Postnatal maturation of gamma-aminobutyric acidA and B-mediated inhibition in the CA3 hippocampal region of the rat

GABAB receptor-mediated modulation in the developing dorsal nucleus of the lateral lemniscus

The dorsal nucleus of the lateral lemniscus (DNLL) is a GABAergic, reciprocally connected auditory brainstem structure that continues to develop postnatally in rodents. One key feature of the DNLL is the generation of a strong, prolonged, ionotropic, GABAA receptor-mediated inhibition. Possible GABAB receptor-mediated signalling is unexplored in the DNLL. Here, we used Mongolian gerbils of either sex to describe GABAB receptor-mediated modulation of postsynaptic potassium currents and synaptic inputs in postnatal (P) animals of days 10/11 and 23-28. Throughout development, we observed the presence of a Baclofen-activated GABAB receptor-enhanced potassium outward conductance that is capable of suppressing action potential generation. In P10/11, old gerbils GABAB receptor activation enhances glutamatergic and suppresses ionotropic GABAergic synaptic transmission. During development, this differential modulation becomes less distinct, because in P22-28, old animals Baclofen-activated GABAB receptors rather enhance ionotropic GABAergic synaptic transmission, whereas glutamatergic transmission is both enhanced and suppressed. Blocking GABAB receptors causes an increase in ionotropic GABAergic transmission in P10/11 old gerbils that was independent on stimulation frequency but depended on the type of short-term plasticity. Together with the lack of Baclofen-induced changes in the synaptic paired-pulse ratio of either input type, we suggest that GABAB receptor-mediated modulation is predominantly postsynaptic and activates different signalling cascades. Thus, we argue that in DNLL neurons, the GABAB receptor is a post-synaptically located signalling hub that alters signalling cascades during development for distinct targets.

Differentiation of Myoblasts in Culture: Focus on Serum and Gamma-Aminobutyric Acid

There are many facts about the possible role of gamma-aminobutyric acid (GABA) in the development and differentiation of cells not only in nervous but also in muscle tissue. In the present study, a primary culture of rat skeletal muscle myocytes was used to evaluate the correlation between the content of GABA in the cytoplasm and the processes of myocyte division and their fusion into myotubes. The effect of exogenous GABA on the processes of culture development was also estimated. Since the classical protocol for working with myocyte cultures involves the use of fetal bovine serum (FBS) to stimulate cell division (growth medium) and horse serum (HS) to activate the differentiation process (differentiation medium), the studies were carried out both in the medium with FBS and with HS. It was found that cells grown in medium supplemented with FBS contain more GABA compared to cultures growing in medium supplemented with HS. Addition of exogeneous GABA leads to a decrease in the number of myotubes formed in both media, while the addition of an amino acid to the medium supplemented with HS had a more pronounced inhibitory effect. Thus, we have obtained data indicating that GABA is able to participate in the early stages of skeletal muscle myogenesis by modulating the fusion process.

Interaction of GABAA and GABAB antagonists after status epilepticus in immature rats

Among neurotransmitter systems affected by status epilepticus (SE) in adult rats are both GABAergic systems. To analyze possible changes of GABA_A and GABA_B systems in developing rats lithium-pilocarpine SE was induced at postnatal day 12 (P12). Seizures were elicited by a GABA_A antagonist pentylenetetrazol (PTZ) 3, 6, 9, and 13 days after SE (i.e., in P15, P18, P21, and P25 rats), and their possible potentiation by a GABA_B receptor antagonist CGP46381 was studied. Pilocarpine was replaced by saline in control animals (lithium-paraldehyde [LiPAR]). Pentylenetetrazol in a dose of 50 mg/kg s.c. elicited generalized seizures in nearly all 15-day-old naive rats and in 40% of 18-day-old ones but not in older animals. After SE, PTZ no longer elicited seizures in these two younger groups, i.e., sensitivity of GABA_A system was diminished. The GABA_B antagonist exhibited proconvulsant effect in P15 and P18 SE as well as LiPAR rats returning the incidence of PTZ-induced seizures to values of control animals. A decrease in the incidence of minimal clonic seizures was seen in P21 LiPAR animals; these seizures in the oldest group were not affected. Change of the effect from proconvulsant to anticonvulsant (or at least to no action) took place before postnatal day 21. Both SE and LiPAR animals exhibited similar changes but their intensity differed, effects in LiPAR controls were usually more expressed than in SE rats.

Role of fast inhibitory synaptic transmission in neonatal respiratory rhythmogenesis and pattern formation

Several studies have investigated the general role of chloride-based neurotransmission (GABA_A and glycinergic signaling) in respiratory rhythmogenesis and pattern formation. In several brain regions, developmental alterations in these signaling pathways have been shown to be mediated by changes in cation-chloride cotransporter (CC) expression. For instance, CC expression changes during the course of neonatal development in medullary respiratory nuclei and other brain/spinal cord regions in a manner which decreases the cellular import, and increases the export, of chloride ions, shifting reversal potentials for chloride to progressively more negative values with maturation. In slice preparations of the same, this is related to an excitatory-to-inhibitory shift of GABA_A- and glycinergic signaling. In medullary slices, GABA_A-/glycinergic signaling in the early neonatal period is excitatory, becoming inhibitory over time. Additionally, blockade of the Na⁺/K⁺/2Cl^- cotransporter, which imports these ions via secondary active transport, converts excitatory response to inhibitory ones. These effects have not yet been demonstrated at the individual respiratory-related neuron level to occur in intact (in vivo or in situ) animal preparations, which in contrast to slices, possess normal network connectivity and natural sources of tonic drive. Developmental changes in respiratory rhythm generating and pattern forming pontomedullary respiratory circuitry may contribute to critical periods, during which there exist increased risk for perinatal respiratory disturbances of central, obstructive, or hypoxia/hypercapnia-induced origin, including the sudden infant death syndrome. Thus, better characterizing the neurochemical maturation of the central respiratory network will enhance our understanding of these conditions, which will facilitate development of targeted therapies for respiratory disturbances in neonates and infants.

Long-Term Effects of Early Life Seizures on Endogenous Local Network Activity of the Mouse Neocortex

Understanding the long term impact of early life seizures (ELS) is of vital importance both for researchers and clinicians. Most experimental studies of how seizures affect the developing brain have drawn their conclusions based on changes detected at the cellular or behavioral level, rather than on intermediate levels of analysis, such as the physiology of neuronal networks. Neurons work as part of networks and network dynamics integrate the function of molecules, cells and synapses in the emergent properties of brain circuits that reflect the balance of excitation and inhibition in the brain. Therefore, studying network dynamics could help bridge the cell-to-behavior gap in our understanding of the neurobiological effects of seizures. To this end we investigated the long-term effects of ELS on local network dynamics in mouse neocortex. By using the pentylenetetrazole (PTZ)-induced animal model of generalized seizures, single or multiple seizures were induced at two different developmental stages (P9-15 or P19-23) in order to examine how seizure severity and brain maturational status interact to affect the brain's vulnerability to ELS. Cortical physiology was assessed by comparing spontaneous network activity (in the form of recurring Up states) in brain slices of adult (>5 mo) mice. In these experiments we examined two distinct cortical regions, the primary motor (M1) and somatosensory (S1) cortex in order to investigate regional differences in vulnerability to ELS. We find that the effects of ELSs vary depending on (i) the severity of the seizures (e.g., single intermittent ELS at P19-23 had no effect on Up state activity, but multiple seizures induced during the same period caused a significant change in the spectral content of spontaneous Up states), (ii) the cortical area examined, and (iii) the developmental stage at which the seizures are administered. These results reveal that even moderate experiences of ELS can have long lasting age- and region-specific effects in local cortical network dynamics.

Postsynaptic GABA(B) Receptors Contribute to the Termination of Giant Depolarizing Potentials in CA3 Neonatal Rat Hippocampus

During development, hippocampal CA3 network generates recurrent population bursts, so-called Giant Depolarizing Potentials (GDPs). GDPs are characterized by synchronous depolarization and firing of CA3 pyramidal cells followed by afterhyperpolarization (GDP-AHP). Here, we explored the properties of GDP-AHP in CA3 pyramidal cells using gramicidin perforated patch clamp recordings from neonatal rat hippocampal slices. We found that GDP-AHP occurs independently of whether CA3 pyramidal cells fire action potentials (APs) or remain silent during GDPs. However, the amplitude of GDP-AHP increased with the number of APs the cells fired during GDPs. The reversal potential of the GDP-AHP was close to the potassium equilibrium potential. During voltage-clamp recordings, current-voltage relationships of the postsynaptic currents activated during GDP-AHP were characterized by reversal near the potassium equilibrium potential and inward rectification, similar to the responses evoked by the GABA(B) receptor agonists. Finally, the GABA(B) receptor antagonist CGP55845 strongly reduced GDP-AHP and prolonged GDPs, eventually transforming them to the interictal and ictal-like discharges. Together, our findings suggest that the GDP-AHP involves two mechanisms: (i) postsynaptic GABA(B) receptor activated potassium currents, which are activated independently on whether the cell fires or not during GDPs; and (ii) activity-dependent, likely calcium activated potassium currents, whose contribution to the GDP-AHP is dependent on the amount of firing during GDPs. We propose that these two complementary inhibitory postsynaptic mechanisms cooperate in the termination of GDP.

Postnatal maturation of GABAergic modulation of sensory inputs onto lateral amygdala principal neurons

KEY POINTS

Throughout life, fear learning is indispensable for survival and neural plasticity in the lateral amygdala underlies this learning and storage of fear memories. During development, properties of fear learning continue to change into adulthood, but currently little is known about changes in amygdala circuits that enable these behavioural transitions. In recordings from neurons in lateral amygdala brain slices from infant up to adult mice, we show that spontaneous and evoked excitatory and inhibitory synaptic transmissions mature into adolescence. At this time, increased inhibitory activity and signalling has the ability to restrict the function of excitation by presynaptic modulation, and may thus enable precise stimulus associations to limit fear generalization from adolescence onward. Our results provide a basis for addressing plasticity mechanisms that underlie altered fear behaviour in young animals.

ABSTRACT

Convergent evidence suggests that plasticity in the lateral amygdala (LA) participates in acquisition and storage of fear memory. Sensory inputs from thalamic and cortical areas activate principal neurons and local GABAergic interneurons, which provide feed-forward inhibition that tightly controls LA activity and plasticity via pre- and postsynaptic GABAA and GABAB receptors. GABAergic control is also critical during fear expression, generalization and extinction in adult animals. During rodent development, properties of fear and extinction learning continue to change into early adulthood. Currently, few studies have assessed physiological changes in amygdala circuits that may enable these behavioural transitions. To obtain first insights, we investigated changes in spontaneous and sensory input-evoked inhibition onto LA principal neurons and then focused on GABAB receptor-mediated modulation of excitatory sensory inputs in infant, juvenile, adolescent and young adult mice. We found that spontaneous and sensory-evoked inhibition increased during development. Physiological changes were accompanied by changes in dendritic morphology. While GABAB heteroreceptors were functionally expressed on sensory afferents already early in development, they could only be physiologically recruited by sensory-evoked GABA release to mediate heterosynaptic inhibition from adolescence onward. Furthermore, we found GABAB -mediated tonic inhibition of sensory inputs by ambient GABA that also emerged in adolescence. The observed increase in GABAergic drive may be a substrate for providing modulatory GABA. Our data suggest that GABAB -mediated tonic and evoked presynaptic inhibition can suppress sensory input-driven excitation in the LA to enable precise stimulus associations and limit generalization of conditioned fear from adolescence onward.

Short-term plasticity and modulation of synaptic transmission at mammalian inhibitory cholinergic olivocochlear synapses

The organ of Corti, the mammalian sensory epithelium of the inner ear, has two types of mechanoreceptor cells, inner hair cells (IHCs) and outer hair cells (OHCs). In this sensory epithelium, vibrations produced by sound waves are transformed into electrical signals. When depolarized by incoming sounds, IHCs release glutamate and activate auditory nerve fibers innervating them and OHCs, by virtue of their electromotile property, increase the amplification and fine tuning of sound signals. The medial olivocochlear (MOC) system, an efferent feedback system, inhibits OHC activity and thereby reduces the sensitivity and sharp tuning of cochlear afferent fibers. During neonatal development, IHCs fire Ca²⁺ action potentials which evoke glutamate release promoting activity in the immature auditory system in the absence of sensory stimuli. During this period, MOC fibers also innervate IHCs and are thought to modulate their firing rate. Both the MOC-OHC and the MOC-IHC synapses are cholinergic, fast and inhibitory and mediated by the α9α10 nicotinic cholinergic receptor (nAChR) coupled to the activation of calcium-activated potassium channels that hyperpolarize the hair cells. In this review we discuss the biophysical, functional and molecular data which demonstrate that at the synapses between MOC efferent fibers and cochlear hair cells, modulation of transmitter release as well as short term synaptic plasticity mechanisms, operating both at the presynaptic terminal and at the postsynaptic hair-cell, determine the efficacy of these synapses and shape the hair cell response pattern.

Mechanisms and effects of seizures in the immature brain

The developing immature brain is not simply a small adult brain but rather possesses unique physiological properties. These include neuronal ionic currents that differ markedly from those in the adult brain, typically being longer-lasting and less selective. This enables immature heterogeneous neurons to connect and fire together but at the same time, along with other features may contribute to the enhanced propensity of the developing brain to become epileptic. Indeed, immature neurons tend to readily synchronize and thus generate seizures. Here, we review the differences between the immature and adult brain, with particular focus on the developmental sequence of γ-aminobutyric acid that excites immature neurons while being inhibitory in the normal adult brain. We review the mechanisms underlying the developmental changes to intracellular chloride levels, as well as how epileptiform activity can drive pathologic changes to chloride balance in the brain. We show that regulation of intracellular chloride is one important factor that underlies both the ease with which seizures can be generated and the facilitation of further seizures. We stress in particular the importance of understanding normal developmental sequences and how they are interrupted by seizures and other insults, and how this knowledge has led to the identification of potential novel treatments for conditions such as neonatal seizures.

Anticonvulsant and behavioral effects of GABA(B) receptor positive modulator CGP7930 in immature rats

Possible anticonvulsant action of GABAB receptor positive allosteric modulator CGP7930 was studied in cortical epileptic afterdischarges (ADs) in rat pups 12, 18, and 25 days old. Afterdischarges were induced by six series of stimulation of sensorimotor cortex, and CGP7930 (20 or 40 mg/kgi.p.) was administered after the first AD. In addition, the effects of CGP7930 on sensorimotor performance and behavior in open field and elevated plus maze were assessed. CGP7930 decreased duration of ADs in 12-day-old but not in older rats. Motor phenomena (movements accompanying stimulation and clonic seizures) were not changed. CGP7930 only moderately affected sensorimotor performance, altered slightly spontaneous behavior in the open field, and did not influence behavior in the elevated plus maze in terms of an adaptive form of learning or anxiety-like behavior. Marked anticonvulsant action with subtle deficits in sensorimotor performance in 12-day-old rats suggests a possible use of CGP7930 as an age-specific anticonvulsant.

Contribution of metabotropic GABA(B) receptors to neuronal network construction

In the 1980s, Bowery and colleagues discovered the presence of a novel, bicuculline-resistant and baclofen-sensitive type of GABA receptor on peripheral nerve terminals, the GABA(B) receptor. Since this pioneering work, GABA(B) receptors have been identified in the Central Nervous System (CNS), where they provide an important inhibitory control of postsynaptic excitability and presynaptic transmitter release. GABA(B) receptors have been implicated in a number of important processes in the adult brain such as the regulation of synaptic plasticity and modulation of rhythmic activity. As a result of these studies, several potential therapeutic applications of GABA(B) receptor ligands have been identified. Recent advances have further shown that GABA(B) receptors play more than a classical inhibitory role in adult neurotransmission, and can in fact function as an important developmental signal early in life. Here we summarize current knowledge on the contribution of GABA(B) receptors to the construction and function of developing neuronal networks.

Temporal coding at the immature depolarizing GABAergic synapse

In the developing hippocampus, GABA exerts depolarizing and excitatory actions and contributes to the generation of neuronal network driven giant depolarizing potentials (GDPs). Here, we studied spike time coding at immature GABAergic synapses and its impact on synchronization of the neuronal network during GDPs in the neonatal (postnatal days P2–6) rat hippocampal slices. Using extracellular recordings, we found that the delays of action potentials (APs) evoked by synaptic activation of GABA(A) receptors are long (mean, 65 ms) and variable (within a time window of 10–200 ms). During patch-clamp recordings, depolarizing GABAergic responses were mainly subthreshold and their amplification by persistent sodium conductance was required to trigger APs. AP delays at GABAergic synapses shortened and their variability reduced with an increase in intracellular chloride concentration during whole-cell recordings. Negative shift of the GABA reversal potential (E_GABA) with low concentrations of bumetanide, or potentiation of GABA(A) receptors with diazepam reduced GDPs amplitude, desynchronized neuronal firing during GDPs and slowed down GDPs propagation. Partial blockade of GABA(A) receptors with bicuculline increased neuronal synchronization and accelerated GDPs propagation. We propose that spike timing at depolarizing GABA synapses is determined by intracellular chloride concentration. At physiological levels of intracellular chloride GABAergic depolarization does not reach the action potential threshold and amplification of GABAergic responses by non-inactivating sodium conductance is required for postsynaptic AP initiation. Slow and variable excitation at GABAergic synapse determines the level of neuronal synchrony and the rate of GDPs propagation in the developing hippocampus.

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.

Ethanol modulation of GABAergic transmission: the view from the slice

For almost three decades now, the GABAergic synapse has been the focus of intense study for its putative role in mediating many of the behavioral consequences associated with acute and chronic ethanol exposure. Although it was initially thought that ethanol interacted solely with the postsynaptic GABAA receptors that mediate the majority of fast synaptic inhibition in the mammalian central nervous system (CNS), a number of recent studies have identified novel pre- and postsynaptic mechanisms that may contribute to the acute and long-term effects of ethanol on GABAergic synaptic inhibition. These mechanisms appear to differ in a brain region specific manner and may also be influenced by a variety of endogenous neuromodulatory factors. This article provides a focused review of recent evidence, primarily from in vitro brain slice electrophysiological studies, that offers new insight into the mechanisms through which acute and chronic ethanol exposures modulate the activity of GABAergic synapses. The implications of these new mechanistic insights to our understanding of the behavioral and cognitive effects of ethanol are also discussed.

GABAB receptor subunit mRNAs are differentially regulated in pituitary melanotropes during development and detection of functioning receptors coincides with completion of innervation

This study examines the developmental expression of GABAB receptor subunits (GABAB(1a), GABAB(1b), GABAB(2)) in the pituitary intermediate lobe using in situ hybridization, reverse transcriptase-polymerase chain reaction, immunohistochemistry, and Western blots. Receptor functionality was studied by baclofen-stimulated GTPgammaS binding. In the adult rat pituitary all three transcripts were detected in melanotropes, but not in glia, of the intermediate lobe. No transcripts of any subunit were detected in the neural lobe. Transcripts of GABAB(1a) and GABAB(1b), but not of GABAB(2), were detected in specific subpopulations of cells in the anterior lobe. All three transcripts were detected in melanotropes on gestational day 18 using in situ hybridization. Reverse transcriptase-polymerase chain reactions comparing postnatal day 2 and adult transcript levels in the neurointermediate lobe support in situ hybridization data that GABAB(1a) mRNA levels do not change, GABAB(1b) levels increase, and GABAB(2) levels decrease as the rat matures. Thus, GABAB receptor subunit transcripts are differentially regulated in melanotropes during development. In the adult rat both GABAB(1) and GABAB(2) proteins were detected in the neurointermediate lobe using Western blotting and in melanotropes by immunohistochemistry. Developmentally, GABAB(1) protein was not detected until postnatal day 7, but was clearly expressed by postnatal day 15 while GABAB(2) protein could not be detected until postnatal day 15. Functional receptors were found in the intermediate lobe at postnatal day 15 and in the adult. The demonstration of transcripts for GABAB(1a), GABAB(1b) and GABAB(2) subunits at gestational day 18 contrasted with the failure to detect any protein before postnatal day 7, suggesting that the regulation of GABAB subunit isoforms occurs differentially at both the transcriptional and translational level as development progresses. The disparity in the regulation of the receptor subunits may suggest that GABAB(1) could have other functions besides being part of the GABAB receptor heterodimer.

Seizure spread through the life cycle: optical imaging in combined brain slices from immature, adult, and senile rats in vitro

The semiology of epileptic seizures changes during the lifetime. Hence, it can be assumed that age-related changes in brain plasticity influence the patterns of seizure onset, spread and propagation velocity. We employed the 4-aminopyridine model of epilepsy to study seizure-like events in vitro. Combined entorhinal cortex-hippocampus brain slices from juvenile (10-13 days), adult (2-3 months), and senile (24-27 months) rats were examined using electrophysiological recordings and imaging of intrinsic optical signals. In the juvenile group, seizure onset was multifocal in all slice regions including the hippocampus. Onset in adult animals was confined to the entorhinal cortex and to neocortical regions. In slices from senile animals, there was a preponderance of seizure onsets in the neocortex. Spread patterns were highly variable in the juvenile group and became gradually more monomorph with increasing age. Propagation velocities were highest in the adult group, with maximum values of 1.51 +/- 0.68 mm/s. In the juvenile group, they amounted to 0.97 +/- 0.39 mm/s, and to 1.18 +/- 0.42 mm/s in senile slices. The results of this study indicate that age-related changes in brain plasticity profoundly affect spread patterns, which may contribute to the clinically observed changes in seizure semiology during early childhood, adulthood and senescence.

Further characterization of muscarinic agonist-induced epileptiform bursting activity in immature rat piriform cortex, in vitro

The characteristics of muscarinic acetylcholine receptor agonist-induced epileptiform bursting seen in immature rat piriform cortex slices in vitro were further investigated using intracellular recording, with particular focus on its postnatal age-dependence (P+14-P+30), pharmacology, site(s) of origin and the likely contribution of the muscarinic acetylcholine receptor agonist-induced post-stimulus slow afterdepolarization and gap junction functionality toward its generation. The muscarinic agonist, oxotremorine-M (10 microM), induced rhythmic bursting only in immature piriform cortex slices; however, paroxysmal depolarizing shift amplitude, burst duration and burst incidence were inversely related to postnatal age. No significant age-dependent changes in neuronal membrane properties or postsynaptic muscarinic responsiveness accounted for this decline. Burst incidence was higher when recorded in anterior and posterior regions of the immature piriform cortex. In adult and immature neurones, oxotremorine-M effects were abolished by M1-, but not M2-muscarinic acetylcholine receptor-selective antagonists. Rostrocaudal lesions, between piriform cortex layers I and II, or layer III and endopiriform nucleus in adult or immature slices did not influence oxotremorine-M effects; however, the slow afterdepolarization in adult (but not immature) lesioned slices was abolished. Gap junction blockers (carbenoxolone or octanol) disrupted muscarinic bursting and diminished the slow afterdepolarization in immature slices, suggesting that gap junction connectivity was important for bursting. Our data show that neural networks within layers II-III function as primary oscillatory circuits for burst initiation in immature rat piriform cortex during persistent muscarinic receptor activation. Furthermore, we propose that muscarinic slow afterdepolarization induction and gap junction communication could contribute towards the increased epileptiform susceptibility of this brain area.

Subcellular vesicular aggregations of GABAB R1a and R1b receptors increase with age in neurons of the developing mouse brain

GABA(B) receptors play a critical neuromodulatory role in the central nervous system. It has been suggested that both the functional role and the cellular distribution of GABA(B) receptors in the neuronal network change during post-natal maturation. In the present study, the cellular and subcellular distribution patterns of the GABA(B) R1a/b receptors have been analysed in different brain regions of the mouse using immunocytochemistry with isoform-specific antisera. GABA(B) R1-immunoreactivity (IR) was present from the first post-natal day (P0) on in most regions of the brain. Neurones exhibited diffuse GABA(B) R1-IR labelling throughout somata and larger proximal dendrites as well as some fine neuronal processes. After P5, distinct punctuated staining was apparent. The number of such GABA(B) IR granules per cell increased with age in a sigmoidal manner from P5 to P60. Electron microscopy revealed GABA(B) IR as clusters of small clear vesicles of 30-50 nm diameter within the cytoplasm and close to the cell membrane at extrasynaptic locations, as well as at pre-synaptic and post-synaptic specialisations. The increase in GABA(B) R1-IR punctuate staining during brain maturation points to increasing functional participation and heterogeneity of GABA(B) receptors as the complexity of the central nervous system expands with growth and development.

Independent maturation of the GABA(B) receptor subunits GABA(B1) and GABA(B2) during postnatal development in rodent brain

GABA(B) receptors mediate slow inhibitory GABAergic neurotransmission. They are encoded by two distinct subunits, GABA(B1) (GBR1) and GABA(B2) (GBR2), with two major isoforms of GBR1, GBR1a and GBR1b, arising from differential promoter usage. Heterodimerization of GBR1 and GBR2 is essential for GABA(B) receptor function, as shown in recombinant expression systems and in GBR1(-/-) mice. GABA(B) receptors are highly expressed during ontogeny, prior to synaptogenesis, but their developmental function remains elusive. Here we investigated the postnatal development of GABA(B) receptors in rodent brain, focusing on potential differences in the spatial and temporal expression pattern of GBR1 and GBR2. Immunohistochemistry with subunit-specific antibodies revealed a widespread staining for GBR1a and GBR2 in neonatal rodent brain. During the first 2 weeks, these two subunits exhibited largely overlapping regional distribution, but with profound distinctions in cellular and subcellular localization. The adult-like pattern was established during the third week, with a prominent up-regulation of GBR1b, extensively codistributed with GBR2. Several unexpected features were noted at early stages, notably, a selective GBR2 staining of axonal tracts, such as the corticothalamic projection, and a prominent GBR1 expression in astrocytes. The specificity of the antibody labeling was verified in GBR1- and GBR2-knockout mice. In addition, the analysis of these mutants revealed a partial preservation of GBR2 staining in GBR1(-/-) mice and vice versa. Altogether, the results suggest a functional role for GBR1 and GBR2 proteins in immature brain in addition to their contribution to dimeric GABA(B) receptor complexes.

Binge ethanol exposure delays development of GABAergic miniature postsynaptic currents in septal neurons

Whole cell GABA(A)R currents of septal neurons isolated from rat pups increase rapidly during the first weeks of life when inhibitory synapses are forming. Early postnatal binge ethanol intubation on days 4-9 delays this maturational up-regulation in septal neurons isolated several days later suggesting inhibitory synapse formation could be disrupted [S.-H. Hsiao, J.L. Acevedo, D.W. DuBois, K.R. Smith, J.R. West, G.D. Frye, Early postnatal ethanol intubation blunts GABA(A) receptor upregulation and modifies 3alpha-hydroxy-5alpha-pregnan-20-one sensitivity in rat MS/DB neurons, Brain Res. Dev. Brain Res. 130 (2001) 25-40]. Surprisingly, whole cell GABA(A)R function does not increase rapidly when septal neurons are grown for the same period in vitro and is not blunted by comparable ethanol exposure of the cultures [S.-H. Hsiao, D.W. DuBois, R.C. Miranda, G.D. Frye, Critically timed ethanol exposure reduces GABA(A)R function on septal neurons developing in vivo but not in vitro, Brain Res Dev. Brain Res. 1008 (2004) 69-80]. Because GABAergic miniature postsynaptic currents (mPSCs) show parallel patterns of maturation whether cortical neurons are growing in vivo or in vitro [D.D. Dunning, C.L. Hoover, I. Soltesz, M.A. Smith, D.K. ODowd, GABA(A) receptor-mediated miniature postsynaptic currents and alpha-subunit expression in developing cortical neurons, J. Neurophysiol. 82 (1999) 3286-3297], we examined the impact of binge ethanol exposure on synaptic receptors activated by these currents in septal cultures. Binge ethanol treatment of embryonic septal neurons over 6-11 days in vitro (DIV) slightly reduced GABA(A)R-mediated mPSC amplitude and frequency, but also substantially slowed decay kinetics when mPSCs were recorded later on DIV 13-18. Decreased frequency and slowed mPSC decay kinetics after ethanol were consistent with parameters measured in immature neurons. Untreated septal neurons exhibited decreased mPSC amplitude and frequency with acute 30-100 mM ethanol, without changing decay kinetics suggesting a direct inhibition of postsynaptic receptors. Sustained inhibition of GABA(A)Rs with 100 microM picrotoxin on DIV 6-11 decreased mPSC amplitude and frequency and slowed decay kinetics similar to binge ethanol exposure. These results suggest that binge ethanol exposure delays mPSC maturation by interfering with trophic postsynaptic GABA(A)R signaling during the early development of septal neurons.

The GABAergic phenotype of the “glutamatergic” granule cells of the dentate gyrus

The granule cells of the dentate gyrus (DG), origin of the mossy fibers (MFs), have been considered to be glutamatergic. However, data obtained with different experimental approaches in recent years may be calling for a redefinition of their phenotype. Although they indeed release glutamate for fast neurotransmission, immunohistological and molecular biology evidence has revealed that these glutamatergic cells also express GABAergic markers. The granule cell expression of a GABAergic phenotype is developmentally regulated. Electrophysiological studies reveal that during the first 3 weeks of age, mossy fiber stimulation provokes monosynaptic fast inhibitory transmission mediated by GABA, besides the monosynaptic excitatory glutamatergic transmission, onto their targets in CA3. After this age, mossy fiber GABAergic transmission abruptly disappears and the GABAergic markers are undetected. In the adult, the GABAergic markers are upregulated and GABA-mediated transmission emerges after induction of hyperexcitability. The simultaneous glutamate- and GABA-mediated signals share the same plastic and pharmacological characteristics that correspond to neurotransmission of mossy fiber origin. This intriguing evidence gives rise to two fundamental points of discussion. The first is the plausible fact that glutamate and GABA, two neurotransmitters of opposing actions, are coreleased from the mossy fibers. The second relates to its functional implications that can be immediately inferred, as the dentate gyrus can exert direct GABA-mediated excitatory actions early in life and inhibitory actions in young and adult hippocampus. This evidence poses the need to reevaluate and reinterpret some aspects of the physiology of the mossy fiber pathway under normal and pathological conditions. This work reviews the recent evidence that supports the assumption that glutamate and GABA can be coreleased from a single pathway, the mossy fibers, and makes some considerations about its functional implications.

Plasticity of the GABAergic phenotype of the "glutamatergic" granule cells of the rat dentate gyrus

The "glutamatergic" granule cells of the dentate gyrus transiently express a GABAergic phenotype when a state of hyperexcitability is induced in the adult rat. Consequently, granule cell (GC) activation provokes monosynaptic GABAergic responses in their targets of area CA3. Because GABA exerts a trophic action on neonatal CA3 and mossy fibers (MF) constitute its main input, we hypothesized that the GABAergic phenotype of the MF could also be transiently expressed early in life. We addressed this possibility with a multidisciplinary approach. Electrophysiological recordings in developing rats revealed that, until day 22-23 of age, glutamate receptor antagonists block the excitatory response evoked in pyramidal cells by GCs, isolating a fast metabotropic glutamate receptor-sensitive GABAergic response. In a clear-cut manner from day 23-24 of age, GC activation in the presence of glutamatergic antagonists was unable to evoke synaptic responses in CA3. Immunohistological experiments showed the presence of GABA and GAD67 (glutamate decarboxylase 67 kDa isoform) in the developing GCs and their MF, and, using reverse transcription-PCR, we confirmed the expression of vesicular GABA transporter mRNA in the developing dentate gyrus and its downregulation in the adult. The GABAergic markers were upregulated and MF inhibitory transmission reappeared when hyperexcitability was induced in adult rats. Our data evidence for the first time a developmental and activity-dependent regulation of the complex phenotype of the GC. At early ages, the GABAergic input from the MF may add to the interneuronal input to CA3 to foster development, and, in the adult, it can possibly protect the system from enhanced excitability.

Effects of osmotic changes on the chemoreceptor cell of rat carotid body

The carotid body plays a crucial role in cardiorespiratory regulation. In the present study we investigated the effect of osmotic changes on cytoplasmic calcium concentration ([Ca(2+)](c)) and pH (pH(i)) of isolated chemoreceptor cells of the rat carotid body. In CO(2)/HCO(3)(-)-buffered medium, reduction of osmolality from the control level of 300 mosmol kg(-1) to 250-285 mosmol kg(-1) resulted in a rise in [Ca(2+)](c), as measured with Indo-1, whereas elevation of osmolality to 350 mosmol kg(-1) had no effect. The Ca(2+) response required extracellular Ca(2+) and was reduced by application of the L-type Ca(2+) channel antagonist nifedipine (10 microM). The hyposmosis-induced Ca(2+) response could be prevented by application of niflumic acid (300 microM), an inhibitor of the swelling-activated Cl(-) channel. In whole-cell patch-clamp experiments niflumic acid abolished the swelling-activated Cl(-) current but only slightly depressed the Ca(2+) current. The inhibition of Ca(2+) current by niflumic acid does not account for its action in preventing of hyposmosis-induced Ca(2+) response, which seems to be initiated by Cl(-)-mediated depolarisation. Withdrawal of CO(2)/HCO(3)(-) also prevented the Ca(2+) response. Reduction of the osmotic concentration by 50 mosmol kg(-1) induced a small but sustained decrease in pH(i), while elevation by 50 mosmol kg(-1) had an inverse effect, as measured fluorimetrically with carboxy SNARF-1. Our conclusion is that in the rat chemoreceptor cell the activation of Cl(-) channels, e.g. by hyposmotic challenge, induces depolarisation, which, in turn, activates voltage-gated Ca(2+) channels.

Dose-, time-, age-, and sex-response profiles for the anticonvulsant effects of deoxycorticosterone in 15-day-old rats

Recently, we have shown that a single high dose of the adrenal steroid precursor hormone deoxycorticosterone (DOC) has potent anticonvulsant effects in 15-day-old rats. To better define the actions of DOC, the present study established dose-, time-, age-, and sex-response curves for the anticonvulsant actions of DOC. Methods. Dose- and time-response studies were done using two different seizure models: (1) maximal pentylenetetrazol seizures (MMT) and (2) maximal electroconvulsive shock (MES) seizures. Subsequently, age- and sex-response studies were done using MMT seizures and two different DOC doses, one low (nonsedating) and one high (sedating). Results. In dose-response studies, DOC suppressed MMT seizures with an ED(50) of about 5 mg/kg (sc). Higher doses were necessary to suppress MES seizures, where the ED(50) was about 20 mg/kg. In time-response studies, DOC's effects were rapid in onset. Complete suppression of seizures was seen by 5 min in the MES model and by 15 min in the MMT model. In developmental studies, both a low nonsedating and a high sedating dose of DOC suppressed MMT seizures in neonatal, infant, weanling, and juvenile rats of either sex. The suppressive effects of low-dose DOC were lost after puberty, however. The suppressive effects of high-dose DOC also declined after puberty, especially in males. Conclusion. DOC has significant anticonvulsant actions that occur in prepubertal, but not postpubertal subjects. DOC might have clinical importance in the future treatment of childhood seizure disorders.

Differential ontogeny of GABA(B)-receptor-mediated pre- and postsynaptic modulation of GABA and glycine transmission in respiratory rhythm-generating network in mouse

Rhythm generation in mature respiratory networks is influenced strongly by synaptic inhibition. In early neonates, GABA(A)-receptor- and glycine-receptor-mediated inhibition is not present, thus the question arises as to whether GABA(B)-receptor-mediated inhibition plays an important role. Using brainstem slices of neonatal mice (postnatal day, P0-P15), we analysed the role of GABA(B)-mediated modulation of GABA and glycine synaptic transmission in the respiratory network. Blockade of GABA uptake by nipecotic acid (0.25-2 mM) reduced the respiratory frequency. This reduction was prevented by the selective GABA(B) receptor antagonist CGP55845A (CGP) alone at P0-P3, but by bicuculline as well as CGP at P7-P15. Blockade of GABA(B) receptors by CGP increased the respiratory frequency at P0-P3, whereas it caused a reduction of frequency in older animals. The effect of CGP on respiratory frequency was diminished in the presence of bicuculline and strychnine in older but not in younger animals. The relative contribution of GABA(B)-receptor-mediated pre- and postsynaptic modulation was examined by analysing the effect of GABA(B) receptors on spontaneous and miniature IPSCs. In younger animals (P0-P3), the GABA(B) receptor agonist baclofen had no detectable effect on IPSC frequency, but caused a significant decrease in the amplitude. In older animals (P7-P15), baclofen decreased both the frequency and amplitude of spontaneous and miniature IPSCs. These results demonstrate that GABA(B)-receptor-mediated postsynaptic modulation plays an important role in the respiratory network from P0 on. GABA(B)-receptor-mediated presynaptic modulation develops with a longer postnatal latency, and becomes predominant within the first postnatal week.

Prolonged prenatal psychotropic medication exposure alters neonatal acute pain response

Selective serotonin reuptake inhibitors (SSRIs) and benzodiazepines are frequently used to treat maternal depression during pregnancy, however the effect of increased serotonin (5HT) and gamma-amino-butyric acid (GABA) agonists in the fetal human brain remains unknown. 5HT and GABA are active during fetal neurologic growth and play early roles in pain modulation, therefore, if prolonged prenatal exposure alters neurodevelopment this may become evident in altered neonatal pain responses. To examine biologic and behavioral effects of prenatal exposure, neonatal responses to acute pain (phenylketonuria heel lance) in infants with prolonged prenatal exposure were examined. Facial action (Neonatal Facial Coding System) and cardiac autonomic reactivity derived from the relationship between respiratory activity and short term variations of heart rate (HRV) were compared between 22 infants with SSRI exposure (SE) [fluoxetine (n = 7), paroxetine (n = 11), sertraline (n = 4)]; 16 infants exposed to SSRIs and clonazepam (SE+) [paroxetine (n = 14), fluoxetine (n = 2)]; and 23 nonexposed infants during baseline, lance, and recovery periods of a heel lance. Length of maternal SSRI use did not vary significantly between exposure groups-[mean (range)] SE:SE+ 183 (31-281):141 (54-282) d (p > 0.05). Infants exposed to SE and SE+ displayed significantly less facial activity to heel lance than control infants. Mean HR increased with lance, but was significantly lower in SE infants during recovery. Using measures of HRV and the transfer relationship between heart rate and respiration, SSRI infants had a greater return of parasympathetic cardiac modulation in the recovery period, whereas a sustained sympathetic response continued in the control group. Prolonged prenatal SSRI exposure appears to be associated with reduced behavioral pain responses and increased parasympathetic cardiac modulation in recovery following an acute neonatal noxious event. Possible 5HT-mediated pain inhibition, pharmacologic factors and the developmental course remain to be studied.

Early development of neuronal activity in the primate hippocampus in utero

Morphological studies suggest that the primate hippocampus develops extensively before birth, but little is known about its functional development. Patch-clamp recordings of hippocampal neurons and reconstruction of biocytin-filled pyramidal cells were performed in slices of macaque cynomolgus fetuses delivered by cesarean section. We found that during the second half of gestation, axons and dendrites of pyramidal cells grow intensively by hundreds of micrometers per day to attain a high level of maturity near term. Synaptic currents appear around midgestation and are correlated with the level of morphological differentiation of pyramidal cells: the first synapses are GABAergic, and their emergence correlates with the growth of apical dendrite into stratum radiatum. A later occurrence of glutamatergic synaptic currents correlates with a further differentiation of the axodendritic tree and the appearance of spines. Relying on the number of dendritic spines, we estimated that hundreds of new glutamatergic synapses are established every day on a pyramidal neuron during the last third of gestation. Most of the synaptic activity is synchronized in spontaneous slow ( approximately 0.1 Hz) network oscillations reminiscent of the giant depolarizing potentials in neonatal rodents. Epileptiform discharges can be evoked by the GABA(A) receptor antagonist bicuculline by the last third of gestation, and postsynaptic GABA(B) receptors contribute to the termination of epileptiform discharges. Comparing the results obtained in primates and rodents, we conclude that the template of early hippocampal network development is conserved across the mammalian evolution but that it is shifted toward fetal life in primate.

Human and murine phenotypes associated with defects in cation-chloride cotransport

The diuretic-sensitive cotransport of cations with chloride is mediated by the cation-chloride cotransporters, a large gene family encompassing a total of seven Na-Cl, Na-K-2Cl, and K-Cl cotransporters, in addition to two related transporters of unknown function. The cation-chloride cotransporters perform a wide variety of physiological roles and differ dramatically in patterns of tissue expression and cellular localization. The renal-specific Na-Cl cotransporter (NCC) and Na-K-2Cl cotransporter (NKCC2) are involved in Gitelman and Bartter syndrome, respectively, autosomal recessive forms of metabolic alkalosis. The associated phenotypes due to loss-of-function mutations in NCC and NKCC2 are consistent, in part, with their functional roles in the distal convoluted tubule and thick ascending limb, respectively. Other cation-chloride cotransporters are positional candidates for Mendelian human disorders, and the K-Cl cotransporter KCC3, in particular, may be involved in degenerative peripheral neuropathies linked to chromosome 15q14. The characterization of mice with both spontaneous and targeted mutations of several cation-chloride cotransporters has also yielded significant insight into the physiological and pathophysiological roles of several members of the gene family. These studies implicate the Na-K-2Cl cotransporter NKCC1 in hearing, salivation, pain perception, spermatogenesis, and the control of extracellular fluid volume. Targeted deletion of the neuronal-specific K-Cl cotransporter KCC2 generates mice with a profound seizure disorder and confirms the central role of this transporter in modulating neuronal excitability. Finally, the comparison of human and murine phenotypes associated with loss-of-function mutations in cation-chloride cotransporters indicates important differences in physiology of the two species and provides an important opportunity for detailed physiological and morphological analysis of the tissues involved.

Postnatal maturation of GABA(A) and GABA(C) receptor function in the mammalian superior colliculus

In the stratum griseum superficiale (SGS) of the mammalian superior colliculus, GABA(C) receptors seem to control the excitability of projection neurons by selective inactivation of local GABAergic interneurons. As the onset of visual responses to SC begins well after birth in the rat, it is possible to study developmental changes in GABAergic mechanisms that are linked to the onset of visual information processing. In order to analyse postnatal changes in inhibitory mechanisms that involve GABA receptor function, we used extracellular field potential (FP) recordings and single cell patch-clamp techniques in slices from postnatal day 4 (P4) to P32 and examined the effects of GABA and muscimol on electrically evoked SGS cell activity. While GABA(A) receptor activation affected FP amplitudes throughout postnatal development, GABA(C) receptor activation did not significantly change FP amplitudes until the third postnatal week. Results from patch-clamping single cells, however, clearly demonstrate that GABA(C) receptors are already functional at P4--similar to GABA(A) receptors. Throughout postnatal development, activation of GABA(C) receptors leads to a strong inhibition of inhibitory postsynaptic activity, indicating that GABA(C) receptors are expressed by inhibitory interneurons. Furthermore, the proportion of neurons that show decreased excitatory postsynaptic activity during GABA(C) receptor activation correlates with the proportion of GABAergic interneurons in SGS. Our patch-clamp results indicate that the functional expression of GABA(C) receptors by GABAergic interneurons does not change significantly during postnatal development. However, our measurements of FP amplitudes indicate that the maturation of the efferent connections of these GABAergic neurons within SGS during the third postnatal week strongly changes GABA(C) receptor function.

Early postnatal ethanol intubation blunts GABA(A) receptor up-regulation and modifies 3alpha-hydroxy-5alpha-pregnan-20-one sensitivity in rat MS/DB neurons

Previously we found postnatal binge-like ethanol exposure using an artificial-rearing method in the rat delayed developmental up-regulation of GABA(A) receptors (GABA(A)Rs) in both medial septum/diagonal band (MS/DB) and cerebellar Purkinje neurons. In the present study, the impact of ethanol on developing GABA(A)Rs in MS/DB neurons was further tested under conditions not requiring anesthesia or maternal deprivation. Nursing rat pups received ethanol (4.5-5.25 g/kg/day) on postnatal days (PD) 4-9, which was administrated manually by oral intragastric intubation. This treatment caused dose-dependent blunting of peak GABA(A) receptor whole cell currents in acutely dissociated MS/DB cells on PD 12-15. The threshold with oral intubation was slightly higher than previously observed for artificial-rearing (4.9 vs. 4.5 g/kg/day). The previously observed reduced sensitivity of GABA(A)Rs to Zn(2+)-inhibition after ethanol was not found with the intubation model. In studies only carried out using the intubation method, 3alpha-hydroxy-5alpha-pregnan-20-one (3alpha-OH-DHP) caused an allosteric concentration-dependent potentiation of currents activated by non-saturated concentrations of GABA. A bicuculline sensitive direct activation of GABA(A)Rs also occurred with higher concentrations of 3alpha-OH-DHP alone. Ethanol intubation up-regulated allosteric neurosteroid potentiation with low concentrations of GABA, but did not change direct agonist actions of 3alpha-OH-DHP. Finally, 3alpha-OH-DHP did not prime ethanol insensitive GABA(A)Rs to become sensitivity to acute ethanol potentiation. These results indicate ethanol consistently blunts postnatal GABA(A) receptor up-regulation across early postnatal binge-type ethanol exposure models and may increase positive modulation of GABA(A) receptors by endogenous neurosteroids.

Complex trait analysis of the hippocampus: mapping and biometric analysis of two novel gene loci with specific effects on hippocampal structure in mice

Notable differences in hippocampal structure are associated with intriguing differences in development and behavioral capabilities. We explored genetic and environmental factors that modulate hippocampal size, structure, and cell number using sets of C57BL/6J (B6) and DBA/2J (D2) mice; their F1 and F2 intercrosses (n = 180); and 35 lines of BXD recombinant inbred (RI) strains. Hippocampal weights of the parental strains differ by 20%. Estimates of granule cell number also differ by approximately 20%. Hippocampal weights of RI strains range from 21 to 31 mg, and those of individual F2 mice range from 23 to 36 mg (bilateral weights). Volume and granule cell number are well correlated (r = 0.7-0.8). Significant variation is associated with differences in age and sex. The hippocampus increases in weight by 0.24 mg per month, and those of males are 0.55 mg heavier (bilateral) than those of females. Heritability of variation is approximately 50%, and half of this genetic variation is generated by two quantitative trait loci that map to chromosome 1 (Hipp1a: genome-wide p < 0.005, between 65 and 100 cM) and to chromosome 5 (Hipp5a, p < 0.05, between 15 and 40 cM). These are among the first gene loci known to produce normal variation in forebrain structure. Hipp1a and Hipp5a individually modulate hippocampal weight by 1.0-2.0 mg, an effect size greater than that generated by age or sex. The Hipp gene loci modulate neuron number in the dentate gyrus, collectively shifting the population up or down by as much as 200,000 cells. Candidate genes for the Hipp loci include Rxrg and Fgfr3.

Epileptogenesis and enhanced prepulse inhibition in GABA(B1)-deficient mice

The recent cloning of two GABA(B) receptor subunits, GABA(B1) and GABA(B2), has raised the possibility that differences in GABA(B) receptor subunit composition may give rise to pharmacologically or functionally distinct receptors. If present, such molecular diversity could permit the selective targeting of GABA(B) receptor subtypes specifically involved in pathologies such as drug addiction, spasticity, pain, and epilepsy. To address these issues we have developed a GABA(B1) subunit knockout mouse using gene targeting techniques. In the brains of GABA(B1) null mice, all pre- and postsynaptic GABA(B) receptor function was absent demonstrating that the GABA(B1) subunit is essential for all GABA(B) receptor-mediated mechanisms. Despite this, GABA(B1) null mice appeared normal at birth, although by postnatal week four their growth was retarded and they developed a generalized epilepsy that resulted in premature death. In addition, GABA(B1) heterozygote animals showed enhanced prepulse inhibition responses compared to littermate controls, suggesting that GABA(B1) deficient mice exhibit increased sensorimotor gating mechanisms. These data suggest that GABA(B) receptor antagonists may be of benefit in the treatment of psychiatric and neurological disorders in which attentional processing is impaired.

Developing networks play a similar melody

During development, when synapses start to be established, a primitive form of network-driven activity provides most of the synaptic activity. This pattern enables a high degree of synchrony in immature neurons in spite of the small number of functional synapses and could participate in activity-dependent growth and synapse formation. Relying on the giant depolarizing potentials that provide most of the synaptic activity in the developing hippocampus, this article reviews the common properties and generating mechanisms of these patterns, and particularly the role of the early depolarizing action of GABA(A) and glycine receptors and the sequential expression of GABA and glutamate synapses. Patterns similar to giant depolarizing potentials have been observed in a wide range of structures and species suggesting that there is a temporal template throughout evolution that constitutes an essential step in the formation of functional networks.

Morphology of CA3 non-pyramidal cells in the developing rat hippocampus

Although several investigations have shown that the local GABAergic circuit in the rat hippocampus is functional very early in development, this result has not been yet completed by the investigation of the full dendritic and axonal arborization of the neonatal interneurones. In the present study, intracellular injection of biocytin was used to assess the branching pattern of interneurones in the hippocampal CA3 region of rat between 2 and 6 days of age. Based on their dendritic morphology, the biocytin-filled interneurones were divided into four classes: bipolar, stellate, pyramidal-like and fusiform interneurones. About half of the biocytin-filled neonatal interneurones exhibited dendritic or somatic filopodial processes. The axonal arbors of the filled-interneurones were widely spread into the CA3 region, and in four out of nine cases extended beyond the CA3 region to branch into the CA1 region. These results show that, despite immature features, the filopodial processes, the hippocampal interneurones are well developed early in development at a time when their target cells, the pyramidal neurones, are still developing. These observations are consistent with a trophic role that GABA may play early in development.

Ontogeny of synaptic transmission in the rat hippocampus

Electrophysiological characteristics of the hippocampal slices of juvenile (14-27 days) or young (36-40 days) Wistar rats have been compared. In the juvenile rats measurements were taken daily, from postnatal day (PN) 14 to PN27. Input-output curves were used to quantify the ontogeny of excitatory processes. The dynamic of population spike (PS) maturation was not even during the investigated postnatal period. After day 19 transient decrease of PS amplitude was observed until day 22. There were also some differences between the shape of input-output curves from the slices of rats of different ages. In general, PS was saturated at lower intensities in younger animals. The slices from 19-day-old rats did not display saturated input-output curve with 2-20 V stimuli intensities. But input-output curves on PN20-22 were rather similar to that obtained before PN19. The periods of gradual increase and subsequent decrease of PS amplitudes during early ontogeny correlate with the appearance of certain forms of behaviour. This fact suggests that hippocampal PS amplitude depression may be relevant functionally.

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.

Dorsal column nuclei neurons recorded in a brain stem-spinal cord preparation: characteristics and their responses to dorsal root stimulation

Recordings were obtained from dorsal column nucleus (DCN) neurons in a neonatal rat brain stem-spinal cord preparation to study their basic electrophysiological properties and responses to stimulation of a dorsal root. Whole-cell patch-clamp recordings were made from 21 neurons that responded to dorsal root stimulation with a fast excitatory postsynaptic potential (EPSP). These neurons were located lateral to, but at the level of, the area postrema at depths of 100-268 microm below the dorsal surface of the brain. The neurons could be divided into groups according to the shape of their action potentials or voltage responses to hyperpolarizing current steps; however, the response profiles of the groups of neurons to dorsal root stimulation were not significantly different and all neurons were considered together. Dorsal root stimulation elicited excitatory postsynaptic potentials (EPSPs) in all neurons with a very low variability in onset latency and an ability to follow 100-Hz stimulation, indicating that they were mediated by activation of a monosynaptic pathway. The peak amplitude of the EPSP increased with membrane hyperpolarization, and applications of the non-NMDA receptor antagonists 6-nitro-7-sulfamoylbenzo[f]quinoxaline-2, 3-dione (NBQX) and 6,7-dinitroquinoxaline-2,3-dione (DNQX) decreased the amplitude of the EPSP to 14.2% of the control response (n = 6). The descending phase of the EPSP decreased with membrane hyperpolarization and was reduced by the N-methyl-D-aspartate (NMDA) receptor antagonist AP-5 (n = 2). The EPSPs were also reduced in amplitude by applications of the gamma-aminobutyric acid-B (GABA(B)) receptor agonist baclofen, which had no effect on membrane potential or input resistance. These results show that fast EPSPs in DCN neurons elicited by dorsal root stimulation are mediated by an excitatory amino acid acting at both non-NMDA and, to a lesser extent, NMDA receptors. In addition, GABA acting at presynaptic GABA(B) receptors can inhibit these responses.

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.

The establishment of GABAergic and glutamatergic synapses on CA1 pyramidal neurons is sequential and correlates with the development of the apical dendrite

We have performed a morphofunctional analysis of CA1 pyramidal neurons at birth to examine the sequence of formation of GABAergic and glutamatergic postsynaptic currents (PSCs) and to determine their relation to the dendritic arborization of pyramidal neurons. We report that at birth pyramidal neurons are heterogeneous. Three stages of development can be identified: (1) the majority of the neurons (80%) have small somata, an anlage of apical dendrite, and neither spontaneous nor evoked PSCs; (2) 10% of the neurons have a small apical dendrite restricted to the stratum radiatum and PSCs mediated only by GABA(A) receptors; and (3) 10% of the neurons have an apical dendrite that reaches the stratum lacunosum moleculare and PSCs mediated both by GABA(A) and glutamate receptors. These three groups of pyramidal neurons can be differentiated by their capacitance (C(m) = 17.9 +/- 0.8; 30.2 +/- 1.6; 43.2 +/- 3.0 pF, respectively). At birth, the synaptic markers synapsin-1 and synaptophysin labeling are present in dendritic layers but not in the stratum pyramidale, suggesting that GABAergic peridendritic synapses are established before perisomatic ones. The present observations demonstrate that GABAergic and glutamatergic synapses are established sequentially with GABAergic synapses being established first most likely on the apical dendrites of the principal neurons. We propose that different sets of conditions are required for the establishment of functional GABA and glutamate synapses, the latter necessitating more developed neurons that have apical dendrites that reach the lacunosum moleculare region.

GABA and epileptogenesis: comparing gabrb3 gene-deficient mice with Angelman syndrome in man

The GABAergic system has long been implicated in epilepsy with defects in GABA neurotransmission being linked to epilepsy in both experimental animal models and human syndromes (Olsen and Avoli, 1997). However, to date no human epileptic syndrome has been directly attributed to an altered GABAergic system. The observed defects in GABA neurotransmission in human epileptic syndromes may be the indirect result of a brain besieged by seizures. The use of animal models of epilepsy has sought to address these matters. The advent of gene targeting methodologies in mice now allows for a more direct assessment of GABA's involvement in epileptogenesis. To date several genes associated with the GABAergic system have been disrupted. These include the genes for glutamic acid decarboxylase, both the 65- and 67-kDa isoforms (GAD65 and GAD67), the tissue non-specific alkaline phosphatase gene (TNAP) and genes for the GABA(A) receptor subunits alpha6, beta3, gamma2, and delta (gabra6, gabrb3, gabrg2, and gabrd respectively). Gene disruptions of either GAD67 or gabrg2 result in neonatal lethality, while others, GAD65, TNAP, and gabrb3 exhibit increased mortality and spontaneous seizures. GABA receptor expression has been found to be both regionally and developmentally regulated. Thus in addition to their obvious role in controlling excitability in adult brain, a deficit in GABAergic function during development could be expected to elicit pleiotropic neurodevelopmental abnormalities perhaps including epilepsy. The GABA(A) receptor beta3 subunit gene, gabrb3/GABRB3 (mouse/human), is of particular interest because of its expression early in development and its possible role in the neurodevelopmental disorder Angelman syndrome. Individuals with this syndrome exhibit severe mental retardation and epilepsy. Mice with the gabrb3 gene disrupted likewise exhibit electroencephalograph (EEG) abnormalities, seizures, and behavioral characteristics typically associated with Angelman syndrome. These gabrb3 gene knockout mice provide direct evidence that a reduction of a specific subunit of the GABA(A) receptor system can result in epilepsy and support a GABAergic role in the pathophysiology of Angelman syndrome.

Postnatal development of GABAB receptor-mediated modulation of voltage-activated Ca2+ currents in mouse brain-stem neurons

GABAB receptors modulate respiratory rhythm generation in adult mammals. However, little is currently known of their functional significance during postnatal development. In the present investigation, the effects of GABAB receptor activation on voltage-activated Ca2+ currents were examined in rhythmically active neurons of the pre-Bötzinger complex (PBC). Both low- (LVA) and high-voltage-activated (HVA) Ca2+ currents were present from the first postnatal day (P1). The density of LVA Ca2+ currents increased during the first week, whilst the density of HVA Ca2+ currents increased after the first week. In the second postnatal week, the HVA Ca2+ currents were composed of L- (47 +/- 10%) and N-type (21 +/- 8%) currents plus a 'residual' current, whilst there were no N-type currents detectable in the first few days. The GABAB receptor agonist baclofen (30 microM) increased LVA Ca2+ currents (30 +/- 11%) at P1-P3, but it decreased the currents (35 +/- 11%) at P7-P15 without changing its time course. At all ages, baclofen (30 microM) decreased the HVA Ca2+ currents by approximately 54%. Threshold of baclofen effects on both LVA and HVA Ca2+ currents was 5 microM at P1-P3 and lower than 1 microM at P7-P15. The effect of baclofen was abolished in the presence of the GABAB receptor antagonist CGP 55845A (50 nM). We conclude that both LVA and HVA Ca2+ currents increased postnatally. The GABAB receptor-mediated modulation of these currents undergo marked developmental changes during the first two postnatal weeks, which may contribute essentially to modulation of respiratory rhythm generation.

Postnatal ethanol exposure blunts upregulation of GABAA receptor currents in Purkinje neurons

Recently, we found that early postnatal ethanol exposure inhibits the maturation of GABAA receptors (GABAARs) in developing medial septum/diagonal band (MS/DB) neurons, suggesting that these receptors may represent a target for ethanol related to fetal alcohol syndrome (FAS). To determine whether GABAARs on other neurons are also sensitive to a postnatal ethanol insult, postnatal day (PD) 4-9, rat pups were artificially reared and exposed to ethanol (4.5 g kg-1 day-1, 10.2% v/v). The pharmacological profile of acutely dissociated cerebellar Purkinje cell GABAARs from untreated, artificially reared controls and ethanol-treated animals was examined with conventional whole-cell patch clamp recordings during PD 12-16 (juveniles) and PD 25-35 (young adults). For untreated animals, GABA (0.3-100 microM) consistently induced inward Cl- currents in a concentration-dependent manner showing an age-related increase in maximum response without change in EC50 or slope value. Acute ethanol (100 mM) consistently inhibited 3 microM GABA currents (10-20%); positive modulators, pentobarbital (10 microM), midazolam (1 microM) and loreclezole (10 microM), consistently potentiated; the negative modulator, Zn2+ (30 microM), inhibited GABA currents across both juvenile and young adult groups. Loreclezole potentiation increased while Zn2+ inhibition decreased with age in untreated Purkinje neurons. Postnatal ethanol exposure (PD 4-9) decreased GABAAR maximum current density in young adult Purkinje cells but not in juvenile neurons. However, sensitivity to allosteric modulators did not change after ethanol. These data are consistent with the hypothesis that postnatal ethanol exposure during the brain growth spurt can disturb GABAAR development across the brain, although the mechanism(s) underlying this action remains to be determined.

GABA induces proliferation of immature cerebellar granule cells grown in vitro

The presence of GABA and its receptors early in rodent nervous system development has lead to speculation on the role of this transmitter system in neuroblast proliferation, migration and differentiation. We studied the effect of GABA and GABA agonists on immature cerebellar granule cell proliferation and survival. Cerebellar granule cell suspensions were obtained from 6-8-day-old rats and grown in culture for up to 7 days in serum-containing or serum-free medium. The addition of GABA (0.1-100 microM) or muscimol (0.01-10 microM) 2 h after inoculation and harvested 22 h later, lead to an increase in 3H-thymidine incorporation over control samples with the correspondent increase in granule cells number assayed 48 h later. The effect on cell proliferation exerted by GABAA agonists was blocked by MgCl2 and nifedipine, as well as by the chloride channel blocker, picrotoxin (50 microM), and the GABAA receptor specific blocker, bicuculline (50 microM). The increase on cell proliferation induced by GABA also was blocked by PD98059 (75 microM), a specific inhibitor of the mitogen-activated protein kinase kinase (MAPKK). GABAA receptor-mediated proliferation was consistently seen in cells inoculated in serum-containing medium supplemented with 25 mM KCl but not seen in serum-free medium, with 5 mM or 25 mM KCl. The presence of serum did not enhance the survival of cerebellar granule cells grown for 7 days in either 5 mM or 25 mM KCl. Additionally, neither GABA nor muscimol applied from day 2 to day 7 in vitro affected cell survival in any culture condition. We conclude that GABA and GABAA receptor agonists influence granule cell proliferation but not survival and that this effect is mediated by a calcium influx via voltage-dependent calcium channel activation, with a subsequent activation of the MAPK cascade.

Development of GABAA receptors on medial septum/diagonal band (MS/DB) neurons after postnatal ethanol exposure

The impact of 'binge-like' ethanol exposure on postnatal days (PD) 4-9 was examined on development of gamma-aminobutyric acid type A receptors (GABAAR) during the first month of life in the rat. Whole-cell patch-clamp recordings in acutely isolated medial septum/diagonal band (MS/DB) neurons were used to define effects of rapidly applied ethanol and other allosteric modulators on bicuculline-sensitive GABA currents. Three age groups were examined including 'pups' (PD 4-10), 'juveniles' (PD 11-16) and 'young adults' (PD 25-35). In untreated neurons, maximum responses to GABA and the apparent GABA EC50 increased approximately 2-fold during the first month of life. Potentiation of GABA responses by pentobarbital, midazolam, and loreclezole all increased with age, while Zn2+ inhibition declined. Initial inhibition by ethanol switched to potentiation of GABA responses during this time. In vivo, binge-like ethanol treatment (4.5 g kg-1 day-1 divided into two doses, 2 h apart on PD 4-9) reduced both the GABA maximal response and GABA EC50 measured on PD 11-16. These measures returned to control levels by PD 25-35. After binge-like postnatal ethanol exposure, age-dependent loss of Zn2+ inhibition of GABA responses was increased, while potentiating actions of in vitro ethanol were blocked. GABAAR modulation by other drugs was unaffected. These data suggest that early postnatal ethanol exposure disrupts the expected developmental pattern of GABAAR function in MS/DB neurons, an action that could contribute to neurobehavioral deficits associated with the fetal alcohol syndrome. Whether these changes are due to cellular damage, delayed gene expression or post-translational modification needs to be determined.