L-type calcium channels and NMDA receptors: a determinant duo for short-term nociceptive plasticity

In the dorsal horn of the spinal cord, pain-transmitting neurons exhibit action potential windup, a form of short-term plasticity, which consists of a progressive increase in neuronal response during repetitive stimulation of nociceptive input fibers. Windup depends on N-methyl-D-aspartate (NMDA) receptor activation, but previous in vitro studies indicated that windup also relies on intrinsic plateau properties of spinal neurons. In the present study, we considered the possible involvement of these properties in windup in vivo. For this purpose, we first studied a nociceptive flexion reflex in the rat. We showed that windup of the reflex is actually suppressed by blockers of L-type calcium current and Ca(2+)-activated non-specific cationic current (Ican), the two main depolarizing conductances of plateau potentials. We further showed that, during windup, NMDA receptors provide a critical excitatory component in a dynamic balance of excitatory and inhibitory inputs which ultimately activates L-type calcium channels. The nociceptive reflex involves at least two neuronal groups, which may express intrinsic amplification properties, motor neurons and dorsal horn neurons. By means of extracellular recordings in the dorsal horn, we showed that windup of dorsal horn neuron discharge was sensitive to the modulators of L-type calcium current. Altogether, our results suggest that, in vivo, windup also depends on the amplification properties of spinal neurons, the triggering of which requires previous activation of NMDA receptors.

The kinetics of inhibition of rat recombinant heteromeric alpha1beta glycine receptors by the low-affinity antagonist SR-95531

The GABA(A) antagonist SR-95531 (gabazine) is known to block glycine receptors, albeit with low affinity. We have studied the effect of SR-95531 on rat recombinant alpha1beta glycine receptors expressed in human embryonic kidney (HEK293) cells by recording macroscopic currents elicited by rapid glycine application to outside-out patches. SR-95531 has a fast unbinding rate (k(offSR), about 3000 s(-1)) and this means that the time course of its unbinding is comparable to the expected time course of the transmitter in the cleft. We also found that equilibrium applications of SR-95531 reduced the response to brief glycine applications by an amount inversely proportional to the duration of glycine application. The fast unbinding rate of SR-95531 from the glycine receptor will make it useful for establishing the time course of glycine concentration at glycinergic synapses.

Vesicular glutamate transporter 2 is required for central respiratory rhythm generation but not for locomotor central pattern generation

Glutamatergic excitatory neurotransmission is dependent on glutamate release from presynaptic vesicles loaded by three members of the solute carrier family, Slc17a6-8, which function as vesicular glutamate transporters (VGLUTs). Here, we show that VGLUT2 (Slc17a6) is required for life ex utero. Vglut2 null mutant mice die immediately after birth because of the absence of respiratory behavior. Investigations at embryonic stages revealed that neural circuits in the location of the pre-Bötzinger (PBC) inspiratory rhythm generator failed to become active. However, neurons with bursting pacemaker properties and anatomical integrity of the PBC area were preserved. Vesicles at asymmetric synapses were fewer and malformed in the Vglut2 null mutant hindbrain, probably causing the complete disruption of AMPA/kainate receptor-mediated synaptic activity in mutant PBC cells. The functional deficit results from an inability of PBC neurons to achieve synchronous activation. In contrast to respiratory rhythm generation, the locomotor central pattern generator of Vglut2 null mutant mice displayed normal rhythmic and coordinated activity, suggesting differences in their operating principles. Hence, the present study identifies VGLUT2-mediated signaling as an obligatory component of the developing respiratory rhythm generator.

Distinct roles for glycine and GABA in shaping the response properties of neurons in the superior paraolivary nucleus of the rat

The superior paraolivary nucleus (SPON) is a prominent periolivary cell group of the superior olivary complex. SPON neurons use gamma-aminobutyric acid (GABA) as their neurotransmitter and are contacted by large numbers of glycinergic and GABAergic punctate profiles, representing a dense inhibitory innervation from the medial nucleus of the trapezoid body (MNTB) and from collaterals of SPON axons, respectively. SPON neurons have low rates of spontaneous activity, respond preferentially to the offset of pure tones, and phase-lock to amplitude-modulated tones. To determine the roles of glycine and GABA in shaping SPON responses, we recorded from single units in the SPON of anesthetized rats before, during, and after application of the glycine receptor antagonist strychnine, the GABA(A) receptor antagonist bicuculline, or both drugs applied simultaneously. Strychnine caused a major increase in spike counts during the stimulus presentation, followed by the disappearance of offset spikes. In half of the recorded units, bicuculline caused moderately increased firing during the stimulus. However, in 86% of units bicuculline also caused a large increase in the magnitude of the offset response. Application of the drug cocktail caused increased spontaneous activity, dramatically increased spike counts during the stimulus presentation, and eliminated the offset response in most units. We conclude that glycinergic inhibition from the MNTB suppresses SPON spiking during sound stimulation and is essential in generating offset responses. GABAergic inhibition, presumably from intrinsic SPON collaterals, plays a subtler role, contributing in some cells to suppression of firing during the stimulus and in most cells to restrict firing after stimulus offset.

Reactive oxygen species mediate central cardiorespiratory network responses to acute intermittent hypoxia

Although oxidative stress and reactive oxygen species generation is typically associated with localized neuronal injury, reactive oxygen species have also recently been shown to act as a physiological signal in neuronal plasticity. Here we define an essential role for reactive oxygen species as a critical stimulus for cardiorespiratory reflex responses to acute episodic hypoxia in the brain stem. To examine central cardiorespiratory responses to episodic hypoxia, we used an in vitro medullary slice that allows simultaneous examination of rhythmic respiratory-related activity and synaptic neurotransmission to cardioinhibitory vagal neurons. We show that whereas continuous hypoxia does not stimulate excitatory neurotransmission to cardioinhibitory vagal neurons, acute intermittent hypoxia of equivalent duration incrementally recruits an inspiratory-evoked excitatory neurotransmission to cardioinhibitory vagal neurons during intermittent hypoxia. This recruitment was dependent on the generation of reactive oxygen species. Further, we demonstrate that reactive oxygen species are incrementally generated in glutamatergic neurons in the ventrolateral medulla during intermittent hypoxia. These results suggest a neurochemical basis for the pronounced bradycardia that protects the heart against injury during intermittent hypoxia and demonstrates a novel role of reactive oxygen species in the brain stem.

THE GLYCINE REUPTAKE INHIBITOR ORG 25935 DECREASES ETHANOL INTAKE AND PREFERENCE IN MALE WISTAR RATS

Previous findings from our group indicate that accumbal glycine receptors (GlyRs) are involved in mediating the dopamine (DA) activating effects of ethanol (EtOH), and that administration of glycine locally into the nucleus accumbens (nAc) reduces EtOH consumption in EtOH high-preferring rats.

AIMS

The present study examines the influence of a systemically administered glycine reuptake inhibitor, Org 25935, on EtOH preference and intake, in male Wistar rats with an EtOH preference >60% (during continuous access to a bottle of EtOH, 6% v/v, and a bottle of water), called EP>60 rats, as well as in animals with an EtOH preference <60%, called EP<60 rats. Org 25935 is an inhibitor of the glycine transporter 1 (GlyT1) protein with negligible action on the glycine transporter 2 (GlyT2) protein.

METHODS

Both EP>60 and EP<60 rats were limited to drink 2.5 h/day. Org 25935 or vehicle was administered intraperitoneally approximately 40 min before the rats were presented to a choice of drinking EtOH or water.

RESULTS

Org 25935 decreased EtOH intake and EtOH preference, as compared with vehicle, whereas water intake was unaffected. This effect was dose-dependent, developed gradually and was sustained for up to 40 days, also after introduction of an alcohol deprivation period.

CONCLUSION

It is suggested that Org 25935, and possibly also other GlyT1 inhibitors, can represent a new pharmacological treatment principle for alcohol dependence or abuse.

Histaminergic and glycinergic modulation of GABA release in the vestibular nuclei of normal and labyrinthectomised rats

Vestibular compensation (the behavioural recovery that follows unilateral vestibular de-afferentation), is facilitated by histamine, and is associated with increased central histamine release and alterations in histamine H(3) receptor expression in the vestibular nuclei. However, little is known of the effects of histamine on neurotransmission in the vestibular nuclei, and the mechanisms by which histamine may influence compensation are unclear. Here we examined the modulatory effects of histaminergic agents on the release of amino acid neurotransmitters in slices of the medial vestibular nucleus (MVN) prepared from normal and labyrinthectomised rats. The release of GABA, but not glutamate, glycine or aspartate, was robustly and reproducibly evoked by a high-K(+) stimulus applied to normal MVN slices. Histamine inhibited the evoked release of GABA, both through a direct action on presynaptic H(3) receptors (presumably located on GABAergic terminals), and through a novel, indirect pathway that involved the increased release of glycine by activation of postsynaptic H(1)/H(2) receptors (presumably on glycinergic neurons). After unilateral labyrinthectomy (UL), the direct H(3) receptor-mediated inhibition of GABA release was profoundly downregulated in both ipsi-lesional and contra-lesional MVNs. This effect appeared within 25 h post-UL and persisted for at least 3 weeks post-UL. In addition, at 25 h post-UL the indirect glycinergic pathway caused a marked suppression of GABA release in the contra-lesional but not ipsi-lesional MVN, which was overcome by strychnine. Stimulation of histamine H(3) receptors at 25 h post-UL restored contra-lesional GABA release to normal, suggesting that acutely after UL H(3) receptors may strongly modulate glycinergic and GABAergic neurotransmission in the MVN. These findings are the first to demonstrate the modulatory actions of the histaminergic system on neurotransmission in the vestibular nuclei, and the changes that occur during vestibular system plasticity. During vestibular compensation, histaminergic modulation of glycine and GABA release may contribute to the rebalancing of neural activity in the vestibular nuclei of the lesioned and intact sides.

Chloride homeostasis differentially affects GABA(A) receptor- and glycine receptor-mediated effects on spontaneous circuit activity in hippocampal cell culture

The potassium-chloride cotransporter 2 (KCC2)-dependent intracellular chloride level determines whether neurons respond to GABA and/or glycine by depolarization or hyperpolarization. However, still unknown is the role of KCC2-dependent chloride homeostasis in regulating the spontaneous activity of neuronal circuits via GABA(A) receptor (GABA(A)R) and the glycine receptor (GlyR). In this study, patch-clamp recordings were performed to measure the change of spontaneous neuronal activity in cultured hippocampal neurons. Our results showed that inhibition of KCC2 with furosemide, as well as blockade of GABA(A)R with bicuculline, significantly enhanced circuit activity. Perfusion with bicuculline further enhanced the effects of furosemide on spontaneous circuit activity, while furosemide did not alter the effects of bicuculline. Surprisingly, blockade of GlyR not only induced obvious tonic currents, but also significantly decreased spontaneous synaptic activity. Moreover, inhibition of KCC2 did not change the depressive effect of strychnine on neuronal circuits. Our findings suggest that KCC2-dependent chloride homeostasis is mainly involved in GABA(A)R-mediated synaptic inhibition whereas GlyR-mediated tonic action plays a totally different role in regulating hippocampal circuit activity.

Intrathecal or dietary glycine inhibits bladder and urethral activity in rats with spinal cord injury

PURPOSE

We examined the influence of intrathecal or dietary glycine on bladder and urethral activity in rats with spinal cord injury.

MATERIALS AND METHODS

A total of 20 female Sprague-Dawley rats were used 4 weeks after lower thoracic spinal cord injury. The rats were divided into standard and 1% glycine diet groups. In the standard diet group isovolumetric cystometry and urethral pressure measurement were performed before and after intrathecal injection of glycine. In the 1% glycine diet group bladder and urethral activity were compared with control recordings in the standard diet group.

RESULTS

In the standard diet group intrathecal injection of glycine prolonged the interval and decreased the amplitude of bladder contractions, decreased baseline urethral pressure and altered urethral activity during bladder contraction from a pattern of detrusor-sphincter dyssynergia to detrusor-sphincter synergy at 100 mug glycine. In the 1% glycine diet group the interval and amplitude of bladder contractions were prolonged and decreased, respectively, compared with those in the standard diet group. Baseline urethral pressure was lower than in the standard diet group even after intrathecal injection of 100 mug glycine. Urethral pressure did not change during bladder contraction and it was the same as baseline pressure. Residual urine volume was lower than in the standard diet group.

CONCLUSIONS

Intrathecal or dietary glycine inhibits bladder and urethral activity, and improves detrusor hyperreflexia and detrusor-sphincter dyssynergia.

Absence of GABA type A signaling in adult medial habenular neurons

Neural inhibition in the brain is mainly mediated by ionotropic GABA type A receptors. Apart from the GABA type A receptors, both K(+)-Cl(-) cotransporter isoform 2 and the GABA-synthesizing enzyme, glutamic acid decarboxylase, are essential determinants for GABA type A receptor-mediated inhibition. By using immunofluorescent staining, we observed that K(+)-Cl(-) cotransporter isoform 2, GABA type A receptor beta2/3 subunits and a presynaptically localized glutamic acid decarboxylase isoform, glutamic acid decarboxylase 65, were all absent in adult Sprague-Dawley rat medial habenular nucleus, while immunopositive staining for glutamic acid decarboxylase 67, GABA and GABA type B receptor type 2 subunit were present in the medial habenular nucleus. Consistent with the lack of GABA type A signaling as detected by immunohistochemistry, GABA (100 muM) evoked no measurable currents in the medial habenular nucleus but induced bicuculline-sensitive currents in the lateral habenular nucleus and in the CA1 area of hippocampus. We also failed to record miniature inhibitory postsynaptic currents in medial habenular nucleus neurons. These results support the idea that GABAergic transmission in medial habenular nucleus is probably not mediated by any of the most common GABA type A receptor subtypes. Our data suggest that GABA type B receptor-mediated inhibition may play a role in balancing neuronal excitation in this special region. Further exploration for factors determining medial habenular nucleus neural inhibition will lead to a more complete understanding of control of synaptic balance in the CNS.

Blockade of inhibitory neurotransmission evoked seizure-like firing of cardiac parasympathetic neurons in brainstem slices of newborn rats: Implications for sudden deaths in patients of epilepsy

In patients of epilepsy a proportion of unexplained sudden deaths had been attributed to neurogenic arrhythmias. Although some evidence has suggested that epileptogenic activation of the cardiac parasympathetic nerves, which is revealed by ictal bradyarrhythmias or cardiac asystole, might be very critical in causing sudden deaths of patients of epilepsy the firing behavior of cardiac parasympathetic neurons (CPNs) during epileptic attack is not known. In the present study fluorescent tracer was injected into the cardiac sac of newborn rats to retrogradely label the parasympathetic neurons in the nucleus ambiguus (NA). The fluorescence-labeled NA neurons were further examined using whole-cell patch-clamp method in medulla slices with respiratory-like rhythm, and those with an inspiratory-related increase of the mixed inhibitory synaptic activity were identified as CPNs. We have demonstrated that blockade of the GABAergic and the glycinergic receptors in medulla slices evoked intermittent seizure-like firing of CPNs under current-clamp configuration, and evoked intermittent excitatory inward currents (IEICs) under voltage-clamp configuration. These results have given new evidence that CPNs might fire in a seizure-like pattern during epileptic attack, which might be responsible for the neurogenic ictal bradyarrhythmias, cardiac asystole, or even the sudden deaths of patients of epilepsy.

Fast cholinergic efferent inhibition in guinea pig outer hair cells

Hair cells of inner ear are suggested to be inhibited by the activation of the alpha9-containing nicotinic acetylcholine (ACh) receptors (alpha9-containing nAChRs). Several studies have suggested that the native nicotinic-like ACh receptors (nAChRs) in hair cells display a significant permeability of Ca(2+) ions and unusual pharmacological properties. The activation of native nAChRs will initiate the hyperpolarization of hair cells by activation of the small conductance, Ca(2+)-activated K(+) channels (SK). In this work, the properties of the ACh-sensitive potassium current (IK(ACh)) in outer hair cells (OHCs) of guinea pigs were investigated by employing whole-cell patch-clamp. Followed by perfusion of ACh, OHCs displayed a rapid desensitized current with an N-shaped current-voltage curve (I-V) and a reversal potential of - 66 +/- 7 mV. The IK(ACh) was still present during perfusion of either iberiotoxin (IBTX, 200 nM) or TEA (5 mM) but was potently inhibited by apamin (1 muM), TEA (30 mM). The IK(ACh) demonstrated a strong sensitivity to alpha-bungarotoxin (alpha-BgTx), bicuculline and strychnine. These results suggested that OHCs display the well-known SK current, which might be gated by the alpha9-containing nAChRs. Two important changes were present after lowering the Ca(2+) concentration in the external conditions from 2 mM to 0.2 mM: one was a flattened N-shape I-V relationship with a maximum shifted toward hyperpolarized potentials from -20 approximately -30 mV approximately -40 to -50 mV, the other was a significant reduction in the agonist maximal response (percentage of maximal response 10.5 +/- 5.4). These results indicated that native nAChRs are both permeable to and modulated by extracellular Ca(2+) ions. Taken together, this work provides direct evidences that SK channels in OHCs of guinea pigs are gated by alpha9-containing nAChRs, which play an important role in the fast cholinergic efferent inhibition. This fast inhibition is both potently dependent on the permeability of Ca(2+) ions through the native nAChRs and modulated by Ca(2+) ions.

Protection of renal tubular cells against the cytotoxicity of cadmium by glycine

Glycine treatment is reported to protect against the nephrotoxicity of cadmium (Cd) in rats. The purpose of the present study was to explore the mechanism of this protection using a renal epithelial cell line, LLC-PK(1). The cells were incubated with 10-30 microM Cd in serum-free DMEM and cytotoxicity was evaluated by LDH leakage into the incubation medium. Under these conditions, 20 and 30 microM Cd concentrations were cytotoxic. As compared to the non-Cd-exposed cells, the LDH release was elevated more than six-fold in cells exposed to 30 microM Cd for 24h. Co-treatment with 5-50mM glycine was cytoprotective in a concentration-dependent manner. Prior treatment with 50 mM glycine for 16 h, or co-treatment for 24h, reduced LDH leakage due to 30 microM Cd exposure by 60 and 66%, respectively. Co-incubation with 50 mM alanine was also protective but only about half as effective as with glycine. During the first 4h, prior to the onset of any significant cell membrane damage, the Cd-exposed cells accumulated 0.55 microg Cd/mg protein. Glycine pre-treatment or co-treatment reduced Cd accumulation by about one-quarter or one-half, respectively. To delineate the mechanism of glycine's effect on Cd accumulation, the efflux of Cd was studied after a 30 min pulse exposure. The results suggested that pre-treatment reduced Cd accumulation by increasing its efflux from the cells. In contrast, co-treatment reduced Cd efflux, suggesting that the co-treatment lowered Cd accumulation by suppressing its uptake. When co-incubated, Cd and glycine formed a complex that was apparently responsible for the marked reduction in Cd uptake. It is concluded that, regardless of the mode of treatment, glycine is cytoprotective against Cd and that it may do so by lowering the intracellular Cd burden.

Distinct physiological mechanisms underlie altered glycinergic synaptic transmission in the murine mutants spastic, spasmodic, and oscillator

Spastic (spa), spasmodic (spd), and oscillator (ot) mice have naturally occurring glycine receptor (GlyR) mutations, which manifest as motor deficits and an exaggerated "startle response." Using whole-cell recording in hypoglossal motoneurons, we compared the physiological mechanisms by which each mutation alters GlyR function. Mean glycinergic miniature IPSC (mIPSC) amplitude and frequency were dramatically reduced (>50%) compared with controls for each mutant. mIPSC decay times were unchanged in spa/spa (4.5 +/- 0.3 vs 4.7 +/- 0.2 ms), reduced in spd/spd (2.7 +/- 0.2 vs 4.7 +/- 0.2 ms), and increased in ot/ot (12.3 +/- 1.2 vs 4.8 +/- 0.2 ms). Thus, in spastic, GlyRs are functionally normal but reduced in number, whereas in spasmodic, GlyR kinetics is faster. The oscillator mutation results in complete absence of alpha1-containing GlyRs; however, some non-alpha1-containing GlyRs persist at synapses. Fluctuation analysis of membrane current, induced by glycine application to outside-out patches, showed that mean single-channel conductance was increased in spa/spa (64.2 +/- 4.9 vs 36.1 +/- 1.4 pS), but unchanged in spd/spd (32.4 +/- 2.1 vs 35.3 +/- 2.1 pS). GlyR-mediated whole-cell currents in spa/spa exhibited increased picrotoxin sensitivity (27 vs 71% block for 100 microM), indicating alpha1 homomeric GlyR expression. The picrotoxin sensitivity of evoked glycinergic IPSCs and conductance of synaptic GlyRs, as determined by nonstationary variance analysis, were identical for spa/spa and controls. Together, these findings show the three mutations disrupt GlyR-mediated inhibition via different physiological mechanisms, and the spastic mutation results in "compensatory" alpha1 homomeric GlyRs at extrasynaptic loci.

Medullary lateral tegmental field neurons influence the timing and pattern of phrenic nerve activity in cats

In an effort to characterize the role of the medullary lateral tegmental field (LTF) in regulating respiration, we tested the effects of selective blockade of excitatory (EAA) and inhibitory amino acid (IAA) receptors in this region on phrenic nerve activity (PNA) of vagus-intact and vagotomized cats anesthetized with dial-urethane. We found distinct patterns of changes in central respiratory rate, duration of inspiratory and expiratory phases of PNA (Ti and Te, respectively), and I-burst amplitude after selective blockade of EAA and IAA receptors in the LTF. First, blockade of N-methyl-D-aspartate (NMDA) receptors significantly (P < 0.05) decreased central respiratory rate primarily by increasing Ti but did not alter I-burst amplitude. Second, blockade of non-NMDA receptors significantly reduced I-burst amplitude without affecting central respiratory rate. Third, blockade of GABAA receptors significantly decreased central respiratory rate by increasing Te and significantly reduced I-burst amplitude. Fourth, blockade of glycine receptors significantly decreased central respiratory rate by causing proportional increases in Ti and Te and significantly reduced I-burst amplitude. These changes in PNA were markedly different from those produced by blockade of EAA or IAA receptors in the pre-Bötzinger complex. We propose that a proper balance of excitatory and inhibitory inputs to several functionally distinct pools of LTF neurons is essential for maintaining the normal pattern of PNA in anesthetized cats.

Characterization of the glycinergic input to bipolar cells of the mouse retina

Glycine and gamma-aminobutyric acid (GABA) are the major inhibitory transmitters of the mammalian retina, and bipolar cells receive GABAergic and glycinergic inhibition from multiple amacrine cell types. Here we evaluated the functional properties and subunit composition of glycine receptors (GlyRs) in bipolar cells. Patch-clamp recordings were performed from retinal slices of wild-type, GlyRalpha1-deficient (Glra1(spd-ot)) and GlyRalpha3-deficient (Glra3(-/-)) mice. Whole-cell currents following glycine application and spontaneous inhibitory postsynaptic currents (IPSCs) were analysed. During the recordings the cells were filled with Alexa 488 and, thus, unequivocally identified. Glycine-induced currents of bipolar cells were picrotoxinin-insensitive and thus represent heteromeric channels composed of alpha and beta subunits. Glycine-induced currents and IPSCs were absent from all bipolar cells of Glra1(spd-ot) mice, indicating that GlyRalpha1 is an essential subunit of bipolar cell GlyRs. By comparing IPSCs of bipolar cells in wild-type and Glra3(-/-) mice, no statistically significant differences were found. OFF-cone bipolar (CB) cells receive a strong glycinergic input from AII amacrine cells, that is preferentially based on the fast alpha1beta-containing channels (mean decay time constant tau = 5.9 +/- 1.4 ms). We did not observe glycinergic IPSCs in ON-CB cells and could elicit only small, if any, glycinergic currents. Rod bipolar cells receive a prominent glycinergic input that is mainly mediated by alpha1beta-containing channels (tau = 5.5 +/- 1.6 ms). Slow IPSCs, the characteristic of GlyRs containing the alpha2 subunit, were not observed in bipolar cells. Thus, different bipolar cell types receive kinetically fast glycinergic inputs, preferentially mediated by GlyRs composed of alpha1 and beta subunits.

Gycine and GABA interact to regulate the nitric oxide/cGMP signaling pathway in the turtle retina

Nitric oxide (NO) is a free radical that is important in retinal signal transduction and cyclic guanosine monophosphate (cGMP) is a critical downstream messenger of NO. The NO/cGMP signaling pathway has been shown to modulate neurotransmitter release and gap junction coupling in horizontal cells and amacrine cells, and increase the gain of the light response in photoreceptors. However, many of the mechanisms controlling the production of NO and cGMP remain unclear. Previous studies have shown activation of NO/cGMP production in response to stimulation with N-methyl-d-aspartate (NMDA) or nicotine, and the differential modulation of cGMP production by GABA(A) and GABA(C) receptors (GABA(A)Rs and GABA(C)Rs). This study used cGMP immunocytochemistry and NO imaging to investigate how the inhibitory GABAergic and glycinergic systems modulate the production of NO and cGMP. Our data show that blocking glycine receptors (GLYR) with strychnine (STRY) produced moderate increases in cGMP-like immunoreactivity (cGMP-LI) in select types of amacrine and bipolar cells, and strong increases in NO-induced fluorescence (NO-IF). TPMPA, a selective GABACR antagonist, greatly reduced the increases in cGMP-LI stimulated by STRY, but did not influence the increase in NO-IF stimulated by STRY. Bicuculline (BIC), a GABA(A)R antagonist, however, enhanced the increases in both the cGMP-LI and NO-IF stimulated by STRY. CNQX, a selective antagonist for alpha-Amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid hydrobromide/kainic acid (AMPA/KA) receptors, eliminated both the increases in cGMP-LI and NO-IF stimulated by STRY, while MK801, a selective antagonist for NMDA receptors, slightly increased the cGMP-LI and slightly decreased the NO-IF stimulated by STRY. Finally, double labeling of NO-stimulated cGMP and either GLY or GABA indicated that cGMP predominantly colocalized with GLY. Taken together, these findings support the hypothesis that GLY and GABA interact in the regulation of the NO/cGMP signaling pathway, where GLY primarily inhibits NO production and GABA has a greater effect on cGMP production. Such interacting inhibitory pathways could shape the course of signal transduction of the NO/cGMP pathway under different physiological situations.

Differential control of central cardiorespiratory interactions by hypercapnia and the effect of prenatal nicotine

Hypercapnia evokes a strong cardiorespiratory response including gasping and a pronounced bradycardia; however, the mechanism responsible for these survival responses initiated in the brainstem is unknown. To examine the effects of hypercapnia on the central cardiorespiratory network, we used an in vitro medullary slice that allows simultaneous examination of rhythmic respiratory-related activity and inhibitory synaptic neurotransmission to cardioinhibitory vagal neurons (CVNs). Hypercapnia differentially modulated inhibitory neurotransmission to CVNs; whereas hypercapnia selectively depressed spontaneous glycinergic IPSCs in CVNs without altering respiratory-related increases in glycinergic neurotransmission, it decreased both spontaneous and inspiratory-associated GABAergic IPSCs. Because maternal smoking is the highest risk factor for sudden infant death syndrome (SIDS) and prenatal nicotine exposure is proposed to be the link between maternal smoking and SIDS, we examined the cardiorespiratory responses to hypercapnia in animals exposed to nicotine in the prenatal and perinatal period. In animals exposed to prenatal nicotine, hypercapnia evoked an exaggerated depression of GABAergic IPSCs in CVNs with no significant change in glycinergic neurotransmission. Hypercapnia altered inhibitory neurotransmission to CVNs at both presynaptic and postsynaptic sites. Although the results obtained in this study in vitro cannot be extrapolated with certainty to in vivo responses, the results of this study provide a likely neurochemical mechanism for hypercapnia-evoked bradycardia and the dysregulation of this response with exposure to prenatal nicotine, creating a higher risk for SIDS.

Insulin-like growth factor-1 inhibits adult supraoptic neurons via complementary modulation of mechanoreceptors and glycine receptors

In the CNS, insulin-like growth factor-1 (IGF-1) is mainly known for its trophic effect both during development and in adulthood. Here, we show than in adult rat supraoptic nucleus (SON), IGF-1 receptor immunoreactivity is present in neurons, whereas IGF-1 immunoreactivity is found principally in astrocytes and more moderately in neurons. In vivo application of IGF-1 within the SON acutely inhibits the activity of both vasopressin and oxytocin neurons, the two populations of SON neuroendocrine cells. Recordings of acutely isolated SON neurons showed that this inhibition occurs through two rapid and reversible mechanisms, both involving the neuronal IGF-1 receptor but different intracellular messengers. IGF-1 inhibits Gd3+-sensitive and osmosensitive mechanoreceptor cation current via phosphatidylinositol-3 (PI3) kinase activation. IGF-1 also potentiates taurine-activated glycine receptor (GlyR) Cl- currents by increasing the agonist sensitivity through a extremely rapid (within a second) PI3 kinase-independent mechanism. Both mechanoreceptor channels and GlyR, which form the excitatory and inhibitory components of SON neuron osmosensitivity, are active at rest, and their respective inhibition and potentiation will both be inhibitory, leading to strong decrease in neuronal activity. It will be of interest to determine whether IGF-1 is released by neurons, thus participating in an inhibitory autocontrol, or astrocytes, then joining the growing family of glia-to-neuron transmitters that modulate neuronal and synaptic activity. Through the opposite and complementary acute regulation of mechanoreceptors and GlyR, IGF-1 appears as a new important neuromodulator in the adult CNS, participating in the complex integration of neural messages that regulates the level of neuronal excitability.

Induction of delta-opioid receptor function in the midbrain after chronic morphine treatment

Delta-opioid receptor (DOPr) activation fails to produce cellular physiological responses in many brain regions, including the periaqueductal gray (PAG), despite neural expression of high densities of the receptor. Previous histochemical studies have demonstrated that a variety of stimuli, including chronic morphine treatment, induce the translocation of DOPr from intracellular pools to the surface membrane of CNS neurons. PAG neurons in slices taken from untreated mice exhibited mu-opioid receptor (MOPr) but not DOPr-mediated presynaptic inhibition of GABAergic synaptic currents. In contrast, after 5-6 d of chronic morphine treatment, DOPr stimulation inhibited synaptic GABA release onto most neurons. Shorter exposure to morphine in vitro (upto 4 h) or in vivo (18 h) did not induce functional DOPr responses. DOPr-mediated presynaptic inhibition could not be induced in slices from untreated animals by increasing synaptic activity in vitro using high extracellular potassium concentrations or activation of protein kinase A. Induction of functional DOPr signaling by chronic morphine required MOPr expression, because no DOPr receptor responses were observed in MOPr knock-out mice. DOPr agonists also had no effect on miniature IPSCs in beta-arrestin-2 knock-out mice after chronic morphine. These results suggest that induction of DOPr-mediated actions in PAG by chronic morphine requires prolonged MOPr stimulation and expression of beta-arrestin-2.

Proposal of a method for the genetic transformation of Gordonia jacobaea

AIMS

Gordonia jacobaea is a recently isolated bacterial species with potential industrial application on account of its ability to store large quantities of trans-canthaxanthin. Its genetic manipulation is, however, difficult and cumbersome owing to the presence of mycolic acids in the cell wall and, especially, because of current lack of knowledge about its basic genetics. The present work describes a method for the genetic transformation of G. jacobaea.

METHODS AND RESULTS

Gordonia jacobaea was grown in media supplemented with different glycine, penicillin G and isoniazid concentrations. The temperature, carbon source, growth phase and ultrasounds were analyzed for improving the method efficiency. The cells were finally transformed by electroporation. Finally, the method was applied to Brevibacteriumlactofermentum and Gordonia bronchialis.

CONCLUSIONS

The growth of G. jacobaea in the presence of glycine and isoniazid is essential for obtaining electrocompetents cells. The temperature, growth phase and ultrasounds appeared as the main factors for increasing the transformation efficiency. The use of shuttle plasmids became necessary. The method described can be used with other Corynebacteria species.

SIGNIFICANCE AND IMPACT OF THE STUDY

Because of the importance of the CNM group (Corynebacteria, Nocardia and Mycobacteria genera) in different areas such as industry, bioremediation improve the knowledge of their molecular mechanisms are becoming essential. The method described here improves the genetic manipulation of this group of bacteria.

Structural difference between heteromeric somatic and homomeric axonal glycine receptors in the hypothalamo-neurohypophysial system

Glycine receptors are ionotropic receptors formed by either the homomeric assembly of ligand-binding alpha subunits or the heteromeric combination of an alpha subunit and the auxiliary beta subunit. Glycine receptors in the brain are found at either pre- or post-synaptic sites. Rat supraoptic nucleus neurons express glycine receptors on the membrane of both their soma and dendrites within the supraoptic nucleus, and their axon terminals in the neurohypophysis. Taking advantage of the well-separated cellular compartments of this system, we correlated the structural properties of the receptors to their subcellular localization. Immunohistochemical study using the generic mAb4a antibody revealed that somatodendritic receptors were clustered, whereas axonal glycine receptors showed a more diffuse distribution. This was paralleled by the presence of clusters of the glycine receptor aggregating protein gephyrin in the supraoptic nucleus and its complete absence in the neurohypophysis. Moreover, another antibody recognizing the alpha1/alpha2 subunits similarly labeled the axonal glycine receptors, but did not recognize the somatodendritic receptor clusters of supraoptic nucleus neurons, indicative of structural differences between somatic and axonal glycine receptors. Furthermore, the subunits composing the somatic and axonal receptors have different molecular weight. Functional study further differentiated the two types of glycine receptors on the basis of their sensitivity to picrotoxin, identifying somatic receptors as alpha/beta heteromers, and axonal receptors as alpha homomers. These results indicate that targeting of glycine receptors to axonal or somatodendritic compartment is directly related to their subunit composition, and set the hypothalamo-neurohypophysial system as an excellent model to study the mechanisms of targeting of proteins to various neuronal cellular compartments.

Is glycine effective against elevated blood pressure?

PURPOSE OF REVIEW

Glycine, a non-essential amino acid, has been found to protect against oxidative stress in several pathological situations, and it is required for the biosynthesis of structural proteins such as elastin. As hypertension is a disease in which free radicals and large vessel elasticity are involved, this article will examine the possible mechanisms by which glycine may protect against high blood pressure.

RECENT FINDINGS

The addition of glycine to the diet reduces high blood pressure in a rat model of the metabolic syndrome. Also, glycine supplemented to the low protein diet of rat dams during pregnancy has a beneficial effect on blood pressure in their offspring. The mechanism by which glycine decreases high blood pressure can be attributed to its participation in the reduction of the generation of free radicals, increasing the availability of nitric oxide. In addition, as glycine is required for a number of critical metabolic pathways, such as the synthesis of the structural proteins collagen and elastin, the perturbation of these leads to impaired elastin formation in the aorta. This involves changes in the aorta's elastic properties, which would contribute to the development of hypertension.

SUMMARY

The use of glycine to lower high blood pressure could have a significant clinical impact in patients with the metabolic syndrome and with limited resources. On the other hand, more studies are needed to explore the beneficial effect of glycine in other models of hypertension and to investigate possible side-effects of treatment with glycine.

[Three types of miniature inhibitory potentiaes in the frog spinal cord motoneurons: the possibility of GABA and glycine co-release]

Miniature inhibitory postsynaptic potentials (mlPSPs) were recorded from motoneurons of the frog isolated spinal cord after blocking action potentials and ionotropic glutamate receptors (TTX 1 mcm: CNQX 25 mcm, D-AP5 50 mcm). Three types of mlPSPs were selected by their time characteristics) fast, slow and mlPSPs with two decay time constants. We classified 8.7% of mlPSPs as dual-component, 64.5% as fast mlPSPs, and 26.8% as slow mlPSPs. The GABA(A)R blocker bicuculline (20 mcm) diminished the number of the slow and dual-component events while the number of mlPSP with a fast kinetics was increased. The GlyR antagonist strychnine (1 mcm) reduced the frequency of fast mlPSPs and increased this parameter of slow mlPSPs. These data suggest existence of three different mlPSP groups distinguished by their kinetics and sensitivity to receptor antagonists: fast events mediated by glycine, slow events mediated by GABA and dual-component mlPSPs corresponding to glycine and GABA co-release.

Reduction of all-trans-retinoic acid–induced teratogenesis in the rat by glycine administration

BACKGROUND

Prenatal rat embryo exposure to retinoids induces severe malformations in various organs; the most active and teratogenic metabolite is all-trans-retinoic acid (atRA). The mechanisms of this embryopathy are only partly known. In the present study, the influence of glycine on the teratogenicity of atRA was investigated.

METHODS

Embryos from 5 groups of white rats were studied: Group 1 remained untreated; Group 2 received glycine 2% in drinking water ad libitum from the first gestational day (GD 1); Group 3 was administered vehicle (corn oil); Group 4 was treated with atRA (50 mg/kg of body weight) injected (IP); and Group 5 was treated with atRA (50 mg/kg of body weight IP) plus glycine 2% in drinking water ad libitum from GD 1. atRA was administrated daily from GD 8-10. Dams were killed on the 21st day of pregnancy, and their fetuses were examined to detect external, visceral, and skeletal malformations.

RESULTS

The results show that the atRA-administered dose is not toxic for the dams, and that although fetal death was not observed, it produced abnormalities in the fetuses. Glycine reduced atRA-induced teratogenic effects (external and skeletal defects).

CONCLUSIONS

The results indicate that glycine effectively reduces the teratogenic effects of atRA. Thus, glycine might be useful for the prevention of vitamin A teratogenicity.