G-Protein-coupled modulation of presynaptic calcium currents and transmitter release by a GABAB receptor

Activation of metabotropic GABA receptors increases the energy barrier for vesicle fusion

Neurotransmitter release from presynaptic terminals is under the tight control of various metabotropic receptors. We report here that in addition to the regulation of Ca(2+) channel activity, metabotropic GABA(B) receptors (GABA(B)Rs) at murine hippocampal glutamatergic synapses utilize an inhibitory pathway that directly targets the synaptic vesicle release machinery. Acute application of the GABA(B)R agonist baclofen rapidly and reversibly inhibits vesicle fusion, which occurs independently of the SNAP-25 C-terminus. Using applications of hypertonic sucrose solutions, we find that the size of the readily releasable pool remains unchanged by GABA(B)R activation, but the sensitivity of primed vesicles to hypertonic stimuli appears lowered as the response amplitudes at intermediate sucrose concentrations are smaller and release kinetics are slowed. These data show that presynaptic GABA(B)Rs can inhibit neurotransmitter release directly by increasing the energy barrier for vesicle fusion.

Dynamics of binaural processing in the mammalian sound localization pathway--the role of GABA(B) receptors

The initial binaural processing in the superior olive represents the fastest computation known in the entire mammalian brain. Although the binaural system has to perform under very different and often highly dynamic acoustic conditions, the integration of binaural information in the superior olivary complex (SOC) has not been considered to be adaptive or dynamic itself. Recent evidence, however, shows that the initial processing of interaural level and interaural time differences relies on well-adjusted interactions of both the excitatory and the inhibitory projections, respectively. Under static conditions, these inputs seem to be tightly balanced, but may also require dynamic adjustment for proper function when the acoustic environment changes. GABA(B) receptors are at least one mechanism rendering the system more dynamic than considered so far. A comprehensive description of how binaural processing in the SOC is dynamically regulated by GABA(B) receptors in adults and in early development is important for understanding how spatial auditory processing changes with acoustic context.

Modulation and control of synaptic transmission across the MNTB

The aim of this review is to consider the various forms and functions of transmission across the calyx of Held/MNTB synapse and how its modulation might contribute to auditory processing. The calyx of Held synapse is the largest synapse in the mammalian brain which uses the conventional excitatory synaptic transmitter, glutamate. It is sometimes portrayed as the 'ultimate' in synaptic signalling: it is a synaptic relay in which a single axon forms one synaptic terminal onto one specific target neuron. Questions that are often raised are: "Why does such a large and secure synapse need any form of modulation? Surely it is built simply to guarantee firing an action potential in the target neuron? If this synapse is so secure, why is a synapse needed at all?" Investigating these questions explains some general limitations of transmission at synapses and provides insight into the ionic basis of neuronal function by bringing together in vivo and in vitro approaches. We will start by defining the firing behaviour of MNTB neurons in vitro (in response to synaptic stimulation or current injection) and in vivo (in response to sound) and examining the reasons for different types of firing under the two conditions. Then we will consider some of the mechanisms by which transmission can be regulated. We will finish by discussing the following hypothesis: modulation and adaptation of presynaptic and postsynaptic conductances at the calyx of Held relay synapse are aimed at maximising the security of sound onset encoding while providing secondary information on frequency spectrum, harmonic envelope and duration of sound throughout the later part of the response.

The level and distribution of the GABA(B)R2 receptor subunit in the rat's central auditory system

The GABA(B) receptor is important for the function of auditory neurons. We used Western blotting and immunohistochemical methods to examine the level and localization of GABA(B)R2, a required subunit of a functional GABA(B) receptor, in the rat's central auditory system. Results revealed that this subunit was expressed throughout the auditory system with the level being high in the layers I-V of the auditory cortex, medial geniculate nucleus, dorsomedial and lateral parts of the inferior colliculus, and the molecular and fusiform cell layers of the dorsal cochlear nucleus. Labeled cell bodies were found in all the areas showing immunoreactivity. Neuropil labeling was strong in areas with high overall levels of immunoreactivity. Regional distributions of the receptor subunit revealed clear boundaries of some auditory subnuclei including the dorsal and ventral cochlear nuclei and the lateral superior olivary nucleus. Differences in immunoreactivity were found between the central nucleus and the dorsal cortex of the inferior colliculus and between the dorsal and ventral parts of the ventral nucleus of the lateral lemniscus, although no clear boundaries were observed. No differences in immunoreactivity were found between the core and the belt areas of the auditory cortex and among the subdivisions of the medial geniculate nucleus. The regional distribution of the receptor subunit in auditory structures is consistent with inputs to these structures and the cellular localization of the receptor in auditory neurons supports the contribution of the GABA(B) receptor to synaptic responses in these neurons.

Involvement of protein kinase C and protein kinase A in the enhancement of L-type calcium current by GABAB receptor activation in neonatal hippocampus

In the early neonatal period activation of GABAB receptors attenuates calcium current through N-type calcium channels while enhancing current through L-type calcium channels in rat hippocampal neurons. The attenuation of N-type calcium current has been previously demonstrated to occur through direct interactions of the βγ subunits of Gi/o G-proteins, but the signal transduction pathway for the enhancement of L-type calcium channels in mammalian neurons remains unknown. In the present study, calcium currents were elicited in acute cultures from postnatal day 6-8 rat hippocampi in the presence of various modulators of protein kinase A (PKA) and protein kinase C (PKC) pathways. Overnight treatment with an inhibitor of Gi/o (pertussis toxin, 200 ng/ml) abolished the attenuation of calcium current by the GABAB agonist, baclofen (10 μM) with no effect on the enhancement of calcium current. These data indicate that while the attenuation of N-type calcium current is mediated by the Gi/o subtype of G-protein, the enhancement of L-type calcium current requires activation of a different G-protein. The enhancement of the sustained component of calcium current by baclofen was blocked by PKC inhibitors, GF-109203X (500 nM), chelerythrine chloride (5 μM), and PKC fragment 19-36 (2 μM) and mimicked by the PKC activator phorbol-12-myristate-13-acetate (1 μM). The enhancement of the sustained component of calcium current was blocked by PKA inhibitors H-89 (1 μM) and PKA fragment 6-22 (500 nM) but not Rp-cAMPS (30 μM) and it was not mimicked by the PKA activator, 8-Br-cAMP (500 μM-1 mM). The data suggest that activation of PKC alone is sufficient to enhance L-type calcium current but that PKA may also be involved in the GABAB receptor mediated effect.

Allosteric modulation of family C G-protein-coupled receptors: from molecular insights to therapeutic perspectives

Allosteric receptor modulation is an attractive concept in drug targeting because it offers important potential advantages over conventional orthosteric agonism or antagonism. Allosteric ligands modulate receptor function by binding to a site distinct from the recognition site for the endogenous agonist. They often have no effect on their own and therefore act only in conjunction with physiological receptor activation. This article reviews the current status of allosteric modulation at family C G-protein coupled receptors in the light of their specific structural features on the one hand and current concepts in receptor theory on the other hand. Family C G-protein-coupled receptors are characterized by a large extracellular domain containing the orthosteric agonist binding site known as the "venus flytrap module" because of its bilobal structure and the dynamics of its activation mechanism. Mutational analysis and chimeric constructs have revealed that allosteric modulators of the calcium-sensing, metabotropic glutamate and GABA(B) receptors bind to the seven transmembrane domain, through which they modify signal transduction after receptor activation. This is in contrast to taste-enhancing molecules, which bind to different parts of sweet and umami receptors. The complexity of interactions between orthosteric and allosteric ligands is revealed by a number of adequate biochemical and electrophysiological assay systems. Many allosteric family C GPCR modulators show in vivo efficacy in behavioral models for a variety of clinical indications. The positive allosteric calcium sensing receptor modulator cinacalcet is the first drug of this type to enter the market and therefore provides proof of principle in humans.

Formation and maturation of the calyx of Held

Sound localization requires precise and specialized neural circuitry. A prominent and well-studied specialization is found in the mammalian auditory brainstem. Globular bushy cells of the ventral cochlear nucleus (VCN) project contralaterally to neurons of the medial nucleus of the trapezoid body (MNTB), where their large axons terminate on cell bodies of MNTB principal neurons, forming the calyces of Held. The VCN-MNTB pathway is necessary for the accurate computation of interaural intensity and time differences; MNTB neurons provide inhibitory input to the lateral superior olive, which compares levels of excitation from the ipsilateral ear to levels of tonotopically matched inhibition from the contralateral ear, and to the medial superior olive, where precise inhibition from MNTB neurons tunes the delays of binaural excitation. Here we review the morphological and physiological aspects of the development of the VCN-MNTB pathway and its calyceal termination, along with potential mechanisms that give rise to its precision. During embryonic development, VCN axons grow towards the midline, cross the midline into the region of the presumptive MNTB and then form collateral branches that will terminate in calyces of Held. In rodents, immature calyces of Held appear in MNTB during the first few days of postnatal life. These calyces mature morphologically and physiologically over the next three postnatal weeks, enabling fast, high fidelity transmission in the VCN-MNTB pathway.

GABAB receptor coupling to G-proteins and ion channels

GABA(B) receptors have been found to play a key role in regulating membrane excitability and synaptic transmission in the brain. The GABA(B) receptor is a G-protein coupled receptor (GPCR) that associates with a subset of G-proteins (pertussis toxin sensitive Gi/o family), that in turn regulate specific ion channels and trigger cAMP cascades. In this review, we describe the relationships between the GABA(B) receptor, its effectors and associated proteins that mediate GABA(B) receptor function within the brain. We discuss a unique feature of the GABA(B) receptor, the requirement for heterodimerization to produce functional receptors, as well as an increasing body of evidence that suggests GABA(B) receptors comprise a macromolecular signaling heterocomplex, critical for efficient targeting and function of the receptors. Within this complex, GABA(B) receptors associate specifically with Gi/o G-proteins that regulate voltage-gated Ca(2+) (Ca(V)) channels, G-protein activated inwardly rectifying K(+) (GIRK) channels, and adenylyl cyclase. Numerous studies have revealed that lipid rafts, scaffold proteins, targeting motifs in the receptor, and regulators of G-protein signaling (RGS) proteins also contribute to the function of GABA(B) receptors and affect cellular processes such as receptor trafficking and activity-dependent desensitization. This complex regulation of GABA(B) receptors in the brain may provide opportunities for new ways to regulate GABA-dependent inhibition in normal and diseased states of the nervous system.

Neuron-astrocyte interactions in the medial nucleus of the trapezoid body

The calyx of Held (CoH) synapse serves as a model system to analyze basic mechanisms of synaptic transmission. Astrocyte processes are part of the synaptic structure and contact both pre- and postsynaptic membranes. In the medial nucleus of the trapezoid body (MNTB), midline stimulation evoked a current response that was not mediated by glutamate receptors or glutamate uptake, despite the fact that astrocytes express functional receptors and transporters. However, astrocytes showed spontaneous Ca²⁺ responses and neuronal slow inward currents (nSICs) were recorded in the postsynaptic principal neurons (PPNs) of the MNTB. These currents were correlated with astrocytic Ca²⁺ activity because dialysis of astrocytes with BAPTA abolished nSICs. Moreover, the frequency of these currents was increased when Ca²⁺ responses in astrocytes were elicited. NMDA antagonists selectively blocked nSICs while D-serine degradation significantly reduced NMDA-mediated currents. In contrast to previous studies in the hippocampus, these NMDA-mediated currents were rarely synchronized.

GABAB receptors modulate NMDA receptor calcium signals in dendritic spines

Metabotropic GABA(B) receptors play a fundamental role in modulating the excitability of neurons and circuits throughout the brain. These receptors influence synaptic transmission by inhibiting presynaptic release or activating postsynaptic potassium channels. However, their ability to directly influence different types of postsynaptic glutamate receptors remains unresolved. Here we examine GABA(B) receptor modulation in layer 2/3 pyramidal neurons from the mouse prefrontal cortex. We use two-photon laser-scanning microscopy to study synaptic modulation at individual dendritic spines. Using two-photon optical quantal analysis, we first demonstrate robust presynaptic modulation of multivesicular release at single synapses. Using two-photon glutamate uncaging, we then reveal that GABA(B) receptors strongly inhibit NMDA receptor calcium signals. This postsynaptic modulation occurs via the PKA pathway and does not affect synaptic currents mediated by AMPA or NMDA receptors. This form of GABA(B) receptor modulation has widespread implications for the control of calcium-dependent neuronal function.

Down-regulated GABAergic expression in the olfactory bulb layers of the mouse deficient in monoamine oxidase B and administered with amphetamine

This study explores primarily the role of the activity of monoamine oxidase B (MAOB) in the regulation of glutamic acid decarboxylase(67) (GAD(67)) expression in distinct layers of main olfactory bulb (OlfB), which links the limbic system. Moreover, the response of GAD(67) was investigated to amphetamine perturbation in the absence of MAOB activity. Immunocytochemical analysis was performed on OlfB sections prepared from the adult wild type (WT) and the MAOB gene-knocked-out (KO) mice after receiving repeated intraperitoneal injections (two doses per day, total seven doses) of saline or amphetamine, 5 mg/kg. The levels of the GAD(67) immunoreactivity were approximate 25 and 38% lower in respective glomerular (GloL) and mitral cell layers (ML) of saline-treated KO mice than that of WT, whereas similar in the external plexiform or granule cell layers (GraL) of the KO and WT. In the GloL, the level of tyrosine hydroxylase was 39% lower in the KO mice than WT, implicating different dopamine content in the KO from WT. The amphetamine exposure down-regulated the levels of GAD(67) in the WT layers by 46 to 52%, and in KO layers 65 to 71%, except ML. The GraL GAD(67) level may be regulated by the activation of CREB, as the phosphorylated (p) CREB coexisted with GAD(67), and the percentage of GAD(67)-expressing pCREB neurons was decreased by the amphetamine exposure. The data indicate that the activity of MAOB could modulate the regular and amphetamine-perturbed expression of GAD(67) and pCREB. Thus, interactions are suggested among the MAOB activity, GABA content of OlfB, and olfaction.

GABAB receptor agonism as a novel therapeutic modality in the treatment of gastroesophageal reflux disease

Defined pharmacologically by its insensitivity to the GABA(A) antagonist bicuculline and sensitivity to the GABA analogue baclofen, the G protein-linked gamma-aminobutyric acid type B (GABA(B)) receptor couples to adenylyl cyclase, voltage-gated calcium channels, and inwardly-rectifying potassium channels. On the basis of a wealth of preclinical data in conjunction with early clinical observations that baclofen improves symptoms of gastroesophageal reflux disease (GERD), the GABA(B) receptor has been proposed as a therapeutic target for a number of diseases including GERD. Subsequently, there has been a significant effort to develop a peripherally-restricted GABA(B) agonist that is devoid of the central nervous system side effects that are observed with baclofen. In this article we review the in vitro and in vivo pharmacology of the peripherally-restricted GABA(B) receptor agonists and the preclinical and clinical development of lesogaberan (AZD3355, (R)-(3-amino-2-fluoropropyl) phosphinic acid), a potent and predominately peripherally-restricted GABA(B) receptor agonist with a preclinical therapeutic window superior to baclofen.

GABA transporter GAT1: a crucial determinant of GABAB receptor activation in cortical circuits?

The GABA transporter 1 (GAT1), the main plasma membrane GABA transporter in brain tissue, mediates translocation of GABA from the extracellular to the intracellular space. Whereas GAT1-mediated uptake could generally terminate the synaptic effects of GABA, recent studies suggest a more complex physiological role. This chapter reviews evidence suggesting that in hippocampal and neocortical circuits, GAT1-mediated GABA transport regulates the electrophysiological effects of GABA(B) receptor (GABA(B)R) activation by synaptically-released GABA. Contrasting with synaptic GABA(A) receptors, GABA(B)Rs display high GABA binding affinity, slow G protein-coupled mediated signaling, and a predominantly extrasynaptic localization. Such GABA(B)R properties determine production of slow inhibitory postsynaptic potentials (IPSPs) and slow presynaptic effects. Such effects possibly require diffusion of GABA far away from the release sites, and consequently both GABA(B)R-mediated IPSPs and presynaptic effects are strongly enhanced when GAT1-mediated uptake is blocked. Studies are reviewed here which indicate that GABA(B)R-mediated IPSPs seem to be produced by dendrite-targeting GABA neurons including specifically, although perhaps not exclusively, the neurogliaform cell class. In contrast, the GABA interneuron subtypes that synapse onto the perisomatic membrane of pyramidal cells mostly signal via synaptic GABA(A)Rs. This chapter reviews data suggesting that neurogliaform cells produce electrophysiological effects onto other neurons in the cortical cell network via GABA(B)R-mediated volume transmission that is highly regulated by GAT1 activity. Therefore, the role of GAT1 in controlling GABA(B)R-mediated signaling is markedly different from its regulation of GABA(A)R-mediated fast synaptic transmission.

Influx of calcium through L-type calcium channels in early postnatal regulation of chloride transporters in the rat hippocampus

During the early postnatal period, GABA(B) receptor activation facilitates L-type calcium current in rat hippocampus. One developmental process that L-type current may regulate is the change in expression of the K(+)Cl(-) co-transporter (KCC2) and N(+)K(+)2Cl(-) co-transporter (NKCC1), which are involved in the maturation of the GABAergic system. The present study investigated the connection between L-type current, GABA(B) receptors, and expression of chloride transporters during development. The facilitation of L-type current by GABA(B) receptors is more prominent in the second week of development, with the highest percentage of cells exhibiting facilitation in cultures isolated from 7 day old rats (37.5%). The protein levels of KCC2 and NKCC1 were investigated to determine the developmental timecourse of expression as well as expression following treatment with an L-type channel antagonist and a GABA(B) receptor agonist. The time course of both chloride transporters in culture mimics that seen in hippocampal tissue isolated from various ages. KCC2 levels increased drastically in the first two postnatal weeks while NKCC1 remained relatively stable, suggesting that the ratio of the chloride transporters is important in mediating the developmental change in chloride reversal potential. Treatment of cultures with the L-type antagonist nimodipine did not affect protein levels of NKCC1, but significantly decreased the upregulation of KCC2 during the first postnatal week. In addition, calcium current facilitation occurs slightly before the large increase in KCC2 expression. These results suggest that the expression of KCC2 is regulated by calcium influx through L-type channels in the early postnatal period in hippocampal neurons.

The physiological role of pre- and postsynaptic GABA(B) receptors in membrane excitability and synaptic transmission of neurons in the rat's dorsal cortex of the inferior colliculus

In the inferior colliculus (IC), GABAergic inhibition mediated by GABA(A) receptors has been shown to play a significant role in regulating physiological responses, but little is known about the physiological role of GABA(B) receptors in IC neurons. In the present study, we used whole-cell patch clamp recording in vitro to investigate the effects of activation of GABA(B) receptors on membrane excitability and synaptic transmission of neurons in the rat's dorsal cortex of the inferior colliculus (ICD). Repetitive stimulation of GABAergic inputs to ICD neurons at high frequencies could elicit a slow and long-lasting postsynaptic response, which was reversibly abolished by the GABA(B) receptor antagonist, CGP 35348. The results suggest that postsynaptic GABA(B) receptors can directly mediate inhibitory synaptic transmission in ICD. The role of postsynaptic GABA(B) receptors in regulation of membrane excitability was further investigated by application of the GABA(B) receptor agonist, baclofen. Baclofen hyperpolarized the cell, reduced the membrane input resistance and firing rate, increased the threshold for generating action potentials (APs), and decreased the amplitude of the AP and its associated after-hyperpolarization. The Ca2+-mediated rebound depolarization following hyperpolarization and the depolarization hump at the beginning of membrane depolarization were also suppressed by baclofen. In voltage clamp experiments, baclofen induced inward rectifying K+ current and reduced low- and high-threshold Ca2+ currents, which may account for the suppression of membrane excitability by postsynaptic GABA(B) receptors. Application of baclofen also reduced excitatory synaptic responses mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, and inhibitory synaptic responses mediated by GABA(A) receptors. Baclofen increased the ratios of 2nd/1st excitatory and inhibitory postsynaptic currents to paired-pulse stimulation of the synaptic inputs. These results suggest that fast glutamatergic and GABAergic synaptic transmission in ICD can be modulated by presynaptic GABA(B) receptors.

Subcellular compartment-specific molecular diversity of pre- and post-synaptic GABA-activated GIRK channels in Purkinje cells

Activation of G protein-gated inwardly-rectifying K(+) (GIRK or Kir3) channels by metabotropic gamma-aminobutyric acid (B) (GABA(B)) receptors is an essential signalling pathway controlling neuronal excitability and synaptic transmission in the brain. To investigate the relationship between GIRK channel subunits and GABA(B) receptors in cerebellar Purkinje cells at post- and pre-synaptic sites, we used biochemical, functional and immunohistochemical techniques. Co-immunoprecipitation analysis demonstrated that GIRK subunits are co-assembled with GABA(B) receptors in the cerebellum. Immunoelectron microscopy showed that the subunit composition of GIRK channels in Purkinje cell spines is compartment-dependent. Thus, at extrasynaptic sites GIRK channels are formed by GIRK1/GIRK2/GIRK3, post-synaptic densities contain GIRK2/GIRK3 and dendritic shafts contain GIRK1/GIRK3. The post-synaptic association of GIRK subunits with GABA(B) receptors in Purkinje cells is supported by the subcellular regulation of the ion channel and the receptor in mutant mice. At pre-synaptic sites, GIRK channels localized to parallel fibre terminals are formed by GIRK1/GIRK2/GIRK3 and co-localize with GABA(B) receptors. Consistent with this morphological evidence we demonstrate their functional interaction at axon terminals in the cerebellum by showing that GIRK channels play a role in the inhibition of glutamate release by GABA(B) receptors. The association of GIRK channels and GABA(B) receptors with excitatory synapses at both post- and pre-synaptic sites indicates their intimate involvement in the modulation of glutamatergic neurotransmission in the cerebellum.

Pre-synaptic GABA receptors inhibit glutamate release through GIRK channels in rat cerebral cortex

Neuronal G protein-gated inwardly rectifying potassium (GIRK) channels mediate the slow inhibitory effects of many neurotransmitters post-synaptically. However, no evidence exists that supports that GIRK channels play any role in the inhibition of glutamate release by GABA(B) receptors. In this study, we show for the first time that GABA(B) receptors operate through two mechanisms in nerve terminals from the cerebral cortex. As shown previously, GABA(B) receptors reduces glutamate release and the Ca(2+) influx mediated by N-type Ca(2+) channels in a mode insensitive to the GIRK channel blocker tertiapin-Q and consistent with direct inhibition of this voltage-gated Ca(2+) channel. However, by means of weak stimulation protocols, we reveal that GABA(B) receptors also reduce glutamate release mediated by P/Q-type Ca(2+) channels, and that these responses are reversed by the GIRK channel blocker tertiapin-Q. Consistent with the functional interaction between GABA(B) receptors and GIRK channels at nerve terminals we demonstrate by immunogold electron immunohistochemistry that pre-synaptic boutons of asymmetric synapses co-express GABA(B) receptors and GIRK channels, thus suggesting that the functional interaction of these two proteins, found at the post-synaptic level, also occurs at glutamatergic nerve terminals.

Tonic activation of GABAB receptors reduces release probability at inhibitory connections in the cerebellar glomerulus

In the cerebellum, granule cells are inhibited by Golgi cells through GABAergic synapses generating complex responses involving both phasic neurotransmitter release and the establishment of ambient gamma-aminobutyric acid (GABA) levels. Although at this synapse the mechanisms of postsynaptic integration have been clarified to a considerable extent, the mechanisms of neurotransmitter release remained largely unknown. Here we have investigated the quantal properties of release during repetitive neurotransmission, revealing that tonic GABA(B) receptor activation by ambient GABA regulates release probability. Blocking GABA(B) receptors with CGP55845 enhanced the first inhibitory postsynaptic current (IPSC) and short-term depression in a train while reducing trial-to-trial variability and failures. The changes caused by CGP55845 were similar to those caused by increasing extracellular Ca(2+) concentration, in agreement with a presynaptic GABA(B) receptor modulation of release probability. However, the slow tail following IPSC peak demonstrated a remarkable temporal summation and was not modified by CGP55845 or extracellular Ca(2+) increase. This result shows that tonic activation of presynaptic GABA(B) receptors by ambient GABA selectively regulates the onset of inhibition bearing potential consequences for the dynamic regulation of signal transmission through the mossy fiber-granule cell pathway of the cerebellum.

Retrograde GABA signaling adjusts sound localization by balancing excitation and inhibition in the brainstem

Central processing of acoustic cues is critically dependent on the balance between excitation and inhibition. This balance is particularly important for auditory neurons in the lateral superior olive, because these compare excitatory inputs from one ear and inhibitory inputs from the other ear to compute sound source location. By applying GABA(B) receptor antagonists during sound stimulation in vivo, it was revealed that these neurons adjust their binaural sensitivity through GABA(B) receptors. Using an in vitro approach, we then demonstrate that these neurons release GABA during spiking activity. Consequently, GABA differentially regulates transmitter release from the excitatory and inhibitory terminals via feedback to presynaptic GABA(B) receptors. Modulation of the synaptic input strength, by putative retrograde release of neurotransmitter, may enable these auditory neurons to rapidly adjust the balance between excitation and inhibition, and thus their binaural sensitivity, which could play an important role as an adaptation to various listening situations.

Interactions between multiple sources of short-term plasticity during evoked and spontaneous activity at the rat calyx of Held

Sustained activity at most central synapses is accompanied by a number of short-term changes in synaptic strength which act over a range of time scales. Here we examine experimental data and develop a model of synaptic depression at the calyx of Held synaptic terminal that combines many of these mechanisms (acting at differing sites and across a range of time scales). This new model incorporates vesicle recycling, facilitation, activity-dependent vesicle retrieval and multiple mechanisms affecting calcium channel activity and release probability. It can accurately reproduce the time course of experimentally measured short-term depression across different stimulus frequencies and exhibits a slow decay in EPSC amplitude during sustained stimulation. We show that the slow decay is a consequence of vesicle release inhibition by multiple mechanisms and is accompanied by a partial recovery of the releasable vesicle pool. This prediction is supported by patch-clamp data, using long duration repetitive EPSC stimulation at up to 400 Hz. The model also explains the recovery from depression in terms of interaction between these multiple processes, which together generate a stimulus-history-dependent recovery after repetitive stimulation. Given the high rates of spontaneous activity in the auditory pathway, the model also demonstrates how these multiple interactions cause chronic synaptic depression under in vivo conditions. While the magnitude of the depression converges to the same steady state for a given frequency, the time courses of onset and recovery are faster in the presence of spontaneous activity. We conclude that interactions between multiple sources of short-term plasticity can account for the complex kinetics during high frequency stimulation and cause stimulus-history-dependent recovery at this relay synapse.

Role of GABA receptors in nitric oxide inhibition of dorsolateral periaqueductal gray neurons

Nitric oxide (NO) affects neuronal activity of the midbrain periaqueductal gray (PAG). The purpose of this report was to investigate the role of GABA receptors in NO modulation of neuronal activity through inhibitory and excitatory synaptic inputs within the dorsolateral PAG (dl-PAG). First, spontaneous miniature inhibitory postsynaptic currents (mIPSCs) and excitatory postsynaptic currents (mEPSCs) were recorded using whole cell voltage-clamp methods. Increased NO by either S-nitroso-N-acetyl-penicillamine (SNAP, 100 microM) or L-arginine (50 microM) significantly augmented the frequency of mIPSCs of the dl-PAG neurons without altering their amplitudes or decay time constants. The effects were eliminated after bath application of carboxy-PTIO (NO scavenger), and 1-(2-trifluorom-ethylphenyl) imidazole (NO synthase inhibitor). In contrast, SNAP and L-arginine did not alter mEPSCs in dl-PAG neurons. However the frequency of mEPSCs was significantly increased with prior application of the GABA(B) receptors antagonist, CGP55845. In addition, NO significantly decreased the discharge rate of spontaneous action potentials in the dl-PAG neurons and the effect was reduced in the presence of the GABA(A) receptor antagonist, bicuculline. Our data show that within the dl-PAG NO potentiates the synaptic release of GABA, while NO-induced GABA presynaptically inhibits glutamate release through GABA(B) receptors. Overall, NO suppresses neuronal activity of the dl-PAG via a potentiation of GABAergic synaptic inputs and via GABA(A) receptors.

Presynaptic signaling by heterotrimeric G-proteins

G-proteins (guanine nucleotide-binding proteins) are membrane-attached proteins composed of three subunits, alpha, beta, and gamma. They transduce signals from G-protein coupled receptors (GPCRs) to target effector proteins. The agonistactivated receptor induces a conformational change in the G-protein trimer so that the alpha-subunit binds GTP in exchange for GDP and alpha-GTP, and betagamma-subunits separate to interact with the target effector. Effector-interaction is terminated by the alpha-subunit GTPase activity, whereby bound GTP is hydrolyzed to GDP. This is accelerated in situ by RGS proteins, acting as GTPase-activating proteins (GAPs). Galpha-GDP and Gbetagamma then reassociate to form the Galphabetagamma trimer. G-proteins primarily involved in the modulation of neurotransmitter release are G(o), G(q) and G(s). G(o) mediates the widespread presynaptic auto-inhibitory effect of many neurotransmitters (e.g., via M2/M4 muscarinic receptors, alpha(2) adrenoreceptors, micro/delta opioid receptors, GABAB receptors). The G(o) betagamma-subunit acts in two ways: first, and most ubiquitously, by direct binding to CaV2 Ca(2+) channels, resulting in a reduced sensitivity to membrane depolarization and reduced Ca(2+) influx during the terminal action potential; and second, through a direct inhibitory effect on the transmitter release machinery, by binding to proteins of the SNARE complex. G(s) and G(q) are mainly responsible for receptor-mediated facilitatory effects, through activation of target enzymes (adenylate cyclase, AC and phospholipase-C, PLC respectively) by the GTP-bound alpha-subunits.

The metabotropic glutamate receptor 7 (mGluR7) allosteric agonist AMN082 modulates nucleus accumbens GABA and glutamate, but not dopamine, in rats

The group III metabotropic glutamate receptor 7 (mGluR7) has been implicated in many neurological and psychiatric diseases, including drug addiction. However, it is unclear whether and how mGluR7 modulates nucleus accumbens (NAc) dopamine (DA), L-glutamate or gamma-aminobutyric acid (GABA), important neurotransmitters believed to be involved in such neuropsychiatric diseases. In the present study, we found that systemic or intra-NAc administration of the mGluR7 allosteric agonist N,N'-dibenzyhydryl-ethane-1,2-diamine dihydrochloride (AMN082) dose-dependently lowered NAc extracellular GABA and increased extracellular glutamate, but had no effect on extracellular DA levels. Such effects were blocked by (R,S)-alpha-methylserine-O-phosphate (MSOP), a group III mGluR antagonist. Intra-NAc perfusion of tetrodotoxin (TTX) blocked the AMN082-induced increases in glutamate, but failed to block the AMN082-induced reduction in GABA, suggesting vesicular glutamate and non-vesicular GABA origins for these effects. In addition, blockade of NAc GABAB receptors by 2-hydroxy-saclofen itself elevated NAc extracellular glutamate. Intra-NAc perfusion of 2-hydroxy-saclofen not only abolished the enhanced extracellular glutamate normally produced by AMN082, but also decreased extracellular glutamate in a TTX-resistant manner. We interpret these findings to suggest that the increase in glutamate is secondary to the decrease in GABA, which overcomes mGluR7 activation-induced inhibition of non-vesicular glutamate release. In contrast to its modulatory effect on GABA and glutamate, the mGluR7 receptor does not appear to modulate NAc DA release.

G-protein-coupled GABAB receptors inhibit Ca2+ channels and modulate transmitter release in descending turtle spinal cord terminal synapsing motoneurons

Presynaptic gamma-aminobutyric acid type B receptors (GABA(B)Rs) regulate transmitter release at many central synapses by inhibiting Ca(2+) channels. However, the mechanisms by which GABA(B)Rs modulate neurotransmission at descending terminals synapsing on motoneurons in the spinal cord remain unexplored. To address this issue, we characterized the effects of baclofen, an agonist of GABA(B)Rs, on the monosynaptic excitatory postsynaptic potentials (EPSPs) evoked in motoneurons by stimulation of the dorsolateral funiculus (DLF) terminals in a slice preparation from the turtle spinal cord. We found that baclofen depressed neurotransmission in a dose-dependent manner (IC(50) of approximately 2 microM). The membrane time constant of the motoneurons did not change, whereas the amplitude ratio of the evoked EPSPs in response to a paired pulse was altered in the presence of the drug, suggesting a presynaptic mechanism. Likewise, the use of N- and P/Q-type Ca(2+) channel antagonists (omega-conotoxin GVIA and omega-agatoxin IVA, respectively) also depressed EPSPs significantly. Therefore, these channels are likely involved in the Ca(2+) influx that triggers transmitter release from DLF terminals. To determine whether the N and P/Q channels were regulated by GABA(B)R activation, we analyzed the action of the toxins in the presence of baclofen. Interestingly, baclofen occluded omega-conotoxin GVIA action by approximately 50% without affecting omega-agatoxin IVA inhibition, indicating that the N-type channels are the target of GABA(B)Rs. Lastly, the mechanism underlying this effect was further assessed by inhibiting G-proteins with N-ethylmaleimide (NEM). Our data show that EPSP depression caused by baclofen was prevented by NEM, suggesting that GABA(B)Rs inhibit N-type channels via G-protein activation.

Changes in synaptic transmission properties due to the expression of N-type calcium channels at the calyx of Held synapse of mice lacking P/Q-type calcium channels

P/Q-type and N-type calcium channels mediate transmitter release at rapidly transmitting central synapses, but the reasons for the specific expression of one or the other in each particular synapse are not known. Using whole-cell patch clamping from in vitro slices of the auditory brainstem we have examined presynaptic calcium currents (I(pCa)) and glutamatergic excitatory postsynaptic currents (EPSCs) at the calyx of Held synapse from transgenic mice in which the alpha(1A) pore-forming subunit of the P/Q-type Ca(2+) channels is ablated (KO). The power relationship between Ca(2+) influx and quantal output was studied by varying the number of Ca(2+) channels engaged in triggering release. Our results have shown that more overlapping Ca(2+) channel domains are required to trigger exocytosis when N-type replace P/Q-type calcium channels suggesting that P/Q type Ca(2+) channels are more tightly coupled to synaptic vesicles than N-type channels, a hypothesis that is verified by the decrease in EPSC amplitudes in KO synapses when the slow Ca(2+) buffer EGTA-AM was introduced into presynaptic calyces. Significant alterations in short-term synaptic plasticity were observed. Repetitive stimulation at high frequency generates short-term depression (STD) of EPSCs, which is not caused by presynaptic Ca(2+) current inactivation neither in WT or KO synapses. Recovery after STD is much slower in the KO than in the WT mice. Synapses from KO mice exhibit reduced or no EPSC paired-pulse facilitation and absence of facilitation in their presynaptic N-type Ca(2+) currents. Simultaneous pre- and postsynaptic double patch recordings indicate that presynaptic Ca(2+) current facilitation is the main determinant of facilitation of transmitter release. Finally, KO synapses reveal a stronger modulation of transmitter release by presynaptic GTP-binding protein-coupled receptors (gamma-aminobutyric acid type B receptors, GABA(B), and adenosine). In contrast, metabotropic glutamate receptors (mGluRs) are not functional at the synapses of these mice. These experiments reinforce the idea that presynaptic Ca(2+) channels expression may be tuned for speed and modulatory control through differential subtype expression.

4-Chloro-m-cresol, an activator of ryanodine receptors, inhibits voltage-gated K(+) channels at the rat calyx of Held

4-Chloro-m-cresol (4-CmC) is thought to be a specific activator of ryanodine receptors (RyRs). Using this compound, we examined whether the RyR-mediated Ca(2+) release is involved in transmitter release at the rat calyx of Held synapse in brainstem slices. Bath application of 4-CmC caused a dramatic increase in the amplitude of excitatory postsynaptic currents (TIFCs) with the half-maximal effective concentration of 0.12 mm. By making direct patch-clamp whole-cell recordings from presynaptic terminals, we investigated the mechanism by which 4-CmC facilitates transmitter release. 4-CmC markedly prolonged the duration of action potentials, with little effect on their rise time kinetics. In voltage-clamp recordings, 4-CmC inhibited voltage-gated presynaptic K(+) currents (I(pK)) by 53% (at +20 mV) but had no effect on voltage-gated presynaptic Ca(2+) currents (I(pCa)). In simultaneous pre- and postsynaptic recordings, 4-CmC had no effect on the TIFC evoked by I(pCa). Although immunocytochemical study of the calyceal terminals showed immunoreactivity to type 3 RyRs, ryanodine (0.02 mm) had no effect on the 4-CmC-induced TIFC potentiation. We conclude that the facilitatory effect of 4-CmC on nerve-evoked transmitter release is mediated by its inhibitory effect on I(pK).

Adenosine modulates transmission at the hippocampal mossy fibre synapse via direct inhibition of presynaptic calcium channels

The modulation of synaptic transmission by presynaptic ionotropic and metabotropic receptors is an important means to control and dynamically adjust synaptic strength. Even though synaptic transmission and plasticity at the hippocampal mossy fibre synapse are tightly controlled by presynaptic receptors, little is known about the downstream signalling mechanisms and targets of the different receptor systems. In the present study, we identified the cellular signalling cascade by which adenosine modulates mossy fibre synaptic transmission. By means of electrophysiological and optical recording techniques, we found that adenosine activates presynaptic A1 receptors and reduces Ca2+ influx into mossy fibre terminals. Ca2+ currents are directly modulated via a membrane-delimited pathway and the reduction of presynaptic Ca2+ influx can explain the inhibition of synaptic transmission. Specifically, we found that adenosine modulates both P/Q- and N-type presynaptic voltage-dependent Ca2+ channels and thereby controls transmitter release at the mossy fibre synapse.

Presynaptic inhibition of excitatory afferents to hilar mossy cells

The hippocampus contains one very strong recurrent excitatory network formed by associational connections between CA3 pyramidal cells and another that depends largely on a disynaptic excitatory pathway between dentate granule cells. The recurrent excitatory network in CA3 has long been considered a possible location of autoassociative memory storage, whereas changes in the level and arrangement of recurrent excitation between granule cells are strongly implicated in epileptogenesis. Hilar mossy cells are likely to receive collateral input from CA3 pyramidal cells and they are key intermediaries (by mossy fiber inputs) in the recurrent excitatory network between granule cells. The current study uses minimal stimulation techniques in an in vitro preparation of the rat dentate gyrus to examine presynaptic modulation of both mossy fiber and non-mossy fiber inputs to hilar mossy cells. We report that both mossy fiber and non-mossy fiber inputs to hilar mossy cells express presynaptic gamma-aminobutyric acid type B (GABA(B)) receptors that are subject to tonic inhibition by ambient GABA. We further find that only non-mossy fiber inputs express presynaptic muscarinic acetylcholine receptors, but that bath application of cholinergic agonists produces action potential-dependent increases in ambient GABA that can indirectly inhibit mossy fiber inputs. Finally, we demonstrate that mossy cells express high-affinity postsynaptic GABA(A) receptors that are also capable of detecting changes in ambient GABA produced by cholinergic agonists. Our results are among the first to directly characterize these important collateral inputs to hilar mossy cells and may help facilitate informed comparison between primary and collateral projections in two major excitatory pathways.

Activation of pedunculopontine tegmental PKA prevents GABAB receptor activation-mediated rapid eye movement sleep suppression in the freely moving rat

The pedunculopontine tegmental (PPT) GABAergic system plays a crucial role in the regulation of rapid eye movement (REM) sleep. I recently reported that the activation of PPT GABA(B) receptors suppressed REM sleep by inhibiting REM-on cells. One of the important mechanisms for GABA(B) receptor activation-mediated physiological action is the inhibition of the intracellular cAMP-dependent protein kinase A (cAMP-PKA) signaling pathway. Accordingly, I hypothesized that the PPT GABA(B) receptor activation-mediated REM sleep suppression effect could be mediated through inhibition of cAMP-PKA activation. To test this hypothesis, a GABA(B) receptor selective agonist, baclofen hydrochloride (baclofen), cAMP-PKA activator, Sp-adenosine 3',5'-cyclic monophosphothioate triethylamine (SpCAMPS), and vehicle control were microinjected into the PPT in selected combinations to determine effects on sleep-waking states of chronically instrumented, freely moving rats. Microinjection of SpCAMPS (1.5 nmol) induced REM sleep within a short latency (12.1 +/- 3.6 min) compared with vehicle control microinjection (60.0 +/- 6.5 min). On the contrary, microinjection of baclofen (1.5 nmol) suppressed REM sleep by delaying its appearance for approximately 183 min; however, the suppression of REM sleep by baclofen was prevented by a subsequent microinjection of SpCAMPS. These results provide evidence that the activation of cAMP-PKA within the PPT can successfully block the GABA(B) receptor activation-mediated REM sleep suppression effect. These findings suggest that the PPT GABA(B) receptor activation-mediated REM sleep regulating mechanism involves inactivation of cAMP-PKA signaling in the freely moving rat.

Activity-dependent changes in temporal components of neurotransmission at the juvenile mouse calyx of Held synapse

The temporal fidelity of synaptic transmission is constrained by the reproducibility of time delays such as axonal conduction delay and synaptic delay, but very little is known about the modulation of these distinct components. In particular, synaptic delay is not generally considered to be modifiable under physiological conditions. Using simultaneous paired patch-clamp recordings from pre- and postsynaptic elements of the calyx of Held synapse, in juvenile mouse auditory brainstem slices, we show here that synaptic activity (20-200 Hz) leads to activity-dependent increases in synaptic delay and its variance as well as desynchronization of evoked responses. Such changes were most robust at 200 Hz in 2 mM extracellular Ca(2+) ([Ca(2+)](o)), and could be attenuated by lowering [Ca(2+)](o) to 1 mM, increasing temperature to 35 degrees C, or application of the GABA(B)R agonist baclofen, which inhibits presynaptic Ca(2+) currents (I(Ca)). Conduction delay also exhibited slight activity-dependent prolongation, but this prolongation was only sensitive to temperature, and not to [Ca(2+)](o) or baclofen. Direct voltage-clamp recordings of I(Ca) evoked by repeated action potential train template (200 Hz) revealed little jitter in the timing and kinetics of I(Ca) under various conditions, suggesting that increases in synaptic delay and its variance occur downstream of Ca(2+) entry. Loading the Ca(2+) chelator EGTA-AM into terminals reduced the progression rate, the extent of activity-dependent increases in various delay components, and their variance, implying that residual Ca(2+) accumulation in the presynaptic nerve terminal induces these changes. Finally, by applying a test pulse at different intervals following a 200 Hz train (150 ms), we demonstrated that prolongation in the various delay components reverses in parallel with recovery in synaptic strength. These observations suggest that a depletion of the readily releasable pool of SVs during high-frequency activity may downregulate not only synaptic strength but also decrease the temporal fidelity of neurotransmission at this and other central synapses.

Signaling mechanisms of angiotensin II-induced attenuation of GABAergic input to hypothalamic presympathetic neurons

The hypothalamic paraventricular nucleus (PVN) is an important site for the regulation of sympathetic outflow. Angiotensin II (Ang II) can activate AT(1) receptors to stimulate PVN presympathetic neurons through inhibition of GABAergic input. However, little is known about the downstream pathway involved in this presynaptic action of Ang II in the PVN. In this study, using whole cell recording from retrogradely labeled PVN neurons in rat brain slices, we determined the signaling mechanisms responsible for the effect of Ang II on synaptic GABA release to spinally projecting PVN neurons. Bath application of Ang II reproducibly decreased the frequency of GABAergic miniature postsynaptic inhibitory currents (mIPSCs) in fluorescence-labeled PVN neurons. Ang II failed to change the frequency of mIPSCs in labeled PVN neurons treated with pertussis toxin. However, Ang II-induced inhibition of mIPSCs persisted in the presence of either CdCl(2), a voltage-gated Ca(2+) channel blocker, or 4-aminopyridine, a blocker of voltage-gated K(+) channels. Interestingly, inhibition of superoxide with superoxide dismutase or Mn(III) tetrakis (4-benzoic acid) prophyrin completely blocked Ang II-induced decrease in mIPSCs. By contrast, inhibition of hydroxyl radical formation with the ion chelator deferoxamine did not significantly alter the effect of Ang II. These findings suggest that the presynaptic action of Ang II on synaptic GABA release in the PVN is mediated by the pertussis toxin-sensitive G(i/o) proteins but not by voltage-gated Ca(2+) and K(+) channels. Ang II attenuates GABAergic input to PVN presympathetic neurons through reactive oxygen species, especially superoxide anions.

Differential effects of chronic amphetamine and baclofen administration on cAMP levels and phosphorylation of CREB in distinct brain regions of wild type and monoamine oxidase B-deficient mice

Roles of GABA(B) transmission were explored in the action of amphetamine (Amph) on the brain. Adult male wild type (WT) and monoamine oxidase B-knocked out (MAOBKO) mice received i.p. injections of saline, d-Amph (5 mg/kg), plus baclofen (GABA(B) receptor agonist, 10 mg/kg), or baclofen and Amph, twice daily for 3 days and single treatments on day 4, followed by immuno-cyclic-AMP (cAMP) and immunoblotting assays on the brain tissue. The WT mice responded with higher levels of behavioral responses than the KO to the daily Amph injection; however, baclofen blocked the Amph-induced behavioral hyperactivity of both WT and KO mice. After the last treatment, levels of cAMP and phosphorylated (p) cyclic-AMP response element binding protein (CREB) were up-regulated in the striatum and somatosensory cortex of Amph-treated WT mice, while similar to the saline-controls in the baclofen+Amph-treated group, indicating the blockade by baclofen to Amph. Baclofen similarly suppressed the Amph-induced increases in pCREB levels of WT hippocampus and amygdala, and decreases of olfactory bulb and thalamus. For MAOBKO mice, baclofen hindered the Amph-generated increases in motor cortical cAMP and pCREB, and amygdaloid pCREB, and the decrease in olfactory bulb pCREB, whereas did not affect the Amph-raised hippocampal pCREB. Furthermore, the levels of CREB were variably modified in distinct regions by the drug exposures. The data reveal that the GABA(B)-mediated intracellular signaling differentially participates in mechanisms underlying Amph perturbation to various regions, and may thereby contribute explanations to the behavioral consequences. Moreover, MAOB is region-dependently involved in responses of the brain to Amph and baclofen, supporting interactions between GABA and monoamines.

5-HT1Breceptor-mediated presynaptic inhibition at the calyx of Held of immature rats

5-hydroxytryptamine (5-HT) inhibits transmitter release via activating GTP-binding proteins, but the target of 5-HT receptors in the nerve terminal is not determined. We addressed this question at the calyx of Held synapse in the brainstem slice of immature rats. Bath-application of 5-HT attenuated the amplitude of nerve-evoked excitatory postsynaptic currents (EPSCs) associated with an increase in the paired-pulse ratio, whereas it had no effect on the amplitude of spontaneous miniature EPSCs. The 5-HT1B receptor agonist CP93129 mimicked the inhibitory effect of 5-HT, but the 5-HT1A agonist (R)-(+)-8-hydroxy-DPAT (8-OHDPAT) had no effect. The 5-HT1B receptor antagonist NAS-181 blocked the inhibitory effect of 5-HT. These results suggest that 5-HT activated 5-HT1B receptors in calyceal nerve terminals, thereby inhibiting transmitter release. In direct whole-cell recordings from calyceal nerve terminals, 5-HT attenuated voltage-dependent Ca2+ currents, but had no effect on voltage-dependent K+ currents. When EPSCs were evoked by presynaptic Ca2+ currents during simultaneous pre- and postsynaptic recordings, the magnitude of the 5-HT-induced inhibition of Ca2+ currents fully explained that of EPSCs. Upon repetitive applications, 5-HT showed tachyphylaxis, with its effect on both EPSCs and presynaptic Ca2+ currents becoming weaker in the second application. 1,2-bis(o-aminophenoxy)ethane-N-N'-N'-N'-tetraacetic acid (BAPTA; 10 mm) loaded into the nerve terminal abolished this tachyphylaxis. The presynaptic inhibitory effect of 5-HT was prominent at postnatal day 5, but became weaker as animals matured. We conclude that activation of 5-HT1B receptors inhibits voltage-gated Ca2+ channels, thereby inhibiting transmitter release at immature calyceal nerve terminals, and that 5-HT1B receptors undergo Ca2+-dependent tachyphylaxis on repetitive activations.

GABAB receptors and synaptic modulation

GABA(B) receptors modulate transmitter release and postsynaptic membrane potential at various types of central synapses. They function as heterodimers of two related seven-transmembrane domain receptor subunits. Trafficking, activation and signalling of GABA(B) receptors are regulated both by allosteric interactions between the subunits and by the binding of additional proteins. Recent studies have shed light on the roles of GABA(B) receptors in plasticity processes at excitatory synapses. This review summarizes our knowledge of the localization, structure and function of GABA(B) receptors in the central nervous system and their use as drug targets for neurological and psychiatric disorders.

The calyx of Held

The calyx of Held is a large glutamatergic synapse in the mammalian auditory brainstem. By using brain slice preparations, direct patch-clamp recordings can be made from the nerve terminal and its postsynaptic target (principal neurons of the medial nucleus of the trapezoid body). Over the last decade, this preparation has been increasingly employed to investigate basic presynaptic mechanisms of transmission in the central nervous system. We review here the background to this preparation and summarise key findings concerning voltage-gated ion channels of the nerve terminal and the ionic mechanisms involved in exocytosis and modulation of transmitter release. The accessibility of this giant terminal has also permitted Ca(2+)-imaging and -uncaging studies combined with electrophysiological recording and capacitance measurements of exocytosis. Together, these studies convey the panopoly of presynaptic regulatory processes underlying the regulation of transmitter release, its modulatory control and short-term plasticity within one identified synaptic terminal.

Regulation of synaptic input to hypothalamic presympathetic neurons by GABA(B) receptors

The hypothalamic paraventricular (PVN) neurons projecting to the spinal cord and brainstem play an important role in the control of homeostasis and the sympathetic nervous system. Although GABA(B) receptors are present in the PVN, their function in the control of synaptic inputs to PVN presympathetic neurons is not clear. Using retrograde tracing and whole-cell patch-clamp recordings in rat brain slices, we determined the role of presynaptic GABA(B) receptors in regulation of glutamatergic and GABAergic inputs to spinally projecting PVN neurons. The GABA(B) receptor agonist baclofen (1-50 microM) dose-dependently decreased the frequency but not the amplitude of spontaneous excitatory postsynaptic currents (sEPSCs) and inhibitory postsynaptic currents (sIPSCs). The effect of baclofen on sEPSCs and sIPSCs was completely blocked by 10 microM CGP52432, a selective GABA(B) receptor antagonist. Baclofen also significantly reduced the frequency of both miniature excitatory and miniature inhibitory postsynaptic currents (mEPSCs and mIPSCs). Furthermore, uncoupling pertussis toxin-sensitive G(i/o) proteins with N-ethylmaleimide abolished baclofen-induced inhibition of mEPSCs and mIPSCs. However, the inhibitory effect of baclofen on the frequency of mIPSCs and mEPSCs persisted in the presence of either Cd2+, a voltage-gated Ca2+ channel blocker, or 4-aminopyridine, a blocker of voltage-gated K+ channels. Our results suggest that activation of presynaptic GABA(B) receptors inhibits synaptic GABA and glutamate release to PVN presympathetic neurons. This presynaptic action of GABA(B) receptors is mediated by the N-ethylmaleimide-sensitive G(i/o) proteins, but independent of voltage-gated Ca2+ and K+ channels.

Presynaptic GABABReceptors Regulate Retinohypothalamic Tract Synaptic Transmission by Inhibiting Voltage-Gated Ca2+Channels

Presynaptic GABABreceptor activation inhibits glutamate release from retinohypothalamic tract (RHT) terminals in the suprachiasmatic nucleus (SCN). Voltage-clamp whole cell recordings from rat SCN neurons and optical recordings of Ca2+-sensitive fluorescent probes within RHT terminals were used to examine GABAB-receptor modulation of RHT transmission. Baclofen inhibited evoked excitatory postsynaptic currents (EPSCs) in a concentration-dependent manner equally during the day and night. Blockers of N-, P/Q-, T-, and R-type voltage-dependent Ca2+channels, but not L-type, reduced the EPSC amplitude by 66, 36, 32, and 18% of control, respectively. Joint application of multiple Ca2+channel blockers inhibited the EPSCs less than that predicted, consistent with a model in which multiple Ca2+channels overlap in the regulation of transmitter release. Presynaptic inhibition of EPSCs by baclofen was occluded by ω-conotoxin GVIA (≤72%), mibefradil (≤52%), and ω-agatoxin TK (≤15%), but not by SNX-482 or nimodipine. Baclofen reduced both evoked presynaptic Ca2+influx and resting Ca2+concentration in RHT terminals. Tertiapin did not alter the evoked EPSC and baclofen-induced inhibition, indicating that baclofen does not inhibit glutamate release by activation of Kir3 channels. Neither Ba2+nor high extracellular K+modified the baclofen-induced inhibition. 4-Aminopyridine (4-AP) significantly increased the EPSC amplitude and the charge transfer, and dramatically reduced the baclofen effect. These data indicate that baclofen inhibits glutamate release from RHT terminals by blocking N-, T-, and P/Q-type Ca2+channels, and possibly by activation of 4-AP–sensitive K+channels, but not by inhibition of R- and L-type Ca2+channels or by Kir3 channel activation.

GABAB receptor-mediated presynaptic inhibition of glutamatergic transmission in the inferior colliculus

Whole-cell patch clamp recordings were made from ICC neurons in brain slices of 9-16 day old rats. Postsynaptic currents were evoked by electrical stimulation of the lemniscal inputs. Excitatory postsynaptic currents (EPSCs) were isolated pharmacologically by blocking GABA(A) and glycine receptors. EPSCs were further dissected into AMPA and NMDA receptor-mediated responses by adding the receptor antagonists, APV and CNQX, respectively. The internal solution in the recording electrodes contained CsF and TEA to block K(+) channels that might be activated by postsynaptic GABA(B) receptors. The modulatory effects of GABA(B) receptors on EPSCs in ICC neurons were examined by bath application of the GABA(B) receptor agonist, baclofen, and the antagonist, CGP 35348. The amplitudes of EPSCs in ICC neurons were reduced to 34.4+/-3.2% of the control by baclofen (5-10 microM). The suppressive effect by baclofen was concentration-dependent. The reduction of the EPSC amplitude was reversed by CGP35348. The ratio of the 2nd to 1st EPSCs evoked by paired-pulse stimulation was significantly increased after application of baclofen. These results suggest that glutamatergic excitation in the ICC can be modulated by presynaptic GABA(B) receptors. In addition, baclofen reduced NMDA EPSCs more than AMPA EPSCs. The GABA(B) receptor-mediated modulation of glutamatergic excitation in the ICC provides a likely mechanism for preventing overstimulation and/or regulating the balance of excitation and inhibition involved in processing auditory information.

Endogenous activation of adenosine A1 receptors, but not P2X receptors, during high-frequency synaptic transmission at the calyx of Held

Activation of presynaptic receptors plays an important role in modulation of transmission at many synapses, particularly during high-frequency trains of stimulation. Adenosine-triphosphate (ATP) is coreleased with several neurotransmitters and acts at presynaptic sites to reduce transmitter release; such presynaptic P2X receptors occur at inhibitory and excitatory terminals in the medial nucleus of the trapezoid body (MNTB). We have investigated the mechanism of purinergic modulation during high-frequency repetitive stimulation at the calyx of Held synapse. Suppression of calyceal excitatory postsynaptic currents (EPSCs) by ATP and ATPgammaS (100 microM) was mimicked by adenosine application and was blocked by DPCPX (10 microM), indicating mediation by adenosine A1 receptors. DPCPX enhanced EPSC amplitudes during high-frequency synaptic stimulation, suggesting that adenosine has a physiological role in modulating transmission at the calyx. The Luciferin-Luciferase method was used to probe for endogenous ATP release (at 37 degrees C), but no release was detected. Blockers of ectonucleotidases also had no effect on endogenous synaptic depression, suggesting that it is adenosine acting on A1 receptors, rather than degradation of released ATP, which accounts for presynaptic purinergic suppression of synaptic transmission during physiological stimulus trains at this glutamatergic synapse.

Muscimol and baclofen differentially suppress retinotopic and nonretinotopic responses in visual cortex

This study relates to local field potentials and single-unit responses in cat visual cortex elicited by contrast reversal of bar gratings that were presented in single, double, or multiple discrete patch (es) of the visual field. Concurrent stimulation of many patches by means of the pseudorandom, binary m-sequence technique revealed interactions between their respective responses. An analysis identified two distinct components of local field potentials: a fast local component (FLC) and a slow distributed component (SDC). The FLC is thought to be a primarily postsynaptic response, as judged by its relatively short latency. It is directly generated by thalamocortical volleys following retinotopic stimulation of receptive fields of a small cluster of single cells, combined with responses to recurrent excitation and inhibition derived from the cells under study and immediately neighboring cells. In contrast, the SDC is thought to be an aggregate of dendritic potentials related to the long-range lateral connections (i.e. long-range coupling). We compared the suppressive effects of a GABAA-receptor agonist, muscimol, on the FLC and SDC with those of a GABAB-receptor agonist, baclofen, and found that muscimol more strongly suppressed the FLC than the SDC, and that the reverse was the case for baclofen. The differential suppression of the FLC and SDC found in the present study is consistent with the notion that intracortical electrical signals related to the FLC terminate on the somata and proximal/basal dendrites, while those related to the SDC terminate on distal dendrites.

Cholecystokinin-2 receptors couple to cAMP–protein kinase A to depress excitatory synaptic currents in rat nucleus accumbens in vitro

We recently reported that the activation of cholecystokinin-2 receptors depress evoked excitatory postsynaptic currents (EPSCs) in nucleus accumbens (NAc) indirectly through γ-aminobutyric acid (GABA) acting on γ-aminobutyric acid-B (GABAB) receptors. Here, we determined the second messenger system that couples cholecystokinin-2 receptors to the observed synaptic depression. Using in vitro forebrain slices of rats and whole-cell patch recording, we tested the hypothesis that cholecystokinin-2 receptors are coupled to cAMP and protein kinase A signaling pathway. Cholecystokinin-8S induced inward currents and depressed evoked EPSCs. Forskolin, an activator of adenylyl cyclase and rolipram that is an inhibitor of phosphodiesterase type IV, independently increased EPSC amplitude and blocked the inward current and synaptic depression induced by cholecystokinin-8S. Furthermore, the membrane-permeable cAMP analog, 8-bromo-cAMP, blocked the cholecystokinin-8S effects. H89, a protein kinase A inhibitor, also blocked cholecystokinin-8S effects. However, depression of the evoked EPSC by baclofen, a GABABreceptor agonist, was not blocked by H89 or forskolin. These findings indicate that cholecystokinin-2, but not GABAB, receptors are coupled to the adenylyl cyclase – cAMP – protein kinase A signaling pathway in the NAc to induce inward currents and cause synaptic depression.