The K+/Cl- co-transporter KCC2 renders GABA hyperpolarizing during neuronal maturation

Involvement of K(+)-Cl(-)-cotransport in the apoptosis induced by N-ethylmaleimide in HepG2 human hepatoblastoma cells

The role of K(+)-Cl(-)-cotransport in apoptosis in human cancer cells was investigated. N-Ethylmaleimide, a K(+)-Cl(-)-cotransport activator, induced apoptosis in a dose-dependent manner in HepG2 human hepatoblastoma cells. N-Ethylmaleimide induced Cl(-)-dependent K(+) efflux, indicating that K(+)-Cl(-)-cotransport is functionally present in HepG2 cells. Calyculin-A and genistein, inhibitors of K(+)-Cl(-)-cotransport, significantly prevented both K(+)-Cl(-)-cotransport activation and apoptosis induced by N-ethylmaleimide. These results demonstrate, for the first time, a novel role for K(+)-Cl(-)-cotransport in apoptosis in human hepatoma cells. These results further suggest that K(+)-Cl(-)-cotransport may be a valuable target for therapeutic interventions for human hepatoma.

Differential expression of synaptic and nonsynaptic mechanisms underlying stimulus-induced gamma oscillations in vitro

Gamma frequency oscillations occur in hippocampus in vitro after brief tetani delivered to afferent pathways. Previous reports have characterized these oscillations as either (1) trains of GABA(A) inhibitory synaptic events mediated by depolarization of both pyramidal cells and interneurons at least in part mediated by metabotropic glutamate and acetylcholine receptors, or (2) field potential oscillations occurring in the near absence of an inhibitory synaptic oscillation when cells are driven by depolarizing GABA responses and local synchrony is produced by field effects. The aim of this study was to investigate factors involved in the differential expression of these synaptically and nonsynaptically gated oscillations. Field effects were undetectable in control recordings but manifested when slices were perfused with hypo-osmotic solutions or a reduced level of normal perfusate. These manipulations also reduced the amplitude of the train of inhibitory synaptic events associated with an oscillation and enhanced the depolarizing GABA component underlying the post-tetanic depolarization. The resulting field oscillation was still dependent, at least in part, on inhibitory synaptic transmission, but spatiotemporal aspects of the oscillation were severely disrupted. These changes were also accompanied by an increase in estimated [K(+)](o) compared with control. We suggest that nonsynaptic oscillations occur under conditions also associated with epileptiform activity and constitute a phenomenon that is distinct from synaptically gated oscillations. The latter remain a viable model for in vivo oscillations of cognitive relevance.

Cl(-)-ATPase in rat brain and kidney

Cl(-)-stimulated ATPase/ATP-dependent Cl(-) pump (Cl(-)-ATPase/pump) has been found as a candidate for an active outwardly directed Cl(-) transporter in brain neurons. (1) A 520-kDa protein complex with Cl(-)-ATPase/pump activity was isolated from rat brain. It consisted of four protein subunits (51, 55, 60, and 62 kDa proteins), the 51-kDa protein being a covalent phosphorylenzyme subunit. (2) An antiserum against the 51-kDa protein inhibited Cl(-)-ATPase/pump activity. Western blot analysis showed an immunoreactive 51-kDa protein in the brain, spinal cord, and kidney. By enzyme histochemistry and immunohistochemistry, Cl(-)-ATPase-like activity or immunoreactivity was observed on the plasma membranes of brain neurons, and on the baso-lateral membranes of type A intercalated cells of renal collecting ducts. (3) Reconstituted Cl(-)-ATPase/pump activity was highest in liposomes with phosphatidylinositol-4-monophosphate. LiCl, an inhibitor of inositolphosphatase, reduced Cl(-)-ATPase activity and increased intracellular Cl(-) concentrations in cultured rat hippocampal neurons with increased phosphatidylinositol turnover. (4) In the brains of patients with Alzheimer's disease (AD), where phosphatidylinositol 4-kinase activity is reduced, Cl(-)-ATPase activity was also reduced. Thus, Cl(-)-ATPase is likely an outwardly directed ATP-dependent Cl(-) transporter that consists of four subunits and is regulated by phosphatidylinositol-4-monophosphate. Changes in Cl(-)-ATPase activity may be related to the pathophysiology of human neurodegenerative diseases. J. Exp. Zool. 289:224-231, 2001.

gamma-Aminobutyric acid(A) neurotransmission and cerebral ischemia

In this review, we present evidence for the role of gamma-aminobutyric acid (GABA) neurotransmission in cerebral ischemia-induced neuronal death. While glutamate neurotransmission has received widespread attention in this area of study, relatively few investigators have focused on the ischemia-induced alterations in inhibitory neurotransmission. We present a review of the effects of cerebral ischemia on pre and postsynaptic targets within the GABAergic synapse. Both in vitro and in vivo models of ischemia have been used to measure changes in GABA synthesis, release, reuptake, GABA(A) receptor expression and activity. Cellular events generated by ischemia that have been shown to alter GABA neurotransmission include changes in the Cl(-) gradient, reduction in ATP, increase in intracellular Ca(2+), generation of reactive oxygen species, and accumulation of arachidonic acid and eicosanoids. Neuroprotective strategies to increase GABA neurotransmission target both sides of the synapse as well, by preventing GABA reuptake and metabolism and increasing GABA(A) receptor activity with agonists and allosteric modulators. Some of these strategies are quite efficacious in animal models of cerebral ischemia, with sedation as the only unwanted side-effect. Based on promising animal data, clinical trials with GABAergic drugs are in progress for specific types of stroke. This review attempts to provide an understanding of the mechanisms by which GABA neurotransmission is sensitive to cerebral ischemia. Furthermore, we discuss how dysfunction of GABA neurotransmission may contribute to neuronal death and how neuronal death can be prevented by GABAergic drugs.

Male germ cells and photoreceptors, both dependent on close cell-cell interactions, degenerate upon ClC-2 Cl(-) channel disruption

The functions of some CLC Cl(-) channels are evident from human diseases that result from their mutations, but the role of the broadly expressed ClC-2 Cl(-) channel is less clear. Several important functions have been attributed to ClC-2, but contrary to these expectations ClC-2-deficient mice lacked overt abnormalities except for a severe degeneration of the retina and the testes, which led to selective male infertility. Seminiferous tubules did not develop lumina and germ cells failed to complete meiosis. Beginning around puberty there was a massive death of primary spermatocytes and later also of spermatogonia. Tubules were filled with abnormal Sertoli cells, which normally express ClC-2 in patches adjacent to germ cells. In the retina, photoreceptors lacked normal outer segments and degenerated between days P10 and P30. The current across the retinal pigment epithelium was severely reduced at P36. Thus, ClC-2 disruption entails the death of two cell types which depend on supporting cells that form the blood-testes and blood-retina barriers. We propose that ClC-2 is crucial for controlling the ionic environment of these cells.

Developmental increase in asynchronous GABA release in cultured hippocampal neurons

Developmental changes in GABAergic synaptic transmission were examined in cultured hippocampal neurons using patch-clamp recordings and Ca(2+) imaging. In paired recordings, tetanization of the presynaptic GABAergic neuron with 80 pulses at either 40 or 80Hz was accompanied by tetanic depression of inhibitory postsynaptic responses. In neurons that had been cultured for more than two weeks, asynchronous inhibitory postsynaptic currents often appeared during the tetanus and continued for several seconds following stimulation. There was little asynchronous activity in neurons that had been cultured for shorter times. However, no age-related changes were observed in the amplitude of single synchronous inhibitory postsynaptic currents, paired-pulse depression or post-tetanic potentiation of inhibitory postsynaptic currents. Following equimolar replacement of extracellular Ca(2+) with strontium ions (Sr(2+)), single autaptic inhibitory postsynaptic currents were depressed in amplitude and asynchronous inhibitory postsynaptic currents were present on the decaying phase. Sr(2+)-induced asynchronous inhibitory postsynaptic currents showed no dependence on age in culture. Imaging of Ca(2+) in single GABAergic boutons was performed by including Fluo-3 in the patch pipette. During action potential firing induced by stimulating at 80Hz for 1s, intracellular calcium [Ca(2+)](i) increased rapidly in individual boutons. Following the stimulus, [Ca(2+)](i) decayed back to baseline within 10-15s. The half-time of decay increased from 1. 7+/-0.2s at 15days in vitro to 4.0+/-0.2s at 30days in vitro (P<0. 05), with a developmental profile that closely matched the increase in asynchronous inhibitory postsynaptic currents. We propose that the increase in tetanus-induced asynchronous GABA-release during the first month of synapse maturation in vitro is caused by a slowing of the Ca(2+)-clearing mechanisms in the GABAergic boutons. This results in larger and more prolonged elevations of [Ca(2+)](i) during tetanic stimulation, which leads to enhanced asynchronous transmitter release. We propose that the results of this study demonstrate a potentially important aspect of synapse maturation during development, and also imply that GABA release is up-regulated in conditions of decreased Ca(2+) buffering and clearing.

Developmental profile of excitatory GABA(A) responses in cultured rat cerebellar granule cells

GABA induced a transient increase in cytosolic free Ca2+ in cerebellar granule cells, which decreased from 3 to 8 days in vitro (DIV). Cytosolic Ca2+ changes induced by glutamate/glycine were comparable at 3 and 7 DIV. The GABA response was ascribed to GABA(A)-receptor mediated depolarization activating L-type Ca2+ channels since the response was inhibited by bicuculline or nifedipine. GABA-mediated Ca2+ rise at 4 DIV was potentiated by pentobarbital or by the neurosteroid 5beta-pregnan-3alpha-ol-20-one, or by decreasing the extracellular Cl- concentration. Neurons cultured for > 7 DIV showed no rise in intracellular Ca2+ in response to GABA regardless of the Cl- gradient. GABA(A) receptor-mediated cytosolic Ca2+ rise suggests an important role for the excitatory activity of GABA in developing cerebellar granule neurons.

Distribution of chloride channel-2-immunoreactive neuronal and astrocytic processes in the hippocampus

The chloride homeostasis of neurons and non-neuronal cells is maintained in part by a voltage-sensitive inwardly rectifying chloride conductance through the chloride channel-2. This channel is activated by hyperpolarization and extracellular hypotonicity. In the present study, hippocampal sections were immunostained for chloride channel-2, and somata and dendrites of both pyramidal and non-pyramidal cells were found to be immunoreactive. In addition, glial processes in the vicinity of small blood vessels were also immunostained, whereas the neuropil of strata pyramidale and lacunosum-moleculare contained chloride channel-2-positive punctate structures. Electron microscopy and double immunostaining using antibodies against chloride channel-2 and glial fibrillary acidic protein confirmed that the dense network of chloride channel-2-positive processes corresponds to the end feet of astrocytes. The distribution of chloride channel-2-immunoreactive astrocytes was inhomogeneous throughout the hippocampus: strata oriens, pyramidale and lacunosum-moleculare of CA1-CA3 and the outer molecular layer of the dentate gyrus contained the majority of immunoreactive end feet, whereas the other layers showed sparse labeling. Subcellular studies demonstrated that, in addition to astrocytes, chloride channel-2 was localized in the membrane of dendrites, dendritic spines, cell bodies and axon initial segments of neurons, frequently close to, or within active zones of, symmetrical synapses.Thus, chloride channel-2 appears to be involved in transmembrane chloride movements associated with GABAergic synaptic transmission. The specific laminar distribution of chloride channel-2-positive astroglial processes coinciding with that of GABAergic axon terminals suggests that the network of astrocytes may be able to siphon and deliver Cl(-) ions to layers with intense GABAergic transmission, thereby increasing the efficacy of GABA(A) receptor-mediated inhibition.

Optical imaging reveals cation--Cl(-) cotransporter-mediated transient rapid decrease in intracellular Cl(-) concentration induced by oxygen--glucose deprivation in rat neocortical slices

In brain slices from young (postnatal day (P) 10--15) rat somatosensory cortex, real-time neuronal intracellular Cl(-) concentration ([Cl(-)](i)) recordings were made by an optical technique measuring 6-methoxy-N-ethlquinolinium iodide (MEQ) fluorescence. Oxygen--glucose deprivation (in vitro model of ischemia) induced a long-lasting [Cl(-)](i) increase preceded by a rapid, transient [Cl(-)](i) decrease that could not be inhibited by blockers of Cl(-) pumps, Cl(-) channels, or Cl(-) antiporters, but was sensitive to cation-Cl(-) cotransporter inhibitors (bumetanide and furosemide). Use of low external Na(+) or high external K(+) revealed that the Na(+),K(+)-2Cl(-) cotransporter was inhibited by bumetanide and furosemide, whereas the K(+)-Cl(-) cotransporter was preferentially inhibited by furosemide under our experimental conditions. With a reduced inward driving force for Na(+) (reducing Na(+),K(+)-2Cl(-) cotransport), the transient [Cl(-)](i) decrease was only rarely induced by oxygen-glucose deprivation. In contrast, with a reduced outward driving force for K(+) (reducing K(+)-Cl(-) cotransport), the transient [Cl(-)](i) decrease still occurred. These results suggest that the transient [Cl(-)](i) decrease was primarily mediated by a rapid inhibition of the inwardly directed Na(+),K(+)-2Cl(-) cotransporter. Reverse transcriptase-polymerase chain reaction (RT-PCR) experiments suggested that the isoform involved is NKCC1. We hypothesize that the initial rapid Cl(-) efflux might effectively delay the irreversible Cl(-) influx that mediates neuronal injury.

Cellular abnormalities and synaptic plasticity in seizure disorders of the immature nervous system

The nervous system has an enhanced capacity to generate seizures during a restricted phase of postnatal development. Studies in animals and particularly in in vitro brain slices from hippocampus and neocortex have been instrumental in furthering an understanding of the underlying processes. Developmental alterations in glutaminergic excitatory synaptic transmission appear to play a key role in the enhanced seizure susceptible of rodents during the second and third week of life. Prior to this period, the number of excitatory synapses is relatively low. The scarcity of connections and the inability of the existing synapses to release glutamate when activated at high frequencies likely contribute importantly to the resistance of neonates to seizures. However, at the beginning of week 2, a dramatic outgrowth of excitatory synapses occurs, and these synapses are able to faithfully follow activation at high frequencies. These changes, coupled with the prolonged nature of synaptic potentials in early life, likely contribute to the ease of seizure generation. After this time, seizure susceptibility declines, patterns of local synaptic connectivity remodel, and some synapses are pruned. Concurrently, the duration of excitatory postsynaptic potentials shortens due at least in part to a switch in the subunit composition of postsynaptic receptors. Other studies have examined the mechanisms underlying chronic epilepsy initiated in early life. Models of both cortical dysplasia and recurrent early-life seizures suggest that alterations in the normal development of excitatory synaptic transmission can contribute importantly to chronic epileptic conditions. In the recurrent early-life seizure model, abnormal use-dependent selection of subpopulations of excitatory synapses may play a role. In experimental cortical dysplasia, alterations in the molecular composition of postsynaptic receptor are observed that favor subunit combinations characteristic of infancy.

Role of bicarbonate and chloride in GABA- and glycine-induced depolarization and [Ca2+]i rise in fetal rat motoneurons in situ

Ca(2+) imaging and (perforated) patch recording were used to analyze the mechanism of GABA- and glycine-induced depolarizations in lumbar motoneurons of spinal cord slices from fetal rats. In fura-2 ester-loaded cells, the agonist-induced depolarizations increased [Ca(2+)](i) by up to 100 nm. The GABA- and glycine-evoked [Ca(2+)](i) transients were suppressed by bicuculline and strychnine, respectively. Their magnitude decreased by approximately 50% between embryonic days 15.5 and 19.5. The [Ca(2+)](i) increases were abolished by Ca(2+)-free superfusate and attenuated by approximately 65% by nifedipine, showing that the responses were mediated by voltage-activated Ca(2+) channels. The [Ca(2+)](i) rises were potentiated by >300% immediately after removal of Cl(-) from the superfusate but recovered to values of 50-200% of control during repeated agonist administration in Cl(-)-free saline. Bumetanide gradually suppressed the [Ca(2+)](i) increases by >75%. Subsequent removal of Cl(-) reconstituted the responses and increased, upon repeated agonist application, the peak [Ca(2+)](i) rises to values above control. Removal of HCO(3)(-) from the Cl(-)-free (bumetanide-containing) superfusate reversibly abolished both the agonist-induced [Ca(2+)](i) rises and depolarizations that were reestablished by formate anions. In Cl(-)-containing superfusate, removal of HCO(3)(-) decreased both the peak and duration of the agonist-evoked membrane depolarization and [Ca(2+)](i) response. Our findings show that HCO(3)(-) efflux has a major contribution to depolarizations mediated by GABA(A) and glycine receptor-coupled anion channels in prenatal neurons. We hypothesize that the HCO(3)(-)-dependent depolarizing component, which is likely to produce an intracellular acidosis, might play an important role during the early postnatal period when the Cl(-)-dependent component gradually shifts to hyperpolarization.

Potassium-coupled chloride cotransport controls intracellular chloride in rat neocortical pyramidal neurons

Chloride (Cl(-)) homeostasis is critical for many cell functions including cell signaling and volume regulation. The action of GABA at GABA(A) receptors is primarily determined by the concentration of intracellular Cl(-). Developmental regulation of intracellular Cl(-) results in a depolarizing response to GABA in immature neocortical neurons and a hyperpolarizing or shunting response in mature neocortical neurons. One protein that participates in Cl(-) homeostasis is the neuron-specific K(+)-Cl(-) cotransporter (KCC2). Thermodynamic considerations predict that in the physiological ranges of intracellular Cl(-) and extracellular K(+) concentrations, KCC2 can act to either extrude or accumulate Cl(-). To test this hypothesis, we examined KCC2 function in pyramidal cells from rat neocortical slices in mature (18-28 d postnatal) and immature (3-6 d postnatal) rats. Intracellular Cl(-) concentration was estimated from the reversal potential of whole-cell currents evoked by local application of exogenous GABA. Both increasing and decreasing the extracellular K(+) concentration resulted in a concomitant change in intracellular Cl(-) concentration in neurons from mature rats. KCC2 inhibition by furosemide caused a change in the intracellular Cl(-) concentration that depended on the concentration of pipette Cl(-); in recordings with low pipette Cl(-), furosemide lowered intracellular Cl(-), whereas in recordings with elevated pipette Cl(-), furosemide raised intracellular Cl(-). In neurons from neonatal rats, manipulation of extracellular K(+) had no effect on intracellular Cl(-) concentration, consistent with the minimal KCC2 mRNA levels observed in neocortical neurons from immature animals. These data demonstrate a physiologically relevant and developmentally regulated role for KCC2 in Cl(-) homeostasis via both Cl(-) extrusion and accumulation.

GABAergic inhibition suppresses paroxysmal network activity in the neonatal rodent hippocampus and neocortex

In the adult cerebral cortex, the neurotransmitter GABA is strongly inhibitory, as it profoundly decreases neuronal excitability and suppresses the network propensity for synchronous activity. When fast, GABA(A) receptor (GABA(A)R)-mediated neurotransmission is blocked in the mature cortex, neuronal firing is synchronized via recurrent excitatory (glutamatergic) synaptic connections, generating population discharges manifested extracellularly as spontaneous paroxysmal field potentials (sPFPs). This epileptogenic effect of GABA(A)R antagonists has rarely been observed in the neonatal cortex, and indeed, GABA in the neonate has been proposed to have an excitatory, rather than inhibitory, action. In contrast, we show here that when fast GABAergic neurotransmission was blocked in slices of neonatal mouse and rat hippocampus and neocortex, sPFPs occurred in nearly half the slices from postnatal day 4 (P4) to P7 neocortex and in most slices from P2 to P7 hippocampus. In Mg(2+)-free solution, GABA(A)R antagonists elicited sPFPs in nearly all slices of P2 and older neocortex and P0 and older hippocampus. Mg(2+)-free solution alone induced spontaneous events in the majority of P2 and older slices from both regions; addition of GABA(A)R antagonists caused a dramatic increase in the mean amplitude, but not frequency, of these events in the hippocampus and in their mean frequency, but not amplitude, in the neocortex. In the hippocampus, GABA(A)R agonists suppressed amplitudes, but not frequency, of sPFPs, whereas glutamate antagonists suppressed frequency but not amplitudes. We conclude that neonatal rodent cerebral cortex possesses glutamatergic circuits capable of generating synchronous network activity and that, as in the adult, tonic GABA(A)R-mediated inhibition prevents this activity from becoming paroxysmal.

Neurobiological mechanisms of the onset of puberty in primates

An increase in pulsatile release of LHRH is essential for the onset of puberty. However, the mechanism controlling the pubertal increase in LHRH release is still unclear. In primates the LHRH neurosecretory system is already active during the neonatal period but subsequently enters a dormant state in the juvenile/prepubertal period. Neither gonadal steroid hormones nor the absence of facilitatory neuronal inputs to LHRH neurons is responsible for the low levels of LHRH release before the onset of puberty in primates. Recent studies suggest that during the prepubertal period an inhibitory neuronal system suppresses LHRH release and that during the subsequent maturation of the hypothalamus this prepubertal inhibition is removed, allowing the adult pattern of pulsatile LHRH release. In fact, y-aminobutyric acid (GABA) appears to be an inhibitory neurotransmitter responsible for restricting LHRH release before the onset of puberty in female rhesus monkeys. In addition, it appears that the reduction in tonic GABA inhibition allows an increase in the release of glutamate as well as other neurotransmitters, which contributes to the increase in pubertal LHRH release. In this review, developmental changes in several neurotransmitter systems controlling pulsatile LHRH release are extensively reviewed.

The differential expression patterns of messenger RNAs encoding K-Cl cotransporters (KCC1,2) and Na-K-2Cl cotransporter (NKCC1) in the rat nervous system

Cation-chloride cotransporters have been considered to play pivotal roles in controlling intracellular and extracellular ionic environments of neurons and hence controlling neuronal function. We investigated the total distributions of K-Cl cotransporter 1 (KCC1), KCC2 (KCC2), and Na-K-2Cl cotransporter 1 (NKCC1) messenger RNAs in the adult rat nervous system using in situ hybridization histochemistry. KCC2 messenger RNA was abundantly expressed in most neurons throughout the nervous system. However, we could not detect KCC2 messenger RNA expression in the dorsal root ganglion and mesencephalic trigeminal nucleus, where primary sensory neurons show depolarizing responses to GABA, suggesting that the absence of KCC2 is necessary for this phenomenon. Furthermore, KCC2 messenger RNA was also not detected in the dorsolateral part of the paraventricular nucleus, dorsomedial part of the suprachiasmatic nucleus, and ventromedial part of the supraoptic nucleus where vasopressin neurons exist, and in the reticular thalamic nucleus. As vasopressin neurons in the suprachiasmatic nucleus and neurons in the reticular thalamic nucleus produce their intrinsic rhythmicity, the lack of KCC2 messenger RNA expression in these regions might be involved in the genesis of rhythmicity through the control of intracellular chloride concentration. The expression levels of KCC1 and NKCC1 messenger RNAs were relatively low, however, positive neurons were observed in several regions, including the olfactory bulb, hippocampus, and in the granular layer of the cerebellum. In addition, positive signals were seen in the non-neuronal cells, such as choroid plexus epithelial cells, glial cells, and ependymal cells, suggesting that KCC1 and NKCC1 messenger RNAs were widely expressed in both neuronal and non-neuronal cells in the nervous system. These results clearly indicate a wide area- and cell-specific variation of cation chloride cotransporters, emphasizing the central role of anionic homeostasis in neuronal function and communication.

Luteinizing hormone-releasing hormone (LHRH) neurons: mechanism of pulsatile LHRH release

Many types of neurons and glia exhibit oscillatory changes in membrane potentials and cytoplasmic Ca2+ concentrations. In neurons and neuroendocrine cells an elevation of intracellular Ca2+ concentration is associated with neurosecretion. Since both oscillatory membrane potentials and intracellular Ca2+ oscillations have been described in primary LHRH neurons and in GT1 cells, it is evident that an endogenous pulse-generator/oscillator is present in the LHRH neuron in vitro. The hourly rhythms of LHRH neurosecretion appear to be the synchronization of a population of LHRH neurons. How a network of LHRH neurons synchronizes their activity, i.e., whether by the result of synaptic mechanisms or electrical coupling through gap junctions or through a diffusible substance(s), remains to be clarified. Even though LHRH neurons themselves possess an endogenous pulse-generating mechanism, they may be controlled by other neuronal and nonneuronal elements in vivo. NE, NPY, glutamate, and GABA are neurotransmitters possibly controlling pulsatile LHRH release, and NO, cAMP, and ATP may be diffusible substances involved in pulsatile LHRH release without synaptic input. Although synaptic inputs to the perikarya of LHRH neurons could control the activity of LHRH neurons, a line of evidence suggests that direct neuronal and nonneuronal inputs, especially those from astrocytes to LHRH neuroterminals, appear to be more important for pusatile LHRH release.

Construction of gene-targeting vectors: a rapid Mu in vitro DNA transposition-based strategy generating null, potentially hypomorphic, and conditional alleles

Gene targeting into mammalian genomes by means of homologous recombination is a powerful technique for analyzing gene function through generation of transgenic animals. Hundreds of mouse strains carrying targeted alleles have already been created and recent modifications of the technology, in particular generation of conditional alleles, have extended the usefulness of the methodology for a variety of special purposes. Even though the standard protocols, including the construction of gene-targeting vector plasmids, are relatively straightforward, they typically involve time-consuming and laborious gene mapping and/or sequencing steps. To produce various types of gene-targeting constructions rapidly and with minimum effort, we developed a strategy, that utilizes a highly efficient in vitro transposition reaction of phage Mu, and tested it in a targeting of the mouse Kcc2 gene locus. A vast number and different types of targeting constructions can be generated simultaneously with little or no prior sequence knowledge of the gene locus of interest. This quick and efficient general strategy will facilitate easy generation of null, potentially hypomorphic, and conditional alleles. Especially useful it will be in the cases when effects of several exons within a given gene are to be studied, a task that necessarily will involve generation of multiple constructions. The strategy extends the use of diverse recombination reactions for advanced genome engineering and complements existing recombination-based approaches for generation of gene-targeting constructions.

GABA, not glutamate, a primary transmitter driving action potentials in developing hypothalamic neurons

Neuronal activity is critical for many aspects of brain development. It has often been assumed that the primary excitatory transmitter driving this activity is glutamate. In contrast, we report that during early development, synaptic release of GABA, the primary inhibitory neurotransmitter in the mature brain, is not only excitatory but in addition plays a more robust role than glutamate in generating spike activity in mouse hypothalamic neurons. Based on gramicidin perforated whole cell and extracellular recording, which leave intracellular Cl(-) unperturbed in brain slices and cultures, the GABA(A) receptor antagonist bicuculline induced a dramatic decrease in spike frequency (83% decrease) in developing neurons, three times greater than that generated by glutamate receptor antagonists 2-amino-5-phosphono-pentanoic acid and 6-cyano-7-nitroquinoxalene-2,3-dione. Thus a number of factors related to spike-dependent stabilization of neuronal connections, including Hebbian mechanisms, that are generally applied to glutamate transmission may also participate in stabilization of GABA circuits.

Theta rhythm of hippocampal CA1 neuron activity: gating by GABAergic synaptic depolarization

Information processing and memory consolidation during exploratory behavior require synchronized activity known as hippocampal theta (theta) rhythm. While it is well established that the theta activity depends on cholinergic inputs from the medial septum/vertical limb of the diagonal band nucleus (MS/DBv) and theta discharges of GABAergic interneurons, and can be induced with cholinergic receptor agonists, it is not clear how the increased excitation of pyramidal cells could occur with increased discharges of GABAergic interneurons during theta waves. Here, we show that the characteristic theta activity in adult rat hippocampal CA1 pyramidal cells is associated with GABAergic postsynaptic depolarization and a shift of the reversal potential from Cl(-) toward HCO(3)(-) (whose ionic gradient is regulated by carbonic anhydrase). The theta activity was abolished by GABA(A) receptor antagonists and carbonic anhydrase inhibitors, but largely unaffected by blocking glutamate receptors. Carbonic anhydrase inhibition also impaired spatial learning in a water maze without affecting other sensory/locomotor behaviors. Thus HCO(3)(-)-mediated signaling, as regulated by carbonic anhydrase, through reversed polarity of GABAergic postsynaptic responses is implicated in both theta and memory consolidation in rat spatial maze learning. We suggest that this mechanism may be important for the phase forward shift of the place cell discharges for each theta cycle during the animal's traversal of the place field for that cell.

Chromosomal localization of SLC12A5/Slc12a5, the human and mouse genes for the neuron-specific K(+)-Cl(-) cotransporter (KCC2) defines a new region of conserved homology

K(+)-Cl(-) cotransporters (KCCs) constitute a branch of the cation-chloride cotransporter (CCC) family. To date, four KCC isoforms (KCC1-KCC4) have been identified and they all mediate obligatorily coupled, electroneutral transmembrane movement of K(+) and Cl(-) ions. KCC2 (gene symbol SLC12A5) is expressed exclusively in neurons within the central nervous system and abnormalities in its expression have been proposed to play a role in pathological conditions such as epilepsy and neuronal trauma. Here we have determined chromosome location of both the human and the mouse genes encoding KCC2, which may assist in future efforts to determine the contribution of KCC2 to inherited human disorders. We assigned human SLC12A5 to 20q12-->q13.1 and its murine homolog, Slc12a5, to 5G2-G3 by fluorescence in situ hybridization (FISH). These mapping data are contradictory to the previously reported human-mouse conserved synteny relationships disrupting an exceptionally well-conserved homology segment between human Chr 20 and mouse Chr 2. We hence suggest the first region of conserved homology between human Chr 20 and mouse Chr 5.

Cation-Chloride Cotransporters in Neuronal Communication

Two isoforms of the cation-Cl(-) cotransporter family are expressed in neurons and modulate neurotransmission. NKCC1, a Na(+)-K(+)-2Cl(-) cotransporter, by raising internal Cl(-), is responsible for excitatory GABAergic activity in immature brain and in adult sensory neurons. KCC2, a neuronal-specific isoform of the K(+)-Cl(-) cotransporter, by lowering internal Cl(-), is critical for inhibitory GABA responses in mature central nervous system neurons.

Development of inhibitory synaptic transmission to motoneurons

The inhibitory effects of the neurotransmitters glycine and gamma-aminobutyric acid (GABA) on motoneurons and their role in mediating the timing of motor output have been understood for some years. Recent work, however, has revealed that these neurotransmitters function very differently in developing motor circuits. Most strikingly, both GABA and glycine depolarize neonatal motoneurons, and, in many instances, provide excitatory drive to developing motor networks. Additionally, the relative contributions of GABA and glycine to inhibitory synaptic transmission in a circuit or, indeed, within the same synapse, change with postnatal development. Here, we review three fundamental properties of inhibitory neurotransmission that are altered postnatally and may be important in shaping the unique behaviors of these synapses early in development.

Formation of the central pattern generator for locomotion in the rat and mouse

It is well known that in the neonatal rat spinal cord preparation, alternating rhythmic bursts in the left and right ventral roots in a given lumbar segment can be induced by bath-application of N-methyl-D-aspartate or 5-hydroxytryptamine. Alternation between L2 and L5 ventral roots on the same side, representing the activity of flexor and extensor muscles, respectively, can be observed as well. In the prenatal period in the rat, alternation between the left and right ventral roots is established between embryonic day (E) 16.5 and E18.5. The alternation between the L2 and L5 ventral roots emerges at E20.5. Recent findings show that locomotor-like rhythmic activity with similar characteristics can be induced in the neonatal mouse preparation. In the lumbar spinal cord in the neonatal mouse, it is likely that the rhythm-generating network is distributed throughout the lumbar region with a rostro-caudal gradient, a situation similar to that in the neonatal and fetal rat spinal cord. With this review we hope to highlight the dramatic changes that neuronal networks generating locomotor-like activity undergo during the prenatal development of the rat. Moreover, the distribution of the neuronal network generating the locomotor rhythm in the neonatal rat and mouse spinal cord is compared.

From egg to action

The development of the neural control circuitry underlying different patterns of behavior is rapidly expanding area of interest in neuroscience. New important insights have been gained over the last few years at different molecular, cellular, network, and behavioral levels. Based primarily on the issues presented in the review articles in this issue, I have added some reflections as to what I find most challenging and important.

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

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

Basolateral K-Cl cotransporter regulates colonic potassium absorption in potassium depletion

Active potassium absorption in the rat distal colon is electroneutral, Na(+)-independent, partially chloride-dependent, and energized by an apical membrane H,K-ATPase. Both dietary sodium and dietary potassium depletion substantially increase active potassium absorption. We have recently reported that sodium depletion up-regulates H,K-ATPase alpha-subunit mRNA and protein expression, whereas potassium depletion up-regulates H,K-ATPase beta-subunit mRNA and protein expression. Because overall potassium absorption is non-conductive, K-Cl cotransport (KCC) at the basolateral membrane may also be involved in potassium absorption. Although KCC1 has not been cloned from the colon, we established, in Northern blot analysis with mRNA from the rat distal colon using rabbit kidney KCC1 cDNA as a probe, the presence of an expected size mRNA in the rat colon. This KCC1 mRNA is substantially increased by potassium depletion but only minimally by sodium depletion. KCC1-specific antibody identified a 155-kDa protein in rat colonic basolateral membrane. Potassium depletion but not sodium depletion resulted in an increase in KCC1 protein expression in basolateral membrane. The increase of colonic KCC1 mRNA abundance and KCC1 protein expression in potassium depletion of the rat colonic basolateral membrane suggests that K-Cl cotransporter: 1) is involved in transepithelial potassium absorption and 2) regulates the increase in potassium absorption induced by dietary potassium depletion. We conclude that active potassium absorption in the rat distal colon involves the coordinated regulation of both apical membrane H,K-ATPase and basolateral membrane KCC1 protein.

Functional comparison of the K+-Cl- cotransporters KCC1 and KCC4

The K(+)-Cl(-) cotransporters (KCCs) are members of the cation-chloride cotransporter gene family and fall into two phylogenetic subgroups: KCC2 paired with KCC4 and KCC1 paired with KCC3. We report a functional comparison in Xenopus oocytes of KCC1 and KCC4, widely expressed representatives of these two subgroups. KCC1 and KCC4 exhibit differential sensitivity to transport inhibitors, such that KCC4 is much less sensitive to bumetanide and furosemide. The efficacy of these anion inhibitors is critically dependent on the concentration of extracellular K(+), with much higher inhibition in 50 mm K(+) versus 2 mm K(+). KCC4 is also uniquely sensitive to 10 mm barium and to 2 mm trichlormethiazide. Kinetic characterization reveals divergent affinities for K(+) (K(m) values of approximately 25.5 and 17.5 mm for KCC1 and KCC4, respectively), probably due to variation within the second transmembrane segment. Although the two isoforms have equivalent affinities for Cl(-), they differ in the anion selectivity of K(+) transport (Cl(-) > SCN(-) = Br(-) > PO(4)(-3) > I(-) for KCC1 and Cl(-) > Br(-) > PO(4)(-3) = I(-) > SCN(-) for KCC4). Both KCCs express minimal K(+)-Cl(-) cotransport under isotonic conditions, with significant activation by cell swelling under hypotonic conditions. The cysteine-alkylating agent N-ethylmaleimide activates K(+)-Cl(-) cotransport in isotonic conditions but abrogates hypotonic activation, an unexpected dissociation of N-ethylmaleimide sensitivity and volume sensitivity. Although KCC4 is consistently more volume-sensitive, the hypotonic activation of both isoforms is critically dependent on protein phosphatase 1. Overall, the functional comparison of these cloned K(+)-Cl(-) cotransporters reveals important functional, pharmacological, and kinetic differences with both physiological and mechanistic implications.

Electroneutral chloride-coupled co-transporters

The family of electroneutral chloride-coupled co-transporters has been expanded in the past year by the identification of new genes or new alternatively spliced isoforms of already known genes. In this review, we discuss the molecular cloning of new members of the family, the knowledge revealed by the production of transgenic animals and some new aspects of the functional characteristics of co-transporters.

Dependence of KCC2 K-Cl cotransporter activity on a conserved carboxy terminus tyrosine residue

K-Cl cotransporters (KCC) play fundamental roles in ionic and osmotic homeostasis. To date, four mammalian KCC genes have been identified. KCC2 is expressed exclusively in neurons. Injection of Xenopus oocytes with KCC2 cRNA induced a 20-fold increase in Cl(-)-dependent, furosemide-sensitive K(+) uptake. Oocyte swelling increased KCC2 activity 2-3 fold. A canonical tyrosine phosphorylation site is located in the carboxy termini of KCC2 (R1081-Y1087) and KCC4, but not in other KCC isoforms. Pharmacological studies, however, revealed no regulatory role for phosphorylation of KCC2 tyrosine residues. Replacement of Y1087 with aspartate or arginine dramatically reduced K(+) uptake under isotonic and hypotonic conditions. Normal or near-normal cotransporter activity was observed when Y1087 was mutated to phenylalanine, alanine, or isoleucine. A tyrosine residue equivalent to Y1087 is conserved in all identified KCCs from nematodes to humans. Mutation of the Y1087 congener in KCC1 to aspartate also dramatically inhibited cotransporter activity. Taken together, these results suggest that replacement of Y1087 and its congeners with charged residues disrupts the conformational state of the carboxy terminus. We postulate that the carboxy terminus plays an essential role in maintaining the functional conformation of KCC cotransporters and/or is involved in essential regulatory protein-protein interactions.

A genetically encoded ratiometric indicator for chloride: capturing chloride transients in cultured hippocampal neurons

We constructed a novel optical indicator for chloride ions by fusing the chloride-sensitive yellow fluorescent protein with the chloride-insensitive cyan fluorescent protein. The ratio of FRET-dependent emission of these fluorophores varied in proportion to the concentration of Cl and was used to measure intracellular chloride concentration ([Cl-]i) in cultured hippocampal neurons. [Cl-]i decreased during neuronal development, consistent with the shift from excitation to inhibition during maturation of GABAergic synapses. Focal activation of GABAA receptors caused large changes in [Cl-]i that could underlie use-dependent depression of GABA-dependent synaptic transmission. GABA-induced changes in somatic [Cl-]i spread into dendrites, suggesting that [Cl-]i can signal the location of synaptic activity. This genetically encoded indicator will permit new approaches ranging from high-throughput drug screening to direct recordings of synaptic Cl- signals in vivo.

Consequences of epilepsy in the developing brain: implications for surgical management

The developing brain is highly susceptible to seizures, as demonstrated by both human and animal studies. Until recently, the brain has been considered to be relatively resistant to damage induced by seizures early in life. Accumulating evidence in animal models now suggests that early seizures can cause structural and physiologic changes in developing neural circuits that result in permanent alterations in the balance between neuronal excitation and inhibition, deficits in cognitive function, and increased susceptibility to additional seizures. The disruption of normal neuronal activity by seizures can affect multiple developmental processes, resulting in these long-lasting changes. These data should be considered in the clinical approach to children with intractable epilepsy and suggest that early intervention may avoid some of these long-term neurologic deficits.

Long-lasting inhibitory synaptic depression is age- and calcium-dependent

The developmental refinement of excitatory synapses is often influenced by neuronal activity, and underlying synaptic mechanisms have been suggested. In contrast, few studies have asked whether inhibitory synapses are reorganized during development and whether this is accompanied by use-dependent changes of inhibitory synaptic strength. The topographic inhibitory projection from the medial nucleus of the trapezoid body (MNTB) to the lateral superior olive (LSO) undergoes synapse elimination during development (Sanes and Takács, 1993). To determine whether there is an associated period of synaptic plasticity, whole-cell recordings were obtained from developing LSO neurons of gerbils in a brain slice preparation. In current-clamp recordings, low-frequency stimulation of the MNTB led to a decline in IPSP amplitude by 43%. In voltage-clamp recordings, hyperpolarized LSO neurons also exhibited a long-lasting depression of MNTB-evoked inhibitory synaptic currents (34%) after low-frequency stimulation. When LSO neurons were depolarized, low-frequency stimulation of the MNTB produced a significantly larger inhibitory synaptic depression (59%). This synaptic plasticity declined dramatically by postnatal days 17-19. Similar to well studied forms of excitatory synaptic plasticity, inhibitory depression depended on postsynaptic calcium. We propose that such activity-dependent synaptic depression may support the developmental rearrangement of inhibitory terminals as they compete with neighboring excitatory and/or inhibitory inputs.

Early postnatal maturation of GABAA-mediated inhibition in the brainstem respiratory rhythm-generating network of the mouse

It is well established that GABAA-mediated postsynaptic potentials are excitatory in many brain regions during embryonic and early postnatal life. The pre-Bötzinger complex (PBC) in the brainstem is an essential component of the respiratory rhythm-generating network, where GABAA-mediated inhibition plays a critical role in generating a stable respiratory rhythm in adult animals. In the present study, using the perforated patch technique, we investigated the maturation of GABAA receptor-mediated effects on rhythmically active PBC neurons and on the motor output in slice preparations from P0-15 neonatal mice. The reversal potential of GABAA receptor-mediated current (EGABA-A) switched from depolarizing to hyperpolarizing within the first postnatal week. EGABA-A was -13.7 +/- 9.8 mV at P0, then it changed to -44.8 +/- 7.0 mV at P2 and -71.5 +/- 6.8 mV at P4. Perfusion of bicarbonate-free saline has no detectable influence on EGABA-A, indicating that a lack of Cl- extrusion during P0-3 is mainly responsible for early GABAA-ergic excitation. At the network level, blockade of GABAA receptors with bicuculline did not significantly change the frequency of rhythmic bursts recorded from hypoglossal nerve roots before P3, whereas it increased the coefficient of variation. After P3, bicuculline increased burst frequency with little effect on the coefficient of variation. Thus, chloride-mediated inhibition, which appears in PBC neurons after P3, coincides with the appearance of GABAA-mediated modulation of the respiratory rhythm. GABAA receptor-activated inhibition may therefore be necessary for frequency modulation in the respiratory network beginning on the fourth postnatal day in the mouse brainstem.

An excitatory GABAergic plexus in developing neocortical layer 1

Layer 1 of the developing rodent somatosensory cortex contains a dense, transient GABAergic fiber plexus. Axons arising from the zona incerta (ZI) of the ventral thalamus contribute to this plexus, as do axons of intrinsic GABAergic cells of layer 1. The function of this early-appearing fiber plexus is not known, but these fibers are positioned to contact the apical dendrites of most postmigratory neurons. Here we show that electrical stimulation of layer 1 results in a GABA(A)-mediated postsynaptic current (PSC) in pyramidal neurons. Gramicidin perforated patch recording demonstrates that the GABAergic layer 1 synapse is excitatory and can trigger action potentials in cortical neurons. In contrast to electrical stimulation, activation of intrinsic layer 1 neurons with a glutamate agonist fails to produce PSCs in pyramidal cells. In addition, responses can be evoked by stimulation of layer 1 at relatively large distances from the recording site. These findings are consistent with a contribution of the widely projecting incertocortical pathway, the only described GABAergic projection to neonatal cortex. Recording of identified neonatal incertocortical neurons reveals a population of active cells that exhibit high frequencies of spontaneous action potentials and are capable of robustly activating neonatal cortical neurons. Because the fiber plexus is confined to layer 1, this pathway provides a spatially restricted excitatory GABAergic innervation of the distal apical dendrites of pyramidal neurons during the peak period of cortical synaptogenesis.

Reversibility and cation selectivity of the K(+)-Cl(-) cotransport in rat central neurons

The reversibility and cation selectivity of the K(+)-Cl(-) cotransporter (KCC), which normally extrudes Cl(-) out of neurons, was investigated in dissociated lateral superior olive neurons of rats using the gramicidin perforated patch technique. Intracellular Cl(-) activity (alpha[Cl(-)](i)) was maintained well below electrochemical equilibrium as determined from the extracellular Cl(-) activity and the holding potential, where the pipette and external solutions contained 150 mM K(+) ([K(+)](pipette)) and 5 mM K(+) ([K(+)](o)), respectively. Extracellular application of 1 mM furosemide or elevated [K(+)](o) increased alpha[Cl(-)](i). When the pipette solution contained 150 mM Cs(+) ([Cs(+)](pipette)), alpha[Cl(-)](i) increased to a value higher than the passive alpha[Cl(-)](i). An increase of alpha[Cl(-)](i) with the [Cs(+)](pipette) was not due to the simple blockade of net KCC by the intracellular Cs(+) since alpha[Cl(-)](i), with the pipette solution containing 75 mM Cs(+) and 75 mM K(+), reached a value between those obtained using the [K(+)](pipette) and the [Cs(+)](pipette). The higher-than-passive alpha[Cl(-)](i) with the [Cs(+)](pipette) was reduced by 1 mM furosemide, but not by 20 microM bumetanide or Na(+)-free external solution, indicating that the accumulation of [Cl(-)](i) in the [Cs(+)](pipette) was mediated by a KCC operating in a reversed mode rather than by Na(+)-dependent, bumetanide-sensitive mechanisms. Replacement of K(+) in the pipette solution with either Li(+) or Na(+) mimicked the effect of Cs(+) on alpha[Cl(-)](i). On the other hand, Rb(+) mimicked K(+) in the pipette solution. These results indicate that K(+) and Rb(+), but not Cs(+), Li(+), or Na(+), can act as substrates of KCC in LSO neurons.

Paradoxical aggravation of paroxysmal dystonia during chronic treatment with phenobarbital in a genetic rodent model

Recent studies in mutant hamsters (dt(sz)), an animal model of primary paroxysmal dystonia, indicated that altered function of the gamma-aminobutyric acid (GABA)ergic system plays a critical role in the pathogenesis of dystonia. In the present study, dt(sz) hamsters were chronically treated with phenobarbital, which has been found to exert antidystonic effects in mutant hamsters after acute administration. In untreated dt(sz) hamsters, the severity of dystonia follows an age-dependent time course with a maximum between the 30th and 40th day of life, followed by a continuous decline of severity until complete remission occurs at the age of about 70 days. In contrast to acute effects, chronic treatment with phenobarbital via drinking water starting at an age of 21 days (i.e., after weaning) worsened dystonia and retarded the spontaneous remission. The unexpected prodystonic effect was more marked after administration of higher doses and when chronic treatment with phenobarbital started at an age of 1 day (neonatal administration via breast milk). After withdrawal of phenobarbital at the age of 70 days, the severity rapidly declined in all treated groups. When phenobarbital was readministered 1 week later, the hamsters again exhibited severe dystonia. The mechanism of these unexpected findings is unknown. Tentatively, activity-dependent GABA-mediated excitation caused by chronic treatment with phenobarbital may be important for the prodystonic effects under pathological conditions in dt(sz) hamsters.

GABA(A)-mediated toxicity of hippocampal neurons in vitro

In the present study, we examined whether the elevation of GABA by gamma-vinyl-GABA protects cultured rat fetal hippocampal neurons against toxicity induced by a 20-min incubation with 100 microM L-glutamate. Neither a 24-h pretreatment nor posttreatment with gamma-vinyl-GABA (100 microM) had any neuroprotective effects, as determined by counting microtubule-associated protein-2 positive cells and lactate dehydrogenase assay 24 h after the glutamate treatment. Unexpectedly, gamma-vinyl-GABA alone induced a 20% loss of microtubule-associated protein-2-positive cells in a culture that was grown in medium containing 25 mM KCl. The toxic effect of gamma-vinyl-GABA was mimicked by a 24-h treatment with GABA (100 microM) and the GABA(A) receptor agonist, muscimol (10 microM), but not the GABA(B) receptor agonist, baclofen (10 microM). The GABA(A) receptor antagonist, bicuculline (10 microM), protected against gamma-vinyl-GABA and GABA-evoked toxicity. Neither gamma-vinyl-GABA nor GABA was toxic in culture medium containing 15 mM KCl. These data indicate that, under depolarizing conditions, an increased GABA level is toxic for a subpopulation of developing hippocampal neurons in vitro. The effect is GABA(A) receptor-mediated. These data provide a new view for understanding neurodegenerative processes, and raise a question of the safety of therapies aimed at increasing GABA concentration following brain insults, especially in immature brains.

Agents which block potassium-chloride cotransport prevent sound-triggered seizures in post-ischemic audiogenic seizure-prone rats

The purpose of this study was to determine whether the loop diuretics furosemide, bumetanide and ethacrynic acid, which block the KCC1 potassium-chloride transporter in the kidney loop of Henle and the KCC2 potassium-chloride transporter in neuronal membranes, would prevent sound-triggered seizures in post-ischemic audiogenic seizure-prone rats. The rats were infused with the test agent via tail vein shortly before being tested for seizure susceptibility by exposure to loud noise (an alarm bell) for 60 s. Sound exposures were repeated at intervals to determine the time course of the seizure suppression effect. All three loop diuretics suppressed sound-triggered seizures in post-ischemic rats tested 2 days to 4 weeks after the ischemic exposure. Furosemide 200 mg/kg had no effect in 4/4 rats made acutely audiogenic seizure-prone by infusion of bicuculline into the inferior colliculus, indicating that the effect was not due to general anti-seizure activity. Mannitol 2 g/kg had no effect in 6/6 post-ischemic rats, indicating that the effect was not due to diuresis or fluid shifts. These results are consistent with the hypothesis that the exposure to global ischemia caused an upregulation of the potassium-chloride transporter KCC2 in neurons which persisted for at least 4 weeks.

Physiology and pharmacology of native glycine receptors in developing rat ventral tegmental area neurons

Properties of whole-cell glycine-activated currents (I(Gly)) of neurons freshly isolated from ventral tegmental area of rats between postnatal day (P) 0 and 40 (P0-P40) were examined using the gramicidin-perforated patch technique. I(Gly) was present throughout the period of investigation and displayed age-related alternations in its kinetics, reversal potentials (E(Gly)) and sensitivity to antagonists. During development, the time constant for desensitization of I(Gly) decreased about two-fold, and the E(Gly) shifted to more negative potentials. Specifically, E(Gly) was -29 mV in neonatal (P0-P7) and decreased to -50 mV in adult (P24-P40) neurons. While the neonatal glycine receptors (GlyRs) are resistant to strychnine, the adult GlyRs are sensitive to STR. The values for 50% inhibition (IC(50)) of I(Gly) was 110 and 12 nM in P0-P3 and P24-P40 neurons, respectively. Picrotoxin, on the other hand, was more effective in blocking neonatal neurons. The IC(50) values were 17 and 231 microM for the neonatal and adult neurons, respectively. These observations are discussed in terms of developmental changes of subunit composition of GlyRs and of intracellular Cl(-) concentrations.

Afferent regulation of inhibitory synaptic transmission in the developing auditory midbrain

To determine whether afferent innervation regulates the strength of inhibitory connections in the gerbil auditory midbrain, both cochleas were surgically removed in postnatal day 7 animals, before sound-driven activity is first observed. Inhibitory synaptic currents were measured in a brain slice preparation 1-7 d after the ablations. Whole-cell and gramicidin-perforated patch recordings were obtained from inferior colliculus neurons, and IPSCs were evoked by stimulation of the commissure of the inferior colliculus (CIC) or the ipsilateral lateral lemniscus (LL) in the presence of kynurenic acid. Deafferentation led to a 24 mV depolarizing shift in the IPSC equilibrium potential within 1 d of deafferentation. As a consequence, there was a large reduction of IPSC amplitude at a holding potential of -20 mV in neurons from bilaterally ablated animals. Furthermore, both afferent pathways displayed a 50% reduction of the inhibitory synaptic conductance after deafferentation, indicating that driving force was not solely responsible for the decline in IPSC amplitude. When paired pulses were delivered to the LL or CIC pathway in control neurons, the evoked IPSCs exhibited facilitation. However, paired pulse facilitation was nearly eliminated after deafferentation. Thus, normal innervation affects inhibitory synaptic strength by regulating postsynaptic chloride homeostasis and presynaptic transmitter release properties.

Chronic NMDA exposure accelerates development of GABAergic inhibition in the superior colliculus

Maturation of excitatory synaptic connections depends on the amount and pattern of their activity, and activity can affect development of inhibitory synapses as well. In the superficial visual layers of the superior colliculus (sSC), developmental increases in the effectiveness of gamma-aminobutyric acid (GABA(A)) receptor-mediated inhibition may be driven by the maturation of visual inputs. In the rat sSC, GABA(A) receptor currents significantly jump in amplitude between postnatal days 17 and 18 (P17 and P18), approximately when the effects of cortical inputs are first detected in collicular neurons. We manipulated the development of these currents in vivo by implanting a drug-infused slice of the ethylene-vinyl acetate copolymer Elvax over the superior colliculus of P8 rats to chronically release from this plastic low levels of N-methyl-D-aspartate (NMDA). Sham-treated control animals received a similar implant containing only the solvent for NMDA. To examine the effects of this treatment on the development of GABA-mediated neurotransmission, we used whole cell voltage-clamp recording of spontaneous synaptic currents (sPSCs) from sSC neurons in untreated, NMDA-treated, and sham-treated superior colliculus slices ranging in age from 10 to 20 days postnatal. Both amplitude and frequency of sPSCs were studied at holding potentials of +50 mV in the presence and absence of the GABA(A) receptor antagonist, bicuculline methiodide (BMI). The normal developmental increase in GABA(A) receptor currents occurred on schedule (P18) in sham-treated sSC, but NMDA treatment caused premature up-regulation (P12). The average sPSCs in early NMDA-treated neurons were significantly larger than in age-matched sham controls or in age-matched, untreated neurons. No differences in average sPSC amplitudes across treatments or ages were present in BMI-insensitive, predominantly glutamatergic synaptic currents of the same neurons. NMDA treatment also significantly increased levels of glutamate decarboxylase (GAD), measured by quantitative western blotting with staining at P13 and P19. Cell counting using the dissector method for MAP 2 and GAD(67) at P13 and P19 indicated that the differences in GABAergic transmission were not due to increases in the proportion of inhibitory to excitatory neurons after NMDA treatment. However, chronic treatments begun at P8 with Elvax containing both NMDA and BMI significantly decreased total neuron density at P19 ( approximately 15%), suggesting that the NMDA-induced increase in GABA(A) receptor currents may protect against excitotoxicity.

Localization and developmental expression patterns of the neuronal K-Cl cotransporter (KCC2) in the rat retina

The processing of signals by integrative neurons in the retina and CNS relies strongly on inhibitory synaptic inputs, principally from GABAergic and glycinergic neurons that serve primarily to hyperpolarize postsynaptic neurons. Recent evidence indicates that the neuron-specific K-Cl cotransporter 2 (KCC2) is the major chloride extrusion system permitting hyperpolarizing inhibitory responses. It has been hypothesized that depolarizing GABA responses observed in immature neurons are converted to hyperpolarizing responses in large part by the expression of KCC2 during the second week of postnatal development. The cell-specific localization and developmental expression of KCC2 protein have been examined in relatively few neural tissues and have never been studied in retina, of which much is known physiologically and morphologically about inhibitory synaptic circuits. We examined the localization of KCC2 in adult rat retina with immunohistochemical techniques and determined the time course of its postnatal expression. KCC2 expression was localized in horizontal cells, bipolar cells, amacrine cells, and, most likely, ganglion cells, all of which are known to express GABA receptor subtypes. Developmentally, KCC2 expression in the retina increased gradually from postnatal day 1 (P1) until P14 in the inner retina, whereas expression was delayed in the outer plexiform layer until P7 but reached its adult level by P14. These data support the hypothesis that the function of KCC2 is intimately involved in GABAergic synaptic processing. Furthermore, the delayed temporal expression of KCC2 in the outer plexiform layer indicates that GABAergic function may be differentially regulated in retina during postnatal development and that GABA may produce depolarizing responses in the outer plexiform layer at times when it generates hyperpolarizing responses in the inner plexiform layer.

Fast network oscillations in the newborn rat hippocampus in vitro

Spontaneous neural activity is crucial for the formation of the intricate patterns of cortical connectivity during development. In particular, temporal correlations in presynaptic and postsynaptic activity have been hypothesized to be a critical determinant in the selection of neurons that are to become wired together. To date, however, temporally correlated activity in the neonatal brain has been believed to take place with a precision of tens of milliseconds to seconds. Here we describe a novel type of a fast network oscillation associated with millisecond synchronization of pyramidal cell firing in newborn rat hippocampus in vitro. Individual pyramidal neurons fired mainly at lower gamma frequencies (20-40 Hz) but were synchronized into a high-frequency (100-400 Hz) population oscillation that was reflected in field potential spikes and intracellular AMPA-kainate receptor-mediated currents. The high-frequency population oscillation was patterned by a gamma-frequency modulatory oscillation. The gamma modulation was imposed by GABAergic currents, which exerted an inhibitory action on pyramidal neurons. Patterned activity based on GABAergic inhibition and glutamatergic excitation thus occurs already in newborn hippocampus. The network oscillations described here may be a mechanism for selective coincidence detection with a millisecond range temporal precision to shape the patterns of connectivity within the emerging hippocampal synaptic circuitry.