Aminooxyacetic acid induced accumulation of GABA in the rat brain. Interaction with GABA receptors and distribution in compartments

Overproduction of hydrogen sulfide, generated by cystathionine β-synthase, disrupts brain wave patterns and contributes to neurobehavioral dysfunction in a rat model of down syndrome

Using a novel rat model of Down syndrome (DS), the functional role of the cystathionine-β-synthase (CBS)/hydrogen sulfide (H₂S) pathway was investigated on the pathogenesis of brain wave pattern alterations and neurobehavioral dysfunction. Increased expression of CBS and subsequent overproduction of H₂S was observed in the brain of DS rats, with CBS primarily localizing to astrocytes and the vasculature. DS rats exhibited neurobehavioral defects, accompanied by a loss of gamma brain wave activity and a suppression of the expression of multiple pre- and postsynaptic proteins. Aminooxyacetate, a prototypical pharmacological inhibitor of CBS, increased the ability of the DS brain tissue to generate ATP in vitro and reversed the electrophysiological and neurobehavioral alterations in vivo. Thus, the CBS/H₂S pathway contributes to the pathogenesis of neurological dysfunction in DS, most likely through dysregulation of cellular bioenergetics and gene expression.

Impact of Pharmacological Inhibition of Hydrogen Sulphide Production in the SOD1G93A-ALS Mouse Model

A number of factors can trigger amyotrophic lateral sclerosis (ALS), although its precise pathogenesis is still uncertain. In a previous study done by us, poisonous liquoral levels of hydrogen sulphide (H₂S) in sporadic ALS patients were reported. In the same study very high concentrations of H₂S in the cerebral tissues of the familial ALS (fALS) model of the SOD1G93A mouse, were measured. The objective of this study was to test whether decreasing the levels of H₂S in the fALS mouse could be beneficial. Amino-oxyacetic acid (AOA)—a systemic dual inhibitor of cystathionine-β-synthase and cystathionine-γ lyase (two key enzymes in the production of H₂S)—was administered to fALS mice. AOA treatment decreased the content of H₂S in the cerebral tissues, and the lifespan of female mice increased by approximately ten days, while disease progression in male mice was not affected. The histological evaluation of the spinal cord of the females revealed a significant increase in GFAP positivity and a significant decrease in IBA1 positivity. In conclusion, the results of the study indicate that, in the animal model, the inhibition of H₂S production is more effective in females. The findings reinforce the need to adequately consider sex as a relevant factor in ALS.

Hydrogen sulfide selectively potentiates central preganglionic fast nicotinic synaptic input in mouse superior mesenteric ganglion

Hydrogen sulfide (H2S) plays important roles in the enteric system in the wall of the gastrointestinal tract. There have been no studies on whether H2S is endogenously generated in peripheral sympathetic ganglia and, if so, its effect on synaptic transmission. In this study, we examined the effect of H2S on cholinergic excitatory fast synaptic transmission in the mouse superior mesenteric ganglion (SMG). Our study revealed that NaHS and endogenously generated H2S selectively potentiated cholinergic fast EPSPs (F-EPSPs) evoked by splanchnic nerve stimulation but not F-EPSPs evoked by colonic nerve stimulation. The H2S-producing enzyme cystathionine-γ-lyase (CSE) was expressed in both neurons and glial cells. The CSE blocker PAG (dl-propargylglycine) significantly reduced the amplitude of F-EPSPs evoked by splanchnic nerve stimulation but not F-EPSPs evoked by colonic nerve stimulation. Inhibiting the breakdown of endogenously generated H2S with stigmatellin potentiated the amplitude of F-EPSPs evoked by splanchnic nerve stimulation but not F-EPSPs evoked by colonic nerve stimulation. Splanchnic F-EPSPs but not colonic F-EPSPs were reduced in CSE knock-out (KO) mice. Functional studies showed that NaHS enhanced the inhibitory effect of splanchnic nerve stimulation on colonic motility. Colonic motility in CSE-KO mice was significantly higher than colonic motility in wild-type mice. We conclude that endogenously generated H2S acted selectively on presynaptic terminals of splanchnic nerves to modulate fast cholinergic synaptic input and that this effect of H2S modulates CNS control of gastrointestinal motility. Our results show for the first time that the facilitatory effect of endogenous H2S in the mouse SMG is pathway specific.

Hydrogen sulfide augments synaptic neurotransmission in the nucleus of the solitary tract

Within the brain stem, the nucleus tractus solitarii (NTS) serves as a principal central site for sensory afferent integration from the cardiovascular and respiratory reflexes. Neuronal activity and synaptic transmission in the NTS are highly pliable and subject to neuromodulation. In the central nervous system, hydrogen sulfide (H₂S) is a gasotransmitter generated primarily by the enzyme cystathionine-β-synthase (CBS). We sought to determine the role of H₂S, and its generation by CBS, in NTS excitability. Real-time RT-PCR, immunoblot, and immunohistochemistry analysis identified the presence of CBS in the NTS. Patch-clamp electrophysiology in brain stem slices examined excitatory postsynaptic currents (EPSCs) and membrane properties in monosynaptically driven NTS neurons. Confocal imaging of labeled afferent synaptic terminals in NTS slices monitored intracellular calcium. Exogenous H₂S significantly increased the amplitude of evoked solitary tract (TS)-EPSCs, frequency of miniature (m)EPSCs, and presynaptic terminal calcium fluorescence in the NTS. H₂S did not alter action potential discharge or postsynaptic properties. On the other hand, the CBS inhibitor aminooxyacetate (AOA) significantly reduced the amplitude of TS-EPSCs and presynaptic terminal calcium fluorescence in the NTS without altering postsynaptic properties. Taken together, these data support a presynaptic role for endogenous H₂S in modulation of excitatory neurotransmission in the NTS.

Hydrogen sulphide in the hypothalamus causes an ATP-sensitive K+ channel-dependent decrease in blood pressure in freely moving rats

Hydrogen sulfide (H2S) is a naturally occurring gas that may act as an endogenous signaling molecule. In the brain, H2S is mainly produced by cystathionine beta-synthase (CBS) and its cellular effects have been attributed to interactions with N-methyl-D-aspartate (NMDA) receptors and cyclic adenosine 3',5'-monophosphate (cAMP). In contrast, direct vasodilator actions of H2S are most probably mediated by opening smooth muscle ATP-sensitive K+ (K(ATP)) channels. In the hypothalamus, K(ATP) channel-dependent mechanisms are involved in CNS-mediated regulation of blood pressure. In this report, we investigated the hypothesis that H2S may act via K(ATP) channels in the hypothalamus to regulate blood pressure. Mean arterial blood pressure (MAP) and heart rate were monitored in freely moving rats via a pressure transducer placed in the femoral artery. Drugs were infused via a cannula placed in the posterior hypothalamus. Infusion of 200 microM sodium hydrogen sulfide (NaHS), an H2S donor, into the hypothalamus of freely moving rats reduced MAP and heart rate. Infusion of 300 nM to 3 microM gliclazide dose-dependently blocked the effect of 200 microM NaHS. Infusion of the CBS activator, s-adenosyl-L-methionine (0.1 mM and 1 mM), likewise decreased MAP. Infusion of the CBS inhibitors aminooxyacetic acid (10 mM) and hydroxylamine (20 mM) increased MAP but did not block the effects of infusion of 200 microM NaHS. These data indicate that actions of H2S in the hypothalamus decrease blood pressure and heart rate in freely moving rats. This effect appears to be mediated by a K(ATP) channel-dependent mechanism and mimicked by endogenous H2S.

Reversal by apomorphine of the gabaculine-induced GABA accumulation in mouse cortex

To test the assumption that in the mice cortex the rate of accumulation of gamma-aminobutyric acid (GABA) after irreversible inhibition of 4-aminobutyrate: 2-oxoglutarate aminotransferase (EC 2.6.1.19; GABA-T) represents an index of GABA turnover, we examined whether the reversal of the gabaculine-induced accumulation of GABA elicited by apomorphine was due to a decrease in GABA turnover or to a modulation of the activity of the GABA-T inhibitor. Therefore, we simultaneously measured the action of apomorphine on gabaculine-induced accumulation of GABA and on GABA-T activity. In vitro, apomorphine (3 and 30 microM) did not alter the concentration-dependent inhibition of GABA-T by gabaculine. Ex vivo, apomorphine (2 x 0.5 mg/kg s.c.) markedly decreased (69%) gabaculine-induced (150 mg/kg i.p.) accumulation of GABA. This drug had no direct effect on GABA-T activity, but significantly reduced from 83 to 71% the inhibition of GABA-T by gabaculine. The linear correlation found between GABA levels and GABA-T activity allowed the quantification of the decrease in GABA turnover elicited by apomorphine. The results showed that apomorphine decreased significantly (P less than 0.001) the rate of GABA synthesis from 7.48 to 3.36 micromol GABA/g per h, if the partial reversal of gabaculine-induced inhibition of GABA-T is considered and 2.44 micromol/g per h if not. Apomorphine effect on GABA accumulation is mainly due to a decrease of the rate of GABA synthesis and to a lesser extent to a reversal of the inhibitory activity of gabaculine. Thus, inhibition of GABA-T by gabaculine is a sensitive and reliable method for the estimation of the rate of synthesis.

Blockade of the LH surge and ovulation by GABA-T inhibitory drugs that increase brain GABA levels in rats

The present experiments were designed to evaluate the effects of GABA increase in the brain on the LH surge and ovulation in female rats. GABA-transaminase (GABA-T) inhibitory drugs were administered intraperitoneally in the early afternoon hours on the day before the expected ovulation. Gamma-acetylenic GABA (GAG) prevented the proestrus LH surge and ovulation in freely moving adult female rats. In a similar manner, GAG blunted the enhanced LH release found at 1800 h on the preovulatory day and the subsequent ovulation in PMSG-treated 32-day-old rats. LH secretion and ovulation in these animals was restored by administration of the selective GABA antagonist, bicuculline. The other GABA-T inhibitors, amino-oxyacetic acid and gamma-vinyl GABA, produced similar effects to GAG in the PMSG-treated rats. These results indicate that the increase of brain GABA levels during the preovulatory day alters LH release and prevents ovulation in rats.

Non involvement of gamma-aminobutyric acid in catechol-induced seizures

The effects of certain anticonvulsant agents, namely, valproate, diazepam and phenobarbitone were investigated on catechol-induced spontaneous and evoked convulsions, in anaesthetized rats and mice. Valproate and diazepam significantly reduced the intensity of spontaneous convulsions and the frequency of occurrence of the longer-latency components (M2 and M3) of the evoked muscle response. Phenobarbitone significantly reduced spontaneous convulsions and the M3 component of the evoked muscle response. None of the drugs affected the short latency M1 component indicating a supra-spinal site of action of these drugs. Agents which modify gamma-aminobutyric acid (GABA)-mediated transmission were without effect on the frequency of occurrence of M1, M2 or M3. The results suggest that the convulsant action of catechol is not dependent on antagonism of GABA-mediated inhibition.

Quasi-morphine withdrawal behaviour: indication for a specific involvement of the GABA-ergic nerve terminal compartment

Wet-dog-shaking resembling morphine withdrawal behaviour can be evoked in rats by administration of dipropylacetate (DPA). It has been postulated that DPA elicits the withdrawal like behaviour through specifically inhibiting the GABA-degradation in nerve terminals. Inhibition of GABA-transaminase (GABA-T) by aminooxyacetic acid (AOAA) in other compartments of the brain would result in an inhibition of the stimulated GABA-release via feedback on autoreceptors and therefore suppress the DPA-evoked behaviour. This hypothesis has been tested using the GABA-T inhibitors gamma-acetylenic-GABA (GAG), gamma-vinyl-GABA (GVG) and ethanolamine-O-sulphate (EOS). Although GAG and AOAA were found to suppress the body shakes, both GVG and EOS had no effect. Both GAG and AOAA have possibly also effect on glutamate decarboxylase (GAD) whereas GVG and EOS did not affect this enzyme, suggesting a nerve terminal-specific effect on DPA-induced behaviour. GVG stimulated DPA-evoked body shakes after 36 and 60 h, when a specific GABA-increase in nerve terminals will be present.