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PubMed2004

Two distinct classes of muscarinic action on hippocampal inhibitory synapses: M2-mediated direct suppression and M1/M3-mediated indirect suppression through endocannabinoid signalling.

The cholinergic system in the CNS plays important roles in higher brain functions, primarily through muscarinic acetylcholine receptors. At cellular levels, muscarinic activation produces various effects including modulation of synaptic transmission. Here we report that muscarinic activation suppresses hippocampal inhibitory transmission through two distinct mechanisms, namely a cannabinoid-dependent and cannabinoid-independent mechanism. We made paired whole-cell recordings from cultured hippocampal neurons of rats and mice, and monitored inhibitory postsynaptic currents (IPSCs). When cannabinoid receptor type 1 (CB1) was blocked, oxotremorine M (oxo-M), a muscarinic agonist, suppressed IPSCs in a subset of neuron pairs. This suppression was associated with an increase in paired-pulse ratio, blocked by the M(2)-preferring antagonist gallamine, and was totally absent in neuron pairs from M(2)-knockout mice. When CB1 receptors were not blocked, oxo-M suppressed IPSCs in a gallamine-resistant manner in cannabinoid-sensitive pairs. This suppression was associated with an increase in paired-pulse ratio, blocked by the CB1 antagonist AM281, and was completely eliminated in neuron pairs from M(1)/M(3)-compound-knockout mice. Our immunohistochemical examination showed that M(2) and CB1 receptors were present at inhibitory presynaptic terminals of mostly different origins. These results indicate that two distinct mechanisms mediate the muscarinic suppression. In a subset of synapses, activation of M(2) receptors at presynaptic terminals suppresses GABA release directly. In contrast, in a different subset of synapses, activation of M(1)/M(3) receptors causes endocannabinoid production and subsequent suppression of GABA release by activating presynaptic CB1 receptors. Thus, the muscarinic system can influence hippocampal functions by controlling different subsets of inhibitory synapses through the two distinct mechanisms.

Animals↗
PubMed1996

Muscarinic inhibition of glutamatergic transmissions onto rat magnocellular basal forebrain neurons in a thin-slice preparation.

We have examined excitatory and inhibitory transmission in visually identified rat magnocellular basal forebrain neurons using whole-cell patch-clamp recordings in a thin-slice preparation of the rat brain. In most cells, spontaneous excitatory and inhibitory synaptic activities could be recorded from their resting membrane potential. Following focal stimulation within the basal forebrain nucleus or directly onto visualized neighbouring neurons, postsynaptic currents were elicited in magnocellular basal forebrain cells held at -70 mb (a value close to their resting membrane potential). The synaptic responses were complex, consisting either mainly of excitatory postsynaptic currents (EPSCs), or inhibitory postsynaptic currents (IPSCs), or an EPSC-IPSC sequence. The EPSC component was consistent with the activation of AMPA/KA receptors, as it could be selectively blocked by CNQX. The IPSC component resulted in the activation of GAGAA receptors, and could be blocked by bicuculline. Since GABA-mediated trasmissions were not frequently recorded, we focused on the glutamate-mediated transmission. Studies using specific calcium channel blockers suggested that both omega-conotoxin GVIA-sensitive and omega-agatoxin VIA-sensitive calcium channels contribute to the glutamatergic transmission onto magnocellular basal forebrain neurons. Carbachol (0.3-30 microM) had no observable effect on holding current, but produced a dose-dependent inhibition of the amplitude of evoked EPSCs. This cholinergic modulation was mediated by muscarinic receptors, as it could be antagonized by atropine. The inhibitory effect of carbachol on the amplitude of EPSCs could be significantly antagonized by 100 nM methoctramine, an M2-receptor antagonist. In contrast, only a small degree of antagonism could be obtained with pirenzepine, and M1-muscarinic receptor antagonist, when present at relatively high concentration of 1 microM. Moreover, the action of carbachol was presynaptic, since the frequency of miniature postsynaptic currents was reduced without affecting their amplitude. In conclusion, the present findings indicate that glutamate-mediated transmission onto magnocellular basal forebrain neurons appeared to involve both N- and P/Q-type calcium channels, and that muscarinic modulation of glutamatergic transmission to MBF neurons is mediated by a presynaptic M2-muscarinic receptor subtypes.

Animals↗
PubMed2005

Voltage-gated Ca2+ channel subtypes mediating GABAergic transmission in the rat supraoptic nucleus.

The supraoptic nucleus receives an abundant gamma-aminobutyric acid (GABA)ergic input which is inhibited by activation of various presynaptic metabotropic receptors. We here analysed the subtypes of voltage-gated Ca2+ channels intervening in the control of transmitter release at these synapses. To address this issue, we tested various specific inhibitors of Ca2+ channels on evoked inhibitory postsynaptic currents (IPSCs). Blocking N- and P-type voltage-gated Ca2+ channels with 1 micromomega-conotoxin-GVIA and 20 nmomega-agatoxin-IVA, respectively, dramatically reduced IPSC amplitude. Q- and L-type Ca2+ channels also contributed to GABAergic transmission, although to a lesser extent, as revealed by applications of 200 nmomega-agatoxin-IVA and of the dihydropyridines nifedipine (10 microm) and nimodipine (10 microm). Evoked IPSCs were insensitive to SNX-482 (300 nm), a blocker of some R-type Ca2+ channels. Analysis of selective blockade by the various antagonists suggested that multiple types of Ca2+ channels synergistically interact to trigger exocytosis at some individual GABA release sites. We next investigated whether inhibition of GABA release in response to the activation of metabotropic glutamate, GABA and adenosine receptors involved the modulation of these presynaptic Ca2+ channels. This was not the case, as the inhibitory actions of selective agonists of these receptors were unaffected by the presence of the different Ca2+ channel antagonists. This finding suggests that these metabotropic receptors modulate GABAergic transmission through a different mechanism, downstream of Ca2+ entry in the terminals, or upstream through the activation of K+ channels.

Adenosine↗
PubMed2005

Cell-type specific GABA synaptic transmission and activity-dependent plasticity in rat hippocampal stratum radiatum interneurons.

Abstract In hippocampal pyramidal cells, the efficacy of synaptic transmission at gamma-aminobutyric acid (GABA)ergic synapses, is modulated by activity. However, whether such plasticity occurs at inhibitory synapses on interneurons remains largely unknown. Using whole-cell voltage-clamp recordings of inhibitory postsynaptic currents (IPSCs) in Sprague-Dawley rat hippocampal slices, we examined whether GABA synapses of stratum radiatum interneurons were affected by stimulation protocols known to alter efficacy at inhibitory synapses of CA1 pyramidal cells. Monosynaptically evoked IPSCs (eIPSCs) exhibited different properties with significantly faster kinetics, higher coefficients of variation, a current-voltage (I-V) relationship shifted to depolarized values and a smaller paired-pulse depression, in interneurons than in pyramidal cells. GABA synapses on interneurons also showed a different capacity for plasticity. Indeed, theta-burst stimulation induced a long-term potentiation of eIPSCs in both cell types, but the induction mechanisms differed in interneurons, as it was not affected by antagonists of GABAB receptors and group I/II metabotropic glutamate receptors (mGluRs). Furthermore, 100-Hz tetanization selectively elicited a short-term depression of eIPSCs in pyramidal cells. A postsynaptic depolarization produced a transient suppression of eIPSCs (depolarization-induced suppression of inhibition) in pyramidal cells but not in interneurons. Spontaneous IPSCs were similarly reduced following depolarization in pyramidal cells, but not in interneurons. These results indicate that GABA synapses of stratum radiatum interneurons exhibit different properties and capacity for activity-dependent plasticity than those of pyramidal cells. This cell-type specific mode of transmission and adaptive regulation of GABA synapses may contribute to hippocampal plasticity and functions.

Animals↗
PubMed2006

N-ethylmaleimide increases release probability at GABAergic synapses in layer I of the mouse visual cortex.

The sulphydryl alkylating agent N-ethylmaleimide (NEM) has been often used as an uncoupler of pertussis toxin-sensitive G-proteins. However, the effects of NEM on gamma-aminobutyric acid (GABA)ergic synaptic transmission remain controversial. Using the whole-cell patch-clamp technique, GABA(A) receptor-mediated postsynaptic currents (IPSCs) have been recorded from Cajal-Retzius (CR) cells in layer I of the neonatal mouse visual cortex. NEM increased the frequencies of both spontaneous and miniature IPSCs (mIPSCs) without an effect on the median mIPSC amplitudes or mIPSC kinetics. The NEM actions on mIPSCs did not depend on the extracellular Ca(2+), Ca(2+) release from intracellular stores, adenylyl cyclase and protein kinase A activities. NEM increased the mean amplitudes of evoked IPSCs and strongly decreased the paired-pulse ratio. The size of the readily releasable pool of presynaptic vesicles (RRP) was estimated using a high-frequency stimulation protocol. The RRP size was not affected by NEM. In addition, NEM significantly decreased the latency between the stimulus and the onset of GABA release. These results suggest that NEM selectively increases GABA release probability. At postnatal day 2, mIPSCs were observed only in about 30% of CR cells. NEM application revealed, however, that more than 90% of CR cells receive GABAergic inputs. Therefore, NEM seems to be a useful tool to verify the existence of 'silent' GABAergic synapses.

Animals↗
PubMed1998

Voltage-dependent deactivation and desensitization of GABA responses in cultured murine cerebellar granule cells.

1. Electrophysiological recordings of GABAergic IPSCs and responses to applications of exogenous GABA were made from cultured murine cerebellar granule cells. In both the presence and absence of tetrodotoxin, depolarization of the postsynaptic cell consistently produced a broadening of the IPSC. This voltage-dependent change in kinetics arose entirely from a slowing of the rate of current decay. The duration of miniature IPSCs was increased by a significant but lesser amount by the GABA uptake inhibitor nipecotic acid (300 microM). 2. Five millisecond applications of 1 mM GABA elicited rapidly activating, biexponentially deactivating currents in patches derived from granule cell bodies. Deactivation of these responses was slowed by membrane depolarization. This effect arose from an increased fractional participation of the slow component of deactivation. The benzodiazepine flunitrazepam (1 microM) slowed deactivation at a holding potential of -70 mV but not at +50 mV. 3. Longer-lasting applications of GABA produced substantial biexponential macroscopic desensitization. The rate of desensitization was faster at a holding potential of +50 mV than at -70 mV. The speeding of desensitization at depolarized membrane potentials arose from an increase in the fractional contribution of the fast component of desensitization. 4. When two 5 ms, 1 mM GABA applications were made at an interstimulus latency of 150 ms, the second response was consistently smaller than the first. The depression of the second response was significantly heightened when the membrane potential was depolarized from -70 to +50 mV. 5. The degree of desensitization produced was closely linked to receptor occupancy. The rate of current deactivation was also voltage dependent when non-saturating, and therefore less desensitizing, applications of GABA were analysed. In contrast, both the GABA EC50 (approximately 30 microM) and the current activation kinetics at near EC50 agonist concentrations appeared to be voltage independent.

Animals↗
PubMed1998

Lanthanum-mediated modification of GABAA receptor deactivation, desensitization and inhibitory synaptic currents in rat cerebellar neurons.

1. We investigated La3+ effects on recombinant and native gamma-aminobutyric acid A (GABAA) receptors using rapid agonist applications and on inhibitory synaptic currents (IPSCs) in granule and stellate neurons of rat cerebellar slices. 2. Rapid desensitization of currents elicited by 200 ms pulses of 1 mM GABA to small lifted cells transfected with alpha1beta3gamma2 cDNAs was greatly decreased by the coapplication of 100 microM LaCl3. 3. GABA responses were unaffected when coapplication lasted only 2 ms. In contrast, with LaCl3 pre-perfusion, a significant slowing of deactivation in response to 2 ms applications was observed. LaCl3 pre-perfusion also prolonged the duration of responses to 20 mM taurine. 4. Outside-out patches excised from cells transfected with alpha1beta3gamma2 subunit cDNAs were briefly exposed to a saturating concentration of GABA, eliciting a transient activation of single channel currents with a main conductance of 30 pS. Opening and burst durations increased by pre-equilibration of patches with LaCl3. 5. LaCl3 depressed the peak amplitude without affecting the slow deactivation and desensitization of GABA responses in cells transfected with alpha6beta3gamma2 and alpha6beta3delta cDNAs. No significant difference in La3+ modulation of GABA-gated currents was observed between alpha1beta3gamma2 and alpha1beta3delta receptors. 6. The effects of LaCl3 on deactivation and desensitization of GABA responses observed in nucleated patches excised from rat cerebellar granule and stellate neurons were comparable to those in the cells transfected with alpha1beta3gamma2 cDNAs. In addition, La3+ clearly prolonged the spontaneous IPSC time course without changing the amplitude. 7. Our results indicate that La3+ has a dual action on GABA-gated currents: it decreases desensitization and increases channel opening duration. These actions depend on receptor subunit composition and contribute to the prolongation of IPSCs.

Animals↗
PubMed1999

Modulation by K+ channels of action potential-evoked intracellular Ca2+ concentration rises in rat cerebellar basket cell axons.

1. Action potential-evoked [Ca2+]i rises in basket cell axons of rat cerebellar slices were studied using two-photon laser scanning microscopy and whole-cell recording, to identify the K+ channels controlling the shape of the axonal action potential. 2. Whole-cell recordings of Purkinje cell IPSCs were used to screen K+ channel subtypes which could contribute to axonal repolarization. alpha-Dendrotoxin, 4-aminopyridine, charybdotoxin and tetraethylammonium chloride increased IPSC rate and/or amplitude, whereas iberiotoxin and apamin failed to affect the IPSCs. 3. The effects of those K+ channel blockers that enhanced transmitter release on the [Ca2+]i rises elicited in basket cell axons by action potentials fell into three groups: 4-aminopyridine strongly increased action potential-evoked [Ca2+]i; tetraethylammonium and charybdotoxin were ineffective alone but augmented the effects of 4-aminopyridine; alpha-dendrotoxin had no effect. 4. We conclude that cerebellar basket cells contain at least three pharmacologically distinct K+ channels, which regulate transmitter release through different mechanisms. 4-Aminopyridine-sensitive, alpha-dendrotoxin-insensitive K+ channels are mainly responsible for repolarization in basket cell presynaptic terminals. K+ channels blocked by charybdotoxin and tetraethylammonium have a minor role in repolarization. alpha-Dendrotoxin-sensitive channels are not involved in shaping the axonal action potential waveform. The two last types of channels must therefore exert control of synaptic activity through a pathway unrelated to axonal action potential broadening.

4-Aminopyridine↗
PubMed2001

Cannabinoid actions on rat superficial medullary dorsal horn neurons in vitro.

1. This study examined the cellular actions of cannabinoids on neurons in the substantia gelatinosa of the spinal trigeminal nucleus pars caudalis, using whole-cell and perforated patch recording in brain slices. 2. The cannabinoid agonist WIN55,212-2 (3 microM) decreased the amplitude of both GABAergic and glycinergic electrically evoked inhibitory postsynaptic currents (IPSCs) by 35 and 41 %, respectively. This inhibition was completely reversed by the CB(1) receptor-selective antagonist N-piperidino-5-(4-chlorophenyl)-l-(2,4-dichlorophenyl)-4-methyl-3-pyrazole-carboxamide) (SR141716A, 3 microM). WIN55,212-2 also produced relative facilitation of the second evoked IPSC to paired stimuli. 3. WIN55,212-2 decreased the rate of both GABAergic and glycinergic miniature IPSCs by 44 and 34 %, respectively, without changing their amplitude distributions or kinetics. 4. WIN55,212-2 did not affect the amplitude of electrically evoked non-NMDA glutamatergic excitatory postsynaptic currents (EPSCs). 5. WIN55,212-2 produced no postsynaptic membrane current and had no significant effect on membrane conductance over a range of membrane potentials (-60 to -130 mV). 6. These results suggest that, within the superficial medullary dorsal horn, cannabinoids presynaptically inhibit GABAergic and glycinergic neurotransmission. At the cellular level, the analgesic action of cannabinoids on these medullary dorsal horn neurons therefore differs from that of mu-opioids, which have both pre- and postsynaptic actions.

Animals↗
PubMed1983

Baclofen blocks postsynaptic inhibition but not the effect of muscimol in the olfactory cortex.

1 The olfactory cortex slice preparation from the guinea-pig brain was used to study the effects of baclofen on inhibition using intracellular recording. Stimulation of the lateral olfactory tract activities sequentially excitatory and inhibitory pathways. Inhibition is manifest as a period of increased membrane conductance (termed postsynaptic inhibitory conductance, IPSC). 2 Bath application of baclofen (0.2-500 muM) reversibly blocked the IPSC. Baclofen also produced a secondary increase in the amplitude and duration of the initial excitatory postsynaptic potential. 3 Baclofen (0.5-500 muM) slightly augmented the ability of bath-applied muscimol to increase the resting membrane conductance. Baclofen had no effect on cell excitability and membrane potential and no effect on the action of gamma-aminobutyric acid (GABA), noradrenaline, glycine, taurine or 5-hydroxytrypamine. 4 These results confirm previous suggestions that baclofen at low concentrations acts outside the GABA receptor mediating the IPSC perhaps by reducing the release of the excitatory transmitter activating the inhibitory interneurones.

Animals↗
PubMedDate not supplied

Effect of a gamma-aminobutyric acid uptake inhibitor, NNC-711, on spontaneous postsynaptic currents in cultured rat hippocampal neurons: implications for antiepileptic drug development.

The effect of a novel gamma-aminobutyric acid (GABA) uptake inhibitor, NNC-711, on spontaneous postsynaptic currents was studied in cultured rat hippocampal neurons by the whole cell patch clamp method. NNC-711 decreased the amplitude of inhibitory postsynaptic currents (IPSCs) and did not prolong the decay. NNC-711 also decreased the amplitude of excitatory PSCs (EPSCs). The GABAB receptor antagonist 2-OH saclofen abolished the effect on both IPSCs and EPSCs. NNC-711 itself induced no current and had no effect on currents induced by exogenously applied GABA. These findings suggest that duration of GABAA-receptor mediated IPSCs is not determined by GABA uptake and that GABA uptake inhibitors may work by allowing GABA to remain in the synaptic area long enough to activate presynaptic GABAB receptors.

Animals↗
PubMed2006

Three brief epileptic seizures reduce inhibitory synaptic currents, GABA(A) currents, and GABA(A)-receptor subunits.

PURPOSE: Cellular mechanisms activated during seizures may exacerbate epilepsy. gamma-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter in brain, and we hypothesized that brief epileptic seizures may reduce GABA function. METHODS: We used audiogenic seizures (AGSs) in genetically epilepsy-prone rats (GEPRs) to investigate effects of seizures on GABA-mediated inhibition in the presence of epilepsy. GEPRs are uniformly susceptible to AGSs beginning at 21 postnatal days. AGSs are brief convulsions lasting approximately 20 s, and they begin in inferior colliculus (IC). We evoked three seizures in GEPRs and compared the results with those in seizure-naive GEPRs and nonepileptic Sprague-Dawley (SD) rats, the GEPR parent strain. RESULTS: Whole-cell recording in IC slices showed that GABA-mediated monosynaptic inhibitory postsynaptic currents (IPSCs) were reduced 55% by three brief epileptic seizures. Whole-cell recording in IC neuronal cultures showed that currents elicited by GABA were reduced 67% by three seizures. Western blotting for the alpha1 and alpha4 subunits of the GABA(A) receptor showed no statistically significant effects. In contrast, three brief epileptic seizures reduced gamma2 subunit levels by 80%. CONCLUSIONS: The effects of the very first seizures, in animals known to be epileptic, in an area of brain known to be critical to the seizure network, were studied. The results indicate that even brief epileptic seizures can markedly reduce IPSCs and GABA currents and alter GABA(A)-receptor subunit protein levels. The cause of the reductions in IPSCs and GABA currents is likely to be altered receptor subunit composition, with reduced gamma2 levels causing reduced GABA(A)-receptor sensitivity to GABA. Seizure-induced reductions in GABA-mediated inhibition could exacerbate epilepsy.

Acoustic Stimulation↗
PubMed1989

Patch clamp techniques used for studying synaptic transmission in slices of mammalian brain.

Procedures are described for recording postsynaptic currents from neurones in slices of rat brain using patch clamp techniques. The method involves cutting brain slices (120-300 microns thick) with a vibrating microtome followed by localization of cell somata, which can be clearly seen with Nomarski differential interference contrast optics in the light microscope. Tissue covering the identified cell is then removed mechanically and standard patch clamp techniques are applied. Using these methods, spontaneously occurring and stimulus-evoked inhibitory postsynaptic currents (IPSCs) were recorded from neurons in rat hippocampus at greatly improved resolution. In the presence of tetrodotoxin, to block presynaptic action potentials, spontaneous IPSCs seldom exceeded 25 pA. Evoked IPSCs elicited by constant electrical stimulation of a presynaptic neurone were larger and fluctuated in their amplitudes. Single-channel currents, activated by the putative inhibitory transmitter gamma-aminobutyric acid (GABA), had a size of about 1 pA. The number of postsynaptic channels activated by a packet of inhibitory transmitter is probably not more than thirty, nearly two orders of magnitude smaller than previously reported estimates for CNS synapses. This might reflect matching of synaptic efficacy to the high input resistance of hippocampal neurones and could be a requirement for fine tuning of inhibition.

Animals↗
PubMed1992

Excitatory and inhibitory synaptic currents and receptors in rat medial septal neurones.

1. A thin-slice preparation was used to study the postsynaptic potentials and the underlying currents of visually identified rat medial septal (MS) neurones under tight-seal voltage- and current-clamp conditions. 2. Upon stimulation of the afferent fibres, all MS neurones exhibited a sequence of excitatory-inhibitory postsynaptic potentials (EPSP-IPSP). Under voltage clamp, with potassium glutamate as internal solution and at negative holding potentials (Vh), this synaptic pattern appeared as an initial inward current followed by a longer lasting outward current. 3. The inward postsynaptic current was completely abolished by 5 microM-6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) whereas the outward current disappeared in the presence of 10 microM-bicuculline. Thus the major excitatory and inhibitory synaptic inputs were identified as being due to activation of quisqualate/kainate glutamatergic and gamma-aminobutyric acid (GABAA) receptors, respectively. 4. At positive Vh a CNQX-resistant component of the excitatory postsynaptic current (EPSC) was revealed. This component was slower than the one mediated by the quisqualate receptor and was abolished by 3-3(2-carboxypiperazine-4-yl)propyl-1-phosphonate (CPP), indicating that N-methyl-D-aspartate (NMDA) receptors are involved in excitatory synaptic transmission in MS cells. The existence of the two main subtypes (NMDA and non-NMDA) of glutamatergic receptors in MS neurones was also confirmed by the responses of the neurones to bath application of the different agonists (glutamate, quisqualate, kainate and NMDA). 5. The CNQX-sensitive EPSC had a reversal potential near 0 mV. The fast rise time (approximately 0.7 ms) indicates a somatic location of the excitatory synapses. The relaxation kinetics of the fast EPSC were fitted by a single exponential function with a time constant of 1.13 +/- 0.1 ms. This parameter was independent of Vh. Fast EPSCs were blocked by CNQX in a dose-dependent manner (dissociation constant, KD = 0.2 microM). 6. Inhibitory postsynaptic currents (IPSCs) were studied in symmetrical chloride solutions after blockade of the excitatory receptors. The current-voltage relation was linear and reversed at 0 mV. The IPSCs had a fast rise time and their decay was best fitted by the sum of two exponentials with time constant of approximately 20 and 50 ms (Vh = -60 mV). The IPSCs were abolished by bicuculline (KD = 1 microM), a selective antagonist of GABAA receptors. As expected, bath application of GABA produced large whole-cell currents. 7. In many cells, in addition to the usual EPSP-IPSP sequence, failures of either the EPSP or the IPSP were frequently observed during the experimental protocol.(ABSTRACT TRUNCATED AT 400 WORDS)

6-Cyano-7-nitroquinoxaline-2,3-dione↗
PubMed1992

Cholinergic excitation of GABAergic interneurons in the rat hippocampal slice.

1. Intracellular recordings were made from CA1 pyramidal cells in the rat hippocampal slice to study the cholinergic modulation of GABAergic inhibition. The cholinergic receptor agonist, carbamylcholine (carbachol), depressed evoked excitatory postsynaptic potentials (EPSPs) and evoked inhibitory postsynaptic potentials (IPSPs), but enhanced small spontaneously occurring membrane potential fluctuations that resembled IPSPs. Both atropine (1 microM) and picrotoxin (25-60 microM) abolished the small fluctuations. 2. Recording from cells with potassium or caesium chloride (KCl or CsCl)-filled microelectrodes enhanced and inverted spontaneous Cl(-)-dependent GABAA-mediated IPSPs. These events appeared to result from the spontaneous firing of GABAergic interneurons since they could be inhibited by picrotoxin or bicuculline and nearly eliminated by tetrodotoxin. 3. Muscarinic acetylcholine (ACh) receptor activation significantly increased the frequency of spontaneous-activity-dependent IPSPs from 1.7 +/- 0.4 s (mean +/- S.E.M.) in control saline to 7.0 +/- 1.1 s in carbachol (10-50 microM)-containing saline, although evoked IPSPs were inhibited. All effects of carbachol were completely reversed by atropine. 4. The increase in frequency of spontaneous IPSPs observed in carbachol was not secondary to changes in the postsynaptic cell and was not blocked by high doses of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 5-10 microM) and 2-amino-5-phosphonovaleric acid (APV, 10-20 microM), which abolished evoked excitatory transmission. Amplitude histograms showed an increase in mean size as well as of frequency of spontaneous IPSCs in carbachol. 5. Stimulation of cholinergic afferents in stratum oriens in the presence of the acetylcholinesterase inhibitor eserine (1 microM) also increased spontaneous IPSP frequency, and the time course of this response was similar to that of the muscarinic slow EPSP. Postsynaptic factors or the activation of glutamatergic excitatory pathways could not account for this effect. 6. Evoked monosynaptic IPSCs in CNQX and APV were diminished by carbachol. 7. We conclude that GABAergic inhibitory interneurons possess muscarinic receptors, that activation of these receptors increases the excitability of the interneurons and that synaptically released ACh increases interneuronal activity. Cholinergic reduction of the monosynaptic IPSC may point to additional complexity in cholinergic regulation of the GABA system.

2-Amino-5-phosphonovalerate↗
PubMed1994

A branching dendritic model of a rodent CA3 pyramidal neurone.

1. We constructed a branching dendritic compartmental model of a CA3 pyramidal neurone, using experimental data from guinea-pig and rat cells obtained in vitro. The goal was to understand interactions between synaptic events impinging on dendritic branches and voltage- and calcium-dependent currents. The model contained sixty-four soma-dendrite (SD) compartments, an axon initial segment (IS), and four axonal compartments. There were six active conductances in the SD membrane, including a sodium conductance (gNa) and a high-threshold calcium conductance (gCa), with kinetic properties similar to those reported in a previous study. 2. The distribution of conductance densities across the IS and SD was adjusted by testing the model response to antidromic stimulation and current pulses or sustained currents injected into the soma or apical dendrites. As before, gNa was concentrated on and near the soma with lower density in the dendrites, while gCa had a higher density in apical dendrites than at the soma. 3. The model predicts that CA3 pyramidal neurones in media blocking synaptic transmission should fire a burst of action potentials following antidromic stimulation. This was confirmed experimentally in hippocampal slices. 4. Both in the model and in guinea-pig neurones, dendritic IPSCs can delay the onset of bursting. If an IPSC begins soon enough after the first fast action potential, the later burst envelope is attenuated. This effect results from suppression of dendritic Ca2+ electrogenesis. 5. The model predicts that an appropriately timed dendritic IPSC (after the first somatic spike but before the dendritic Ca2+ spike) may suppress the transient local [Ca2+] signal, while having a negligible effect on the electrical output of the neurone. This phenomenon has been reported in guinea-pig Purkinje cells. 6. We conclude that active dendritic currents are critical for regulation of the electrical output of CA3 pyramidal neurones. We suggest also that dendritic [Ca2+] signals might be controlled in individual dendrites independently of action potential outputs, an effect of possible importance for synaptic plasticity.

Action Potentials↗
PubMed1996

Volatile anaesthetic enhancement of paired-pulse depression investigated in the rat hippocampus in vitro.

1. A prominent in vivo effect of general anaesthetics, including volatile anaesthetics such as halothane, is the prolonging of paired-pulse depression of the hippocampal CA1 population spike. The mechanisms by which volatile anaesthetics produce this effect were investigated in the hippocampal brain slice preparation by testing the effect of halothane on several long-lasting inhibitory processes, including the calcium-activated potassium current that underlies the slow after-hyperpolarization (IAHP), the GABAB-mediated potassium current that underlies the late IPSP, and the fast and slow components of the early GABAA-mediated IPSP. 2. Halothane produced a dose-dependent block of IAHP at concentrations between 0.5 and 1.5%. This block was manifested as a reduction in spike frequency adaptation, a reduction in the amplitude of the slow after-hyperpolarization following a train of action potentials, and a reduction in the amplitude of the voltage-clamped current following a calcium spike elicited in the presence of tetraethylammonium and tetrodotoxin. The effect did not appear to be caused by blockade of voltage-sensitive calcium channels, since halothane markedly reduced IAHP at a concentration (1.5%) that had little effect on the depolarization-evoked calcium spike. 3. Halothane reduced the amplitude of the late GABAB-mediated IPSP by approximately 50% at concentrations between 1 and 2%. Similar results were obtained for polysynaptic and monosynaptic responses, and with current-clamp and voltage-clamp recordings. However, halothane, at concentrations up to 3%, had no effect on the presynaptic GABAB response, as indicated by no reduction in paired-pulse depression of the monosynaptic GABAA response. 4. Halothane (2%) and enflurane (4%) prolonged the decay phase of the slow component of the monosynaptic GABAA-mediated IPSC approximately twofold, but did not alter the amplitude of the response. Halothane also prolonged the decay phase of the fast component of the GABAA-mediated IPSC, with no effect on the amplitude. However, enflurane markedly reduced the amplitude of the fast component of the GABAA IPSC, so that only a small slow current remained in response to a selective stimulus. 5. It is concluded that the effects of halothane on IAHP and on GABAB responses cannot account for its effects on paired-pulse depression, but that volatile anaesthetics enhance paired-pulse depression by prolonging the decay of the slow dendritic GABAA response. Furthermore, it is speculated that the proconvulsant property of enflurane is related to its depression of the fast somatic component of GABAA inhibition.

Action Potentials↗
PubMed2002

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

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

Aging↗