Preparation and maintenance of organotypic slice cultures of CNS tissue

Organotypic slice cultures are the in vitro method of choice for applications requiring long-term survival of the preparation and a high degree of cellular differentiation and organization resembling that of the original tissue. Long-term survival is achieved by culturing slices at the air/liquid interface, either by continuously rotating the preparation (roller-tube cultures) or by culturing them on semiporous membranes (stationary interface cultures). Both culture techniques yield nerve cells which are highly differentiated in terms of their morphological and physiological characteristics. Because most of these cultures are prepared from 1-week-old postnatal animals, in which the cellular and tissue organization is already relatively advanced, the original cytoarchitecture is often remarkably well maintained. Moreover, the presence of a full complement of glial and nerve cells is thought to provide a microenvironment facilitating differentiation of neurons. Slice culture also offers unique advantages for recording from pairs of cells, as a consequence of the high degree of connectivity between nerve cells. Recently, new applications have emerged such as the cultivation of slices from knock-out animals with limited postnatal survival time or alteration of gene expression by viral vectors.

AMPA-receptor activation regulates the diffusion of a membrane marker in parallel with dendritic spine motility in the mouse hippocampus

Dendritic spines are the site of most excitatory connections in the hippocampus. We have investigated the diffusibility of a membrane-bound green fluorescent protein (mGFP) within the inner leaflet of the plasma membrane using Fluorescence Recovery After Photobleaching. In dendritic spines the diffusion of mGFP was significantly retarded relative to the dendritic shaft. In parallel, we have assessed the motility of dendritic spines, and found an inverse correlation between spine motility and the rate of diffusion of mGFP. We then tested the influence of glutamate receptor activation or blockade, and the involvement of the actin cytoskeleton (using latrunculin A) on spine motility and mGFP diffusion. These results show that glutamate receptors regulate the mobility of molecules in the inner leaflet of the plasma membrane through an action upon the actin cytoskeleton, suggesting a novel mechanism for the regulation of postsynaptic receptor density and composition.

Climbing Fibre Responses in Olivo-cerebellar Slice Cultures. II. Dynamics of Cytosolic Calcium in Purkinje Cells

The generation of climbing fibre responses in cerebellar Purkinje cells has been analysed in co-cultured slices of rat cerebellum and inferior olive. Complex spikes were evoked in Purkinje cells by climbing fibre activation or by intrasomatic injection of depolarizing current pulses. Microfluorometric measurements of cytosolic free calcium ([Ca2+]i) by means of intracellularly injected fura-2 combined with intracellular recordings revealed that both types of complex spikes were accompanied by a transient rise in [Ca2+]i, which was most prominent at dendritic locations. Synaptically induced Ca2+ transients were completely and reversibly abolished by 6-cyano-7-nitroquinoxaline-2-3-dione (CNQX, 5 microM), an antagonist of the ionotropic action mediated by non-N-methyl-d-aspartate excitatory amino acid receptors. Ca2+ transients evoked by injections of depolarizing current pulses were not affected by CNQX. These observations indicate that Ca2+ transients induced by climbing fibre activity are generated by voltage-gated Ca2+ channels, which are activated by a CNQX-sensitive synaptic depolarization.

Climbing Fibre Responses in Olivo-cerebellar Slice Cultures. I. Microelectrode Recordings from Purkinje Cells

Cerebellar slices prepared from newborn rats were co-cultured with slices derived from the inferior olive of 4-day-old rats. After several weeks in vitro olivary fibres projecting into the cerebellar tissue could be assessed by anterograde labelling with the fluorescent dye 1,1-dioctadecyl-3,3,3,3'-tetramethylindocarbocyanine perchlorate (Dil). Following electrical field stimulation of the olivary tissue, all-or-nothing complex spikes were generated in Purkinje cells, which closely resembled climbing fibre responses as seen in situ. These responses were completely and reversibly abolished by 6-cyano-7-nitroquinoxaline-2-3-dione (CNQX, 5 microM), an antagonist of non-N-methyl-d-aspartate excitatory amino acid receptors. Wash in of smaller concentrations of CNQX (0.5 - 2 microM) resulted in a graded dose-dependent depression of the climbing fibre-induced postsynaptic potentials and in a consecutive failure of distinct active components of the complex spikes. With climbing fibre synaptic transmission blocked by CNQX, complex spike-like potentials could, however, still be evoked by intrasomatic injection of depolarizing current pulses. Increasing the concentration of Mg2+ in the bathing solution from 0.5 to up to 8 mM depressed regenerative complex-spike components. Olivary stimulation elicited only monophasic postsynaptic potentials in Purkinje cells under these conditions. These observations indicate that voltage-gated conductances which are substantially involved in the generation of the complex spike, are gated by the climbing fibre synaptic depolarization rather than directly by the climbing fibre transmitter.

Reduction of Potassium Conductances Mediated by Metabotropic Glutamate Receptors in Rat CA3 Pyramidal Cells Does Not Require Protein Kinase C or Protein Kinase A

Metabotropic glutamate receptors, unlike ionotropic receptors, exert their actions on ion channels via G-proteins coupled to second messenger systems. In the hippocampus stimulation of metabotropic receptors can lead to decreased potassium channel conductance, decreased accommodation of cell firing and inhibition of the slow calcium-dependent afterhyperpolarizing current (IAHP). Using the single-electrode voltage-clamp technique in hippocampal slice cultures of the rat, the role of protein kinases in mediating these metabotropic glutamate responses was investigated. In the presence of staurosporin, protein kinase C activation by phorbol esters and protein kinase A activation by 8-bromo-cyclic adenosine monophosphate were blocked. Under these conditions, the inhibition of IAHP by 1-amino-cyclopentyl-trans-dicarboxylate (ACPD), a metabotropic agonist, was unchanged, whilst the inward current elicited by ACPD was enhanced. These results demonstrate that, in the hippocampus, metabotropic glutamate responses persist during inhibition of protein kinase A and C activation. Furthermore, these responses are insensitive to pertussis toxin, confirming previous observations.

NMDA receptor activation limits the number of synaptic connections during hippocampal development

Activity-dependent synaptic plasticity triggered by N-methyl-d-aspartate (NMDA) receptor activation is a fundamental property of many glutamatergic synapses and may be critical for the shaping and refinement of the structural and functional properties of neuronal circuits during early postnatal development. Using a combined morphological and electrophysiological approach, we showed that chronic blockade of NMDA receptors in hippocampal slice cultures during the first two weeks of postnatal development leads to a substantial increase in synapse number and results in a more complex dendritic arborization of CA1 pyramidal cells. Thus, the development of excitatory circuitry in the hippocampus is determined by two opposing processes: NMDA receptor-independent synapse formation and NMDA receptor-dependent attenuation of synaptogenesis.

Fast synaptic transmission mediated by P2X receptors in CA3 pyramidal cells of rat hippocampal slice cultures

1. A fast ATP-mediated synaptic current was identified in CA3 pyramidal cells in organotypic hippocampal slice cultures. In the presence of inhibitors for ionotropic glutamate and GABA receptors, extracellular stimulation in the pyramidal cell layer evoked fast synaptic currents that reversed near 0 mV, reflecting an increase in a non-selective cationic conductance. This response was mimicked by focal application of ATP. Antagonists of ionotropic P2X receptors reduced both synaptic and ATP-induced currents. 2. Using a pharmacological approach, the source of synaptically released ATP was determined. Synaptic ATP responses were insensitive to presynaptic blockade of GABAergic transmission between interneurons and CA3 pyramidal cells with the mu-opioid receptor agonist D-Ala(2),MePhe(4),Met(O)(5)-ol-enkephalin (FK33-824), but were blocked by adenosine, which inhibits glutamate release from synaptic terminals in the hippocampus. However, selective inhibition of mossy fibre glutamatergic transmission with the metabotropic glutamate receptor group II agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG IV) did not affect the response. This result points to the associational fibres as the source of the ATP-mediated synaptic response. 3. These results suggest that ATP, coreleased with glutamate, induces a synaptic response in CA3 pyramidal cells that is observed mainly under conditions of synchronous discharge from multiple presynaptic inputs.

Acute decrease in net glutamate uptake during energy deprivation

The extracellular glutamate concentration ([glu](o)) rises during cerebral ischemia, reaching levels capable of inducing delayed neuronal death. The mechanisms underlying this glutamate accumulation remain controversial. We used N-methyl-D-aspartate receptors on CA3 pyramidal neurons as a real-time, on-site, glutamate sensor to identify the source of glutamate release in an in vitro model of ischemia. Using glutamate and L-trans-pyrrolidine-2,4-dicarboxylic acid (tPDC) as substrates and DL-threo-beta-benzyloxyaspartate (TBOA) as an inhibitor of glutamate transporters, we demonstrate that energy deprivation decreases net glutamate uptake within 2-3 min and later promotes reverse glutamate transport. This process accounts for up to 50% of the glutamate accumulation during energy deprivation. Enhanced action potential-independent vesicular release also contributes to the increase in [glu](o), by approximately 50%, but only once glutamate uptake is inhibited. These results indicate that a significant rise in [glu](o) already occurs during the first minutes of energy deprivation and is the consequence of reduced uptake and increased vesicular and nonvesicular release of glutamate.

Heterogeneity of synaptic plasticity at unitary CA3-CA1 and CA3-CA3 connections in rat hippocampal slice cultures

Long-term potentiation (LTP) of unitary EPSPs, generated by pairs of monosynaptically connected CA3 and CA1 pyramidal cells, was compared with LTP of extracellularly evoked, multi-unitary EPSPs in rat hippocampal slice cultures. LTP was induced by repeated, synchronous pairing of low-frequency presynaptic and postsynaptic activity. Three differences were observed. First, LTP of multi-unitary EPSPs displayed two phases: transient (<5 min) and sustained. Potentiation of unitary EPSPs displayed both phases in 42% of experiments; the remainder showed sustained potentiation only. Unitary EPSPs displaying transient-sustained and only sustained potentiation could be recorded from single postsynaptic cells, indicating that excitatory synapses on a given cell are heterogeneous with respect to short-term plasticity. Second, whereas LTP of multi-unitary EPSPs never resulted in greater than twofold increases in amplitude (mean potentiation of 175% of control), maximal LTP of unitary EPSPs was as great as 13-fold (mean potentiation of 250%). Third, LTP could not be induced in 24% of unitary EPSPs. We provide here the first evidence for the coexistence of potentiatable and nonpotentiatable synapses on individual postsynaptic neurons. Thirty-seven percent of connections not displaying LTP exhibited long-term depression (LTD), suggesting that the connections were already maximally potentiated. In the remaining 63% of these pairs, neither LTP nor LTD could be induced, despite the existence of a pharmacologically identified, NMDA receptor-mediated EPSP component. In conclusion, there is considerable heterogeneity in the amplitude and time course of LTP expression at different synaptic connections. A substantial proportion of apparently nonplastic synapses probably accounts for the weaker potentiation displayed by compound EPSPs.

Apamin-sensitive conductance mediates the K(+) current response during chemical ischemia in CA3 pyramidal cells

Pyramidal cells typically respond to ischemia with initial transient hyperpolarization, which may represent a neuroprotective response. To identify the conductance underlying this hyperpolarization in CA3 pyramidal neurons of rat hippocampal organotypic slice cultures, recordings were obtained using the single-electrode voltage-clamp technique. Brief chemical ischemia (2 mM 2-deoxyglucose and 3 mM NaN(3), for 4 min) induced a response mediated by an increase in K(+) conductance. This current was blocked by intracellular application of the Ca(2+) chelator, bis-(o-aminophenoxy)-N,N,N', N'-tetraacetic acid (BAPTA), reduced with low external [Ca(2+)], and inhibited by a selective L-type Ca(2+) channel inhibitor, isradipine, consistent with the activation of a Ca(2+)-dependent K(+) conductance. Experiments with charybdotoxin (10 nM) and tetraethylammonium (TEA; 1 mM), or with the protein kinase C activator, phorbol 12,13-diacetate (PDAc; 3 microM), ruled out an involvement of a large conductance-type or an apamin-insensitive small conductance, respectively. In the presence of apamin (1 microM), however, the outward current was significantly reduced. These results demonstrate that in rat hippocampal CA3 pyramidal neurons an apamin-sensitive Ca(2+)-dependent K(+) conductance is activated in response to brief ischemia generating a pronounced outward current.

G-protein-independent signaling mediated by metabotropic glutamate receptors

Synaptically released glutamate activates ionotropic and metabotropic receptors at central synapses. Metabotropic glutamate receptors (mGluRs) are thought to modulate membrane conductances through transduction cascades involving G proteins. Here we show, in CA3 pyramidal cells from rat hippocampus, that synaptic activation of type 1 mGluRs by mossy fiber stimulation evokes an excitatory postsynaptic response independent of G-protein function, while inhibiting an afterhyperpolarization current through a G-protein-coupled mechanism. Experiments using peptide activators and specific inhibitors identified a Src-family protein tyrosine kinase as a component of the G-protein-independent transduction pathway. These results represent the first functional evidence for a dual signaling mechanism associated with a heptahelical receptor such as mGluR1, in which intracellular transduction involves activation of either G proteins or tyrosine kinases.

Selective glutamate receptor antagonists can induce or prevent axonal sprouting in rat hippocampal slice cultures

After the transection of the Schaffer collateral pathway in hippocampal slice cultures, reactive sprouting is induced in the CA3 area, and eventually synaptic transmission between areas CA1 and CA3 is restored. Using this model, we have studied the role of ionotropic glutamate receptors in the initiation of axonal sprouting and the regeneration of functional synapses. We show that neither reactive sprouting nor functional recovery of synaptic transmission occur in the presence of the non-N-methyl-D-aspartate (NMDA) receptor antagonist 6-nitro-7-sulfamoylbenzoquinoxaline-2,3-dione (CNQX). In contrast, the NMDA receptor antagonists methyl-10, 11-dihydro-5-H-dibenzocyclohepten-5,10-imine (MK-801) or 3-(RS)-2-carboxypiperazine-4-yl)-propyl-1-phosphonic acid (CPP) did not interfere with these processes. Moreover, we observed that the application of NMDA receptor antagonists induced massive axonal sprouting and an increase in the frequency of miniature excitatory postsynaptic currents in unlesioned cultures. Our results thus indicate that NMDA and non-NMDA receptors exert a differential effect on reactive sprouting and the recovery of synaptic transmission after injury in the hippocampus. Activation of non-NMDA receptors appears necessary for these processes to occur, whereas activation of NMDA receptors suppresses growth-associated protein -43 expression and axonal outgrowth.

Activation of intrinsic hippocampal theta oscillations by acetylcholine in rat septo-hippocampal cocultures

1. Oscillatory electro-encephalographic activity at theta frequencies (4-15 Hz) can be recorded from the hippocampus in vivo and depends on intact septal projections. The hypothesis that these oscillations are imposed on the hippocampus by rhythmically active septal inputs was tested using dual intracellular recordings from CA1 and CA3 pyramidal cells in septo-hippocampal cocultures. 2. Septo-hippocampal cocultures displayed spontaneous oscillatory synaptic activity at theta frequencies. In CA3 cells, EPSP/IPSP sequences predominated, whereas only EPSPs were apparent in CA1 cells. Synaptic potentials in CA3 cells preceded those in CA1 cells by 5-10 ms. 3. Oscillatory synaptic activity was blocked in cocultures by the muscarinic antagonist atropine (0.1 microM), facilitated but unchanged in frequency upon application of the acetylcholinesterase inhibitor neostigmine (1 microM), and not seen in hippocampal monocultures. 4. The muscarinic agonist methacholine (5-20 nM) induced oscillatory synaptic activity at 4-15 Hz in hippocampal monocultures, which was identical to that occurring spontaneously in septo-hippocampal cocultures. 5. Synaptic theta activity was observed in cocultures of septal tissue with subdissected hippocampal slices containing area CA3 alone, but not in septo-CA1 cocultures. 6. We conclude that oscillatory synaptic activity at theta frequencies, with similar characteristics to theta activity in vivo, can be generated by the hippocampal network in response to activation of muscarinic receptors by synaptically released acetylcholine from septal afferents. Furthermore, the oscillatory activity is determined by mechanisms intrinsic to the hippocampal circuitry, particularly area CA3. Rhythmic septal input is not required.

Inhibition of uptake unmasks rapid extracellular turnover of glutamate of nonvesicular origin

Maintaining glutamate at low extracellular concentrations in the central nervous system is necessary to protect neurons from excitotoxic injury and to ensure a high signal-to-noise ratio for glutamatergic synaptic transmission. We have used DL-threo-beta-benzyloxyaspartate (TBOA), an inhibitor of glutamate uptake, to determine the role of glutamate transporters in the regulation of extracellular glutamate concentration. By using the N-methyl-D-aspartate receptors of patched CA3 hippocampal neurons as "glutamate sensors," we observed that application of TBOA onto organotypic hippocampal slices led to a rapid increase in extracellular glutamate concentration. This increase was Ca(2+)-independent and was observed in the presence of tetrodotoxin. Moreover, prevention of vesicular glutamate release with clostridial toxins did not affect the accumulation of glutamate when uptake was inhibited. Inhibition of glutamine synthase, however, increased the rate of accumulation of extracellular glutamate, indicating that glial glutamate stores can serve as a source in this process. TBOA blocked synaptically evoked transporter currents in astrocytes without inducing a current mediated by the glutamate transporter. This indicates that this inhibitor is not transportable and does not release glutamate by heteroexchange. These results show that under basal conditions, the activity of glutamate transporters compensates for the continuous, nonvesicular release of glutamate from the intracellular compartment. As a consequence, acute disruption of transporter activity immediately results in significant accumulation of extracellular glutamate.

Recombinant Semliki Forest virus and Sindbis virus efficiently infect neurons in hippocampal slice cultures

Gene transfer into nervous tissue is a powerful tool for the analysis of gene function. By using a rat hippocampal slice culture preparation, we show here that Semliki Forest virus (SFV) and Sindbis virus (SIN) vectors are useful for the effective infection of neurons. The stratum pyramidale and/or the granular cell layer were injected with recombinant virus encoding beta-galactosidase (LacZ) or green fluorescent protein (GFP). By using low concentrations of injected SFV-LacZ or SIN-LacZ, we detected LacZ staining of pyramidal cells, interneurons, and granule cells. About 60% of the infected cells showed clear neuronal morphology; thus, relatively few glial cells expressed the transgene. Expression of GFP from SFV and SIN vectors gave similar results, with an even higher percentage (>90%) of the GFP-positive cells identified as neurons. Infected pyramidal cells were readily recognized in living slices, displaying GFP fluorescence in dendrites of up to fourth order and in dendritic spines. They appeared morphologically normal and viable at 1-5 days postinfection. We conclude that both SFV and SIN vectors efficiently transfer genes into neurons in hippocampal slice cultures. In combination with the GFP reporter, SFV and SIN vectors will allow the physiological examination of identified neurons that have been modified by overexpression or suppression of a specific gene product.

Excitatory synaptic transmission and its modulation by PKC is unchanged in the hippocampus of GAP-43-deficient mice

We compared excitatory synaptic transmission between hippocampal pyramidal cells in dissociated hippocampal cell cultures and in area CA3 of hippocampal slice cultures derived from wild-type mice and mice with a genetic deletion of the presynaptic growth associated protein GAP-43. The basal frequency and amplitude of action potential-dependent and -independent spontaneous excitatory postsynaptic currents were similar in both groups. The probability that any two CA3 pyramidal cells in wild-type or GAP-43 knockout (-/-) slice cultures were synaptically connected was assessed with paired recordings and was not different. Furthermore, unitary synaptic responses were similar in the two genotypes. Bath application of phorbol 12,13-diacetate (0.6-3 microM) elicited a comparable increase in the frequency of miniature excitatory synaptic currents in wild-type and GAP-43 (-/-) cultures. This effect was blocked by the protein kinase C inhibitor, bisindolylmaleimide I (1.2 microM). Finally, 3 microM phorbol 12,13-diacetate potentiated the amplitude of unitary synaptic currents to a comparable extent in wild-type and GAP-43 (-/-) slice cultures. We conclude that GAP-43 is not required for normal excitatory synaptic transmission or the potentiation of presynaptic glutamate release mediated by activation of protein kinase C in the hippocampus.

Miniature synaptic events maintain dendritic spines via AMPA receptor activation

We investigated the influence of synaptically released glutamate on postsynaptic structure by comparing the effects of deafferentation, receptor antagonists and blockers of glutamate release in hippocampal slice cultures. CA1 pyramidal cell spine density and length decreased after transection of Schaffer collaterals and after application of AMPA receptor antagonists or botulinum toxin to unlesioned cultures. Loss of spines induced by lesion or by botulinum toxin was prevented by simultaneous AMPA application. Tetrodotoxin did not affect spine density. Synaptically released glutamate thus exerts a trophic effect on spines by acting at AMPA receptors. We conclude that AMPA receptor activation by spontaneous vesicular glutamate release is sufficient to maintain dendritic spines.

L-Type Ca2+ channels mediate the slow Ca2+-dependent afterhyperpolarization current in rat CA3 pyramidal cells in vitro

Single-electrode voltage-clamp recordings were obtained from CA3 pyramidal cells in rat hippocampal organotypic slice cultures, and the slow Ca2+-dependent K+ current or afterhyperpolarization current (IAHP) was elicited with brief depolarizing voltage jumps. The slow IAHP was suppressed by the selective L-type Ca2+ channel antagonists isradipine (2 microM) or nifedipine (10 microM). In contrast, neither omega-conotoxin MVIIA (1 microM) nor omega-agatoxin IVA (200 nM), N-type and P/Q-type Ca2+ channel antagonists, respectively, attenuated this slow outward current. The slow IAHP was significantly reduced by thapsigargin (10 microM), a Ca2+ ATPase inhibitor that depletes intracellular Ca2+ stores, and by ryanodine (10-100 microM), which blocks Ca2+-induced Ca2+ release from intracellular compartments. At this concentration thapsigargin did not modify high-threshold Ca2+ current, which was, however, blocked by isradipine. Thus, in hippocampal CA3 pyramidal cells, Ca2+ influx through L-type Ca2+ channels is necessary to trigger the slow IAHP. Furthermore, intracellular Ca2+-activated Ca2+ stores represent a critical component in the transduction pathway leading to the generation of the slow IAHP.

Target cell-specific modulation of transmitter release at terminals from a single axon

In the hippocampus, a CA3 pyramidal cell forms excitatory synapses with thousands of other pyramidal cells and inhibitory interneurons. By using sequential paired recordings from three connected cells, we show that the presynaptic properties of CA3 pyramidal cell terminals, belonging to the same axon, differ according to the type of target cell. Activation of presynaptic group III metabotropic glutamate receptors decreases transmitter release only at terminals contacting CA1 interneurons but not CA1 pyramidal cells. Furthermore, terminals contacting distinct target cells show different frequency facilitation. On the basis of these results, we conclude that the pharmacological and physiological properties of presynaptic terminals are determined, at least in part, by the target cells.

Effects of transient oxygen-glucose deprivation on G-proteins and G-protein-coupled receptors in rat CA3 pyramidal cells in vitro

The role of guanosine triphosphate-binding proteins (G-proteins) in the generation of the outward current during transient oxygen-glucose deprivation (OGD) was investigated in CA3 pyramidal cells in rat hippocampal organotypic slice cultures using the single-electrode voltage-clamp technique with KMeSO4-filled microelectrodes. To simulate ischaemia, brief chemical OGD (2 mM 2-deoxyglucose and 3 mM NaN3 for 4-9 min) was used, which induced an outward K+ current associated with an increase in input conductance. OGD failed to induce the outward current under conditions where G-protein function was disrupted by loading cells with guanosine 5'-O-(2-thiodiphosphate) [GDPbetaS] or after prolonged injection of guanosine 5'-O(3-thiotdphosphate) [GTPgammaS]. However, in slices treated with pertussis toxin (PTX), OGD still elicited the outward current, indicating that PTX-insensitive G-proteins are involved. Consistent with this insensitivity to PTX, neither adenosine receptors nor GABA(B) (gamma-aminobutyric acid) receptors, which operate via PTX-sensitive G-proteins, mediate the OGD-induced outward current. When adenosine receptors or GABA(B) receptors were blocked with 1,3-dipropyl-8-psulphophenylxanthine (DPSPX, 5 microM) or CGP 52 432 (10 microM), respectively, the OGD-induced response was not modified. The response also persisted following pretreatment of slice cultures with tetanus toxin to prevent vesicular release of neurotransmitters and neuromodulators from presynaptic terminals. Both PTX-sensitive and PTX-insensitive G-protein-mediated responses were suppressed during OGD. The inward current induced by the metabotropic glutamate receptor agonist 1 S, 3R-1-aminocyclopentane-1,3-dicarboxylate (1S,3R-ACPD) and the outward current elicited by adenosine or baclofen were strongly or completely attenuated. In contrast, the ionotropic alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) response was not affected. These findings suggest that during OGD there is a functional uncoupling of receptors from G-proteins, and a direct receptor-independent activation of PTX-insensitive G-proteins leading to an increase in membrane K+ conductance.

Long-term synaptic plasticity between pairs of individual CA3 pyramidal cells in rat hippocampal slice cultures

1. Long-term potentiation (LTP) and depression (LTD) were investigated at synapses formed by pairs of monosynaptically connected CA3 pyramidal cells in rat hippocampal slice cultures. 2. An N-methyl-D-aspartate (NMDA) receptor-mediated component of the unitary EPSP, elicited at the resting membrane potential in response to single action potentials in an individual CA3 cell, could be isolated pharmacologically. 3. Associative LTP was induced when single presynaptic action potentials were repeatedly paired with 240 ms postsynaptic depolarizing pulses that evoked five to twelve action potentials or with single postsynaptic action potentials evoked near the peak of the unitary EPSP. LTP induction was prevented by an NMDA receptor antagonist. 4. Associative LTD was induced when single presynaptic action potentials were repeatedly elicited with a certain delay after either 240 ms postsynaptic depolarizing pulses or single postsynaptic action potentials. The time window within which presynaptic activity had to occur for LTD induction was dependent on the amount of postsynaptic depolarization. LTD was induced if single pre- and postsynaptic action potentials occurred synchronously. 5. Homosynaptic LTD was induced by 3 Hz tetanization of the presynaptic neuron for 3 min and was blocked by an NMDA receptor antagonist. 6. Depotentiation was produced with stimulation protocols that elicit either homosynaptic or associative LTD. 7. Recurrent excitatory synapses between CA3 cells display associative potentiation and depression. The sign of the change in synaptic strength is a function of the relative timing of pre- and postsynaptic action potentials.

Ca2+ or Sr2+ partially rescues synaptic transmission in hippocampal cultures treated with botulinum toxin A and C, but not tetanus toxin

Botulinum (BoNT/A-G) and tetanus toxins (TeNT) are zinc endopeptidases that cleave proteins associated with presynaptic terminals (SNAP-25, syntaxin, or VAMP/synaptobrevin) and block neurotransmitter release. Treatment of hippocampal slice cultures with BoNT/A, BoNT/C, BoNT/E, or TeNT prevented the occurrence of spontaneous or miniature EPSCs (sEPSCs or mEPSCs) as well as the [Ca2+]o-independent increase in their frequency induced by phorbol ester, 0.5 nM alpha-latrotoxin, or sucrose. [Ca2+]o-independent and -dependent release thus requires that the target proteins of clostridial neurotoxins be uncleaved. In contrast, significant increases in mEPSC frequency were produced in BoNT-treated, but not TeNT-treated, cultures by application of the Ca2+ ionophore ionomycin in the presence of 10 mM [Ca2+]o. The frequency of sEPSCs was increased in BoNT-treated, but not TeNT-treated, cultures by increasing [Ca2+]o from 2.8 to 5-10 mM or by applying 5 mM Sr2+. Large Ca2+ and Sr2+ influxes thus can rescue release after BoNT treatment, albeit less than in control cultures. The nature of the toxin-induced modification of Ca2+-dependent release was assessed by recordings from monosynaptically coupled CA3 cell pairs. The paired-pulse ratio of unitary EPSCs evoked by two presynaptic action potentials in close succession was 0.5 in control cultures, but it was 1.4 and 1.2 in BoNT/A- or BoNT/C-treated cultures when recorded in 10 mM [Ca2+]o. Log-log plots of unitary EPSC amplitude versus [Ca2+]o were shifted toward higher [Ca2+]o in BoNT/A- or BoNT/C-treated cultures, but their slope was unchanged and the maximal EPSC amplitudes were reduced. We conclude that BoNTs reduce the Ca2+ sensitivity of the exocytotic machinery and the number of quanta released.

Organotypic slice cultures: a technique has come of age

Slices of CNS tissue prepared from young rodents can be maintained in culture for many weeks to months. The basic requirements are simple: a stable substratum, culture medium, sufficient oxygenation and incubation at a temperature of about 36 degrees C. Under these conditions, nerve cells continue to differentiate and to develop a tissue organization that closely resembles that observed in situ. Several alternative culturing methods have been developed recently. Slices maintained in stationary culture with the interface method are ideally suited for questions requiring a three-dimensional structure, whereas slices cultured in roller-tubes remain the method of choice for experiments that require optimal optical conditions. In this report, three typical experiments are discussed that illustrate the potential of the slice-culture technique. The first example indicates that, due to their high neuronal connectivity, slice cultures provide a very useful tool for studying the properties of synaptic transmission between monosynaptically coupled cell pairs. The other two studies show how long-term application of substances to slice cultures can be used to examine the consequences of epileptic discharges in vitro, as well as the effects of slowly acting clostridial neurotoxins on synaptic transmission.

Action-potential propagation gated by an axonal I(A)-like K+ conductance in hippocampus

Integration of membrane-potential changes is traditionally reserved for neuronal somatodendritic compartments. Axons are typically considered to transmit reliably the result of this integration, the action potential, to nerve terminals. By recording from pairs of pyramidal cells in hippocampal slice cultures, we show here that the propagation of action potentials to nerve terminals is impaired if presynaptic action potentials are preceded by brief or tonic hyperpolarization. Action-potential propagation fails only when the presynaptic action potential is triggered within the first 15-20ms of a depolarizing step from hyperpolarized potentials; action-potential propagation failures are blocked when presynaptic cells are impaled with electrodes containing 4-aminopyridine, indicating that a fast-inactivating, A-type K+ conductance is involved. Propagation failed between some, but not all, of the postsynaptic cells contacted by a single presynaptic cell, suggesting that the presynaptic action potentials failed at axonal branch points. We conclude that the physiological activation of an I(A)-like potassium conductance can locally block propagation of presynaptic action potentials in axons of the central nervous system. Thus axons do not always behave as simple electrical cables: their capacity to transmit action potentials is determined by a time-dependent integration of recent membrane-potential changes.

Lesion-induced axonal sprouting and hyperexcitability in the hippocampus in vitro: implications for the genesis of posttraumatic epilepsy

The delayed development of recurring seizures is a common consequence of traumatic head injury; the cause of such epilepsy is unknown. We demonstrate here that transection of the mature axons of CA3 pyramidal cells in hippocampal slice cultures leads to the formation by CA3 pyramidal cells of new axon collaterals that are immunoreactive with the growth-associated protein GAP-43. Individual CA3 cell axons had an elevated number of presynaptic boutons 14 days after the lesion, and dual intracellular recordings revealed an increased probability that any two CA3 pyramidal cells were connected by an excitatory synapse. Lesioned cultures were hyperexcitable and synaptic responses often displayed unusual prolonged polysynaptic components. We thus demonstrate that recurrent axon collaterals are newly sprouted by pyramidal cells as a consequence of axonal injury and suggest that this underlies the development of posttraumatic epilepsy.

Organotypic cultures of the rat anterior pituitary: morphology, physiology and cell-to-cell communication

Organotypic cultures, prepared from young rats, were used to investigate the neuroendocrine properties of anterior pituitary cells. Pituitary cells maintained the features of endocrine cells, up to 7 weeks in vitro. Secretory granules could be seen with electron microscopy, and cells contained immunocytochemically detectable levels of adenohypophyseal hormones. Significant levels of prolactin (PRL), growth hormone and luteinizing hormone were present in the culture media after several weeks in vitro and PRL release could be modulated by dopaminergic agonists or forskolin. The electrophysiological properties of pituitary cells were investigated with both intracellular and patch-clamp recordings after 2 to 7 weeks in vitro. Cellular resting membrane potentials were approximately -50 mV, and spontaneous or depolarization-induced action potentials were found in approximately 50% of cells. Records of voltage-dependent outward membrane currents showed that cells expressed functional voltage-gated channels. Cells remained responsive to hypothalamic neuropeptides, as shown by the outward membrane current triggered by thyrotropin-releasing hormone. Intracellularly injected Lucifer Yellow readily diffused between neighboring cells, suggesting the presence of gap junctions. These data confirm the viability of organotypic cultures of the anterior pituitary gland, and demonstrate that the characteristic properties of this excitable endocrine tissue are conserved. This neuroendocrine preparation is suitable for studying the mechanisms regulating cell-to-cell communication under conditions resembling the in vivo tissue organization.

Bidirectional associative plasticity of unitary CA3-CA1 EPSPs in the rat hippocampus in vitro

Associative long-term potentiation (LTP) and depression of compound and unitary CA3-CA excitatory postsynaptic potentials (EPSPs) were investigated in rat hippocampal slice cultures. The induction of LTP with synchronous pairing of synaptic activation and postsynaptic depolarization resulted in an increase in the amplitude of EPSPs to the same absolute level, regardless of whether the input was naive or had been previously depressed by asynchronous pairing of pre- and postsynaptic activity. Saturated LTP of compound and unitary EPSPs was reversed by asynchronous pairing and could be reinduced by synchronous pairing. The likelihood that an action potential in a presynaptic CA3 cell failed to trigger an unitary EPSP in a postsynaptic CA1 cell decreased after induction of associative potentiation and increased after induction of associative depotentiation. These changes in the rate of transmission failures were accompanied by large changes in the amplitude of nonfailure EPSPs. We conclude that the same CA3-CA1 synapses can alternatively undergo associative potentiation and depression, perhaps through opposite changes in a single expression mechanism.

Properties of spontaneous miniature GABAA receptor mediated synaptic currents in area CA3 of rat hippocampal slice cultures

Miniature, gamma-aminobutyric acid A receptor mediated inhibitory postsynaptic currents (mIPSCs) were recorded from CA3 pyramidal cells in hippocampal slice cultures using whole-cell techniques in the presence of tetrodotoxin. The kinetics and amplitudes of the mIPSCs were analyzed with the aim of determining whether subclasses of events arising from distinct populations of presynaptic interneurons could be distinguished. Histograms of mIPSC amplitude, rise time constant, and decay time constant were all positively skewed, but discrete subsets of events could not be distinguished. The positive skew did not appear to result from electrotonic filtering of distal synaptic currents because there was no correlation among mIPSC amplitudes and the kinetic parameters. Analysis of the intervals between mIPSCs indicated that each event occurred independently. The analysis of spontaneous mIPSCs does not provide evidence of the innervation of pyramidal cells by heterogeneous interneurons.

G-protein-mediated desensitization of metabotropic glutamatergic and muscarinic responses in CA3 cells in rat hippocampus

1. Desensitization of a metabotropic response was investigated in CA3 pyramidal neurons in hippocampal slice cultures using the patch-clamp technique. 2. 1S,3R-1-aminocyclopentane-1,3-dicarboxylate (1S,3R-ACPD), an agonist at metabotropic glutamate receptors (mGluRs), and metacholine (MCh), an agonist at muscarinic receptors, induced a cationic current that appears to be activated through a G-protein-independent transduction process, as previously shown. Prolonged or repetitive bath application of agonists led to rapid desensitization of the cationic current with a time constant of approximately 20 s. 3. Complete recovery from desensitization was observed within 6 min. 4. These responses mediated by mGluRs and muscarinic receptors cross-desensitized. 5. Preventing the activation of G-proteins by loading cells with GDP beta S strongly reduced or suppressed desensitization, and resulted in a sustained inward cationic current. When cells were filled with GTP gamma S to irreversibly activate G-proteins, the desensitization process was enhanced such that a first application of agonist caused a markedly reduced response. 6. These results show that a cationic current induced by metabotropic agonists in hippocampal pyramidal cells undergoes apparent desensitization and suggests that this process occurs through a G-protein-mediated inhibition of the underlying membrane conductance.

Either N- or P-type calcium channels mediate GABA release at distinct hippocampal inhibitory synapses

Transmitter release at most central synapses depends on multiple types of calcium channels. Identification of the channels mediating GABA release in hippocampus is complicated by the heterogeneity of interneurons. Unitary IPSPs were recorded from pairs of inhibitory and pyramidal cells in hippocampal slice cultures. The N-type channel antagonist omega-conotoxin MVIIA abolished IPSPs generated by interneurons in st. radiatum, whereas the P/Q-type antagonist omega-agatoxin IVA had no effect. In contrast, omega-agatoxin IVA abolished IPSPs generated by st. lucidum and st. oriens interneurons, but omega-conotoxin MVIIA had no effect. After unitary IPSPs were blocked by toxin, transmission could not be restored by increasing presynaptic calcium entry. The axons of the two types of interneurons terminated within distinct strata of area CA3. Thus, GABA release onto pyramidal cells, unlike glutamate release, is mediated entirely by either N- or P-type calcium channels, depending on the presynaptic cell and the postsynaptic location of the synapse.

Low-threshold Ca2+ currents in dendritic recordings from Purkinje cells in rat cerebellar slice cultures

Voltage-dependent Ca2+ conductances were investigated in Purkinje cells in rat cerebellar slice cultures using the whole-cell and cell-attached configurations of the patch-clamp technique. In the presence of 0.5 mM Ca2+ in the extracellular solution, the inward current activated with a threshold of -55 +/- 1.5 mV and reached a maximal amplitude of 2.3 +/- 0.4 nA at -31 +/- 2 mV. Decay kinetics revealed three distinct components: a fast (24.6 +/- 2 msec time constant), a slow (304 +/- 46 msec time constant), and a nondecaying component. Rundown of the slow and sustained components of the current, or application of antagonists for the P/Q-type Ca2+ channels, allowed isolation of the fast-inactivating Ca2+ current, which had a threshold for activation of -60 mV and reached a maximal amplitude of 0.7 nA at a membrane potential of -33 mV. Both activation and steady-state inactivation of this fast-inactivating Ca2+ current were described with Boltzmann equations, with half-activation and inactivation at -51 mV and -86 mV, respectively. This Ca2+ current was nifedipine-insensitive, but its amplitude was reduced reversibly by bath-application of NiCl2 and amiloride, thus allowing its identification as a T-type Ca2+ current. Channels with a conductance of 7 pS giving rise to a fast T-type ensemble current (insensitive to omega-Aga-IVA) were localized with a high density on the dendritic membrane. Channel activity responsible for the ensemble current sensitive to omega-Aga-IVA was detected with 10 mM Ba2+ as the charge carrier. These channels were distributed with a high density on dendritic membranes and in rare cases were also seen in somatic membrane patches.

1S, 3R-ACPD induces a region of negative slope conductance in the steady-state current-voltage relationship of hippocampal pyramidal cells

Synaptic responses mediated by metabotropic glutamate receptors (mGluRs) display a marked voltage-dependent increase in amplitude when neurons are moderately depolarized beyond membrane potential. We have investigated the basis for this apparent nonlinear behavior by activating mGluRs with 1S, 3R-1-aminocyclopentane-1, 3-dicarboxylate (1S, 3R-ACPD; 10 microM) in CA3 pyramidal cells from rat hippocampal slice cultures with the use of the single-electrode voltage-clamp technique. Under control conditions, cells depolarized from resting potential by 10-20 mV responded with delayed outwardly rectifying currents due to activation of voltage- and Ca(2+)-dependent K+ conductances. In contrast, in the continuous presence of 1S, 3R-ACPD, small depolarizations (10-20 mV) induced a delayed inward current. The steady-state current-voltage relationship for this response displayed a region of negative slope conductance at potentials between -55 and -40 mV. The reversal potential of the corresponding 1S,3R-ACPD-sensitive tail currents (-93.0 +/- 2.2 mV, mean +/- SE) was close to the potassium reversal potential, consistent with an mGluR-mediated suppression of K+ current. When external K+ concentration was increased to 8 mM, there was a positive shift in reversal potential to -76.9 +/- 5.1 mV. The depolarization-induced inward current in the presence of 1S,3R-ACPD was blocked by Ba2+ (1 mM). The response was not dependent on changes in intracellular Ca2+ concentration and was insensitive to bath-applied Cs+ (1 mM), ruling out a contribution of Ca(2+)-dependent currents or the inward rectifier lQ. Furthermore, the effect of 1S,3R-ACPD was not mimicked by inhibiting afterhyperpolarizing current and M current with low-Ca2+ saline (0.5 mM Ca2+, 10 mM Mg2+) containing 10 mM tetraethylammonium chloride. A comparison of the responses induced by 1S,3R-ACPD and N-methyl-D-aspartate showed that both induce an inward current with small depolarizations from resting potential but with different kinetics and Mg2+ sensitivity. These results indicate that the suppression of K+ currents in response to activation of mGluRs is markedly voltage dependent, increasing at depolarized potentials and decreasing at hyperpolarized potentials. The negative slope conductance at membrane voltages positive to resting potential may underlie the amplification of mGluR-mediated responses when the membrane potential approaches action potential threshold.

Calcium-independent actions of alpha-latrotoxin on spontaneous and evoked synaptic transmission in the hippocampus

1. The black widow spider venom component, alpha-latrotoxin (alpha-LTx) (< 0.5 nM), increased the frequency of miniature excitatory postsynaptic currents (mEPSCs) in hippocampal CA3 pyramidal cells 14-fold, without changing their amplitude. 2. This action of alpha-LTx was not affected by application of Ca(2+)-free/ethylene glycol-bis(b-aminoethyl ether)-N,N,N',N'-tetraacetic acid-containing saline, 100 microM Cd2+, or 50 microM Gd3+. The increase in mEPSC frequency was thus not due to an influx of Ca2+ into the axon terminal via voltage-dependent Ca2+ channels or alpha-LTx-induced pores. 3. alpha-LTx did not increase spontaneous release when synaptic transmission had been impaired by botulinum toxin/F. 4. alpha-LTx reduced the amplitude of EPSCs, elicited with stimulation of mossy fibers, without affecting paired-pulse facilitation. 5. The Ca2+ ionophore ionomycin (2-2.5 microM) also enhanced the frequency of mEPSCs, but unlike alpha-LTx, potentiated evoked EPSCs and reduced paired-pulse facilitation. Application of N-methyl-D-aspartate elicited a high frequency of Ca(2+)-dependent, tetrodotoxin-sensitive spontaneous EPSCs, but did not affect evoked EPSC amplitude. Agents that stimulate vesicular release by increasing presynaptic Ca2+ influx thus do not mimic the alpha-LTx-induced depression of evoked EPSCs. 6. We conclude that entry of Ca2+ into presynaptic axon terminals is not responsible for the effects of low concentrations of alpha-LTx on either spontaneous or evoked transmitter release in the hippocampus. 7. Potential presynaptic mechanisms that could mediate the opposing actions of alpha-LTx on spontaneous and evoked transmitter release in the hippocampus (i.e., alpha-LTx-induced ionic pores, depletion of synaptic vesicles, actions on exocytotic proteins) are discussed.

Cooperative interactions in the induction of long-term potentiation and depression of synaptic excitation between hippocampal CA3-CA1 cell pairs in vitro

The requirement for cooperative interactions between multiple synaptic inputs in the induction of long-term potentiation (LTP) and long-term depression (LTD) has been tested at Schaffer collateral synapses with paired recordings from monosynaptically coupled CA3-CA1 cell pairs in rat hippocampal slice cultures. Tetanization of single presynaptic neurons at 50 Hz (repeated 5-7 times for 300-500 ms each) induced only a transient potentiation (< 3 min) of excitatory postsynaptic potentials (EPSPs). Persistent potentiation (> 15 min) was induced only when single presynaptic action potentials were synchronously paired with directly induced postsynaptic depolarizing pulses (repeated 50-100 times). Tetanus-induced potentiation of extracellularly evoked EPSPs lasting > 4 min could only be obtained if the EPSP was > 4 mV. Because unitary EPSP amplitudes average approximately 1 mV, we conclude that high-frequency discharge must occur synchronously] in 4-5 CA3 cells for LTP to be induced in a common postsynaptic CA1 cell. Asynchronous pairing of presynaptic action potentials with postsynaptic depolarizing current pulses (preceding each EPSP by 800 ms) depressed both naive and previously potentiated unitary EPSPs. Likewise, homosynaptic LTD of unitary EPSPs was induced when the presynaptic cell was tetanized at 3 Hz for 3 min, regardless of their amplitude (0.3-3.2 mV). Homosynaptic LTD of extracellularly evoked Schaffer collateral EPSPs < 4 mV could be induced if no inhibitory postsynaptic potential was apparent, but was prevented by eliciting a large inhibitory postsynaptic potential or by injection of hyperpolarizing current in the postsynaptic cell. We conclude that cooperative interactions among multiple excitatory inputs are not required for induction of homosynaptic LTD of unitary EPSPs.

Mechanisms underlying the neuropathological consequences of epileptic activity in the rat hippocampus in vitro

Blockage of gamma-aminobutyric acid (GABA)ergic synaptic transmission in mature hippocampal slice cultures for a period of 3 days with convulsants was shown previously to induce chronic epileptiform activity and to mimic many of the degenerative changes observed in the hippocampi of epileptic humans. The cellular mechanisms underlying the induction of this degeneration were examined in the present study by comparing the effects of GABA blockers with the effects produced by the K+ channel blocker tetraethylammonium (2 mM). Both types of convulsant caused a comparable decrease in the number of Nissl-stained pyramidal cells in areas CA1 and CA3. No significant cell loss was induced by tetraethylammonium when epileptiform discharge was reduced by simultaneous exposure of cultures to tetrodotoxin (0.5 microM) or to the anticonvulsants pentobarbital (50 microM) or tiagabine (50 microM). We conclude that this degeneration was mediated by convulsant-induced epileptiform discharge itself. The hypothesis that N-methyl-d-aspartate (NMDA) receptor-mediated excitotoxicity underlies cell death in this model was tested by applying convulsants together with specific antagonists of glutamate receptors. Whereas coapplication of antagonists of both non-NMDA and NMDA receptors strongly reduced the degeneration induced by the convulsants, application of either class of antagonist alone did not. Application of exogenous NMDA produced potent cell death, and this degeneration was blocked by the NMDA receptor antagonist methyl-10,11-dihydro-5-H-dibenzocyclohepten-5,10-imine (MK-801). Convulsants also induced a loss of dendritic spines that could be partially prevented by NMDA or non-NMDA receptor antagonists. We conclude that NMDA receptor activation is not solely responsible for the neuronal pathology resulting as a consequence of epileptiform discharge.

Distinct modes of channel gating underlie inactivation of somatic K+ current in rat hippocampal pyramidal cells in vitro

1. We have used the cell-attached configuration of the patch-clamp recording method to characterize the biophysical properties of the voltage-gated K+ channel underlying a 4-aminopyridine (4-AP)- and tetraethylammonium (TEA)-sensitive K+ current (IK(AT)) in pyramidal cells of hippocampal slice cultures. 2. The unitary conductance of channels carrying IK(AT) current (KAT channels) was 19.1 +/- 5.1 pS with a physiological K+ gradient (2.7 mM external K+) and 39.0 +/- 3.6 pS with high external K+ (140 mM). The reversal potential changed with the external K+ concentration as expected for a channel with a dominant K+ selectivity. Channel activity was blocked under both conditions by either external application of 4-AP at 100 microM or by including 20 mM TEA in the pipette solution. 3. An analysis of kinetic behaviour showed that open times were distributed as a single exponential. The mean open time (+/- S.D.) was 4.4 +/- 1.4 ms at a voltage 30 mV positive to resting potential and increased with further depolarization to reach a value of 16.2 +/- 7.4 ms at 70 mV positive to the resting potential. At this depolarized potential, we observed bursts of channel openings with a mean burst duration around 100 ms. 4. With repeated depolarizing pulses, response failures of the KAT channel occurred in a non-random manner and were grouped (referred to as mode 0). This mode was associated with a voltage-dependent inactivation process of the channel and was favoured when the opening probability of the channel was reduced by increasing steady-state inactivation or by bath application of 4-AP. This is consistent with the localization of the binding site for 4-AP at or near the inactivation gate of the channel. 5. When KAT channel openings were elicited by 500 ms depolarizing steps, activity was either transient or it persisted throughout the duration of the pulse. These two modes of activity alternated in a random manner or occurred in groups giving rise to transient (time constant, 20-100 ms) or sustained ensemble currents. In the presence of low concentrations of 4-AP (20-40 microM), the transient pattern of activity was more frequently observed. 6. In addition to mode 0, we propose the existence of at least two further gating modes for KAT channels: mode T (transient current) and mode S (sustained current) that underlie the three decaying components of the IK(AT) ensemble current. These gating modes are probably under the control of intracellular factors that remain to be identified.

Somatic voltage-gated potassium currents of rat hippocampal pyramidal cells in organotypic slice cultures

1. The dominant voltage-gated K+ currents in the somatic membrane of CA3 pyramidal cells from hippocampal slice cultures were characterized using the cell-attached configuration of the patch-clamp recording method. The kinetics, the voltage dependence of activation and inactivation, and the pharmacological properties of the current were determined from ensemble averages of large numbers of episodes from multichannel patches. 2. Steady-state analysis revealed that this current was half-inactivated at the resting membrane potential (Vr), and fully inactivated when patches were held 40 mV positive to Vr. Inactivation was removed when patches were hyperpolarized by 50 mV from Vr. Inactivation was well described by the Boltzmann equation with a slope factor of 12.6 mV. Removal of inactivation of the peak outward current could be described by a time-dependent monoexponential function with a time constant of the order of 100 ms. In contrast, the time course of inactivation was very slow: a +40 mV depolarization relative to Vr of several seconds was required for complete inactivation of the total outward current. 3. When steady-state inactivation was removed by hyperpolarization, the outward current activated with a threshold 10 mV positive to Vr and was half-activated at a potential 57 mV positive to Vr. The conductance can be described in terms of a single Boltzmann equation with a slope factor of 13.5 mV. Activation and inactivation properties of the somatic conductance produce a small window current between +10 and +20 mV relative to Vr. 4. The outward current activated in a voltage-dependent manner in less than 10 ms with 500 ms depolarizing steps. A kinetic analysis of its decay revealed at least three components, with the following time constants: a fast (17 ms), a slowly (approximately 150 ms), and a very slowly inactivating component (in the range of seconds). 5. External application of 4-aminopyridine (4-AP) induced a dose-dependent block of the peak outward current with an IC50 of 28 microM. The inhibitory effect of 4-AP saturated at a concentration of 200 microM which blocked 80% of the total current. The slowly and very slowly inactivating components of the current were not observed with 20 mM tetraethylammonium (TEA) in the pipette solution. A fast transient ensemble current (mean decay time constant, 24 ms) persisted in the presence of extracellular TEA in 29% of the patches. 6. In summary, at least two distinct voltage-gated K+ currents were present at the somatic level of hippocampal pyramidal cells. The dominant one, which we named IK(AT), is sensitive to micromolar concentrations of 4-AP and millimolar concentrations of TEA, and contributes three kinetic components to the total outward current. The second is TEA insensitive, and contributes only a fast transient component of outward current probably corresponding to the classic A-type K+ current. Intracellular recordings in CA3 pyramidal cells showed that IK(AT) plays an important role in regulating the duration of the action potential.

Functional characterization and modulation of feedback inhibitory circuits in area CA3 of rat hippocampal slice cultures

Feedback inhibitory circuits were characterized electrophysiologically in the CA3 region of organotypic rat hippocampal cultures. Pyramidal cells were impaled with sharp microelectrodes and brief depolarizing current pulses were injected intracellularly to elicit single action potentials. An inhibitory postsynaptic potential (IPSP) was observed at fixed latency after the action potential in 27% of impaled cells (n = 131). These IPSPs were fully blocked by bicuculline, indicating that they were mediated solely by gamma-aminobutyric acid type A (GABAA) receptors. They were also blocked by 6-cyano-7-nitro-quinoxaline-2, 3-dione but not D-2-amino-5-phosphonovalerate, indicating that non-N-methyl-D-aspartate receptors were necessary and sufficient for activating interposed GABAergic interneurons. Adenosine (0.1-5 microM) increased the percentage of action potentials that were not followed by IPSPs by reducing the probability of glutamatergic activation of the interneurons. In 18 of 21 experiments adenosine also decreased the mean amplitude of successfully elicited IPSPs, indicating that more than one interneuron participated in the feedback inhibition of those pyramidal cells. In three experiments the non-failure IPSP amplitude was not affected by adenosine, suggesting that only one interneuron participated. Repetitive stimulation at 2-4 Hz decreased the amplitude of non-failure feedback IPSPs and usually increased the number of failures of transmission. These effects were transient and insensitive to the GABAB antagonist CGP 35348. We conclude that both the excitation of interneurons and the release of GABA from interneurons are modulated by repetitive stimulation.

Presynaptic inhibition of calcium-dependent and -independent release elicited with ionomycin, gadolinium, and alpha-latrotoxin in the hippocampus

1. Presynaptic inhibition of synaptic transmission in the hippocampus was investigated by comparing the effects of several agonists on miniature excitatory and inhibitory postsynaptic currents (mEPSCs and mIPSCs). 2. The Ca2+ ionophore ionomycin increased the frequency of mEPSCs and mIPSCs but did not affect their amplitude. Ionomycin-induced release required extracellular Ca2+ and was prevented by pretreatment with botulinum neurotoxin serotype F, like evoked synaptic transmission. Unlike evoked transmission, however, this increase did not involve activation of voltage-dependent Ca2+ channels because it was insensitive to Cd2+. 3. Both the lanthanide gadolinium and alpha-latrotoxin produced increases in the frequency of mEPSCs and mIPSCs, but their actions were independent of extracellular Ca2+. 4. Adenosine, the gamma-aminobutyric acid-B (GABAB) receptor agonist baclofen, and a mu-opioid receptor agonist strongly reduced the frequency of synaptic currents triggered by all three secretagogues. 5. We conclude that activation of these presynaptic receptors can reduce high frequencies of vesicular glutamate and GABA release by directly impairing transmitter exocytosis. Presynaptic inhibition of gadolinium- and alpha-latrotoxin-induced release indicates that this impairment occurs without changes in intraterminal Ca2+ homeostasis and when vesicle fusion is rendered Ca2+ independent, respectively. 6. The inhibition of ionomycin-induced release provides additional evidence for a direct, neurotransmitter receptor-mediated modulation of the proteins underlying vesicular docking or fusion as an important component of presynaptic inhibition of evoked synaptic transmission.

Paired-pulse facilitation and depression at unitary synapses in rat hippocampus: quantal fluctuation affects subsequent release

1. Excitatory synaptic transmission between pairs of monosynaptically coupled pyramidal cells was examined in rat hippocampal slice cultures. Action potentials were elicited in single CA3 pyramidal cells impaled with microelectrodes and unitary excitatory postsynaptic currents (EPSCs) were recorded in whole-cell voltage-clamped CA1 or CA3 cells. 2. The amplitude of successive unitary EPSCs in response to single action potentials varied. The amplitude of EPSCs was altered by adenosine or changes in the [Mg2+]/[CA2+] ratio. We conclude that single action potentials triggered the release of multiple quanta of glutamate. 3. When two action potentials were elicited in the presynaptic cell, the amplitude of the second EPSC was inversely related to the amplitude of the first. Paired-pulse facilitation (PPF) was observed when the first EPSC was small, i.e. the second EPSC was larger than the first, whereas paired-pulse depression (PPD) was observed when the first EPSC was large. 4. The number of trials displaying PPD was greater when release probability was increased, and smaller when release probability was decreased. 5. PPD was not postsynaptically mediated because it was unaffected by decreasing ionic flux with 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) or receptor desensitization with aniracetam. 6. PPF was maximal at an interstimulus interval of 70 ms and recovered within 500 ms. Recovery from PPD occurred within 5 s. 7. We propose that multiple release sites are formed by the axon of a CA3 pyramidal cell and a single postsynaptic CA1 or CA3 cell. PPF is observed if the first action potential fails to release transmitter at most release sites. PPD is observed if the first action potential successfully triggers release at most release sites. 8. Our observations of PPF are consistent with the residual calcium hypothesis. We conclude that PPD results from a decrease in quantal content, perhaps due to short-term depletion of readily releasable vesicles.

Presynaptic GABAB receptors modulate IPSPs evoked in neurons of deep cerebellar nuclei in vitro

1. Recording from deep cerebellar nuclei neurons, we investigated the role of presynaptic gamma-aminobutyric acid-B (GABAB) receptors in the modulation of monosynaptic inhibitory postsynaptic potentials (IPSPs) evoked by stimulation of Purkinje cells in rat slice cultures. 2. Bath application of the GABAB receptor agonist, baclofen (10 and 100 microM) induced two effects in cerebellar nuclei neurons: a postsynaptic hyperpolarization of 4.2 +/- 1.7 (SD) mV and a reduction in the amplitude of evoked IPSPs (30 +/- 10%). 3. When the postsynaptic GABAB response was blocked by filling the electrode with cesium methanesulfonate (2 M), or with a solution containing QX 314 (50 mM), bath application of baclofen (10 microM) reversibly depressed the evoked IPSPs by 36.7 +/- 18.7% and 42 +/- 20.3%, respectively. Under these experimental conditions, baclofen (10 microM) also reduced the amplitude of spontaneous IPSPs (10.2 +/- 9.5%) and decreased their frequency by 45.6 +/- 8.8%, suggesting a presynaptic site of action. 4. The presynaptic action of baclofen was not due to activation of receptors on the somata of Purkinje cells: baclofen (100 microM) failed to alter membrane holding current in Purkinje cells, and it had no effect on the rate of spontaneous action-potential discharge in Purkinje cells in the presence of ionotropic glutamate receptor antagonists (6-cyano-7-nitroquinoxaline-2,3-dione, 20 microM; D-2-amino-5-phosphonovalerate, 40 microM). 5. IPSPs could be evoked by extracellular stimulation of the Purkinje cell layer or by direct stimulation of the fiber bundle connecting Purkinje cells to deep cerebellar neurons. In both situations, baclofen (10 microM) reduced the amplitude of evoked IPSPs by 32.7 +/- 8.8% and 31.2 +/- 10.2%, respectively. 6. These results demonstrate that GABAB receptors are present on the terminals of Purkinje cells. Their activation causes a decrease in the amplitude of evoked IPSPs recorded in deep cerebellar nuclei and also reduces the frequency of spontaneous inhibitory events.

A slowly inactivating potassium current in CA3 pyramidal cells of rat hippocampus in vitro

The time- and voltage-dependent properties of a slowly inactivating K+ current were investigated by using the single-electrode current- and voltage-clamp recording technique in CA3 hippocampal cells of organotypic slice cultures. After a period of prolonged hyperpolarization, the onset of action-potential discharge in response to depolarizing current injection was delayed by several seconds. The conductances underlying this delay were identified in voltage-clamp recordings. A biphasically decaying outward current was evoked when the membrane potential was stepped back to -60 mV after a 30 sec period of hyperpolarization. The fast component was identified as the previously described D-current and was blocked by 100 microM 4-aminopyridine (4-AP). The slow component, which we refer to as IK(slow), appeared to be mediated by K+ ions, because its reversal potential shifted in a Nernstian manner with changes in extracellular K+ concentration. It decayed with a time constant of 7.5 sec and required a hyperpolarizing prepulse below -95 mV for 5.5 sec for 50% recovery from inactivation. IK(slow) was found to be voltage-dependent, with 50% activation occurring at -65 mV and 50% steady-state inactivation occurring at -84 mV. It displayed minimal or no sensitivity to the K(+)-channel blockers 4-AP (0.1-5 mM), Cs+ (1 mM), tetraethylammonium (10-50 mM), Ba2+ (1 mM), dendrotoxin-alpha (5-10 microM), charybdotoxin (0.5-2.5 microM), or glibenclamide (5-10 microM) and was not affected by preventing increases in intracellular Ca2+ concentration with Ca2+ chelators. IK(slow) was reduced by activation of metabotropic glutamatergic and cholinergic receptors. In summary, the biophysical characteristics of IK(slow) suggest a role in determining discharge onset after a period of membrane hyperpolarization, and its modulation by G-protein-coupled receptors reveals an additional function for these receptors in the control of cellular excitability.

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

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

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

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

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

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

Activation of a nonselective cationic conductance by metabotropic glutamatergic and muscarinic agonists in CA3 pyramidal neurons of the rat hippocampus

We have characterized a cationic membrane conductance activated by metabotropic glutamatergic and muscarinic cholinergic agonists in CA3 neurons in hippocampal slice cultures using the patch-clamp technique. When the potassium concentration in the superfusing fluid was raised above 5 mM, a biphasic current was observed in cells held at -60 mV in response to stimulation of postsynaptic metabotropic glutamate receptors (mGluRs) with 1S,3R-ACPD (50 microM) or muscarinic receptors with methacholine (MCh, 5 microM). The initial inward component was due to an increase in a cationic membrane conductance as determined by its reversal potential and its sensitivity to changes in extracellular K+ or Na+. The conductance underlying this current displayed no apparent voltage sensitivity over the range -120 to -50 mV. The response was reduced by extracellular application of Ba2+, Cd2+, Mg2+, or TEA, whereas extracellular Cs+ or loading cells with BAPTA or Cs+ did not affect the current. The effects of 1S,3R-ACPD were reversibly inhibited by bath-applied MCPG, an antagonist at mGluRs. Experiments with atropine and pirenzepine indicated that non-M1 muscarinic receptors mediated the MCh-induced current. A decrease in a resting leak potassium conductance (IK,leak) was responsible for the late component of the 1S,3R-ACPD- and MCh-induced response, seen as an outward current in the bathing solution with high K+ concentration. Loading cells with GDP beta S, GTP gamma S, or GTP did not alter the cationic current, while, in the same cells, the reduction in IKleak was abolished or irreversibly activated. Single-channel recordings of cationic channel activity in the cell-attached configuration provided evidence for the requirement of second messengers in coupling these receptors to the cationic channels. The data indicate that in addition to the previously described reduction of IK,leak, IM, and IAHP, both 1S,3R-ACPD and MCh activate a nonselective cationic conductance that is clearly revealed upon elevating external K+ concentration. This current is mediated by activation of metabotropic receptors, although no evidence could be obtained to show an involvement of G-proteins.

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

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

Characterization of synaptic connections between cortex and deep nuclei of the rat cerebellum in vitro

Intracellular recordings were used to characterize the inhibitory synapses formed by Purkinje cells on neurons in the deep cerebellar nuclei of the rat. This work was performed on organotypic cerebellar cultures where functional connections between Purkinje cells and deep cerebellar neurons are formed de novo. After blocking ionotropic excitatory amino acid, and GABAA receptors with 6-cyano-7-nitro-quinoxaline-2,3-dione,D-2-amino-5-phosphonovalerate and bicuculline, respectively, the majority of deep cerebellar neurons fired spontaneously without accommodation. This tonic firing was linearly dependent on membrane potential and was abolished with hyperpolarization. Bath application of muscimol and baclofen reversibly hyperpolarized deep cerebellar nuclei cells. In the presence of excitatory amino acid receptor antagonists, field stimulation within the Purkinje cell layer induced monosynaptic inhibitory potentials in deep cerebellar neurons that were graded and completely blocked by bicuculline. Inhibitory potential amplitudes were not markedly reduced during fast repetitive stimulation of Purkinje cells, and the resulting hyperpolarization was not affected by the competitive GABAB receptor antagonist CGP 35348. A single inhibitory potential temporarily interrupted trains of action potentials induced in deep cerebellar cells by short depolarizing pulses. Trains of five inhibitory postsynaptic potentials, evoked at 20 Hz, induced a hyperpolarization which transiently blocked the spontaneous firing of deep cerebellar cells. The efficiency to block action potential discharges depended on the frequency of evoked inhibitory potentials. Bath application of bicuculline induced burst discharges in the control solution. When the excitatory amino acid receptors were pharmacologically blocked, bicuculline depolarized deep cerebellar neurons inducing sustained action potential discharges. In the presence of tetrodotoxin, bicuculline abolished miniature inhibitory postsynaptic potentials and resulted in a membrane depolarization of deep cerebellar cells. We conclude that deep cerebellar neurons isolated from synaptic inputs display a pacemaker-like activity. Although these neurons possess GABAA and GABAB receptors, we confirm that only GABAA receptors were involved in the generation of inhibitory postsynaptic potentials, even with high frequency stimulation. The amplitude of evoked inhibitory potentials was weakly frequency-dependent, thus allowing a powerful inhibition of the pacemaker-like activity by trains of evoked inhibitory postsynaptic potentials. Additionally, spontaneous and miniature inhibitory potentials control the excitability of deep cerebellar neurons by exerting a continuous hyperpolarizing tone.