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PubMed2001

Postsynaptic mechanism may contribute to inhibitory acetylcholine effect on GABAergic synaptic transmission in hippocampal cell cultures.

The effect of acetylcholine (ACh) on evoked GABAergic inhibitory postsynaptic currents (IPSCs) was studied in cell cultures of dissociated hippocampal neurons with established synaptic connections. Spontaneous IPSCs and IPSCs evoked by extracellular stimulation of a single presynaptic neuron were recorded. ACh inhibited the evoked IPSCs in most of the connections, although facilitation was also observed. Regardless of inhibitory or facilitatory effects on the evoked IPSCs, an enhanced spontaneous synaptic input to the postsynaptic neurons was usually observed. ACh-induced changes in the evoked IPSCs were usually accompanied by changes in paired pulse depression (PPD), which are thought to reflect presynaptic mechanisms of modulation. However, the time course of PPD changes did not always match that of the IPSC changes, suggesting a contribution of other, possibly postsynaptic, mechanism(s). To analyze this possibility, effects of ACh on responses to direct application of exogenous GABA were studied. In a proportion of the neurons (40%) ACh reversibly decreased GABA responses, indicating that postsynaptic mechanisms may also contribute to the inhibitory ACh effect on GABAergic transmission. We conclude that several different modulatory mechanisms of ACh action participate in the regulation of GABAergic transmission at the level of synaptic connection of a single GABAergic neuron.

Acetylcholine↗
PubMed1991

Reduction of GABAA receptor-mediated inhibition by the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione in cultured neurons of rat brain.

The action of the non-N-methyl-D-aspartate (non-NMDA) receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) on gamma-aminobutyric acid-A (GABAA) receptor-mediated currents was studied in dissociated rat midbrain and hypothalamic cultures using whole-cell recording. Spontaneous synaptic activity consisted of excitatory (EPSCs) and inhibitory postsynaptic currents (IPSCs). Bicuculline (20 microM) blocked IPSCs and increased the frequency of EPSCs. CNQX (1 microM) reduced both EPSCs and IPSCs. In the presence of 0.3 microM tetrodotoxin (TTX), CNQX (1-20 microM) blocked miniature EPSCs and reduced IPSCs. In TTX, increasing K+ (20 mM) evoked EPSCs and IPSCs in a Ca-dependent manner. CNQX (10 microM) blocked evoked EPSCs and diminished evoked IPSCs similarly as miniature IPSCs. Muscimol-(0.2-5 microM) induced currents were dose-dependently reduced by CNQX (10-50 microM). It is concluded that CNQX reduces GABAA receptor-mediated inhibition primarily by reducing the excitatory drive in the evolving network, but, in addition, has a significant blocking effect on the GABAA receptor-channel complex.

6-Cyano-7-nitroquinoxaline-2,3-dione↗
PubMed2004

Calcium-independent inhibition of GABA(A) current by caffeine in hippocampal slices.

Although inhibitory postsynaptic currents (IPSCs) mediated by GABA(A) receptor is thought to be affected by intracellular calcium ion concentration ([Ca2+]i), origin or route of [Ca2+]i increment has not been well elucidated. Reports on the effect of [Ca2+]i elevation on GABA(A)ergic IPSCs per se are also controversial. In this study, effects of caffeine and several other [Ca2+]i-mobilizing drugs were examined on the IPSCs in acute slices of rat hippocampus. Using the patch clamp recording method, spontaneous and evoked currents were recorded from CA3 neurons. Caffeine strongly inhibited both extra-synaptic and synaptic GABAergic IPSCs, regardless of the presence or absence of extracellular Ca2+. This inhibition was not relieved by the intracellular application of EGTA or 1,2-bis(2-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA). This inhibition by caffeine was not prevented by preequilibration with caffeine. Ca2+ store depletion caused by thapsigargin or repetitive stimulation by caffeine could not prevent the inhibition. Moreover, ruthenium red and ryanodine could not overcome the inhibition. On the contrary, GABA(A)ergic currents were not inhibited by stimulation with several Ca2+-mobilizing agonists. Forskolin could not mimic the effect of caffeine on the IPSC, and caffeine inhibited the IPSC in the presence of adenosine. These results suggest that intracellular Ca2+ mobilization through ryanodine-sensitive store stimulation does not significantly affect GABAergic IPSCs, and most of the inhibitory effect of caffeine is independent of [Ca2+]i elevation under the present experimental conditions.

Acetylcholine↗
PubMed2002

Cannabinoids inhibit striatonigral GABAergic neurotransmission in the mouse.

The substantia nigra pars reticulata (SNR) belongs to the brain regions with the highest density of CB(1) cannabinoid receptors. Anatomical studies indicate that the great majority of CB(1) receptors in the SNR are localized on terminals of GABAergic axons arriving from the caudate-putamen (striatonigral axons). The aim of the present experiments was to clarify the role of CB(1) receptors on terminals of striatonigral axons. Oblique sagittal slices, including the caudate-putamen and the substantia nigra, were prepared from brains of young mice. Electrical stimulation in the caudate-putamen elicited GABAergic inhibitory postsynaptic currents (IPSCs) in the SNR, which were studied by patch-clamp techniques. The long latency of IPSCs (14+/-1 ms) suggests that striatonigral axons were indeed activated within the caudate-putamen. The synthetic CB(1)/CB(2) cannabinoid receptor agonist WIN55212-2 (R(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazin-yl]-(1-naphthalenyl)methanone mesylate; 10(-5) M) decreased the amplitude of IPSCs by 93+/-1%. CP55940 ((-)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-trans-4-(3-hydroxypropyl)cyclohexanol; 10(-5) M), another CB(1)/CB(2) receptor agonist, also reduced IPSC amplitude, by 76+/-4%. The CB(1) cannabinoid receptor antagonist SR141716A (N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-3-pyrazole-carboxamide; 10(-6) M) prevented the inhibition produced by WIN55212-2 (10(-5) M). Depolarization of SNR neurons led to suppression of IPSCs; this suppression was prevented by SR141716A (10(-6) M). Three observations indicate that the agonists inhibited neurotransmission presynaptically. (1) CP55940 (10(-5) M) enhanced the ratio of amplitudes of two IPSCs which were elicited by two electrical stimuli 100 ms apart (paired pulses). (2) WIN55212-2 (10(-5) M) did not change the amplitude of miniature IPSCs recorded in the presence of tetrodotoxin. (3) WIN55212-2 (10(-5) M) also had no effect on currents elicited in SNR neurons by ejection of the GABA(A) receptor agonist muscimol from a pipet. In summary, we have established a method which allows selective examination of GABAergic neurotransmission between striatonigral axons and SNR neurons. Using this method, the function of CB(1) cannabinoid receptors on terminals of striatonigral axons was unequivocally clarified. Activation of these receptors causes strong presynaptic inhibition of GABAergic neurotransmission between striatonigral axons and SNR neurons. This effect may be one explanation of the catalepsy observed in animals after cannabinoid administration. Endocannabinoids released from SNR neurons can modulate striatonigral neurotransmission by inhibiting GABA release from terminals of striatonigral axons.

Animals↗
PubMed2002

Glycinergic synaptic transmission to bullfrog retinal bipolar cells is input-specific.

Glycinergic inhibitory postsynaptic currents (IPSCs) focally elicited at the dendrites and axon terminals were recorded from bipolar cells in the bullfrog retinal slice, using the whole-cell clamp technique. IPSCs driven by input from interplexiform cells at bipolar cell dendrites (ipc-IPSCs) had a much slower decay time constant (25.2 +/- 7.8 ms) than IPSCs driven by input from amacrine cells at bipolar cell axon terminals (ac-IPSCs) (14.7 +/- 5.5 ms). Furthermore, peak-scaled non-stationary noise analysis revealed that the weighted mean single-channel conductance of the glycine receptors underlying bipolar cell dendritic ipc-IPSCs (20.8 +/- 6.6 pS) was significantly larger than that of those underlying bipolar cell axon terminal ac-IPSCs (12.9 +/- 2.9 pS). These results demonstrate that glycinergic synaptic transmission with different properties at bipolar cell dendrites and axon terminals differentially mediates intraretinal centrofugal signal transfer from the inner retina to the outer retina provided by interplexiform cells and lateral inhibition offered by amacrine cells in the inner retina.

Animals↗
PubMed2002

Differences in GABAergic transmission between two inputs into the perirhinal cortex.

We have investigated properties of GABAergic synaptic transmission in perirhinal cortex evoked by stimulation of temporal and entorhinal cortex sides. GABAA IPSCs were isolated by blockade of glutamatergic transmission in slices of adult perirhinal cortex; IPSC decay was best fitted with two exponentials. Interestingly, temporal IPSCs had a larger slow component of decay (P < 0.05) compared to entorhinal IPSCs. Depression of IPSCs by the GABAB receptor agonist baclofen was greater (P < 0.05) in the temporal input (79 +/- 4% depression) than the entorhinal input (65 +/- 3% depression). Furthermore, baclofen abolished the slow component of IPSC decay in both inputs. Activity-dependent depression of IPSCs at 5 Hz was greater (P < 0.05) in the temporal input [paired pulse ratio (PPR) 0.5 +/- 0.04] compared to that in the entorhinal input (PPR 0.67 +/- 0.02, n = 10). The differences in paired pulse depression between the inputs were removed by the GABAB receptor antagonist CGP55845A. This study demonstrates several differences in GABA transmission between temporal and entorhinal inputs including the differential activation of presynaptic GABAB receptors and differential regulation of inhibitory synaptic transmission. These properties may be important in the control of neuronal activity within perirhinal cortex.

Afferent Pathways↗
PubMed1999

Mechanisms underlying kainate receptor-mediated disinhibition in the hippocampus.

Kainate (KA) receptor activation depresses stimulus-evoked gamma-aminobutyric acid (GABA-mediated) synaptic transmission onto CA1 pyramidal cells of the hippocampus and simultaneously increases the frequency of spontaneous GABA release through an increase in interneuronal spiking. To determine whether these two effects are independent, we examined the mechanism by which KA receptor activation depresses the stimulus-evoked, inhibitory postsynaptic current (IPSC). Bath application of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid (AMPA)/KA receptor agonist KA in the presence of the AMPA receptor antagonist GYKI 53655 caused a large increase in spontaneous GABA release and a coincident depression of the evoked IPSC. The depressant action on the evoked IPSC was reduced, but not abolished, by the GABA(B) receptor antagonist SCH 50911, suggesting that the KA-induced increase in spontaneous GABA release depresses the evoked IPSC through activation of presynaptic GABA(B) receptors. KA had no resolvable effect on the potassium-induced increase in miniature IPSC frequency, suggesting that KA does not act through a direct effect on the release machinery or presynaptic calcium influx. KA caused a decrease in pyramidal cell input resistance, which was reduced by GABA(A) receptor antagonists. KA also caused a reduction in the size of responses to iontophoretically applied GABA, which was indistinguishable from the SCH 50911-resistant, residual depression of the evoked IPSC. These results suggest that KA receptor activation depresses the evoked IPSC indirectly by increasing interneuronal spiking and GABA release, leading to activation of presynaptic GABA(B) receptors, which depress GABA release, and postsynaptic GABA(A) receptors, which increase passive shunting.

Hippocampus↗
PubMed2000

Pre- and postsynaptic modulation of the electrical activity of rat supraoptic neurones.

The release of vasopressin and oxytocin is regulated by the electrical activity of magnocellular neurosecretory cells in the supraoptic and paraventricular nuclei, which is under the control of a great variety of neurotransmitters and neuromodulators. The major neural signals to the supraoptic nucleus are from excitatory glutamate inputs and inhibitory GABA inputs. In recent studies, the voltage-clamp mode of the whole-cell patch-clamp technique has been applied to slice preparations from rat hypothalamus to monitor synaptic inputs to supraoptic neurones. Spontaneous excitatory and inhibitory postsynaptic currents (EPSCs and IPSCs) are abolished by CNQX and picrotoxin, respectively, but are insensitive to tetrodotoxin, indicating that they represent quantal release of glutamate and GABA, respectively, from nerve terminals of presynaptic neurones. GABA and glutamate show remarkable suppressive effects on both EPSCs and IPSCs via presynaptic GABA(B) and mGlu receptors, respectively. Noradrenaline, which excites supraoptic neurones via postsynaptic alpha1-receptors, also suppresses IPSCs and potentiates EPSCs. On the other hand, prostaglandin E2, which excites supraoptic neurones via postsynaptic prostaglandin E2 (EP) receptors of the EP4 subclass, also suppresses IPSCs via EP3 receptors but has little effect on EPSCs. Thus pre- and postsynaptic mechanisms may act cooperatively to excite supraoptic neurones. Nitric oxide, which inhibits supraoptic neurones, potentiates IPSCs without affecting EPSCs. This provides another example for the preferential modulation of IPSCs of supraoptic neurones. On the other hand, PACAP, which causes a long-lasting increase in the firing frequency via the postsynaptic receptors, has no effect on EPSCs and IPSCs, suggesting that some ligands act only at postsynaptic receptors. Thus multiple patterns for pre- and postsynaptic modulation are present in the supraoptic nucleus, and the electrical activity of supraoptic neurones is regulated via complex mechanisms at both pre- and postsynaptic sites.

Animals↗
PubMed1998

Glycine and GABAA receptor-mediated synaptic transmission in rat substantia gelatinosa: inhibition by mu-opioid and GABAB agonists.

1. Bicuculline-sensitive and strychnine-sensitive inhibitory postsynaptic currents (IPSCs) could be evoked in neurones of the rat substantia gelatinosa of the spinal trigeminal nucleus pars caudalis. 2. Spontaneous tetrodotoxin (TTX)-insensitive-mediated miniature IPSCs (mIPSCs) blocked by strychnine or bicuculline were also present in many neurones. The decay of the glycine receptor-mediated mIPSCs was fitted by a single exponential, whereas the decay of the GABAA receptor-mediated mIPSCs could in some instances be fitted by a single exponential, but in other instances required two exponentials. 3. An increase in baseline current noise developed during the course of the recording. This noise was abolished by strychnine (1 microM) but was insensitive to bicuculline (10 microM), TTX (0.5 microM), [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO, 1 microM) or baclofen (30 microM). The single-channel conductance underlying the noise was estimated to be 21 pS. 4. The mu-opioid agonist DAMGO (1-10 microM) reduced the amplitude of the evoked glycine receptor-mediated IPSC and the evoked GABAA receptor-mediated IPSC. The mu-opioid antagonist D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP, 1 microM) reversed the DAMGO inhibition. 5. The GABAB agonist baclofen (30 microM) reduced the amplitude of the evoked glycine receptor-mediated IPSC and the GABAA receptor-mediated IPSC. The inhibition was reversed by the selective GABAB antagonist 3-N[1-(S)-(3,4-dichlorophenyl)ethyl]amino- 2-(S)-hydroxypropyl-P-benzyl-phosphinic acid (CGP 55845A, 1 microM). 6. Both DAMGO and baclofen reduced the frequency of glycine and GABAA receptor-mediated mIPSCs without affecting average amplitude, and increased the percentage of failures of the evoked glycine and GABAA receptor-mediated IPSCs, suggesting a presynaptic site of action.

Analgesics, Opioid↗
PubMed1993

Beta-adrenergic enhancement of inhibitory synaptic activity in rat cerebellar stellate and Purkinje cells.

1. Using the tight-seal whole-cell recording technique, we studied the effects of noradrenaline (NA) on the spontaneous inhibitory synaptic currents (IPSCs) of stellate and Purkinje cells in rat cerebellar slices. 2. In both types of cells, NA (10 microM) induced a marked increase in the frequency of the IPSCs. This effect was observed both in the absence and in the presence of TTX in the saline bathing the cerebellar slices. 3. The NA-induced increase in frequency of IPSCs and miniature IPSCs (mIPSCs) in the two cell types was mimicked by bath applications of isoprenaline (10 microM) and of the adenylyl cyclase activator forskolin (20 microM). Neither phenylephrine nor clonidine changed the frequency of IPSCs in stellate or Purkinje cells. 4. In stellate cells, the beta-agonists and forskolin had variable effects on the amplitudes of both IPSCs and mIPSCs. None of these compounds altered the amplitude of mIPSCs in Purkinje cells. 5. The responses to local applications of GABA to Purkinje cells were unchanged by bath applications of beta-adrenergic agonists or forskolin. A decrease in the response to GABA after treatment with these agents was observed in half the stellate cells examined. 6. We conclude that the major effect of NA on stellate and Purkinje cells is an increase in the frequency of occurrence of spontaneous inhibitory synaptic currents. This action is exerted through the activation of beta-adrenergic receptors and is probably mediated by an intracellular mechanism involving cAMP. The beta-adrenergic modulation of IPSC frequency takes place at the presynaptic level and may involve a change in the process of transmitter release.

Action Potentials↗
PubMed1993

The physiological regulation of synaptic inhibition by GABAB autoreceptors in rat hippocampus.

1. Intracellular recording techniques were used to study the effects of repetitive stimulation on monosynaptically activated inhibitory postsynaptic currents (IPSCs) in rat hippocampal slices. This was achieved by stimulation in stratum radiatum close to a recorded CA1 pyramidal neurone after pharmacological blockade of excitatory synaptic responses, using a combination of the N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor antagonists D-2-amino-5-phosphonopentanoate (AP5; 0.04-0.1 mM) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 0.02-0.04 mM), respectively. 2. Fixed-intensity stimulation at frequencies of less than 0.1 Hz evoked biphasic IPSCs of constant amplitude and waveform. In contrast, when two shocks (paired pulse) or longer trains of ten or more stimuli (i.e. tetani) were delivered at frequencies of between 0.2 and 20 Hz there was marked depression of both phases of every IPSC (by 60-100%) relative to the first or 'priming' IPSC evoked. 3. The gamma-aminobutyric acid (GABA)B receptor antagonists phaclofen (0.4-2 mM), 2-hydroxy-saclofen (0.02-0.4 mM) and 3-aminopropyl(diethoxymethyl)phosphinic acid (CGP 35348; 0.01-1 mM) reduced or abolished, in a concentration-dependent and reversible manner, both the late phase of the IPSC (IPSCB) and paired-pulse depression of the early phase of the IPSC (IPSCA). Expressed in terms of IC50 values, all three antagonists were 5-10 times more potent at blocking IPSCB than paired-pulse depression. 4. Paired-pulse depression, at 5 and 10 Hz, has been shown to be mediated by GABA acting on presynaptic GABAB receptors (i.e. GABAB autoreceptors). We now show that GABAB receptor antagonists reverse paired-pulse depression over the entire range of frequencies (0.1-50 Hz) that it occurs. 5. GABAB receptor antagonists reversed substantially the depression of IPSCs during tetani delivered at 5 or 10 Hz. However at 20 Hz, GABAB receptor antagonists appeared to be less effective. At 100 Hz they appeared to be ineffective at reversing the depression of IPSCA; since the antagonists block IPSCB the net effect was to reduce the level of outward current. 6. At frequencies of 20 Hz or more, there was also the appearance of a slow inward current which increased in size in proportion to the frequency and number of shocks in the tetanus. This current (termed here IPSCI) was more pronounced at hyperpolarized membrane potentials and was blocked by picrotoxin (0.1 mM) or bicuculline (0.05 mM). 7. 'Priming' is considered to represent a more physiological pattern of activity than a tetanus.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1995

Presynaptic metabotropic glutamatergic regulation of inhibitory synapses in rat cerebellar slices.

1. The effects of the metabotropic glutamate agonist trans-ACPD (t-ACPD) were investigated in various locations of the inhibitory network made by GABAergic interneurones in the molecular layer of rat cerebellar slices. 2. t-ACPD exerted complex effects on spontaneous IPSCs in Purkinje cells. IPSC frequency was transiently inhibited during short (< 1 min) applications, and enhanced upon washing. During prolonged exposure to t-ACPD, IPSCs became organized in high-frequency bursts interspersed with periods of deep inhibition. 3. In interneurones, the frequency of spontaneous IPSCs was enhanced by t-ACPD. As in Purkinje cells, spontaneous IPSCs had a tendency to cluster in bursts in the presence of t-ACPD. 4. Evoked IPSCs recorded either in interneurones or in Purkinje cells upon stimulation of presynaptic interneurones were reversibly inhibited by t-ACPD. 5. Miniature IPSCs (mIPSCs) were recorded in the presence of tetrodotoxin both in Purkinje cells and in interneurones. t-ACPD did not alter the mean amplitude of mIPSCs in either cell type. It reduced the frequency of mIPSCs in Purkinje cells, but did not alter their rate in interneurones. 6. In cell-attached recordings on interneurone somata, t-ACPD was found to induce clustering of action potentials and to enhance the mean rate of firing. These results apply whether t-ACPD was tested in normal saline, in the presence of glutamatergic ionotrophic blockers, or in the presence of a mixture of glutamatergic and GABAergic blockers. 7. The results suggest that t-ACPD has at least two different modes of action. One effect is to alter the intrinsic firing rate of interneurones, presumably through an action on somatic conductance mechanisms. The other is to decrease the efficacy of the interneurone-interneurone and interneurone-Purkinje cell synapses, presumably through an action on axonal conductance systems and/or vesicle release mechanisms.

Action Potentials↗
PubMed1996

Retrograde signalling in depolarization-induced suppression of inhibition in rat hippocampal CA1 cells.

1. We have investigated the phenomenon of 'depolarization-induced suppression of inhibition' (DSI) using whole-cell voltage-clamp techniques in Ca1 pyramidal cells of rat hippocampal slices. DSI was induced by eliciting voltage-dependent calcium (Ca2+) currents with 1 s voltage steps of +60 to +90 mV from the holding potential. DSI was apparent as a reduction in synaptic GABAA responses for a period of about 1 min following the voltage step. 2. TTX-sensitive spontaneous IPSCs (sIPSCs) were susceptible to DSI, while TTX-resistant miniature inhibitory postsynaptic current (mIPSCs) were not. Miniature IPSCs are ordinarily infrequent and independent of external Ca2+ in the CA1 region. To increase the frequency of mIPSCs and to induce a population of Ca(2+)-sensitive mIPSCs, we increased the bath K+ concentration to 15 mM. The increased mIPSCs were also insensitive to DSI, however. 3. T whole-cell pipette-filling solution contained 5 mM 2(triethylamino-N-(2,6-dimethyl-phenyl)acetamide (QX-314) to block voltage-dependent Na+ currents and caesium to block K+ currents. Nevertheless, bath application of 50 microM 4-aminopyridine (4-AP) or 250 nM veratridine both clearly reduced DSI, evidently by acting at presynaptic sites. 4. The amplitudes of monosynaptically evoked IPSCs (elicited in the presence of 10 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 50 microM 2-amino-5-phosphonovaleric acid (APV)) were dramatically reduced during the DSI period. Weak stimulation produced small IPSCs and occasional 'failures' of transmission during the control period. The percentage of failures increased markedly during the DSI period. Moderate-intensity stimulation produced larger IPSCs that were often composed of distinguishable multiquantal components. All-or-none failures of multiquantal IPSC components also occurred during DSI. 5. The degree of paired-pulse IPSC depression did not change during DSI, whereas it was decreased, as expected, by baclofen. 6. We conclude that the data represent novel evidence that DSI is mediated by a retrograde signalling process possibly involving presynaptic axonal conduction block.

Animals↗
PubMed2005

Heterogeneous chloride homeostasis and GABA responses in the median preoptic nucleus of the rat.

The median preoptic nucleus (MnPO) is an integrative structure of the hypothalamus receiving periphery-derived information pertinent to hydromineral and cardiovascular homeostasis. In this context, excitability of MnPO neurones is controlled by fast GABAergic, glutamatergic and angiotensinergic projection from the subfornical organ (SFO). Taking advantage of a brain slice preparation preserving synaptic connection between the SFO and the MnPO, and appropriate bicarbonate-free artificial cerebrospinal fluid (CSF), we investigated a possible implication of an active outward Cl- transport in regulating efficacy of the GABA(A) receptor-mediated inhibitory response at the SFO-MnPO synapse. When somata of the MnPO neurones was loaded with 18 mm chloride, stimulation of the SFO evoked outward inhibitory postsynaptic currents (IPSCs) in 81% of the MnPO neurones held at -60 mV. Accordingly, E(IPSC) was found 25 mV hyperpolarized from the theoretical value calculated from the Nernst equation, indicating that IPSC polarity and amplitude were driven by an active Cl- extrusion system in these neurones. E(IPSC) estimated with gramicidin-based perforated-patch recordings amounted -89.2 +/- 4.3 mV. Furosemide (100 microm), a pharmacological compound known to block the activity of the neurone-specific K(+)-Cl- cotransporter, KCC2, reversed IPSC polarity and shifted E(IPSC) towards its theoretical value. Presence of the KCC2 protein in the MnPO was further detected with immunohistochemistry, revealing a dense network of KCC2-positive intermingled fibres. In the presence of a GABA(B) receptor antagonist, high-frequency stimulation (5 Hz) of the SFO evoked a train of IPSCs or inhibitory postsynaptic potentials (IPSPs), whose amplitude was maintained throughout the sustained stimulation. Contrastingly, similar 5 Hz stimulation carried out in the presence of furosemide (50 microm) evoked IPSCs/IPSPs, whose amplitude collapsed during the high-frequency stimulation. Similar reduction in inhibitory neurotransmission was also observed in MnPO neurones lacking the functional Cl- extrusion mechanism. We conclude that a majority of MnPO neurones were characterized by a functional Cl- transporter that ensured an efficient activity-dependent Cl- transport rate, allowing sustained synaptic inhibition of these neurones. Pharmacological and anatomical data strongly suggested the involvement of KCC2, as an essential postsynaptic determinant of the inhibitory neurotransmission afferent to the MnPO, a key-structure in the physiology of the hydromineral and cardiovascular homeostasis.

Animals↗
PubMed2006

Modulation of GABAergic inhibition in the rat superior colliculus by a presynaptic group II metabotropic glutamate receptor.

Previous work has indicated that metabotropic glutamate receptors (mGluRs) modulate visual responses of superior colliculus (SC) neurones in vivo in a variety of ways, in a manner that can be dependent upon visual stimulus properties. How this occurs remains unclear. In this study we aimed to determine how activation of mGluR2 and mGluR3 receptors (Group II) might modulate visual responses, by using field potential and whole-cell patch clamp recording techniques in rat SC slice. Stimulation within the superficial layers of the SC, in the presence of ionotropic glutamate receptor antagonists, evoked IPSCs that were blocked by bicuculline indicating that they are mediated via GABAA receptors. It is likely that these IPSCs were of heterogeneous origin as they showed substantial variation in paired-pulse behaviour. Nevertheless, activation of Group II mGluRs with the group-selective agonist LY354740 (300 nM, bath application) resulted in a reduction of these IPSCs (to 56% of control amplitude), and this was associated with a decrease in paired-pulse depression. At the same concentration, LY354740 did not reduce the EPSC or field-EPSP evoked by stimulation of the retinal input to the SC. The effects of LY354740 on IPSCs were not mimicked by the mGluR3-selective agonist N-acetyl-aspartyl-glutamate (NAAG, 200-500 microM). Stimulation of IPSCs with trains of impulses (10 at 20 Hz) in order to mimic natural activation patterns resulted in sequences of IPSCs that were reduced in amplitude towards the end of the stimulus train. Application of the Group II antagonist LY341495 (100 nM) under these conditions resulted in an increase in later IPSCs in a third of neurones tested. These findings indicate that mGluR2 (but not mGluR3) can selectively modulate GABAergic inhibition in SC, probably via a presynaptic mechanism. Furthermore, these receptors may be activated by synaptically released transmitter during patterns of activation similar to those seen during visual processing. Thus mGluR2 receptors could have a function in activity-dependent modulation of inhibitory processing during visual responses.

Animals↗
PubMed2004

Differential GABAB Receptor Modulation of Ethanol Effects on GABA(A) synaptic activity in hippocampal CA1 neurons.

We tested the hypothesis that differential sensitivity to ethanol of synaptic GABA(A) somatic and dendritic inhibitory postsynaptic currents (IPSCs) in hippocampal CA1 pyramidal neurons could be due to differences in the extent of GABA(B) receptor activity at GABAergic synapses in these two hippocampal subfields. Our present results show that dendritic (distally evoked) GABA IPSCs contain a larger GABA(B) IPSC component of the total GABA IPSC than the somatic (proximally evoked) subfield. The inhibition of GABA(B) receptors by pretreatment of hippocampal slices with CGP-52432 [3[[(3,4-dichlorophenyl)methyl]amino]propyl](diethoxymethyl) phosphinic acid], a selective GABA(B) receptor antagonist, changes the basal ethanol-insensitive, distally evoked GABA(A) IPSCs to become more sensitive to ethanol. In addition, paired-pulse stimulation of the proximal and distal subfields of hippocampal pyramidal neurons shows that ethanol alone increases the probability of GABA release at proximal but not distal regions. Changes by ethanol on the probability of GABA release are only seen at distal locations during GABA(B) blockade. Finally, when the modulation of presynaptic GABA(B) receptors is minimized by the local application of 10 mM GABA directly onto somatic or dendritic GABAergic synaptic regions, postsynaptic GABA(B) receptors seem to exert significant negative (inhibiting) influence on the effects of ethanol on GABA(A) IPSCs in the distal subfields of CA1 pyramidal neurons. Together, our data suggest that differences in both presynaptic and postsynaptic GABA(B) receptor activity at these GABAergic synapses may modulate the differential ethanol sensitivity of proximal and distal GABA IPSCs(A) in hippocampal CA1 pyramidal neurons.

Animals↗
PubMed2002

Development of GABA(A) receptor-mediated inhibitory postsynaptic currents in hippocampus.

Hippocampal CA1 pyramidal cells receive two kinetic classes of GABA(A) receptor-mediated inhibition: slow dendritic inhibitory postsynaptic currents (GABA(A,slow) IPSCs) and fast perisomatic (GABA(A,fast)) IPSCs. These two classes of IPSCs are likely generated by two distinct groups of interneurons, and we have previously shown that the kinetics of the IPSCs have important functional consequences for generating synchronous firing patterns. Here, we studied developmental changes in the properties of GABA(A,fast) and GABA(A,slow) spontaneous, miniature, and evoked IPSCs (sIPSCs, mIPSCs, and eIPSCs, respectively) using whole cell voltage-clamp recordings in brain slices from animals aged P10-P35. We found that the rate of GABA(A,slow) sIPSCs increased by over 70-fold between P11 and P35 (from 0.0017 to 0.12 s(-1)). Over this same age range, we observed a >3.5-fold increase in the maximal amplitude of GABA(A,slow) eIPSCs evoked by stratum lacunosum-moleculare (SL-M) stimuli. However, the rate and amplitude of GABA(A,slow) mIPSCs remained unchanged between P10 and P30, suggesting that the properties of GABA(A,slow) synapses remained stable over this age range, and that the increase in sIPSC rate and in eIPSC amplitude was due to increased excitability or excitation of GABA(A,slow) interneurons. This hypothesis was tested using bath application of norepinephrine (NE), which we found at low concentrations (1 microM) selectively increased the rate of GABA(A,slow) sIPSCs while leaving GABA(A,fast) sIPSCs unchanged. This effect was observed in animals as young as P13 and was blocked by coapplication of tetrodotoxin, suggesting that NE was acting to increase the spontaneous firing rate of GABA(A,slow) interneurons and consistent with our hypothesis that developmental changes in GABA(A,slow) IPSCs are due to changes in presynaptic excitability. In contrast to the changes we observed in GABA(A,slow) IPSCs, the properties of GABA(A,fast) sIPSCs remained largely constant between P11 and P35, whereas the rate, amplitude, and kinetics of GABA(A,fast) mIPSCs showed significant changes between P10 and P30, suggesting counterbalancing changes in action potential-dependent GABA(A,fast) sIPSCs. These observations suggest differential developmental regulation of the firing properties of GABA(A,fast) and GABA(A,slow) interneurons in CA1 between P10 and P35.

Aging↗
PubMed2006

Kappa opioid inhibition of somatodendritic dopamine inhibitory postsynaptic currents.

In the midbrain, dopamine neurons can release dopamine somatodendritically. This results in an inhibitory postsynaptic current (IPSC) within adjacent dopamine cells that occurs by the activation of inhibitory D(2) autoreceptors. Kappa, but not mu/delta, opioid receptors inhibit this IPSC. The aim of the present study was to determine the mechanism by which kappa-opioid receptors inhibit the dopamine IPSC. In both the ventral tegmental area (VTA) and substantia nigra compacta (SNc) the kappa-receptor agonist U69593 inhibited the IPSC, but not the current induced by the exogenous iontophoretic application of dopamine. The endogenous peptide dynorphin A (1-13) also inhibited IPSCs in the VTA and SNc, but also the dopamine iontophoretic current in the VTA. Although both kappa agonists induced a postsynaptic outward current in the VTA, the current induced by dynorphin was dramatically larger. This suggests that the decrease in iontophoretic dopamine current was the result of occlusion. Occlusion alone, however, could not completely account for suppression of the IPSC. The kappa opioid inhibition of the IPSC was not affected by global increases or decreases in dopamine cell activity within the slice. These findings suggest that, although kappa opioid receptors can hyperpolarize dopamine neurons, they also suppress dopamine release by direct actions at the release site. The results thus demonstrate both pre- and postsynaptic actions of kappa receptor agonists. The actions of dynorphin indicate that VTA dopamine cells are selectively regulated by kappa receptors.

Analgesics, Opioid↗