Synaptic and non-synaptic plasticity between individual pyramidal cells in the rat hippocampus in vitro

We report here two forms of activity-dependent plasticity of the transfer of neuronal information between pairs of monosynaptically coupled pyramidal cells. In a first part, we discuss the induction of long-term bidirectional changes in excitatory synaptic transmission following defined regimes of neuronal activity. In a second part, we provide evidence that the conditions in which the presynaptic action potential is elicited determine whether it will successfully propagate along the presynaptic axon.

Spine loss in experimental epilepsy: quantitative light and electron microscopic analysis of intracellularly stained CA3 pyramidal cells in hippocampal slice cultures

The sequence of neuronal alterations resulting from epileptic activity is poorly understood. In the hippocampus of some epileptic patients, there is a loss of certain neuronal types in the hilar region and in CA3. The neuronal alterations preceding this degeneration probably affect synaptic structures. Here we have estimated the number of dendritic spines, major postsynaptic elements of hippocampal neurons, in defined dendritic segments of identified (intracellularly stained) CA3 pyramidal neurons in "epileptic" slice cultures of hippocampus and in control cultures. Slice cultures were prepared from five- or six-day-old rat pups and maintained in vivo for 23 days before epileptic activity was induced by application of the convulsants bicuculline and picrotoxin for three days. Individual CA3 pyramidal neurons were then intracellularly injected with horseradish peroxidase, and the number of dendritic spines was counted in proximodistal dendritic segments by applying the Sholl method. In addition, the total dendritic length was measured and the branching index evaluated. The number of spines on CA3 pyramidal cell dendrites in the "epileptic" cultures was found to be decreased by 40%. This spine loss affected proximal and peripheral dendritic segments of the CA3 pyramidal neurons to a similar extent. No significant differences were observed between control and "epileptic" cultures in dendritic length or in the branching index. Quantitative electron microscopic analysis did not reveal differences between "epileptic" cultures and control cultures in the spine area of the labelled CA3 pyramidal cells, indicating that there was a real spine loss, not just a reduction in the size of the spines. We conclude that epileptic activity causes morphological alterations in defined postsynaptic compartments of hippocampal pyramidal cells surviving under these conditions.

Associative long-term depression in the hippocampus in vitro

An association between the test and conditioning stimuli is critical for determining the nature of their interaction during learning and memory. Two experimental protocols which result in the induction of associative forms of long-term depression (LTD) at Schaffer collateral synapses onto CA1 pyramidal cells in vitro are reviewed in this article. The requirements for the induction of LTD with these protocols are discussed, as well as the relationship between these forms of associative LTD and so-called homosynaptic LTD. In particular, the biological basis of the experimentally demonstrated necessity for temporal and spatial conjunction between the test and conditioning stimuli is examined.

Modulation of synaptic GABAA receptor function by benzodiazepines in area CA3 of rat hippocampal slice cultures

The effects of the benzodiazepine agonist midazolam on GABAA receptor-mediated inhibition were investigated in area CA3 of hippocampal slice cultures. Midazolam (100 nM) increased the decay time constant (tau OFF) of miniature inhibitory postsynaptic currents (mIPSCs) recorded from pyramidal cells by approximately 40%, but did not significantly affect their activation rate or amplitude, consistent with saturation of postsynaptic GABAA receptors by a quantum of GABA. Non-stationary variance analysis of mIPSCs revealed that the unitary conductance of synaptic GABAA channels (approximately 31 pS) was unaffected by midazolam. Midazolam increased not only the tau OFF (51%), but also the amplitude (23%) of unitary IPSPs, recorded from pairs of monosynaptically connected inhibitory and pyramidal cells. Simulation of unitary IPSPs indicated that the increased amplitude was primarily due to the slow time constant of pyramidal cells. Finally, the mean amplitude, tau OFF, and single-channel conductance of mIPSCs recorded in cultures chronically exposed to midazolam (0.1-10 microM) for 2 weeks were not different from control mIPSCs, nor was their response to midazolam. We conclude that benzodiazepines increase synaptic GABAA channel open time, as described previously, and that this results in an increase in both the amplitude and duration of IPSPs in pyramidal cells.

Bilateral simultaneous trabeculectomy. A review of outcome and a survey of ophthalmologists' attitudes

In this study the attitudes of experienced ophthalmologists to bilateral simultaneous trabeculectomy were canvased using a postal questionnaire. Only 16% of respondents ever performed bilateral simultaneous trabeculectomy (BST). The main reason for not performing BST was fear of bilateral simultaneous complications. The notes of 95 patients who had undergone BST were reviewed. Post-operative complications occurred in 41 eyes (43.6%) during the first post-operative week, but only 8 patients (8.5%) hand complications in both eyes. At 3 months post-operatively, 5 patients (5.5%) had bilateral raised intraocular pressures with or without medication, and at 12 months this figure was 8 patients (9.3%). Twenty patients (21.3%) had a drop in visual acuity of two Snellen lines or more in both eyes at one week. At 3 months this figure was 5 patients (5.5%), and at 12 months 3 patients (3.5%) had lost two or more lines of vision in both eyes. Although BST is not commonly performed, the incidence of bilateral post-operative complications is low.

Presynaptic inhibition of excitatory synaptic transmission by muscarinic and metabotropic glutamate receptor activation in the hippocampus: are Ca2+ channels involved?

Activation of either muscarinic cholinergic or metabotropic glutamatergic presynaptic receptors inhibits evoked excitatory synaptic responses in the hippocampus. We have investigated two possible mechanisms underlying these actions using whole-cell recording from CA3 pyramidal cells in hippocampal slice cultures. Application of either methacholine (MCh, 10 microM) or trans-aminocyclopentane-1,3-dicarboxylic acid (t-ACPD, 10 microM) was found to reduce the frequency of miniature excitatory postsynaptic currents (mEPSCs) by roughly 50%, without changing their mean amplitude. The voltage-dependent Ca2+ channel blocker Cd2+ (100 microM), in contrast, had no effect on the mEPSC frequency. When the extracellular [K+] was increased from 2.7 to 16 mM, the mEPSC frequency increased from 1.7 to 4.9 Hz. This increase could be completely reversed by applying Cd2+, indicating that it was triggered by voltage-dependent Ca2+ influx. MCh and t-ACPD each decreased the mEPSC frequency by roughly 50% under these conditions. Because the agonists were equally effective in inhibiting spontaneous release whether voltage-dependent channels were activated or not, we conclude that presynaptic cholinergic and glutamatergic inhibition is not mediated by inhibition of presynaptic Ca2+ channels, but rather by a direct interference in the neurotransmitter release process at some point subsequent to Ca2+ influx.

Screening scale predicts patients successfully receiving long-term implantable left ventricular assist devices

BACKGROUND

Although use of long-term implantable left ventricular assist devices (LVAD) is becoming more popular, further reduction of the mortality rate accompanying device insertion through improved patient selection would make this alternative even more appealing. We sought to develop a scoring system that was based on criteria obtainable at the time of evaluation and predictive of successful early outcome and simple to apply.

METHODS AND RESULTS

Patients (n = 56) undergoing LVAD insertion between 1990 and 1994 were screened for easily obtainable preoperative risk factors. To test the association between survival and each risk factor, a chi 2 analysis was performed, and relative risks were estimated. Oliguria, ventilator dependence, elevated central venous pressure, elevated prothrombin time, and reoperation stats had low probability values and high estimated relative risks. On the basis of these relations, a risk factor-selection scale (RFSS) (range, 0 to 10) was developed by computing appropriate weights for each risk factor. The distribution of patients for each scale score reveal that with RFSS > or = 5, most device recipients will die (P < .001). The average RFSS (+/- SD) of survivors (n = 42) was 2.45 +/- 1.73 compared with 5.43 +/- 2.85 in nonsurvivors (n = 14) (P < .0001). Univariate logistical regression was also significant (score statistic, 16.2; df = 1; P = .001).

CONCLUSIONS

The RFSS is simple, easy to apply, and statistically valid. Physicians could use the scale as a starting point in discussing the suitability for LVAD implantation in a specific patient and as a basis for comparing patient outcomes.

Alterations in glutamate but not GABAA receptor subunit expression as a consequence of epileptiform activity in vitro

The consequences of epileptiform discharge on the expression of glutamate and GABA receptors were examined by in situ hybridization histochemistry after treatment of rat hippocampal slice cultures with convulsants. Application of 500 microM picrotoxin for two days led to decreases in the messenger RNA levels for the N-methyl-D-aspartate receptor subunits, NR2A and NR2B, and for the non-N-methyl-D-aspartate receptor subunits, glutamate receptors 1 and glutamate receptors 2, to about 50% of the levels seen in control cultures. Messenger RNA levels for the N-methyl-D-aspartate receptor subunit, NR1; the non-N-methyl-D-aspartate receptor subunits, glutamate receptors 3 and 4; the high-affinity kainate receptor subunits 1 and 2; and the GABAA receptor subunits, alpha 2, beta 2, gamma 2 were unchanged. Decreased levels of expression were no longer seen five days after removal of convulsant. The down-regulation could be prevented by co-application of both the non-N-methyl-D-aspartate and N-methyl-D-aspartate receptor antagonists, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and dizocilpine maleate, but not by applying each alone. Application of CNQX or dizocilpine maleate in the absence of picrotoxin also resulted in changes in glutamate receptor expression. We suggest that the convulsant-induced reduction in glutamate receptor expression leads to a decreased excitability in these cultures, and that this down-regulation represents a compensatory reaction of hippocampal pyramidal cells to enhanced excitatory input.

Mast cells and histamine contribute to bile acid-stimulated secretion in the mouse colon

Certain dihydroxy bile acids cause secretory diarrhea when present in the colonic lumen at inappropriately high concentrations. However, the mechanism underlying the secretagogue activity has not been fully elucidated. Experiments were performed to test whether mast cells and one of their major mediators, histamine, might contribute to the secretory effect. Chenodeoxycholic acid, a secretory bile acid, and ursodeoxycholic acid, a nonsecretory, hydrophilic bile acid, were compared for their ability to induce chloride secretion across segments of mouse colon mounted in Ussing chambers. Chenodeoxycholic acid, but not ursodeoxycholic acid, induced dose-dependent, biphasic chloride secretion that was greater after serosal than mucosal addition and was greater in distal versus proximal colonic segments. The secretory effect of chenodeoxycholic acid was inhibited by H1 histamine receptor antagonists and modified by the cyclooxygenase inhibitor indomethacin. However, it was unaffected by an H2 histamine receptor antagonist or by atropine. Secretory effects of chenodeoxycholic acid were diminished in magnitude and delayed in colonic tissues from mice with a genetic deficiency of tissue mast cells. Concentrations of chenodeoxycholic acid inducing secretion also released histamine from tissue segments. These data indicate that mast cells and histamine-mediated processes contribute significantly to the secretory effects of dihydroxy bile acids in the murine colon.

Brief report: adherence-facilitating behaviors of a multidisciplinary pediatric rheumatology staff

Investigated the behaviors of pediatric rheumatology health care providers that were expected to be related to patient or parent adherence. Medical charts of 108 patients ages 1 to 20 years diagnosed with Juvenile Rheumatoid Arthritis were examined. The 473 outpatient visits over 15 months yielded a total of 2,578 treatment recommendations, but only 1,390 adherence-facilitating behaviors by medical staff were documented. Providing information about how often to perform the recommendation was the most common staff behavior. In contrast, care providers rarely indicated that they addressed their patients' concerns and barriers to implementing the recommendations, or employed behavior modification strategies to increase adherence. Implications of these findings for development of programs designed to increase treatment adherence in children with chronic diseases requiring time-consuming, intrusive medical regimens are discussed.

Physiology and pharmacology of unitary synaptic connections between pairs of cells in areas CA3 and CA1 of rat hippocampal slice cultures

1. Paired intracellular recordings were made in rat hippocampal slice cultures, with the use of either sharp microelectrodes or the whole cell configuration of the patch-clamp technique. Unitary synaptic connections were studied between pyramidal and nonpyramidal cells within and between areas CA1 and CA3. 2. Monosynaptic excitatory synaptic responses between CA3 pyramidal neurons were found in 56% of cell pairs (n = 91, 28 postsynaptic cells). Monosynaptic connections from a CA3 cell to a CA1 cell were observed in 76% of cell pairs (n = 125, 26 postsynaptic cells), but from CA1 to CA3 neurons in only 8% of cell pairs (n = 13, 13 postsynaptic cells). Monosynaptic excitatory connections were found in only 16% of CA1/CA1 cell pairs (n = 25, 10 postsynaptic cells). 3. Disynaptic inhibition was commonly observed between CA3 cell pairs (43%), but rarely found between CA3-CA1 pyramidal cell pairs (2%). In 50% of CA3 pyramidal cell pairs, synchronous inhibitory postsynaptic potentials (IPSPs) in both cells could be triggered by an action potential in one pyramidal cell. Reciprocal monosynaptic connections were found between 75% of interneuron and pyramidal cell pairs within area CA3. 4. The latency of monosynaptic CA3- to CA1-cell responses was significantly longer than for responses between two CA3 cells. Within area CA3 the latencies for inhibitory synaptic responses between interneurons and pyramidal cells were significantly shorter than those for excitatory responses between pyramidal cells. Monosynaptic excitatory postsynaptic potentials (EPSPs) in interneurons had a significantly shorter time-to-peak than those recorded in pyramidal neurons. 5. 6-Cyano-7-nitroquinoxaline-2,3-dione (CNQX)- and D-2-amino-5-phosphonovalerate (AP5)-sensitive components were identified in unitary monosynaptic EPSPs in CA3-CA3 and CA3-CA1 pyramidal cell pairs. The CNQX-sensitive component had a mean time-to-peak and duration of 6.2 +/- 0.3 (SE) ms and 61.2 +/- 2.0 ms, respectively, and an amplitude of approximately 1 mV (n = 93). The AP5-sensitive component of EPSPs was only detected when the cell was depolarized with respect to the resting potential, had a mean time-to-peak of 41 +/- 5 ms and duration of 121 +/- 11 ms (n = 6), and increased in amplitude with postsynaptic depolarization. 6. Unitary monosynaptic IPSPs between an interneuron and a pyramidal cell had a mean amplitude of approximately 1 mV and were fully blocked by gamma-aminobutyric acid-A (GABAA) receptor antagonists (n = 3). 7. Unitary inhibitory responses were found only within, but not between, areas CA3 or CA1.(ABSTRACT TRUNCATED AT 400 WORDS)

Presynaptic enhancement of inhibitory synaptic transmission by protein kinases A and C in the rat hippocampus in vitro

The protein kinase C activator phorbol 12,13-dibutyrate (0.5 microM, PDBu) and the protein kinase A activator forskolin (20 microM) each increased evoked monosynaptic inhibitory postsynaptic current (IPSC) amplitude, without affecting its reversal potential, and increased the frequency of miniature IPSCs (mIPSCs), without affecting their amplitude or kinetics, as assessed with whole-cell recording form CA3 pyramidal cells in hippocampal slice cultures. The effects of forskolin and PDBu on both evoked IPSC amplitude and mIPSC frequency were additive and were antagonized by inhibitors of protein kinases A and C, respectively. The kinase activator-induced increases in mIPSC frequency were quantitatively comparable to the increases in evoked IPSC amplitude. The increases in mIPSC frequency were not attenuated by the voltage-dependent calcium channel blocker Cd2+ (100 microM). We conclude that stimulation of protein kinases A and C potentiates hippocampal inhibitory synaptic transmission through independent presynaptic mechanisms of action. Kinase-induced potentiation of spontaneous release does not require modulation of axon terminal Ca2+ channels. This mechanism may also contribute substantially to the potentiation of evoked release.

Calcium: A Trigger for Long-Term Depression and Potentiation in the Hippocampus

Two opposing types of plasticity at excitatory synapses in the hippocampus, long-term potentiation and depression, require N-methyl-D-aspartate receptor activation and Ca2+ influx for their induction.The direction of the change in synaptic strength is determined by a balance between phosphorylation and dephosphorylation, as regulated by protein kinases and phosphatases that are activated selectively by different levels of intracellular Ca2+.

Role of excitatory amino acid and GABAB receptors in the generation of epileptiform activity in disinhibited hippocampal slice cultures

Selective excitatory amino acid- and GABAB-receptor antagonists were used to examine the role these receptors play in epileptiform burst discharge elicited by blocking GABAA receptor-mediated inhibition in hippocampal slice cultures of the rat. Application of bicuculline caused a single ictal burst followed by interictal bursting. The N-methyl-D-aspartate receptor antagonist, D-2-amino-5-phosphonovalerate, reduced the depolarizing envelope underlying interictal discharge, and accentuated the appearance of concomitant slow oscillatory potentials, which occurred synchronously in all CA3 cells. The non-N-methyl-D-aspartate receptor antagonists, 6-nitro-7sulphamoyl-benzo(F) quinoxaline and 6-cyano-7-nitro-quinoxaline-2,3-dione, blocked interictal bursting at high concentrations, and low concentrations of 6-cyano-7-nitro-quinoxaline-2,3-dione selectively eliminated the slow oscillations in an all-or-none manner, leaving the depolarizing envelope. No effects of either metabotropic glutamate receptor antagonists or of dihydropyridine Ca2+ channel agonists or antagonists on evoked interictal discharge were observed. 6-Cyano-7-nitro-quinoxaline-2,3-dione-resistant interictal-like discharge could be obtained in the presence of bicuculline when the external Mg2+ concentration was reduced from 1.5-0.5 mM. The GABAB receptor antagonist CGP 35348 prolonged individual evoked interictal bursts, and caused the appearance of spontaneous ictal-like discharges. The implications of these results are discussed with regard to the mechanisms of epileptogenesis and to potential therapeutic intervention.

Asynchronous pre- and postsynaptic activity induces associative long-term depression in area CA1 of the rat hippocampus in vitro

Associative long-term depression (LTD) was induced in hippocampal slice cultures with repeated low-frequency (0.3 Hz) stimulation of the Schaffer collateral pathway, only when such stimuli were preceded by intracellular injection of brief depolarizing current pulses in the postsynaptic CA1 pyramidal cell. The decrease in excitatory postsynaptic potential amplitude lasted > 30 min, could be reversed by induction of potentiation, could be induced at previously potentiated inputs, was input-specific, and did not require activation or potentiation of other inputs. The magnitude of the depression depended upon the time interval between depolarization and stimulation and upon the duration of the depolarization pulse. LTD was not observed in neurons impaled with electrodes containing a Ca2+ chelator. LTD could not be induced in the presence of an N-methyl-D-aspartate receptor antagonist, suggesting that voltage-dependent Ca2+ influx is necessary but not sufficient for LTD induction. We conclude that associative LTD results when synaptic activity follows postsynaptic depolarization within a circumscribed time window.

Presynaptic inhibition of excitatory synaptic transmission mediated by alpha adrenergic receptors in area CA3 of the rat hippocampus in vitro

We have investigated the action of norepinephrine (NE) on excitatory synaptic transmission in the hippocampus by recording from CA3 pyramidal cells in organotypic slice cultures. NE (5 microM) was found to decrease the amplitude of pharmacologically isolated EPSPs elicited with stimulation of mossy fibers or recurrent axon collaterals (mean decrease in EPSP amplitude, 44%). Desensitization was observed with repetitive applications. NE did not affect the sensitivity of CA3 cells to iontophoretically applied AMPA, and did not affect the amplitude distribution of TTX-resistant, miniature excitatory synaptic currents. These data suggest that NE acts at presynaptic receptors to decrease glutamate release. This action of NE was blocked by the alpha receptor antagonist phentolamine and the specific alpha 1 receptor antagonist prazosine, but not by the beta receptor antagonist timolol or the alpha 2 receptor antagonist idazoxan. Inhibition of EPSPs by NE was prevented by pretreatment of cultures with pertussis toxin, indicating that G-proteins couple these receptors to their effectors. Stimulation of protein kinase C with phorbol ester blocked the action of NE on EPSPs. This effect, as well as the desensitization of NE responses, was reduced by application of the protein kinase inhibitor staurosporin. Presynaptic inhibition of excitatory synaptic transmission, mediated by alpha adrenergic receptors, represents a novel modulatory action of NE in the hippocampus.

Consequence of epileptic activity in vitro

Hippocampal sclerosis: a cause or consequence of epileptic activity? This question has concerned neurologists and pathologists for over 150 years. This paper reviews data from an in vitro model system regarding the consequences of epileptic activity of known origin. Exposure of organotypic hippocampal slice cultures to convulsants, such as bicuculline or picrotoxin for three days leads to pronounced neuronal degeneration and a reversible loss of dendritic spines. A similar pathology has been described in hippocampal tissue removed from patients suffering from severe, drug refractory epilepsy. The consequences of such pathological changes are not self-sustaining epileptic activity, as might be expected if such sclerosis caused epilepsy, but rather a selective decrease in synaptic excitation. Inhibitory synaptic transmission and GABAergic interneurons, in contrast, are preserved. At least two mechanisms contribute to the depression of synaptic excitation: morphological changes in dendritic spines and a decrease in the expression of genes for some glutamatergic receptors. It is hoped that this model will allow the characterization of the mechanisms underlying the pathological consequences of epileptic activity, and lead to useful therapeutic strategies.

Mechanism of mu-opioid receptor-mediated presynaptic inhibition in the rat hippocampus in vitro

1. The electrophysiological action of the mu-opioid receptor-preferring agonist D-Ala2, MePhe4, Met(O)5-ol-enkephalin (FK 33-824) on synaptic transmission has been studied in area CA3 of organotypic rat hippocampal slice cultures. 2. FK 33-824 (1 microM) had no effect on the amplitude of pharmacologically isolated N-methyl-D-aspartate (NMDA) or non-NMDA receptor-mediated EPSPs. 3. FK 33-824 (10 nM to 10 microM) reduced the amplitude of monosynaptic inhibitory postsynaptic potentials (IPSPs) that were elicited in pyramidal cells with local stimulation after pharmacological blockade of excitatory amino acid receptors. This effect was reversible, dose-dependent, and sensitive to naloxone and the mu-receptor antagonist Cys2,Tyr3,Orn5,Pen7-amide (CTOP). FK 33-824 at 1 microM caused a mean reduction in the amplitude of the monosynaptic IPSP of 70%. 4. Neither delta- nor kappa-receptor-preferring agonists had any effect on excitatory or inhibitory synaptic potentials. 5. The disinhibitory action of FK 33-824 was blocked by incubating the cultures with pertussis toxin (500 ng/ml for 48 h) or by stimulation of protein kinase C with phorbol 12,13-dibutyrate (PDBu, 0.5 microM). 6. The depression of monosynaptic IPSPs by FK 33-824 was unaffected by extracellular application of the K+ channel blockers Ba2+ or Cs+ (1 mM each). 7. FK 33-824 produced a decrease in the frequency of miniature, action potential-independent, spontaneous inhibitory synaptic currents (mIPSCs) recorded with whole-cell voltage-clamp techniques, but did not change their mean amplitude. Application of the Ca2+ channel blocker Cd2+ (100 microM) or of nominally Ca(2+)-free solutions did not alter either the frequency and amplitude of mIPSCs or the reduction of mIPSC frequency induced by FK 33-824. 8. The effect of FK 33-824 on spontaneous mIPSCs was prevented by naloxone, and by incubation of cultures with pertussis toxin. 9. These results indicate that mu-opioid receptors decrease GABA release presynaptically by a G protein-mediated inhibition of the vesicular GABA release process, and not by changes in axon terminal K+ or Ca2+ conductances that are sensitive to extracellular Ba2+, Cs+ or Cd2+.

Presynaptic inhibition in the hippocampus

Presynaptic receptors for virtually all transmitters have been identified throughout the nervous system. Recent studies in the hippocampus provide new insights into the mechanisms by which the activation of these receptors leads to presynaptic inhibition of transmitter release, and characterize the second messengers involved in coupling presynaptic receptors to their effectors. Presynaptic receptors also provide a tractable route via which the amount of transmitter release may be selectively regulated in therapeutically useful ways.

Reversible loss of dendritic spines and altered excitability after chronic epilepsy in hippocampal slice cultures

The morphological and functional consequences of epileptic activity were investigated by applying the convulsants bicuculline and/or picrotoxin to mature rat hippocampal slice cultures. After 3 days, some cells in all hippocampal subfields showed signs of degeneration, including swollen somata, vacuolation, and dendritic deformities, whereas others displayed only a massive reduction in the number of their dendritic spines. Intracellular recordings from CA3 pyramidal cells revealed a decrease in the amplitude of evoked excitatory synaptic potentials. gamma-Aminobutyric acid-releasing interneurons and inhibitory synaptic potentials were unaffected. Seven days after withdrawal of convulsants, remaining cells possessed a normal number of dendritic spines, thus demonstrating a considerable capacity for recovery. The pathological changes induced by convulsants are similar to those found in the hippocampi of human epileptics, suggesting that they are a consequence, rather than a cause, of epilepsy.

Presynaptic inhibition of miniature excitatory synaptic currents by baclofen and adenosine in the hippocampus

Presynaptic inhibition of neurotransmitter release is thought to be mediated by a reduction of axon terminal Ca2+ current. We have compared the actions of several known inhibitors of evoked glutamate release with the actions of the Ca2+ channel antagonist Cd2+ on action potential-independent synaptic currents recorded from CA3 neurons in hippocampal slice cultures. Baclofen and adenosine decreased the frequency of miniature excitatory postsynaptic currents (mEPSCs) without affecting the distribution of their amplitudes. Cd2+ blocked evoked synaptic transmission, but had no effect on the frequency or amplitude of either mEPSCs or inhibitory postsynaptic currents (IPSCs). Inhibition of presynaptic Ca2+ current therefore appears not to be required for the inhibition of glutamate release by adenosine and baclofen. Baclofen had no effect on the frequency of miniature IPSCs, indicating that gamma-aminobutyric acid B-type receptors exert distinct presynaptic actions at excitatory and inhibitory synapses.

Effects of the GABA uptake inhibitor tiagabine on inhibitory synaptic potentials in rat hippocampal slice cultures

1. The effects of the gamma-aminobutyric acid (GABA) uptake blocker tiagabine on inhibitory synaptic potentials (IPSPs) were examined with microelectrode and whole-cell recording from CA3 pyramidal cells in rat hippocampal slice cultures. 2. Tiagabine (10-25 microM) greatly prolonged the duration of monosynaptic IPSPs elicited in the presence of excitatory amino acid antagonists but had no effect on their amplitude. Part of the prolonged time course resulted from a GABAB receptor-mediated component that was not detectable under control conditions. 3. The mean decay time constant of the underlying GABAA receptor-mediated synaptic current was increased from 16 to 250 ms. Spontaneous miniature IPSPs recorded with whole-cell clamp were unaffected by tiagabine. Pentobarbital sodium, in contrast, increased the decay time constant of both evoked and spontaneous GABAA-mediated currents. 4. Tiagabine (25 microM) inhibited spontaneous and evoked epileptiform bursting induced by increasing the extracellular potassium concentration to 8 mM. 5. We conclude that GABA uptake plays a significant role in determining the time course of evoked IPSPs and also limits the likelihood that GABAB receptors are activated.