GABA and glycine are protective to mature but toxic to immature rat cortical neurons under hypoxia

Chromosome-level genome assembly of the deep-sea snail Phymorhynchus buccinoides provides insights into the adaptation to the cold seep habitat

BACKGROUND

The deep-sea snail Phymorhynchus buccinoides belongs to the genus Phymorhynchus (Neogastropoda: Raphitomidae), and it is a dominant specie in the cold seep habitat. As the environment of the cold seep is characterized by darkness, hypoxia and high concentrations of toxic substances such as hydrogen sulfide (H2S), exploration of the diverse fauna living around cold seeps will help to uncover the adaptive mechanisms to this unique habitat. In the present study, a chromosome-level genome of P. buccinoides was constructed and a series of genomic and transcriptomic analyses were conducted to explore its molecular adaptation mechanisms to the cold seep environments.

RESULTS

The assembled genome size of the P. buccinoides was approximately 2.1 Gb, which is larger than most of the reported snail genomes, possibly due to the high proportion of repetitive elements. About 92.0% of the assembled base pairs of contigs were anchored to 34 pseudo-chromosomes with a scaffold N50 size of 60.0 Mb. Compared with relative specie in the shallow water, the glutamate regulative and related genes were expanded in P. buccinoides, which contributes to the acclimation to hypoxia and coldness. Besides, the relatively high mRNA expression levels of the olfactory/chemosensory genes in osphradium indicate that P. buccinoides might have evolved a highly developed and sensitive olfactory organ for its orientation and predation. Moreover, the genome and transcriptome analyses demonstrate that P. buccinoides has evolved a sulfite-tolerance mechanism by performing H2S detoxification. Many genes involved in H2S detoxification were highly expressed in ctenidium and hepatopancreas, suggesting that these tissues might be critical for H2S detoxification and sulfite tolerance.

CONCLUSIONS

In summary, our report of this chromosome-level deep-sea snail genome provides a comprehensive genomic basis for the understanding of the adaptation strategy of P. buccinoides to the extreme environment at the deep-sea cold seeps.

Evaluation of cytotoxic effects of amitraz and fipronil on digestive, reproductive and neural processes of engorged Rhipicephalus microplus (Acari: Ixodidae) female

Fipronil and amitraz are potentially toxic compounds used for controlling ticks infesting pet and livestock. The use of fipronil on large animals was limited because of its high costs while amitraz is still persisting in the market since its introduction over four decades ago. Though resistance in ticks against these pesticides has been reported worldwide since 2000, the toxicity of these chemicals at cellular level in ticks is still poorly understood. The present study aimed to examine the gross and cellular impact of fipronil and amitraz on the gut, ovaries and synganglion of engorged Rhipicephalus microplus females. Fipronil and amitraz treated tick groups showed formation of a large number of vacuoles of different size throughout the cytoplasm of generative cells whereas sessile, residual and detached digestive cells were very low in numbers. The treatment of ticks resulted in the formation of vacuolations at periphery of all oocytes. Ultra-thin sections of the synganglion revealed severe rupture of neural lamella and perineurium with apoptosis of neural cells after fipronil treatment whereas in the amitraz treated ticks, severe destruction of neuropile region and extensive vacuolation of type I and II cells of cortical region as compared to the unexposed ticks were noted.

A synthetic BBB-permeable tripeptide GCF confers neuroprotection by increasing glycine in the ischemic brain

Background: We and others have previously demonstrated that glycine is neuroprotective in cerebral ischemia-reperfusion injury. But glycine has low permeability to the blood–brain barrier (BBB). To deliver glycine into the ischemic brain to confer neuroprotection, we designed a novel glycine-containing and BBB-permeable tripeptide, the H-glycine-cysteine-phenylalanine-OH (GCF).

Methods: For the synthesis of GCF, phenylalanine was included to increase the BBB permeability of the tripeptide. Cysteine was conjugated with glycine to enable the release of glycine from GCF. With the use of immunofluorescence labeling and HPLC assays, we measured the distribution and level of GCF. We used TTC labeling, LDH release, and MTT assays to evaluate the neuroprotective effect of GCF.

Results: Following intravenous injection in a rat model of cerebral ischemia-reperfusion injury, GCF was intensively distributed in the ischemic neurons. Intravenous injection of GCF, but not the non-cleavable acetyl-GCF, resulted in the elevation of glycine in the ischemic brain. GCF but not acetyl-GC conferred neuroprotection in ischemic stroke animals.

Conclusion: GCF protects against cerebral ischemia-reperfusion injury in the rat. In contrast to peptide drugs that exert therapeutic effect by interfering with signaling interaction, GCF acts as a BBB shuttle and prodrug to deliver glycine to confer neuroprotection, representing a novel therapeutic strategy for acute ischemic stroke.

Intermittent Hypoxia and Effects on Early Learning/Memory: Exploring the Hippocampal Cellular Effects of Pediatric Obstructive Sleep Apnea

This review provides an update on the neurocognitive phenotype of pediatric obstructive sleep apnea (OSA). Pediatric OSA is associated with neurocognitive deficits involving memory, learning, and executive functioning. Adenotonsillectomy (AT) is presently accepted as the first-line surgical treatment for pediatric OSA, but the executive function deficits do not resolve postsurgery, and the timeline for recovery remains unknown. This finding suggests that pediatric OSA potentially causes irreversible damage to multiple areas of the brain. The focus of this review is the hippocampus, 1 of the 2 major sites of postnatal neurogenesis, where new neurons are formed and integrated into existing circuitry and the mammalian center of learning/memory functions. Here, we review the clinical phenotype of pediatric OSA, and then discuss existing studies of OSA on different cell types in the hippocampus during critical periods of development. This will set the stage for future study using preclinical models to understand the pathogenesis of persistent neurocognitive dysfunction in pediatric OSA.

Glycine Protects against Hypoxic-Ischemic Brain Injury by Regulating Mitochondria-Mediated Autophagy via the AMPK Pathway

Hypoxic-ischemic encephalopathy (HIE) is detrimental to newborns and is associated with high mortality and poor prognosis. Thus, the primary aim of the present study was to determine whether glycine could (1) attenuate HIE injury in rats and hypoxic stress in PC12 cells and (2) downregulate mitochondria-mediated autophagy dependent on the adenosine monophosphate- (AMP-) activated protein kinase (AMPK) pathway. Experiments conducted using an in vivo HIE animal model and in vitro hypoxic stress to PC12 cells revealed that intense autophagy associated with mitochondrial function occurred during in vivo HIE injury and in vitro hypoxic stress. However, glycine treatment effectively attenuated mitochondria-mediated autophagy. Additionally, after identifying alterations in proteins within the AMPK pathway in rats and PC12 cells following glycine treatment, cyclosporin A (CsA) and 5-aminoimidazole-4-carboxamide-1-b-4-ribofuranoside (AICAR) were administered in these models and indicated that glycine protected against HIE and CoCl₂ injury by downregulating mitochondria-mediated autophagy that was dependent on the AMPK pathway. Overall, glycine attenuated hypoxic-ischemic injury in neurons via reductions in mitochondria-mediated autophagy through the AMPK pathway both in vitro and in vivo.

Metabolite changes in the ipsilateral and contralateral cerebral hemispheres in rats with middle cerebral artery occlusion

Cerebral ischemia not only causes pathological changes in the ischemic areas but also induces a series of secondary changes in more distal brain regions (such as the contralateral cerebral hemisphere). The impact of supratentorial lesions, which are the most common type of lesion, on the contralateral cerebellum has been studied in patients by positron emission tomography, single photon emission computed tomography, magnetic resonance imaging and diffusion tensor imaging. In the present study, we investigated metabolite changes in the contralateral cerebral hemisphere after supratentorial unilateral ischemia using nuclear magnetic resonance spectroscopy-based metabonomics. The permanent middle cerebral artery occlusion model of ischemic stroke was established in rats. Rats were randomly divided into the middle cerebral artery occlusion 1-, 3-, 9- and 24-hour groups and the sham group. 1H nuclear magnetic resonance spectroscopy was used to detect metabolites in the left and right cerebral hemispheres. Compared with the sham group, the concentrations of lactate, alanine, γ-aminobutyric acid, choline and glycine in the ischemic cerebral hemisphere were increased in the acute stage, while the concentrations of N-acetyl aspartate, creatinine, glutamate and aspartate were decreased. This demonstrates that there is an upregulation of anaerobic glycolysis (shown by the increase in lactate), a perturbation of choline metabolism (suggested by the increase in choline), neuronal cell damage (shown by the decrease in N-acetyl aspartate) and neurotransmitter imbalance (evidenced by the increase in γ-aminobutyric acid and glycine and by the decrease in glutamate and aspartate) in the acute stage of cerebral ischemia. In the contralateral hemisphere, the concentrations of lactate, alanine, glycine, choline and aspartate were increased, while the concentrations of γ-aminobutyric acid, glutamate and creatinine were decreased. This suggests that there is a difference in the metabolite changes induced by ischemic injury in the contralateral and ipsilateral cerebral hemispheres. Our findings demonstrate the presence of characteristic changes in metabolites in the contralateral hemisphere and suggest that they are most likely caused by metabolic changes in the ischemic hemisphere.

The role of glycine in regulated cell death

The cytoprotective effects of glycine against cell death have been recognized for over 28 years. They are expressed in multiple cell types and injury settings that lead to necrosis, but are still not widely appreciated or considered in the conceptualization of cell death pathways. In this paper, we review the available data on the expression of this phenomenon, its relationship to major pathophysiologic pathways that lead to cell death and immunomodulatory effects, the hypothesis that it involves suppression by glycine of the development of a hydrophilic death channel of molecular dimensions in the plasma membrane, and evidence for its impact on disease processes in vivo.

Beta Amyloid Differently Modulate Nicotinic and Muscarinic Receptor Subtypes which Stimulate in vitro and in vivo the Release of Glycine in the Rat Hippocampus

Using both in vitro (hippocampal synaptosomes in superfusion) and in vivo (microdialysis) approaches we investigated whether and to what extent β amyloid peptide 1-40 (Aβ 1-40) interferes with the cholinergic modulation of the release of glycine (GLY) in the rat hippocampus. The nicotine-evoked overflow of endogenous GLY in hippocampal synaptosomes in superfusion was significantly inhibited by Aβ 1-40 (10 nM) while increasing the concentration to 100 nM the inhibitory effect did not further increase. Both the Choline (Ch; α7 agonist; 1 mM) and the 5-Iodo-A-85380 dihydrochloride (5IA85380, α4β2 agonist; 10 nM)-evoked GLY overflow were inhibited by Aβ 1-40 at 100 nM but not at 10 nM concentrations. The KCl evoked [(3)H]GLY and [(3)H]Acetylcholine (ACh) overflow were strongly inhibited in presence of oxotremorine; however this inhibitory muscarinic effect was not affected by Aβ 1-40. The effects of Aβ 1-40 on the administration of nicotine, veratridine, 5IA85380, and PHA543613 hydrochloride (PHA543613; a selective agonist of α7 subtypes) on hippocampal endogenous GLY release in vivo were also studied. Aβ 1-40 significantly reduced (at 10 μM but not at 1 μM) the nicotine-evoked in vivo release of GLY. Aβ 1-40 (at 10 μM but not at 1 μM) significantly inhibited the PHA543613 (1 mM)-elicited GLY overflow while was ineffective on the GLY overflow evoked by 5IA85380 (1 mM). Aβ 40-1 (10 μM) did not produce any inhibitory effect on nicotine-evoked GLY overflow both in the in vitro and in vivo experiments. Our results indicate that (a) the cholinergic modulation of the release of GLY occurs by the activation of both α7 and α4β2 nicotinic ACh receptors (nAChRs) as well as by the activation of inhibitory muscarinic ACh receptors (mAChRs) and (b) Aβ 1-40 can modulate cholinergic evoked GLY release exclusively through the interaction with α7 and the α4β2 nAChR nicotinic receptors but not through mAChR subtypes.

Glycine, a simple physiological compound protecting by yet puzzling mechanism(s) against ischaemia-reperfusion injury: current knowledge

Ischaemia is amongst the leading causes of death. Despite this importance, there are only a few therapeutic approaches to protect from ischaemia-reperfusion injury (IRI). In experimental studies, the amino acid glycine effectively protected from IRI. In the prevention of IRI by glycine in cells and isolated perfused or cold-stored organs (tissues), direct cytoprotection plays a crucial role, most likely by prevention of the formation of pathological plasma membrane pores. Under in vivo conditions, the mechanism of protection by glycine is less clear, partly due to the physiological presence of the amino acid. Here, inhibition of the inflammatory response in the injured tissue is considered to contribute decisively to the glycine-induced reduction of IRI. However, attenuation of IRI recently achieved in experimental animals by low-dose glycine treatment regimens suggests additional/other (unknown) protective mechanisms. Despite the convincing experimental evidence and the large therapeutic width of glycine, there are only a few clinical trials on the protection from IRI by glycine with ambivalent results. Thus, both the mechanism(s) behind the protection of glycine against IRI in vivo and its true clinical potential remain to be addressed in future experimental studies/clinical trials.

Effects of inhibitory amino acids on expression of GABAA Rα and glycine Rα1 in hypoxic rat cortical neurons during development

Recent studies suggest that GABA and glycine are protective to mature but toxic to immature cortical neurons during prolonged hypoxia. Since the action of these inhibitory amino acids is mediated by GABA and glycine receptors, the expression of these receptors is a critical factor in determining neuronal response to GABA(A) and glycine in hypoxia. Therefore, we asked whether in rat cortical neurons, 1) hypoxia alters the expression of the GABA and glycine receptors; 2) inhibitory amino acids change the course of GABA and glycine receptor expression; and 3) there are any differences between the immature and mature neurons. In cultured rat cortical neurons from day 4 (four days in vitro or DIV 4) to day 20 (DIV 20), we observed that 1) GABA(A)Rα and GlyRα1 underwent differential changes in expression during the development in vitro; 2) hypoxia for 3 days decreased GABA(A)Rα and GlyRα1 density in the neurons in-between DIV 4 and DIV 20, but did not induce a major change in immature (DIV 4) and mature (DIV 20) neurons; 3) during normoxia GABA, glycine and taurine decreased GABA(A)Rα and GlyRα1 density in the immature neurons, but had a tendency to increase the density in the mature neurons, except for taurine; 4) under hypoxia, all these amino acids decreased GABA(A)Rα and GlyRα1 density in most groups of the immature neurons with a slight effect on the mature neurons; and 5) δ-opioid receptor activation with DADLE increased GABA(A)Rα and GlyRα1 density in both the immature and mature neurons under normoxia and in the mature neurons under hypoxic condition. These data suggest that inhibitory amino acids differentially regulate the expression of GABA(A) and glycine receptors in rat cortical neurons under normoxic and hypoxic conditions with major differences between the immature and mature neurons.

Effect of delta-opioid receptor over-expression on cortical expression of GABAA receptor alpha1-subunit in hypoxia

Recent studies show that both delta-opioid receptors (DOR) and GABA receptors play a neuroprotective role in the mature cortex. Since we have observed that DOR over-expression renders the cortex more tolerant to hypoxic stress, we asked whether DOR over-expression affects GABA receptors expression in the cortex under hypoxia. As the first step, we investigated the expression of GABAA receptor alpha1-subunit (GABAA Ralpha1, the most abundant alpha-subunit of GABA receptors in the adult brain) in the mouse cortex with transgenic DOR over-expression after hypoxia. The results showed that GABAA Ralpha1 expression was lower in the transgenic than wild-type cortex, suggesting that DOR overexpression induces an inhibitory effect on GABAA receptor expression. Hypoxia for 1-3 days significantly increased GABAA Ralpha1 expression in the wild-type cortex, which may be an adaptive strategy for protecting the cortex against hypoxic stress. Interestingly, such increase was not found in the transgenic cortex with DOR over-expression. This may represent an interactive regulation in the transgenic cortex to efficiently balance energy production and consumption for better adaptation to hypoxic environment. Since DOR over-expression increases cortical tolerance to hypoxia, an increase in GABA receptors expression (an energy-costing process) may not be necessary in the cortex with DOR over-expression.

Neurovascular interaction and the pathophysiology of diabetic retinopathy

Diabetic retinopathy (DR) is the most severe of the several ocular complications of diabetes, and in the United States it is the leading cause of blindness among adults 20 to 74 years of age. Despite recent advances in our understanding of the pathogenesis of DR, there is a pressing need to develop novel therapeutic treatments that are both safe and efficacious. In the present paper, we identify a key mechanism involved in the development of the disease, namely, the interaction between neuronal and vascular activities. Numerous pathological conditions in the CNS have been linked to abnormalities in the relationship between these systems. We suggest that a similar situation arises in the diabetic retina, and we propose a logical strategy aimed at therapeutic intervention.

Selective vulnerability of adult cochlear nucleus neurons to de-afferentation by mechanical compression

It is well established that the cochlear nucleus (CN) of developing species is susceptible to loss of synaptic connections from the auditory periphery. Less information is known about how de-afferentation affects the adult auditory system. We investigated the effects of de-afferentation to the adult CN by mechanical compression. This experimental model is quantifiable and highly reproducible. Five weeks after mechanical compression to the axons of the auditory neurons, the total number of neurons in the CN was evaluated using un-biased stereological methods. A region-specific degeneration of neurons in the dorsal cochlear nucleus (DCN) and posteroventral cochlear nucleus (PVCN) by 50% was found. Degeneration of neurons in the anteroventral cochlear nucleus (AVCN) was not found. An imbalance between excitatory and inhibitory synaptic transmission after de-afferentation may have played a crucial role in the development of neuronal cell demise in the CN. The occurrence of a region-specific loss of adult CN neurons illustrates the importance of evaluating all regions of the CN to investigate the effects of de-afferentation. Thus, this experimental model may be promising to obtain not only the basic knowledge on auditory nerve/CN degeneration but also the information relevant to the application of cochlear or auditory brainstem implants.

Effect of the inhibitory neurotransmitter glycine on slow destructive processes in brain cortex slices under anoxic conditions

Slow destructive processes in brain cortex were studied under deep hypoxia (anoxia). Study of the character and dynamics of DNA destruction showed that apoptosis and necrosis run in parallel under the experimental conditions. These processes typically develop in tens of hours. A similar conclusion was reached from electron microscopic study of the tissue ultrastructure. More detailed study revealed that a relatively rare type of apoptosis not involving cytochrome c release from the intermembrane space of mitochondria and not associated with opening of the mitochondrial nonspecific pore occurs under the experimental conditions. As this is occurring, the process can be slowed by high concentrations of glycine, an inhibitory neurotransmitter. The study of DNA destruction demonstrated that high concentrations of glycine selectively slow apoptosis but have almost no effect on necrosis. Glycine also drastically decreases changes in the tissue ultrastructure, particularly of mitochondria, arising under anoxia. Glycine does not notably influence the mitochondrial oxidative phosphorylation system. Study of impairment of mitochondrial function demonstrated that the oxidative phosphorylation system is not disturbed for 1 h, which is several times longer than the inhibition time of brain function under deep hypoxia. The mitochondrial respiratory system is preserved for a relatively long time (24 h). Malate oxidase activity is deactivated after 48 h. The succinate oxidase fragment of the mitochondrial respiratory chain proved especially resistant; it retains activity under anoxia for more than 72 h. A possible mechanism of the effect of high glycine concentrations is discussed.

Direct correlation between ischemic injury and extracellular glycine concentration in mice with genetically altered activities of the glycine cleavage multienzyme system

BACKGROUND AND PURPOSE

Ischemia elicits the rapid release of various amino acid neurotransmitters. A glutamate surge activates N-methyl-d-aspartate (NMDA) glutamate receptors, triggering deleterious processes in neurons. Although glycine is a coagonist of the NMDA receptor, the effect of extracellular glycine concentration on ischemic injury remains controversial. To approach this issue, we examined ischemic injury in mice with genetically altered activities of the glycine cleavage multienzyme system (GCS), which plays a fundamental role in maintaining extracellular glycine concentration.

METHODS

A mouse line with increased GCS activity (340% of C57BL/6 control mice) was generated by transgenic expression of glycine decarboxylase, a key GCS component (high-GCS mice). Another mouse line with reduced GCS activity (29% of controls) was established by transgenic expression of a dominant-negative mutant of glycine decarboxylase (low-GCS mice). We examined neuronal injury after transient occlusion of the middle cerebral artery in these mice by measuring extracellular amino acid concentrations in microdialysates.

RESULTS

High-GCS and low-GCS mice had significantly lower and higher basal concentrations of extracellular glycine than did controls, respectively. In low-GCS mice, the extracellular glycine concentration reached 2-fold of control levels during ischemia, and infarct volume was significantly increased by 69% with respect to controls. In contrast, high-GCS mice had a significantly smaller infarct volume (by 21%). No significant difference was observed in extracellular glutamate concentrations throughout the experiments. An antagonist for the NMDA glycine site, SM-31900, attenuated infarct size, suggesting that glycine operated via the NMDA receptor.

CONCLUSIONS

There is a direct correlation between ischemic injury and extracellular glycine concentration maintained by the GCS.

Endogenous opiates and behavior: 2005

This paper is the 28th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2005 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).

Gene transcription of neuroglobin is upregulated by hypoxia and anoxia in the brain of the anoxia-tolerant turtle Trachemys scripta

Neuroglobin is a heme protein expressed in the vertebrate brain in mammals, fishes, and birds. The physiological role of neuroglobin is not completely understood but possibilities include serving as an intracellular oxygen-carrier or oxygen-sensor, as a terminal oxidase to regenerate NAD(+) under anaerobic conditions, or involvement in NO or ROS metabolism. As the vertebrate nervous system is particularly sensitive to hypoxia, an intracellular protein that helps sustain cellular respiration would aid hypoxic survival. However, the regulation of Neuroglobin (Ngb) under conditions of varying oxygen is controversial. This study examines the regulation of Ngb in an anoxia-tolerant vertebrate under conditions of hypoxia and anoxia. The freshwater turtle Trachemys scripta can withstand complete anoxia for days, and adaptations that permit neuronal survival have been extensively examined. Turtle neuroglobin specific primers were employed in RT-PCR for determining the regulation of neuroglobin mRNA expression in turtles placed in normoxia, hypoxia (4 h), anoxia (1 and 4 h), and anoxia-reoxygenation. Whole brain expression of neuroglobin is strongly upregulated by hypoxia and post-anoxic-reoxygenation in T. scripta, with a lesser degree of upregulation at 1 and 4 h anoxia. Our data implicate neuroglobin in mediating brain anoxic survival.

Enhancement of Neuroprotection of Mulberry Leaves (Morus alba L.) Prepared by the Anaerobic Treatment against Ischemic Damage

Several neurological disorders such as Alzheimer's and Parkinson's diseases have been attributed to gamma-aminobutyric acid (GABA) depletion in the brain. In order to provide a pharmacological basis for the neuroprotective actions of the enhanced accumulation of GABA in mulberry leaves (ML) against cerebral ischemia in vitro and in vivo, a process was developed to enhance the accumulation of GABA in mulberry leaves (GAML) as a result of the various anaerobic treatments. The GABA concentrations were changed by N(2) gas purging, the reaction temperature, reaction time, pH and the leaf size. GABA enhanced the potential of neuroprotection in the PC12 cells damaged by H(2)O(2)-induced oxidation. GAML reduced the cytotoxicity in the PC12 cells against oxygen glucose deprivation-induced cerebral ischemic condition. The neuroprotective effect of GAML was further demonstrated in vivo using middle cerebral artery occlusion brain injury model. GAML significantly decreased the infarct volume of the brain compared with than control group. Overall, these results suggest that the anaerobic treatment of ML makes GAML enhance the neuroprotection effect against in vivo cerebral ischemia such as in vitro.

Glycine-gated chloride channels depress synaptic transmission in rat hippocampus

An inhibitory role for strychnine-sensitive glycine-gated chloride channels (GlyRs) in mature hippocampus is beginning to be appreciated. We have reported previously that CA1 pyramidal cells and GABAergic interneurons recorded in 3- to 4-wk-old rat hippocampal slices express functional GlyRs, dispelling previous misconceptions that GlyR expression ceases in early development. However, the effect of GlyR activation on cell excitability and synaptic circuits in hippocampus has not been fully explored. Using whole cell current-clamp recordings, we show that activation of strychnine-sensitive GlyRs through exogenous glycine application causes a significant decrease in input resistance and prevents somatically generated action potentials in both CA1 pyramidal cells and interneurons. Furthermore, GlyR activation depresses the synaptic network by reducing suprathreshold excitatory postsynaptic potentials (EPSPs) to subthreshold events in both cell types. Blockade of postsynaptic GlyRs with the chloride channel blocker 4, 4'-diisothiocyanatostilbene-2-2'-disulfonic acid (DIDS) or altering the chloride ion driving force in recorded cells attenuates the synaptic depression, strongly indicating that a postsynaptic mechanism is responsible. Increasing the local glycine concentration by blocking reuptake causes a strychnine-sensitive synaptic depression in interneuron recordings, suggesting that alterations in extracellular glycine will impact excitability in hippocampal circuits. Finally, using immunohistochemical methods, we show that glycine and the glycine transporter GlyT2 are co-localized selectively in GABAergic interneurons, indicating that interneurons contain both inhibitory neurotransmitters. Thus we report a novel mechanism whereby activation of postsynaptic GlyRs can function to depress activity in the synaptic network in hippocampus. Moreover, the co-localization of glycine and GABA in hippocampal interneurons, similar to spinal cord, brain stem, and cerebellum, suggests that this property is likely to be a general characteristic of inhibitory interneurons throughout the CNS.