Functional properties of glycine receptors expressed in primary cultures of mouse cerebellar granule cells

Hyperammonemia alters glycinergic neurotransmission and modulation of the glutamate-nitric oxide-cGMP pathway by extracellular glycine in cerebellum in vivo

The glutamate-nitric oxide (NO)-cGMP pathway modulates some forms of learning. How glycine modulates this pathway is unclear. Glycine could modulate the pathway biphasically, enhancing its function through NMDA receptor activation or reducing it through glycine receptor activation. Chronic hyperammonemia impairs the glutamate-NO-cGMP pathway in the cerebellum and induces cognitive impairment. The possible alterations in hyperammonemia of glycinergic neurotransmission and of glutamate-NO-cGMP pathway modulation by glycine remain unknown. The aims were to assess, by in vivo microdialysis in cerebellum: (i) the effects of different glycine concentrations, administered through the microdialysis probe, on the glutamate-NO-cGMP pathway function; (ii) the effects of tonic glycine receptors activation on the pathway function, by blocking them with strychnine; (iii) whether hyperammonemia alters the pathway modulation by glycine; (iv) and whether hyperammonemia alters extracellular glycine concentration and/or glycine receptor membrane expression. In control rats, low glycine levels reduce the pathway function, likely by activating glycine receptors, while 20 μM glycine enhances the pathway function, likely by enhancing NMDA receptor activation. In hyperammonemic rats, glycine did not reduce the pathway function, but enhanced it when administered at 1-20 μM. Hyperammonemia reduces extracellular glycine concentration by approximately 50% and glycine receptor membrane expression. However, tonic glycine receptor activation seems to be enhanced in hyperammonemic rats, as indicated by the larger increase in extracellular cGMP induced by strychnine. These data show that glycine modulates the glutamate-NO-cGMP pathway biphasically and that hyperammonemia strongly alters glycinergic neurotransmission and modulation by glycine of the glutamate-NO-cGMP pathway. These alterations may contribute to the cerebellar aspects of cognitive alterations in hyperammonemia. The findings reported in this study show that hyperammonemia alters glycinergic neurotransmission and the glutamate-NO-cGMP pathway modulation by glycine. In control rats, low glycine levels reduced the pathway function, likely by activating glycine receptors, while 20 μM glycine enhanced the pathway, likely by enhancing NMDA receptor activation. In hyperammonemic rats, glycine (administered at 1-20 μM) enhances the pathway, likely by activating NMDA receptors.

Propofol depresses cerebellar Purkinje cell activity via activation of GABA(A) and glycine receptors in vivo in mice

Propofol is an intravenous sedative-hypnotic agen, which causes rapid and reliable loss of consciousness. Under in vitro conditions, propofol activates GABAA and glycine receptors in spinal cord, hippocampus and hypothalamus neurons. However, the effects of propofol on the cerebellar neuronal activity under in vivo conditions are currently unclear. In the present study, we examined the effects of propofol on the spontaneous activity of Purkinje cells (PCs) in urethane-anesthetized mice by cell-attached recording and pharmacological methods. Our results showed that cerebellar surface perfusion of propofol (10-1000 μM) induced depression of the PC simple spike (SS) firing rate in a dose-dependent manner, but without significantly changing the properties of complex spikes (CS). The IC50 of propofol for inhibiting SS firing of PCs was 144.5 μM. Application of GABAA receptor antagonist, SR95531 (40 μM) or GABAB receptor antagonist, saclofen (20 μM), as well as glycine receptor antagonist, strychnine (10 μM) alone failed to prevent the propofol-induced inhibition of PCs spontaneous activity. However, application the mixture of SR95531 (40 μM) and strychnine (10 μM) completely blocked the propofol-induced inhibition of PC SS firing. These data indicated that cerebellar surface application of propofol depressed PC SS firing rate via facilitation of GABAA and functional glycine receptors activity in adult cerebellar PCs under in vivo conditions. Our present results provide a new insight of the anesthetic action of propofol in cerebellar cortex, suggesting that propofol depresses the SS outputs of cerebellar PCs which is involved in both GABAA and glycine receptors activity.

Novel expression of a functional glycine receptor chloride channel that attenuates contraction in airway smooth muscle

Airway smooth muscle (ASM) contraction is an important component of the pathophysiology of asthma. Taurine, an agonist of glycine receptor chloride (GlyR Cl(-)) channels, was found to relax contracted ASM, which led us to question whether functional GlyR Cl(-) channels are expressed in ASM. Messenger RNA for β (GLRB), α1 (GLRA1), α2 (GLRA2), and α4 (GLRA4) subunits were found in human (Homo sapiens) and guinea pig (Cavia porcellus) tracheal smooth muscle. Immunoblotting confirmed the protein expression of GLRA1 and GLRB subunits in ASM. Electrical activity of cultured human ASM cells was assessed using a fluorescent potentiometric dye and electrophysiological recordings. Glycine increased current and significantly increased fluorescence in a dose-dependent manner. The GlyR Cl(-) channel antagonist strychnine significantly blocked the effects of glycine on potentiometric fluorescence in ASM cells. Guinea pig airway ring relaxation of ACh-induced contractions by isoproterenol was significantly left-shifted in the presence of glycine. This effect of glycine was blocked by pretreatment with the GlyR Cl(-) channel antagonist strychnine. Glycine treatment during tachykinin- and acetylcholine-induced contractions significantly decreased the maintenance of muscle force compared to control. GlyR Cl(-) channels are expressed on ASM and regulate smooth muscle force and offer a novel target for therapeutic relaxation of ASM.

Glycine receptors and glycinergic synaptic transmission in the deep cerebellar nuclei of the rat: a patch-clamp study

To clarify possible glycinergic transmission in the cerebellum, principal neurons in deep cerebellar nuclei (DCN) of sliced cerebella (200 microm in thickness) from rats (aged 2-14 days) were studied using whole cell patch-clamp techniques. When glycine (100 microM) was applied to the DCN neurons from a "Y tube," large outward currents were induced (average peak amplitude of about 600 pA at -40 mV). The currents were blocked by strychnine (1 microM) and showed a reversal potential of -62 mV, which was approximately the estimated Cl- equilibrium potential. The dose-response relation of the currents showed an apparent dissociation constant of 170 microM for glycine and Hill coefficient of 1.6. In the presence of 6-cyano-7-nitroquinoziline-2, 3-dione (CNQX), d-(-)-2-amino-5-phosphonovaleric acid (APV) and bicuculline, which antagonize amino-3-hydroxy-5-methyl-isoxazol-propionate (APMA), N-methyl-d-aspartate (NMDA), and GABAA receptors, respectively, postsynaptic currents sensitive to strychnine (1 microM) were induced in DCN neurons by external perfusion of 20 mM K+ saline. Electrical stimulation of surrounding tissues in DCN evoked definite inhibitory postsynaptic currents (IPSCs) in these neurons. The IPSCs had a reversal potential of -62 mV and showed sensitivities to strychnine and tetrodotoxin. Thus this study has revealed that strychnine-sensitive glycine receptors are expressed in neurons of the DCN of rats and that glycinergic transmission mediated by these receptors is functional in these neurons from stages immediately after birth. The glycinergic innervations are presumably supplied by small interneurons located in the DCN.

Glycine facilitates transmitter release at developing synapses: a patch clamp study from Purkinje neurons of the newborn rat

Synaptic currents in immature Purkinje cells from rats on postnatal days 0-14 (P0-P14) were studied using whole-cell patch-electrodes applied to cerebellar slices (200 micro m in thickness). Purkinje cells (held at -40 mV) showed excitatory postsynaptic currents (EPSCs) and inhibitory postsynaptic currents (IPSCs) spontaneously. From P2 to P12 the frequencies of miniature EPSCs and miniature IPSCs in the Purkinje cells increased by 10-fold or more, suggesting progressive formation of functional synapses during this period. Application of glycine (100 micro M) to an immature Purkinje cell at P3-10 immediately increased the frequencies of both EPSCs and IPSCs. The effects of glycine showed maximum at P5-6 for EPSCs and at P9-10 for IPSCs and decreased thereafter. Facilitatory effects of glycine were suppressed by strychnine (1 micro M), a specific blocker of the ionotropic glycine receptor, while the effects were also induced by other glycinergic agonists, including alpha-L-alanine (1 mM), L-serine (1 mM) and taurine (500 micro M). The site of glycinergic effects was studied by removing the action potential generation in cerebellar slices. Following the addition of tetrodotoxin (TTX, 1 micro M), the glycine-induced facilitation of EPSC almost disappeared, while that of IPSC remained (i.e. miniature IPSCs) and reached more than half of the value without TTX. These findings suggest that the ionotropic glycinergic receptors are expressed transiently but profoundly in the developing cerebellum, and that the distributions of these receptors causing excitation are different at excitatory and inhibitory presynaptic neurons. The glycine receptors may play distinct roles in the maturation and organization of cerebellar neural circuits.

The organochlorine pesticides gamma-hexachlorocyclohexane (lindane), alpha-endosulfan and dieldrin differentially interact with GABA(A) and glycine-gated chloride channels in primary cultures of cerebellar granule cells

The neurotoxic organochlorine pesticides gamma-hexachlorocyclohexane, alpha-endosulfan and dieldrin induce in mammals a hyperexcitability syndrome accompanied by convulsions. They reduce the GABA-induced Cl(-) flux. The strychnine-sensitive glycine receptor also regulates Cl(-)-flux inhibitory responses. We studied the effects of these compounds on Cl(-) channels associated with glycine receptors in cultured cerebellar granule cells in comparison to the GABA(A) receptor. Both GABA (EC(50): 5 microM) and glycine (EC(50): 68 microM) increased (36)Cl(-) influx. This increase was antagonized by bicuculline and strychnine, respectively. Lindane inhibited with similar potency both GABA(A) (IC(50): 6.1 microM) and glycine (5.0 microM) receptors. alpha-Endosulfan and dieldrin inhibited the GABA(A) receptor (IC(50) values: 0.4 microM and 0.2 microM, respectively) more potently than the glycine receptor (IC(50) values: 3.5 microM and 3 microM, respectively). Picrotoxinin also inhibited the glycine receptor, although with low potency (IC(50)>100 microM). A 3D pharmacophore model, consisting of five hydrophobic regions and one hydrogen bond acceptor site in a specific three-dimensional arrangement, was developed for these compounds by computational modelling. We propose that the hydrogen bond acceptor moiety and the hydrophobic region were responsible for the affinity of these compounds at the GABA(A) receptor whereas only the hydrophobic region of the molecules was responsible for their interaction with the glycine receptors. In summary, these compounds could produce neuronal hyperexcitability by blocking glycine receptors besides the GABA(A) receptor. We propose that two zones of the polychlorocycloalkane pesticide molecules (a lipophilic zone and a polar zone) differentially contribute to their binding to GABA(A) and glycine receptors.

Glycine activates myenteric neurones in adult guinea-pigs

1. We studied the effects of glycine on myenteric neurones and muscle activity in the colon and stomach of adult guinea-pigs. 2. Intracellular recordings revealed that myenteric neurones responded to local microejection of glycine (1 mM) with a fast, transient membrane potential depolarisation (57 % of 191 colonic neurones and 26 % of 50 gastric neurones). Most glycine-sensitive neurones had ascending projections and were choline acetyltransferase immunoreactive. Glycine preferentially activated neurones with a late afterhyperpolarisation (AH-neurones) and tonic spiking neurones with fast synaptic inputs (tonic S-neurones) but less frequently phasic S-neurones and inexcitable (non-spiking) neurones. The depolarisation had a reversal potential at -19 +/- 13 mV, which was increased by 18 +/- 10 % upon lowering extracellular chloride concentration and decreased by 38 +/- 14 % in furosemide (frusemide, 2 mM). 3. Strychnine (300 nM) reversibly abolished the glycine-induced depolarisation and the Cl(-) channel blocker picrotoxin (100 microM) reduced the amplitude of the depolarisation by 55 +/- 5 %. The glycine effect was a postsynaptic response because it was not changed after nerve blockade with tetrodotoxin (1 microM) or blockade of synaptic transmission in reduced extracellular [Ca(2+)]. The effect was specific since the response was not changed by the nicotinic antagonists hexamethonium (200 microM) and mecamylamine (100 microM), the GABA(A) receptor antagonist bicuculline (10 microM), the NMDA antagonist MK-801 (20 microM) or the 5-HT(3) antagonist ICS 205930 (1 microM). 4. Glycine (1 mM) induced a tetrodotoxin- and strychnine-sensitive contractile response in the colon; the contractile response in the stomach was tetrodotoxin insensitive. 5. Glycine activated myenteric neurones in the adult enteric nervous system through strychnine-sensitive mechanisms. The glycine-evoked depolarisation was caused by Cl(-) efflux and the maintenance of relatively high intracellular chloride concentrations involved furosemide-sensitive cation-chloride co-transporters.

Molecular determinants of desensitization and assembly of the chimeric GABA(A) receptor subunits (alpha1/gamma2) and (gamma2/alpha1) in combinations with beta2 and gamma2

Two gamma-aminobutyric acid(A) (GABA(A)) receptor chimeras were designed in order to elucidate the structural requirements for GABA(A) receptor desensitization and assembly. The (alpha1/gamma2) and (gamma2/alpha1) chimeric subunits representing the extracellular N-terminal domain of alpha1 or gamma2 and the remainder of the gamma2 or alpha1 subunits, respectively, were expressed with beta2 and beta2gamma2 in Spodoptera frugiperda (Sf-9) cells using the baculovirus expression system. The (alpha1/gamma2)beta2 and (alpha1/gamma2)beta2gamma2 but not the (gamma2/alpha1)beta2 and (gamma2/alpha1)beta2gamma2 subunit combinations formed functional receptor complexes as shown by whole-cell patch-clamp recordings and [3H]muscimol and [3H]flunitrazepam binding. Moreover, the surface immunofluorescence staining of Sf-9 cells expressing the (alpha1/gamma2)-containing receptors was pronounced, as opposed to the staining of the (gamma2/alpha1)-containing receptors, which was only slightly higher than background. To explain this, the (alpha1/gamma2) and (gamma2/alpha1) chimeras may act like alpha1 and gamma2 subunits, respectively, indicating that the extracellular N-terminal segment is important for assembly. However, the (alpha1/gamma2) chimeric subunit had characteristics different from the alpha1 subunit, since the (alpha1/gamma2) chimera gave rise to no desensitization after GABA stimulation in whole-cell patch-clamp recordings, which was independent of whether the chimera was expressed in combination with beta2 or beta2gamma2. Surprisingly, the (alpha1/gamma2)(gamma2/alpha1)beta2 subunit combination did desensitize, indicating that the C-terminal segment of the alpha1 subunit may be important for desensitization. Moreover, desensitization was observed for the (alpha1/gamma2)beta2gamma2 receptor with respect to the direct activation by pentobarbital. This suggests differences in the mechanism of channel activation for pentobarbital and GABA.

Characterization of gamma-aminobutyrate type A receptors with atypical coupling between agonist and convulsant binding sites in discrete brain regions

Gamma-ainobutyric acid type A (GABA(A)) receptor ionophore ligand t-[35S]butylbicyclophosphorothionate ([35S]TBPS) was used in an autoradiographic assay on brain cryostat sections to visualize and characterize atypical GABA-insensitive [35S]TBPS binding previously described in certain recombinant GABA(A) receptors and the cerebellar granule cell layer. Picrotoxinin-sensitive but 1-mM GABA-insensitive [35S]TBPS binding was present in the rat cerebellar granule cell layer, many thalamic nuclei, subiculum and the internal rim of the cerebral cortex, amounting in these regions up to 6% of the basal binding determined in the absence of exogenous GABA. Similar binding properties were detected also in human and chicken brain sections. Like the GABA-sensitive [35S]TBPS binding, GABA-insensitive binding was profoundly decreased by pentobarbital, pregnanolone, loreclezole and Mg2+. The binding was reversible and apparently dependent on Cl- ions. Localization of the GABA-insensitive [35S]TBPS binding was not identical to that of high-affinity [3H]muscimol binding and diazepam-insensitive [3H]Ro 15-4513 binding, two previously established receptor subtype-dependent binding heterogeneities in the rat brain. The present study reveals a component of the GABA-ionophore enriched in the thalamus and cerebellar granule cells, possibly representing poorly desensitized or desensitizing receptors.

Enhanced taurine release in cultured cerebellar granule cells in cell-damaging conditions

The release of taurine from cultured cerebellar granule neurons was studied in different cell-damaging conditions, including hypoxia, hypoglycemia, ischemia, oxidative stress and in the presence of free radicals. The effects of both ionotropic and metabotropic glutamate receptor agonists on the release were likewise investigated. The release of [3H]taurine from the glutamatergic granule cells was increased by K+ (50 mM) and veratridine (0.1 mM), the effect of veratridine being the greater. Hypoxia and ischemia produced an initial increase in release compared to normoxia but resulted in a diminished response to K+. Hypoglycemia, oxidative stress and free radicals enhanced taurine release, and subsequent K+ treatment exhibited a correspondingly greater stimulation. A common feature of taurine release in all the above conditions was a slow response to the stimulus evoked by K+ and particularly to that evoked by veratridine. All ionotropic glutamate receptor agonists potentiated taurine release, but only the action of kainate seemed to be receptor-mediated. Metabotropic receptor agonists of group I slightly stimulated the release. The prolonged taurine release seen in both normoxia and cell-damaging conditions may be of importance in maintaining homeostasis in the cerebellum and reducing excitability for a longer period than other neuroprotective mechanisms.

Functional integrity of green fluorescent protein conjugated glycine receptor channels

The alpha subunit (alphaZ1) of the zebrafish glycine receptor (GlyR) has been N-terminus fused with green fluorescent protein (GFP). We found that both pharmacological and electrophysiological properties of this chimeric alphaZ1-GFP are indistinguishable from those of the wild-type receptor when expressed in Xenopus oocytes and cell lines. The apparent affinities of this receptor for agonists (glycine, taurine and GABA), and the antagonist (strychnine) are unchanged, and single channel kinetics are not altered. In the same expression systems, alphaZ1-GFP was visualized using fluorescence microscopy. Fluorescence was distributed anisotropically across cellular membranes. In addition to the Golgi apparatus and endoplasmic reticulum, its presence was also detected on the plasmalemma, localized at discrete hot-spots which were identified as sites of high membrane turnover. Overall, the preservation in alphaZ1-GFPs of the wild type receptor functional properties makes it a promising new tool for further in situ investigations of GlyR expression, distribution and function.