Presynaptic GABA(B) receptors inhibit synaptic inputs to rat subthalamic neurons.

Effects of baclofen on synaptic transmission were studied in rat subthalamic neurons using whole-cell patch clamp recording from brain slices. Focal electrical stimulation of the brain slice evoked GABAergic inhibitory postsynaptic currents and glutamatergic excitatory postsynaptic currents. Baclofen reduced the amplitude of evoked inhibitory postsynaptic currents in a concentration-dependent manner with an IC(50) of 0.6+/-0.2 microM. Evoked excitatory postsynaptic currents were also reduced by baclofen concentration-dependently (IC(50) of 1.6+/-0.2 microM), but baclofen was more potent at reducing the GABA(A) receptor inhibitory postsynaptic currents. The GABA(B) receptor antagonist CGP 35348 blocked these inhibitory effects of baclofen on evoked inhibitory and excitatory postsynaptic currents. Baclofen increased the paired-pulse ratios of evoked inhibitory and excitatory postsynaptic currents. Furthermore, baclofen reduced the frequency of spontaneous miniature excitatory postsynaptic currents, but had no effect on their amplitude.These results provide evidence for presence of presynaptic GABA(B) receptors that modulate both GABA and glutamate release from afferent terminals in the subthalamus.     

Presynaptic inhibition of synaptic transmission by adenosine in rat subthalamic nucleus in vitro

Whole-cell patch clamp recordings were made from the subthalamic nucleus in rat brain slice preparations to examine the effect of adenosine on inhibitory and excitatory synaptic transmission. Adenosine reversibly inhibited both GABA-mediated inhibitory and glutamate-mediated excitatory postsynaptic currents. Adenosine at 100 microM reduced the amplitude of inhibitory and excitatory postsynaptic currents by 42+/-5% and 34+/-6%, respectively. Reductions in the amplitude of both inhibitory and excitatory postsynaptic currents were accompanied by increases in paired-pulse ratios. In addition, adenosine decreased the frequency of spontaneous miniature excitatory postsynaptic currents but had no effect on their amplitude. These results are consistent with a presynaptic site of action. The adenosine A(1) receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine completely reversed the adenosine-induced attenuation of inhibitory and excitatory postsynaptic currents, but 8-cyclopentyl-1,3-dipropylxanthine alone had no effect on synaptic currents evoked at 0.1 Hz. However, 8-cyclopentyl-1,3-dipropylxanthine inhibited a time-dependent depression of excitatory postsynaptic currents that was normally observed in response to a 5 Hz train of stimuli, suggesting that endogenous adenosine could be released during higher frequencies of stimulation. These results suggest that adenosine inhibits synaptic release of GABA and glutamate by stimulation of presynaptic A(1) receptors in the subthalamic nucleus.     

Development of the quantal properties of evoked and spontaneous synaptic currents at a brain synapse

In many studies of central synaptic transmission, the quantal properties of miniature synaptic events do not match those derived from synaptic events evoked by action potentials. Here we show that at mossy fiber-granule cell (MF-gc) synapses of mature cerebellum, evoked excitatory postsynaptic currents (EPSCs) are multiquantal, and their amplitudes vary in discrete steps, whereas miniature (m)EPSCs are monoquantal or multiquantal with quantal parameters identical to those of the EPSCs. In contrast, at immature MF-gc synapses, EPSCs are multiquantal, but their amplitudes do not vary in discrete steps, whereas most mEPSCs seem to be monoquantal with a broad and skewed amplitude distribution. The results demonstrate that quantal variance decreases during synaptic development. They also directly confirm the quantal hypothesis of neurotransmission at a mature brain synapse.     

Reciprocal modulation of glutamate and GABA release may underlie the anticonvulsant effect of phenytoin

Although conventional wisdom suggests that the effectiveness of phenytoin as an anticonvulsant is due to blockade of Na+-channels this is unlikely to be it's sole mechanism of action. In the present paper we examined the effects of phenytoin on evoked and spontaneous transmission at excitatory (glutamate) and inhibitory (GABA) synapses, in the rat entorhinal cortex in vitro. Evoked excitatory postsynaptic potentials at glutamate synapses exhibited frequency-dependent enhancement, and phenytoin reduced this enhancement without altering responses evoked at low frequency. In whole-cell patch-clamp recordings the frequency of excitatory postsynaptic currents resulting from the spontaneous release of glutamate was reduced by phenytoin, with no change in amplitude, rise time or decay time. Similar effects were seen on miniature excitatory postsynaptic currents, recorded in the presence of tetrodotoxin. Evoked inhibitory postsynaptic potentials at GABA synapses displayed a frequency-dependent decrease in amplitude. Phenytoin caused a reduction in this decrement without affecting the responses evoked at low frequency. The frequency of spontaneous GABA-mediated inhibitory postsynaptic currents, recorded in whole-cell patch mode, was increased by phenytoin, and this was accompanied by the appearance of much larger amplitude events. The effect of phenytoin on the frequency of inhibitory postsynaptic currents persisted in the presence of tetrodotoxin, but the change in amplitude distribution largely disappeared. These results demonstrate for the first time that phenytoin can cause a simultaneous reduction in synaptic excitation and an increase in inhibition in cortical networks. The shift in balance in favour of inhibition could be a major factor in the anticonvulsant action of phenytoin.     

Reciprocal modulation of glutamate and GABA release may underlie the anticonvulsant effect of phenytoin

Although conventional wisdom suggests that the effectiveness of phenytoin as an anticonvulsant is due to blockade of Na⁺-channels this is unlikely to be it's sole mechanism of action. In the present paper we examined the effects of phenytoin on evoked and spontaneous transmission at excitatory (glutamate) and inhibitory (GABA) synapses, in the rat entorhinal cortex in vitro. Evoked excitatory postsynaptic potentials at glutamate synapses exhibited frequency-dependent enhancement, and phenytoin reduced this enhancement without altering responses evoked at low frequency. In whole-cell patch-clamp recordings the frequency of excitatory postsynaptic currents resulting from the spontaneous release of glutamate was reduced by phenytoin, with no change in amplitude, rise time or decay time. Similar effects were seen on miniature excitatory postsynaptic currents, recorded in the presence of tetrodotoxin. Evoked inhibitory postsynaptic potentials at GABA synapses displayed a frequency-dependent decrease in amplitude. Phenytoin caused a reduction in this decrement without affecting the responses evoked at low frequency. The frequency of spontaneous GABA-mediated inhibitory postsynaptic currents, recorded in whole-cell patch mode, was increased by phenytoin, and this was accompanied by the appearance of much larger amplitude events. The effect of phenytoin on the frequency of inhibitory postsynaptic currents persisted in the presence of tetrodotoxin, but the change in amplitude distribution largely disappeared.These results demonstrate for the first time that phenytoin can cause a simultaneous reduction in synaptic excitation and an increase in inhibition in cortical networks. The shift in balance in favour of inhibition could be a major factor in the anticonvulsant action of phenytoin.     

Regulatory role of the cell adhesion molecule nectin‐1 in GABAergic inhibitory synaptic transmission in the CA3 region of mouse hippocampus

Proper operation of a neural circuit relies on both excitatory and inhibitory synapses. We previously showed that cell adhesion molecules nectin‐1 and nectin‐3 are localized at puncta adherentia junctions of the hippocampal mossy fiber glutamatergic excitatory synapses and that they do not regulate the excitatory synaptic transmission onto the CA3 pyramidal cells. We studied here the roles of these nectins in the GABAergic inhibitory synaptic transmission onto the CA3 pyramidal cells using nectin‐1‐deficient and nectin‐3‐deficient cultured mouse hippocampal slices. In these mutant slices, the amplitudes and frequencies of miniature excitatory postsynaptic currents were indistinguishable from those in the control slices. In the nectin‐1‐deficient slices, but not in the nectin‐3‐deficient slices, however, the amplitude of miniature inhibitory postsynaptic currents (mIPSCs) was larger than that in the control slices, although the frequency of the mIPSCs was not different between these two groups of slices. In the dissociated culture of hippocampal neurons from the nectin‐1‐deficient mice, the amplitude and frequency of mIPSCs were indistinguishable from those in the control neurons. Nectin‐1 was not localized at or near the GABAergic inhibitory synapses. These results indicate that nectin‐1 regulates the neuronal activities in the CA3 region of the hippocampus by suppressing the GABAergic inhibitory synaptic transmission.     

Synaptic activity in chronically injured,epileptogenic sensory-motor neocortex

We recorded spontaneous and evoked synaptic currents in pyramidal neurons of layer V in chronically injured, epileptogenic neocortex to assess changes in the efficacy of excitatory and inhibitory neurotransmission that might promote cortical hyperexcitability. Partial sensory-motor neocortical isolations with intact blood supply ("undercuts") were made in 20 rats on postnatal day 21-25 and examined 2-6 wk later in standard brain slice preparations using whole cell patch-clamp techniques. Age-matched, uninjured naive rats (n = 20) were used as controls. Spontaneous and miniature excitatory and inhibitory postsynaptic currents (s- and mEPSCs; s- and mIPSCs) were recorded using patch-clamp techniques. The average frequency of s- and mEPSCs was significantly higher, while that of s- and mIPSCs was significantly lower in neurons of undercuts versus controls. The increased frequency of excitatory events was due to an increase in both s- and mEPSC frequency, suggesting an increased number of excitatory contacts and/or increased release probability at excitatory terminals. No significant difference was observed in 10-90% rise time of these events. The input-output slopes of fast, short-latency, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid/kainate (AMPA/KA) receptor-mediated components of evoked EPSCs were steeper in undercuts than in controls. The peak amplitude of the AMPA/KA component of EPSCs evoked by supra-threshold stimuli was significantly greater in the partially isolated neocortex. In contrast, the N-methyl-D-aspartate receptor-mediated component of evoked EPSCs was not significantly different in neurons of injured versus control cortex, suggesting that the increased AMPA/KA component was due to postsynaptic alterations. Results support the conclusion that layer V pyramidal neurons receive increased AMPA/KA receptor-mediated excitatory synaptic drive and decreased GABA(A) receptor-mediated inhibition in this chronically injured, epileptogenic cortex. This shift in the balance of excitatory and inhibitory synaptic activation of layer V pyramidal cells toward excitation might be maladaptive and play a critical role in epileptogenesis.     

Brain-derived neurotrophic factor modulates GABAergic synaptic transmission by enhancing presynaptic glutamic acid decarboxylase 65 levels, promoting asynchronous release and reducing the number of activated postsynaptic receptors

Brain-derived neurotrophic factor is known to modulate the function of GABAergic synapses, but the site of brain-derived neurotrophic factor action is still a matter of controversy. This study was aimed at further dissecting the functional alterations produced by brain-derived neurotrophic factor treatment of GABAergic synaptic connections in cultures of the murine superior colliculus. The functional consequences of long-term brain-derived neurotrophic factor treatment were assessed by analysis of unitary evoked and delayed inhibitory postsynaptic currents in response to high frequency stimulation of single axons. It was found that brain-derived neurotrophic factor facilitated the asynchronous release, but had no effect on the probability of evoked release, the size of the readily releasable pool, and the paired-pulse behavior of evoked inhibitory postsynaptic currents. However, the amplitudes of evoked inhibitory postsynaptic currents, delayed inhibitory postsynaptic currents and miniature inhibitory postsynaptic currents were significantly reduced. Non-stationary fluctuation analysis revealed a decrease in the open channel number at the miniature/evoked inhibitory postsynaptic current peak, but no effect on the mean GABA(A) receptor single channel conductance. Quantitative immunocytochemistry uncovered a significant elevation of presynaptic levels of glutamic acid decarboxylase 65. Together, these findings indicate that brain-derived neurotrophic factor treatment induces pre- as well as postsynaptic changes. What effect predominates will depend on the presynaptic activity pattern: at low activation rates brain-derived neurotrophic factor-treated synapses display a pronounced postsynaptic depression, but at high frequencies this depression is fully compensated by an enhancement of asynchronous release.     

Serotonin modulates synaptic transmission in immature rat ventrolateral medulla neurons in vitro.

Patch-clamp recordings in whole-cell configuration were made from ventrolateral medulla neurons of brainstem slices from 8-12-day-old rats. 5-Hydroxytryptamine (3-30 microM) concentration-dependently suppressed excitatory and inhibitory postsynaptic currents evoked by focal stimulation. An augmentation of inhibitory synaptic currents by 5-hydroxytryptamine was noted in a small number of neurons. 5-Hydroxytryptamine depressed synaptic currents with or without causing a significant change in holding currents and membrane conductances; the inward or outward currents induced by exogenously applied glutamate or GABA/glycine were also not significantly changed by 5-hydroxytryptamine. In paired-pulse paradigms designed to evaluate a presynaptic site of action, 5-hydroxytryptamine suppressed synaptic currents but enhanced the paired-pulse facilitation. 5-Hydroxytryptamine reduced the frequency of miniature excitatory postsynaptic currents without significantly affecting the amplitude. 5-Carboxamidotryptamine, 8-hydroxy-2(di-n-propylamino)tetralin, sumatriptan and N-(3-trifluoromethylphenyl)piperazine which exhibit 5-hydroxytryptamine1 receptor agonist activity, depressed synaptic currents with different potencies, with 5-carboxamidotryptamine being the most potent. The non-selective 5-hydroxytryptamine1 receptor antagonist pindolol attenuated the presynaptic effect of 5-hydroxytryptamine, whereas the 5-hydroxytryptamine1A antagonist pindobind-5-hydroxytryptamine1A and 5-hydroxytryptamine2 receptor antagonist ketanserin were ineffective. Our results indicate that 5-hydroxytryptamine suppressed synaptic transmission in ventrolateral medulla neurons by activating presynaptic 5-hydroxytryptamine1 receptors, probably the 5-hydroxytryptamine1B/5-hydroxytryptamine1D subtype. In addition, 5-hydroxytryptamine augmented inhibitory synaptic currents in a small number of neurons the site and mechanism of this potentiating action are not known.     

Differential modulation of nucleus accumbens synapses

The nucleus accumbens (NAcc) is a brain region involved in functions ranging from motivation and reward to feeding and drug addiction. The NAcc is typically divided into two major subdivisions, the shell and the core. The primary output neurons of both of these areas are medium spiny neurons (MSNs), which are quiescent at rest and depend on the relative input of excitatory and inhibitory synapses to determine when they fire action potentials. These synaptic inputs are, in turn, regulated by a number of neurochemical signaling agents that can ultimately influence information processing in the NAcc. The present study characterized the ability of three major signaling pathways to modulate synaptic transmission in NAcc MSNs and compared this modulation across different synapses within the NAcc. The opioid [Met](5)enkephalin (ME) inhibited excitatory postsynaptic currents (EPSCs) in shell MSNs, an effect mediated primarily by micro-opioid receptors. Forskolin, an activator of adenylyl cyclase, potentiated shell EPSCs. An analysis of miniature EPSCs indicated a primarily presynaptic site of action, although a smaller postsynaptic effect may have also contributed to the potentiation. Adenosine and an adenosine A(1)-receptor agonist inhibited shell EPSCs, although no significant tonic inhibition by endogenous adenosine was detected. The effects of these signaling agents were then compared across four different synapses in the NAcc: glutamatergic EPSCs and GABAergic inhibitory postsynaptic currents (IPSCs) in both the core and shell subregions. ME inhibited all four of these synapses but produced a significantly greater inhibition of shell IPSCs than the other synapses. Forskolin produced an increase in transmission at each of the synapses tested. However, analysis of miniature IPSCs in the shell showed no sign of a postsynaptic contribution to this potentiation, in contrast to the shell miniature EPSCs. Tonic inhibition of synaptic currents by endogenous adenosine, which was not observed in shell EPSCs, was clearly present at the other three synapses tested. These results indicate that neuromodulation can vary between the different subregions of the NAcc and between the different synapses within each subregion. This may reflect differences in neuronal interconnections and functional roles between subregions and may contribute to the effects of drugs acting on these systems.     

Miniature postsynaptic currents recorded from identified rat spinal dorsal horn projection neurons in thin-slice preparations.

Whole-cell voltage-clamp recordings were made from spinothalamic and spinomesencephalic tract neurons in thin-slice preparations of rat spinal cord. In the presence of tetrodotoxin, spontaneous inward and outward postsynaptic currents were observed near the resting membrane potential. These currents were divided into miniature excitatory postsynaptic currents (mEPSCs) mediated by glutamate, and miniature inhibitory postsynaptic currents (mIPSCs) mediated by glycine or gamma-aminobutyric acid (GABA). Glutamatergic mEPSCs had two components mediated by NMDA and non-NMDA receptors. Analyzing these miniature synaptic currents, valuable information concerning the pre- and postsynaptic mechanisms underlying modulation of synaptic transmission in the spinal dorsal horn could be obtained.     

Possibility of multiquantal transmission at single inhibitory synapse in cultured rat hippocampal neurons.

Miniature, spontaneous and evoked inhibitory postsynaptic currents were studied using the whole-cell patch-clamp technique on synaptically connected cultured hippocampal neurons, at a holding potential of -75 mV. All experiments were done in tetrodotoxin-containing solution to exclude an action potential generation. Spontaneous miniature inhibitory postsynaptic currents were observed in Ca2+-free solution. The distribution of miniature inhibitory postsynaptic currents was skewed to larger current amplitudes and could be fitted reliably by one Gaussian with the mean at 10.0 +/- 1.2 pA (n = 7). Spontaneously occurring whole-cell spontaneous inhibitory postsynaptic currents were recorded in physiological solution (Ca2+ 2 mM). The average amplitude of spontaneously occurring currents depended on membrane potential and reversed at -18 +/- 5 mV (n = 5). The amplitude distribution of spontaneous inhibitory postsynaptic currents had one peak clearly detectable with the mean of 20.0 +/- 2.0 pA (n = 6) or 10.0 +/- 2.0 pA (n = 2). Inhibitory postsynaptic stimulus-evoked currents arose in responses to gradual activation of neurotransmitter release by direct extracellular electrical stimulation of a single presynaptic bouton by short depolarizing pulses. The current-voltage relation of the averaged amplitudes of evoked inhibitory postsynaptic currents was linear and reversed at potential predicted by the Nernst equation for corresponding intra- and extracellular Cl- concentrations. The time-course of decay of miniature, spontaneous and evoked inhibitory postsynaptic currents was fitted by a sum of two exponents and their time-constants were the same in the range of standard deviation. The stimulus-evoked inhibitory postsynaptic currents fluctuated with regard to the discrete aliquot values of their peak amplitudes in all the investigated synapses from a measurable minimum of about 8 pA to 200 pA. The evoked inhibitory postsynaptic currents were assumed as superimposition of statistically independent quantal events. Fitting amplitude histograms of evoked inhibitory postsynaptic currents with several Gaussian curves resulted in peaks that were equidistant with the mean space of 20 +/- 3 pA (n = 10), which was assumed as one quantum (quantum size) to construct the Poisson's distribution. A possibility of simultaneous multiquantal release at single inhibitory synapses of rat hippocampal neurons was discussed.     

Differences in glycinergic mIPSCs in the auditory brain stem of normal and congenitally deaf neonatal mice

We have investigated the fundamental properties of central auditory glycinergic synapses in early postnatal development in normal and congenitally deaf (dn/dn) mice. Glycinergic miniature inhibitory postsynaptic currents (mIPSCs) were recorded using patch-clamp methods in neurons from a brain slice preparation of the medial nucleus of the trapezoid body (MNTB), at 12-14 days postnatal age. Our results show a number of significant differences between normal and deaf mice. The frequency of mIPSCs is greater (50%) in deaf versus normal mice. Mean mIPSC amplitude is smaller in deaf mice than in normal mice (mean mIPSC amplitude: deaf, 64 pA; normal, 106 pA). Peak-scaled fluctuation analysis of mIPSCs showed that mean single channel conductance is greater in the deaf mice (deaf, 64 pS; normal, 45 pS). The mean decay time course of mIPSCs is slower in MNTB neurons from deaf mice (mean half-width: deaf, 2.9 ms; normal, 2.3 ms). Light- and electron-microscopic immunolabeling results showed that MNTB neurons from deaf mice have more (30%) inhibitory synaptic sites (postsynaptic gephyrin clusters) than MNTB neurons in normal mice. Our results demonstrate substantial differences in glycinergic transmission in normal and congenitally deaf mice, supporting a role for activity during development in regulating both synaptic structure (connectivity) and the fundamental (quantal) properties of mIPSCs at central glycinergic synapses.     

Frequency-selective augmenting responses by short-term synaptic depression in cat neocortex

Thalamic stimulation at frequencies between 5 and 15 Hz elicits incremental or 'augmenting' cortical responses. Augmenting responses can also be evoked in cortical slices and isolated cortical slabs in vivo . Here we show that a realistic network model of cortical pyramidal cells and interneurones including short-term plasticity of inhibitory and excitatory synapses replicates the main features of augmenting responses as obtained in isolated slabs in vivo . Repetitive stimulation of synaptic inputs at frequencies around 10 Hz produced postsynaptic potentials that grew in size and carried an increasing number of action potentials resulting from the depression of inhibitory synaptic currents. Frequency selectivity was obtained through the relatively weak depression of inhibitory synapses at low frequencies, and strong depression of excitatory synapses together with activation of a calcium-activated potassium current at high frequencies. This network resonance is a consequence of short-term synaptic plasticity in a network of neurones without intrinsic resonances. These results suggest that short-term plasticity of cortical synapses could shape the dynamics of synchronized oscillations in the brain.     

Differences between the scaling of miniature IPSCs and EPSCs recorded in the dendrites of CA1 mouse pyramidal neurons

Anatomical studies have described inhibitory synaptic contacts on apical dendrites, and an abundant number of GABAergic synapses on the somata and proximal dendrites of CA1 pyramidal cells of the hippocampus. The number of inhibitory contacts decreases dramatically with distance from the soma, but the local electrophysiological characterization of these synapses at their site of origin in the dendrites is missing. We directly recorded dendritic GABA receptor-mediated inhibitory synaptic events in adult mouse hippocampal CA1 pyramidal neurons and compared them to excitatory synaptic currents recorded at the same sites. Miniature GABAergic events were evoked using localized application of a hyperosmotic solution to the apical dendrites in the vicinity of the dendritic whole-cell recording pipette. Glutamatergic synaptic events were blocked by kynurenic acid, leaving picrotoxin-sensitive IPSCs. We measured the amplitude and kinetic properties of mIPSCs at the soma and at three different dendritic locations. The amplitude of mIPSCs recorded at the various sites was similar along the somato-dendritic axis. The rise- and decay-times of local mIPSCs were also independent of the location of the synapses. The frequency of mIPSCs was 5 Hz at the soma, in contrast to < 0.5 Hz at dendritic sites, which could be increased to 10–20 Hz and 6–10 Hz, respectively, by our hyperosmotic stimulation protocol. Miniature glutamatergic events were evoked with the same protocol after blocking inhibitory synapses by bicucculine. The measured amplitudes increased along the somato-dendritic axis proportionally with their distance from the soma. The measured kinetic properties were independent of location. Consistent with the idea that IPSCs may have a restricted local effect in the dendrites, our data show a lack of distance-dependent scaling of miniature inhibitory synaptic events, in contrast to the scaling of excitatory events recorded at the same sites.     

Role of protein kinase C in modulation of excitability of CA1 pyramidal neurons in the rat

Biochemical and in situ hybridization studies demonstrated that the levels of protein kinase C variants were significantly increased in the hippocampus of the experimental models of epilepsy in rats. In addition it has been demonstrated that protein kinase C plays an important role in modulating synaptic transmission in the hippocampus. We examined the effects of activating of protein kinase C on the excitability of CA1 pyramidal neurons and synaptic transmission, using whole-cell current-clamp and extracellular field potential recording techniques. Indolactam V (1 microM) a novel protein kinase C activator, increased the excitability of CA1 neurons acting at both pre- and post-synaptic sites. Indolactam V, acting postsynaptically, significantly reduced the threshold for initiation of action potential from -42+/-3.8 mV to -51+/-3.1 mV and selectively inhibited the slow afterhyperpolarizing potential. Indolactam V also altered the neuronal firing properties in response to prolonged depolarizing pulse by eliminating the spike frequency accommodation. Our data indicate that indolactam V potentiated both amplitudes of Shaffer-collateral stimulation evoked excitatory postsynaptic currents and disynaptically evoked inhibitory evoked postsynaptic currents. However, the potentiation of inhibitory evoked postsynaptic currents amplitudes was not observed after blockade of NMDA and AMPA/kainate currents suggesting it was due to excitatory activity driving inhibitory neurons. The results indicate that the potentiation of pharmacologically isolated excitatory postsynaptic currents (215% of control) and amplitudes of population spikes (290% of control) was due to action of indolactam V presynaptically since the agonist reduced the paired-pulse ratio and the potentiating effect was not blocked by dialyzing the postsynaptic neuron through the recording electrode with a specific protein kinase C inactivator calphostin C. These findings suggest that protein kinase C increases the amplitude of epileptiform activity by causing potentiation of excitatory synaptic transmission, increasing the excitability of postsynaptic neurons and reducing negative feed back provided by slow afterhyperpolarizing potential.     

Postsynaptic activation of calcium/calmodulin-dependent protein kinase II promotes coordinated pre- and postsynaptic maturation of Drosophila neuromuscular junctions

The interaction between a neuron and its target cell(s) is essential for the development of synapses. To elucidate the role of target cells in synaptogenesis, the activity of postsynaptic calcium/calmodulin-dependent protein kinase II (CaMKII) was manipulated in a mosaic manner and its specific effect was examined at the developing Drosophila neuromuscular junction. We found that postsynaptic expression of constitutively active CaMKII augmented the amplitude of excitatory synaptic currents (ESCs) and the frequency of miniature ESCs. It also promoted morphological maturation of presynaptic as well as postsynaptic specializations, which presumably underlie the enhancement of synaptic activities. Expression of an inhibitory peptide of CaMKII in the postsynaptic cell partially affected the synaptic maturation. These results suggest two significant functions of postsynaptic CaMKII in synaptogenesis-retrograde modulation of presynaptic properties and coordinated regulation of pre- and postsynaptic maturation.     

Differences in amplitude-voltage relations between minimal and composite mossy fibre responses of rat CA3 hippocampal neurons support the existence of intrasynaptic ephaptic feedback in large synapses

Computer simulations and electrophysiological experiments have been performed to test the hypothesis on the existence of an ephaptic interaction in purely chemical synapses. According to this hypothesis, the excitatory postsynaptic current would depolarize the presynaptic release site and further increase transmitter release, thus creating an intrasynaptic positive feedback. For synapses with the ephaptic feedback, computer simulations predicted non-linear amplitude-voltage relations and voltage dependence of paired-pulse facilitation. The deviation from linearity depended on the strength of the feedback determined by the value of the synaptic cleft resistance. The simulations showed that, in the presence of the intrasynaptic feedback, recruitment of imperfectly clamped synapses and synapses with linear amplitude-voltage relations tended to reduce the non-linearity and voltage dependence of paired-pulse facilitation. Therefore, the simulations predicted that the intrasynaptic feedback would particularly affect small excitatory postsynaptic currents induced by activation of electrotonically close synapses with long synaptic clefts. In electrophysiological experiments performed on hippocampal slices, the whole-cell configuration of the patch-clamp technique was used to record excitatory postsynaptic currents evoked in CA3 pyramidal cells by activation of large mossy fibre synapses. In accordance with the simulation results, minimal excitatory postsynaptic currents exhibited "supralinear" amplitude-voltage relations at hyperpolarized membrane potentials, decreases in the failure rate and voltage-dependent paired-pulse facilitation. Composite excitatory postsynaptic currents evoked by activation of a large amount of presynaptic fibres typically bear linear amplitude-voltage relationships and voltage-independent paired-pulse facilitation. These data are consistent with the hypothesis on a strong ephaptic feedback in large mossy fibre synapses. The feedback would provide a mechanism whereby signals from large synapses would be amplified. The ephaptic feedback would be more effective on synapses activated in isolation or together with electrotonically remote inputs. During synchronous activation of a large number of neighbouring inputs, suppression of the positive intrasynaptic feedback would prevent abnormal boosting of potent signals.     

Mechanisms underlying rule learning-induced enhancement of excitatory and inhibitory synaptic transmission

Training rats to perform rapidly and efficiently in an olfactory discrimination task results in robust enhancement of excitatory and inhibitory synaptic connectivity in the rat piriform cortex, which is maintained for days after training. To explore the mechanisms by which such synaptic enhancement occurs, we recorded spontaneous miniature excitatory and inhibitory synaptic events in identified piriform cortex neurons from odor-trained, pseudo-trained, and naive rats. We show that olfactory discrimination learning induces profound enhancement in the averaged amplitude of AMPA receptor-mediated miniature synaptic events in piriform cortex pyramidal neurons. Such physiological modifications are apparent at least 4 days after learning completion and outlast learning-induced modifications in the number of spines on these neurons. Also, the averaged amplitude of GABA(A) receptor-mediated miniature inhibitory synaptic events was significantly enhanced following odor discrimination training. For both excitatory and inhibitory transmission, an increase in miniature postsynaptic current amplitude was evident in most of the recorded neurons; however, some neurons showed an exceptionally great increase in the amplitude of miniature events. For both excitatory and inhibitory transmission, the frequency of spontaneous synaptic events was not modified after learning. These results suggest that olfactory discrimination learning-induced enhancement of synaptic transmission in cortical neurons is mediated by postsynaptic modulation of AMPA receptor-dependent currents and balanced by long-lasting modulation of postsynaptic GABA(A) receptor-mediated currents.     

Excitatory and inhibitory postsynaptic currents in a rat model of epileptogenic microgyria

Developmental cortical malformations are common in patients with intractable epilepsy; however, mechanisms contributing to this epileptogenesis are currently poorly understood. We previously characterized hyperexcitability in a rat model that mimics the histopathology of human 4-layered microgyria. Here we examined inhibitory and excitatory postsynaptic currents in this model to identify functional alterations that might contribute to epileptogenesis associated with microgyria. We recorded isolated whole cell excitatory postsynaptic currents and GABA(A) receptor-mediated inhibitory currents (EPSCs and IPSCs) from layer V pyramidal neurons in the region previously shown to be epileptogenic (paramicrogyral area) and in homotopic control cortex. Epileptiform-like activity could be evoked in 60% of paramicrogyral (PMG) cells by local stimulation. The peak conductance of both spontaneous and evoked IPSCs was significantly larger in all PMG cells compared with controls. This difference in amplitude was not present after blockade of ionotropic glutamatergic currents or for miniature (m)IPSCs, suggesting that it was due to the excitatory afferent activity driving inhibitory neurons. This conclusion was supported by the finding that glutamate receptor antagonist application resulted in a significantly greater reduction in spontaneous IPSC frequency in one PMG cell group (PMG(E)) compared with control cells. The frequency of both spontaneous and miniature EPSCs was significantly greater in all PMG cells, suggesting that pyramidal neurons adjacent to a microgyrus receive more excitatory input than do those in control cortex. These findings suggest that there is an increase in numbers of functional excitatory synapses on both interneurons and pyramidal cells in the PMG cortex perhaps due to hyperinnervation by cortical afferents originally destined for the microgyrus proper.