GABA B receptor modulation of excitatory and inhibitory synaptic transmission onto rat CA3 hippocampal interneurons

GABA transporter GAT1 prevents spillover at proximal and distal GABA synapses onto primate prefrontal cortex neurons

The plasma membrane GABA transporter GAT1 is thought to mediate uptake of synaptically released GABA. In the primate dorsolateral prefrontal cortex (DLPFC), GAT1 expression changes significantly during development and in schizophrenia. The consequences of such changes, however, are not well understood because GAT1's role has not been investigated in primate neocortical circuits. We thus studied the effects of the GAT1 blocker 1,2,5,6-tetrahydro-1-[2-[[(diphenylmethylene)amino]oxy]ethyl]-3-pyridinecarboxylic acid hydrochloride (NO711) on GABA transmission onto pyramidal neurons of monkey DLPFC. As in rat cortex, in monkey DLPFC NO711 did not substantially alter miniature GABA transmission, suggesting that GAT1 does not regulate single-synapse transmission. In rat cortical circuits, between-synapse GABA spillover produced by NO711 clearly prolongs the inhibitory postsynaptic currents, but whether NO711 also prolongs the inhibitory postsynaptic potentials (IPSPs) is unclear. Moreover, whether spillover differentially affects perisomatic versus dendritic inputs has not been examined. Here we found that NO711 prolonged the GABAA receptor-mediated IPSPs (GABAAR-IPSPs) evoked by stimulating perisomatic synapses. Dendritic, but not perisomatic, synapse stimulation often elicited a postsynaptic GABAB receptor-mediated IPSP that was enhanced by NO711. Blocking GABAB receptors revealed that NO711 prolonged the GABAAR-IPSPs evoked by stimulation of dendrite-targeting inputs. We conclude that a major functional role for GAT1 in primate cortical circuits is to prevent the effects of GABA spillover when multiple synapses are simultaneously active. Furthermore, we report that, at least in monkey DLPFC, GAT1 similarly restricts GABA spillover onto perisomatic or dendritic inputs, critically controlling the spatiotemporal specificity of inhibitory inputs onto proximal or distal compartments of the pyramidal cell membrane.

The sushi domains of secreted GABA(B1) isoforms selectively impair GABA(B) heteroreceptor function

GABA(B) receptors are the G-protein-coupled receptors for gamma-aminobutyric acid (GABA), the main inhibitory neurotransmitter in the brain. GABA(B) receptors are promising drug targets for a wide spectrum of psychiatric and neurological disorders. Receptor subtypes exhibit no pharmacological differences and are based on the subunit isoforms GABA(B1a) and GABA(B1b). GABA(B1a) differs from GABA(B1b) in its ectodomain by the presence of a pair of conserved protein binding motifs, the sushi domains (SDs). Previous work showed that selectively GABA(B1a) contributes to heteroreceptors at glutamatergic terminals, whereas both GABA(B1a) and GABA(B1b) contribute to autoreceptors at GABAergic terminals or to postsynaptic receptors. Here, we describe GABA(B1j), a secreted GABA(B1) isoform comprising the two SDs. We show that the two SDs, when expressed as a soluble protein, bind to neuronal membranes with low nanomolar affinity. Soluble SD protein, when added at nanomolar concentrations to dissociated hippocampal neurons or to acute hippocampal slices, impairs the inhibitory effect of GABA(B) heteroreceptors on evoked and spontaneous glutamate release. In contrast, soluble SD protein neither impairs the activity of GABA(B) autoreceptors nor impairs the activity of postsynaptic GABA(B) receptors. We propose that soluble SD protein scavenges an extracellular binding partner that retains GABA(B1a)-containing heteroreceptors in proximity of the presynaptic release machinery. Soluble GABA(B1) isoforms like GABA(B1j) may therefore act as dominant-negative inhibitors of heteroreceptors and control the level of GABA(B)-mediated inhibition at glutamatergic terminals. Of importance for drug discovery, our data also demonstrate that it is possible to selectively impair GABA(B) heteroreceptors by targeting their SDs.

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.

ATP-gated P2X receptors on excitatory nerve terminals onto interneurons initiate a form of asynchronous glutamate release

Previous work has shown that ATP-gated P2X2 receptors are expressed in excitatory nerve terminals onto stratum radiatum interneurons in the mouse hippocampal CA1 region. At these synapses receptor activation results in calcium-dependent facilitation of miniature and spontaneous EPSC frequency. In this study I determined if activation of presynaptic P2X receptors produces these effects by utilizing the vesicles underlying action potential dependent release. Brief trains of electrical stimuli caused short-term synaptic depression of excitatory synapses onto interneurons, in a manner consistent with depletion of the readily releasable pool of vesicles. P2X receptor activation increased the frequency of spontaneous EPSCs, but unexpectedly evoked little effect on synaptic depression. This suggests that P2X receptor activation does not markedly draw on the vesicles underlying action potential dependent glutamate release. However asynchronous EPSCs were increased following synaptic depression and a component of these appeared to be initiated by endogenously released ATP acting on presynaptic P2X receptors. Unexpectedly, the data suggest P2X receptor activation initiates a form of asynchronous glutamate release, rather than detectably affecting the vesicles underlying action potential evoked release.

Neurotensin Enhances GABAergic Activity in Rat Hippocampus CA1 Region by Modulating L-Type Calcium Channels

Neurotensin (NT) is a tridecapeptide that interacts with three NT receptors; NTS1, NTS2, and NTS3. Although NT has been reported to modulate GABAergic activity in the brain, the underlying cellular and molecular mechanisms of NT are elusive. Here, we examined the effects of NT on GABAergic transmission and the involved cellular and signaling mechanisms of NT in the hippocampus. Application of NT dose-dependently increased the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) recorded from CA1 pyramidal neurons with no effects on the amplitude of sIPSCs. NT did not change either the frequency or the amplitude of miniature (m)IPSCs recorded in the presence of tetrodotoxin. Triple immunofluorescent staining of recorded interneurons demonstrated the expression of NTS1 on GABAergic interneurons. NT increased the action potential firing rate but decreased the afterhyperpolarization (AHP) amplitude in identified CA1 interneurons. Application of L-type calcium channel blockers (nimodipine and nifedipine) abolished NT-induced increases in action potential firing rate and sIPSC frequency and reduction in AHP amplitude, suggesting that the effects of NT are mediated by interaction with L-type Ca2+channels. NT-induced increase in sIPSC frequency was blocked by application of the specific NTS1 antagonist SR48692, the phospholipase C (PLC) inhibitor U73122, the IP3receptor antagonist 2-APB, and the protein kinase C inhibitor GF109203X, suggesting that NT increases γ-aminobutyric acid release via a PLC pathway. Our results provide a cellular mechanism by which NT controls GABAergic neuronal activity in hippocampus.

Reciprocal inhibition of G-protein signaling is induced by CB(1) cannabinoid and GABA(B) receptor interactions in rat hippocampal membranes

Cannabinoid CB(1) and the metabotropic GABA(B) receptors have been shown to display similar pharmacological effects and co-localization in certain brain regions. Previous studies have reported a functional link between the two systems. As a first step to investigate the underlying molecular mechanism, here we show cross-inhibition of G-protein signaling between GABA(B) and CB(1) receptors in rat hippocampal membranes. The CB(1) agonist R-Win55,212-2 displayed high potency and efficacy in stimulating guanosine-5'-O-(3-[(35)S]thio)triphosphate, [(35)S]GTPgammaS binding. Its effect was completely blocked by the specific CB(1) antagonist AM251 suggesting that the signaling was via CB(1) receptors. The GABA(B) agonists baclofen and SKF97541 also elevated [(35)S]GTPgammaS binding by about 60%, with potency values in the micromolar range. Phaclofen behaved as a low potency antagonist with an ED(50) approximately 1mM. However, phaclofen at low doses (1 and 10nM) slightly but significantly attenuated maximal stimulation of [(35)S]GTPgammaS binding by the CB(1) agonist R-Win55,212-2. The observation that higher concentrations of phaclofen had no such effect rule out the possibility of its direct action on CB(1) receptors. The pharmacologically inactive stereoisomer S-Win55,212-3 had no effect either alone or in combination with phaclofen establishing that the interaction is stereospecific in hippocampus. The specific CB(1) antagonist AM251 at a low dose (1 nM) also inhibited the efficacy of G-protein signaling of the GABA(B) receptor agonist SKF97541. Cross-talk of the two receptor systems was not detected in either spinal cord or cerebral cortex membranes. It is speculated that the interaction might occur via an allosteric interaction between a subset of GABA(B) and CB(1) receptors in rat hippocampal membranes. Although the exact molecular mechanism of the reciprocal inhibition between CB(1) and GABA(B) receptors will have to be explored by future studies it is intriguing that the cross-talk might be involved in balance tuning the endocannabinoid and GABAergic signaling in hippocampus.

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.

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.

Adrenergic facilitation of GABAergic transmission in rat entorhinal cortex

Whereas the entorhinal cortex (EC) receives noradrenergic innervations from the locus coeruleus of the pons and expresses adrenergic receptors, the function of norepinephrine (NE) in the EC is still elusive. We examined the effects of NE on GABA(A) receptor-mediated synaptic transmission in the superficial layers of the EC. Application of NE dose-dependently increased the frequency and amplitude of spontaneous inhibitory postsynaptic currents (IPSCs) recorded from the principal neurons in layer II/III through activation of alpha(1) adrenergic receptors. NE increased the frequency and not the amplitude of miniature IPSCs (mIPSCs) recorded in the presence of TTX, suggesting that NE increases presynaptic GABA release with no effects on postsynaptic GABA(A) receptors. Application of Ca(2+) channel blockers (Cd(2+) and Ni(2+)), omission of Ca(2+) in the extracellular solution, or replacement of extracellular Na(+) with N-methyl-D-glucamine (NMDG) failed to alter NE-induced increase in mIPSC frequency, suggesting that Ca(2+) influx through voltage-gated Ca(2+) or other cationic channels is not required. Application of BAPTA-AM, thapsigargin, and ryanodine did not change NE-induced increase in mIPSC frequency, suggesting that Ca(2+) release from intracellular stores is not necessary for NE-induced increase in GABA release. Whereas alpha(1) receptors are coupled to G(q/11) resulting in activation of the phospholipase C (PLC) pathway, NE-mediated facilitation of GABAergic transmission was independent of PLC, protein kinase C, and tyrosine kinase activities. Our results suggest that NE-mediated facilitation of GABAergic function contributes to its antiepileptic effects in the EC.

Muscarinic M2 and M1 receptors reduce GABA release by Ca2+ channel modulation through activation of PI3K/Ca2+ -independent and PLC/Ca2+ -dependent PKC

We measured pharmacologically isolated GABAergic currents from layer II/III neurons of the rat auditory cortex using patch-clamp recording. Activation of muscarinic receptors by muscarine (1 microM) or oxotremorine (10 microM) decreased the amplitude of electrically evoked inhibitory postsynaptic currents to about one third of their control value. Neither miniature nor exogenously evoked GABAergic currents were altered by the presence of muscarinic agonists, indicating that the effect was spike-dependent and not mediated postsynaptically. The presence of the N- or P/Q-type Ca(2+) channel blockers omega-conotoxin GVIA (1 microM) or omega-AgaTx TK (200 nM) greatly blocked the muscarinic effect, suggesting that Ca(2+)-channels were target of the muscarinic modulation. The presence of the muscarinic M(2) receptor (M(2)R) antagonists methoctramine (5 muM) or AF-DX 116 (1 microM) blocked most of the muscarinic evoked inhibitory postsynaptic current (eIPSC) reduction, indicating that M(2)Rs were responsible for the effect, whereas the remaining component of the depression displayed M(1)R-like sensitivity. Tissue preincubation with the specific blockers of phosphatidyl-inositol-3-kinase (PI(3)K) wortmannin (200 nM), LY294002 (1 microM), or with the Ca(2+)-dependent PKC inhibitor Gö 6976 (200 nM) greatly impaired the muscarinic decrease of the eIPSC amplitude, whereas the remaining component was sensitive to preincubation in the phospholipase C blocker U73122 (10 microM). We conclude that acetylcholine release enhances the excitability of the auditory cortex by decreasing the release of GABA by inhibiting axonal V-dependent Ca(2+) channels, mostly through activation of presynaptic M(2)Rs/PI(3)K/Ca(2+)-independent PKC pathway and-to a smaller extent-by the activation of M(1)/PLC/Ca(2+)-dependent PKC.

Prostaglandin E2 (PGE2) inhibits glutamatergic synaptic transmission in dorsolateral periaqueductal gray (dl-PAG)

The purpose of this study was to determine the role of prostaglandin E(2) (PGE(2)) in modulating neuronal activity of the dorsolateral periaqueductal gray (dl-PAG) through excitatory and inhibitory synaptic inputs. First, whole cell voltage-clamp recording was performed to obtain excitatory and inhibitory postsynaptic currents (EPSCs and IPSCs) of the dl-PAG neurons. Our results show that PGE(2) significantly decreased the frequency of miniature EPSCs and amplitude of evoked EPSCs. The effects were mimicked by sulprostone, an agonist to PGE(2) EP(3) receptors. In contrast, PGE(2) had no distinct effect on IPSCs. In addition, spontaneous action potential of the dl-PAG neurons was recorded using whole cell current-clamp methods. PGE(2) significantly attenuated the discharge rate of the dl-PAG neurons. The decreased firing activity was abolished in the presence of glutamate NMDA and non-NMDA receptor antagonists. The results from the current study provide the first evidence indicating that PGE(2) inhibits the neuronal activity of the dl-PAG via selective attenuation of glutamatergic synaptic inputs, likely due to the activation of presynaptic EP(3) receptors.

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.

Depression of glutamate and GABA release by presynaptic GABAB receptors in the entorhinal cortex in normal and chronically epileptic rats

Presynaptic GABA(B) receptors (GABA(B)R) control glutamate and GABA release at many synapses in the nervous system. In the present study we used whole-cell patch-clamp recordings of spontaneous excitatory and inhibitory synaptic currents in the presence of TTX to monitor glutamate and GABA release from synapses in layer II and V of the rat entorhinal cortex (EC)in vitro. In both layers the release of both transmitters was reduced by application of GABA(B)R agonists. Quantitatively, the depression of GABA release in layer II and layer V, and of glutamate release in layer V was similar, but glutamate release in layer II was depressed to a greater extent. The data suggest that the same GABA(B)R may be present on both GABA and glutamate terminals in the EC, but that the heteroreceptor may show a greater level of expression in layer II. Studies with GABA(B)R antagonists suggested that neither the auto- nor the heteroreceptor was consistently tonically activated by ambient GABA in the presence of TTX. Studies in EC slices from rats made chronically epileptic using a pilocarpine model of temporal lobe epilepsy revealed a reduced effectiveness of both auto- and heteroreceptor function in both layers. This could suggest that enhanced glutamate and GABA release in the EC may be associated with the development of the epileptic condition.

The selective effects of somatostatin- and GABA-mediated transmissions on voltage gated Ca2+ channel immunoreactivity in the gerbil hippocampus

To identify whether altered expressions of voltage gated Ca(2+) channel (VGCC) are linked to inhibitory transmission abnormalities in the gerbil hippocampus, we investigated the effects of GABA receptor or somatostatin receptor (SST) antagonists/agonists on VGCC immunoreactivity in vivo. VGCC immunoreactivities in the hippocampus were significantly higher in seizure sensitive (SS) gerbils than in seizure resistant (SR) gerbils. P/Q-type VGCC immunoreactivity in the gerbil hippocampus was reduced by enhancement in GABA(A) and GABA(B) receptor-mediated transmission, but not by SST-mediated transmission. N-type VGCC immunoreactivity was reduced only by a SST agonist, whereas L-type (alpha1C) VGCC immunoreactivity was reduced only by a GABA(A) receptor agonist, and L-type (alpha1D) VGCC immunoreactivity was modulated by the GABA(B) receptor acting drugs. These findings provide a comprehensive description of the differential responses of VGCC subunits to alteration in GABAergic or somatostatinergic transmission. These findings also suggest that up-regulated VGCC immunoreactivity may be consequence of the neuronal excitability caused by a reduction in inhibitory neurotransmission in the gerbil hippocampus.

Thyrotropin-releasing hormone increases GABA release in rat hippocampus

Thyrotropin-releasing hormone (TRH) is a tripeptide that is widely distributed in the brain including the hippocampus where TRH receptors are also expressed. TRH has anti-epileptic effects and regulates arousal, sleep, cognition, locomotion and mood. However, the cellular mechanisms underlying such effects remain to be determined. We examined the effects of TRH on GABAergic transmission in the hippocampus and found that TRH increased the frequency of GABAA receptor-mediated spontaneous IPSCs in each region of the hippocampus but had no effects on miniature IPSCs or evoked IPSCs. TRH increased the action potential firing frequency recorded from GABAergic interneurons in CA1 stratum radiatum and induced membrane depolarization suggesting that TRH increases the excitability of interneurons to facilitate GABA release. TRH-induced inward current had a reversal potential close to the K+ reversal potential suggesting that TRH inhibits resting K+ channels. The involved K+ channels were sensitive to Ba2+ but resistant to other classical K+ channel blockers, suggesting that TRH inhibits the two-pore domain K+ channels. Because the effects of TRH were mediated via Galphaq/11, but were independent of its known downstream effectors, a direct coupling may exist between Galphaq/11 and K+ channels. Inhibition of the function of dynamin slowed the desensitization of TRH responses. TRH inhibited seizure activity induced by Mg2+ deprivation, but not that generated by picrotoxin, suggesting that TRH-mediated increase in GABA release contributes to its anti-epileptic effects. Our results demonstrate a novel mechanism to explain some of the hippocampal actions of TRH.

Differential compartmentalization and distinct functions of GABAB receptor variants

GABAB receptors are the G protein-coupled receptors for the main inhibitory neurotransmitter in the brain, gamma-aminobutyric acid (GABA). Molecular diversity in the GABAB system arises from the GABAB1a and GABAB1b subunit isoforms that solely differ in their ectodomains by a pair of sushi repeats that is unique to GABAB1a. Using a combined genetic, physiological, and morphological approach, we now demonstrate that GABAB1 isoforms localize to distinct synaptic sites and convey separate functions in vivo. At hippocampal CA3-to-CA1 synapses, GABAB1a assembles heteroreceptors inhibiting glutamate release, while predominantly GABAB1b mediates postsynaptic inhibition. Electron microscopy reveals a synaptic distribution of GABAB1 isoforms that agrees with the observed functional differences. Transfected CA3 neurons selectively express GABAB1a in distal axons, suggesting that the sushi repeats, a conserved protein interaction motif, specify heteroreceptor localization. The constitutive absence of GABAB1a but not GABAB1b results in impaired synaptic plasticity and hippocampus-dependent memory, emphasizing molecular differences in synaptic GABAB functions.

Differential compartmentalization and distinct functions of GABAB receptor variants

GABAB receptors are the G protein-coupled receptors for the main inhibitory neurotransmitter in the brain, gamma-aminobutyric acid (GABA). Molecular diversity in the GABAB system arises from the GABAB1a and GABAB1b subunit isoforms that solely differ in their ectodomains by a pair of sushi repeats that is unique to GABAB1a. Using a combined genetic, physiological, and morphological approach, we now demonstrate that GABAB1 isoforms localize to distinct synaptic sites and convey separate functions in vivo. At hippocampal CA3-to-CA1 synapses, GABAB1a assembles heteroreceptors inhibiting glutamate release, while predominantly GABAB1b mediates postsynaptic inhibition. Electron microscopy reveals a synaptic distribution of GABAB1 isoforms that agrees with the observed functional differences. Transfected CA3 neurons selectively express GABAB1a in distal axons, suggesting that the sushi repeats, a conserved protein interaction motif, specify heteroreceptor localization. The constitutive absence of GABAB1a but not GABAB1b results in impaired synaptic plasticity and hippocampus-dependent memory, emphasizing molecular differences in synaptic GABAB functions.

Homeostatic regulation of glutamate release in response to depolarization

Proper nervous system function requires a balance between excitation and inhibition. Systems of homeostasis may have evolved in neurons to help maintain or restore balance between excitation and inhibition, presumably because excessive excitation can cause dysfunction and cell death. This article reviews evidence for homeostatic mechanisms within the hippocampus that lead to differential regulation of glutamate and gamma-aminobutyric acid release in response to conditions of excess depolarization. We recently found differential effects on glutamate release at the level of action potential coupling to transmitter release, vesicular release probability, and vesicle availability. Such differential regulation may help to prevent excitotoxicity and runaway excitation.

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.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

Molecular diversity, trafficking and subcellular localization of GABAB receptors

GABAB receptors are the G-protein coupled receptors for the main inhibitory neurotransmitter in the brain, gamma-aminobutyric acid (GABA). While native studies predicted pharmacologically distinct GABAB receptor subtypes, molecular studies failed to identify the expected receptor varieties. Mouse genetic experiments therefore addressed whether the cloned receptors can account for the classical electrophysiological, biochemical and behavioral GABAB responses or whether additional receptors exist. Among G-protein coupled receptors, GABAB receptors are unique in that they require 2 distinct subunits for functioning. This atypical receptor structure triggered a large body of work that investigated the regulation of receptor assembly and trafficking. With the availability of molecular tools, substantial progress was also made in the analysis of the receptor protein distribution in neuronal compartments. Here, we review recent studies that shed light on the molecular diversity, the subcellular distribution and the cell surface dynamics of GABAB receptors.

Galanin modulates neuronal and synaptic properties in the rat supraoptic nucleus in a use and state dependent manner

The magnocellular neurons of the hypothalamic supraoptic nucleus (SON) synthesize and secrete oxytocin (OXT) and vasopressin (AVP) from their dendrites. These peptides, and several other neurotransmitters, have been shown to modulate afferent glutamatergic neurotransmission in the SON. The neuropeptide, galanin (GAL) is also localized in SON magnocellular neurons and in afferent fibers in the nucleus. We show that GAL dose-dependently reduces evoked excitatory postsynaptic currents (eEPSCs), alters paired pulse ratio and decreases mEPSC frequency, but not amplitude or decay kinetics in both OXT and AVP neurons. GAL therefore modulates excitatory neurotransmission at a likely presynaptic receptor. Neither OXT/AVP, GABA(B) nor cannabinoid antagonists blocked this effect. A GAL2/3 agonist mimicked GAL's action while GAL1 antagonist did not block GAL's effect, suggesting that GAL2/3 receptors mediate the presynaptic effect. In nondehydrated rats GAL causes a small postsynaptic response, as assessed by input resistance measurements. When the rats were water deprived for 2 days the presynaptic response to GAL was unaltered; however, the postsynaptic decrease in input resistance and hyperpolarization was increased, an effect consistent with a previously described increase in GAL1 receptor expression in dehydration. A GAL1 receptor antagonist blocked the postsynaptic effects. Last, when a train of eEPSCs was elicited, GAL was found to inhibit the earlier events in a train but not the latter. This indicates that GAL may modulate a single synaptic event more effectively than trains of synaptic inputs, thereby acting as a high-pass filter.

Developmentally regulated actions of alcohol on hippocampal glutamatergic transmission

Ethanol exposure during fetal development is a leading cause of learning disabilities. Studies suggest that it alters learning and memory by permanently damaging the hippocampus. It is generally assumed that this is mediated, in part, via alterations in glutamatergic transmission. Although NMDA receptors are presumed to be the most sensitive targets of ethanol in immature neurons, this issue has not been explored in the developing hippocampus. We performed whole-cell patch-clamp recordings in hippocampal slices from neonatal rats. Unexpectedly, we found that acute ethanol (10-50 mM) exposure depresses inward currents elicited by local application of exogenous AMPA, but not NMDA, in CA3 pyramidal neurons. These findings revealed a direct effect of ethanol on postsynaptic AMPA receptors. Ethanol significantly decreased the amplitude of both AMPA and NMDA receptor-mediated EPSCs evoked by electrical stimulation. This effect was associated with an increase in the paired-pulse ratio and a decrease in the frequency of miniature EPSCs driven by depolarization of axonal terminals. These findings demonstrate that ethanol also acts at the presynaptic level. Omega-conotoxin-GVIA occluded the effect of ethanol on NMDA EPSCs, indicating that ethanol decreases glutamate release via inhibition of N-type voltage-gated Ca2+ channels. In more mature rats, ethanol did not affect the probability of glutamate release or postsynaptic AMPA receptor-mediated currents, but it did inhibit NMDA-mediated currents. We conclude that the mechanism by which ethanol inhibits glutamatergic transmission is age dependent and challenge the view that postsynaptic NMDA receptors are the primary targets of ethanol early in development.

Differential Modulation by the GABABReceptor Allosteric Potentiator 2,6-Di-tert-butyl-4-(3-hydroxy-2,2-dimethylpropyl)-phenol (CGP7930) of Synaptic Transmission in the Rat Hippocampal CA1 Area

The recently discovered GABAB receptor-positive allosteric modulators enhanced the potency and efficacy of GABAB receptor agonists in in vitro experiments. These GABAB modulators also attenuated reward and anxiety in behavioral experiments without causing the untoward side effects associated with GABAB receptor activation by agonist administration and hence exhibited potential therapeutic utility. However, the underlying molecular mechanisms enabling the GABAB allosteric modulators to dissociate from the GABAB agonistic side effects remain elusive. To address this question, we have examined the effects of a typical GABAB modulator, 2,6-di-tert-butyl-4-(3-hydroxy-2,2-dimethylpropyl)-phenol (CGP7930), on GABAB receptor-mediated modulations of both the excitatory and the delayed inhibitory components of hippocampal CA1 synaptic transmission. Using baclofen as an agonist and a multielectrode recording system, we recorded GABAB receptor-mediated modulations of both the field excitatory postsynaptic potentials and the population spikes simultaneously, as well as the paired-pulse inhibition of the population spike. We found that CGP7930 selectively enhanced the baclofen-induced modulation of synaptic inhibition without having any significant effects on the synaptic excitation. Our experiments have therefore revealed a pathway-selective differential modulation of synaptic transmission by CGP7930. This finding provides a synaptic mechanism to support the hypothesis that GABAB potentiators may be a better therapeutic alternative than GABAB agonists for central nervous system disorders.

Bidirectional modulation of GABAergic transmission by cholecystokinin in hippocampal dentate gyrus granule cells of juvenile rats

Cholecystokinin (CCK) interacts with two types of G protein-coupled receptors in the brain: CCK-A and CCK-B receptors. Both CCK and CCK-B receptors are widely distributed in the hippocampal formation, but the functions of CCK there have been poorly understood. In the present study, we initially examined the effects of CCK on GABA(A) receptor-mediated synaptic transmission in the hippocampal formation and then explored the underlying cellular mechanisms by focusing on the dentate gyrus region, where the highest levels of CCK-binding sites have been detected. Our results indicate that activation of CCK-B receptors initially and transiently increased spontaneous IPSC (sIPSC) frequency, followed by a persistent reduction. The effects of CCK were more evident in juvenile rats, suggesting that they are developmentally regulated. Cholecystokinin failed to modulate the miniature IPSCs recorded in the presence of TTX and the amplitude of the evoked IPSCs, but produced a transient increase followed by a reduction in action potential firing frequency recorded from GABAergic interneurons, suggesting that CCK acts by modulating the excitability of the interneurons to regulate GABA release. Cholecystokinin reduced the amplitude of the after-hyperpolarization of the action potentials, and application of paxilline or charybdotoxin considerably reduced CCK-mediated modulation of sIPSC frequency, suggesting that the effects of CCK are related to the inhibition of Ca(2+)-activated K(+) currents (I(K(Ca))). The effects of CCK were independent of the functions of phospholipase C, intracellular Ca(2+) release, protein kinase C or phospholipase A(2), suggesting a direct coupling between the G proteins of CCK-B receptors and I(K(Ca)). Our results provide a novel mechanism underlying CCK-mediated modulation of GABA release.

Enhancement of the synchronized firing of CA1 pyramidal cells by medial septum preconditioning: Time-dependent involvement of muscarinic cholinoceptors and GABAB receptors

Activation of the medial septum (MS) enhances the synchronized firing of pyramidal cells in the hippocampus. At least two distinct mechanisms might be recruited in this process: GABAergic inhibition of hippocampal inhibitory interneurons and cholinergic enhancement of pyramidal cell excitability. In the present study, a field potential recording in urethane anaesthetised rats was used to show that MS preconditioning with brief high-frequency burst stimulation markedly enhanced Schaffer collateral/commissural (SC/C) synaptically evoked population spikes in the CA1 area without significant alteration in the initial slope of SC/C field excitatory postsynaptic potentials (fEPSPs). An early phase (< approximately 125 ms) of the facilitation of neuronal discharge was inhibited by intracerebroventricular (i.c.v.) injection of the GABA(B) receptor antagonist CGP 52432 (200 nmol/5 microl). In contrast, the muscarinic cholinoceptor antagonist scopolamine (20 nmol/5 microl) inhibited a later phase ( approximately 150-300 ms) of the population spike facilitation. These findings support an important role for both cholinergic and GABAergic mediated septal drive in the tuning of signal conversion within the hippocampus as well as in securing the cortical feedback from the hippocampus.

Space matters: local and global dendritic Ca2+ compartmentalization in cortical interneurons

Dendrites of pyramidal neurons are complex, electrically active structures that can produce local and global Ca(2+) compartments. Recent studies indicate that dendrites of cortical GABAergic interneurons are also highly specialized, and that different subtypes vary in their morphology, in their intrinsic and synaptic conductances and in the Ca(2+) signals they generate. Because interneurons play a major role in oscillations, understanding their dendrites could offer key insights into rhythmogenesis. Different interneuron subtypes have different synaptic integration properties and generate differentially timed inhibition at distinct sites of the pyramidal neuraxis. In addition, interneuron dendrites generate diverse Ca(2+) signals that reflect this circuit function and probably also implement subclass-specific plasticity and homeostasis.

cAMP response element-binding protein, activating transcription factor-4, and upstream stimulatory factor differentially control hippocampal GABABR1a and GABABR1b subunit gene expression through alternative promoters

Expression of metabotropic GABA(B) receptors is essential for slow inhibitory synaptic transmission in the CNS, and disruption of GABA(B) receptor-mediated responses has been associated with several disorders, including neuropathic pain and epilepsy. The location of GABA(B) receptors in neurons determines their specific role in synaptic transmission, and it is believed that sorting of subunit isoforms, GABA(B)R1a and GABA(B)R1b, to presynaptic or postsynaptic membranes helps to determine this role. GABA(B)R1a and GABA(B)R1b are thought to arise by alternative splicing of heteronuclear RNA. We now demonstrate that alternative promoters, rather than alternative splicing, produce GABA(B)R1a and GABA(B)R1b isoforms. Our data further show that subunit gene expression in hippocampal neurons is mediated by the cAMP response element-binding protein (CREB) by binding to unique cAMP response elements in the alternative promoter regions. Double-stranded oligonucleotide decoys selectively alter levels of endogenous GABA(B)R1a and GABA(B)R1b in primary hippocampal neurons, and CREB knock-out mice show changes in levels of GABA(B)R1a and GABA(B)R1b transcripts, consistent with decoy competition experiments. These results demonstrate a critical role of CREB in transcriptional mechanisms that control GABA(B)R1 subunit levels in vivo. In addition, the CREB-related factor activating transcription factor-4 (ATF4) has been shown to interact directly with GABA(B)R1 in neurons, and we show that ATF4 differentially regulates GABA(B)R1a and GABA(B)R1b promoter activity. These results, together with our finding that the depolarization-sensitive upstream stimulatory factor (USF) binds to a composite CREB/ATF4/USF regulatory element only in the absence of CREB binding, indicate that selective control of alternative GABA(B)R1 promoters by CREB, ATF4, and USF may dynamically regulate expression of their gene products in the nervous system.

GABA(B) receptor activation modulates GABA(A) receptor-mediated inhibition in chicken nucleus magnocellularis neurons

Neurons of nucleus magnocellularis (NM), a division of avian cochlear nucleus that performs precise temporal encoding, receive glutamatergic excitatory input solely from the eighth nerve and GABAergic inhibitory input primarily from the ipsilateral superior olivary nucleus. GABA activates both ligand-gated Cl- channels [GABA(A) receptors (GABA(A)Rs)] and G protein-coupled receptors (GABA(B) receptors). The net effect of GABA(A)R-mediated input to NM is inhibitory, although depolarizing. Several studies have shown that this shunting, inhibitory GABAergic input can evoke action potentials in postsynaptic NM neurons, which could interfere with their temporal encoding. While this GABA-mediated firing is limited by a low-voltage-activated K+ conductance, we have found evidence for a second mechanism. We investigated modulation of GABA(A)R-mediated responses by GABA(B)Rs using whole cell recording techniques. Bath-applied baclofen, a GABA(B)R agonist, produced dose-dependent suppression of evoked inhibitory postsynaptic currents (eIPSCs). This suppression was blocked by CGP52432, a potent and selective GABA(B)R antagonist. Baclofen reduced the frequency but not the amplitude of miniature IPSCs (mIPSCs) and did not affect postsynaptic currents elicited by puff application of a specific GABA(A)R agonist muscimol, suggesting a presynaptic mechanism for the GABA(B)R-mediated modulation. Firing of NM neurons by synaptic stimulation of GABAergic inputs to NM was eliminated by baclofen. However, endogenous GABA(B)R activity in the presynaptic inhibitory terminals was not observed. We propose that presynaptic GABA(B)Rs function as autoreceptors, regulating synaptic strength of GABA(A)R-mediated inhibition, and prevent NM neurons from generating firing during activation of the inhibitory inputs.

GABA(B)-receptor modulation of short-term synaptic depression at an excitatory input to murine hippocampal CA3 pyramidal neurons

GABA(B) agonists inhibit excitatory transmission to hippocampal CA3 neurons during low frequency stimulation. We examined whether GABA(B) receptor activation can also enhance synaptic efficacy, when investigated at an input with high initial release probability. Short-term depression of field excitatory postsynaptic potential (EPSP) amplitude was observed during trains of stimuli applied to associational/commissural inputs (10-50 Hz; 22 degrees C). Baclofen (10 microM) reduced the amplitude of initial EPSPs in a train, and also reduced the degree of short-term depression. EPSPs recorded late in a train were significantly larger in baclofen than those recorded in control solution. These dual effects were mimicked by another selective GABA(B) agonist (SKF 97541, 10 microM), and abolished by a GABA(B)-selective antagonist (SCH 50911, 20 microM). The effects of baclofen were similar at a higher recording temperature (32 degrees C), where short-term depression was observed at higher stimulation frequencies. These results are consistent with the idea that a reduction of transmitter release probability could increase the fidelity of high-frequency transmission at this input, an effect that could help account for excitatory effects of GABA(B) agonists in some seizure models.

Mechanism of GABAB receptor-mediated inhibition of spontaneous GABA release onto cerebellar Purkinje cells

gamma-Aminobutyric acid (GABA)(B) receptor-mediated modulation of spontaneous GABA release onto Purkinje cells was investigated in cerebellar slices from 3- to 5-week-old mice. The GABA(B) receptor agonists baclofen and CGP 44533 each reduced the frequency of miniature inhibitory postsynaptic currents (mIPSCs), with no significant effect on mIPSC amplitude; together, consistent with a presynaptic site of action. The GABA(B) receptor antagonist CGP 55845 blocked baclofen-induced inhibition. The sulphydryl alkylating agent N-ethylmaleimide occluded baclofen effects, implicating G(i/o) subunits in mediating a GABA(B) G protein-coupled receptor pathway. Baclofen-induced inhibition persisted in the presence of Ba(2+), a blocker of K(+) channels, and Cd(2+), a blocker of Ca(2+) channel-mediated GABA release. Application of nominally Ca(2+)-free extracellular solutions reduced mIPSC frequency and amplitude; however, baclofen produced a significant inhibition in mIPSC frequency, further suggesting that this pathway was independent of Ca(2+) influx. Spontaneous GABA release was increased by the adenylate cyclase activator, forskolin, and the phorbol ester, phorbol 12,13-dibutyrate. However, baclofen-induced inhibition was not significantly changed in either condition. Baclofen action was also not affected by the adenylate cyclase inhibitor SQ 22536 or the protein kinase C inhibitor chelerythrine chloride. Baclofen still reduced mIPSC frequency in the presence of the polyvalent cation ruthenium red, which acts as a secretagogue here; however, baclofen-induced inhibition was reduced significantly. Furthermore, baclofen produced no clear inhibition during high-frequency mIPSCs bursts induced by the potent secretagogue alpha-Latrotoxin. Together, these results suggest that GABA(B) inhibition occurs downstream of Ca(2+) influx and may be mediated, in part, by an inhibition of the vesicular release mechanism.

Inhibition of gamma-aminobutyric acid uptake: anatomy, physiology and effects against epileptic seizures

The transport of gamma-aminobutyric (GABA) limits the overspill from the synaptic cleft and serves to maintain a constant extracellular level of GABA. Two transporters, GABA transporter-1 (GAT-1) and GAT-3, are the most likely candidates for regulating GABA transport in the brain. Drugs acting either selectively or nonselectively at GATs exert distinct anticonvulsant effects, presumably because of distinct regions of action. Here I shall give a brief review of the localization and physiology of GATs and describe effects of selective and nonselective inhibitors thereof in different animal models of epilepsy.

Presynaptic GABAB receptors modulate thalamic excitation of inhibitory and excitatory neurons in the mouse barrel cortex

Cortical inhibition plays an important role in the processing of sensory information, and the enlargement of receptive fields by the in vivo application of GABAB receptor antagonists indicates that GABAB receptors mediate some of this cortical inhibition. Although there is evidence of postsynaptic GABAB receptors on cortical neurons, there is no evidence of GABAB receptors on thalamocortical terminals. Therefore to determine if presynaptic GABAB receptors modulate the thalamic excitation of layer IV inhibitory neurons and excitatory neurons in layers II-III and IV of the somatosensory "barrel" cortex of mice, we used a thalamocortical slice preparation and patch-clamp electrophysiology. Stimulation of the ventrobasal thalamus elicited excitatory postsynaptic currents (EPSCs) in cortical neurons. Bath application of baclofen, a selective GABAB receptor agonist, reversibly decreased AMPA receptor-mediated and N-methyl-D-aspartate (NMDA) receptor-mediated EPSCs in inhibitory and excitatory neurons. The GABAB receptor antagonist, CGP 35348, reversed the inhibition produced by baclofen. Blocking the postsynaptic GABAB receptor-mediated effects with a Cs+ -based recording solution did not affect the inhibition, suggesting a presynaptic effect of baclofen. Baclofen reversibly increased the paired-pulse ratio and the coefficient of variation, consistent with the presynaptic inhibition of glutamate release. Our results indicate that the presynaptic activation of GABAB receptors modulates thalamocortical excitation of inhibitory and excitatory neurons and provide another mechanism by which cortical inhibition can modulate the processing of sensory information.

Substance P presynaptically depresses the transmission of sensory input to bronchopulmonary neurons in the guinea pig nucleus tractus solitarii

Substance P modulates the reflex regulation of respiratory function by its actions both peripherally and in the CNS, particularly in the nucleus tractus solitarii (NTS), the first central site for synaptic contact of the lung and airway afferent fibres. There is considerable evidence that the actions of substance P in the NTS augment respiratory reflex output, but the precise effects on synaptic transmission have not yet been determined. Therefore, we determined the effects of substance P on synaptic transmission at the first central synapses by using whole-cell voltage clamping in an NTS slice preparation. Studies were performed on second-order neurons in the slice anatomically identified as receiving monosynaptic input from sensory nerves in the lungs and airways. This was done by the fluorescent labelling of terminal boutons after 1,1'-dioctadecyl-3,3,3',3'-tetra-methylindocarbo-cyanine perchlorate (DiI) was applied via tracheal instillation. Substance P (1.0, 0.3 and 0.1 microM) significantly decreased the amplitude of excitatory postsynaptic currents (eEPSCs) evoked by stimulation of the tractus solitarius, in a concentration-dependent manner. The decrease was accompanied by an increase in the paired-pulse ratio of two consecutive eEPSCs, and a decrease in the frequency, but not the amplitude, of spontaneous EPSCs and miniature EPSCs, findings consistent with a presynaptic site of action. The effects were consistently and significantly attenuated by a neurokinin-1 (NK1) receptor antagonist (SR140333, 3 muM). The data suggest a new site of action for substance P in the NTS (NK1 receptors on the central terminals of sensory fibres) and a new mechanism (depression of synaptic transmission) for regulating respiratory reflex function.

Two Loci of expression for long-term depression at hippocampal mossy fiber-interneuron synapses

Two distinct forms of long-term depression (LTD) exist at mossy fiber synapses between dentate gyrus granule cells and hippocampal CA3 stratum lucidum interneurons. Although induction of each form of LTD requires an elevation of postsynaptic intracellular Ca2+, at Ca2+-impermeable AMPA receptor (CI-AMPAR) synapses, induction is NMDA receptor (NMDAR) dependent, whereas LTD at Ca2+-permeable AMPA receptor (CP-AMPAR) synapses is NMDAR independent. However, the expression locus of either form of LTD is not known. Using a number of criteria, including the coefficient of variation, paired-pulse ratio, AMPA-NMDA receptor activity, and the low-affinity AMPAR antagonist gamma-D-glutamyl-glycine, we demonstrate that LTD expression at CP-AMPAR synapses is presynaptic and results from reduced transmitter release, whereas LTD expression at CI-AMPAR synapses is postsynaptic. The N-ethylmaleimide-sensitive fusion protein-AP2-clathrin adaptor protein 2 inhibitory peptide pep2m occluded LTD expression at CI-AMPAR synapses but not at CP-AMPAR synapses, confirming that CI-AMPAR LTD involves postsynaptic AMPAR trafficking. Thus, mossy fiber innervation of CA3 stratum lucidum interneurons occurs via two parallel systems targeted to either Ca2+-permeable or Ca2+-impermeable AMPA receptors, each with a distinct expression locus for long-term synaptic plasticity.

Lamina-specific differences in GABA(B) autoreceptor-mediated regulation of spontaneous GABA release in rat entorhinal cortex

Spontaneous synaptic inhibition plays an important role in regulating the excitability of cortical networks. Here we have investigated the role of GABA(B) autoreceptors in regulating spontaneous GABA release in the entorhinal cortex (EC), a region associated with temporal lobe epilepsies. We have previously shown that the level of spontaneous inhibition in superficial layers of the EC is much greater than that seen in deeper layers. In the present study, using intracellular and whole cell patch clamp recordings in rat EC slices, we have demonstrated that evoked GABA responses are controlled by feedback inhibition via GABA(B) autoreceptors. Furthermore, recordings of spontaneous, activity-independent inhibitory postsynaptic currents in layer II and layer V neurones showed that the GABA(B) receptor agonist, baclofen, reduced the frequency of GABA-mediated currents indicating the presence of presynaptic GABA(B) receptors in both layers. Application of the antagonist, CGP55845, blocked the effects of baclofen and also increased the frequency of GABA-mediated events above baseline, but the latter effect was restricted to layer V. This demonstrates that GABA(B) autoreceptors are tonically activated by synaptically released GABA in layer V, and this may partly explain the lower level of spontaneous GABA release in the deep layer.

Subcellular localization of metabotropic GABA(B) receptor subunits GABA(B1a/b) and GABA(B2) in the rat hippocampus

Metabotropic GABA(B) receptors mediate slow inhibitory effects presynaptically and postsynaptically. Using preembedding immunohistochemical methods combined with quantitative analysis of GABA(B) receptor subunit immunoreactivity, this study provides a detailed description of the cellular and subcellular localization of GABA(B1a/b) and GABA(B2) in the rat hippocampus. At the light microscopic level, an overlapping distribution of GABA(B1a/b) and GABA(B2) was revealed in the dendritic layers of the hippocampus. In addition, expression of the GABA(B1a/b) subunit was found in somata of CA1 pyramidal cells and of a subset of GABAergic interneurons. At the electron microscopic level, immunoreactivity for both subunits was observed on presynaptic and, more abundantly, on postsynaptic elements. Presynaptically, subunits were mainly detected in the extrasynaptic membrane and occasionally over the presynaptic membrane specialization of putative glutamatergic and, to a lesser extent, GABAergic axon terminals. Postsynaptically, the majority of GABA(B) receptor subunits were localized to the extrasynaptic plasma membrane of spines and dendritic shafts of principal cells and shafts of interneuron dendrites. Quantitative analysis revealed enrichment of GABA(B1a/b) around putative glutamatergic synapses on spines and an even distribution on dendritic shafts of pyramidal cells contacted by GABAergic boutons. The association of GABA(B) receptors with glutamatergic synapses at both presynaptic and postsynaptic sides indicates their intimate involvement in the modulation of glutamatergic neurotransmission. The dominant extrasynaptic localization of GABA(B) receptor subunits suggests that their activation is dependent on spillover of GABA requiring simultaneous activity of populations of GABAergic cells as it occurs during population oscillations or epileptic seizures.

Contribution of Kir3.1, Kir3.2A and Kir3.2C subunits to native G protein-gated inwardly rectifying potassium currents in cultured hippocampal neurons

G protein-gated inwardly rectifying potassium (GIRK) channels are found in neurons, atrial myocytes and neuroendocrine cells. A characteristic feature is their activation by stimulation of Gi/o-coupled receptors. In central neurons, for example, they are activated by adenosine and GABA and, as such, they play an important role in neurotransmitter-mediated regulation of membrane excitability. The channels are tetrameric assemblies of Kir3.x subunits (Kir3.1-3.4 plus splice variants). In this study I have attempted to identify the channel subunits which contribute to the native GIRK current recorded from primary cultured rat hippocampal pyramidal neurons. Reverse transcriptase-polymerase chain reaction revealed the expression of mRNA for Kir3.1, 3.2A, 3.2C and 3.3 subunits and confocal immunofluorescence microscopy was used to investigate their expression patterns. Diffuse staining was observed on both cell somata and dendrites for Kir3.1 and Kir3.2A yet that for Kir3.2C was weaker and punctate. Whole-cell patch clamp recordings were used to record GIRK currents from hippocampal pyramidal neurons which were identified on the basis of inward rectification, dependence of reversal potential on external potassium concentration and sensitivity to tertiapin. The GIRK currents were enhanced by the stimulation of a number of Gi/o-coupled receptors and were inhibited by pertussis toxin. In order to ascertain which Kir3.x subunits were responsible for the native GIRK current I compared the properties with those of the cloned Kir3.1 + 3.2A and Kir3.1 + 3.2C channels heterologously expressed in HEK293 cells.

Interneuron diversity series: containing the detonation--feedforward inhibition in the CA3 hippocampus

Feedforward inhibitory circuits are involved both in the suppression of excitability and timing of action potential generation in principal cells. In the CA3 hippocampus, a single mossy fiber from a dentate gyrus granule cell forms giant boutons with multiple release sites, which are capable of detonating CA3 principal cells. By contrast, mossy fiber terminals form a larger number of Lilliputian-sized synapses with few release sites onto local circuit interneurons, with distinct presynaptic and postsynaptic properties. This dichotomy between the two synapse types endows the circuit with exquisite control over pyramidal cell discharge. Under pathological conditions where feedforward inhibition is compromised, focal excitation is no longer contained, rendering the circuit susceptible to hyperexcitability.

Regulation of exocytosis from single visualized GABAergic boutons in hippocampal slices

Regulation of GABA release is crucial for normal brain functioning, and GABAA-mediated IPSCs are strongly influenced by repetitive stimulation and neuromodulation. However, GABA exocytosis has not been examined directly in organized tissue. Important issues remain outside the realm of electrophysiological techniques or are complicated by postsynaptic factors. For example, it is not known whether all presynaptic modulators affect release from all boutons in the same way, or whether modulator effects depend on the presence of certain types of voltage-gated calcium channels (VGCCs). To address such issues, we used confocal imaging and styryl dyes to monitor exocytosis from identified GABAergic boutons in organotypic hippocampal slice cultures. Repetitively evoked IPSCs declined more rapidly and completely than exocytosis, suggesting that depletion of filled vesicles cannot fully account for IPSC depression and underscoring the usefulness of directly imaging exocytosis. Stimulation at 10 Hz produced a transient facilitation of exocytosis that was dependent on L-type VGCCs. Using specific toxins, we found that release mediated via N-type and P-type VGCCs had similar properties. Neither baclofen nor a cannabinoid receptor agonist, CP55940, affected all boutons uniformly; they slowed release from some but completely prevented detectable release from others. Increasing stimulus frequency overcame this blockade of release. However, baclofen and CP55940 did not act identically, because only baclofen reduced facilitation and affected bouton releasing via P/Q-type VGCCs. Direct observation thus revealed novel features of GABAergic exocytosis and its regulation that would have been difficult or impossible to detect electrophysiologically. These features advance the understanding of the regulation of synapses and networks by presynaptic inhibition.

Subcellular localization of metabotropic GABA(B) receptor subunits GABA(B1a/b) and GABA(B2) in the rat hippocampus

Metabotropic GABA(B) receptors mediate slow inhibitory effects presynaptically and postsynaptically. Using preembedding immunohistochemical methods combined with quantitative analysis of GABA(B) receptor subunit immunoreactivity, this study provides a detailed description of the cellular and subcellular localization of GABA(B1a/b) and GABA(B2) in the rat hippocampus. At the light microscopic level, an overlapping distribution of GABA(B1a/b) and GABA(B2) was revealed in the dendritic layers of the hippocampus. In addition, expression of the GABA(B1a/b) subunit was found in somata of CA1 pyramidal cells and of a subset of GABAergic interneurons. At the electron microscopic level, immunoreactivity for both subunits was observed on presynaptic and, more abundantly, on postsynaptic elements. Presynaptically, subunits were mainly detected in the extrasynaptic membrane and occasionally over the presynaptic membrane specialization of putative glutamatergic and, to a lesser extent, GABAergic axon terminals. Postsynaptically, the majority of GABA(B) receptor subunits were localized to the extrasynaptic plasma membrane of spines and dendritic shafts of principal cells and shafts of interneuron dendrites. Quantitative analysis revealed enrichment of GABA(B1a/b) around putative glutamatergic synapses on spines and an even distribution on dendritic shafts of pyramidal cells contacted by GABAergic boutons. The association of GABA(B) receptors with glutamatergic synapses at both presynaptic and postsynaptic sides indicates their intimate involvement in the modulation of glutamatergic neurotransmission. The dominant extrasynaptic localization of GABA(B) receptor subunits suggests that their activation is dependent on spillover of GABA requiring simultaneous activity of populations of GABAergic cells as it occurs during population oscillations or epileptic seizures.

Depolarization-induced long-term depression at hippocampal mossy fiber-CA3 pyramidal neuron synapses

Hippocampal CA3 pyramidal neurons receive two types of excitatory afferent innervation: mossy fibers (MFs) from granule cells of the dentate gyrus and recurrent collateral fibers (CFs) from other CA3 pyramidal neurons. At CF-CA3 pyramidal neuron synapses, membrane depolarization paired with low (0.33 Hz) presynaptic stimulation generated a heterogeneous response that ranged from long-term potentiation (LTP), long-term depression (LTD), to no alteration of synaptic strength. However, the same induction paradigm applied at MF-CA3 pyramidal neuron synapses consistently induced LTD. This novel form of LTD was independent of NMDARs, mGluRs, cannabinoid receptors, opioid receptors, or coincident synaptic activity, but was dependent on postsynaptic Ca2+ elevation through L-type Ca2+ channels and release from inositol 1,4,5-trisphosphate receptor-sensitive intracellular stores. Ca2+ imaging of both proximal and distal CA3 pyramidal neuron dendrites demonstrated that the depolarizing induction paradigm differentially elevated intracellular Ca2+ levels. L-type Ca2+ channel activation was observed only at the most proximal locations where mossy fibers make synapses. Depolarization-induced LTD did not occlude the conventional 1 Hz-induced LTD or vice versa, suggesting independent mechanisms underlie each form of plasticity. The paired-pulse ratio and coefficient of variation of synaptic transmission were unchanged after LTD induction, suggesting that the expression locus of LTD is postsynaptic. Moreover, peak-scaled nonstationary variance analysis indicated that depolarization-induced LTD correlated with a reduction in postsynaptic AMPA receptor numbers without a change in AMPA receptor conductance. Our results suggest that this novel form of LTD is selectively expressed at proximal dendritic locations closely associated with L-type Ca2+ channels.

Regulation of exocytosis from single visualized GABAergic boutons in hippocampal slices

Regulation of GABA release is crucial for normal brain functioning, and GABAA-mediated IPSCs are strongly influenced by repetitive stimulation and neuromodulation. However, GABA exocytosis has not been examined directly in organized tissue. Important issues remain outside the realm of electrophysiological techniques or are complicated by postsynaptic factors. For example, it is not known whether all presynaptic modulators affect release from all boutons in the same way, or whether modulator effects depend on the presence of certain types of voltage-gated calcium channels (VGCCs). To address such issues, we used confocal imaging and styryl dyes to monitor exocytosis from identified GABAergic boutons in organotypic hippocampal slice cultures. Repetitively evoked IPSCs declined more rapidly and completely than exocytosis, suggesting that depletion of filled vesicles cannot fully account for IPSC depression and underscoring the usefulness of directly imaging exocytosis. Stimulation at 10 Hz produced a transient facilitation of exocytosis that was dependent on L-type VGCCs. Using specific toxins, we found that release mediated via N-type and P-type VGCCs had similar properties. Neither baclofen nor a cannabinoid receptor agonist, CP55940, affected all boutons uniformly; they slowed release from some but completely prevented detectable release from others. Increasing stimulus frequency overcame this blockade of release. However, baclofen and CP55940 did not act identically, because only baclofen reduced facilitation and affected bouton releasing via P/Q-type VGCCs. Direct observation thus revealed novel features of GABAergic exocytosis and its regulation that would have been difficult or impossible to detect electrophysiologically. These features advance the understanding of the regulation of synapses and networks by presynaptic inhibition.

Depolarization-induced long-term depression at hippocampal mossy fiber-CA3 pyramidal neuron synapses

Hippocampal CA3 pyramidal neurons receive two types of excitatory afferent innervation: mossy fibers (MFs) from granule cells of the dentate gyrus and recurrent collateral fibers (CFs) from other CA3 pyramidal neurons. At CF-CA3 pyramidal neuron synapses, membrane depolarization paired with low (0.33 Hz) presynaptic stimulation generated a heterogeneous response that ranged from long-term potentiation (LTP), long-term depression (LTD), to no alteration of synaptic strength. However, the same induction paradigm applied at MF-CA3 pyramidal neuron synapses consistently induced LTD. This novel form of LTD was independent of NMDARs, mGluRs, cannabinoid receptors, opioid receptors, or coincident synaptic activity, but was dependent on postsynaptic Ca2+ elevation through L-type Ca2+ channels and release from inositol 1,4,5-trisphosphate receptor-sensitive intracellular stores. Ca2+ imaging of both proximal and distal CA3 pyramidal neuron dendrites demonstrated that the depolarizing induction paradigm differentially elevated intracellular Ca2+ levels. L-type Ca2+ channel activation was observed only at the most proximal locations where mossy fibers make synapses. Depolarization-induced LTD did not occlude the conventional 1 Hz-induced LTD or vice versa, suggesting independent mechanisms underlie each form of plasticity. The paired-pulse ratio and coefficient of variation of synaptic transmission were unchanged after LTD induction, suggesting that the expression locus of LTD is postsynaptic. Moreover, peak-scaled nonstationary variance analysis indicated that depolarization-induced LTD correlated with a reduction in postsynaptic AMPA receptor numbers without a change in AMPA receptor conductance. Our results suggest that this novel form of LTD is selectively expressed at proximal dendritic locations closely associated with L-type Ca2+ channels.

GABA transporter-1 (GAT1)-deficient mice: differential tonic activation of GABAA versus GABAB receptors in the hippocampus

After its release from interneurons in the CNS, the major inhibitory neurotransmitter GABA is taken up by GABA transporters (GATs). The predominant neuronal GABA transporter GAT1 is localized in GABAergic axons and nerve terminals, where it is thought to influence GABAergic synaptic transmission, but the details of this regulation are unclear. To address this issue, we have generated a strain of GAT1-deficient mice. We observed a large increase in a tonic postsynaptic hippocampal GABAA receptor-mediated conductance. There was little or no change in the waveform or amplitude of spontaneous inhibitory postsynaptic currents (IPSCs) or miniature IPSCs. In contrast, the frequency of quantal GABA release was one-third of wild type (WT), although the densities of GABAA receptors, GABAB receptors, glutamic acid decarboxylase 65 kDa, and vesicular GAT were unaltered. The GAT1-deficient mice lacked a presynaptic GABAB receptor tone, present in WT mice, which reduces the frequency of spontaneous IPSCs. We conclude that GAT1 deficiency leads to enhanced extracellular GABA levels resulting in an overactivation of GABAA receptors responsible for a postsynaptic tonic conductance. Chronically elevated GABA levels also downregulate phasic GABA release and reduce presynaptic signaling via GABAB receptors thus causing an enhanced tonic and a diminished phasic inhibition.

Rapid and long-term alterations of hippocampal GABAB receptors in a mouse model of temporal lobe epilepsy

Alterations of gamma-aminobutyric acid (GABA)B receptor expression have been reported in human temporal lobe epilepsy (TLE). Here, changes in regional and cellular expression of the GABAB receptor subunits R1 (GBR1) and R2 (GBR2) were investigated in a mouse model that replicates major functional and histopathological features of TLE. Adult mice received a single, unilateral injection of kainic acid (KA) into the dorsal hippocampus, and GABAB receptor immunoreactivity was analysed between 1 day and 3 months thereafter. In control mice, GBR1 and GBR2 were distributed uniformly across the dendritic layers of CA1-CA3 and dentate gyrus. In addition, some interneurons were labelled selectively for GBR1. At 1 day post-KA, staining for both GBR1 and GBR2 was profoundly reduced in CA1, CA3c and the hilus, and no interneurons were visible anymore. At later stages, the loss of GABAB receptors persisted in CA1 and CA3, whereas staining increased gradually in dentate gyrus granule cells, which become dispersed in this model. Most strikingly, a subpopulation of strongly labelled interneurons reappeared, mainly in the hilus and CA3 starting at 1 week post-KA. In double-staining experiments, these cells were selectively labelled for neuropeptide Y. The number of GBR1-positive interneurons also increased contralaterally in the hilus. The rapid KA-induced loss of GABAB receptors might contribute to epileptogenesis because of a reduction in both presynaptic control of transmitter release and postsynaptic inhibition. In turn, the long-term increase in GABAB receptors in granule cells and specific subtypes of interneurons may represent a compensatory response to recurrent seizures.