Monosynaptic GABA-mediated inhibitory postsynaptic potentials in CA1 pyramidal cells of hyperexcitable hippocampal slices from kainic acid-treated rats

To examine the mechanisms underlying chronic epileptiform activity, field potentials were first recorded to identify hyperexcitable hippocampal slices from kainic acid-treated rats. Intracellular recordings were then obtained from CA1 pyramidal cells in the hyperexcitable areas. Twenty-two of the 47 cells responded to electrical stimulation of the stratum radiatum with a burst of two or more action potentials and reduced early inhibitory postsynaptic potentials, and were considered hyperexcitable. The remaining 25 cells were not hyperexcitable, displaying a single action potential and biphasic inhibitory postsynaptic potentials after stimulation, like control cells (n = 20). A long duration, voltage-sensitive component was associated with subthreshold excitatory postsynaptic potentials in the majority of hyperexcitable (12/15) and non-hyperexcitable (3/5) cells examined from kainic acid-treated animals, but not from cells (1/10) of control animals. Stimulation of stratum radiatum during pharmacological blockade of ionotropic excitatory amino acid synaptic transmission elicited biphasic monosynaptic inhibitory postsynaptic potentials in all hyperexcitable (n = 9) and non-hyperexcitable (n = 9) cells tested from kainate-treated animals, as well as in control cells (n = 8). The mean amplitude, latency to peak, equilibrium potential, and conductance changes of early and late monosynaptic inhibitory postsynaptic potentials were not different between cells of kainic acid-treated and control animals. In seven hyperexcitable cells tested, the early component of monosynaptic inhibitory postsynaptic potentials was significantly reduced by the GABAA receptor antagonist bicuculline (100-200 microM). The late component was significantly decreased by the GABAB receptor antagonist 2-hydroxysaclofen (1-2 mM; n = 3). Comparable effects were observed on early and late monosynaptic inhibitory postsynaptic potentials in non-hyperexcitable cells (n = 4) from kainic acid-treated animals and control cells (n = 5). These results suggest that GABAergic synapses on hyperexcitable hippocampal pyramidal cells of kainate-treated rats are intact and functional. Therefore, epileptiform activity in the kainate-lesioned hippocampus may not arise from a disconnection of GABAergic synapses made by inhibitory interneurons on pyramidal cells. The hyperexcitability may be due to underactivation of inhibitory interneurons and/or reorganization of excitatory inputs to pyramidal cells since, in kainate-treated animals, pyramidal cells appear to express additional excitatory mechanisms.     

Net interaction between different forms of short-term synaptic plasticity and slow-IPSPs in the hippocampus and auditory cortex

Paired-pulse plasticity is typically used to study the mechanisms underlying synaptic transmission and modulation. An important question relates to whether, under physiological conditions in which various opposing synaptic properties are acting in parallel, the net effect is facilitatory or depressive, that is, whether cells further or closer to threshold. For example, does the net sum of paired-pulse facilitation (PPF) of excitatory postsynaptic potentials (EPSPs), paired-pulse depression (PPD) of inhibitory postsynaptic potentials (IPSPs), and the hyperpolarizing slow IPSP result in depression or facilitation? Here we examine how different time-dependent properties act in parallel and examine the contribution of gamma-aminobutyric acid-B (GABAB) receptors that mediate two opposing processes, the slow IPSP and PPD of the fast IPSP. Using intracellular recordings from rat CA3 hippocampal neurons and L-II/III auditory cortex neurons, we examined the postsynaptic responses to paired-pulse stimulation (with intervals between 50 and 400 ms) of the Schaffer collaterals and white matter, respectively. Changes in the amplitude, time-to-peak (TTP), and slope of each EPSP were analyzed before and after application of the GABAB antagonist CGP-55845. In both CA3 and L-II/III neurons the peak amplitude of the second EPSP was generally depressed (further from threshold) compared with the first at the longer intervals; however, these EPSPs were generally broader and exhibited a longer TTP that could result in facilitation by enhancing temporal summation. At the short intervals CA3 neurons exhibited facilitation of the peak EPSP amplitude in the absence and presence of CGP-55845. In contrast, on average L-II/III cells did not exhibit facilitation at any interval, in the absence or presence of CGP-55845. CGP-55845 generally "erased" short-term plasticity, equalizing the peak amplitude and TTP of the first and second EPSPs at longer intervals in the hippocampus and auditory cortex. These results show that it is necessary to consider all time-dependent properties to determine whether facilitation or depression will dominate under intact pharmacological conditions. Furthermore our results suggest that GABAB-dependent properties may be the major contributor to short-term plasticity on the time scale of a few hundred milliseconds and are consistent with the hypothesis that the balance of different time-dependent processes can modulate the state of networks in a complex manner and could contribute to the generation of temporally sensitive neural responses.     

Differential effects of a benzodiazepine on synaptic transmissions in rat hippocampal neurons in vitro

The effects of midazolam, one of the most popular benzodiazepines, on synaptic transmissions were compared with intracellular recordings between CA1 pyramidal cells (CA1-PCs) and dentate gyrus granule cells (DG-GCs) in rat hippocampal slices. First, we studied the effects of midazolam on orthodromically evoked spikes, membrane properties and synaptic potentials. Secondly, the effects of a GABA(A) receptor agonist, muscimol, were examined on membrane properties to determine whether or not the densities of GABA(A) receptors are different between CA1-PCs and DG-GCs. Midazolam (75 microM) markedly depressed orthodromically evoked spikes in CA1-PCs, compared with those in DG-GCs. A GABA(A) receptor antagonist, bicuculline (10 microM), almost completely antagonized the depressant effects of midazolam on spike generation in CA1-PCs, whereas it had little effect on midazolam in dentate gyrus granule cells. Midazolam produced either depolarizing or hyperpolarizing effects on resting membrane potentials (Vm) with an input resistance decrease in CA1-PCs, whereas it produced depolarized Vm in DG-GCs. Midazolam significantly increased the amplitude of monosynaptic inhibitory postsynaptic potentials in CA1-PCs, whereas midazolam slightly decreased these in DG-GCs. Midazolam significantly decreased the amplitude of excitatory postsynaptic potentials both in CA1-PCs and DG-GCs. Muscimol (100 microM) produced either depolarizing or hyperpolarizing effects on Vm with an input resistance decrease in CA1-PCs, and it depolarized Vm with an input resistance decrease in DG-GCs. These results demonstrate that midazolam has differential effects on excitatory and inhibitory synaptic transmissions in hippocampal neurons. The mechanism of this difference could be partly due to the different types of GABA(A) receptors between CA1-PCs and DG-GCs.     

Adrenoceptor-mediated elevation of ambient GABA levels activates presynaptic GABA(B) receptors in rat sensorimotor cortex

At inhibitory synapses in the mature neocortex and hippocampus in vitro, spontaneous action-potential-dependent and -independent release of gamma-aminobutyric acid (GABA) activates postsynaptic GABA(A) receptors but not pre- or postsynaptic GABA(B) receptors. Elevation of synaptic GABA levels with pharmacological agents or electrical stimulation can cause activation of GABA(B) receptors, but the physiological conditions under which such activation occurs need further elucidation. In rodent sensorimotor cortex, epinephrine produced a depression in the amplitude of evoked monosynaptic inhibitory postsynaptic currents (IPSCs) and a concomitant, adrenoceptor-mediated increase in the frequency of spontaneous IPSCs. Blockade of GABA(B) receptors prevented the depression of evoked IPSC amplitude by epinephrine but did not affect the increase in spontaneous IPSC frequency. These data show that adrenoceptor-mediated increases in spontaneous IPSCs can cause activation of presynaptic GABA(B) receptors and indirectly modulate impulse-related GABA release, presumably through elevation of synaptic GABA levels.     

Bradykinin-induced vasodilation of human coronary arteries in vivo: role of nitric oxide and angiotensin-converting enzyme

Voltage-clamped GABA(A fast) and GABA(A slow) inhibitory postsynaptic currents (IPSCs) were selectively elicited in hippocampal area CA1 pyramidal neurons. Clinically relevant concentrations of halothane (1.2 vol.%) prolonged both GABA(A fast) and GABA(A slow) IPSC decay times approximately 2.5 fold, while having little to no effect on current amplitudes or rise times. Current-voltage analysis revealed that IPSC reversal potentials (-70 to -75 mV) remained constant in the presence of halothane. Under control conditions, GABA(A slow) IPSC decay times increased linearly with membrane depolarization, and this IPSC decay time voltage dependence was not significantly altered by halothane. These results confirm the existence of separable GABA(A fast) and GABA(A slow) IPSCs in hippocampus, and further elucidate the effects of halothane on these currents.     

Neuroactive steroids induce GABA(A) receptor-mediated depolarizing postsynaptic potentials in hippocampal CA1 pyramidal cells of the rat

Intracellular recordings were performed in area CA1 pyramidal cells of rat hippocampal slices to determine the effects of certain steroids on inhibitory postsynaptic potentials/currents (IPSP/Cs) mediated by GABA(A) receptors. Following application of the steroids 5alpha-pregnan-3alpha,21-diol-20-one (5alpha-THDOC), alphaxalone and 5beta-pregnan-3alpha-ol-20-one (pregnanolone) hyperpolarizing PSPs developed into biphasic responses consisting of an early hyperpolarizing and a late depolarizing PSP sequence. Steroid-induced depolarizing PSPs could be elicited in the presence of antagonists to non-NMDA, NMDA, and GABA(B) receptors, indicating that these receptor types do not contribute significantly to the initiation of these responses. Depolarizing PSPs were completely blocked by both GABA(A) receptor antagonists bicuculline and t-butylbicyclophosphorothionat (TBPS) providing evidence for their mediation by GABA(A) receptors. The reversal potential of steroid-induced late inward PSCs, measured in single-electrode voltage clamp, was -29.9+/-5.3 mV, whereas the early outward current, which corresponded to the early hyperpolarizing component of PSPs, reversed at -68.2+/-1.5 mV. Depolarizing PSPs and late inward PSCs were sensitive to reduction of extracellular [HCO3-] and block of carbonic anhydrase by application of acetazolamide. The results suggest that certain neuroactive steroids can induce GABA(A) receptor-mediated depolarizing PSPs, which are dependent on HCO3-.     

The K+/Cl- co-transporter KCC2 renders GABA hyperpolarizing during neuronal maturation

GABA (gamma-aminobutyric acid) is the main inhibitory transmitter in the adult brain, and it exerts its fast hyperpolarizing effect through activation of anion (predominantly Cl-)-permeant GABA(A) receptors. However, during early neuronal development, GABA(A)-receptor-mediated responses are often depolarizing, which may be a key factor in the control of several Ca2+-dependent developmental phenomena, including neuronal proliferation, migration and targeting. To date, however, the molecular mechanism underlying this shift in neuronal electrophysiological phenotype is unknown. Here we show that, in pyramidal neurons of the rat hippocampus, the ontogenetic change in GABA(A)-mediated responses from depolarizing to hyperpolarizing is coupled to a developmental induction of the expression of the neuronal (Cl-)-extruding K+/Cl- co-transporter, KCC2. Antisense oligonucleotide inhibition of KCC2 expression produces a marked positive shift in the reversal potential of GABAA responses in functionally mature hippocampal pyramidal neurons. These data support the conclusion that KCC2 is the main Cl- extruder to promote fast hyperpolarizing postsynaptic inhibition in the brain.     

Outward chloride/potassium co-transport in insect neurosecretory cells (DUM neurones)

The mechanism underlying outward chloride transport in the cell body and in the neuritic field of cockroach Dorsal Unpaired Median (DUM) neurones was assessed using the intracellular microelectrode technique. The chloride equilibrium potential was indirectly estimated from the reversal potentials of responses to gamma-aminobutyric acid (GABA) pressure ejections and of inhibitory postsynaptic potential (IPSP) evoked by electrical stimulation of the anterior connectives. Changes in intracellular chloride concentration [Cl-]i following various treatments were estimated from the amplitude changes of soma GABA responses and IPSP. Decreasing external Cl- concentration reduced the amplitude of GABA-mediated inhibitory events without affecting the membrane potential. Cl-/K+ co-transport was assessed by increasing external K+ concentration. The rate of outward Cl- movement was reduced furosemide but not by SITS or DIDS. All these results suggest that Cl- is not passively distributed in DUM neurones and that an active outwardly directed Cl-/K+ co-transport is implicated in the regulation of [Cl-]i.     

Alteration in the sensitivity of GABA(A) receptors to allosteric modulatory drugs in rat hippocampus after chronic intermittent ethanol treatment

Chronic intermittent ethanol (CIE)-treated rats exhibited a kindling-like persistent increase in withdrawal hyperexcitability. The alteration of GABA(A) receptor (GABA(A)R) function in the hippocampus was suggested as a possible mechanism underlying the hyperexcitability observed in CIE rats, because (1) GABA(A)R agonist (muscimol)-evoked 36Cl- efflux was decreased; (2) paired-pulse inhibition in the CA1 area, predominantly due to GABA(A)R-mediated recurrent inhibition, was persistently decreased; and (3) GABA(A)R subunit expression was altered in the hippocampus from CIE rats. To further characterize the functional alteration of GABA(A)R after CIE treatment, their sensitivity to acute ethanol, a steroid anesthetic (alphaxalone), and a benzodiazepine inverse agonist (DMCM; methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate) were studied using either synaptically evoked GABA(A)R responses or exogenously applied muscimol-evoked responses in hippocampal slices. Bath application of ethanol (60 mM) enhanced the area of GABA(A)R-mediated inhibitory postsynaptic potentials in the hippocampal CA1 region from control and CIE rats, and this potentiation was significantly (p = 0.027) greater in CIE rats (98%) than in control rats (53%). The positive modulatory effect of alphaxalone (1 microM) on GABA(A)R-inhibitory postsynaptic potentials was not significantly different between control and CIE rats (p = 0.375), whereas alphaxalone allosterically increased [3H]flunitrazepam binding in the CA1 area only in CIE rats (by 20 to 25%, p < 0.01), but not in controls. On the other hand, the negative modulatory effect of DMCM (1 microM) on muscimol-evoked responses was significantly larger in CIE rats (p = 0.002). These results suggest that the sensitization of GABA(A)R to acute ethanol and benzodiazepine inverse agonists, and possibly neurosteroids, may underlie ethanol dependence after multiple ethanol withdrawal episodes. These altered pharmacological properties are most consistent with changes in the subunit composition in the CA1 area of this rat model of alcohol dependence.     

Vasoactive intestinal polypeptide enhances the GABAergic synaptic transmission in cultured hippocampal neurons

The whole-cell mode of patch-clamp techniques was used to investigate the effect of vasoactive intestinal polypeptide (VIP) on spontaneous gamma-aminobutyric acid (GABA)-mediated inhibitory postsynaptic currents (IPSCs) of cultured hippocampal neurons. Application of VIP caused a significant increase in the frequency of spontaneous IPSCs with a reversible and dose-dependent manner. VIP had no effect on the mean amplitude and kinetic parameters of spontaneous IPSCs. In the presence of tetrodotoxin, VIP increased the frequency of miniature inhibitory postsynaptic currents (mIPSCs) without affecting their mean magnitude. Forskolin, but not its inactive analog 1,9-dideoxyforskolin, mimicked the stimulatory effect of VIP on spontaneous IPSCs and mIPSCs. VIP and forskolin failed to modulate GABAergic IPSCs in the presence of Rp-cAMPs, a cell permeable antagonist of cAMP-dependent protein kinase (PKA). Calcium channel blocker CdCl2 did not prevent VIP and forskolin from increasing the frequency of mIPSCs. These results suggest that the activation of presynaptic VIP receptor enhances the GABAergic synaptic transmission in cultured hippocampal neurons through the cAMP-PKA pathway and that VIP is likely to increase GABA release by directly stimulating the vesicular release apparatus.     

Operative GABAergic inhibition in hippocampal CA1 pyramidal neurons in experimental epilepsy

Patch-clamp recordings of CA1 interneurons and pyramidal cells were performed in hippocampal slices from kainate- or pilocarpine-treated rat models of temporal lobe epilepsy. We report that gamma-aminobutyric acid (GABA)ergic inhibition in pyramidal neurons is still functional in temporal lobe epilepsy because: (i) the frequency of spontaneous GABAergic currents is similar to that of control and (ii) focal electrical stimulation of interneurons evokes a hyperpolarization that prevents the generation of action potentials. In paired recordings of interneurons and pyramidal cells, synchronous interictal activities were recorded. Furthermore, large network-driven GABAergic inhibitory postsynaptic currents were present in pyramidal cells during interictal discharges. The duration of these interictal discharges was increased by the GABA type A antagonist bicuculline. We conclude that GABAergic inhibition is still present and functional in these experimental models and that the principal defect of inhibition does not lie in a complete disconnection of GABAergic interneurons from their glutamatergic inputs.     

Modulation by substance P of synaptic transmission in the mouse hippocampal slice

The modulatory action of substance P on synaptic transmission of CA1 neurons was studied using intra- or extracellular recording from the mouse hippocampal slice preparation. Bath-applied substance P (2-4 microM) or the selective NK1 receptor agonist substance P methylester (SPME, 10 nM-5 microM) depressed field potentials (recorded from stratum pyramidale) evoked by focal stimulation of Schaffer collaterals. This effect was apparently mediated via NK1 receptors since it was completely blocked by the selective NK1 antagonist SR 140333. The field potential depression by SPME was significantly reduced in the presence of bicuculline. Intracellular recording from CA1 pyramidal neurons showed that evoked excitatory postsynaptic potentials (EPSPs) and evoked inhibitory postsynaptic potentials (IPSPs) were similarly depressed by SPME, which at the same time increased the frequency of spontaneous GABAergic events and reduced that of spontaneous glutamatergic events. The effects of SPME on spontaneous and evoked IPSPs were prevented by the ionotropic glutamate receptor blocker kynurenic acid. In tetrodotoxin (TTX) solution, no change in either the frequency of spontaneous GABAergic and glutamatergic events or in the amplitude of responses of pyramidal neurons to 4 microM alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) or 10 microM N-methyl-D-aspartate (NMDA) was observed. On the same cells, SPME produced minimal changes in passive membrane properties unable to account for the main effects on synaptic transmission. The present data indicate that SPME exerted its action on CA1 pyramidal neurons via a complex network mechanism, which is hypothesized to involve facilitation of a subset of GABAergic neurons with widely distributed connections to excitatory and inhibitory cells in the CA1 area.     

Neuronal diversity in the subiculum: correlations with the effects of somatostatin on intrinsic properties and on GABA-mediated IPSPs in vitro

1. We used intracellular current-clamp techniques to record from 33 ventral subicular neurons in slices or rat hippocampal formation. Presumed pyramidal neurons were characterized by their responses to depolarizing current pulses as either intrinsically burst firing (IB) or regular spiking (RS). Within the subiculum, IB cells were encountered most frequently in the deep cell layer, whereas RS cells were encountered most frequently in the superficial cell layer. IB cells had more depolarized resting potentials, lower input resistances, and more sag in their voltage responses to hyperpolarizing current pulses. 2. Somatostatin (5 microM) applied in the bathing medium caused a hyperpolarization and reduction in input resistance. These effects were of greater magnitude in IB cells. Somatostatin had no effect on sag in either cell type. These effects of somatostatin were unchanged in the presence of gamma-aminobutyric acid (GABA) receptor antagonists. 3. In a series of experiments conducted in RS cells only, somatostatin reduced the amplitude of the late but not the early component of evoked biphasic inhibitory postsynaptic potentials (IPSPs). 4. A second series of experiments was conducted in RS and IB cells. Somatostatin reduced the amplitude of pharmacologically isolated GABAA IPSPS in both cell types. In IB cells but not RS cells there was a correlation between this effect and the somatostatin-induced hyperpolarization. Somatostatin also reduced the amplitude of isolated GABAB IPSPS in both cell types, but more so in IB cells. 5. Somatostatin had no effect on the reversal potential of either IPSP in either cell type and no effect on the GABAA-mediated conductance in either cell type. In contrast, the GABAB-mediated conductance was reduced, especially in IB cells. 6. The effects of somatostatin on GABAA IPSPS are principally a result of membrane shunting and reductions in ionic driving force, but these mechanisms do not account for the reduction in GABAB IPSPS. 7. We suggest that the combined effects of somatostatin are likely to alter the balance between fast and slow inhibition and to do so more in IB cells than in RS cells.     

L-arginine potentiates GABA-mediated synaptic transmission by a nitric oxide-independent mechanism in rat dopamine neurons

Effects of L-arginine in the nervous system are often attributed to nitric oxide. Using whole-cell patch pipettes to record membrane currents in voltage-clamp from dopamine neurons in the rat midbrain slice, the present studies found that L-arginine potentiates GABA-dependent membrane currents via a nitric oxide-independent mechanism. L-Arginine (0.3-10 mM) increased the peak amplitude, half-width duration and time constant of decay of GABA(B) receptor-mediated inhibitory postsynaptic currents in a concentration-dependent manner. In the presence of CGP 35348 (300 microM), a GABA(B) receptor antagonist, L-arginine also prolonged the duration of inhibitory postsynaptic currents mediated by GABA(A) receptors, but their amplitudes were reduced. L-Arginine (10 mM) also evoked 17+/-3 pA of outward current (at -60 mV) which was significantly increased in the presence of exogenous GABA (100 microM). Pressure-ejection of GABA from micropipettes produced outward currents mediated by GABA(B) receptors (recorded in bicuculline) or GABA(A) receptors (recorded in CGP 35348); both types of receptor-mediated currents were increased by L-arginine (10 mM). In contrast, outward currents evoked by baclofen, a GABA(B) receptor agonist, were not potentiated by L-arginine. The GABA transport inhibitors NO 711 (1 microM) and nipecotic acid (1 mM) significantly increased the half-width duration and time-constant of decay of GABA(B)-mediated inhibitory postsynaptic currents, thus mimicking effects of L-arginine. However, nitric oxide donors failed to mimic effects of L-arginine on GABA(B) inhibitory postsynaptic currents, and inhibitors of nitric oxide synthesis failed to selectively block the action of L-arginine. These findings suggest that L-arginine potentiates GABA synaptic transmission by a nitric oxide-independent mechanism. Similarities between effects of L-arginine, NO 711 and nipecotic acid suggest that L-arginine inhibits a GABA transporter.     

Glutamate mediates an inhibitory postsynaptic potential in dopamine neurons

Rapid information transfer within the brain depends on chemical signalling between neurons that is mediated primarily by glutamate and GABA (gamma-aminobutyric acid), acting at ionotropic receptors to cause excitatory or inhibitory postsynaptic potentials (EPSPs or IPSPs), respectively. In addition, synaptically released glutamate acts on metabotropic receptors to excite neurons on a slower timescale through second-messenger cascades, including phosphoinositide hydrolysisl. We now report a unique IPSP mediated by the activation of metabotropic glutamate receptors. In ventral midbrain dopamine neurons, activation of metabotropic glutamate receptors (mGluR1) mobilized calcium from caffeine/ryanodine-sensitive stores and increased an apamin-sensitive potassium conductance. The underlying potassium conductance and dependence on calcium stores set this IPSP apart from the slow IPSPs described so far. The mGluR-induced hyperpolarization was dependent on brief exposure to agonist, because prolonged application of exogenous agonist desensitized the hyperpolarization and caused the more commonly reported depolarization. The rapid rise and brief duration of synaptically released glutamate in the extracellular space can therefore mediate a rapid excitation through activation of ionotropic receptors, followed by inhibition through the mGluR1 receptor. Thus the idea that glutamate is solely an excitatory neurotransmitter must be replaced with a more complex view of its dual function in synaptic transmission.     

Tetanic stimulation induces short-term potentiation of inhibitory synaptic activity in the rostral nucleus of the solitary tract

Whole cell recordings from neurons in the rostral nucleus of the solitary tract (rNST) were made to explore the effect of high-frequency tetanic stimulation on inhibitory postsynaptic potentials (IPSPs). IPSPs were elicited in the rNST by local electrical stimulation after pharmacological blockade of excitatory synaptic transmission. Tetanic stimulation at frequencies of 10-30 Hz resulted in sustained hyperpolarizing IPSPs that had a mean amplitude of -68 mV. The hyperpolarization resulted in a decrease in neuronal input resistance and was blocked by the gamma-aminobutyric acid-A (GABAA) antagonist bicuculline. For most of the neurons (n = 87/102), tetanic stimulation resulted in a maximum hyperpolarization immediately after initiation of the tetanic stimulation, but for some neurons the maximum was achieved after three or more consecutive shock stimuli in the tetanic train of stimuli. When the extracellular Ca2+ concentration was reduced, the maximum IPSP amplitude was reached after several consecutive shock stimuli in the tetanic train for all neurons. Tetanic stimulation at frequencies of 30 Hz and higher resulted in IPSPs that were not sustained but decayed to a more positive level of hyperpolarization. In some neurons the decay was sufficient to become depolarizing and resulted in a biphasic IPSP. It was possible to evoke this biphasic IPSP in all the neurons tested if the cells were hyperpolarized to -75 to -85 mV. The ionic mechanism of the depolarizing IPSPs was examined and was found to be due to an elevation of the extracellular K+ concentration and accumulation of intracellular Cl-. Tetanic stimulation increased the mean 80-ms decay time constant of a single shock-evoked IPSP up to 8 s. The length of the IPSP decay time constant was dependent on the duration and frequency of the tetanic stimulation as well as the extracellular Ca2+ concentration. Afferent sensory input to the rNST consists of trains of relatively high-frequency spike discharges similar to the tetanic stimulation frequencies used to elicit the IPSPs in the brain slices. Thus the short-term changes in inhibitory synaptic activity in the slice preparation probably occur in vivo and may play a key role in taste processing by facilitating synaptic integration.     

Radiation therapy of esophageal carcinoma: a report on 180 cases]

To determine whether the charybdotoxin-sensitive subtypes of voltage-gated K+ channels (Kv1.2 and Kv1.3) exist in inhibitory pre-synaptic terminals, effects of K+ channel blockers including TEA, charybdotoxin (ChTX), iberiotoxin (IbTX), kaliotoxin (KTX) and margatoxin (MgTX) on the inhibitory transmission were examined with cultured rat hippocampal neurons. Monosynaptic inhibitory postsynaptic currents (IPSCs) evoked by electrical stimulation of single presynaptic neurons were recorded from the whole-cell clamped postsynaptic neurons. In the presence of TEA, application of ChTX greatly increased the amplitude of IPSCs. A specific maxi-K+ channel blocker IbTX failed to augment IPSCs. KTX and MgTX, both of which block Kv1.3 but not Kv1.2, mimicked the facilitating effect of ChTX. In the absence of TEA, application of ChTX increased the IPSC amplitude significantly, while IbTX was without effect. These results indicate that the ChTX-sensitive subtypes of voltage-gated K+ channels, most likely Kv1.3, contribute to the repolarization of action potentials at presynaptic terminals of hippocampal inhibitory neurons, and that the ChTX-induced facilitation of the transmission can be explained by its effects on the Kv channels rather than maxi-K+ channels.     

Prostaglandin E2 facilitates excitatory synaptic transmission in the nucleus tractus solitarii of rats

Intrinsic membrane properties and synaptic responses of neocortical neurons located lateral to photochemically induced ischemic lesions were investigated using neocortical slice preparation. In comparison to neurons from control slices, these neurons had a significantly less negative resting membrane potential without any significant change in input resistance. In addition, gamma-aminobutyric acid (GABA) mediated synaptic inhibition was found to be less efficient; the conductances of both the early and late inhibitory postsynaptic potentials (IPSPs) were significantly smaller, and the reversal potential of the early IPSP was shifted to a more positive value. In some of the neurons, 'epileptiform' postsynaptic potentials could be elicited, which were abolished after wash-in of the N-methyl-D-aspartic acid (NMDA)-receptor antagonist D-2-amino-5-phosphonovaleric acid (AP-5). The results provide a possible explanation for the hyperexcitability found in the vicinity of cortical infarcts.     

Modulation of calcium by inhibitory systems in the developing auditory midbrain

Inhibitory synaptic transmission is of fundamental importance during the maturation of central auditory circuits, and their subsequent ability to process acoustic information. The present study investigated the manner in which inhibitory transmission regulates intracellular free calcium levels in the gerbil inferior colliculus using a brain slice preparation. Inhibitory and excitatory postsynaptic potentials were evoked by electrical stimulation of the ascending afferents at the level of the dorsal nucleus of the lateral lemniscus. Pharmacologically isolated inhibitory synaptic potentials were able to attenuate a calcium rise in collicular neurons that was generated by depolarizing current injection. In addition, GABA(A) and glycine receptor antagonists typically led to an increase of calcium in collicular neurons during electrical stimulation of the ascending afferent pathway at the level of the dorsal nucleus of the lateral lemniscus. Bath application of GABA or muscimol, a GABA(A) receptor agonist, evoked a brief hyperpolarization followed by a long-lasting depolarization in inferior colliculus neurons. This treatment also induced a transient calcium increase that correlated with the membrane depolarization phase. Baclofen, a GABA(B) receptor agonist, had no effect on either membrane potential or calcium levels. Ratiometric measures indicated that the muscimol-evoked rise in calcium was approximately 150 nM above basal levels. The muscimol-evoked responses were completely antagonized by bicuculline and attenuated by picrotoxin. Together, these results suggest that inhibitory synaptic transmission participates in the regulation of postsynaptic calcium during the developmental period. Inhibitory transmission may attenuate a calcium influx that is evoked by excitatory synapses, but it can also produce a modest influx of calcium when activated alone. These mechanisms may help to explain the influence of inhibitory transmission on the development of postsynaptic properties.     

Postsynaptic spike firing reduces synaptic GABAA responses in hippocampal pyramidal cells

Using intracellular recording techniques in CA1 cells in the hippocampal slice, we studied the responses of cells to synaptically released and iontophoretically applied GABA. With high-resistance, Cl(-)-filled electrodes, which inverted and enlarged the responses at normal resting potentials, we examined spontaneous GABA-mediated IPSPs. Usually we recorded the spontaneous events in the presence of carbachol (10-25 microM), which significantly increased IPSP frequency and blocked potentially confounding K+ conductances. Following a train of action potentials, spontaneous IPSPs were transiently suppressed. This suppression could not be accounted for by membrane conductance changes following the train or activation of a recurrent circuit. Whole-cell voltage-clamp recordings in the slice indicated that the amplitudes of the spontaneous GABAA inhibitory postsynaptic currents (IPSCs) were also diminished following the action potential train. In some cases BAY K 8644, a Ca2+ channel agonist, enhanced the suppression of IPSPs, while buffering changes in [Ca2+]i with EGTA or BAPTA prevented it. The monosynaptically evoked IPSC in the presence of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and dl-2-amino-5-phosphonovaleric acid (APN) was also diminished following a train of action potentials; however, iontophoretically applied GABA responses did not change significantly. These studies suggest that localized physiological changes in postsynaptic [Ca2+]i potently modulate synaptic GABAA inputs and that this modulation may be an important regulatory mechanism in mammalian brain.