Excitatory actions of GABA in developing rat hypothalamic neurones

Developmental changes in the expression of GABAA receptor subunits alpha1, alpha2, and alpha3 in brain stem nuclei of rats

Gamma-aminobutyric acid (GABA)(A) receptor subunit switching is a suggested postnatal mechanism for changes in GABA transmission from depolarization to hyperpolarization. Previously, we found an apparent switch between GABA(A) alpha3 and alpha1 subunit expression in the rat pre-Bötzinger complex (PBC) on postnatal day (P) 12, a presumed peak critical period of respiratory nuclei development. The present study aimed at determining if GABA(A) subunit switching occurred in another respiratory nucleus, the ventrolateral subnucleus of the solitary tract nucleus (NTS(VL)), and in a non-respiratory cuneate nucleus (CN) of P0 to P21 rats. In both nuclei: (1) the expression of GABA(A) alpha1 subunit was relatively low at birth but increased with development; (2) that of GABA(A) alpha3 was relatively high at birth but declined with age; and (3) GABA(A) alpha2 remained relatively low and constant throughout development. However, the specific patterns differed between the two nuclei, but were similar between the NTS(VL) and the PBC. In the NTS(VL), GABA(A) alpha1 expression gradually increased from birth and peaked at P12, whereas that in the CN sharply rose from P7 and peaked at P10. GABA(A) alpha3 expression had a prominent decrease from P11 to P12 in the NTS(VL), whereas that in the CN only gradually declined from P10 to P12. The developmental trends of alpha1 and alpha3 in the NTS(VL) intersected close to P12, whereas those in the CN intersected at P10. Despite differences in timing, GABA(A) alpha subunit switching may be a common theme in the brain stem that may mediate different functional properties of GABA transmission.

Selective impairment of GABAergic synaptic transmission in the flurothyl model of neonatal seizures

Neonatal seizures can result in long-term adverse consequences including alteration of seizure susceptibility and impairment in spatial memory. However, little is known about the effects of neonatal seizures on developmental changes occurring in synaptic transmission during the first postnatal weeks. The purpose of the present study was to examine the effect of neonatal seizures on several aspects of gamma-aminobutyric acid (GABA)ergic and glutamatergic synaptic transmission in the developing rat hippocampus. Flurothyl was used to induce multiple recurrent seizures in rat pups during the first postnatal days. Whole-cell patch-clamp recordings from the hippocampal CA3 pyramidal cell and extracellular recordings from the CA3 pyramidal cell layer were made in slice preparations. In rats that experienced neonatal seizures the amplitude of spontaneous inhibitory postsynaptic currents at P15-17 was decreased by 27% compared with controls, whereas neither frequency nor the kinetic properties were altered. Neonatal seizures did not affect the timing of the developmental switch in the GABAA signaling from excitatory to inhibitory. None of the studied parameters of glutamatergic postsynaptic currents was different between the flurothyl and control groups, including the amplitude and frequency of the spontaneous excitatory postsynaptic currents, the ratio of the amplitudes and frequencies of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA)-mediated spontaneous postsynaptic currents, and the kinetics of AMPA and NMDA mediated postsynaptic currents in the age groups P8-10 and P15-17. We suggest that the selective depression of the amplitude of GABAergic synaptic responses may contribute to the adverse neurological and behavioral consequences that occur following neonatal seizures.

A shared vesicular carrier allows synaptic corelease of GABA and glycine

The type of vesicular transporter expressed by a neuron is thought to determine its neurotransmitter phenotype. We show that inactivation of the vesicular inhibitory amino acid transporter (Viaat, VGAT) leads to embryonic lethality, an abdominal defect known as omphalocele, and a cleft palate. Loss of Viaat causes a drastic reduction of neurotransmitter release in both GABAergic and glycinergic neurons, indicating that glycinergic neurons do not express a separate vesicular glycine transporter. This loss of GABAergic and glycinergic synaptic transmission does not impair the development of inhibitory synapses or the expression of KCC2, the K+ -Cl- cotransporter known to be essential for the establishment of inhibitory neurotransmission. In the absence of Viaat, GABA-synthesizing enzymes are partially lost from presynaptic terminals. Since GABA and glycine compete for vesicular uptake, these data point to a close association of Viaat with GABA-synthesizing enzymes as a key factor in specifying GABAergic neuronal phenotypes.

GABAergic signaling in young granule cells in the adult rat and mouse dentate gyrus

Throughout most of the developing brain, including the hippocampus, GABAergic synapses are the first to become functional. Several features of GABAergic signaling change across development, suggesting that this signaling in the immature brain may play important roles in the growth of young neurons and the establishment of networks. To determine whether GABA(A) receptor (GABA(A)R)-containing synapses in new neurons born in the adult dentate gyrus have similar immature features, we examined spontaneous and evoked GABA(A)R-mediated synaptic currents in young (POMC-EGFP or doublecortin-immunostained) granule cells in acute slice preparations from adult mice and rats. Spontaneous inhibitory postsynaptic currents (IPSCs) were observed in nearly all immature granule cells, but their frequency was considerably lower and their decay time constant was nearly two times longer than in neighboring mature (doublecortin-non-immunoreactive or EGFP-non-expressing) granule cells within the sub-granular zone. Evoked IPSCs (eIPSCs) in mature granule cells, but not immature granule cells, were sensitive to zolpidem, suggesting a maturational increase in GABA(A)R alpha1-subunit expression. Perforated-patch recording revealed that eIPSCs depolarized young neurons, but hyperpolarized mature neurons. The early establishment of synaptic GABAergic inputs slow IPSC decay time, and depolarizing action of eIPSCs are remarkably similar to features previously seen in neurons during development, suggesting that they are intrinsic features of immature neurons and not functions of the surrounding circuitry. These developmental features in adult-born granule cells could play a role in maturational processes such as developmental cell death. However, treatment of adult mice with GABA(A)R agonists and an inverse agonist did not significantly alter the number of 4- to 14-day-old BrdU-labeled cells.

GABA(B) receptor-mediated modulation of hypocretin/orexin neurones in mouse hypothalamus

Hypocretin/orexin (Hcrt) is a critical neurotransmitter for the maintenance of wakefulness and has been implicated in several other functions, including energy metabolism and reward. Using whole-cell patch-clamp recordings from transgenic mice in which enhanced green fluorescent protein was linked to the Hcrt promoter, we investigated GABAergic control of the Hcrt neurones in hypothalamic slices. Bath application of GABA or muscimol caused an early hyperpolarization mediated by Cl(-) and a late depolarization mediated by the efflux of bicarbonate. These GABA(A) receptor-mediated responses were blocked by picrotoxin and bicuculline. Under the GABA(A) blockade condition, GABA produced consistent hyperpolarization, decreased firing rate and input resistance. The selective GABA(B) agonist (R)-baclofen caused a similar response with an EC(50) of 7.1 mum. The effects of (R)-baclofen were blocked by the GABA(B) antagonist CGP 52432 but persisted in the presence of tetrodotoxin, suggesting direct postsynaptic effects. The existence of GABA(B) modulation was supported by GABA(B(1)) subunit immunoreactivity on Hcrt cells colabelled with antisera to the Hcrt-2 peptide. Furthermore, GABA(B) receptor activation inhibited the presynaptic release of both glutamate and GABA. (R)-Baclofen depressed the amplitude of evoked excitatory postsynaptic currents (EPSCs) and inhibitory synaptic currents (IPSCs), and also decreased the frequency of both spontaneous and miniature EPSCs and IPSCs with a modest effect on their amplitudes. These data suggest that GABA(B) receptors modulate Hcrt neuronal activity via both pre- and postsynaptic mechanisms, which may underlie the promotion of non-rapid eye movement sleep and have implications for the use of GABA(B) agonists in the treatment of substance addiction through direct interaction with the Hcrt system.

Late onset of NMDA receptor-mediated ventilatory control during early development in zebrafish (Danio rerio)

Increased ventilation frequency (fV) in response to hypoxia in adult fish depends on ionotropic N-methyl-D-aspartate (NMDA) receptors. Nonetheless, the ontogeny of central control mechanisms mediating hypoxic ventilatory chemoreflexes in lower vertebrates has not been studied. Therefore, the aim of this study was to determine when the hypoxic ventilatory response during zebrafish (Danio rerio) development is mediated via NMDA receptors, by performing physiological experiments and western blot analysis of NMDA receptor subunits. Zebrafish larvae at stages 4-16 days post-fertilisation (dpf) were exposed to an hypoxic pulse in control groups and in groups treated with MK801 (NMDA receptor antagonist). The hypoxic increase in fV was present at all larval stages, and it matured during development. The reflex became MK801 sensitive at 8 dpf, but did not completely rely on a glutamatergic transmission until 13 dpf. This, together with changing subunit composition during the different stages (increasing amounts of NMDAR1 subunits and appearance of NMDAR2A subunits in adults), suggests that the amount of functional NMDA receptors needed to achieve a fully developed reflex is not attained until later stages. Furthermore, our results suggest that other non-NMDA receptor mechanisms are responsible for the hypoxia-induced increase in fV during the earlier developmental stages.

Electrophysiological properties and thermosensitivity of mouse preoptic and anterior hypothalamic neurons in culture

Responses of mouse preoptic and anterior hypothalamic neurons to variations of temperature are key elements in regulating the setpoint of homeotherms. The goal of the present work was to assess the relevance of culture preparations for investigating the cellular mechanisms underlying thermosensitivity in hypothalamic cells. Our working hypothesis was that some of the main properties of preoptic/anterior hypothalamic neurons in culture are similar to those reported by other authors in slice preparations. Indeed, cultured preoptic/anterior hypothalamic neurons share many of the physiological and morphological properties of neurons in hypothalamic slices. They display heterogenous dendritic arbors and somatic shapes. Most of them are GABAergic and their activity is synaptically driven by the activation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate receptors. Active membrane properties include a depolarizing "sag" in response to hyperpolarization, and a low threshold spike, which is present in a majority of cells and is generated by T-type Ca2+ channels. In a fraction of the cells, the low threshold spike repeats rhythmically, either spontaneously, or in response to depolarization. The background synaptic noise in cultured neurons is characterized by the presence of numerous postsynaptic potentials which can be easily distinguished from the baseline, thus providing an opportunity for assessing their possible roles in thermosensitivity. An unexpected finding was that GABA-A receptors can generate both hyper- and depolarizing postsynaptic potentials in the same neuron. About 20% of the spontaneously firing preoptic/anterior hypothalamic neurons are warm-sensitive. Warming (32-41 degrees C) depolarizes some cells, a phenomenon which is Na+-dependent and tetrodotoxin-insensitive. The increased firing rate of warm-sensitive cells in response to warming can be prepotential and/or synaptically driven. Overall, our data suggest that a warm-sensitive phenotype is already developed in cultured cells. Therefore, and despite obvious differences in their networks, cultured and slice preparations of hypothalamic neurons can complement each other for further studies of warm-sensitivity at the cellular and molecular level.

Initiation and Propagation of Neuronal Coactivation in the Developing Hippocampus

Correlated neuronal activity is ubiquitous in developing nervous systems, where it may introduce spatiotemporal coherence and contribute to the organization of functional circuits. In this report, we used voltage-sensitive dyes and optical imaging to examine the spatiotemporal pattern of a spontaneous network activity, giant depolarizing potentials (GDPs), in rat hippocampal slices during the first postnatal week. The propagation pattern of the GDP is closely correlated to the anatomical organization of the network. In the hilus, where mossy cells and interneurons are not organized in layers, GDPs propagate at the same velocity in all directions. In CA3 and CA1, the activation is synchronous along the axis of the pyramidal cells' dendritic tree. The velocity of wave propagation is significantly different in three hippocampal subfields: it is slowest in the hilus, faster in CA3, and fastest in CA1. The velocity of horizontal propagation (along the axis of the pyramidal layer) has a large variation from trial to trial, suggesting that the horizontal velocity is determined to some extent by dynamic network factors. Imaging revealed that each GDP event is initiated from a small focus. The location of the initiation focus differs from event to event. All together, our data suggest that GDP is a propagating excitation wave, initiated from a small site, and propagating to the whole hippocampus. The spatiotemporal patterns of the wave in CA3 and CA1 areas show better synchrony along the pyramidal cell dendritic trees and progressive activation along the axis of the pyramidal cell layer.

Theta bursts set up glutamatergic as well as GABA-ergic plasticity in neonatal rat hippocampal CA1 neurons

gamma-Aminobutyric acid (GABA) is inhibitory in adult, but excitatory in neonatal, neurons. The switch from excitatory to inhibitory action is due to a negative shift in the equilibrium potential for the GABA(A) receptor-mediated postsynaptic current (E(GABA-PSC)). Here, we report that, in neonatal rat hippocampal CA1 neurons, presynaptic theta-burst activation induces not only a shift in E(GABA-PSC) towards that in adult neurons, but also a recruitment of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-receptor-mediated postsynaptic currents.

Shift from depolarizing to hyperpolarizing glycine action occurs at different perinatal ages in superior olivary complex nuclei

The inhibitory transmitters glycine and GABA undergo a developmental shift from depolarizing to hyperpolarizing action (D/H-shift). To analyse this shift in functionally related nuclei of the rat superior olivary complex (SOC), we employed voltage-sensitive dye recordings in auditory brainstem slices. Complementarily, we analysed single neurons in gramicidin perforated-patch recordings. Our results show a differential timing of the D/H-shift in the four SOC nuclei analysed. In the medial superior olive (MSO), the shift occurred at postnatal day (P) 5-9. In the superior paraolivary nucleus (SPN), it occurred between embryonic day (E) 18 and P1. No D/H-shift was observed in the medial nucleus of the trapezoid body (MNTB) until P10. This is in line with the finding that most of the patched MNTB neurons displayed glycine-induced depolarizations between P0-9. While no regional differences regarding the D/H-shift were found within the MSO, SPN, and MNTB, we observed such differences in the lateral superior olive (LSO). All LSO regions showed a D/H-shift at P4-5. However, in the high-frequency regions, hyperpolarizations were large already at P6, yet amplitudes of this size were not present until P8 in the low-frequency regions, suggesting a delayed development in the latter regions. Our physiological results demonstrate that D/H-shifts in SOC nuclei are staggered in time and occur over a period of almost two weeks. Membrane-associated immunoreactivity of the Cl- outward transporter KCC2 was found in every SOC nucleus already at times when glycine was still depolarizing. This implies that the mere presence of KCC2 does not correlate with functional Cl- outward transport.

Activity-mediated shift in reversal potential of GABA-ergic synaptic currents in immature neurons

Gamma-aminobutyric acid (GABA), which is inhibitory in the adult central nervous system, can be excitatory in the developing brain. The change from excitatory to inhibitory action of GABA during development is caused by a negative shift in its reversal potential. Here, we report a presynaptic activity-mediated negative shift in the reversal potential of the GABA-mediated synaptic currents in immature deep cerebellar nuclei neurons. This shift appears to be due to an increased expression and activation of the K+-Cl- co-transporter type 2 (KCC-2) through the activation of protein kinase A, protein synthesis and activation of protein phosphatases. Thus, maturation of the GABA response may rely on an activity-dependent up-regulation of KCC-2.

Simultaneous glutamate and GABA(A) receptor agonist administration increases calbindin levels and prevents hippocampal damage induced by either agent alone in a model of perinatal brain injury

Perinatal brain injury is associated with the release of amino acids, principally glutamate and GABA, resulting in massive increases in intracellular calcium and eventual cell death. We have previously demonstrated that independent administration of kainic acid (KA), an AMPA/kainate receptor agonist, or muscimol, a GABA(A) receptor agonist, to newborn rats results in hippocampal damage [Hilton, G.D., Ndubuizu, A., and McCarthy, M.M., 2004. Neuroprotective effects of estradiol in newborn female rat hippocampus. Dev. Brain Res. 150, 191-198; Hilton, G. D., Nunez, J.L. and McCarthy, M.M., 2003. Sex differences in response to kainic acid and estradiol in the hippocampus of newborn rats. Neuroscience. 116, 383-391; Nunez, J.L. and McCarthy, M.M., 2003. Estradiol exacerbates hippocampal damage in a model of preterm infant brain injury. Endocrinology. 144, 2350-2359; Nunez, J.L., Alt, J.J. and McCarthy, M.M., 2003. A new model for prenatal brain damage. I. GABA(A) receptor activation induces cell death in developing rat hippocampus. Exp. Neurol. 181, 258-269]. We now report that KA or muscimol alone administered to immature hippocampal neurons in culture induces significant cell death as evidenced by TUNEL assay. Surprisingly, simultaneous administration of equimolar quantities of these two agonists blocks the effect of either one alone. Moreover, treatment of newborn pups results in less damage compared to either muscimol or KA alone. We further observed that immunoreactivity for the calcium-binding protein, calbindin D(28K), is increased in the brains of pups simultaneously administered KA and muscimol as compared to either alone.

Sex differences in estrogenic regulation of neuronal activity in neonatal cultures of ventromedial nucleus of the hypothalamus

Estrogenic effects have been implicated in sexual differentiation of brain and behavior, in part by affecting neuronal activity in the ventromedial nucleus of the hypothalamus (VMN). We report here a remarkable sex difference in estrogenic regulation of neuronal activity in male vs. female neural networks. Spontaneous synaptic currents originating from a population of neurons were recorded in primary VMN cultures using the whole-cell patch-clamp technique. Treatment with 17beta-estradiol (E2, 10 nM) for 24 h induced opposite effects in the two sexes: the frequency of spontaneous synaptic events decreased significantly in neurons derived from males but increased in those from females. Interestingly, the 24-hour E2 effect was partially reversed by an acute application (5 min) of a second dose of E2 (10 nM), suggesting an interaction between extended (24-hr) and acute (5-min) effects of E2 in VMN neurons. To understand the underlying mechanism of this sexually dimorphic action of E2, we analyzed the E2 effect on GABAergic neurotransmission by recording miniature inhibitory postsynaptic currents. After 24-hour E2 treatment, both the amplitude and frequency of miniature inhibitory postsynaptic currents increased in neurons derived from males but decreased in those from females. These results suggest that E2-induced changes in GABAergic inhibition could at least partially explain E2 effects on neuronal activity. We conclude that E2 may have sexually dimorphic effects on the synaptic output of VMN neurons by modulating GABAergic neurotransmission.

The regulation of alpha-MSH release by GABA is mediated by a chloride-dependent [Ca2+]c increase in frog melanotrope cells

In frog melanotrope cells, gamma-aminobutyric acid (GABA) induces a biphasic effect, i.e. a transient stimulation followed by a more sustained inhibition of alpha-MSH release, and both phases of the GABA effect are mediated by GABAA receptors. We have previously shown that the stimulatory phase evoked by GABAA receptor agonists can be accounted for by calcium entry. In the present study, we have investigated the involvement of the chloride flux on GABA-induced [Ca2+]c increase and alpha-MSH release. We show that GABA evokes a concentration-dependent [Ca2+]c rise through specific activation of the GABAA receptor. The GABA-induced [Ca2+]c increase results from opening of voltage-activated L- and N-type calcium channels, and sodium channels. Variations of the extracellular Cl- concentration revealed that GABA-induced [Ca2+]c rise and alpha-MSH release both depend on the Cl- flux direction and driving force. These observations suggest for the first time that GABA-gated Cl- efflux provokes an increase in [Ca2+]c increase that is responsible for hormone secretion.

Activation of presynaptic GABAA receptors increases spontaneous glutamate release onto noradrenergic neurons of the rat locus coeruleus

In order to further explore how GABA can modulate the excitability of noradrenergic neurons of the locus coeruleus (LC), we investigated the presence of GABA(A) receptors on glutamatergic nerve terminals and the functional consequences of their activation. We used mechanically dissociated immature rat LC neurons with adherent nerve terminals and patch-clamp recordings of spontaneous excitatory postsynaptic currents. Activation of presynaptic GABA(A) receptors by muscimol facilitated spontaneous glutamate release by activating tetrodotoxin-sensitive Na(+) channels and high-threshold Ca(2+) channels. Bumetanide (10 microM), a potent blocker of Na(+)-K(+)-Cl(-) cotransporter, diminished the muscimol-induced facilitatory action of glutamate release. Our results indicate that the Na(+)-K(+)-Cl(-) cotransporter accumulates Cl(-) inside the nerve terminals so that activation of presynaptic GABA(A) receptors causes depolarization. This GABA(A)-receptor-mediated modulation of spontaneous glutamatergic transmission is another mechanism by which GABA and its analogues can regulate the excitability and activity of noradrenergic neurons in the LC.

Glycinergic and GABAergic synaptic activity differentially regulate motoneuron survival and skeletal muscle innervation

GABAergic and glycinergic synaptic transmission is proposed to promote the maturation and refinement of the developing CNS. Here we provide morphological and functional evidence that glycinergic and GABAergic synapses control motoneuron development in a region-specific manner during programmed cell death. In gephyrin-deficient mice that lack all postsynaptic glycine receptor and some GABA(A) receptor clusters, there was increased spontaneous respiratory motor activity, reduced respiratory motoneuron survival, and decreased innervation of the diaphragm. In contrast, limb-innervating motoneurons showed decreased spontaneous activity, increased survival, and increased innervation of their target muscles. Both GABA and glycine increased limb-innervating motoneuron activity and decreased respiratory motoneuron activity in wild-type mice, but only glycine responses were abolished in gephyrin-deficient mice. Our results provide genetic evidence that the development of glycinergic and GABAergic synaptic inputs onto motoneurons plays an important role in the survival, axonal branching, and spontaneous activity of motoneurons in developing mammalian embryos.

Feed-forward facilitation of glutamate release by presynaptic GABA(A) receptors

Disynaptic GABAergic inputs from Schaffer collateral (SC) afferents on to the soma of glutamatergic CA1 pyramidal neurons are involved in feed-forward inhibition in the hippocampal neural circuits. Here we report the functional roles of presynaptic GABA(A) receptors on SC afferents projecting to CA1 pyramidal neurons. Muscimol (0.5 microM), a selective GABA(A) receptor agonist, increased SC-evoked EPSC amplitude and decreased paired-pulse ratio in the slice preparation, in addition, it facilitated spontaneous glutamate release on to mechanically dissociated CA1 pyramidal neurons in an external Ca2+-dependent manner. In field recordings, muscimol at low concentrations (< or = 0.5 microM) increased not only the excitability of SC afferents but glutamate release, however, it at high concentrations (> or = 1 microM) changed bidirectionally. These results suggest that the moderate activation of presynaptic GABA(A) receptors depolarizes SC afferents and enhances SC-mediated glutamatergic transmission. When endogenous GABA was disynaptically released by brief trains of stimulation of SC afferents, the axonal excitability in addition to glutamate release was increased. The effects of endogenous GABA on the excitability of SC afferents were blocked by either SR95531 or AMPA receptor blockers, which would be expected to block disynaptic feed-forward neural circuits. The present results provide a novel form of presynaptic modulation (feed-forward facilitation) of glutamatergic transmission by presynaptic GABA(A) receptors within the intrinsic hippocampal neural circuits.

Expression and developmental regulation of the K+-Cl- cotransporter KCC2 in the cochlear nucleus

KCC2 is a neuron-specific Cl- transporter whose role in adult central neurons is to maintain low intracellular Cl- concentrations and, therefore, generate an inward-directed electrochemical gradient for Cl- needed for the hyperpolarizing responses to the inhibitory amino acids GABA and glycine. We report that the KCC2 protein is intensely expressed in CN neurons and preferentially associated with plasma membrane domains, consistent with GABA and glycinergic-mediated inhibition in this auditory nucleus. Postnatal KCC2 expression and distribution patterns are similar in developing and adult CN neurons and do not match the time course of GABergic or glycinergic synaptogenesis. Therefore, in the CN, neither KCC2 protein upregulation nor progressive integration in the plasma membrane seem to be involved in KCC2 developmental regulation. Considering that GABA and glycine are depolarizing during early postnatal development, it is conceivable that KCC2 is in place but inactive during early postnatal development in the CN and becomes active as inhibitory synaptogenesis proceeds. This notion is supported by the finding that the phosphorylation state of KCC2 differs from developing to adult CN, with the phosphorylated form predominating in the latter.

Anticonvulsant action of GABA in the high potassium-low magnesium model of ictogenesis in the neonatal rat hippocampus in vivo and in vitro

Previous developmental studies in vitro suggested that the inhibitory neurotransmitter GABA exerts depolarizing and excitatory actions on the immature neurons and that depolarizing GABA is causally linked to ictal activity during the first weeks of postnatal life. However, remarkably little is known on the role of GABA in the generation of neonatal seizures in vivo. Here, using extracellular recordings from CA3 hippocampus, we studied the effects of GABA(A)-acting drugs on electrographic seizures induced by local intrahippocampal injection of the epileptogenic agents (high K(+)/low Mg(2+)) in the nonanesthetized rats in vivo and in the hippocampal slices in vitro during the second postnatal week (postnatal days P8-12). We found that in vivo, the induction of ictal-like events was facilitated by co-infusion of high-K(+)/low Mg(2+) together with the GABA(A) antagonist bicuculline or gabazine. Moreover, the infusion of bicuculline alone caused ictal-like activity in approximately 30% of cases. Co-infusion of the GABA(A) receptor agonist isoguvacine or the GABA(A)-positive allosteric modulator diazepam completely prevented high-K(+)/low Mg(2+)-induced seizures. In in vitro studies using hippocampal slices, we also found that high-K(+)/low Mg(2+) produced ictal activity that was exacerbated by bicuculline and gabazine and reduced by isoguvacine. Thus in the model of high-K(+)/low Mg(2+)-induced seizures both in in vivo and in vitro conditions, GABA, acting via GABA(A) receptors, has an anticonvulsant effect during the critical developmental period of enhanced excitability.

Activity-dependent matching of excitatory and inhibitory inputs during refinement of visual receptive fields

The receptive field (RF) of single visual neurons undergoes progressive refinement during development. It remains largely unknown how the excitatory and inhibitory inputs on single developing neurons are refined in a coordinated manner to allow the formation of functionally correct circuits. Using whole-cell voltage-clamp recording from Xenopus tectal neurons, we found that RFs determined by excitatory and inhibitory inputs in more mature tectal neurons are spatially matched, with each spot stimulus evoking balanced synaptic excitation and inhibition. This emerges during development through a gradual reduction in the RF size and a transition from disparate to matched topography of excitatory and inhibitory inputs to the tectal neurons. Altering normal spiking activity of tectal neurons by either blocking or elevating GABA(A) receptor activity significantly impeded the developmental reduction and topographic matching of RFs. Thus, appropriate inhibitory activity is essential for the coordinated refinement of excitatory and inhibitory connections.

In situ Characterization of Gonadotropin- Releasing Hormone-I, -III, and Glutamic Acid Decarboxylase Expression in the Brain of the Sea Lamprey, Petromyzon marinus

The distribution of lamprey gonadotropin-releasing hormone (GnRH)-I and -III has been extensively characterized by immunocytochemistry in the forebrain of the sea lamprey, Petromyzon marinus. However, the cellular location of lamprey GnRH-III mRNA expression by in situ hybridization in the lamprey brain has not been determined. We show for the first time the location of expression of lamprey GnRH-III, as well as provide a more comprehensive in situ study of lamprey GnRH-I and glutamic acid decarboxylase (GAD; GABA-synthesizing enzyme) mRNA expression in the brain of the lamprey in different reproductive life stages. Colorimetric and dual-label fluorescent amplification methods of in situ hybridization were used on brain tissue sections of adult, juvenile, and larval sea lamprey. In each life stage of the lamprey, expression of lamprey GnRH-I was shown in the preoptic area (POA) and the hypothalamus forming the characteristic arc-like cell population extending from the preoptic nucleus (NPO) to the neurohypophysis. Lamprey GnRH-III expression was also seen in the POA of each life stage in close proximity to lamprey GnRH-I mRNA containing neurons. GAD expression was shown in distinct cell clusters in and around the POA, in the olfactory bulb, in the dorsal thalamus beneath the habenular region, and also in the ventral-medial hypothalamus stretching from the periventricular region to the anterior portion of the rhombencephalon. Using dual-label in situ hybridization, we have shown that lamprey GnRH-I and -III mRNA are colocalized in the same cells in the POA in adult lampreys. Dual-label in situ hybridization also showed close proximity of GAD mRNA containing neurons and GnRH containing neurons in the POA. These data suggest that gamma-aminobutyric acid (GABA) may directly affect GnRH release in the brain of the sea lamprey.

Development of GABAergic and glycinergic transmission in the neonatal rat dorsal horn

Cutaneous spinal sensory transmission appears to lack inhibitory control in the newborn spinal cord, but the properties of GABAergic and glycinergic synapses in the neonatal dorsal horn have not been characterized. Whole-cell patch-clamp recordings from rat superficial dorsal horn neurons in spinal cord slices at postnatal day 0 (P0) to P2, P6-P7, and P13-P14 revealed an age-dependent increase in the frequency of spontaneous IPSCs, which were abolished by the GABA(A) receptor (GABA(A)R) antagonist bicuculline between P0 and P7 but not at P14. GABA(A)R-mediated miniature IPSCs (mIPSCs), but not glycinergic mIPSCs, were present at birth, and GABA mIPSCs remained more frequent than glycine mIPSCs at all ages. Sciatic nerve stimulation resulted in IPSCs with both GABAergic and glycinergic components, although a larger contribution arose from GABA(A) receptors at all ages. In gramicidin perforated patch-clamp recordings, exogenous GABA applications produced depolarization in 40% of neurons at P0-P2, but the reversal potential of GABA-evoked currents (E(GABA)) was consistently more negative than action potential threshold at this age. By P6-P7, GABA evoked only membrane hyperpolarization. The GABA(B)R agonist baclofen elicited an outward current in all neurons with peak amplitudes observed by P6-P7 and abolished sciatic nerve-evoked monosynaptic glutamatergic EPSCs in all groups. The results show considerable postnatal development of inhibitory processing in the dorsal horn with GABAergic mechanisms initially dominant over glycinergic events. GABA(A)R-mediated depolarizations during the first postnatal week are likely to be important for the maturation of spinal networks but do not provide a major excitatory drive to the newborn dorsal horn.

Etomidate elevates intracellular calcium levels and promotes catecholamine secretion in bovine chromaffin cells

Etomidate, an intravenous imidazole general anaesthetic, is thought to produce anaesthesia by modulating or activating ionotropic Cl(-)-permeable GABA(A) receptors. Chromaffin cells are known to express functional GABA(A) receptors with properties similar to their neuronal counterparts. We have shown that activation of the GABA(A) receptors, with specific GABA(A) agonists, leads to cellular excitation. Our goal was to determine whether etomidate mimicked this response and to explore the functional consequences of this activation. Imaging experiments with the Ca(2+)-indicator dye fura-2 were used to assay [Ca(2+)](i). Bovine adrenal chromaffin cells were superfused with a variety of GABA(A)-selective drugs to determine their effects on [Ca(2+)](i). Amperometric measurements were used to assay catecholamine release in real-time. We show that bovine adrenal chromaffin cells were excited by etomidate at clinically relevant concentrations. Etomidate directly activated GABA(A) receptors found in chromaffin cells thereby elevating [Ca(2+)](i). The effects of etomidate were mimicked by the specific GABA(A) agonist muscimol and blocked by the specific antagonist bicuculline. Our data show that low concentrations of etomidate modulated GABA(A) receptor activation by muscimol. Blockade of voltage-dependent Ca(2+) channels prevented the elevation of [Ca(2+)](i) by GABA. Application of etomidate directly to the chromaffin cells elicited robust catecholamine secretion from these cells. The data indicate that clinically relevant concentrations of etomidate can directly activate GABA(A) receptors, which, due to the positive anion equilibrium potential, depolarizes chromaffin cells. This depolarization activates voltage-dependent Ca(2+) channels thereby stimulating catecholamine release. Our data suggest that circulating catecholamine levels may be elevated after etomidate application.

GABAA-receptor-mediated increase in intracellular Ca2+ concentration in the regenerating retina of adult newt

We used optical recording with the Ca(2+)-sensitive dye, fura-2, in living slice preparations from the newt retina at different stages of regeneration. gamma-Aminobutyric acid (GABA) induced pronounced [Ca(2+)](i) rise in progenitor cells and differentiating ganglion cells in the 'intermediate' stage of retinal regeneration. This [Ca(2+)](i) rise became less pronounced at the beginning of synapse formation in the late regenerating retina. At the late period of the late regenerating retina with the IPL thickness comparable to that of the control retina, GABA-induced [Ca(2+)](i) rise became undetectable or sometimes a small decrease in [Ca(2+)](i) was observed in regenerated ganglion cells. In contrast, N-methyl-d-aspartate (NMDA)-induced [Ca(2+)](i) rise appeared in premature ganglion cells and became prominent gradually as the regeneration proceeded. The [Ca(2+)](i) rise to GABA was mediated by GABA(A) receptors. This was shown by inhibition of GABA-induced Ca(2+) response with the preincubation of the GABA(A) receptor antagonist, bicuculline. The [Ca(2+)](i) rise due to GABA was suppressed in the absence of extracellular Ca(2+) or in the presence of the L-type voltage-gated Ca(2+) channel blocker, verapamil, suggesting that Ca(2+) may be entered through L-type Ca(2+) channels. Transient appearance of [Ca(2+)](i) rise to GABA during regeneration and origin of GABA-induced [Ca(2+)](i) rise were similar to those in the developing retina [J. Neurobiol. 24 (1993) 1600]. These similarities may suggest that common mechanisms may control neurogenesis and/or synaptogenesis during development and regeneration.

Developmental patterns and plasticities: the hippocampal model

Because developmental activity-dependent synaptic plasticity has been hypothesized to participate in network refinement, leading to the precise mapping of synaptic contacts constituting a functional brain, it is important to investigate the spatio-temporal structure of immature network activities. This article is briefly reviewing 15 years of studies on the immature rat hippocampus which, together with recent results obtained from awake rat pups, represent an important step toward the understanding of spontaneous patterns of activity and their potential implication in network maturation. Due to synergistic excitatory actions of GABA and glutamate receptor mediated signals during early postnatal life, spontaneous patterns of hippocampal activity that have been characterized both in vitro and in vivo are likely to provide hebbian modulation of developing glutamatergic and GABAergic synapses. Together with studies on trophic actions of these transmitters, study of the immature hippocampal network patterns and plasticities allows for multiple technical and conceptual approaches and represents an interesting experimental model for development studies.

Developmental changes in GABAergic actions and seizure susceptibility in the rat hippocampus

The immature brain is prone to seizures but the underlying mechanisms are poorly understood. We explored the hypothesis that increased seizure susceptibility during early development is due to the excitatory action of GABA. Using noninvasive extracellular field potential and cell-attached recordings in CA3 of Sprague-Dawley rat hippocampal slices, we compared the developmental alterations in three parameters: excitatory actions of GABA, presence of the immature pattern of giant depolarizing potentials (GDPs) and severity of epileptiform activity generated by high potassium. The GABA(A) receptor agonist isoguvacine increased firing of CA3 pyramidal cells in neonatal slices while inhibiting activity in adults. A switch in the GABA(A) signalling from excitation to inhibition occurred at postnatal day (P) 13.5 +/- 0.4. Field GDPs were present in the form of spontaneous population bursts until P12.7 +/- 0.3. High potassium (8.5 mm) induced seizure-like events (SLEs) in 35% of slices at P7-16 (peak at P11.3 +/- 0.4), but only interictal activity before and after that age. The GABA(A) receptor antagonist bicuculline reduced the frequency or completely blocked SLEs and induced interictal clonic-like activity accompanied by a reduction in the frequency but an increase in the amplitude of the population spikes. In slices with interictal activity, bicuculline typically caused a large amplitude interictal clonic-like activity at all ages; in slices from P5-16 rats it was often preceded by one SLE at the beginning of bicuculline application. These results suggest that, in the immature hippocampus, GABA exerts dual (both excitatory and inhibitory) actions and that the excitatory component in the action of GABA may contribute to increased excitability during early development.

Structural Effects and Neurofunctional Sequelae of Developmental Exposure to Psychotherapeutic Drugs: Experimental and Clinical Aspects

The advent of psychotherapeutic drugs has enabled management of mental illness and other neurological problems such as epilepsy in the general population, without requiring hospitalization. The success of these drugs in controlling symptoms has led to their widespread use in the vulnerable population of pregnant women as well, where the potential embryotoxicity of the drugs has to be weighed against the potential problems of the maternal neurological state. This review focuses on the developmental toxicity and neurotoxicity of five broad categories of widely available psychotherapeutic drugs: the neuroleptics, the antiepileptics, the antidepressants, the anxiolytics and mood stabilizers, and a newly emerging class of nonprescription drugs, the herbal remedies. A brief review of nervous system development during gestation and following parturition in mammals is provided, with a description of the development of neurochemical pathways that may be involved in the action of the psychotherapeutic agents. A thorough discussion of animal research and human clinical studies is used to determine the risk associated with the use of each drug category. The potential risks to the fetus, as demonstrated in well described neurotoxicity studies in animals, are contrasted with the often negative findings in the still limited human studies. The potential risk fo the human fetus in the continued use of these chemicals without more adequate research is also addressed. The direction of future research using psychotherapeutic drugs should more closely parallel the methodology developed in the animal laboratories, especially since these models have already been used extremely successfully in specific instances in the investigation of neurotoxic agents.

Ontogenic changes of the GABAergic system in the embryonic mouse spinal cord

Numerous studies have demonstrated an excitatory action of GABA early in development, which is likely to play a neurotrophic role. In order to better understand the role of GABA in the mouse spinal cord, we followed the evolution of GABAergic neurons over the course of development. We investigated, in the present study, the ontogeny of GABA immunoreactive (GABA-ir) cell bodies and fibers in the embryonic mouse spinal cord at brachial and lumbar levels. GABA-ir somata were first detected at embryonic day 11.5 (E11.5) exclusively at brachial level in the marginal zone. By E13.5, the number of GABAergic neurons sharply increased throughout the extent of the ventral horn both at brachial and lumbar level. Stained perikarya first appeared in the future dorsal horn at E15.5 and progressively invaded this area while they decreased in number in the presumed ventral gray matter. At E12.5, E13.5 and E15.5, we checked the possibility that ventral GABA-ir cells could belong to the motoneuronal population. Using a GABA/Islet-1/2 double labeling, we did not detect any double-stained neurons indicating that spinal motoneurons do not synthesize GABA during the course of development. GABA-ir fibers also appeared at the E11.5 stage in the presumptive lateral white matter at brachial level. At E12.5 and E13.5, GABA-ir fibers progressively invaded the ventral marginal zone and by E15.5 reached the dorsal marginal zone. At E17.5 and postnatal day 0 (P0), the number of GABA-ir fibers declined in the white matter. Finally, by P0, GABA immunoreactivity that delineated somata was mainly restricted to the dorsal gray matter and declined in intensity and extent. The ventral gray matter exhibited very few GABA-ir cell bodies at this neonatal stage of development. The significance of the migration of somatic GABA immunoreactivity from ventral to the dorsal gray matter is discussed.

Experience-dependent changes in intracellular Cl− regulation in developing auditory neurons

A developmental change in GABA and glycine responses, from a depolarization to a hyperpolarization, have been reported for a range of CNS neurons, and has been demonstrated to be due to a developmental decrease in the intracellular Cl- concentration ([Cl-](i)). We examined [Cl-](i) in isolated rat lateral superior olive (LSO) neurons using patch-clamp recordings of glycine gated Cl- currents and by measuring intracellular Cl- -fluorescence. In neurons from 14-16-day-old rats (P14-P16), which had previously received unilateral or bilateral cochlear ablations before the onset of hearing, there was no developmental decrease in [Cl-](i). No significant differences in [Cl-](i) were observed amongst rats with either ipsi- and contralateral ablations. Implanted strychnine pellets also prevented the decrease in [Cl-](i) in most neurons. In some of these neurons in which [Cl-](i) remained high, there was a lack of expression of the K+-Cl- cotransporter 2 (KCC2) mRNA. These results demonstrate that the developmental decrease in [Cl-](i) in LSO neurons is dependent on neuronal activity and that both GABAergic/glycinergic and glutamatergic afferent activity contribute to this maturation of the Cl- regulatory mechanisms.

Developmental changes in the expression of GABAA receptor subunits alpha1, alpha2, and alpha3 in the rat pre-Botzinger complex

Previously, we reported that the pre-Bötzinger complex (PBC) exhibited a dramatic reduction in cytochrome oxidase activity at postnatal day (P) 12. This coincided in time with decreases in glutamate and NMDA receptor subunit 1 and increases in GABA, GABAB, glycine receptor, and glutamate receptor GluR2. To test our hypothesis that various alpha-subunits of GABAA receptors also undergo changes in their expression during postnatal development, as they do in other brain regions, we undertook an in-depth immunohistochemical study of GABAA receptor subunits alpha1, alpha2, and alpha3 in the PBC of P0 to P21 rats. We found that 1) GABAA alpha3-subunit was expressed at relatively high levels at P0, which then declined with age; 2) GABAA alpha1-subunit was expressed at relatively low levels at P0 but increased with age; 3) the developmental trends of subunits alpha1 and alpha3 intersected at P12; and 4) GABAA alpha2-subunit expression was moderate to light at P0 and remained quite constant during development, being lowest at P21. These findings suggest that the apparent switch in relative expressions of subunits alpha3 and alpha1 during development and the intersection of slopes around P12 may be associated with possible changes in GABAA receptor subtypes that would mediate different functional properties of GABA transmission, such as primarily a less efficient inhibitory transmission before P12 and a more mature inhibitory effect at P12 and thereafter, as suggested by the kinetics of distinct postsynaptic potentials. This mechanism may contribute partially to the dramatic reduction in cytochrome oxidase activity within the PBC at P12, as shown previously.

Role of Cl- co-transporters in the excitation produced by GABAA receptors in juvenile bovine adrenal chromaffin cells

GABA is the primary inhibitory neurotransmitter in the adult mammalian brain. However, in neonatal animals, activation of Cl(-)-permeable GABA receptors is excitatory and appears to depend on the expression of a Na(+)-K(+)-2Cl- cotransporter (NKCC) that elevates intracellular Cl- levels, leading to a depolarized Cl- equilibrium potential (ECl). The change from excitation to inhibition appears to involve the expression of the K+/Cl- co-transporter, KCC2, which lowers intracellular Cl- levels resulting in a hyperpolarized ECl. In this study, we show that bovine chromaffin cells from 4- to 5-mo-old animals are excited by GABA. Activation of GABAA receptors depolarizes the cells, opens voltage-dependent Ca2+ channels, elevates [Ca2+]i, and promotes the release of catecholamines. Blockade of voltage-dependent Ca2+ channels prevents the elevation of [Ca2+]i by GABA. The extrapolated anion reversal potential in these cells is approximately -28 mV, indicating a resting intracellular anion concentration of approximately 50 mM. Expression of KCC2 protein was not detected in the juvenile chromaffin cells. In contrast, clear expression of NKCC1 was observed. Blockade of NKCC1 should reduce the intracellular Cl- concentration and hyperpolarize ECl. Bumetanide, an NKCC1 blocker, reduced the elevation of [Ca2+]i by GABA. In some cells, activation of GABAA receptors inhibits responses to excitatory neurotransmitters, even though GABA itself is depolarizing. Co-activation of cholinergic and GABAA receptors in chromaffin cells produced elevations in [Ca2+]i that were comparable to those produced by cholinergic receptors alone. Our data showing the selective expression of chloride co-transporters and the resulting strongly depolarized anion reversal potential may help explain how activation of GABAA receptors causes sufficient excitation to elicit catecholamine release from chromaffin cells.

Cyclothiazide potently inhibits gamma-aminobutyric acid type A receptors in addition to enhancing glutamate responses

Ionotropic glutamate and gamma-aminobutyric acid type A (GABAA) receptors mediate critical excitatory and inhibitory actions in the brain. Cyclothiazide (CTZ) is well known for its effect of enhancing glutamatergic transmission and is widely used as a blocker for alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptor desensitization. Here, we report that in addition to its action on AMPA receptors, CTZ also exerts a powerful but opposite effect on GABAA receptors. We found that CTZ reversibly inhibited both evoked and spontaneous inhibitory postsynaptic currents, as well as GABA application-induced membrane currents, in a dose-dependent manner. Single-channel analyses revealed further that CTZ greatly reduced the open probability of GABAA receptor channels. These results demonstrate that CTZ interacts with both glutamate and GABAA receptors and shifts the excitation-inhibition balance in the brain by two independent mechanisms. Understanding the molecular mechanism of this double-faceted drug-receptor interaction may help in designing new therapies for neurological diseases.

Ontogeny of gamma-aminobutyric acid-immunoreactive neurons in the rhombencephalon and spinal cord of the sea lamprey

The development of neurons expressing gamma-aminobutyric acid (GABA) in the rhombencephalon and spinal cord of the sea lamprey (Petromyzon marinus) was studied for the first time with an anti-GABA antibody. The earliest GABA-immunoreactive (GABAir) neurons appear in late embryos in the basal plate of the isthmus, caudal rhombencephalon, and rostral spinal cord. In prolarvae, the GABAir neurons of the rhombencephalon appear to be distributed in spatially restricted cellular domains that, at the end of the prolarval period, form four longitudinal GABAir bands (alar dorsal, alar ventral, dorsal basal, and ventral basal). In the spinal cord, we observed only three GABAir longitudinal bands (dorsal, intermediate, and ventral). The larval pattern of GABAir neuronal populations was established by the 30-mm stage, and the same populations were observed in premetamorphic and adult lampreys. The ontogeny of GABAergic populations in the lamprey rhombencephalon and spinal cord is, in general, similar to that previously described in mouse and Xenopus.

GABA receptor-mediated inhibition of neuronal activity in rat SCN in vitro: pharmacology and influence of circadian phase

The effect of gamma-aminobutyric acid (GABA) on neuronal firing rate in rat suprachiasmatic nucleus (SCN) slices was examined using continuous recording methods. GABA inhibited neuronal discharge during both the subjective day and the subjective night in a concentration-dependent manner characterized by two apparent affinity states. The GABAA receptor agonist muscimol caused potent inhibition regardless of circadian time; repeated applications of the agonist did not reverse the direction of effect. The GABAA receptor antagonists bicuculline and picrotoxin increased excitability when applied during either subjective day or subjective night. A significant increase in GABAA receptor- mediated inhibition, as well as endogenous GABAergic tone, was observed on the second day after slice preparation. The GABAB receptor agonist baclofen inhibited cell firing during subjective day and night, but the GABAB antagonist phaclofen had no significant effect. These data provide additional strong support for a predominantly inhibitory role of GABA in the rat SCN, regardless of the time of application in relation to the circadian rhythm, and demonstrate an important level of plasticity of this system in vitro.

Coincident pre- and postsynaptic activity modifies GABAergic synapses by postsynaptic changes in Cl- transporter activity

Coincident pre- and postsynaptic activation is known to induce long-term modification of glutamatergic synapses. We report here that, in both hippocampal cultures and acute hippocampal slices, repetitive postsynaptic spiking within 20 ms before and after the activation of GABAergic synapses also led to a persistent change in synaptic strength. This synaptic modification required Ca2+ influx through postsynaptic L-type Ca2+ channels and was due to a local decrease in K+-Cl- cotransport activity, effectively reducing the strength of inhibition. Thus, GABAergic synapses can detect and be modified by coincident pre- and postsynaptic spiking, allowing the level of inhibition to be modulated in accordance to the temporal pattern of postsynaptic excitation.

Hyperpolarizing inhibition develops without trophic support by GABA in cultured rat midbrain neurons

During a limited period of early neuronal development, GABA is depolarizing and elevates [Ca2+]i, which mediates the trophic action of GABA in neuronal maturation. We tested the attractive hypothesis that GABA itself promotes the developmental change of its response from depolarizing to hyperpolarizing (Ganguly et al. 2001). In cultured midbrain neurons we found that the GABA response changed from depolarizing to hyperpolarizing, although GABAA receptors had been blocked throughout development. In immature neurons prolonged exposure of the cells to nanomolar concentrations of GABA or brief repetitive applications of GABA strongly diminished the elevation of [Ca+]i by GABA. As revealed by gramicidin perforated-patch recording, reduced [Ca2+]i responses were due to a diminished driving force for Cl-. This suggests that immature neurons do not have an efficient inward transport that can compensate the loss of cytosolic Cl-resulting from sustained GABAA receptor activation by ambient GABA. Transient increases in external K+, which can induce voltage-dependent Cl- entry, restored GABA-induced [Ca2+]i elevations. In mature neurons, GABA reduced [Ca2+]i provided that background [Ca2+]i was elevated by the application of an L-type Ca2+ channel agonist. This was probably due to a hyperpolarization of the membrane by Cl- currents. K(+)-Cl- cotransport maintained the gradient for hyperpolarizing Cl-currents. We conclude that in immature midbrain neurons an inward Cl- transport is not effective although the GABA response is depolarizing. Further, GABA itself is not required for the developmental switch of GABAergic responses from depolarizing to hyperpolarizing in cultured midbrain neurons.

Expression and function of chloride transporters during development of inhibitory neurotransmission in the auditory brainstem

Glycine and GABA, the dominant inhibitory neurotransmitters in the CNS, assume a depolarizing role in early development, leading to increased cytoplasmic Ca2+ levels and action potentials. The effect is thought to be of some significance for maturation. The depolarization is caused by Cl- efflux, and chloride transporters contribute to the phenomenon by raising the intracellular Cl- concentration ([Cl-]i) above equilibrium, thereby generating an outward-directed electrochemical gradient for Cl-. In mature neurons, the [Cl-]i is reduced below equilibrium, thus rendering glycine activity hyperpolarizing. Here, we investigated the temporal expression of the K-Cl cotransporter KCC2 and the Na-K-Cl cotransporter NKCC1 in the lateral superior olive (LSO) of rats and mice. The two cation cotransporters normally extrude and accumulate Cl-, respectively. As evidenced by several methods, KCC2 mRNA was present in LSO neurons during both the depolarizing and hyperpolarizing periods. Western blots confirmed a constant level of KCC2 in the brainstem, and immunohistochemistry showed that the protein is diffusely distributed within neonatal LSO neurons, becoming integrated into the plasma membrane only with increasing age. The glycine reversal potential in KCC2 knock-out mice differed significantly from that determined in wild-type controls at postnatal day 12 (P12) but not at P3, demonstrating that KCC2 is not active in neonates, despite its early presence. NKCC1 mRNA was not detected during the depolarizing phase in the LSO, implying that this transporter does not contribute to the high [Cl-]i. Our results reveal major differences in the development of [Cl-]i regulation mechanisms seen in brainstem versus forebrain regions.

Contribution of excitatory chloride conductance in the determination of the direction of traveling waves in an olfactory center

Traveling waves have been found in the CNS of vertebrates and invertebrates. In the olfactory center [procerebrum (PC)] of the terrestrial slug Limax, periodic waves travel from the apex to the base with a frequency of approximately 0.7 Hz. The oscillation and propagation of waves have been thought to be mediated by the mutual connections of bursting neurons in the PC. The direction of the wave is Cl(-) dependent, because lowering the Cl(-) concentration in the medium reverses the direction. The bursting neurons have a Cl(-) channel-coupled glutamate receptor (GluClR), and, using a calcium imaging technique, the receptor was found to be excitatory. Activation of the GluClR with its selective agonist ibotenate resulted in an increased frequency of the oscillatory neural activity recorded as a periodic local field potential. Depletion of cytoplasmic Cl(-) with Cl(-)-free saline abolished all of the ibotenate-induced effects. Perforated-patch-clamp recording in single PC neurons revealed a spatial difference in the Cl(-)-dependent periodic depolarizations in the bursting neurons, with a higher amplitude in the apical region. These results suggest the involvement of excitatory GluClRs in the unidirectional propagation of waves in the PC.

Expression and function of chloride transporters during development of inhibitory neurotransmission in the auditory brainstem

Glycine and GABA, the dominant inhibitory neurotransmitters in the CNS, assume a depolarizing role in early development, leading to increased cytoplasmic Ca2+ levels and action potentials. The effect is thought to be of some significance for maturation. The depolarization is caused by Cl- efflux, and chloride transporters contribute to the phenomenon by raising the intracellular Cl- concentration ([Cl-]i) above equilibrium, thereby generating an outward-directed electrochemical gradient for Cl-. In mature neurons, the [Cl-]i is reduced below equilibrium, thus rendering glycine activity hyperpolarizing. Here, we investigated the temporal expression of the K-Cl cotransporter KCC2 and the Na-K-Cl cotransporter NKCC1 in the lateral superior olive (LSO) of rats and mice. The two cation cotransporters normally extrude and accumulate Cl-, respectively. As evidenced by several methods, KCC2 mRNA was present in LSO neurons during both the depolarizing and hyperpolarizing periods. Western blots confirmed a constant level of KCC2 in the brainstem, and immunohistochemistry showed that the protein is diffusely distributed within neonatal LSO neurons, becoming integrated into the plasma membrane only with increasing age. The glycine reversal potential in KCC2 knock-out mice differed significantly from that determined in wild-type controls at postnatal day 12 (P12) but not at P3, demonstrating that KCC2 is not active in neonates, despite its early presence. NKCC1 mRNA was not detected during the depolarizing phase in the LSO, implying that this transporter does not contribute to the high [Cl-]i. Our results reveal major differences in the development of [Cl-]i regulation mechanisms seen in brainstem versus forebrain regions.

Early establishment of multiple release site connectivity between interneurons and pyramidal neurons in the developing hippocampus

The strength of the synaptic transmission between two neurons critically depends on the number of release sites connecting the neurons. Here we examine the development of connectivity between gamma-aminobutyric acid (GABA)ergic interneurons and CA1 pyramidal neurons in the hippocampus. GABAergic postsynaptic currents (PSCs) were recorded in whole-cell voltage-clamped CA1 pyramidal neurons. By comparing spontaneous and miniature (action potential-independent) GABAergic PSCs, we found that multiple release site connectivity is established already at the first postnatal day and that the degree of connectivity remains unaltered into adulthood. During the same time there is a dramatic increase in the number of GABAergic synapses on each pyramidal neuron as indicated by the increase in frequency of miniature GABAergic PSCs. These results indicate that during development a given interneuron contacts an increasing number of target pyramidal neurons but with the same multiple release site connectivity. It has been shown previously that the connectivity between CA3 and CA1 pyramidal neurons is initially restricted to one release site, and develops gradually. The present result thus suggests different mechanisms to govern the maturation of excitatory and inhibitory synaptic transmissions.

Contribution of excitatory chloride conductance in the determination of the direction of traveling waves in an olfactory center

Traveling waves have been found in the CNS of vertebrates and invertebrates. In the olfactory center [procerebrum (PC)] of the terrestrial slug Limax, periodic waves travel from the apex to the base with a frequency of approximately 0.7 Hz. The oscillation and propagation of waves have been thought to be mediated by the mutual connections of bursting neurons in the PC. The direction of the wave is Cl(-) dependent, because lowering the Cl(-) concentration in the medium reverses the direction. The bursting neurons have a Cl(-) channel-coupled glutamate receptor (GluClR), and, using a calcium imaging technique, the receptor was found to be excitatory. Activation of the GluClR with its selective agonist ibotenate resulted in an increased frequency of the oscillatory neural activity recorded as a periodic local field potential. Depletion of cytoplasmic Cl(-) with Cl(-)-free saline abolished all of the ibotenate-induced effects. Perforated-patch-clamp recording in single PC neurons revealed a spatial difference in the Cl(-)-dependent periodic depolarizations in the bursting neurons, with a higher amplitude in the apical region. These results suggest the involvement of excitatory GluClRs in the unidirectional propagation of waves in the PC.

Excitatory actions of endogenously released GABA contribute to initiation of ictal epileptiform activity in the developing hippocampus

In the developing rat hippocampus, ictal epileptiform activity can be elicited easily in vitro during the first three postnatal weeks. Changes in neuronal ion transport during this time cause the effects of GABA(A) receptor (GABA(A)-R) activation to shift gradually from strongly depolarizing to hyperpolarizing. It is not known whether the depolarizing effects of GABA and the propensity for ictal activity are causally linked. A key question is whether the GABA-mediated depolarization is excitatory, which we defined operationally as being sufficient to trigger action potentials. We assessed the effect of endogenous GABA on ictal activity and neuronal firing rate in hippocampal slices from postnatal day 1 (P1) to P30. In extracellular recordings, there was a strong correlation between the postnatal age at which GABA(A)-R antagonists decreased action potential frequency (P23) and the age at which ictal activity could be induced by elevated potassium (P23). In addition, there was a strong correlation between the fraction of slices in which ictal activity was induced by elevated potassium concentrations and the fractional decrease in action potential firing when GABA(A)-Rs were blocked in the presence of ionotropic glutamate receptor antagonists. Finally, ictal activity induced by elevated potassium was blocked by the GABA(A)-R antagonists bicuculline and SR-95531 (gabazine) and increased in frequency and duration by GABA(A)-R agonists isoguvacine and muscimol. Thus, the propensity of the developing hippocampus for ictal activity is highly correlated with the effect of GABA on action potential probability and reversed by GABA(A) antagonists, indicating that GABA-mediated excitation is causally linked to ictal activity in this developmental window.

Rescue of developing spinal motoneurons from programmed cell death by the GABA(A) agonist muscimol acts by blockade of neuromuscular activity and increased intramuscular nerve branching

Blockade of neuromuscular activity in the chick embryo during the period of programmed cell death of motoneurons results in a complete rescue of these cells. Understanding the cellular mechanisms that mediate this counterintuitive effect is of considerable interest with respect to the regulation of motoneuron survival during development as well as for understanding why motoneurons die pathologically. Although considerable evidence supports the role of a peripheral site of action at the neuromuscular junction in mediating the rescue of motoneurons following activity blockade, some evidence also supports a role for central nervous system (CNS) neurons. For example, the rescue of motoneurons by curare has been reported to be blocked by the GABA(A) agonist muscimol via its actions on CNS neurons. We have carried out a series of studies to further investigate this interesting observation. Surprisingly, we find that: (1) muscimol blocks activity and rescues MNs in a dose-dependent manner, similar to curare; (2) muscimol's effects on MN survival appear to be mediated by its action on intramuscular nerve branching, similar to curare; and (3) although muscimol acts centrally, the effects of muscimol on MN survival and axon branching are mediated peripherally at the neuromuscular junction, similar to curare. Because muscimol reduces MN depolarization these data also suggest that the depolarization of MNs by afferents is not required for promoting MN survival. Taken together, these data provide further evidence in support of a peripheral site of action of activity blockade in rescuing motoneurons from developmental cell death.

Excitatory actions of endogenously released GABA contribute to initiation of ictal epileptiform activity in the developing hippocampus

In the developing rat hippocampus, ictal epileptiform activity can be elicited easily in vitro during the first three postnatal weeks. Changes in neuronal ion transport during this time cause the effects of GABA(A) receptor (GABA(A)-R) activation to shift gradually from strongly depolarizing to hyperpolarizing. It is not known whether the depolarizing effects of GABA and the propensity for ictal activity are causally linked. A key question is whether the GABA-mediated depolarization is excitatory, which we defined operationally as being sufficient to trigger action potentials. We assessed the effect of endogenous GABA on ictal activity and neuronal firing rate in hippocampal slices from postnatal day 1 (P1) to P30. In extracellular recordings, there was a strong correlation between the postnatal age at which GABA(A)-R antagonists decreased action potential frequency (P23) and the age at which ictal activity could be induced by elevated potassium (P23). In addition, there was a strong correlation between the fraction of slices in which ictal activity was induced by elevated potassium concentrations and the fractional decrease in action potential firing when GABA(A)-Rs were blocked in the presence of ionotropic glutamate receptor antagonists. Finally, ictal activity induced by elevated potassium was blocked by the GABA(A)-R antagonists bicuculline and SR-95531 (gabazine) and increased in frequency and duration by GABA(A)-R agonists isoguvacine and muscimol. Thus, the propensity of the developing hippocampus for ictal activity is highly correlated with the effect of GABA on action potential probability and reversed by GABA(A) antagonists, indicating that GABA-mediated excitation is causally linked to ictal activity in this developmental window.

Induction of NMDA and GABAA receptor-mediated Ca2+ oscillations with KCC2 mRNA downregulation in injured facial motoneurons

To clarify the changes that occur in gamma-aminobutyric acid type A (GABA(A)) receptor-mediated effects and contribute to alterations in the network activities after neuronal injury, we studied intracellular Ca(2+) concentration ([Ca(2+)](i)) dynamics in a rat facial-nerve-transection model. In facial motoneurons, an elevation of the resting [Ca(2+)](i), GABA-mediated [Ca(2+)](i) transients, enhancement of the glutamate-evoked [Ca(2+)](i) increases, and spontaneous [Ca(2+)](i) oscillations were induced by axotomy. All these axotomy-induced modifications were abolished by the GABA(A)-receptor antagonist bicuculline and N-methyl-d-aspartate (NMDA)-receptor antagonist d(-)-2-amino-5-phosphonopentanoic acid. A downregulation of K(+)-Cl(-) cotransporter (KCC2) mRNA, an increase in intracellular Cl(-) concentration ([Cl(-)](i)), and transformation of GABAergic hyperpolarization to depolarization were also induced by axotomy. We suggest that in axotomized neurons KCC2 downregulation impairs Cl(-) homeostasis and makes GABA act depolarizing, resulting in endogenous GABA inducing [Ca(2+)](i) oscillations via facilitation of NMDA-receptor activation. Such GABA(A)-receptor-mediated [Ca(2+)](i) oscillations may play a role in neural survival and regeneration.

Characterization of the circuits that generate spontaneous episodes of activity in the early embryonic mouse spinal cord

In the developing nervous system, patterned spontaneous activity affects a variety of developmental processes. Thus, it is important to identify the earliest time that such activity occurs and to characterize the underlying circuitry. In isolated mouse spinal cord-limb preparations, highly rhythmic spontaneous activity occurred as early as embryonic day 11 (E11)-E12, when many lumbosacral motoneurons were still migrating and extending their peripheral projections. This activity required both electrical and chemical transmission, and acetylcholine, rather than glutamate, provided the main excitatory drive. Our data are consistent with motoneurons themselves playing a critical role in generating such activity by making excitatory connections on each other and on GABAergic interneurons via dihydro-beta-erythroidine hydrobromide (DHbetaE)-insensitive nicotinic receptors. This resulted in the generation of local bursts. Consistent with these observations, E12-E12.5 mouse motoneurons retrogradely labeled by HRP were observed to have extensive axon collaterals that projected locally within the lateral motor column and to interneuron-containing regions dorsal and medial of the lateral motor column. Cholinergic axons, presumably from motoneurons, were also observed in the ventral and lateral funiculi. However, for local bursts to propagate throughout the cord, a second DHbetaE-sensitive cholinergic pathway that also involved glycinergic interneurons was required. This circuit characterization should facilitate the use of genetic mutations that alter specific subpopulations of interneurons or cholinergic transmission to determine how modifying different aspects of this early activity affects subsequent development of the spinal motor circuit.

Characterization of the circuits that generate spontaneous episodes of activity in the early embryonic mouse spinal cord

In the developing nervous system, patterned spontaneous activity affects a variety of developmental processes. Thus, it is important to identify the earliest time that such activity occurs and to characterize the underlying circuitry. In isolated mouse spinal cord-limb preparations, highly rhythmic spontaneous activity occurred as early as embryonic day 11 (E11)-E12, when many lumbosacral motoneurons were still migrating and extending their peripheral projections. This activity required both electrical and chemical transmission, and acetylcholine, rather than glutamate, provided the main excitatory drive. Our data are consistent with motoneurons themselves playing a critical role in generating such activity by making excitatory connections on each other and on GABAergic interneurons via dihydro-beta-erythroidine hydrobromide (DHbetaE)-insensitive nicotinic receptors. This resulted in the generation of local bursts. Consistent with these observations, E12-E12.5 mouse motoneurons retrogradely labeled by HRP were observed to have extensive axon collaterals that projected locally within the lateral motor column and to interneuron-containing regions dorsal and medial of the lateral motor column. Cholinergic axons, presumably from motoneurons, were also observed in the ventral and lateral funiculi. However, for local bursts to propagate throughout the cord, a second DHbetaE-sensitive cholinergic pathway that also involved glycinergic interneurons was required. This circuit characterization should facilitate the use of genetic mutations that alter specific subpopulations of interneurons or cholinergic transmission to determine how modifying different aspects of this early activity affects subsequent development of the spinal motor circuit.

Developmental and circadian changes in Ca2+ mobilization mediated by GABAA and NMDA receptors in the suprachiasmatic nucleus

The hypothalamic suprachiasmatic nucleus (SCN) develops as the circadian pacemaker during postnatal life. Although both GABAA and NMDA receptors are expressed in the majority of SCN neurons, postnatal development of their functions has not been analysed. Thus, we studied the receptor-mediated Ca2+ responses in mouse hypothalamic slices prepared on postnatal days (P) 6-16. The NMDA-induced Ca2+ flux was prominent in the SCN and maximal Ca2+ responses in Mg2+-free conditions had no day-night variations in P14-16 mice. At P6-7, extracellular Mg2+ reduced the NMDA-induced Ca2+ flux irrespective of the circadian time whereas, after P9-10, Mg2+ produced a larger reduction at night than during the daytime. Muscimol also significantly increased Ca2+ in the developing SCN. Voltage-sensitive Ca2+ channel blockers inhibited the muscimol-induced Ca2+ increase whereas tetrodotoxin had no effect, suggesting that stimulation of postsynaptic GABAA receptors depolarizes SCN neurons to increase Ca2+. Macroscopic imaging analysis demonstrated a developmental reduction in the muscimol-induced Ca2+ increase preferentially in the nighttime group older than P9-10. The day-night variation in the magnitude of the Ca2+ response was due to two cell populations, one of which exhibited an increase and the other a decrease in Ca2+ in response to muscimol. Because the critical developmental stages for exhibiting day-night variations in the receptor-mediated Ca2+ responses overlapped the maturation of firing rhythms in SCN neurons, the Ca2+ signalling may be necessary for or regulated by the mature circadian clock.

In situ GABAergic modulation of synchronous gonadotropin releasing hormone-1 neuronal activity

Evidence indicates that gonadotropin releasing hormone-1 [GnRH-1, also known as luteinizing hormone releasing hormone (LHRH)] neurons can exhibit synchronized neuroendocrine secretory activity before entrance into the CNS. In this study, we used calcium imaging to evaluate patterns of activity in individual, embryonic, GnRH-1 neurons as well as population dynamics of GnRH-1 neurons in mouse nasal explants maintained for 1 versus 3 weeks. Independent of age, GnRH-1 neurons displayed significant calcium peaks that synchronized at an interval of approximately 20 min across multiple GnRH-1 cells within an explant. Acute tetrodotoxin treatment decreased the amplitude of calcium peaks in individual GnRH-1 neurons and the duration but not the frequency of synchronized activity in the population of GnRH-1 neurons. Acute GABA(B) receptor antagonism increased the frequency of synchronized neuronal activity at both ages, whereas acute GABA(A) receptor antagonism decreased calcium oscillations in individual GNRH-1 cells as well as synchronization of the calcium pulses within the GnRH-1 population at the 1 week time point to background non-GNRH-1 cell levels. These results indicate that developing GnRH-1 neurons rely heavily on GABAergic signaling to initiate synchronized bouts of activity but thereafter, possess an innate capacity for synchronized activity patterns that are modulated by, but not completely dependent on GABAergic signaling.

Changes in Na(+)-K(+)-Cl(-) cotransporter immunoreactivity in the gerbil hippocampus following transient ischemia

We examined alterations in Na(+)-K(+)-Cl(-) cotransporter 1 (NKCC1) immunoreactivity following ischemia. Twelve hours after ischemia, NKCC1 immunoreactivity in the CA1 region and in the hilar region was significantly diminished. Twenty-four hours after ischemia, NKCC1 immunoreactivity was intensified in these hippocampal regions as well as CA2-3. Two days after ischemia, NKCC1 immunoreactivity in the CA1 and the hilar neurons had disappeared, although in the CA2-3 and the granule cell layer NKCC1 immunoreactivities had recovered to the sham level. This finding suggests that NKCC1 may play an important role in the ischemic neuronal injury induced by excitotoxicity as well as neuronal edema.