Neonatal irradiation prevents the formation of hippocampal mossy fibers and the epileptic action of kainate on rat CA3 pyramidal neurons

1. The effects of unilateral gamma-ray irradiation at birth on the properties of adult CA3 pyramidal neurons have been studied in hippocampal slices. 2. Neonatal gamma-ray irradiation reduced by 80% the number of granule cells and prevented the formation of mossy fiber synapses without reducing the number of CA3 pyramidal cells. The destruction of the mossy fibers was also confirmed with extracellular recordings. 3. Excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) evoked by stimulation of the stratum radiatum had similar properties in nonirradiated and irradiated hippocampi: the EPSP reversed polarity near 0 mV, was reduced in amplitude by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM) and D(-)-2-amino-5-phosphonovalerate (APV, 50 microM); the fast and slow IPSPs reversed at -75 and -100 mV, were blocked by bicuculline (10 microM), and reduced by phaclofen (0.5 mM), respectively. 4. Bath application of kainate (300-500 nM) evoked epileptiform activity in 81.5% of nonirradiated hippocampal CA3 regions and only in 29% of the irradiated CA3 regions. In contrast, bath application of high potassium (7 mM) and bicuculline (10 microM) generated spontaneous and evoked epileptiform activity in both nonirradiated and irradiated CA3 regions. 5. In nonirradiated and irradiated CA3 regions, kainate (200-300 nM) reduced the amplitude of the fast and slow IPSPs, reduced spike accommodation, and increased the duration of the action potential generated by a depolarizing pulse. 6. The postsynaptic responses of CA3 neurons to bath application of glutamatergic agonists were similar in nonirradiated and irradiated hippocampi in terms of amplitude, reversal potential, and pharmacology. 7. It is concluded that the most conspicuous effect of neonatal gamma-ray irradiation is to prevent the epileptic action of kainate. We propose that kainate generates epileptiform activity in the intact CA3 region by activating high-affinity binding sites located on the mossy fiber terminals.     

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

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

Prolonged enhancement and depression of synaptic transmission in CA1 pyramidal neurons induced by transient forebrain ischemia in vivo

Evoked postsynaptic potentials of CA1 pyramidal neurons in rat hippocampus were studied during 48 h after severe ischemic insult using in vivo intracellular recording and staining techniques. Postischemic CA1 neurons displayed one of three distinct response patterns following contralateral commissural stimulation. At early recirculation times (0-12 h) approximately 50% of neurons exhibited, in addition to the initial excitatory postsynaptic potential, a late depolarizing postsynaptic potential lasting for more than 100 ms. Application of dizocilpine maleate reduced the amplitude of late depolarizing postsynaptic potential by 60%. Other CA1 neurons recorded in this interval failed to develop late depolarizing postsynaptic potentials but showed a modest blunting of initial excitatory postsynaptic potentials (non-late depolarizing postsynaptic potential neuron). The proportion of recorded neurons with late depolarizing postsynaptic potential characteristics increased to more than 70% during 13-24 h after reperfusion. Beyond 24 h reperfusion, approximately 20% of CA neurons exhibited very small excitatory postsynaptic potentials even with maximal stimulus intensity. The slope of the initial excitatory postsynaptic potentials in late depolarizing postsynaptic potential neurons increased to approximately 150% of control values up to 12 h after reperfusion indicating a prolonged enhancement of synaptic transmission. In contrast, the slope of the initial excitatory postsynaptic potentials in non-late depolarizing postsynaptic potential neurons decreased to less than 50% of preischemic values up to 24 h after reperfusion indicating a prolonged depression of synaptic transmission. More late depolarizing postsynaptic potential neurons were located in the medial portion of CA1 zone where neurons are more vulnerable to ischemia whereas more non-late depolarizing postsynaptic potential neurons were located in the lateral portion of CA1 zone where neurons are more resistant to ischemia. The result from the present study suggests that late depolarizing postsynaptic potential and small excitatory postsynaptic potential neurons may be irreversibly injured while non-late depolarizing postsynaptic potential neurons may be those that survive the ischemic insult. Alterations of synaptic transmission may be associated with the pathogenesis of postischemic neuronal injury.     

Muscarine receptor activation in the substantia gelatinosa of the spinal trigeminal nucleus of the guinea pig

1. Intracellular recordings were made from slices of guinea pig spinal trigeminal nucleus pars caudalis (SG). 2. Muscarine [0.3-30 microM; half maximally effective concentration (EC50) = 2.9 microM] hyperpolarized 61% of SG neurons. The effect was mimicked by carbachol (0.3-30 microM; EC50 = 3.9 microM) and antagonized by pirenzepine (1 microM). Thirty-four percent of the neurons were depolarized by muscarine and carbachol (1-30 microM: EC50 = 5.7 microM), and the effect was antagonized by pirenzepine (100 nM). 3. In approximately 80% of recordings, muscarine (10-30 microM) evoked repetitive spontaneous inhibitory postsynaptic potentials (IPSPs) that were sensitive to bicuculline (10 microM). 4. Muscarine (1-30 microM; EC50 = 3 microM) decreased the amplitude of the majority of evoked excitatory postsynaptic potentials (EPSPs), and the effect was mimicked by carbachol and antagonized by pirenzepine (100 nM). 5. These results indicate that there are at least three mechanisms by which muscarine inhibits SG neurons: 1) hyperpolarization through activation of non-M1 receptors; 2) activation of gamma-amino-butyric acid-containing interneurons that mediate IPSPs in a subset of neurons; and 3) a decrease in evoked EPSP amplitude. Muscarine can also activate SG neurons via interaction with an M1-type receptor.     

Amplification of tumor immunity by gene transfer of the co-stimulatory 4-1BB ligand: synergy with the CD28 co-stimulatory pathway

The superficial cells of the entorhinal cortex (EC), main input to the hippocampus, receive a serotonergic input from the raphe nuclei and express 5-hydroxytryptamine creatine sulfate complex (5-HT) receptors at high density. With the use of intracellular recordings, we investigated the effects of serotonin on synaptic inhibition of layer II and III neurons of the EC. Serotonin reduced both polysynaptic fast and slow inhibitory postsynaptic potentials (IPSPs) in projection neurons of the superficial EC. Polysynaptic fast and slow IPSPs were depressed by serotonin in a dose-dependent manner (0.1-100 microM). Serotonin in a concentration of 1 microM reduced the amplitudes of polysynaptic fast and slow IPSPs by approximately 40 and 50%, respectively. To identify the subtype of the 5-HT-receptor mediating the effects on polysynaptic IPSPs, we applied various 5-HT-receptor agonists and antagonists. Although the serotonin agonists for the 5-HT1B,2C,3 receptors were ineffective, the effects were mimicked by the 5-HT1A-receptor agonists (8-OH-DPAT, 5-CT) and prevented by the 5-HT1A-receptor antagonist NAN-190. To look at the direct effects of 5-HT on inhibitory interneurons, we elicited monosynaptic IPSPs in the absence of excitatory synaptic transmission. In contrast to the polysynaptic IPSPs, monosynaptic IPSPs were not significantly affected by serotonin. Recordings from putative inhibitory interneurons revealed that their excitatory postsynaptic potentials (EPSPs) were reversibly reduced by serotonin. We conclude that serotonin suppresses polysynaptic inhibition in projection neurons of layers II and III of the EC by depression of EPSPs on inhibitory interneurons via 5-HT1A receptors.     

Postsynaptic spike firing reduces synaptic GABAA responses in hippocampal pyramidal cells

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

Mechanisms underlying the enhancement of excitatory synaptic transmission in basolateral amygdala neurons of the kindling rat

To elucidate the mechanism underlying epileptiform discharges in kindled rats, synaptic responses in kindled basolateral amygdala neurons in vitro were compared with those from control rats by using intracellular and whole cell patch-clamp recordings. In kindled neurons, electrical stimulation of the stria terminalis induced epileptiform discharges. The resting potential, apparent input resistance, current-voltage relationship of the membrane, and the threshold, amplitude, and duration of action potentials in kindled neurons were not different from those in control neurons. The electrical stimulation of stria terminalis elicited excitatory postsynaptic potentials (EPSPs) and DL-2-amino-5-phosphonopentanoic acid (AP5)-sensitive and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)-sensitive excitatory postsynaptic currents (EPSCs). The amplitude of evoked EPSPs and of evoked AP5-sensitive and CNQX-sensitive EPSCs were enhanced markedly, whereas fast and slow inhibitory postsynaptic potentials (IPSPs) induced by electrical stimulation of lateral amygdaloid nucleus were not significantly different. The rise time and the decay time constant of the evoked CNQX-sensitive EPSCs were shortened, whereas the rise time of the evoked AP5-sensitive EPSCs was shortened, but the decay time constants were not significantly different. In both tetrodotoxin (TTX)-containing medium and low Ca2+ and TTX-containing medium, the frequency and amplitude of spontaneous EPSCs were increased in kindled neurons. These increases are presumably due to nearly synchronous multiquantal events resulted from the increased probability of Glu release at the nerve terminals. The rise time of evoked CNQX- and AP5-sensitive EPSCs and the decay time constant of evoked CNQX-sensitive EPSCs were shortened, suggesting that excitatory synapses at the proximal dendrite and/or the soma in kindled neurons may contribute more effectively to generate evoked EPSCs than those at distal dendrites. In conclusion, the increases in the amplitudes of spontaneous and evoked EPSCs and in the frequency of spontaneous EPSCs may contribute to the epileptiform discharges in kindled neurons.     

Transplanted embryonic entorhinal neurons make functional synapses in adult host hippocampus

Grafts of embryonic entorhinal cortex (EC) or non-entorhinal cortex (NEC) were placed into the hippocampus of adult rats with transection of the perforant paths. Graft-host connectivity was investigated at 4-6 months post-transplantation by recording extracellular evoked responses in hippocampal slice preparations. Electrical stimulation of the grafts evoked excitatory postsynaptic potentials (EPSPs) in the outer molecular layer of the dentate gyrus, and the stratum lacunosum moleculare of CA1, CA3, and elicited population spikes in the granule cell layer and the pyramidal cell layer of CA1, but not CA3. While the latencies and the forms of these evoked response were similar to those in matched control slices from the normal animals, the amplitudes were smaller than normal controls. However, in the slices with NEC grafts, no such responses were recorded when stimulus was applied in similar position in the grafts. The findings suggest that grafted entorhinal neurons make viable synaptic connections with the host hippocampus.     

Transient neurophysiological changes in CA3 neurons and dentate granule cells after severe forebrain ischemia in vivo

Transient neurophysiological changes in CA3 neurons and dentate granule cells after severe forebrain ischemia in vivo. J. Neurophysiol. 80: 2860-2869, 1998. The spontaneous activities, evoked synaptic responses, and membrane properties of CA3 pyramidal neurons and dentate granule cells in rat hippocampus were compared before ischemia and 相似文献   

5-Hydroxytryptamine2 receptor facilitates GABAergic neurotransmission in rat hippocampus

5-Hydroxytryptamine (5-HT; serotonin) administration enhances GABAergic synaptic activity recorded in pyramidal neurons of the CA1 region of hippocampus. Previous studies have attributed this effect to the activation of HT-5(3) receptors located on GABAergic interneurons. During unrelated experiments, we noticed that under our recording conditions, 5-HT can still increase GABAergic synaptic activity after the complete blockade of 5-HT3 receptors. This indicated the involvement of an additional 5-HT receptor subtype. Therefore, we reinvestigated the effects of 5-HT on GABAergic synaptic activity recorded in pyramidal cells of the CA1 region. The ability of 5-HT to increase GABAergic synaptic activity in the presence of 5-HT3 receptor blockade was mimicked by the selective 5-HT2 agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane and blocked by the selective 5-HT2 antagonist ketanserin. This indicated that the additional 5-HT receptor belongs to 5-HT2 receptor family. 5-HT2 receptor activation resulted in an increase in the frequency of spontaneous inhibitory postsynaptic currents as well as a shift in their amplitude distribution toward larger sizes. These effects were absent in the presence of tetrodotoxin. We interpret these results to indicate that 5-HT2 receptors activate GABAergic interneurons in the slice, leading to an increase in GABAergic synaptic activity onto pyramidal cells of the CA1 region.     

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

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

Operative GABAergic inhibition in hippocampal CA1 pyramidal neurons in experimental epilepsy

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

A purinergic component of the excitatory postsynaptic current mediated by P2X receptors in the CA1 neurons of the rat hippocampus

The pyramidal neurons in the CA1 area of hippocampal slices from 17- to 19-day-old rats have been investigated by means of patch clamp. Excitatory postsynaptic currents (EPSCs) were elicited by stimulating the Schaffer collateral at a frequency below 0.2 Hz. It was found that inhibition of glutamatergic transmission by 20 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 100 microM 2-amino-5-phosphonovaleric acid (D-APV) left a small component of the EPSC uninhibited. The amplitude of this residual EPSC (rEPSC) comprised 25 +/- 11% of the total EPSC when measured at a holding potential of -50 mV. The rEPSC was blocked by selective P2 blocker pyridoxal phosphate-6-azophenyl-2'-4'-disulphonic acid (PPADS) 10 microM and bath incubation with non-hydrolysable ATP analogues, ATP-gamma-S and alpha, beta-methylene-ATP at 50 and 20 microM, respectively. The rEPSC was dramatically potentiated by external Zn2+ (10 microM). In another series of experiments exogenous ATP was applied to the CA1 neurons in situ. An inward current evoked by ATP was inhibited by PPADS to the same extent as the rEPSC. It is concluded that, depending on membrane voltage, about one-fifth to one-quarter of the EPSC generated by the excitatory synaptic input to the hippocampal CA1 neurons of rat is due to the activity of P2X receptors.     

Dendritic spines as basic functional units of neuronal integration

Most excitatory synaptic connections occur on dendritic spines. Calcium imaging experiments have suggested that spines constitute individual calcium compartments, but recent results have challenged this idea. Using two-photon microscopy to image fluorescence with high resolution in strongly scattering tissue, we measured calcium dynamics in spines from CA1 pyramidal neurons in slices of rat hippocampus. Subthreshold synaptic stimulation and spontaneous synaptic events produced calcium accumulations that were localized to isolated spines, showed stochastic failure, and were abolished by postsynaptic blockers. Single somatic spikes induced fast-peaking calcium accumulation in spines throughout the cell. Pairing of spikes with synaptic stimulation was frequently cooperative, that is, it resulted in supralinear calcium accumulations. We conclude: (1) calcium channels exist in spine heads; (2) action potentials invade the spines; (3) spines are individual calcium compartments; and (4) spines can individually detect the temporal coincidence of pre- and postsynaptic activity, and thus serve as basic functional units of neuronal integration.     

Anoxic depression of excitatory and inhibitory postsynaptic potentials in rat neocortical slices

1. The effects of brief anoxia (4-6 min replacement of O2 by N2) on synaptic potentials evoked from layer IV and/or the white matter were studied in pyramidal neurons of layers II-III from rat neocortical slices. 2. The early and late components of excitatory postsynaptic potentials (EPSPs) showed differential sensitivity to anoxia: within 2 min the late EPSP (lEPSP) disappeared, whereas the amplitude of the early EPSP (eEPSP) decreased by 70% at 5 min of anoxia. Recovery was complete within 4-11 min. 3. Both fast and slow inhibitory postsynaptic potentials (IPSPs) were extremely sensitive to lack of O2 and were abolished earlier than the lEPSP evoked by the same stimulus. As well, recovery of the IPSPs was always more delayed than that of the EPSPs. 4. A transient increase in excitability during early anoxia and/or midrecovery, manifested as enhanced probability of spiking in 25% of neurons, is attributed to the higher sensitivity of IPSPs compared with EPSPs. 5. The anoxic-induced depression of the lEPSP and IPSPs, which are generated close to the soma, is not due to depolarization-induced occlusion; however, occlusion may cause an attenuation of the eEPSP at dendritic sites. 6. The depression of the EPSPs is not a result of a decreased transmembrane Na+ gradient after inactivation of Na-K-adenosine triphosphatase (Na-K-ATPase). Although ouabain induced a depolarization similar to that of anoxia, it did not affect EPSP amplitude.(ABSTRACT TRUNCATED AT 250 WORDS)     

Active summation of excitatory postsynaptic potentials in hippocampal CA3 pyramidal neurons

The manner in which the thousands of synaptic inputs received by a pyramidal neuron are summed is critical both to our understanding of the computations that may be performed by single neurons and of the codes used by neurons to transmit information. Recent work on pyramidal cell dendrites has shown that subthreshold synaptic inputs are modulated by voltage-dependent channels, raising the possibility that summation of synaptic responses is influenced by the active properties of dendrites. Here, we use somatic and dendritic whole-cell recordings to show that pyramidal cells in hippocampal area CA3 sum distal and proximal excitatory postsynaptic potentials sublinearly and actively, that the degree of nonlinearity depends on the magnitude and timing of the excitatory postsynaptic potentials, and that blockade of transient potassium channels linearizes summation. Nonlinear summation of synaptic inputs could have important implications for the computations performed by single neurons and also for the role of the mossy fiber and perforant path inputs to hippocampal area CA3.     

Interleukin-1beta does not increase synaptic inhibition in hippocampal CA3 pyramidal and dentate gyrus granule cells of the rat in vitro

Effects of interleukin-1beta (bath-applied; 500 pM) on rat hippocampal CA3 pyramidal and dentate granule cells were studied using intracellular microelectrode recording in vitro. In both cell types membrane input resistance, resting membrane potential and action potential amplitude remained stable throughout. No change was seen in postsynaptic potentials in granule cells. After blocking excitatory synaptic transmission in CA3 pyramids interleukin-1beta was found to consistently decrease synaptic inhibition by about 30%.     

Electrophysical properties, synaptic transmission and neuromodulation in serotonergic caudal raphe neurons

1. We studied electrophysiological properties, synaptic transmission and modulation by 5-hydroxytryptamine (5-HT) of caudal raphe neurons using whole-cell recording in a neonatal rat brain slice preparation; recorded neurons were identified as serotonergic by post-hoc immunohistochemical detection of tryptophan hydroxylase, the 5-HT-synthesizing enzyme. 2. Serotonergic neurons fired spontaneously (approximately 1 Hz), with maximal steady state firing rates of < 4 Hz. 5-Hydroxytryptamine caused hyperpolarization and cessation of spike activity in these neurons by activating inwardly rectifying K+ conductance via somatodendritic 5-HT1A receptors. 3. Unitary glutamatergic excitatory post-synaptic potentials (EPSP) and currents (EPSC) were evoked in serotonergic neurons by local electrical stimulation. Evoked EPSC were potently inhibited by 5-HT, an effect mediated by presynaptic 5-HT1B receptors. 4. In conclusion, serotonergic caudal raphe neurons are spontaneously active in vitro; they receive prominent glutamatergic synaptic inputs. 5-Hydroxytryptamine regulates serotonergic neuronal activity of the caudal raphe by decreasing spontaneous activity via somatodendritic 5-HT1A receptors and by inhibiting excitatory synaptic transmission onto these neurons via presynaptic 5-HT1B receptors. These local modulatory mechanisms provide multiple levels of feedback autoregulation of serotonergic raphe neurons by 5-HT.     

Synaptic inhibition in the isolated respiratory network of neonatal rats

Gramicidin-perforated patch-clamp recording revealed phasic Cl(-)-mediated hyperpolarizations in respiratory neurons of the brainstem-spinal cord preparation from newborn rats. The in vitro respiratory rhythm persisted after block of gamma-aminobutyric acid (GABA), i.e. GABAA, receptor-mediated inhibitory postsynaptic potentials (IPSPs) with bicuculline and/or glycinergic IPSPs with strychnine. In one class of expiratory neurons, bicuculline unmasked inspiration-related excitatory postsynaptic potentials (EPSPs), leading to spike discharge. Bicuculline also blocked hyperpolarizations and respiratory arrest due to bath-applied muscimol, whereas strychnine antagonized similar responses to glycine. The reversal potential of respiration-related IPSPs and responses to GABA, muscimol or glycine was not affected by CO2/HCO3(-)-free solutions, but shifted from about -65 mV to values more positive than -20 mV upon dialysis of the cells with 144 instead of 4 mM Cl-. Impairment of GABA uptake with nipecotic acid or glycine uptake with sarcosine evoked a bicuculline- or strychnine-sensitive decrease of respiratory frequency which could lead to respiratory arrest. Also, the GABAB receptor agonist baclofen led to reversible suppression of respiratory rhythm. This in vitro apnoea was accompanied by a K+ channel-mediated hyperpolarization (reversal potential -88 mV) of tonic cells, whereas membrane potential of neighbouring respiratory neurons remained almost unaffected. Both baclofen-induced hyperpolarization and respiratory depression were antagonised by 2-OH-saclofen, which did not affect respiration-related IPSPs per se. The results show that synaptic inhibition is not essential for rhythmogenesis in the isolated neonatal respiratory network, although (endogenous) GABA and glycine have a strong modulatory action. Hyperpolarizing IPSPs mediated by GABAA and glycine receptors provide a characteristic pattern of membrane potential oscillations in respiratory neurons, whereas GABAB receptors rather appear to be a feature of non-respiratory neurons, possibly providing excitatory drive to the network.     

cDNA of eight nuclear encoded subunits of NADH:ubiquinone oxidoreductase: human complex I cDNA characterization completed

Brief elevation in postsynaptic calcium in hippocampal CA1 neurons leads to prolonged changes in synaptic strength. The calcium may enter the postsynaptic neuron via different routes, such as voltage-gated calcium channels or glutamate receptor channels of N-methyl-D-aspartate type, and/or be released from intracellular stores. The manner in which the synapse is altered, leading to the expression of an enhanced/depressed synaptic strength, is still unclear. The present study, performed using whole-cell recording from CA1 pyramidal cells of three- to five-week-old guinea-pigs, shows that postsynaptic depolarization alone, allowing for calcium influx through voltage-gated calcium channels, leads to a synaptic potentiation characterized by an altered time-course of the evoked excitatory synaptic response, an unaltered coefficient of variation of that response and a decreased paired-pulse facilitation likely related to a postsynaptic mechanism. These characteristics contrasted with those of long-term potentiation induced via activation of N-methyl-D-aspartate receptor channels, where the time-course was unaltered, the coefficient of variation was decreased and no change in paired-pulse facilitation was observed. Synapses can thus have mechanistically separate, but co-existent, potentiations of synaptic transmission initiated from separate sources for postsynaptic calcium.