Integrity of perforant path fibers and the frequency of action potential independent excitatory and inhibitory synaptic events in dentate gyrus granule cells

The Pathway-Selective Dependence of Nitric Oxide for Long-Term Potentiation in the Anterior Cingulate Cortex of Adult Mice

Nitric oxide (NO) is a key diffusible messenger in the mammalian brain. It has been proposed that NO may diffuse in retrograde into presynaptic terminals, contributing to the induction of hippocampal long-term potentiation (LTP). Here, we present novel evidence that NO is selectively required for the synaptic potentiation of the interhemispheric projection in the anterior cingulate cortex (ACC). Unilateral low-frequency stimulation (LFS) induced a short-term synaptic potentiation on the contralateral ACC through the corpus callosum (CC). The use of the antagonists of the NMDA receptor (NMDAR), or the inhibitor of the L-type voltage-dependent Ca2+ channels (L-VDCCs), blocked the induction of this ACC-ACC potentiation. In addition, the inhibitor of NO synthase, or inhibitors for its downstream signaling pathway, also blocked this ACC-ACC potentiation. However, the application of the NOS inhibitor blocked neither the local electric stimulation-induced LTP nor the stimulation-induced recruitment of silent responses. Our results present strong evidence for the pathway-selective roles of NO in the LTP of the ACC.

Expression of green fluorescent protein defines a specific population of lamina II excitatory interneurons in the GRP::eGFP mouse

Dorsal horn excitatory interneurons that express gastrin-releasing peptide (GRP) are part of the circuit for pruritogen-evoked itch. They have been extensively studied in a transgenic line in which enhanced green fluorescent protein (eGFP) is expressed under control of the Grp gene. The GRP-eGFP cells are separate from several other neurochemically-defined excitatory interneuron populations, and correspond to a class previously defined as transient central cells. However, mRNA for GRP is widely distributed among excitatory interneurons in superficial dorsal horn. Here we show that although Grp mRNA is present in several transcriptomically-defined populations, eGFP is restricted to a discrete subset of cells in the GRP::eGFP mouse, some of which express the neuromedin receptor 2 and likely belong to a cluster defined as Glut8. We show that these cells receive much of their excitatory synaptic input from MrgA3/MrgD-expressing nociceptive/pruritoceptive afferents and C-low threshold mechanoreceptors. Although the cells were not innervated by pruritoceptors expressing brain natriuretic peptide (BNP) most of them contained mRNA for NPR1, the receptor for BNP. In contrast, these cells received only ~ 10% of their excitatory input from other interneurons. These findings demonstrate that the GRP-eGFP cells constitute a discrete population of excitatory interneurons with a characteristic pattern of synaptic input.

Stuttering interneurons generate fast and robust inhibition onto projection neurons with low capacity of short term modulation in mouse lateral amygdala

The stuttering interneurons (STi) represent one minor subset of interneuron population and exhibit characteristic stuttering firing upon depolarization current injection. While it has been long held that the GABAergic inhibitory transmission largely varies with the subtype identity of presynaptic interneurons, whether such a rule also applies to STi is largely unknown. Here, by paired recording of interneuron and their neighboring projection neuron in lateral amygdala, we found that relative to the fast spiking and late spiking interneurons, the STi-evoked unitary postsynaptic currents onto the projection neurons had markedly larger amplitude, shorter onset latency and faster rising and decay kinetics. The quantal content and the number of vesicles in the readily releasable pool were also larger in synapses made by STi versus other interneurons. Moreover, the short-term plasticity, as reflected by the paired pulse depression and depolarization-induced suppression of inhibition, was the least prominent in the output synapses of STi. Thus, the fast and robust inhibition together with its low capacity of short term modulation may suggest an important role for STi in preventing the overexcitation of the projection neurons and thus gating the information traffic in amygdala.

TNFα controls glutamatergic gliotransmission in the hippocampal dentate gyrus

VIDEO ABSTRACT

Glutamatergic gliotransmission provides a stimulatory input to excitatory synapses in the hippocampal dentate gyrus. Here, we show that tumor necrosis factor-alpha (TNFα) critically controls this process. With constitutive TNFα present, activation of astrocyte P2Y1 receptors induces localized [Ca(2+)](i) elevations followed by glutamate release and presynaptic NMDA receptor-dependent synaptic potentiation. In preparations lacking TNFα, astrocytes respond with identical [Ca(2+)](i) elevations but fail to induce neuromodulation. We find that TNFα specifically controls the glutamate release step of gliotransmission. In cultured astrocytes lacking TNFα glutamate exocytosis is dramatically slowed down due to altered vesicle docking. Addition of low picomolar TNFα promptly reconstitutes both normal exocytosis in culture and gliotransmission in situ. Alternatively, gliotransmission can be re-established without adding TNFα, by limiting glutamate uptake, which compensates slower release. These findings demonstrate that gliotransmission and its synaptic effects are controlled not only by astrocyte [Ca(2+)](i) elevations but also by permissive/homeostatic factors like TNFα.

Glutamatergic and nonglutamatergic neurons of the ventral tegmental area establish local synaptic contacts with dopaminergic and nondopaminergic neurons

The ventral tegmental area (VTA) contributes to reward and motivation signaling. In addition to the well established populations of dopamine (DA) or GABA VTA neurons, glutamatergic neurons were recently discovered in the VTA. These glutamatergic neurons express the vesicular glutamate transporter 2, VGluT2. To investigate whether VTA glutamatergic neurons establish local synapses, we tagged axon terminals from resident VTA neurons by intra-VTA injection of Phaseolus vulgaris leucoagglutinin (PHA-L) or an adeno-associated virus encoding wheat germ agglutinin (WGA) and by immunoelectron microscopy determined the presence of VGluT2 in PHA-L- or WGA-positive terminals. We found that PHA-L- or WGA-positive terminals from tagged VTA cells made asymmetric or symmetric synapses within the VTA. VGluT2 immunoreactivity was detected in the vast majority of PHA-L- or WGA-positive terminals forming asymmetric synapses. These results indicate that both VTA glutamatergic and nonglutamatergic (likely GABAergic) neurons establish local synapses. To examine the possible DAergic nature of postsynaptic targets of VTA glutamatergic neurons, we did triple immunolabeling with antibodies against VGluT2, tyrosine hydroxylase (TH), and PHA-L. From triple-labeled tissue, we found that double-labeled PHA-L (+)/VGluT2 (+) axon terminals formed synaptic contacts on dendrites of both TH-positive and TH-negative cells. Consistent with these anatomical observations, in whole-cell slice recordings of VTA neurons we observed that blocking action potential activity significantly decreased the frequency of synaptic glutamatergic events in DAergic and non-DAergic neurons. These observations indicate that resident VTA glutamatergic neurons are likely to affect both DAergic and non-DAergic neurotransmission arising from the VTA.

Regionally localized recurrent excitation in the dentate gyrus of a cortical contusion model of posttraumatic epilepsy

Posttraumatic epilepsy is a frequent consequence of brain trauma, but relatively little is known about how neuronal circuits are chronically altered after closed head injury. We examined whether local recurrent excitatory synaptic connections form between dentate granule cells in mice 8-12 wk after cortical contusion injury. Mice were monitored for behavioral seizures shortly after brain injury and < or = 10 wk postinjury. Injury-induced seizures were observed in 15% of mice, and spontaneous seizures were observed weeks later in 40% of mice. Timm's staining revealed mossy fiber sprouting into the inner molecular layer of the dorsal dentate gyrus ipsilateral to the injury in 95% of mice but not contralateral to the injury or in uninjured controls. Whole cell patch-clamp recordings were made from granule cells in isolated hippocampal brain slices. Cells in slices with posttraumatic mossy fiber sprouting had an increased excitatory postsynaptic current (EPSC) frequency compared with cells in slices without sprouting from injured and control animals (P < 0.001). When perfused with Mg(2+)-free artificial cerebrospinal fluid containing 100 microM picrotoxin, these cells had spontaneous bursts of EPSCs and action potentials. Focal glutamate photostimulation of the granule cell layer evoked a burst of EPSCs and action potentials indicative of recurrent excitatory connections in granule cells of slices with mossy fiber sprouting. In granule cells of slices without sprouting from injured animals and controls, spontaneous or photostimulation-evoked epileptiform activity was never observed. These results suggest that a new regionally localized excitatory network forms between dentate granule cells near the injury site within weeks after cortical contusion head injury.

Glutamate exocytosis from astrocytes controls synaptic strength

The release of transmitters from glia influences synaptic functions. The modalities and physiological functions of glial release are poorly understood. Here we show that glutamate exocytosis from astrocytes of the rat hippocampal dentate molecular layer enhances synaptic strength at excitatory synapses between perforant path afferents and granule cells. The effect is mediated by ifenprodil-sensitive NMDA ionotropic glutamate receptors and involves an increase of transmitter release at the synapse. Correspondingly, we identify NMDA receptor 2B subunits on the extrasynaptic portion of excitatory nerve terminals. The receptor distribution is spatially related to glutamate-containing synaptic-like microvesicles in the apposed astrocytic processes. This glial regulatory pathway is endogenously activated by neuronal activity-dependent stimulation of purinergic P2Y1 receptors on the astrocytes. Thus, we provide the first combined functional and ultrastructural evidence for a physiological control of synaptic activity via exocytosis of glutamate from astrocytes.

Activation of NMDA receptors in rat dentate gyrus granule cells by spontaneous and evoked transmitter release

Activation of N-methyl-D-aspartate (NMDA) receptors by synaptically released glutamate in the nervous system is usually studied using evoked events mediated by a complex mixture of AMPA, kainate, and NMDA receptors. Here we have characterized pharmacologically isolated spontaneous NMDA receptor-mediated synaptic events and compared them to stimulus evoked excitatory postsynaptic currents (EPSCs) in the same cell to distinguish between various modes of activation of NMDA receptors. Spontaneous NMDA receptor-mediated EPSCs recorded at 34 degrees C in dentate gyrus granule cells (DGGC) have a frequency of 2.5 +/- 0.3 Hz and an average peak amplitude of 13.2 +/- 0.8 pA, a 10-90% rise time of 5.4 +/- 0.3 ms, and a decay time constant of 42.1 +/- 2.1 ms. The single-channel conductance estimated by nonstationary fluctuation analysis was 60 +/- 5 pS. The amplitudes (46.5 +/- 6.4 pA) and 10-90% rise times (18 +/- 2.3 ms) of EPSCs evoked from the entorhinal cortex/subiculum border are significantly larger than the same parameters for spontaneous events (paired t-test, P < 0.05, n = 17). Perfusion of 50 microM D(-)-2-amino-5-phosphonopentanoic acid blocked all spontaneous activity and caused a significant baseline current shift of 18.8 +/- 3.0 pA, thus identifying a tonic conductance mediated by NMDA receptors. The NR2B antagonist ifenprodil (10 microM) significantly reduced the frequency of spontaneous events but had no effect on their kinetics or on the baseline current or variance. At the same time, the peak current and charge of stimulus-evoked events were significantly diminished by ifenprodil. Thus spontaneous NMDA receptor-mediated events in DGGC are predominantly mediated by NR2A or possibly NR2A/NR2B receptors while the activation of NR2B receptors reduces the excitability of entorhinal afferents either directly or through an effect on the entorhinal cells.

Excitatory actions of endogenously released GABA contribute to initiation of ictal epileptiform activity in the developing hippocampus

In the developing rat hippocampus, ictal epileptiform activity can be elicited easily in vitro during the first three postnatal weeks. Changes in neuronal ion transport during this time cause the effects of GABA(A) receptor (GABA(A)-R) activation to shift gradually from strongly depolarizing to hyperpolarizing. It is not known whether the depolarizing effects of GABA and the propensity for ictal activity are causally linked. A key question is whether the GABA-mediated depolarization is excitatory, which we defined operationally as being sufficient to trigger action potentials. We assessed the effect of endogenous GABA on ictal activity and neuronal firing rate in hippocampal slices from postnatal day 1 (P1) to P30. In extracellular recordings, there was a strong correlation between the postnatal age at which GABA(A)-R antagonists decreased action potential frequency (P23) and the age at which ictal activity could be induced by elevated potassium (P23). In addition, there was a strong correlation between the fraction of slices in which ictal activity was induced by elevated potassium concentrations and the fractional decrease in action potential firing when GABA(A)-Rs were blocked in the presence of ionotropic glutamate receptor antagonists. Finally, ictal activity induced by elevated potassium was blocked by the GABA(A)-R antagonists bicuculline and SR-95531 (gabazine) and increased in frequency and duration by GABA(A)-R agonists isoguvacine and muscimol. Thus, the propensity of the developing hippocampus for ictal activity is highly correlated with the effect of GABA on action potential probability and reversed by GABA(A) antagonists, indicating that GABA-mediated excitation is causally linked to ictal activity in this developmental window.

Zinc-induced augmentation of excitatory synaptic currents and glutamate receptor responses in hippocampal CA3 neurons

Zinc is found throughout the CNS at synapses co-localized with glutamate in presynaptic terminals. In particular, dentate granule cells' (DGC) mossy fiber (MF) axons contain especially high concentrations of zinc co-localized with glutamate within vesicles. To study possible physiological roles of zinc, visualized slice-patch techniques were used to voltage-clamp rat CA3 pyramidal neurons, and miniature excitatory postsynaptic currents (mEPSCs) were isolated. Bath-applied zinc (200 microM) enhanced median mEPSC peak amplitudes to 153.0% of controls, without affecting mEPSC kinetics. To characterize this augmentation further, rapid agonist application was performed on perisomatic outside-out patches to coapply zinc with glutamate extremely rapidly for brief (1 ms) durations, thereby emulating release kinetics of these substances at excitatory synapses. When zinc was coapplied with glutamate, zinc augmented peak glutamate currents (mean +/- SE, 116.6 +/- 2.8% and 143.8 +/- 9.8% of controls at 50 and 200 microM zinc, respectively). This zinc-induced potentiation was concentration dependent, and pharmacological isolation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated currents (AMPAR currents) gave results similar to those observed with glutamate application (mean, 115.0 +/- 5.4% and 132.5 +/- 9.1% of controls at 50 and 200 microM zinc, respectively). Inclusion of the AMPAR desensitization blocker cyclothiazide in the control solution, however, abolished zinc-induced augmentation of glutamate-evoked currents, suggesting that zinc may potentiate AMPAR currents by inhibiting AMPAR desensitization. Based on the results of the present study, we hypothesize that zinc is a powerful modulator of both excitatory synaptic transmission and glutamate-evoked currents at physiologically relevant concentrations. This modulatory role played by zinc may be a significant factor in enhancing excitatory neurotransmission and could significantly regulate function at the mossy fiber-CA3 synapse.

Stable properties of spontaneous EPSCs and miniature retinal EPSCs during the development of ON/OFF sublamination in the ferret lateral geniculate nucleus

Retinal projections to the lateral geniculate nucleus (LGN) in ferrets progressively segregate into eye-specific laminae and subsequently into sublaminae that receive inputs from either ON-center or OFF-center afferents. To study the development of synaptic efficacy during a period of activity-dependent growth and reorganization in the CNS, we recorded spontaneous EPSCs (sEPSCs) from cells of the LGN during ON/OFF sublamination. We also examined retinal inputs specifically by stimulating the optic tract in the presence of strontium and recording evoked miniature EPSCs (emEPSCs). The rise times, areas, half-widths, and decay times of sEPSCs and emEPSCs and interevent intervals of sEPSCs recorded at the beginning of ON/OFF sublamination were not different from those recorded after its completion. Typically EPSC areas were small (10-20 fC) but varied greatly both within and between neurons. The frequency of sEPSCs was also quite variable, ranging from 0.2 to 5 Hz. sEPSCs were equivalent to miniature EPSCs recorded in the presence of tetrodotoxin, and both sEPSCs and emEPSCs were CNQX-sensitive. No difference was observed between sEPSCs recorded at room temperature and those recorded at 34 degreesC, and strontium could be substituted for calcium with no effect on sEPSC shape. These data argue for a remarkable stability in the components of at least AMPA-mediated synaptic transmission during a period of major synaptic rearrangement in the LGN.

GABAergic input to cholinergic nucleus basalis neurons

The potential influence of GABAergic input to cholinergic basalis neurons was studied in guinea-pig basal forebrain slices. GABA and its agonists were applied to electrophysiologically-identified cholinergic neurons, of which some were labelled with biocytin and confirmed to be choline acetyltransferase-immunoreactive. Immunohistochemistry for glutamate decarboxylase was also performed in some slices and revealed GABAergic varicosities in the vicinity of the biocytin-filled soma and dendrites of electrophysiologically-identified cholinergic cells. From rest (average - 63 mV), the cholinergic cells were depolarized by GABA. The depolarization was associated with a decrease in membrane resistance and diminution in firing. The effect was mimicked by muscimol, the specific agonist for GABA(A) receptors, and not by baclofen, the specific agonist for GABA(B) receptors, which had no discernible effect. The GABA- and muscimol-evoked depolarization and decrease in resistance were found to be postsynaptic since they persisted in the presence of solutions containing either high Mg2+/low Ca2+ or tetrodotoxin. They were confirmed as being mediated by a GABA(A) receptor, since they were antagonized by bicuculline. The reversal potential for the muscimol effect was estimated to be approximately -45 mV, which was -15 mV above the resting membrane potential. Finally, in some cholinergic cells, spontaneous subthreshold depolarizing synaptic potentials (average 5 mV in amplitude), which were rarely associated with action potentials, were recorded and found to persist in the presence of glutamate receptor antagonists but to be eliminated by bicuculline. These results suggest that GABAergic input may be depolarizing, yet predominantly inhibitory to cholinergic basalis neurons.

The pro-convulsant actions of corticotropin-releasing hormone in the hippocampus of infant rats

Whole-cell patch-clamp and extracellular field recordings were obtained from 450-microns-thick brain slices of infant rats (10-13 days postnatal) to determine the actions of corticotropin-releasing hormone on glutamate- and GABA-mediated synaptic transmission in the hippocampus. Synthetic corticotropin-releasing hormone (0.15 microM) reversibly increased the excitability of hippocampal pyramidal cells, as determined by the increase in the amplitude of the CA1 population spikes evoked by stimulation of the Schaffer collateral pathway. This increase in population spike amplitude could be prevented by the corticotropin-releasing hormone receptor antagonist alpha-helical (9-41)-corticotropin-releasing hormone (10 microM). Whole-cell patch-clamp recordings revealed that, in the presence of blockers of fast excitatory and inhibitory synaptic transmission, corticotropin-releasing hormone caused only a small (1-2 mV) depolarization of the resting membrane potential in CA3 pyramidal cells, and it did not significantly alter the input resistance. However, corticotropin-releasing hormone, in addition to decreasing the slow afterhyperpolarization, caused an increase in the number of action potentials per burst evoked by depolarizing current pulses. Corticotropin-releasing hormone did not significantly change the frequency, amplitude or kinetics of miniature excitatory postsynaptic currents. However, it increased the frequency of the spontaneous excitatory postsynaptic currents in CA3 pyramidal cells, without altering their amplitude and single exponential rise and decay time constants. Corticotropin-releasing hormone did not change the amplitude of the pharmacologically isolated (i.e. recorded in the presence of GABAA receptor antagonist bicuculline) excitatory postsynaptic currents in CA3 and CA1 pyramidal cells evoked by stimulation of the mossy fibers and the Schaffer collaterals, respectively. Current-clamp recordings in bicuculline-containing medium showed that, in the presence of corticotropin-releasing hormone, mossy fiber stimulation leads to large, synchronized, polysynaptically-evoked bursts of action potentials in CA3 pyramidal cells. In addition, the peptide caused a small, reversible decrease in the amplitude of the pharmacologically isolated (i.e. recorded in the presence of glutamate receptor antagonists) evoked inhibitory postsynaptic currents in CA3 pyramidal cells, but it did not significantly alter the frequency, amplitude, rise and decay time constants of spontaneous or miniature inhibitory postsynaptic currents. These data demonstrate that corticotropin-releasing hormone, an endogenous neuropeptide whose intracerebroventricular infusion results in seizure activity in immature rats, has diverse effects in the hippocampus which may contribute to epileptogenesis. It is proposed that the net effect of corticotropin-releasing hormone is a preferential amplification of those incoming excitatory signals which are strong enough to reach firing threshold in at least a subpopulation of CA3 cells. These findings suggest that the actions of corticotropin-releasing hormone on neuronal excitability in the immature hippocampus may play a role in human developmental epilepsies.

Synaptic communication among hippocampal interneurons: properties of spontaneous IPSCs in morphologically identified cells

The properties of spontaneous IPSCs (sIPSCs) recorded with whole-cell patch-clamp techniques were investigated in various anatomically identified hippocampal CA1 interneurons and were compared with those recorded in pyramidal cells. Neurons labeled with biocytin or neurobiotin were classified on the basis of their dendritic and axonal arborizations, leading to the identification of previously unknown interneuron types projecting to the dendritic region of pyramidal cells. In most interneurons, the average sIPSCs decayed slower than did those observed in pyramidal cells. The properties of sIPSCs were homogeneous within a given morphologically identified neuron type. Many interneurons had comparable somatic size, location, and dendritic arbor but displayed extremely different axonal projections paralleled by distinct sIPSC properties. Thus, physiological comparisons are only meaningful after the complete morphological identification of the recorded cells. The decay of sIPSCs matched for amplitudes and rise times could vary over 10-fold in a given interneuron, consistent with electrotonic filtering and possibly with different GABAA receptor subunit assemblies present at distinct synapses. Our findings demonstrate an extensive connectivity among hippocampal interneurons through GABAA synapses of various properties that may underlie complex network oscillations at different frequencies.

Spontaneous excitatory currents and kappa-opioid receptor inhibition in dentate gyrus are increased in the rat pilocarpine model of temporal lobe epilepsy

Temporal lobe epilepsy is associated with a characteristic pattern of synaptic reorganization in the hippocampal formation, consisting of neuronal loss and aberrant growth of mossy fiber collaterals into the dentate gyrus inner molecular layer. We have used the rat pilocarpine model of temporal lobe epilepsy to study the functional consequences of mossy fiber sprouting on excitatory activity and kappa-opioid receptor-mediated inhibition. Using the whole cell voltage-clamp technique, we found that abnormal excitatory activity was evident in granule cells of the dentate gyrus from pilocarpine-treated rats. The frequency of spontaneous excitatory postsynaptic currents (EPSCs) was increased greatly in cells from tissue in which significant mossy fiber sprouting had developed. In the presence of bicuculline, giant spontaneous EPSCs, with large amplitudes and long durations, were seen only in association with mossy fiber sprouting. Giant EPSCs also could be evoked by low-intensity stimulation of the perforant path. Mossy fibers release not only excitatory amino acids, but also opioid peptides. kappa-Opioid receptor-mediated inhibition in normal Sprague-Dawley rats was seen only in hippocampal sections from the ventral pole. In pilocarpine-treated rats, however, kappa receptor-mediated effects were seen in both ventral and more dorsal sections. Thus in this model of temporal lobe epilepsy, several types of abnormal excitatory activity were observed, thereby supporting the idea that mossy fiber sprouting leads to recurrent excitatory connections. At the same time, inhibition of excitatory activity by kappa-opioid receptors was increased, perhaps representing an endogenous anticonvulsant mechanism.

Adrenergic modulation of hilar neuron activity and granule cell inhibition in the guinea-pig hippocampal slice

To study the effects of norepinephrine on synaptic inhibition in the dentate gyrus, intracellular recordings were made from hilar neurons in the guinea-pig hippocampal slice. The effects of norepinephrine on hilar neurons were compared with changes in the frequency of spontaneous inhibitory postsynaptic potentials recorded from granule cells. Hilar neurons comprised two electrophysiologically distinct groups: type I hilar neurons displayed a pronounced single spike afterhyperpolarization and little spike frequency accommodation, type II hilar neurons had small afterhyperpolarizations and pronounced spike frequency accommodation. The majority of recordings were from type I hilar neurons which are presumably inhibitory to granule cells. In most instances, effects of norepinephrine (2-10 microM) on hilar neurons could be mimicked by the beta-adrenergic agonist isoproterenol (0.1-1 microM). Isoproterenol induced a slight depolarization, blocked a slow afterhyperpolarization and, in type II neurons, reduced spike frequency accommodation. These effects were associated with an increase in the spontaneous discharge rate and an enhancement of spontaneous excitatory and inhibitory postsynaptic potentials. In accordance, isoproterenol and norepinephrine increased the frequency of inhibitory postsynaptic potentials in granule cells. In the presence of the non-N-methyl-D-aspartate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione and the N-methyl-D-aspartate receptor antagonist CGP 37849, isoproterenol and norepinephrine also increased the frequency of Cl- -dependent inhibitory postsynaptic potentials in granule cells. Under this experimental condition, however, norepinephrine reduced the discharge rate of type I hilar neurons through an effect on alpha-receptors. In the presence of GABAA receptor blockers, norepinephrine increased the frequency of spontaneously occurring K(+)-dependent inhibitory postsynaptic potentials in granule cells. Accordingly, the frequency of burst discharges in type I hilar neurons was increased. We suggest that the discrepancy in the effect of norepinephrine on the discharge rate of presumed inhibitory hilar neurons and the frequency of Cl- -dependent inhibitory postsynaptic potentials in granule cells results from a direct effect of norepinephrine on GABAergic terminals because norepinephrine also enhanced the frequency of tetrodotoxin-resistant inhibitory postsynaptic potentials in granule cells. Thus, the net effect of synaptically released norepinephrine on synaptic inhibition in the dentate gyrus will be determined by opposing actions of alpha- versus beta-receptor stimulation at the synapse on hilar neurons.

Tonic inhibition originates from synapses close to the soma

Central neurons are subject to a tonic barrage of randomly occurring spontaneous inhibitory events (mIP-SCs) resulting from the action potential-independent release of gamma-aminobutyric acid (GABA). Do the terminals making synapses onto somatic versus dendritic sites, which arise from specific populations of interneurons, differ in their ability to generate mIPSCs? We have combined the techniques of whole-cell patch-clamp recording and computational simulation to demonstrate that in granule cells of the dentate gyrus, most of the action potential-independent inhibition taking place as mIPSCs originates from proximal sites. Indeed, removal of the bulk (> 50%) of the dendritic tree did not change the characteristics of mIPSCs. These results are consistent with a functional segregation of GABAergic terminals synapsing at proximal versus distal portions of central neurons. Thus, proximal GABAergic terminals are responsible for tonic inhibition targeted at the soma.

The molecular basis of kindling

Kindling is an experimental model of epilepsy that involves activity-dependent changes in neuronal structure and function. During kindling, pathological changes may occur at several organizational levels of the nervous system, from alterations in gene-expression in individual neurons to the loss of specific neuronal populations and rearrangement of synaptic connectivity resulting from sustained stimulation of major excitatory pathways. This review summarizes recent developments in alterations at single neuronal and molecular levels that may be responsible for kindling epileptogenesis.

Modulation of decay kinetics and frequency of GABAA receptor-mediated spontaneous inhibitory postsynaptic currents in hippocampal neurons

Inhibitory postsynaptic currents mediated by spontaneous activation of GABAA receptors were studied using whole-cell voltage-clamp recordings in granule cells of the adult rat (postnatal day 60+) dentate gyrus in 400-microns-thick coronal half-brain slices maintained at 34-35 degrees C. The average amplitude of spontaneous inhibitory postsynaptic currents remained constant during a given recording period (i.e. no rundown was noted). The spontaneous currents had an average conductance between 200-400 pS, were mediated by Cl- flux through GABAA receptor/channels since they reversed at the Cl- equilibrium potential and were blocked by bicuculline or picrotoxin. Their mono-exponential decay time-constants (range: 4.2-7.2 ms) were prolonged by midazolam and pentobarbital in a dose-dependent manner. The effect of midazolam was reversed by the benzodiazepine receptor antagonist flumazenil (RO 15-1788) which, by itself, had no effect on the decay time-constant. The decay time-constant was also dependent on membrane voltage and on temperature. A 132-mV change in membrane potential produced an e-fold prolongation of the decay while the Q10 (between 22-37 degrees C) of the decay rate was 2.1. Within a given neuron, the frequency of spontaneous GABAergic events was remarkably constant over long time-periods, though the mean frequency among different cells showed large variability. Spontaneous miniature inhibitory postsynaptic currents also persisted under experimental conditions such as the presence of extracellular tetrodotoxin (1 microM), Cd2+ (200 microM) or lowered extracellular Ca2+/elevated Mg2+, which effectively abolished all stimulus-evoked GABAergic neurotransmission. The frequency of tetrodotoxin-resistant miniature events was increased by elevating extracellular K+ concentration and was diminished by the GABAB receptor agonist (-)baclofen only at a dose (50 microM) which was an order of magnitude larger than that required to depress stimulus-evoked responses. These findings are consistent with different mechanisms being responsible for the spontaneous and stimulus-evoked release of GABA from interneuron terminals and also identify pre- and postsynaptic modulatory factors of the endogenous, action-potential-independent, GABAergic neurotransmission as being important determinants of the excitability level of mammalian CNS neurons.