Synaptic activation of GABAA receptors induces neuronal uptake of Ca2+ in adult rat hippocampal slices. (1/783)

Synaptically evoked transmembrane movements of Ca2+ in the adult CNS have almost exclusively been attributed to activation of glutamate receptor channels and the consequent triggering of voltage-gated calcium channels (VGCCs). Using microelectrodes for measuring free extracellular Ca2+ ([Ca2+]o) and extracellular space (ECS) volume, we show here for the first time that synaptic stimulation of gamma-aminobutyric acid-A (GABAA) receptors can result in a decrease in [Ca2+]o in adult rat hippocampal slices. High-frequency stimulation (100-200 Hz, 0.4-0.5 s) applied in stratum radiatum close (+info)

Phase-dependent presynaptic modulation of mechanosensory signals in the locust flight system. (2/783)

In the locust flight system, afferents of a wing hinge mechanoreceptor, the hindwing tegula, make monosynaptic excitatory connections with motoneurons of the elevator muscles. During flight motor activity, the excitatory postsynaptic potentials (EPSPs) produced by these connections changed in amplitude with the phase of the wingbeat cycle. The largest changes occurred around the phase where elevator motoneurons passed through their minimum membrane potential. This phase-dependent modulation was neither due to flight-related oscillations in motoneuron membrane potential nor to changes in motoneuron input resistance. This indicates that modulation of EPSP amplitude is mediated by presynaptic mechanisms that affect the efficacy of afferent synaptic input. Primary afferent depolarizations (PADs) were recorded in the terminal arborizations of tegula afferents, presynaptic to elevator motoneurons in the same hemiganglion. PADs were attributed to presynaptic inhibitory input because they reduced the input resistance of the afferents and were sensitive to the gamma-aminobutyric acid antagonist picrotoxin. PADs occurred either spontaneously or were elicited by spike activity in the tegula afferents. In summary, afferent signaling in the locust flight system appears to be under presynaptic control, a candidate mechanism of which is presynaptic inhibition.  (+info)

Synaptic transmission at nicotinic acetylcholine receptors in rat hippocampal organotypic cultures and slices. (3/783)

1. Whole-cell clamp recordings of the compound synaptic current elicited by afferent stimulation of Schaffer collaterals showed that blockade of the NMDA, AMPA and GABAA receptor-mediated components by 6-nitro-7-sulphamoyl- benzo(f)quinoxaline-2,3-dione (NBQX), 3-((R)-2-carboxypiperazine-4-yl)propyl-1-phosphonate (R-CPP) and picrotoxin, respectively, left a small residual current in 39 out of 41 CA1 pyramidal neurones in organotypic cultures and 9 out of 16 CA1 cells in acutely prepared slices. 2. This current represented 2. 9 +/- 0.4 % of the compound evoked synaptic response in organoypic cultures and 1.4 +/- 0.5 % in slices. It was characterized by a slightly rectifying I-V curve and a reversal potential of 3.4 +/- 5. 1 mV. 3. This residual current was insensitive to blockers of GABAB, purinergic, muscarinic and 5-HT3 receptors, but it was essentially blocked by the nicotinic receptor antagonist d-tubocurarine (91 +/- 4 % blockade; 20 microM), and partly blocked by alpha-bungarotoxin (200 nM) and methyllycaconitine (10 nM), two antagonists with a higher selectivity for alpha7 subunit-containing nicotinic receptors (48 +/- 3 % and 55 +/- 11 % blockade, respectively). 4. The residual current was of synaptic origin, since it occurred after a small delay; its amplitude depended upon the stimulation intensity and it was calcium dependent and blocked by the sodium channel antagonist tetrodotoxin. 5. We conclude that afferent stimulation applied in the stratum radiatum evokes in some hippocampal neurones a small synaptic current mediated by activation of neuronal nicotinic receptors.  (+info)

Inhibitory control of LTP and LTD: stability of synapse strength. (4/783)

Although much is known about the induction of synaptic plasticity, the persistence of memories suggests the importance of understanding factors that maintain synaptic strength and prevent unwanted synaptic changes. Here we present evidence that recurrent inhibitory connections in the CA1 region of hippocampus may contribute to this task by modulating the relative ability to induce long-term potentiation and depression (LTP and LTD). Bath application of the gamma-aminobutyric acid (GABA) type A agonist muscimol to hippocampal slices increased the range of frequencies that produce LTD, whereas in the presence of the GABA type A antagonist picrotoxin LTD was induced only at very low stimulation frequencies (0.25-0.5 Hz). Because one source of GABAergic input to CA1 pyramidal cells is via recurrent inhibition, we tested the prediction that elevated postsynaptic spike activity would increase feedback GABA inhibition and favor the induction of LTD. By using an induction stimulation of 8 Hz, which alone produced no net change in synaptic strength, we found that stimulation presented during antidromic activation of pyramidal cell spikes induced LTD. This effect was blocked by picrotoxin. The influence of recurrent inhibition on LTP and LTD displays properties that may decrease the potential for self-reinforcing, runaway changes in synapse strength. A mechanism of this sort may help maintain patterns of synaptic strengths despite the ongoing opportunities for plasticity produced by synapse activation.  (+info)

Inhibition of GABA-gated chloride channels by 12,14-dichlorodehydroabietic acid in mammalian brain. (5/783)

1. 12,14-dichlorodehydroabietic acid (12,14-Cl2DHA) reduced GABA-stimulated uptake of 36Cl- into mouse brain synaptoneurosomes suggesting inhibition of mammalian GABA(A) receptor function. 2. 12,14-Cl2DHA did not affect the binding of [3H]-muscimol to brain membranes but displaced specifically bound [3H]-EBOB. The inhibitory effect on [3H]-EBOB binding was not reversible. 12,14-Cl2DHA reduced the availability of [3H]-EBOB binding sites (Bmax) without changing the KD of the radioligand for remaining sites. 12,14-Cl2DHA did not affect the rate of association of [3H]-EBOB with its chloride channel receptor, but increased the initial rate of [3H]-EBOB dissociation. 3. 12,14-Cl2DHA enhanced the incidence of EPSCs when rapidly applied to cultured rat cortical neurones. Longer exposures produced block of IPSCs with marked increases in the frequency of EPSCs and min EPSCs. 12,14-Cl2DHA also irreversibly suppressed chloride currents evoked by pulses of exogenous GABA in these cells. 4. Ultimately, 12,14-Cl2DHA inhibited all synaptic traffic and action currents in current clamped cells indicating that, in contrast to picrotoxinin (which causes paroxysmal bursting), it is not fully selective for the GABA(A) receptor-chloride channel complex. 5. The depolarizing block seen with 12,14-Cl2DHA in amphotericin-perforated preparations implicates loss of Ca2+ buffering in the polarity change and this may account for inhibition of spontaneous action potentials. 6. Our investigation demonstrates that 12,14-Cl2DHA blocks GABA-dependent chloride entry in mammalian brain and operates as a non-competitive insurmountable GABA(A) antagonist. The mechanism likely involves either irreversible binding of 12,14-Cl2DHA to the trioxabicyclooctane recognition site or a site that is allosterically coupled to it. We cannot exclude, however, the possibility that 12,14-Cl2DHA causes localized proteolysis or more extensive conformational change within a critical subunit of the chloride channel.  (+info)

Actions of 3-[2-phosphonomethyl[1,1-biphenyl]-3-yl]alanine (PMBA) on cloned glycine receptors. (6/783)

1. PMBA is a novel antagonist of strychnine-sensitive glycine receptors in the rat spinal cord, however, its mode of action is unknown. The actions of PMBA on rat glycine receptor alpha1 and alpha2 homomers in Xenopus oocytes were studied under two-electrode voltage-clamp. 2. Co-application of PMBA and glycine to both alpha1 and alpha2 homomers yielded inward currents which decayed to a steady-state. Responses rose slowly to the same steady-state amplitude following a 2 min pre-incubation in PMBA. Strychnine, but not picrotoxinin, showed similar antagonism to PMBA. The potency of PMBA was independent of membrane potential between -100 and 0 mV. 3. When tested against EC50 concentrations of glycine, PMBA was almost equally potent on alpha1 (IC50, 406+/-41 nM: Hill coefficient, 1.5+/-0.2) and alpha2 (IC50, 539+/-56 nM; Hill coefficient, 1.4+/-0.2) homomers. 4. PMBA (1-I0 microM) and strychnine (200 nM) reduced the potency of glycine and the amplitude of the maximal agonist response of alpha1 and alpha2 homomers. In 10 microM PMBA, two distinct classes of glycine response were observed on alpha2, only a single class of responses were observed on alpha1. 5. There are similarities in PMBA and strychnine antagonism, although these compounds are structurally distinct. The possibility that PMBA interacts at two binding sites which differ in alpha1 and alpha2 subunits is discussed. PMBA may provide a lead structure for novel antagonists with which to investigate structural differences in glycine receptor at alpha1 and alpha2 subunits.  (+info)

Sodium action potentials are not required for light-evoked release of GABA or glycine from retinal amacrine cells. (7/783)

Although most CNS neurons require sodium action potentials (Na-APs) for normal stimulus-evoked release of classical neurotransmitters, many types of retinal and other sensory neurons instead use only graded potentials for neurotransmitter release. The physiological properties and information processing capacity of Na-AP-producing neurons appear significantly different from those of graded potential neurons. To classify amacrine cells in this dichotomy, we investigated whether Na-APs, which are often observed in these cells, are required for functional light-evoked release of inhibitory neurotransmitters from these cells. We recorded light-evoked inhibitory postsynaptic currents (IPSCs) from retinal ganglion cells, neurons directly postsynaptic to amacrine cells, and applied TTX to block Na-APs. In control solution, TTX application always led to partial suppression of the light-evoked IPSC. To isolate release from glycinergic amacrine cells, we used either bicuculline, a GABAA receptor antagonist, or picrotoxin, a GABAA and GABAC receptor antagonist. TTX application only partially suppressed the glycinergic IPSC. To isolate release from GABAergic amacrine cells, we used the glycine receptor blocker strychnine. TTX application only partially suppressed the light-evoked GABAergic IPSC. Glycinergic and GABAergic amacrine cells did not obviously differ in the usage of Na-APs for release. These observations, in conjunction with previous studies of other retinal neurons, indicate that amacrine cells, taken as a class, are the only type of retinal neuron that uses both Na-AP-dependent and -independent modes for light-evoked release of neurotransmitters. These results also provide evidence for another parallel between the properties of retinal amacrine cells and olfactory bulb granule cells.  (+info)

Cl- accumulation does not account for the depolarizing phase of the synaptic GABA response in hippocampal pyramidal cells. (8/783)

It has been proposed that the depolarizing phase of the biphasic synaptic GABA response could be mediated by HCO3- passing through GABA(A) channels after dissipation of the transmembrane Cl- gradient due to intracellular Cl- accumulation. To test this hypothesis, giant GABA-mediated postsynaptic currents (GPSCs) were recorded from pyramidal cells in slices of adult guinea pig hippocampus in the presence of 4-aminopyridine. GPSCs consisted of an early outward current (GABA(A) component) followed by a late inward current (GABA(D) component). Spontaneous outward inhibitory postsynaptic currents (IPSCs) occurred during the GABA(D) component of the GPSC. GPSCs that were evoked 1-12 s after the preceding GPSC (short interval, siGPSCs) showed no GABA(D) component even though in many cells the amplitude of the siGPSC was greater than the amplitude of the GABA(A) component of the preceding spontaneous GPSC. In addition, the siGPSC evoked during the GABA(D) component of a spontaneous GPSC was an outward current. To test whether the siGPSC lacked a GABA(D) component because it was generated predominantly at the soma, where less of an increase in [Cl-](i) would occur, picrotoxin was applied to the soma of the pyramidal cell. To the contrary, this focal application of picrotoxin caused less of a reduction in the amplitude of the siGPSC than in the amplitude of the GABA(A) component of the GPSC. Furthermore when a GPSC and siGPSC were evoked 10 s apart using identical stimuli, the area under the outward current curve was sometimes greater for the siGPSC than for the GPSC, and yet the siGPSC had no inward component. This result indicates that even when the location of Cl- entry was the same, more Cl- could enter the cell during the siGPSC than during the outward component of the GPSC and yet not lead to an inward current. In addition, when the second of two identical stimuli was applied during the inward GABA(D) component of the first evoked GPSC, the GABA(A) response it generated was always outward, demonstrating that the equilibrium potential for GABA(A) responses did not become more positive than the holding potential during a GPSC. Finally, evoking GPSCs at a hyperpolarized potential revealed that the siGPSC actually lacked a GABA(D) conductance. These results disprove the Cl- accumulation hypothesis of the synaptic depolarizing GABA response and suggest the possibility that a separate channel type may mediate the GABA(D) component of the GPSC.  (+info)