Presynaptic GABAergic control of the locomotor drive in the isolated spinal cord of neonatal rats

The in vitro newborn rat isolated brain stem/spinal cord preparation was used to study the involvement of presynaptic inhibition in the control of the synaptic locomotor drive. The recording chamber was partitioned with Vaseline walls to separate the L1-L2 locomotor network from the motoneurons in the lower segments. When locomotor like activity was induced by bath applying a mixture of N-methyl-D-L-aspartate and serotonin to the L1-L2 segments, intracellular recordings of L3-L5 motoneurons show an alternating pattern of monosynaptic excitatory glutamatergic and inhibitory glycinergic inputs known as the locomotor drive. Gamma-aminobutyric acid (GABA), baclofen and muscimol (respectively GABA(B) and GABA(A) agonists) superfused on the L3-L5 segments depressed the synaptic locomotor drive of motoneurons during the ongoing activity. On the contrary, the GABA(B) receptor antagonist CGP35348 enhanced the locomotor drive, which suggests that an endogenous release of GABA occurs during locomotor-like activity. Baclofen, unlike muscimol and GABA, did not affect the passive membrane properties and the firing discharge of synaptically isolated motoneurons. Baclofen and muscimol acted on the two phases (inhibitory and excitatory) of the synaptic drive. The effects of GABAergic agonists on the whole locomotor activity were tested. When superfused on the L3-L5 part of the cord, they affected only the L5 burst amplitude. When bath-applied to the L1-L2 network, GABA and muscimol decreased the amplitude of the L2 and L5 bursts and increased the locomotor period while baclofen had significant effects only on the period. It was concluded that GABA modulates the information conveyed by the L1-L2 network to its target motoneurons presynaptically via GABA(B) and possibly GABA(A) receptors and postsynaptically, via GABA(A) receptors.     

GABA is excitatory in adult vasopressinergic neuroendocrine cells

Neuronal excitability in the adult brain is controlled by a balance between synaptic excitation and inhibition mediated by glutamate and GABA, respectively. While generally inhibitory in the adult brain, GABA(A) receptor activation is excitatory under certain conditions in which the GABA reversal potential is shifted positive due to intracellular Cl(-) accumulation, such as during early postnatal development and brain injury. However, the conditions under which GABA is excitatory are generally either transitory or pathological. Here, we reveal GABAergic synaptic inputs to be uniformly excitatory in vasopressin (VP)-secreting magnocellular neurons in the adult hypothalamus under normal conditions. The GABA reversal potential (E(GABA)) was positive to resting potential and spike threshold in VP neurons, but not in oxytocin (OT)-secreting neurons. The VP neurons lacked expression of the K(+)-Cl(-) cotransporter 2 (KCC2), the predominant Cl(-) exporter in the adult brain. The E(GABA) was unaffected by inhibition of KCC2 in VP neurons, but was shifted positive in OT neurons, which express KCC2. Alternatively, inhibition of the Na(+)-K(+)-Cl(-) cotransporter 1 (NKCC1), a Cl(-) importer expressed in most cell types mainly during postnatal development, caused a negative shift in E(GABA) in VP neurons, but had no effect on GABA currents in OT neurons. GABA(A) receptor blockade caused a decrease in the firing rate of VP neurons, but an increase in firing in OT neurons. Our findings demonstrate that GABA is excitatory in adult VP neurons, suggesting that the classical excitation/inhibition paradigm of synaptic glutamate and GABA control of neuronal excitability does not apply to VP neurons.     

Inhibition of spinal or hypoglossal motoneurons of the newborn rat by glycine or GABA

The function of GABA or glycine during early postnatal development remains controversial as their action is reported as either excitatory or inhibitory. The present study addressed the question of the functional role of GABA or glycine on rat motoneurons shortly after birth. For this purpose, using in vitro preparations from immature rats (postnatal age, P0-P4 days), we recorded from lumbar spinal motoneurons and hypoglossal motoneurons. All data were obtained under current clamp conditions (recording with potassium methylsulphate containing electrodes) from cells at about -70 mV resting potential. On spinal motoneurons we used the glycinergic and GABAergic recurrent postsysnaptic potential (PSP) mediated by Renshaw cells to assess its impact on excitatory synaptic inputs from dorsal afferent fibres. Despite its depolarizing nature, the recurrent PSP consistently inhibited synaptic excitation of lumbar motoneurons. On hypoglossal motoneurons, exogenously applied GABA or glycine produced depolarization with decreased input resistance. This response was always associated with inhibition of cell firing induced by intracellular current pulses. Even when the membrane potential was repolarized to resting level in the presence of GABA or glycine, hypoglossal motoneurons failed to generate spikes. Conversely, similar depolarization produced by glutamate consistently facilitated spike firing. GABAergic and glycinergic synaptic potentials evoked by focal stimulation of the reticular formation inhibited firing and/or increased firing latency in the majority of hypoglossal motoneurons. These results indicate that, immediately after birth, rat motoneurons were inhibited by synaptically released or exogenously applied GABA or glycine.     

Deficits in cognition and synaptic plasticity in a mouse model of Down syndrome ameliorated by GABAB receptor antagonists

Cognitive impairment in Down syndrome (DS) is characterized by deficient learning and memory. Mouse genetic models of DS exhibit impaired cognition in hippocampally mediated behavioral tasks and reduced synaptic plasticity of hippocampal pathways. Enhanced efficiency of GABAergic neurotransmission was implicated in those changes. We have recently shown that signaling through postsynaptic GABA(B) receptors is significantly increased in the dentate gyrus of Ts65Dn mice, a genetic model of DS. Here we examined a role for GABA(B) receptors in cognitive deficits in DS by defining the effect of selective GABA(B) receptor antagonists on behavior and synaptic plasticity of adult Ts65Dn mice. Treatment with the GABA(B) receptor antagonist CGP55845 restored memory of Ts65Dn mice in the novel place recognition, novel object recognition, and contextual fear conditioning tasks, but did not affect locomotion and performance in T-maze. The treatment increased hippocampal levels of brain-derived neurotrophic factor, equally in 2N and Ts65Dn mice. In hippocampal slices, treatment with the GABA(B) receptor antagonists CGP55845 or CGP52432 enhanced long-term potentiation (LTP) in the Ts65Dn DG. The enhancement of LTP was accompanied by an increase in the NMDA receptor-mediated component of the tetanus-evoked responses. These findings are evidence for a contribution of GABA(B) receptors to changes in hippocampal-based cognition in the Ts65Dn mouse. The ability to rescue cognitive performance through treatment with selective GABA(B) receptor antagonists motivates studies to further explore the therapeutic potential of these compounds in people with DS.     

GABA sets the tempo for activity-dependent adult neurogenesis

GABA, a major inhibitory neurotransmitter in the adult brain, activates synaptic and extrasynaptic GABA(A) receptors, causing hyperpolarization of mature neurons. As in the embryonic nervous system, GABA depolarizes neural progenitors and immature neurons in the adult brain. Several recent studies have suggested that GABA has crucial roles in regulating different steps of adult neurogenesis, including proliferation of neural progenitors, migration and differentiation of neuroblasts, and synaptic integration of newborn neurons. Here, we review recent findings on how GABA regulates adult neurogenesis in the subventricular zone of the lateral ventricles and in the dentate gyrus of the hippocampus. We also discuss an emerging view that GABA serves as a key mediator of neuronal activity in setting the tempo of adult neurogenesis.     

The mechanisms of the strong inhibitory modulation of long-term potentiation in the rat dentate gyrus

The hippocampus is essential for the formation of certain types of memory, and synaptic plasticity such as long-term potentiation (LTP) is widely accepted as a cellular basis of hippocampus-dependent memory. Although LTP in both perforant path-dentate gyrus (DG) granule cell and CA3-CA1 pyramidal cell synapses is similarly dependent on activation of postsynaptic N-methyl-D-aspartate receptors, several reports suggest that modulation of LTP by γ-aminobutyric acid (GABA) receptor-mediated inhibitory inputs is stronger in perforant path-DG granule cell synapses. However, little is known about how different the mechanism and physiological relevance of the GABAergic modulation of LTP induction are among different brain regions. We confirmed that the action of GABA(A) receptor antagonists on LTP was more prominent in the DG, and explored the mechanism introducing such difference by examining two types of GABA(A) receptor-mediated inhibition, i.e. synaptic and tonic inhibition. As synaptic inhibition, we compared inhibitory vs. excitatory monosynaptic responses and their summation during an LTP-inducing stimulus, and found that the balance of the summated postsynaptic currents was biased toward inhibition in the DG. As tonic inhibition, or sustained activation of extrasynaptic GABA(A) receptors by ambient GABA, we measured the change in holding currents of the postsynaptic cells induced by GABA(A) receptor antagonists, and found that the tonic inhibition was significantly stronger in the DG. Furthermore, we found that tonic inhibition was associated with LTP modulation. Our results suggest that both the larger tonic inhibition and the larger inhibitory/excitatory summation balance during conditioning are involved in the stronger inhibitory modulation of LTP in the DG.     

GABA A,slow: causes and consequences

GABA(A) receptors in the CNS mediate both fast synaptic and tonic inhibition. Over the past decade a phasic current with features intermediate between fast synaptic and tonic inhibition, termed GABA(A,slow), has received increasing attention. This has coincided with an ever-growing appreciation for GABAergic cell type diversity. Compared with classical fast synaptic inhibition, GABA(A,slow) is slower by an order of magnitude. In this review, we summarize recent studies that have enhanced our understanding of GABA(A,slow). These include the discovery of specialized interneuron types from which this current originates, the factors that could underlie its characteristically slow kinetics, its contribution to specific aspects of integrative function and network oscillations, and its potential usefulness as a novel drug target for modulating inhibitory synaptic transmission in the CNS.     

GABA(A) receptor in growth cones: the outline of GABA(A) receptor-dependent signaling in growth cones is applicable to a variety of alpha-subunit species.

The growth cone is responsible for axonal elongation and pathfinding by responding to various modulators for neurite growth, including neurotransmitters. We demonstrated an outline of the gamma aminobutyric acid (GABA)(A)-dependent signaling in growth cones. Here, we examined the effects of the modulators of GABA(A) receptor on the signaling in growth cones. Phenobarbital or propofol, acting on beta-subunit, enhanced the [Cl(-)]infi change and [Ca(2+)](i) elevation by the GABA stimulation to isolated growth cones. Besides, propofol enhanced GABA-dependent phosphorylation of growth-associated protein of 43 kDa (GAP-43) by protein kinase C. In contrast, an alpha-subunit acting agent diazepam did not modulate any of the above signals. Next, we examined the effect of the developmental change of alpha-subunit on the outline of the GABA(A)-dependent signaling in growth cones. We also found that the amounts of several different alpha-subunit isoforms developmentally increased or decreased in growth cone membrane (GCM), but that the affinity and density of the [(3)H]diazepam binding sites were similar to those in adult synaptic membrane. Taken together, our results strongly suggest that each step of GABA(A)-dependent signaling in GCM is not modified by diazepam, indicating that the signaling pathway mediated by GABA(A) receptor in growth cones is applicable to any compositional change of alpha-subunit isoforms.     

Hippocampal endocannabinoids play an important role in induction of long-term potentiation and regulation of contextual fear memory formation

Recent studies show contradictory results regarding the contribution of endocannabinoids in fear memory formation and long-term synaptic plasticity. In this study, we investigated the effects of both cannabinoid receptor type 1 (CB1 receptor) antagonist AM281 and anandamide reuptake inhibitor AM404 on the formation of contextual fear memory in adult mice. Both i.p. and intra-hippocampal injections of AM281 promoted contextual fear memory while a high dose of AM404 inhibited it. These findings demonstrate that CB1 receptor-mediated signaling negatively contributes to contextual fear memory formation. We further investigated the induction of long-term potentiation (LTP) in CA1 pyramidal neurons of hippocampal slices and found that AM281 impaired the induction of LTP. Additionally, the blockade of LTP by AM281 was completely prevented by bath application of picrotoxin, a selective antagonist of GABA(A) receptor. Taken together, these results indicate that activation of CB1 receptor contributes to induction of LTP via a GABA(A) receptor-mediated mechanism.     

Neurotransmitters, calcium signalling and neuronal communication

In this article we show some recent findings that constitute a great progress in the molecular knowledge of synaptic dynamics. To communicate, neurons use a code that includes electrical (action potentials) and chemical signals (neurotransmitters, neuromodulators). At the moment a great variety of molecules are known, whose neurotransmitter function in brain and the peripheral nervous system are out of question. Monoamines like acetylcholine, dopamine, noradrenaline, adrenaline, histamine, serotonin, glutamate, aspartate, glycine, ATP and GABA are good examples. Opioid neuropeptides, vasoactive intestinal peptide (VIP), neurokinines (substance P), somatostatin, neurotensin, neuropeptide Y, cholecystokinine, vasopressin or oxitocin have been related to the control of the stress response, sexual behaviour, food intake, pain, learning and memory, qualities that are also related to nitric oxide (NO). A great part of the molecular structure of the secretory machinery is known to be responsible for fast neurotransmitter release at the synapse, in response to action potentials. Proteins like sinaptobrevin (located in the membrane of the synaptic vesicle), sintaxin and SNAP-25 (both located at the presynaptic plasma membrane) constitute a trimeric complex which is responsible of the vesicular docking at the active sites for exocytosis. From this strategic location, vesicles release their neurotransmitter within few milliseconds, when the action potential invades the nerve terminal and activates the opening of the different subtypes of voltage-dependent Ca2+ channels. The asymmetric geographical distribution of each type of channel, in different neurons, rose the hypothesis that Ca2+ that enters through each subtype of channel is compartmentalised, thus favouring the generation of Ca2+ microdomains, in the cytosol and the nucleus, involved in different cellular functions. This great biochemical synaptic heterogeneity is facilitating the selection of many biological targets to develop drugs with potential therapeutic applications in neuropsychiatric diseases i.e. Alzheimer's, Parkinson, epilepsies, stroke, vascular dementia, depression, schizophrenia, anxiety and so on.     

Postnatal changes in somatic gamma-aminobutyric acid signalling in the rat hippocampus

During postnatal development of the rat hippocampus, γ-aminobutyric acid (GABA) switches its action on CA3 pyramidal cells from excitatory to inhibitory. To characterize the underlying changes in the GABA reversal potential, we used somatic cell-attached recordings of GABA(A) and N -methyl- d -aspartate channels to monitor the GABA driving force and resting membrane potential, respectively. We found that the GABA driving force is strongly depolarizing during the first postnatal week. The strength of this depolarization rapidly declines with age, although GABA remains slightly depolarizing, by a few millivolts, even in adult neurons. Reduction in the depolarizing GABA driving force was due to a progressive negative shift of the reversal potential of GABA currents. Similar postnatal changes in GABA signalling were also observed using the superfused hippocampus preparation in vivo , and in the hippocampal interneurons in vitro . We also found that in adult pyramidal cells, somatic GABA reversal potential is maintained at a slightly depolarizing level by bicarbonate conductance, chloride-extrusion and chloride-loading systems. Thus, the postnatal excitatory-to-inhibitory switch in somatic GABA signalling is associated with a negative shift of the GABA reversal potential but without a hyperpolarizing switch in the polarity of GABA responses. These results also suggest that in adult CA3 pyramidal cells, somatic GABAergic inhibition takes place essentially through shunting rather than hyperpolarization. Apparent hyperpolarizing GABA responses previously reported in the soma of CA3 pyramidal cells are probably due to cell depolarization during intracellular or whole-cell recordings.     

Anatomical evidence for presynaptic modulation by the delta opioid receptor in the ventrolateral periaqueductal gray of the rat

The delta opioid receptor (DOR) modulates nociception and blood pressure in the periaqueductal gray (PAG). To examine the cellular basis for DOR effects, the ultrastructural distribution of DOR immunoreactivity was examined in the caudal ventrolateral PAG. DOR immunoreactivity was located predominantly in axon terminals that formed asymmetric (excitatory-type) synaptic contacts. However, rather then localized to the plasma membrane of synaptic boutons, immunolabeling for the DOR was intracellular, often associated with large dense-core vesicles. This finding suggests that dense-core vesicles may play a role in targeting the DOR, as vesicle fusion would shift the distribution of the DOR to the plasma membrane. To investigate the neural circuits in which DOR may function, dual-immunolabeling was used to determine the relationship of the DOR to an endogenous ligand, enkephalin, and to a potential target, GABAergic neurons. Approximately a third (38 of 127) of DOR containing axons had enkephalin immunoreactivity, indicating DOR may act in part as a presynaptic autoreceptor. Although single axon terminals containing immunoreactivity for both DOR and GABA were not detected, some DOR-immunolabeled axon terminals (26 of 86) contacted soma or dendrites containing GABA. These data suggest that the DOR may act in part as an autoreceptor to regulate synaptic input to GABAergic as well as non-GABAergic PAG neurons. Furthermore, the exposure of the DOR to the extracellular space may be contingent upon dense-core vesicle fusion with the plasma membrane.     

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.     

Perinatal development of inhibitory synapses in the nucleus tractus solitarii of the rat

The nucleus tractus solitarii (NTS) plays a key role in the central control of the autonomic nervous system. In adult rats, both GABA and glycine are used as inhibitory neurotransmitter in the NTS. Using a quantitative morphological approach, we have investigated the perinatal development of inhibitory synapses in the NTS. The density of both inhibitory axon terminals and synapses increased from embryonic day 20 until the end of the second postnatal week (postnatal day 14). Before birth, only GABAergic axon terminals developed and their number increased during the first postnatal week. Mixed GABA/glycine axon terminals appeared at birth and their number increased during the first postnatal week. This suggests the development of a mixed GABA/glycine inhibition in parallel to pure GABA inhibition. However, whereas GABAergic axon terminals were distributed throughout the NTS, mixed GABA/glycine axon terminals were strictly located in the lateral part of the NTS. Established at birth, this specific topography remained in the adult rat. From birth, GABA_A receptors, glycine receptors and gephyrin were clustered in inhibitory synapses throughout the NTS, revealing a neurotransmitter–receptor mismatch within the medial part of the NTS. Together these results suggest that NTS inhibitory networks develop and mature until postnatal day 14. Developmental changes in NTS synaptic inhibition may play an important role in shaping neural network activity during a time of maturation of autonomic functions. The first two postnatal weeks could represent a critical period where the impact of the environment influences the physiological phenotypes of adult rats.     

Possible role of intramembrane receptor-receptor interactions in memory and learning via formation of long-lived heteromeric complexes: focus on motor learning in the basal ganglia

Learning in neuronal networks occurs by instructions to the neurons to change their synaptic weights (i.e., efficacies). According to the present model a molecular mechanism that can contribute to change synaptic weights may be represented by multiple interactions between membrane receptors forming aggregates (receptor mosaics) via oligomerization at both pre- and post-synaptic level. These assemblies of receptors together with inter alia single receptors, adapter proteins, G-proteins and ion channels form the membrane bound part of a complex three-dimensional (3D) molecular circuit, the cytoplasmic part of which consists especially of protein kinases, protein phosphatases and phosphoproteins. It is suggested that this molecular circuit has the capability to learn and store information. Thus, engram formation will depend on the resetting of 3D molecular circuits via the formation of new receptor mosaics capable of addressing the transduction of the chemical messages impinging on the cell membrane to certain sets of G-proteins. Short-term memory occurs by a transient stabilization of the receptor mosaics producing the appropriate change in the synaptic weight. Engram consolidation (long-term memory) may involve intracellular signals that translocate to the nucleus to cause the activation of immediate early genes and subsequent formation of postulated adapter proteins which stabilize the receptor mosaics with the formation of long-lived heteromeric receptor complexes. The receptor mosaic hypothesis of the engram formation has been formulated in agreement with the Hebbian rule and gives a novel molecular basis for it by postulating that the pre-synaptic activity change in transmitter and modulator release reorganizes the receptor mosaics at post-synaptic level and subsequently at pre-synaptic level with the formation of novel 3D molecular circuits leading to a different integration of chemical signals impinging on pre- and post-synaptic membranes hence leading to a new value of the synaptic weight. Engram retrieval is brought about by the scanning of the target networks by the highly divergent arousal systems. Hence, a continuous reverberating process occurs both at the level of the neural networks as well as at the level of the 3D molecular circuits within each neuron of the network until the appropriate tuning of the synaptic weights is obtained and, subsequently, the reappearance of the engram occurs. Learning and memory in the basal ganglia is discussed in the frame of the present hypothesis. It is proposed that formation of long-term memories (consolidated receptor mosaics) in the plasma membranes of the striosomal GABA neurons may play a major role in the motivational learning of motor skills of relevance for survival. In conclusion, long-lived heteromeric receptor complexes of high order may be crucial for learning, memory and retrieval processes, where extensive reciprocal feedback loops give rise to coherent synchronized neural activity (binding) essential for a sophisticated information handling by the central nervous system.     

Reproductive history affects the synaptology of the ageing gonadotropin-releasing hormone system in the male rat

This study is an examination of the density of synaptic input to gonadotropin-releasing hormone (GnRH) neurons in young adult and aged retired breeder male rats. In earlier experiments on aged virgin male rats we observed an increase in synaptic input to this specific neuronal population, ascribable in part to synapses containing flattened vesicles, suggesting GABAergic input. The present study utilized retired breeders in order to dissect the effects of ageing from those associated with reproductive behavioral history. Tissue from the preoptic area was treated for the simultaneous electron microscopic immunocytochemical demonstration of GnRH with tetramethylbenzidine and glutamic acid decarboxylase (the essential enzyme in the production of GABA) using 3,3'-diaminobenzidine. Estimates of the density of synaptic input to the soma of GnRH neurons were made by calculating the percentage of perikaryal membrane with postsynaptic modification. Five GnRH neurons per animal were measured using computerized morpho-metrics and differences in the percent of membrane with synaptic modification between experimental groups were tested using the Mann-Whitney U non-parametric statistic. There was no difference in the total density of synaptic input to GnRH neurons in the young and old animals, or in the proportion of this input that was immunoreactive for glutamic acid decarboxylase. Similar measurements were made on random, non-identified neurons in the same region and a significant decrease with ageing in total synaptic input was found, though the glutamic acid decarboxylase component was unchanged. The present results are in contrast to our earlier findings on virgin males and suggest that reproductive behavioral experience affects the connectivity of GnRH neurons.     

Differential bicuculline-induced epileptogenesis in rat neonatal, juvenile and adult CA3 pyramidal neurons in vitro.

The GABA(A) receptor antagonist bicuculline methiodide (BMI, 10 microM) transformed the evoked synaptic responses, recorded intracellularly from the CA3 area of neonatal (postnatal days 3-7, P3-P7), juvenile (P8-P20) and adult hippocampal slices, into long-lasting paroxysmal depolarizations (PDs), with repetitive action potentials (APs). In the same preparation, GABA(A)-mediated fast-IPSPs were depolarizing at resting membrane potential (RMP), with a reversal potential shifting to a hyperpolarizing direction with age (n=15, P6-P17). BMI provoked also spontaneous PDs in juvenile (20/30) and adult (7/10) but not in neonatal (0/12) neurons. PDs were depressed by either the NMDA receptor antagonist CPP (10 microM) or the non-NMDA antagonist CNQX (10 microM), but were blocked only by the combination of the two (n=6), indicating that activation of either NMDA or non-NMDA receptors can independently sustain PDs in immature hippocampus. In conclusion, these findings show that endogenous GABA tonically inhibits CA3 synaptic responses in neonatal life despite the depolarizing nature of GABA(A)-mediated potentials. Moreover, they suggest that during the 1st postnatal week, disinhibition alone is not sufficient to provoke spontaneous epileptiform discharges in CA3 hippocampal area.     

Tonic GABA(A) receptor-mediated currents in human brain

GABA(A) receptors can mediate both phasic (synaptic) and tonic (extrasynaptic) forms of inhibition. It has been proposed that tonic inhibition plays a critical part in controlling neuronal and network excitability. Although tonic GABA(A) receptor-mediated currents have been well characterized in rodents, their existence in human tissue has yet to be demonstrated. Here we show that tonic currents can be recorded from human tissue obtained from patients undergoing temporal lobectomies. Tonic GABA(A) receptor-mediated currents were present in pyramidal cells and interneurons in layer V-VI of temporal neocortex and granule cells in the dentate gyrus. These tonic currents have cell type-specific pharmacologies, opening up the possibility of targeted therapeutics.     

Selective action of piretanide on primary afferent GABA responses in the frog spinal cord

The effects of piretanide, a drug which blocks active chloride transport in other systems, was examined on amino acid responses and synaptic potentials in the frog spinal cord. It was found that piretanide had no effect on GABA ventral root hyperpolarizing responses, but abolished dorsal root depolarizing responses. Dorsal root glutamate responses were little affected by piretanide. The depression of GABA responses on dorsal root ganglion cells was accompanied by a large decrease in the conductance change elicited by GABA. In addition, there was a small shift of the GABA equilibrium potential toward the resting membrane potential, which is consistent with a reduced inward pumping of chloride ions. Piretanide abolished the dorsal root potential elicited by ventral root stimulation. In addition it reduced the early part of the dorsal root potential evoked by stimulating an adjacent dorsal root but had remarkably little effect on the late phase.     

Isoflurane modulates glutamatergic and GABAergic neurotransmission in the amygdala

Attempts have been made to attribute the particular features of general anaesthesia such as hypnosis, analgesia, amnesia and autonomic stability to certain brain regions. In the present study, we examined the effects of the commonplace volatile anaesthetic isoflurane on synaptic transmission in an in vitro slice preparation of the murine amygdala. Despite the established role of this limbic structure in the formation of aversive memories, conditioned fear and anxiety, as well as pain processing and regulation of sympathetic tone, the influence of volatile anaesthetics on synaptic signalling has not yet been investigated in this region of the brain. Evoked postsynaptic currents were monitored from principal neurons in the basolateral nucleus of the amygdala by means of patch-clamp recording. The mixed postsynaptic currents were mediated by non-NMDA, NMDA, GABA A and GABA B receptors. Isoflurane added to the perfusion medium reduced the strength of synaptic signalling following the activation of non-NMDA, NMDA, and GABA B receptors, whereas the GABA A receptor-mediated responses were enhanced. The overall reduction of neuronal excitability was also reflected in a reduction of field potential amplitudes. Isoflurane neither changed the membrane resting potential nor the input resistance of principal neurons in the amygdala. The present results may contribute to the understanding of how stress reactions and long-lasting neuroplastic processes are suppressed under general anaesthesia.