same model. Corymine inhibited glycine current in a noncompetitive fashion. Co-application with strychnine, a competitive glycine receptor antagonist, or 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), a Cl- channel blocker, corymine decreased the ED50 value of strychnine, but did not change that of DIDS. Moreover, the inhibitory effects of corymine and either strychnine or DIDS were additive. The desensitization phase of glycine current showed two exponentials and corymine preferentially inhibited the fast component, whereas strychnine affected both of them to the same extent and DIDS preferentially inhibited the slow component. When these drugs were applied repeatedly, the inhibitory effects of corymine and strychnine were not use-dependent and reversible, while the effect of DIDS was use-dependent and irreversible. The inhibitory effect of corymine on gamma-aminobutyric acid (GABA) current was less potent than the effect on glycine current, while this alkaloid failed to affect acetylcholine and serotonin currents. These results demonstrate that corymine inhibits glycine-gated CI- channels by interacting with the site different from that of DIDS.

Inhibitory effects of corymine, an alkaloidal component from the leaves of Hunteria zeylanica, on glycine receptors expressed in Xenopus oocytes

We previously reported that corymine, an alkaloidal compound extracted from the leaves of Hunteria zeylanica native to Thailand, potentiated convulsions induced by either picrotoxin or strychnine. Therefore, to clarify the mechanism of action of corymine, the effects of corymine on gamma-aminobutyric acid (GABA) and glycine receptors were examined. We used Xenopus oocytes expressing these receptors and the two-electrode voltage-clamp method. The receptors expressed in oocytes injected with rat brain and spinal cord RNA showed the pharmacological properties of GABAA and glycine receptors, respectively. Corymine (1-100 microM) partially (20-30%) reduced the GABA responses in oocytes injected with rat brain RNA, while marked (up to 80%) dose-dependent reductions were observed in the glycine responses in oocytes injected with rat spinal cord RNA. These observations suggest that corymine was more effective against the glycine receptors than the GABA receptors. The ED50 of corymine on the glycine response was 10.8 microM. Corymine, at 30 microM, caused a shift to the right, with a lower maximal response, of the glycine concentration-response curve. This indicated that the action of corymine on glycine receptors is neither competitive nor purely non-competitive. These observations suggest that a binding site other than the glycine recognition site of the glycine receptors is the site of action of corymine.

Agonist pharmacology of two Drosophila GABA receptor splice variants

1. The Drosophila melanogaster gamma-aminobutyric acid (GABA) receptor subunits, RDLac and DRC 17-1-2, form functional homo-oligomeric receptors when heterologously expressed in Xenopus laevis oocytes. The subunits differ in only 17 amino acids, principally in regions of the N-terminal domain which determine agonist pharmacology in vertebrate ionotropic neurotransmitter receptors. A range of conformationally restricted GABA analogues were tested on the two homo-oligomers and their agonists pharmacology compared with that of insect and vertebrate iontropic GABA receptors. 2. The actions of GABA, isoguvacine and isonipecotic acid on RDLac and DRC 17-1-2 homo-oligomers were compared, by use of two-electrode voltage-clamp. All three compounds were full agonists of both receptors, but were 4-6 fold less potent agonists of DRC 17-1-2 homo-oligomers than of RDLac. However, the relative potencies of these agonists on each receptor were very similar. 3. A more complete agonist profile was established for RDLac homo-oligomers. The most potent agonists of these receptors were GABA, muscimol and trans-aminocrotonic acid (TACA), which were approximately equipotent. RDLac homo-oligomers were fully activated by a range of GABA analogues, with the order of potency: GABA > ZAPA ((Z)-3-[(aminoiminomethyl)thio]prop-2-enoic acid) > isoguvacine > imidazole-4-acetic acid > or = isonipecotic acid > or = cis-aminocrotonic acid (CACA) > beta-alanine. 3-Aminopropane sulphonic acid (3-APS), a partial agonist of RDLac homo-oligomers, was the weakest agonist tested and 100 fold less potent than GABA. 4. SR95531, an antagonist of vertebrate GABAA receptors, competitively inhibited the GABA responses of RDLac homo-oligomers, which have previously been found to insensitive to bicuculline. However, its potency (IC50 500 microM) was much reduced when compared to GABAA receptors. 5. The agonist pharmacology of Drosophila RDLac homo-oligomers exhibits aspects of the characteristic pharmacology of certain native insect GABA receptors which distinguish them from vertebrate GABA receptors. The high potency and efficacy of isoguvacine and ZAPA distinguishes RDLac homo-oligomers from bicuculline-insensitive vertebrate GABAC receptors, while the low potency of SR95531 and 3-APS distinguishes them from GABAA receptors. The differences in the potency of agonists on RDLac and DRC 17-1-2 homo-oligomers observed in the present study may assist in identification of further molecular determinants of GABA receptor function.

Identification and function of glycine receptors in cultured cerebellar granule cells

Poly(A)+ mRNA was isolated from cultured mouse cerebellar granule cells and injected into Xenopus oocytes. This led to the expression of receptors that evoked large membrane currents in response to glycine. Current-responses were also obtained after application of beta-alanine and taurine, but these were very low relative to that of glycine (maximal beta-alanine and taurine responses were 8 and 3% of that of glycine, respectively). The role of glycine receptors on K(+)-evoked transmitter release in cultured cerebellar granule cells was also assayed. Release of preloaded D-[3H]aspartate evoked by 40 mM K+ was dose dependently inhibited by glycine, and the concentration producing half-maximal inhibition was 50 microM. Taurine, beta-alanine, and the specific GABAA receptor agonist isoguvacine also inhibited K(+)-evoked release, and the maximal inhibition was similar for all agonists (approximately 40%). The EC50 value was 200 microM for taurine, 70 microM for beta-alanine, and 4 microM for isoguvacine. Bicuculline (150 microM) antagonized the inhibitory effect of isoguvacine (150 microM) but not that of glycine (1 mM). In contrast, strychnine (20 microM) antagonized the inhibitory effect of glycine (1 mM) but not that of isoguvacine (150 microM). The pharmacology of the responses to beta-alanine and taurine showed that these agonists activate both glycine and GABAA receptors. The results indicate that cultured cerebellar granule cells translate the gene for the glycine receptor and that activation of glycine receptors produces neuronal inhibition.

The effect of high pressure on glycine- and kainate-sensitive receptor channels expressed in Xenopus oocytes

The effect of high pressure on the response to glycine or kainate of voltage-clamped Xenopus oocytes micro-injected with messenger-RNA derived from either rat spinal cord or whole brain, respectively, has been investigated. Current responses were measured at 1 bar (= 10(5) Pa), 50 bar, 100 bar and 150 bar, with PO2 fixed at 1 bar and the balance helium. Glycine elicited a depolarizing current response which was antagonized by nanomolar concentrations of strychnine. The responses reversibly desensitized, with a decay constant of 0.01 s-1, when glycine concentrations greater than 250 microM were used. The decay constant was insensitive to both glycine concentration and pressure. Resensitization was complete within 4 min. Kainate elicited a depolarizing current which was non-desensitizing. The response was slightly sensitive to glutamate diethyl ester (50 microM), which increased the EC50 by 25%. The action of glycine was highly pressure sensitive. The dose-response curves established at 50 bar, 100 bar and 150 bar were shifted progressively to the right, with no effect on the maximal current. The EC50 increased from 216 microM to 296 microM at 50 bar, to 345 microM at 100 bar, and to 425 microM at 150 bar. The action of kainate was unaffected by pressure. No shift in the dose-response curves was established, nor was there any effect on the maximum current. The EC50 was 113 microM at 1 bar, and 111 microM at both 50 bar and 100 bar.(ABSTRACT TRUNCATED AT 250 WORDS)

Structure and function of inhibitory glycine receptors

Ion transport by peptide channels has been the major theme in the work of the late P. Läuger. His theoretical and experimental approaches provided the basis for a deeper understanding of pore-mediated ion permeation through biological membranes. This review on a ligand–gated ion channel protein from the mammalian brain is dedicated to the memory of this outstanding scientist.

Expression of mammalian gamma-aminobutyric acid receptors with distinct pharmacology in Xenopus oocytes

Gamma-Aminobutyric acid (GABA), the major inhibitory neurotransmitter in mammalian brain, is known to interact with two classes of GABA receptors denoted GABAA and GABAB. Using Xenopus oocytes, we compared the electrical and pharmacological properties of GABA receptors expressed by poly(A)+ RNA isolated from mammalian brain and retina. RNA from cerebral cortex expressed GABA responses with features characteristic of currents mediated by GABAA receptors. In contrast, RNA from retina expressed responses mediated by GABAA receptors and, in addition, GABA responses that were insensitive to the GABAA antagonist bicuculline and the GABAB agonist baclofen and showed no modulation by barbiturates or benzodiazepines. The bicuculline/baclofen-insensitive GABA response was a Cl- current that was blocked by picrotoxin but showed little desensitization or outward rectification. Our results suggest that mammalian retina contains RNAs encoding GABA receptors with distinct pharmacology.

Discrimination of heterogenous mRNAs encoding strychnine-sensitive glycine receptors in Xenopus oocytes by antisense oligonucleotides

Three synthetic oligodeoxynucleotides complementary to different parts of an RNA encoding a glycine receptor subunit were used to discriminate heterogenous mRNAs coding for glycine receptors in adult and neonatal rat spinal cord. Injection of the three antisense oligonucleotides into Xenopus oocytes specifically inhibited the expression of glycine receptors by adult spinal cord mRNA. In contrast, the antisense oligonucleotides were much less potent in inhibiting the expression of glycine receptors encoded by neonatal spinal cord mRNA. Northern blot analysis revealed that the oligonucleotides hybridized mostly to an adult cord transcript of approximately 10 kilobases in size. This band was also present in neonatal spinal cord mRNA but its density was about one-fourth of the adult cord message. There was no intense band in the low molecular weight position (approximately 2 kilobases), the existence of which was expected from electrophysiological studies with size-fractionated mRNA of neonatal spinal cord. Our results suggest that in the rat spinal cord there are at least three different types of mRNAs encoding functional strychnine-sensitive glycine receptors.