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.

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.

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.

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.

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

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

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.

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)

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.

Synaptic origin and stimulus dependency of neuronal oscillatory activity in the primary visual cortex of the cat

1. We have studied the oscillatory activity of single neurons (91 recorded extracellularly and 76 intracellularly) in the primary visual cortex of cats and kittens to characterize its origins and its stimulus dependency. A new method for the detection of oscillations was developed in order to maximize the range of detectable frequencies in both types of recordings. Three types of activity were examined: spontaneous background activity, responses to intracellular current steps and visual responses. 2. During spontaneous activity, persistent oscillatory activity was very rare in both types of recordings. However, when intracellular records were made using KCl-filled micropipettes, spontaneous activity appeared rhythmic and contained repeated depolarizing events at a variety of frequencies, suggestive of tonic periodic inhibitory input normally masked at resting potential. 3. Patterns of firing activity in response to intracellular current steps allowed us to classify neurons as regular spiking, intrinsically bursting, and fast-spiking types, as described in vitro. In the case of rhythmically firing cells, the spike frequency increased with the amount of injected current. Subthreshold current-induced oscillations were rarely observed (2 out of 76 cells). 4. Visual stimulation elicited oscillations in one-third of the neurons (55 out of 167), predominantly in the 7-20 Hz frequency range in 93% of the cases. Rhythmicity was observed in both simple and complex cells, and appeared to be more prominent at 5 and 6 weeks of age. 5. Intracellular recordings in bridge mode and voltage clamp revealed that visually evoked oscillations were driven by synaptic activity and did not depend primarily on the intrinsic properties of recorded neurons. Hyperpolarizing the membrane led to an increase in the size of the rhythmic depolarizing events without a change in frequency. In voltage-clamped cells, current responses showed large oscillations at the same frequency as in bridge mode, independently of the actual value of the holding potential. 6. In fourteen intracellularly recorded neurons, oscillations consisted of excitatory events that could be superimposed on a depolarizing or a hyperpolarizing slow wave. In two other neurons, visual responses consisted of excitatory and inhibitory events, alternating with a constant phase shift. 7. Drifting bars were much more efficient in evoking oscillatory responses than flashed bars. Except in three cells, the frequency of the oscillation did not depend on the physical characteristics of the stimulus that were tested (contrast, orientation, direction, ocularity and position in the receptive field). No significant correlation was found between the intensity of the visual response and the strength of the rhythmic component. 8. Although it cannot be excluded that the dominant frequency of oscillations might be related to the type of anaesthetics used, no correlation was found between local EEG and the oscillatory activity elicited by visual stimulation. 9. We conclude that the oscillations observed in the present work are generated by synaptic activity. It is likely that they represent an important mode of transmission in sensory processing, resulting from periodic packets of synchronized activity propagated across recurrent circuits. Their relevance to perceptual binding is further discussed.

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

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

The role of dendritic filtering in associative long-term synaptic plasticity

Several forms of synaptic plasticity in the neocortex and hippocampus depend on the temporal coincidence of presynaptic activity and postsynaptic trains of action potentials (APs). This requirement is consistent with the Hebbian, or correlational, type of cellular learning rule used in many studies of associative synaptic plasticity. Recent experimental evidence suggests that APs initiated in the axosomatic area are actively back-propagated to the dendritic arborization of neocortical and pyramidal cells. High-frequency trains of postsynaptic APs that are used as conditioning stimuli for the induction of Hebbian-like plasticity in both neocortical and hippocampal pyramidal cells display attenuation of the dendritic AP amplitude during the train. This attenuation has been shown to be modulated by neurotransmitters and by electrical activity. We suggest here that both spike train attenuation in the dendrite and its modulation by neurotransmitters and electrical activity may have important functional consequences on the magnitude and/or the sign of the synaptic plasticity induced by a Hebbian pairing procedure.

Activity-dependent regulation of 'on' and 'off' responses in cat visual cortical receptive fields

1. A supervised learning procedure was applied to individual cat area 17 neurons to test the possible role of neuronal co-activity in controlling the plasticity of the spatial 'on-off' organization of visual cortical receptive fields (RFs). 2. Differential pairing between visual input evoked in a fixed position of the RF and preset levels of postsynaptic firing (imposed iontophoretically) were used alternately to boost the 'on' (or 'off') response to a 'high' level of firing (S+ pairing), and to reduce the opponent response (respectively 'off' or 'on') in the same position to a 'low' level (S- pairing). This associative procedure was repeated 50-100 times at a low temporal frequency (0.1-0.15 s-1). 3. Long-lasting modifications of the ratio of 'on-off' responses, measured in the paired position or integrated across the whole RF, were found in 44 % of the conditioned neurons (17/39), and in most cases this favoured the S+ paired characteristic. The amplitude change was on average half of that imposed during pairing. Comparable proportions of modified cells were obtained in 'simple' (13/27) and 'complex' (4/12) RFs, both in adult cats (4/11) and in kittens within the critical period (13/28). 4. The spatial selectivity of the pairing effects was studied by pseudorandomly stimulating both paired and spatially distinct unpaired positions within the RF. Most modifications were observed in the paired position (for 88 % of successful pairings). 5. In some cells (n = 13), a fixed delay pairing procedure was applied, in which the temporal phase of the onset of the current pulse was shifted by a few hundred milliseconds from the presentation or offset of the visual stimulus. Consecutive effects were observed in 4/13 cells, which retained the temporal pattern of activity imposed during pairing for 5-40 min. They were expressed in the paired region only. 6. The demonstration of long-lasting adaptive changes in the ratio of 'on' and 'off' responses, expressed in localized subregions of the RF, leads us to suggest that simple and complex RF organizations might be two stable functional states derived from a common connectivity scheme.

Synaptic origin of rhythmic visually evoked activity in kitten area 17 neurones

Rhythmic patterns in neuronal activity in response to moving stimuli were observed in 28% of cells recorded extracellularly or intracellularly in area 17 of 4-16 week old anaesthetized and paralysed kittens. In both recording modes, oscillation frequencies ranged between 7 and 71 Hz, and were confined for 88% of cells in the 7-20 Hz band of the spectrum. A comparative study of firing autocorrelograms) and subthreshold activity (autocorrelation functions) indicates that the regularity of discharge stemmed from visually evoked oscillations of membrane potential at the same frequency. These oscillations are shown to result from extrinsic excitatory activity, since their amplitude, but not their frequency, depends on the resting membrane potential. The dependency on stimulus configuration supports the hypothesis that oscillations in neuronal output are dictated by periodic activity in afferent circuits selectively recruited by different attributes of the visual input which are not exclusively processed at the cortical level.

Temporal constraints in associative synaptic plasticity in hippocampus and neocortex

We present comparative experimental evidence for the induction of synaptic potentiation and depression in organotypic cultures of hippocampus and in visual cortex in vitro and in vivo. The effects of associative pairings on the efficacy of synaptic transmission are analyzed as a function of the temporal delay between presynaptic activity and post-synaptic changes imposed in membrane potential. Synchronous association at a low temporal frequency (< 0.5 Hz) between presynaptic input and postsynaptic depolarization resulted in homosynaptic potentiation of functionally identified postsynaptic potentials in the three types of preparation. Synchronous pairing of afferent activity with hyperpolarization of the postsynaptic cell resulted in homosynaptic depression in visual cortex in vivo and in vitro. An associative form of depression was induced in hippocampus when the test input was followed repeatedly with a fixed-delay postsynaptic depolarization imposed either by intracellular current injection or synaptically. The latter process might play a significant role in heterosynaptic plasticity in visual cortex in vivo and in vitro, if it is assumed that associative depression still operates in visual cortex a few seconds after the initial surge of calcium in the postsynaptic cell. We conclude that the precise timing between presynaptic activity and polarization changes in postsynaptic membrane potential up- and down-regulates the efficacy of active pathways.

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.

Associative synaptic plasticity in hippocampus and visual cortex: cellular mechanisms and functional implications

Synchronous pre- and postsynaptic neuronal activity results in long-term potentiation (LTP) of excitatory synaptic transmission in the hippocampus and the neocortex. Induction of this form of potentiation requires calcium influx mediated by NMDA receptors. Experimental evidence is reviewed for induction of long-term depression (LTD) of synaptic transmission in the hippocampus in vitro and neocortical neurons in vivo, when the discharge of the postsynaptic neuron is temporally decorrelated from the presynaptic stimulation. Homosynaptic LTD induced by low frequency tetani in the hippocampus in vitro requires NMDA receptor activation and a moderate calcium influx. The role of postsynaptic calcium as a key parameter in the encoding of temporal contiguity of neural activity and its possible implications in the formation of engrams during specific learning tasks are discussed.

Gating of action potential propagation by an axonal A-like potassium conductance in the hippocampus: a new type of non-synaptic plasticity

Synaptic plasticity is usually considered as the main form of activity-dependent functional plasticity in the mammalian brain. Short- and long-term regulation of synaptic transmission have been shown in various types of excitatory synapses including cortical and hippocampal synapses. In this review, we discuss a novel form of non-synaptic plasticity that involves voltage-gated K+ conductances in CA3 pyramidal cell axons. With experimental and theoretical arguments, we show that axons cannot only be considered as a simple structure that transmit reliably the action potential (AP) from the cell body to the nerve terminals. The axon is also able to express conduction failures in specific axonal pathways. We discuss possible physiological conditions in which these axonal plasticity may occur and its incidence on hippocampal network properties.

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.

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.

Associative long-term depression in the hippocampus in vitro

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

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.

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.

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

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