Changes in the properties of developing glycine receptors in cultured mouse spinal neurons

Potentiation of the Glycine Response by Bisphenol A, an Endocrine Disrupter, on the Substantia Gelatinosa Neurons of the Trigeminal Subnucleus Caudalis in Mice

Lamina II, also called the substantia gelatinosa (SG) of the medullary dorsal horn (the trigeminal subnucleus caudalis, Vc), is thought to play an essential role in the control of orofacial nociception because it receives the nociceptive signals from primary afferents, including thin myelinated Aδ- and unmyelinated C-fibers. Glycine, the main inhibitory neurotransmitter in the central nervous system, plays an essential role in the transference of nociceptive messages from the periphery to higher brain regions. Bisphenol A (BPA) is reported to alter the morphological and functional characteristics of neuronal cells and to be an effector of a great number of ion channels in the central nervous system. However, the electrophysiological effects of BPA on the glycine receptors of SG neurons in the Vc have not been well studied. Therefore, in this study, we used the whole-cell patch-clamp technique to determine the effect of BPA on the glycine response in SG neurons of the Vc in male mice. We demonstrated that in early neonatal mice (0-3 postnatal day mice), BPA did not affect the glycine-induced inward current. However, in the juvenile and adult groups, BPA enhanced the glycine-mediated responses. Heteromeric glycine receptors were involved in the modulation by BPA. The interaction between BPA and glycine appears to have a significant role in regulating transmission in the nociceptive pathway.

Presence of Inhibitory Glycinergic Transmission in Medium Spiny Neurons in the Nucleus Accumbens

It is believed that the rewarding actions of drugs are mediated by dysregulation of the mesolimbic dopaminergic system leading to increased levels of dopamine in the nucleus accumbens (nAc). It is widely recognized that GABAergic transmission is critical for neuronal inhibition within nAc. However, it is currently unknown if medium spiny neurons (MSNs) also receive inhibition by means of glycinergic synaptic inputs. We used a combination of proteomic and electrophysiology studies to characterize the presence of glycinergic input into MSNs from nAc demonstrating the presence of glycine transmission into nAc. In D1 MSNs, we found low frequency glycinergic miniature inhibitory postsynaptic currents (mIPSCs) which were blocked by 1 μM strychnine (STN), insensitive to low (10, 50 mM) and high (100 mM) ethanol (EtOH) concentrations, but sensitive to 30 μM propofol. Optogenetic experiments confirmed the existence of STN-sensitive glycinergic IPSCs and suggest a contribution of GABA and glycine neurotransmitters to the IPSCs in nAc. The study reveals the presence of glycinergic transmission in a non-spinal region and opens the possibility of a novel mechanism for the regulation of the reward pathway.

Differential regulation of NMDA receptors by d-serine and glycine in mammalian spinal locomotor networks

We provide evidence that NMDARs within murine spinal locomotor networks determine the frequency and amplitude of ongoing locomotor-related activity in vitro and that NMDARs are regulated by d-serine and glycine in a synapse-specific and activity-dependent manner. In addition, glycine transporter-1 is shown to be an important regulator of NMDARs during locomotor-related activity. These results show how excitatory transmission can be tuned to diversify the output repertoire of spinal locomotor networks in mammals.

Activation of N-methyl-d-aspartate receptors (NMDARs) requires the binding of a coagonist, either d-serine or glycine, in addition to glutamate. Changes in occupancy of the coagonist binding site are proposed to modulate neural networks including those controlling swimming in frog tadpoles. Here, we characterize regulation of the NMDAR coagonist binding site in mammalian spinal locomotor networks. Blockade of NMDARs by d(−)-2-amino-5-phosphonopentanoic acid (d-APV) or 5,7-dichlorokynurenic acid reduced the frequency and amplitude of pharmacologically induced locomotor-related activity recorded from the ventral roots of spinal-cord preparations from neonatal mice. Furthermore, d-APV abolished synchronous activity induced by blockade of inhibitory transmission. These results demonstrate an important role for NMDARs in murine locomotor networks. Bath-applied d-serine enhanced the frequency of locomotor-related but not disinhibited bursting, indicating that coagonist binding sites are saturated during the latter but not the former mode of activity. Depletion of endogenous d-serine by d-amino acid oxidase or the serine-racemase inhibitor erythro-β-hydroxy-l-aspartic acid (HOAsp) increased the frequency of locomotor-related activity, whereas application of l-serine to enhance endogenous d-serine synthesis reduced burst frequency, suggesting a requirement for d-serine at a subset of synapses onto inhibitory interneurons. Consistent with this, HOAsp was ineffective during disinhibited activity. Bath-applied glycine (1–100 µM) failed to alter locomotor-related activity, whereas ALX 5407, a selective inhibitor of glycine transporter-1 (GlyT1), enhanced burst frequency, supporting a role for GlyT1 in NMDAR regulation. Together these findings indicate activity-dependent and synapse-specific regulation of the coagonist binding site within spinal locomotor networks, illustrating the importance of NMDAR regulation in shaping motor output.

NEW & NOTEWORTHY We provide evidence that NMDARs within murine spinal locomotor networks determine the frequency and amplitude of ongoing locomotor-related activity in vitro and that NMDARs are regulated by d-serine and glycine in a synapse-specific and activity-dependent manner. In addition, glycine transporter-1 is shown to be an important regulator of NMDARs during locomotor-related activity. These results show how excitatory transmission can be tuned to diversify the output repertoire of spinal locomotor networks in mammals.

Structure and Pharmacologic Modulation of Inhibitory Glycine Receptors

Glycine receptors (GlyR) are inhibitory Cys-loop ion channels that contribute to the control of excitability along the central nervous system (CNS). GlyR are found in the spinal cord and brain stem, and more recently they were reported in higher regions of the CNS such as the hippocampus and nucleus accumbens. GlyR are involved in motor coordination, respiratory rhythms, pain transmission, and sensory processing, and they are targets for relevant physiologic and pharmacologic modulators. Several studies with protein crystallography and cryoelectron microscopy have shed light on the residues and mechanisms associated with the activation, blockade, and regulation of pentameric Cys-loop ion channels at the atomic level. Initial studies conducted on the extracellular domain of acetylcholine receptors, ion channels from prokaryote homologs-Erwinia chrysanthemi ligand-gated ion channel (ELIC), Gloeobacter violaceus ligand-gated ion channel (GLIC)-and crystallized eukaryotic receptors made it possible to define the overall structure and topology of the Cys-loop receptors. For example, the determination of pentameric GlyR structures bound to glycine and strychnine have contributed to visualizing the structural changes implicated in the transition between the open and closed states of the Cys-loop receptors. In this review, we summarize how the new information obtained in functional, mutagenesis, and structural studies have contributed to a better understanding of the function and regulation of GlyR.

Functional modulation of glycine receptors by the alkaloid gelsemine

BACKGROUND AND PURPOSE

Gelsemine is one of the principal alkaloids produced by the Gelsemium genus of plants belonging to the Loganiaceae family. The extracts of these plants have been used for many years, for a variety of medicinal purposes. Coincidentally, recent studies have shown that gelsemine exerts anxiolytic and analgesic effects on behavioural models. Several lines of evidence have suggested that these beneficial actions were dependent on glycine receptors, which are inhibitory neurotransmitter-gated ion channels of the CNS. However, it is currently unknown whether gelsemine can directly modulate the function of glycine receptors.

EXPERIMENTAL APPROACH

We examined the functional effects of gelsemine on glycine receptors expressed in transfected HEK293 cells and in cultured spinal neurons by electrophysiological techniques.

KEY RESULTS

Gelsemine directly modulated recombinant and native glycine receptors and exerted conformation-specific and subunit-selective effects. Gelsemine modulation was voltage-independent and was associated with differential changes in the apparent affinity for glycine and in the open probability of the ion channel. In addition, the alkaloid preferentially targeted glycine receptors in spinal neurons and showed only minor effects on GABAA and AMPA receptors. Furthermore, gelsemine significantly diminished the frequency of glycinergic and glutamatergic synaptic events without altering the amplitude.

CONCLUSIONS AND IMPLICATIONS

Our results provide a pharmacological basis to explain, at least in part, the glycine receptor-dependent, beneficial and toxic effects of gelsemine in animals and humans. In addition, the pharmacological profile of gelsemine may open new approaches to the development of subunit-selective modulators of glycine receptors.

Ethanol effects on glycinergic transmission: From molecular pharmacology to behavior responses

It is well accepted that ethanol is able to produce major health and economic problems associated to its abuse. Because of its intoxicating and addictive properties, it is necessary to analyze its effect in the central nervous system. However, we are only now learning about the mechanisms controlling the modification of important membrane proteins such as ligand-activated ion channels by ethanol. Furthermore, only recently are these effects being correlated to behavioral changes. Current studies show that the glycine receptor (GlyR) is a susceptible target for low concentrations of ethanol (5-40mM). GlyRs are relevant for the effects of ethanol because they are found in the spinal cord and brain stem where they primarily express the α1 subunit. More recently, the presence of GlyRs was described in higher regions, such as the hippocampus and nucleus accumbens, with a prevalence of α2/α3 subunits. Here, we review data on the following aspects of ethanol effects on GlyRs: (1) direct interaction of ethanol with amino acids in the extracellular or transmembrane domains, and indirect mechanisms through the activation of signal transduction pathways; (2) analysis of α2 and α3 subunits having different sensitivities to ethanol which allows the identification of structural requirements for ethanol modulation present in the intracellular domain and C-terminal region; (3) Genetically modified knock-in mice for α1 GlyRs that have an impaired interaction with G protein and demonstrate reduced ethanol sensitivity without changes in glycinergic transmission; and (4) GlyRs as potential therapeutic targets.

Control of ethanol sensitivity of the glycine receptor α3 subunit by transmembrane 2, the intracellular splice cassette and C-terminal domains

Previous studies have shown that the effect of ethanol onglycine receptors (GlyRs) containing the a1 subunit is affected by interaction with heterotrimeric G proteins (Gβγ). GlyRs containing the α3 subunit are involved in inflammatory pain sensitization and rhythmic breathing and have received much recent attention. For example, it is unknown whether ethanol affects the function of this important GlyR subtype. Electrophysiologic experiments showed that GlyR α3 subunits were not potentiated by pharmacologic concentrations of ethanol or by Gβγ. Thus, we studied GlyR α1–α3 chimeras and mutants to determine the molecular properties that confer ethanol insensitivity. Mutation of corresponding glycine 254 in transmembrane domain 2 (TM2) found in a1 in the α3(A254G) –α1 chimera induced a glycine-evoked current that displayed potentiation during application of ethanol (46 ± 5%, 100 mM) and Gβγ activation (80 ± 17%). Interestingly,insertion of the intracellular α3L splice cassette into GlyR α1 abolished the enhancement of the glycine-activated current by ethanol (5 ± 6%) and activation by Gβγ (21 6 7%). In corporation of the GlyR α1 C terminus into the ethanol-resistant α3S(A254G) mutant produced a construct that displayed potentiation of the glycine-activated current with 100 mM ethanol (40 ± 6%)together with a current enhancement after G protein activation (68 ± 25%). Taken together, these data demonstrate that GlyRα3 subunits are not modulated by ethanol. Residue A254 in TM2, the α3L splice cassette, and the C-terminal domain of α3GlyRs are determinants for low ethanol sensitivity and form the molecular basis of subtype-selective modulation of GlyRs by alcohol.

Activation of glycine receptors modulates spontaneous epileptiform activity in the immature rat hippocampus

While the expression of glycine receptors in the immature hippocampus has been shown, no information about the role of glycine receptors in controlling the excitability in the immature CNS is available. Therefore, we examined the effect of glycinergic agonists and antagonists in the CA3 region of an intact corticohippocampal preparation of the immature (postnatal days 4-7) rat using field potential recordings. Bath application of 100 μM taurine or 10 μM glycine enhanced the occurrence of recurrent epileptiform activity induced by 20 μM 4-aminopyridine in low Mg(2+) solution. This proconvulsive effect was prevented by 3 μM strychnine or after incubation with the loop diuretic bumetanide (10 μM), suggesting that it required glycine receptors and an active NKCC1-dependent Cl(-) accumulation. Application of higher doses of taurine (≥ 1 mM) or glycine (100 μM) attenuated recurrent epileptiform discharges. The anticonvulsive effect of taurine was also observed in the presence of the GABAA receptor antagonist gabazine and was attenuated by strychnine, suggesting that it was partially mediated by glycine receptors. Bath application of the glycinergic antagonist strychnine (0.3 μM) induced epileptiform discharges. We conclude from these results that in the immature hippocampus, activation of glycine receptors can mediate both pro- and anticonvulsive effects, but that a persistent activation of glycine receptors is required to suppress epileptiform activity. In summary, our study elucidated the important role of glycine receptors in the control of neuronal excitability in the immature hippocampus.

Effects of ethanol on glycinergic synaptic currents in mouse spinal cord neurons

Ethanol increased the frequency of miniature glycinergic currents [miniature inhibitory postsynaptic currents (mIPSCs)] in cultured spinal neurons. This effect was dependent on intracellular calcium augmentation, since preincubation with BAPTA (an intracellular calcium chelator) or thapsigargin [a sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) pump inhibitor] significantly attenuated this effect. Similarly, U73122 (a phospholipase C inhibitor) or 2-aminoethoxydiphenyl borate [2-APB, an inositol 1,4,5-trisphosphate (IP₃) receptor (IP3R) inhibitor] reduced this effect. Block of ethanol action was also achieved after preincubation with Rp-cAMPS, inhibitor of the adenylate cyclase (AC)/PKA signaling pathway. These data suggest that there is a convergence at the level of IP₃R that accounts for presynaptic ethanol effects. At the postsynaptic level, ethanol increased the decay time constant of mIPSCs in a group of neurons (30 ± 10% above control, n = 13/26 cells). On the other hand, the currents activated by exogenously applied glycine were consistently potentiated (55 ± 10% above control, n = 11/12 cells), which suggests that ethanol modulates synaptic and nonsynaptic glycine receptors (GlyRs) in a different fashion. Supporting the role of G protein modulation on ethanol responses, we found that a nonhydrolyzable GTP analog [guanosine 5'-O-(3-thiotriphosphate) (GTPγS)] increased the decay time constant in ∼50% of the neurons (28 ± 12%, n = 11/19 cells) but potentiated the glycine-activated Cl(-) current in most of the neurons examined (83 ± 29%, n = 7/9 cells). In addition, confocal microscopy showed that α1-containing GlyRs colocalized with Gβ and Piccolo (a presynaptic cytomatrix protein) in ∼40% of synaptic receptor clusters, suggesting that colocalization of Gβγ and GlyRs might account for the difference in ethanol sensitivity at the postsynaptic level.

Acute effects of ethanol on GABAA and glycine currents in the lateral habenula neurons of young rats

Compelling evidence has shown a pivotal role of dopaminergic function in drug addiction. Recently, the lateral habenula (LHb) has attracted a great deal of attention as another target for abused drugs in the brain because its role in regulating dopaminergic system, among others. GABA and glycine are major inhibitory neurotransmitters. Their corresponding receptors are key targets of ethanol. The properties of these receptors in LHb neurons and their responses to ethanol in particular however, remain unknown. Using the patch clamp techniques, we examined the effects of ethanol on the chloride currents elicited by GABA and glycine in LHb neurons acutely dissociated from 10-20 day-old Sprague-Dawley rats. We show that GABA concentration-dependently elicited a bicuculline sensitive inward current in 96% (130/140) of the neurons tested. Ethanol (43.2 mM) suppressed current elicited by a wide range of concentrations (1-300 μM) of GABA in 74% (35/47) cells tested. Ethanol suppression is dependent on its concentrations but not on membrane potentials of the neurons. Moreover, glycine concentration-dependently elicited an inward current in 94% (112/120) of the neurons tested. Both strychnine and picrotoxin concentration dependently suppressed glycine current with IC₅₀ of 220 nM and 813 μM, respectively. Ethanol (43.2 mM) potentiated current elicited by unsaturated but not saturated concentrations of glycine. Thus, the LHb neurons of young rats contain both functional GABA_A and glycine receptors which are sensitive to ethanol at pharmacologically relevant concentrations. These effects of ethanol might be important in the control of the activity and output of LHb neurons.

Neuronal correlates of the dominant role of GABAergic transmission in the developing mouse locomotor circuitry

GABA and glycine are the primary fast inhibitory neurotransmitters in the mammalian spinal cord, but they differ in their regulatory functions, balancing neuronal excitation in the locomotor circuitry in the mammalian spinal cord. This review focuses on the unique role of GABAergic transmission during the assembly of the locomotor circuitry, from early embryonic stages when GABA(A) receptor-activated membrane depolarizations increase network excitation, to the period of early postnatal development, when GABAergic inhibition plays a primary role in coordinating the patterns of locomotor-like motor activity. To gain insight into the mechanisms that underlie the dominant contribution of GABAergic transmission to network activity during that period, we examined the morphological and electrophysiological properties of a subpopulation of GABAergic commissural interneurons that fit well with their putative function as integrated components of the rhythm-coordinating networks in the mouse spinal cord.

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.

A common molecular basis for exogenous and endogenous cannabinoid potentiation of glycine receptors

Both exogenous and endogenous cannabinoids can allosterically modulate glycine receptors (GlyRs). However, little is known about the molecular basis of cannabinoid-GlyR interactions. Here we report that sustained incubation with the endocannabinoid anandamide (AEA) substantially increased the amplitude of glycine-activated current in both rat cultured spinal neurons and in HEK-293 cells expressing human α1, rat α2 and α3 GlyRs. While the α1 and α3 subunits were highly sensitive to AEA-induced potentiation, the α2 subunit was relatively insensitive to AEA. Switching a serine at 296 and 307 in the TM3 (transmembrane domain 3) of the α1 and α3 subunits with an alanine (A) at the equivalent position in the α2 subunit converted the α1/α3 AEA-sensitive receptors to sensitivity resembling that of α2. The S296 residue is also critical for exogenous cannabinoid-induced potentiation of I(Gly). The magnitude of AEA potentiation decreased with removal of either the hydroxyl or oxygen groups on AEA. While desoxy-AEA was significantly less efficacious in potentiating I(Gly), desoxy-AEA inhibited potentiation produced by both Δ(9)-tetrahydrocannabinol (THC), a major psychoactive component of marijuana, and AEA. Similarly, didesoxy-THC, a modified THC with removal of both hydroxyl/oxygen groups, did not affect I(Gly) when applied alone but inhibited the potentiation of I(Gly) induced by AEA and THC. These findings suggest that exogenous and endogenous cannabinoids potentiate GlyRs via a hydrogen bonding-like interaction. Such a specific interaction likely stems from a common molecular basis involving the S296 residue in the TM3 of the α1 and α3 subunits.

Allosteric modulation of glycine receptors

Inhibitory (or strychnine sensitive) glycine receptors (GlyRs) are anion-selective transmitter-gated ion channels of the cys-loop superfamily, which includes among others also the inhibitory γ-aminobutyric acid receptors (GABA(A) receptors). While GABA mediates fast inhibitory neurotransmission throughout the CNS, the action of glycine as a fast inhibitory neurotransmitter is more restricted. This probably explains why GABA(A) receptors constitute a group of extremely successful drug targets in the treatment of a wide variety of CNS diseases, including anxiety, sleep disorders and epilepsy, while drugs specifically targeting GlyRs are virtually lacking. However, the spatially more restricted distribution of glycinergic inhibition may be advantageous in situations when a more localized enhancement of inhibition is sought. Inhibitory GlyRs are particularly relevant for the control of excitability in the mammalian spinal cord, brain stem and a few selected brain areas, such as the cerebellum and the retina. At these sites, GlyRs regulate important physiological functions, including respiratory rhythms, motor control, muscle tone and sensory as well as pain processing. In the hippocampus, RNA-edited high affinity extrasynaptic GlyRs may contribute to the pathology of temporal lobe epilepsy. Although specific modulators have not yet been identified, GlyRs still possess sites for allosteric modulation by a number of structurally diverse molecules, including alcohols, neurosteroids, cannabinoids, tropeines, general anaesthetics, certain neurotransmitters and cations. This review summarizes the present knowledge about this modulation and the molecular bases of the interactions involved.

Glycine-activated chloride currents of neurons freshly isolated from the prefrontal cortex of young rats

Strychnine-sensitive glycine receptors (GlyR) play a major role in the excitability of CNS neurons and are also a major target of many drugs including some general anesthetics and ethanol. The prefrontal cortex (PFC) is an important substrate responsible for cognitive function and for sedation, as well as hypnosis (unconsciousness) which is induced by general anesthetics and ethanol. However, the functions and the physiological and pharmacological properties of GlyRs in mature PFC neurons have not been well studied. In this study, whole-cell currents induced by glycine (I(Gly)) were recorded from freshly isolated PFC neurons of Sprague-Dawley rats aged 5 to 39 postnatal days (neonatal, P5-12; weanling, P17-21 and peri-adolescent, P30-39). We found that most of the neurons examined were responsive to glycine and the response was concentration dependent. With the increase of age, the sensitivity to glycine was significantly decreased and the sensitivity to picrotoxin was significantly increased. Conversely, the changes in sensitivity to strychnine were not significant. Interestingly, I(Gly) of all age groups was suppressed (to different scope) by low concentrations of picrotoxin (≤ 30 μM), which selectively blocked α homomeric GlyRs. Conversely, about 20-65% of I(Gly) remained in the presence of 300 μM picrotoxin, suggesting the picrotoxin-resistant subtype the αβ heteromeric GlyR, was also present. These data provide the first evidence that there are at least two subtypes of functional GlyRs in the PFC neurons of young rats, and their physiological and pharmacological properties change substantially during maturation.

Molecular requirements for ethanol differential allosteric modulation of glycine receptors based on selective Gbetagamma modulation

It is now believed that the allosteric modulation produced by ethanol in glycine receptors (GlyRs) depends on alcohol binding to discrete sites within the protein structure. Thus, the differential ethanol sensitivity of diverse GlyR isoforms and mutants was explained by the presence of specific residues in putative alcohol pockets. Here, we demonstrate that ethanol sensitivity in two ligand-gated ion receptor members, the GlyR adult α(1) and embryonic α(2) subunits, can be modified through selective mutations that rescued or impaired Gβγ modulation. Even though both isoforms were able to physically interact with Gβγ, only the α(1) GlyR was functionally modulated by Gβγ and pharmacological ethanol concentrations. Remarkably, the simultaneous switching of two transmembrane and a single extracellular residue in α(2) GlyRs was enough to generate GlyRs modulated by Gβγ and low ethanol concentrations. Interestingly, although we found that these TM residues were different to those in the alcohol binding site, the extracellular residue was recently implicated in conformational changes important to generate a pre-open-activated state that precedes ion channel gating. Thus, these results support the idea that the differential ethanol sensitivity of these two GlyR isoforms rests on conformational changes in transmembrane and extracellular residues within the ion channel structure rather than in differences in alcohol binding pockets. Our results describe the molecular basis for the differential ethanol sensitivity of two ligand-gated ion receptor members based on selective Gβγ modulation and provide a new mechanistic framework for allosteric modulations of abuse drugs.

Regulation of excitation by glycine receptors

Our knowledge of glycine receptor (GlyR) regulation of excitation has advanced significantly in recent years. GlyRs are widespread in the CNS, are heterogeneous, and undergo developmental changes. Activation of GlyRs of immature neurons induces outflow of Cl( - ), membrane depolarization, neuronal excitation, calcium influx, and transmitter release, in contrast to the inhibitory effects these receptors have in mature neurons. Thus, GlyRs are important for neuronal excitability in both the developing and the mature CNS. This chapter is an overview of selective studies on the newly discovered roles of GlyRs in regulating neuronal excitation, and inhibition, particularly in the upper brain areas.

Anti-homeostatic synaptic plasticity of glycine receptor function after chronic strychnine in developing cultured mouse spinal neurons

In this study, we describe a novel form of anti-homeostatic plasticity produced after culturing spinal neurons with strychnine, but not bicuculline or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Strychnine caused a large increase in network excitability, detected as spontaneous synaptic currents and calcium transients. The calcium transients were associated with action potential firing and activation of gamma-aminobutyric acid (GABA(A)) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors as they were blocked by tetrodotoxin (TTX), bicuculline, and CNQX. After chronic blockade of glycine receptors (GlyRs), the frequency of synaptic transmission showed a significant enhancement demonstrating the phenomenon of anti-homeostatic plasticity. Spontaneous inhibitory glycinergic currents in treated cells showed a fourfold increase in frequency (from 0.55 to 2.4 Hz) and a 184% increase in average peak amplitude compared with control. Furthermore, the augmentation in excitability accelerated the decay time constant of miniature inhibitory post-synaptic currents. Strychnine caused an increase in GlyR current density, without changes in the apparent affinity. These findings support the idea of a post-synaptic action that partly explains the increase in synaptic transmission. This phenomenon of synaptic plasticity was blocked by TTX, an antibody against brain-derived neurotrophic factor (BDNF) and K252a suggesting the involvement of the neuronal activity-dependent BDNF-TrkB signaling pathway. These results show that the properties of GlyRs are regulated by the degree of neuronal activity in the developing network.

Study of the interaction of antiplasmodial strychnine derivatives with the glycine receptor

Strychnos icaja Baill. (Loganiaceae) is a liana found in Central Africa known to be an arrow and ordeal poison but also used by traditional medicine to treat malaria. Recently, many dimeric or trimeric indolomonoterpenic alkaloids with antiplasmodial properties have been isolated from its rootbark. Since these alkaloids are derivatives of strychnine, it was important, in view of their potential use as antimalarial drugs, to assess their possible convulsant strychnine-like properties. In that regard, their interaction with the strychnine-sensitive glycine receptor was investigated by whole-cell patch-clamp recordings on glycine-gated currents in mouse spinal cord neurons in culture and by [(3)H]strychnine competition assays on membranes from adult rat spinal cord. These experiments were carried out on sungucine (leading compound of the chemical class) and on the antiplasmodial strychnogucine B (dimeric) and strychnohexamine (trimeric). In comparison with strychnine, all compounds interact with a very poor efficacy and only at concentrations >1 microM with both [(3)H]strychnine binding and glycine-gated currents. Furthermore, the effects of strychnine and protostrychnine, a monomeric alkaloid (without antiplasmodial activity) also isolated from S. icaja and differing from strychnine only by a cycle opening, were compared in the same way. The weak interaction of protostrychnine confirms the importance of the G cycle ring structure in strychnine for its binding to the glycine receptor and its antagonist properties.

Distinct roles of glycinergic and GABAergic inhibition in coordinating locomotor-like rhythms in the neonatal mouse spinal cord

The primary objective of our study was to examine the role of the inhibitory neurotransmitters glycine and GABA in modulating spontaneous activity and coordinating neurochemically induced locomotor-like rhythms in the mouse spinal cord. Motor outputs were recorded in lumbar ventral roots of 1-4-day old neonatal mice, and the function of glycinergic and GABAergic synapses in regulating spontaneous and induced activities was examined by suppressing synaptic inhibition using selective glycine or GABAA receptor antagonists. Strychnine (0.5 microM), a glycine receptor antagonist, did not change the pattern of spontaneous activity that consisted of random single spikes and discharges of variable durations and intervals. In contrast, blocking GABAA receptors with either picrotoxin (10 microM) or bicuculline (5 microM) triggered bilaterally synchronous, non-rhythmic discharges. These findings suggested that GABAergic synapses suppressed excitatory synapses, and their disinhibition synchronized spontaneous discharges between the two sides of the spinal cord. Locomotor-like rhythms alternating between the two sides of the spinal cord were triggered by the neurotransmitter agonists 5-HT, N-methyl-D,L-aspartic acid and dopamine. Blocking glycine receptors increased tonic discharges, and in most preparations it reduced the phase correlation between the alternating rhythms. Inhibiting GABAA receptor-mediated synapses synchronized the onset and prolonged the duration of rhythmic discharges. Intraburst alternating peaks were evident and those were suppressed by strychnine, suggesting that they were mediated via glycinergic synapses. Our findings indicated that GABAergic and glycinergic synapses played different roles in modulating neurochemically induced locomotion rhythms. GABAergic inhibition regulated the onset and duration of neurochemically induced locomotor-like rhythms, and glycinergic inhibition stabilized the pattern of the alternating rhythms.

Developmental Regulation of β-Carboline-Induced Inhibition of Glycine-Evoked Responses Depends on Glycine Receptor β Subunit Expression

In this work, we show that beta-carbolines, which are known negative allosteric modulators of GABA(A) receptors, inhibit glycine-induced currents of embryonic mouse spinal cord and hippocampal neurons. In both cell types, beta-carboline-induced inhibition of glycine receptor (GlyR)-mediated responses decreases with time in culture. Single-channel recordings show that the major conductance levels of GlyR unitary currents shifts from high levels (> or = 50 pS) in 2 to 3 days in vitro (DIV) neurons to low levels (<50 pS) in 11 to 14 DIV neurons, assessing the replacement of functional homomeric GlyR by heteromeric GlyR. In cultured spinal cord neurons, the disappearance of beta-carboline inhibition of glycine responses and high conductance levels is almost complete in mature neurons, whereas a weaker decrease in beta-carboline-evoked glycine response inhibition and high conductance level proportion is observed in hippocampal neurons. To confirm the hypothesis that the decreased sensitivity of GlyR to beta-carbolines depends on beta subunit expression, Chinese hamster ovary cells were permanently transfected either with GlyR alpha2 subunit alone or in combination with GlyR beta subunit. Single-channel recordings revealed that the major conductance levels shifted from high levels (> or = 50 pS) in GlyR-alpha2-transfected cells to low levels (<50 pS) in GlyR-alpha2+beta-containing cells. Consistently, both picrotoxin- and beta-carboline-induced inhibition of glycine-gated currents were significantly decreased in GlyR-alpha2+beta-transfected cells compared with GlyR-alpha2-containing cells. In summary, we demonstrate that the incorporation of beta subunits in GlyRs confers resistance not only to picrotoxin but also to beta-carboline-induced inhibition. Furthermore, we also provide evidence that hippocampal neurons undergo in vitro a partial maturation process of their GlyR-mediated responses.

Changes on the properties of glycine receptors during neuronal development

Glycine receptors (GlyRs) play a major role in the excitability of spinal cord and brain stem neurons. During development, several properties of these receptors undergo significant changes resulting in major modifications of their physiological functions. For example, the receptor structure switches from a monomeric alpha or heteromeric alpha 2 beta in immature neurons to an alpha 1 beta receptor type in mature neurons. Together with these changes in receptor subunits, the postsynaptic cluster size increases with development. Parallel to these modifications, the apparent receptor affinity to glycine and strychnine, as well as that of Zn(2+) and ethanol increases with time. The mature receptor is characterized by a slow desensitizing current and high sensitivity to modulation by protein kinase C. Also, the high level of glycinergic transmission in immature spinal neurons modulates neuronal excitability causing membrane depolarization and changes in intracellular calcium. Due to these properties, chronic inhibition of glycinergic transmission affects neurite outgrowth and produces changes in the level of synaptic transmission induced by GABA(A) and AMPA receptors. Finally, the high level of plasticity found in immature GlyRs is likely associated to changes in cytoskeleton dynamics.

Molecular structure and function of the glycine receptor chloride channel

The glycine receptor chloride channel (GlyR) is a member of the nicotinic acetylcholine receptor family of ligand-gated ion channels. Functional receptors of this family comprise five subunits and are important targets for neuroactive drugs. The GlyR is best known for mediating inhibitory neurotransmission in the spinal cord and brain stem, although recent evidence suggests it may also have other physiological roles, including excitatory neurotransmission in embryonic neurons. To date, four alpha-subunits (alpha1 to alpha4) and one beta-subunit have been identified. The differential expression of subunits underlies a diversity in GlyR pharmacology. A developmental switch from alpha2 to alpha1beta is completed by around postnatal day 20 in the rat. The beta-subunit is responsible for anchoring GlyRs to the subsynaptic cytoskeleton via the cytoplasmic protein gephyrin. The last few years have seen a surge in interest in these receptors. Consequently, a wealth of information has recently emerged concerning GlyR molecular structure and function. Most of the information has been obtained from homomeric alpha1 GlyRs, with the roles of the other subunits receiving relatively little attention. Heritable mutations to human GlyR genes give rise to a rare neurological disorder, hyperekplexia (or startle disease). Similar syndromes also occur in other species. A rapidly growing list of compounds has been shown to exert potent modulatory effects on this receptor. Since GlyRs are involved in motor reflex circuits of the spinal cord and provide inhibitory synapses onto pain sensory neurons, these agents may provide lead compounds for the development of muscle relaxant and peripheral analgesic drugs.

Striatal PSA-NCAM+ precursor cells from the newborn rat express functional glycine receptors

Immunocytochemical analysis showed that ionotropic glycine receptors are expressed in neurogenic progenitors purified from the newborn rat striatum and expressing the polysialylated form of the neural cell adhesion molecule, both in vitro and in situ. To ascertain whether glycine receptors were functional in vitro, whole-cell patch-clamp recordings demonstrated that glycine triggers inward strychnine-sensitive currents in the majority of these cells. Moreover, we found that glycine receptors expressed by these neurogenic progenitors display intermediate electrophysiological characteristics between those of glycine receptors expressed by neural stem cells and by mature interneurons from the rat striatum. Altogether, the present data show that functional strychnine-sensitive glycine receptors are expressed in neurogenic progenitors purified from the newborn rat striatum.

A Role for Ligand-Gated Ion Channels in Rod Photoreceptor Development

Neurotransmitter receptors are central to communication at synapses. Many components of the machinery for neurotransmission are present prior to synapse formation, suggesting a developmental role. Here, evidence is presented that signaling through glycine receptor alpha2 (GlyRalpha2) and GABA(A) receptors plays a role in photoreceptor development in the vertebrate retina. The signaling is likely mediated by taurine, which is present at high levels throughout the developing central nervous system (CNS). Taurine potentiates the production of rod photoreceptors, and this induction is inhibited by strychnine, an antagonist of glycine receptors, and bicuculline, an antagonist of GABA receptors. Gain-of-function experiments showed that signaling through GlyRalpha2 induced exit from mitosis and an increase in rod photoreceptors. Furthermore, targeted knockdown of GlyRalpha2 decreased the number of photoreceptors while increasing the number of other retinal cell types. These data support a previously undescribed role for these ligand-gated ion channels during the early stages of CNS development.

The 5-HT3B subunit confers reduced sensitivity to picrotoxin when co-expressed with the 5-HT3A receptor

There are currently no known agents that display selectivity between homomeric 5-hydroxytryptamine type 3A (5-HT3A) and heteromeric 5-HT3A/3B receptors. In the present study, we show that the CNS convulsant picrotoxin selectively interacts with 5-HT3A receptors. In whole-cell patch clamp recordings, the inhibitory effect of PTX was reduced 100-fold in heteromeric mouse 5-HT3A/3B receptors, compared to homomeric 5-HT3A receptors. Picrotoxin should prove to be a useful probe for determining the presence of homomeric vs. heteromeric 5-HT3 receptors in both native tissue and recombinant receptor preparations.

Developmental-dependent action of microtubule depolymerization on the function and structure of synaptic glycine receptor clusters in spinal neurons

Microtubules have been proposed to interact with gephyrin/glycine receptors (GlyRs) in synaptic aggregates. However, the consequence of microtubule disruption on the structure of postsynaptic GlyR/gephyrin clusters is controversial and possible alterations in function are largely unknown. In this study, we have examined the physiological and morphological properties of GlyR/gephyrin clusters after colchicine treatment in cultured spinal neurons during development. In immature neurons (5-7 DIV), disruption of microtubules resulted in a 33 +/- 4% decrease in the peak amplitude and a 72 +/- 15% reduction in the frequency of spontaneous glycinergic miniature postsynaptic currents (mIPSCs) recorded in whole cell mode. However, similar colchicine treatments resulted in smaller effects on 10-12 DIV neurons and no effect on mature neurons (15-17 DIV). The decrease in glycinergic mIPSC amplitude and frequency reflects postsynaptic actions of colchicine, since postsynaptic stabilization of microtubules with GTP prevented both actions and similar reductions in mIPSC frequency were obtained by modifying the Cl(-) driving force to obtain parallel reductions in mIPSC amplitude. Confocal microscopy revealed that colchicine reduced the average length and immunofluorescence intensity of synaptic gephyrin/GlyR clusters in immature (approximately 30%) and intermediate (approximately 15%) neurons, but not in mature clusters. Thus the structural and functional changes of postsynaptic gephyrin/GlyR clusters after colchicine treatment were tightly correlated. Finally, RT-PCR, kinetic analysis and picrotoxin blockade of glycinergic mIPSCs indicated a reorganization of the postsynaptic region from containing both alpha2beta and alpha1beta GlyRs in immature neurons to only alpha1beta GlyRs in mature neurons. Microtubule disruption preferentially affected postsynaptic sites containing alpha2beta-containing synaptic receptors.

Effects of chronic ethanol consumption on rat GABA(A) and strychnine-sensitive glycine receptors expressed by lateral/basolateral amygdala neurons

It is well known that the anxiolytic potential of ethanol is maintained during chronic exposure. We have confirmed this using a light-dark box paradigm following chronic ethanol ingestion via a liquid diet. However, cessation from chronic ethanol exposure is known to cause severe withdrawal anxiety. These opposing effects on anxiety likely result from neuro-adaptations of neurotransmitter systems within the brain regions regulating anxiety. Recent work highlights the importance of amygdala ligand-gated chloride channels in the expression of anxiety. We have therefore examined the effects of chronic ethanol exposure on GABA(A) and strychnine-sensitive glycine receptors expressed by acutely isolated adult rat lateral/basolateral amygdala neurons. Chronic ethanol exposure increased the functional expression of GABA(A) receptors in acutely isolated basolateral amygdala neurons without altering strychnine-sensitive glycine receptors. Neither the acute ethanol nor benzodiazepine sensitivity of either receptor system was affected. We explored the likelihood that subunit composition might influence each receptor's response to chronic ethanol. Importantly, when expressed in a mammalian heterologous system, GABA(A) receptors composed of unique alpha subunits were differentially sensitive to acute ethanol. Likewise, the presence of the beta subunit appeared to influence the acute ethanol sensitivity of glycine receptors containing the alpha(2) subunit. Our results suggest that the facilitation of GABA(A) receptors during chronic ethanol exposure may help explain the maintenance of ethanol's anti-anxiety effects during chronic ethanol exposure. Furthermore, the subunit composition of GABA(A) and strychnine-sensitive glycine receptors may ultimately influence the response of each system to chronic ethanol exposure.

Functional glycine receptor maturation in the absence of glycinergic input in dopaminergic neurones of the rat substantia nigra

The postnatal maturation pattern of glycine receptor channels (GlyRs) expressed by dopaminergic (DA) neurones of the rat substantia nigra pars compacta (SNc) was investigated using single-channel and whole-cell patch-clamp recordings in brain slices from rats aged 7-21 postnatal days (P). In neonatal rats (P7-P10), GlyRs exhibited a main conductance state of 100-110 pS with a mean open time of 16 ms. In juvenile rats (P19-P22), both the GlyR main conductance state (46-55 pS) and the mean open time (6.8 ms) were decreased. In neonatal rats, application of 30 microM picrotoxin, which is known to block homomeric GlyRs, strongly reduced glycine-evoked responses, while it was much less effective in juvenile rats. These results suggest that these GlyRs correspond functionally to alpha(2) homomeric GlyRs in neonatal rats and alpha(1)/beta heteromeric GlyRs in juvenile rats. A drastic but transient decrease in the glycine responsiveness of DA neurones occurred around P17 concomitant to the functional switch from the homomeric state to the heteromeric state. This age corresponds to a maturation phase for DA neurones. The application of 1 microM gabazine blocked spontaneous or evoked inhibitory synaptic current, while the addition of 1 microM strychnine had no effect, suggesting a lack of functional glycinergic synapses on DA neurones. Although it has been proposed that taurine is co-released with GABA at GABAergic synapses on DA neurones, in the present study the stimulation of GABAergic fibres failed to activate GlyRs. Blockade of taurine transporters and applications of high K(+) and hyposmotic solutions were also unable to induce any strychnine-sensitive current. We conclude that functional maturation of GlyRs can occur in the absence of any detectable GlyR activation in DA neurones of the SNc.

Picrotoxin and bicuculline have different effects on lumbar spinal networks and motoneurons in the neonatal rat

Bicuculline is the most commonly used GABA(A) receptor antagonist to investigate the contribution of these receptors in motor control. However, this compound has been shown recently to potentiate the burst firing of neurons in various brain regions by blocking a calcium-activated potassium current underlying the spike after-hyperpolarization (AHP). This effect may distort our understanding of the role of GABA(A) receptors at the network level. In vitro brainstem-spinal cord preparations isolated from neonatal rats were used to compare the effects of bicuculline methiodide (bicuculline-M) and picrotoxin (PTX), another GABA(A) receptor antagonist, on the AHP of lumbar motoneurons as well as on spontaneous and locomotor-like motor activities. Intracellular recordings of lumbar motoneurons showed that bicuculline-M (20 microM) reduced the AHP to 57% of control whereas PTX (20-60 microM) had no significant effect. Bath-application of increasing concentrations of PTX caused an increase in spontaneous ventral root activity, which further increased significantly when bicuculline-M was added. The effects of both antagonists were tested on fictive locomotion. The left-right alternation was disrupted in the presence of bicuculline-M. A slow synchronous bursting activity of large amplitude also appeared in the presence of PTX. This slow rhythm was superimposed on a faster rhythm which still exhibited some degree of left-right alternation. These data demonstrate that bicuculline-M may not reveal accurately the contribution of GABA(A) receptors in motor control and the intrinsic properties of disinhibited networks.

Early expression of glycine and GABA(A) receptors in developing spinal cord neurons. Effects on neurite outgrowth

Using fluorometric and immunocytochemical techniques, we found that high glycine concentrations or blockade of glycine receptors increases neurite outgrowth in developing mouse spinal cord neurons. Glycine- and GABA(A)-activated currents were demonstrated during applications of glycine and GABA (50-100 microM) in 5 days in vitro (DIV) neurons. Long application (> or =10 min) of 100 microM glycine desensitized the membrane response by more than 95%. Application of glutamate in the absence of external Mg(2+), at several membrane potentials, did not produce any detectable membrane response in these cells. Immunocytochemical studies with NR1 and GluR1 antibodies showed a delayed appearance of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors respectively. Spontaneous synaptic activity was readily observed in 5 DIV neurons. The use of various receptor antagonists (strychnine, bicuculline, DL-2-amino-5-phosphonovalerate [APV], 6-cyano-7-nitroquinoxaline-2,3-dione [CNQX]) revealed that this activity was predominantly glycinergic, and to a smaller extent, GABAergic. In the presence of bicuculline, APV and CNQX, we detected abundant spontaneous depolarizing potentials which often reached the action potential threshold. Further evidence for functional synaptic activity was provided by the detection of co-localization of gephyrin and synaptophysin at 5 DIV using confocal microscopy. Fluorometric studies with Fluo-3, a Ca(2+) indicator, in 5 DIV cultures showed the presence of spontaneous fluctuations associated with tetrodotoxin-sensitive synaptic events. The number of neurons displaying these fluctuations was significantly increased (>100%) when the cells were bathed in a strychnine-containing solution. On the other hand, these synaptically mediated Ca(2+) events were blocked by the co-application of strychnine and bicuculline. This suggests that glycine and GABA(A) receptors provide a fundamental regulation of both neuronal excitability and intracellular Ca(2+) at this early time of development.The neurotrophic effects of agonists and antagonists for glycine, GABA(A) and glutamate receptors were examined in neurons cultured for 2 or 5 DIV. From all the agonists used, only high concentrations of glycine increased neurite outgrowth in 5 DIV neurons. We found that strychnine also increased neurite outgrowth, whereas tetrodotoxin (1 microM), nimodipine (4 microM) and bicuculline (20 microM) completely blocked it. On the other hand, APV (50 microM) and CNQX (20 microM) were unable to affect neurite outgrowth. These data suggest that spinal glycine receptors depress neurite outgrowth by shunting neuronal excitability. Outgrowth induction possibly results from the enhanced activity found after the inhibition of glycinergic activity. We postulate that this resets the intracellular calcium at a concentration that favors neurite outgrowth.

Glycine receptors involved in synaptic transmission are selectively regulated by the cytoskeleton in mouse spinal neurons

Using whole cell patch-clamp recordings, we examined the effect of colchicine, a microtubule disrupter, on the properties of glycine receptors (GlyRs) in cultured spinal cord neurons. Confocal microscopy revealed that colchicine treatment effectively altered microtubule bundles and neuronal morphology. Application of colchicine via the culture media or the patch-pipette, however, did not affect the whole cell current rundown (73 +/- 6% of control after 1 h), the sensitivity of the GlyR to glycine (EC(50) = 29 +/- 1 microM), or strychnine inhibition (47 +/- 5% of control after 100 nM strychnine). On the other hand, colchicine dialyzed for 25 min via the patch pipette selectively reduced the quantal amplitude of spontaneous glycinergic miniature inhibitory postsynaptic currents (mIPSCs) to 68 +/- 5% of control. This effect was specific for GlyRs since synaptic events mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and GABA(A) receptors were unchanged. In conclusion, this study indicates that microtubules can regulate the function of GlyRs involved in inhibitory synaptic transmission.

Ethanol inhibition of glycine-activated responses in neurons of ventral tegmental area of neonatal rats

The brain is particularly sensitive to alcohol during the period of its rapid growth. To better understand the mechanism(s) involved, we studied ethanol effects on glycine-activated responses of ventral tegmental area (VTA) neurons isolated from the newborn rat, using whole cell and gramicidin perforated patch-clamp techniques. Previously we reported that 0.1-40 mM ethanol enhances glycine-induced responses of 35% of VTA neurons. We now direct our attention to the inhibitory effects of ethanol observed in 45% (312 of 694) of neonatal VTA neurons. Under current-clamp conditions, 1 mM ethanol had no effect on the membrane potential of these cells, but it decreased glycine-induced membrane depolarization and the frequency of spontaneous action potentials. Under voltage-clamp conditions, 0.1-10 mM ethanol did not elicit a current but depressed the glycine-induced currents. The ethanol-induced inhibition of glycine current was independent of membrane potential (between -60 and +60 mV). Likewise, ethanol did not alter the reversal potential of the glycine-activated currents. Ethanol-mediated inhibition of glycine current depended on the glycine concentration. While ethanol strongly depressed currents activated by 30 microM glycine, it had no appreciable effect on maximal currents activated by 1 mM glycine. In the presence of ethanol (1 mM), the EC(50) for glycine increased from 32 +/- 5 to 60 +/- 3 microM. Thus ethanol may decrease the agonist affinity of glycine receptors. A kinetic analysis indicated that ethanol shortens the time constant of glycine current deactivation but has no effect on activation. In conclusion, by altering VTA neuronal function, ethanol-induced changes in glycine receptors may contribute to neurobehavioral manifestations of the fetal alcohol syndrome.

High-affinity zinc potentiation of inhibitory postsynaptic glycinergic currents in the zebrafish hindbrain

Zinc has been reported to potentiate glycine receptors (GlyR), but the physiological significance of this observation has been put in doubt by the relatively high values of the EC(50), 0.5-1 microM, since such concentrations may not be attained in the synaptic cleft of glycinergic synapses. We have re-evaluated this observation in the frame of the hypothesis that contaminant heavy metals present in usual solutions may have lead to underestimate the affinity of the zinc binding site, and therefore to underestimate the potential physiological role of zinc. Using chelators either to complex heavy metals or to apply zinc at controlled concentrations, we have examined the action of zinc on GlyR kinetics in outside-out patches from 50-h-old zebrafish Mauthner cells. Chelating contaminating heavy metals with tricine or N,N,N',N'-tetrakis-(2-pyridylmethyl)-ethylenediamine (TPEN) decreased the duration of the currents evoked by glycine, confirming that traces of heavy metals alter the GlyR response in control conditions. Using tricine- (10 mM) buffered zinc solution, we then showed that zinc increases the amplitude of outside-out responses evoked by 0.1-0.5 mM glycine with an EC(50) of 15 nM. In contrast zinc had no effect on the amplitude of currents evoked by a saturating concentration (3-10 mM) of glycine. This suggests that zinc enhances GlyR apparent affinity for glycine. The study of the effects of zinc on the kinetics of the response indicates that this increase of apparent affinity is due to a decrease of the glycine dissociation rate constant. We then analyzed the effects of zinc on postsynaptic GlyRs in whole cell recordings of glycinergic miniature inhibitory postsynaptic currents (mIPSCs). Chelation of contaminant heavy metals decreased the amplitude and the duration of the mIPSCs; inverse effects were observed by adding zinc in buffered solutions containing nanomolar free zinc concentrations. Zinc plus tricine or tricine alone did not change the coefficient of variation ( approximately 0.85) of the mIPSC amplitude distributions. These results suggest that postsynaptic GlyRs are not saturated after the release of one vesicle.

Postnatal maturation of gephyrin/glycine receptor clusters on developing Renshaw cells

Adult mammalian Renshaw cells express large and complex postsynaptic gephyrin/glycine receptor clusters on their surface. Larger gephyrin clusters correlate with more "efficacious" inhibitory synapses, in terms of larger postsynaptic quantal size amplitudes, in part because they likely contain more postsynaptic receptors (Lim et al. [1999] J. Physiol. (Lond.) 516:505-512; Oleskevich et al. [1999] J. Neurophysiology 82:312-319). Here, we studied the postnatal development of the gephyrin/glycine receptor cluster size on Renshaw cells. Renshaw cells were identified by their calbindin immunoreactivity, location and morphology, and presence of cholinergic input. The populations of clusters over developing Renshaw cells immunoreactive to gephyrin or glycine receptor alpha1 subunits were comparable in number, size, and complexity and displayed a high degree of colocalization (>90%) at all ages. Quantitative morphologic analysis was performed on gephyrin-immunoreactive clusters. In neonatal animals, Renshaw cells expressed small punctate gephyrin-immunoreactive clusters (mean cluster size +/- SD = 0.19 +/- 0.19 microm(2)at 2 days; 0.22 +/- 0. 19 microm(2)at 5 days). By 10 and 15 days of age, Renshaw cells exhibited gephyrin-immunoreactive clusters that were larger and more complex (0.32 +/- 0.19 microm(2) at 10 days; 0.41 +/- 0.32 microm(2) at 15 days). Cluster growth reached a plateau in 25- and 60-day-old Renshaw cells (0.45 +/- 0.43 microm(2); 0.56 +/- 0.55 microm(2), respectively). By using electron microscopy, we confirmed that gephyrin-immunoreactive clusters were located at postsynaptic sites at both early and late postnatal ages on Renshaw cells. The potential significance of this gephyrin/glycine receptor cluster size maturation that sets Renshaw cells apart from other interneurons is discussed.

Glycine receptor beta subunits play a critical role in potentiation of glycine responses by ICS-205,930

The sensitivity of various types of recombinant glycine receptors (GlyRs) to ICS-205,930 was studied by fast perfusion in Xenopus laevis oocytes. This compound has previously been shown to potentiate glycine responses in rat spinal neurons between 10 nM and 1 microM, independently of its 5-HT(3) antagonist properties. In contrast, submicromolar concentrations of ICS-205,930 failed to affect responses of homomeric GlyRs formed from human alpha1 or alpha2 subunits, and micromolar concentrations (1-20 microM) acted differentially on the two types of homomeric receptors, potentiating the responses to glycine (10-20 microM) of alpha1 homomeric GlyRs and inhibiting the responses of alpha2 homomeric GlyRs. GlyRs beta subunits markedly influenced the modulations induced by ICS-205,930. In oocytes expressing alpha1/beta or alpha2/beta heteromeric GlyRs, low concentrations of ICS-205,930 (20 nM-1 microM) induced a potentiation of glycine responses that was counteracted by an inhibitory effect at higher concentrations. Thus, GlyRs beta subunits reduce by 2 orders of magnitude the concentration range potentiating alpha1-containing GlyRs and are required for potentiation of alpha2-containing GlyRs. These results reveal a new high-affinity potentiating site on GlyRs, to which beta subunits participate. The difference in ICS sensitivity between alpha1 and alpha2 GlyRs cannot be explained by their difference in TM2 segment and extracellular domains partly conserved between glycine and 5-HT(3) receptors are probably involved in the interaction of some 5-HT(3) antagonists with GlyRs.

Direct inhibition of glycine receptors by genistein, a tyrosine kinase inhibitor

Genistein, a tyrosine kinase inhibitor, has been widely used to examine potential effects of protein tyrosine kinase (PTK)-mediated regulation of receptor/channel function. Alteration of ion channel function in the presence of genistein has typically led to the conclusion that PTK regulates the activity of the channel under investigation. In the present report, we have assessed the possibility that genistein directly inhibits the glycine receptor, independent of effects on protein tyrosine kinase. Coapplication of genistein with glycine reversibly inhibited the strychnine-sensitive, glycine-activated current recorded from hypothalamic neurons. The time course of genistein action was rapid (within ms). Equilibration of genistein in the intracellular solution did not affect the ability of extracellularly applied genistein to inhibit the glycine response. Glycine concentration-response profiles generated in the absence and presence of genistein indicated the block was due to non-competitive antagonism. The genistein effect also displayed voltage-dependence. Daidzein, an analog of genistein that does not block protein kinases, also inhibited glycine-activated current. Coapplication of lavendustin A, a specific inhibitor of PTK, had no effect on the glycine response. Our results demonstrate that the tyrosine kinase inhibitor genistein has a direct inhibitory effect on glycine receptors that is not mediated via inhibition of PTK.

Direct measurement of specific membrane capacitance in neurons

The specific membrane capacitance (C(m)) of a neuron influences synaptic efficacy and determines the speed with which electrical signals propagate along dendrites and unmyelinated axons. The value of this important parameter remains controversial. In this study, C(m) was estimated for the somatic membrane of cortical pyramidal neurons, spinal cord neurons, and hippocampal neurons. A nucleated patch was pulled and a voltage-clamp step was applied. The exponential decay of the capacitative charging current was analyzed to give the total membrane capacitance, which was then divided by the observed surface area of the patch. C(m) was 0.9 microF/cm(2) for each class of neuron. To test the possibility that membrane proteins may alter C(m), embryonic kidney cells (HEK-293) were studied before and after transfection with a plasmid coding for glycine receptor/channels. The value of C(m) was indistinguishable in untransfected cells and in transfected cells expressing a high level of glycine channels, indicating that differences in transmembrane protein content do not significantly affect C(m). Thus, to a first approximation, C(m) may be treated as a "biological constant" across many classes of neuron.

Characterization of strychnine-sensitive glycine receptors in acutely isolated adult rat basolateral amygdala neurons

Large concentrations of the beta-amino acid, taurine, can be found in many forebrain areas such as the basolateral amygdala, a portion of the limbic forebrain intimately associated with the regulation of fear/anxiety-like behaviors. In addition to its cytoprotective and osmoregulatory roles, taurine may also serve as an agonist at GABA(A)- and strychnine-sensitive glycine receptors. In this latter context, the present study demonstrates that application of taurine to acutely isolated neurons from the basolateral amygdala of adult rats causes significant alterations in resting membrane current, as measured by whole-cell patch clamp electrophysiology. Using standard pharmacological approaches, we find that currents gated by concentrations of taurine </=3 mM are predominantly mediated by strychnine-sensitive receptors. Furthermore, these strychnine-sensitive receptors are shown to be pharmacologically and biophysically similar to 'classic' strychnine-sensitive, chloride-conducting glycine receptors expressed in brainstem and spinal cord. While amygdala glycine receptors can be distinguished from GABA(A) receptors expressed by the same neurons, these two chloride channels are functionally expressed at comparable levels. Given that a number of clinically relevant compounds are associated with the regulation of GABA(A) receptors in this brain region, the presence of both strychnine-sensitive glycine receptors and their agonist, taurine, in the basolateral amygdala may suggest an important role for these receptors in the limbic forebrain of adult rats.