Whole cell recording from neurons in slices of reptilian and mammalian cerebral cortex

Simultaneous LTP of non-NMDA- and LTD of NMDA-receptor-mediated responses in the nucleus accumbens

The nucleus accumbens (NA), a ventral extension of the striatum, plays a role in several complex behaviour patterns and also is a major site of action of drugs of abuse such as cocaine. Intrinsic NA cells are predominantly quiescent and their activity depends on excitatory input from cortical and subcortical limbic afferents. Here we examine the mechanisms of synaptic plasticity at the synapse between prelimbic cortical afferents and cells in the core region of the NA. Manipulations that induce a Ca(2+)-dependent long-term potentiation (LTP) of non-NMDA (N-methyl-D-aspartate)-receptor-mediated responses also produce a simultaneous long-term depression (LTD) of NMDA-receptor-mediated responses. These results indicate that in a single cell the same change in postsynaptic Ca2+ concentration can have opposite effects on non-NMDA- and NMDA-receptor-mediated synaptic responses. This may be particularly important in the NA, where NMDA receptors are critical for mediating the behavioural actions of drugs of abuse.

Adenosine inhibition of mesopontine cholinergic neurons: implications for EEG arousal

Increased discharge activity of mesopontine cholinergic neurons participates in the production of electroencephalographic (EEG) arousal; such arousal diminishes as a function of the duration of prior wakefulness or of brain hyperthermia. Whole-cell and extracellular recordings in a brainstem slice show that mesopontine cholinergic neurons are under the tonic inhibitory control of endogenous adenosine, a neuromodulator released during brain metabolism. This inhibitory tone is mediated postsynaptically by an inwardly rectifying potassium conductance and by an inhibition of the hyperpolarization-activated current. These data provide a coupling mechanism linking neuronal control of EEG arousal with the effects of prior wakefulness, brain hyperthermia, and the use of the adenosine receptor blockers caffeine and theophylline.

Whole-cell recording of the Ca(2+)-dependent slow afterhyperpolarization in hippocampal neurones: effects of internally applied anions

Using the whole-cell recording technique, we have examined the slow Ca(2+)-activated afterhyperpolarization (AHP) and its underlying current (IAHP) in hippocampal CA1 neurones of brain slices obtained from mature rats. Specifically we have studied the effects of the anion component of various K+ salts commonly used to make the pipette filling solution that dialyses neurones during whole-cell recordings. Among the K+ salts examined which included potassium methylsulfate, potassium methanesulfonate, potassium gluconate, potassium chloride, potassium citrate and potassium glutamate, stable AHPs/IAHP and strong spike firing adaptation could only be observed in neurones recorded with the patch pipette solution containing potassium methylsulfate. These AHPs and firing patterns closely mimicked those recorded with sharp electrodes. Similarly, the sustained component of voltage-activated Ca2+ currents was more stable in neurones dialysed with cesium methanesulfonate than in those dialysed with cesium gluconate or cesium chloride. Although the mechanisms underlying the interaction(s) between internally applied anions and ionic channels need further investigation, the present experiments illustrate that in mammalian brain neurones at 33 degrees C, the Ca(2+)-activated IAHP is dramatically altered by internal anions. We suggest that among anions commonly used in electrode filling solutions for whole-cell recordings, methylsulfate is the least disruptive to intracellular structures or Ca2+ homeostasis and permits stable whole-cell recording of the IAHP and Ca2+ currents in mammalian CNS neurones.

2,3-diphosphoglyceric acid blocks long-term potentiation of excitatory postsynaptic currents in hippocampal CA1 neurons of the rat

The dependence of long-term potentiation on an intact inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] metabolism was investigated with the whole-cell voltage-clamp method in the CA1 region of hippocampal slices. The intracellular application of 2,3-diphosphoglyceric acid (1 mM), an inhibitor of the Ins(1,4,5)P3 5-phosphatase and of Ins(1,4,5)P3 3-kinase eliminated the potentiation of postsynaptic currents in pyramidal cells 30 min after paired pre- and postsynaptic activation. These data suggest a possible role of postsynaptic inositol 1,4-bisphosphate and/or inositol 1,3,4,5-tetra-kisphosphate in synaptic plasticity.

A rise in postsynaptic Ca2+ potentiates miniature excitatory postsynaptic currents and AMPA responses in hippocampal neurons

We have investigated the site of expression of the potentiation of excitatory postsynaptic currents (EPSCs) induced by the activation of postsynaptic voltage-sensitive Ca2+ channels, by examining the effect of depolarizing pulses on miniature (m) EPSCs and responses to AMPA. Application of voltage pulses caused a approximately 2.5-fold increase in the mean amplitude of mEPSCs. This NMDA receptor-independent potentiation of mEPSC amplitudes was transient, returning to control values within 30-40 min. The potentiation was associated with a decrease in the number of small amplitude events and an increase in the number, as well as the maximum amplitude, of the larger events, with no apparent change in mEPSC kinetics. Accompanying the increase in mEPSC amplitudes, there was a 1.6-fold increase in the apparent frequency of events. Voltage pulse-induced potentiation was completely blocked by the inclusion of the Ca2+ chelator BAPTA in the recording pipette. Responses to repeated applications of AMPA were also potentiated following the application of voltage pulses, and the time course of this potentiation was similar to that observed with the mEPSCs. Our data indicate that rises in intracellular Ca2+ that occur independently of NMDA receptor activation can result in a potentiation of quantal size, which is due to an increase in the postsynaptic sensitivity of non-NMDA receptors.

Linearity of summation of synaptic potentials underlying direction selectivity in simple cells of the cat visual cortex

Intracellular recordings from simple cells of the cat visual cortex were used to test linear models for the generation of selectivity for the direction of visual motion. Direction selectivity has been thought to arise in part from nonlinear processes, as suggested by previous experiments that were based on extracellular recordings of action potentials. In intracellular recordings, however, the fluctuations in membrane potential evoked by moving stimuli were accurately predicted by the linear summation of responses to stationary stimuli. Nonlinear mechanisms were not required.

PKA mediates the effects of monoamine transmitters on the K+ current underlying the slow spike frequency adaptation in hippocampal neurons

The Ca(2+)-activated K+ current IAHP, which underlies spike frequency adaptation in cortical pyramidal cells, can be modulated by multiple transmitters and probably contributes to state control of the forebrain by ascending monoaminergic fibers. Here, we show that the modulation of this current by norepinephrine, serotonin, and histamine is mediated by protein kinase A in hippocampal CA1 neurons. Two specific protein kinase A inhibitors, Rp-cAMPS and Walsh peptide, suppressed the effects of these transmitters on IAHP and spike frequency adaptation. The effects of the cyclic AMP analog 8CPT-cAMP were also inhibited, whereas muscarinic and metabotropic glutamate receptor agonists had full effect. Intracellular application of protein kinase A catalytic subunit or a phosphatase inhibitor mimicked the effects of monoamines or 8CPT-cAMP. These results demonstrate that monoaminergic modulation of neuronal excitability in the mammalian CNS is mediated by protein phosphorylation.

4-Aminopyridine-induced synaptic GABAB currents in granule cells of the guinea-pig hippocampus

Sharp-electrode and tight-seal perforated-patch and whole-cell recording techniques were used to evaluate K(+)-dependent inhibitory postsynaptic potentials (K-IPSPs) and currents (K-IPSCs) induced by the convulsant 4-aminopyridine (50 mumol l-1) in granule cells of guinea-pig hippocampal slices. The responses were recorded in the presence of blockers for glutamatergic and GABAA-receptor-mediated synaptic transmission, 6-cyano-7-nitroquinoxaline-2,3-dione, picrotoxin and bicuculline. The input resistance was much larger (approximately 300 M omega) in tight-seal recording than in sharp-electrode recording (approximately 100 M omega), but the amplitudes of K-IPSPs recorded at -65 mV holding potential were similar in all three recording configurations. The 4-aminopyridine-induced currents reversed near the K+ equilibrium potential, and the reversal potentials shifted with changes in [K+]out or [K+]in as expected for a K+ current. Slope conductance measurements indicated a conductance increase during the peak of the K-IPSP up to 5 nS (mean 2.4 nS). The peak conductance was underestimated in whole-cell recordings unless the pipette contained Cs+. Considering the high membrane resistance of granule cells, K-IPSCs induced by 4-aminopyridine hyperpolarize the cells considerably and thereby are likely to contribute to the failure of 4-aminopyridine to induce burst discharges in granule cells.

Heterogeneity in presynaptic regulation of GABA release from hippocampal inhibitory neurons

Release of GABA from the terminals of hippocampal inhibitory neurons is inhibited by activation of GABAB autoreceptors and mu opioid receptors. However, it is not known whether these presynaptic processes affect all inhibitory synapses equally. We examined the effects of the GABAB receptor agonist baclofen and the mu opioid receptor agonist DAGO on postsynaptic currents evoked by minimal stimulation of inhibitory fibers (meIPSCs) in area CA3. Baclofen reversibly depressed approximately half of the meIPSCs evoked in the stratum pyramidale. The remaining meIPSCs were unaffected despite a coincident depression of spontaneous IPSCs. In contrast, all meIPSCs were depressed by DAGO. In addition, minimal stimulation in the stratum radiatum evoked meIPSCs that were always depressed by baclofen. These results indicate that regulation of GABA release by GABAB autoreceptors occurs at a subset of inhibitory synapses and that GABAB-resistant inhibitory synapses are located on pyramidal neuron somata. Hippocampal inhibitory neurons may be heterogeneous with respect to presynaptic receptor-mediated regulation of GABA release.

Different glutamate receptor channels mediate fast excitatory synaptic currents in inhibitory and excitatory cortical neurons

Spontaneous excitatory postsynaptic currents (sEPSCs) and responses to rapid application of glutamate were recorded in excitatory spiny, pyramidal neurons and compared with those recorded in inhibitory aspiny interneurons. The sEPSC decay time constant was faster in aspiny interneurons (2.5 ms) compared with pyramidal neurons (4.6 ms). The decay time constant in response to a brief application (1 ms) of glutamate (10 mM) in patches excised from pyramidal and aspiny interneurons were similar (1.9 and 2.7 ms, respectively). However, the rate of desensitization was faster in patches from interneurons compared with pyramidal neurons (3.4 and 12.0 ms, respectively). In addition, single-channel conductance was larger in aspiny interneurons (27 pS) compared with pyramidal neurons (9 pS). These results indicate that pyramidal neurons and aspiny interneurons express different non-N-methyl-D-aspartate receptors and that selective desensitization of interneuron receptors may contribute to depression of inhibition.

Metabotropic glutamate receptors and calcium signalling in dendrites of hippocampal CA1 neurones

We have combined patch-clamp recording with confocal microscopy to investigate how the synaptic activation of metabotropic glutamate receptors (mGluRs) may participate in the modulation of intracellular free calcium (Ca2+) in the dendrites of single CA1 pyramidal neurones, within hippocampal slices. Tetanic stimulation (100 Hz, 1 sec) of the Schaffer collateral-commissural pathway led to a transient rise in Ca2+ in the dendrites of neurones voltage- clamped at -35 mV, as determined using the fluorescent indicator fluo-3. The specific mGluR antagonist (+)-alpha-methyl-4-carboxyphenylglycine (MCPG), applied at a concentration of 250 or 500 microM, reduced the size of the Ca2+ transient whilst either producing a small reduction or, more commonly, having no effect on the synaptic current evoked by the tetanus. These data suggest that the synaptic activation of mGluRs can contribute to Ca2+ signalling in hippocampal neurones.

Evidence for all-or-none regulation of neurotransmitter release: implications for long-term potentiation

1. We have used the whole-cell patch-clamp recording technique to examine the modulation of dual-component excitatory postsynaptic currents (EPSCs) in CA1 pyramidal cells in guinea-pig hippocampal slices. 2. The dramatic difference in the reported sensitivities of the N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptors to glutamate suggests that changes in transmitter concentration in the synaptic cleft would result in differential modulation of the two components of the EPSC. 3. To test whether presynaptic manipulations change transmitter concentration in the synaptic cleft, pharmacological modulation of transmitter release by the GABAB agonist baclofen or by the adenosine antagonist theophylline was used. These manipulations resulted in parallel changes of NMDA and non-NMDA receptor-mediated components of EPSCs over a sixteen-fold range. 4. Stimuli that induce long-term potentiation (LTP) did not cause a sustained enhancement of isolated NMDA receptor-mediated EPSCs evoked in the presence of the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). 5. To compare directly the effect of LTP on the components of the EPSC, dual-component EPSCs were elicited while the postsynaptic membrane potential was held at +30 mV. Induction of long-term potentiation by delivering low-frequency synaptic stimulation in conjunction with such depolarization led to differential enhancement of the non-NMDA receptor-mediated component of the EPSC. 6. These data support the notion that synaptic transmission at individual boutons occurs in an all-or-none fashion, without changing peak transmitter concentration in the synaptic cleft. Long-term potentiation could occur through a postsynaptic modification of receptors or through a presynaptic change involving increased transmitter concentration in the synaptic cleft, but is difficult to explain by a generalized increase in release probability.

Single-channel and whole-cell recordings from on-neurone glial cells in Helix pomatia ganglia

A procedure is described for performing patch-clamp recordings on satellite glial cells kept in place within the nervous ganglia in the mollusc Helix. Glial cell properties were deduced from whole-cell and cell-attached recordings. The glial membrane was found to contain densely packed inwardly rectifying K+ channels. Activation of the neurones, under either current-clamp or voltage-clamp conditions, depolarized the glial cell layer wrapped around the neurones and induced a delayed persistent increase in the K+ channel opening probability. These results suggest that the glial channels opened in response to a signal emanating from the active neurones. This preparation provides a useful means of detecting and analysing neurone-glial interactions at the cell and unitary channel levels.

Aspects of early postnatal development of cortical neurons that proceed independently of normally present extrinsic influences

To examine the contribution of local versus extrinsic influences on postnatal development of cortical neurons, we compared the maturation of deep (infragranular) layer neurons in isolated slices of neocortex grown in organotypic culture to a similar population of neurons developing in vivo. All slice cultures were prepared from sensorimotor cortices of newborn mice (P0) and neurons in these cultures were examined at daily intervals during the first 9 days in vitro (DIV). The maturational state of neurons developing in vivo over this same time period was assessed in acute slices prepared from animals of equivalent postnatal age, P1-P9. Electrophysiological recordings were obtained from neurons in both cultured and acute slices, using Lucifer yellow filled whole-cell recording electrodes, enabling subsequent morphometric analysis of the labeled cells. We report significant changes in both cellular morphology and electrical membrane properties of these deep layer cortical neurons during the first week in culture. Morphological maturation over this time period was characterized by a two- to three-fold increase in cell body size and total process length, and an increase in dendritic complexity. In this same population of cells a three-fold decrease in input resistance and changes in the action potential waveform, including a two-fold decrease in the AP duration, also occur. The degree of morphological and electrophysiological differentiation of individual neurons was highly correlated across developmental ages, suggesting that the maturational state of a cell is reflected in both cellular morphology and intrinsic membrane properties. A remarkably similar pattern of neuronal maturation was observed in neurons in layers V, VI/SP examined in acute slices prepared from animals between P1-P9. Because our culture system preserves many aspects of the local cortical environment while eliminating normal extrinsic influences (including thalamic, brainstem, and callosal connections), our findings argue that this early phase of neuronal differentiation, including the rate and extent of dendritic growth and development of AP waveform, results from instructive and/or permissive local influences, and appears to proceed independently of the many normally present extrinsic factors.

The role of Ca2+ entry via synaptically activated NMDA receptors in the induction of long-term potentiation

Influx of Ca2+ through the NMDA subtype of glutamate receptor is widely accepted as a trigger for many forms of neural plasticity. However, direct support for this model has been elusive, since indirect activation of dendritic voltage-sensitive Ca2+ channels is difficult to exclude. We have optically measured synaptically induced changes in cytoplasmic free Ca2+ concentration in pyramidal cell dendrites in hippocampal slices. Steady postsynaptic depolarization to the synaptic reversal potential eliminated the effect of voltage-sensitive Ca2+ channels. Under these conditions, synaptically induced Ca2+ transients were observed, which were blocked by the NMDA receptor antagonist APV. In addition, the magnitude of LTP was diminished when induced with the postsynaptic membrane held at progressively more positive potentials. LTP could be completely suppressed at potentials near +100 mV. These results provide important experimental support for a role for Ca2+ influx through NMDA receptors in synaptic plasticity.

Synaptic currents evoked in Purkinje cells by stimulating individual granule cells

Each cerebellar Purkinje cell receives input from about 160,000 glutamatergic granule cells. In anatomically intact preparations this input has hitherto been studied only as a compound synaptic potential or current. Presented here are simultaneous recordings in cerebellar slices of synaptically connected granule cell-Purkinje cell pairs. The mean amplitude of the excitatory synaptic currents evoked by stimulation of individual granule cells ranged from 2 to 60 pA, whereas the great majority of the spontaneous glutamatergic currents in Purkinje cells in the presence of tetrodotoxin were < 40 pA. In several cases, stimulation of a single granule cell evoked a disynaptic inhibitory current. It is estimated that on the order of 50 simultaneously active granule cells are sufficient to excite a Purkinje cell.

Presynaptic mechanism for heterosynaptic, posttetanic depression in area CA1 of rat hippocampus

Conditioning stimulation applied to afferent fibers in stratum radiatum or stratum oriens of hippocampal area CA1 produced heterosynaptic, posttetanic depression (PTD) of excitatory postsynaptic potentials (EPSPs). PTD amounted to a 60-80% reduction of EPSPs and recovered over a 5 min period. Conditioning stimulation also induced a posttetanic hyperpolarization (PTH) averaging 4 mV and decaying over a 1-1.5 min period. PTH was accompanied by a large reduction in input resistance. We sought to determine the pre- or postsynaptic locus of heterosynaptic PTD. Our results suggest that PTD reflects a presynaptic mechanism: (1) PTD was observed for both N-methyl-D-aspartate (NMDA) and non-NMDA receptor mediated EPSPs; (2) Direct depolarization of pyramidal cells, substituted for the synaptic depolarization induced by conditioning stimulation, did not elicit PTD; (3) PTD and PTH were differentially affected by pharmacological and postsynaptic manipulations; (4) Conditioning stimulation depressed responses to pressure applied glutamate, but the magnitude and duration were too small to account for PTD. Since afferent fiber volleys were not depressed following conditioning stimulation, while field EPSPs were, we conclude that conditioning stimulation suppresses synaptic release of glutamate.

Slow and incomplete inactivations of voltage-gated channels dominate encoding in synthetic neurons

Electrically excitable channels were expressed in Chinese hamster ovary cells using a vaccinia virus vector system. In cells expressing rat brain IIA Na+ channels only, brief pulses (< 1 ms) of depolarizing current resulted in action potentials with a prolonged (0.5-3 s) depolarizing plateau; this plateau was caused by slow and incomplete Na+ channel inactivation. In cells expressing both Na+ and Drosophila Shaker H4 transient K+ channels, there were neuron-like action potentials. In cells with appropriate Na+/K+ current ratios, maintaining stimulation produced repetitive firing over a 10-fold range of frequencies but eventually led to "lock-up" of the potential at a positive value after several seconds of stimulation. The latter effect was due primarily to slow inactivation of the K+ currents. Numerical simulations of modified Hodgkin-Huxley equations describing these currents, using parameters from voltage-clamp kinetics studied in the same cells, accounted for most features of the voltage trajectories. The present study shows that insights into the mechanisms for generating action potentials and trains of action potentials in real excitable cells can be obtained from the analysis of synthetic excitable cells that express a controlled repertoire of ion channels.

Characterization of Ca2+ signals induced in hippocampal CA1 neurones by the synaptic activation of NMDA receptors

1. A combination of confocal microscopy, whole-cell patch-clamp recording, intracellular dialysis and pharmacological techniques have been employed to study Ca2+ signalling in CA1 pyramidal neurones, within rat hippocampal slices. 2. In the soma of CA1 neurones, depolarizing steps applied through the patch-pipette resulted in transient increases in the fluorescence emitted by the Ca2+ indicator fluo-3. The intensity of the fluorescence transients was proportional to the magnitude of the Ca2+ currents recorded through the pipette. Both the somatic fluorescence transients and the voltage-activated Ca2+ currents ran down in parallel over a period of between approximately 15-45 min. The fluorescence transients were considered, therefore, to be caused by increases in cytosolic free Ca2+. 3. Under current-clamp conditions, high-frequency (tetanic) stimulation (100 Hz, 1 s) of the Schaffer collateral-commissural pathway led to compound excitatory postsynaptic potentials (EPSPs) and somatic Ca2+ transients. The somatic Ca2+ transients were sensitive to the N-methyl-D-aspartate (NMDA) receptor antagonist D-2-amino-5-phosphonopentanoate (AP5; 100 microM). These transients, but not the EPSPs, disappeared with a time course similar to that of the run-down of voltage-gated Ca2+ currents. Tetanus-induced somatic Ca2+ transients could not be elicited under voltage-clamp conditions. 4. Fluorescence images were obtained from the dendrites of CA1 pyramidal neurones starting at least 30 min after obtaining whole-cell access to the neurone. Measurements were obtained only after voltage-gated Ca2+ channel activity had run down completely. 5. Tetanic stimulation of the Schaffer collateral-commissural pathway resulted in compound EPSPs and excitatory postsynaptic currents (EPSCs), under current- and voltage-clamp, respectively. In both cases, these were invariably associated with dendritic Ca2+ transients. In cells voltage-clamped at -35 mV, the fluorescent signal increased on average 2-fold during the tetanus and decayed to baseline values with a half-time (t1/2) of approximately 5 s. 6. The alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM) partially reduced the tetanus-induced EPSC without affecting the Ca2+ transients. In contrast, AP5, which also depressed the EPSC, substantially reduced or eliminated the Ca2+ transients. 7. In normal (i.e. 1 mM Mg(2+)-containing) medium, NMDA receptor-mediated synaptic currents displayed the typical region of negative slope conductance in the peak I-V relationship (between -90 and -35 mV). The dendritic tetanus-induced Ca2+ transients also displayed a similar anomalous voltage dependence, decreasing in size from -35 to -90 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

Photostimulation using caged glutamate reveals functional circuitry in living brain slices

An approach for high-spatial-resolution mapping of functional circuitry in living mammalian brain slices has been developed. The locations of neurons making functional synaptic connections to a single neuron are revealed by photostimulation of highly restricted areas of the slice (50-100 microns in diameter) while maintaining a whole-cell recording of the neuron of interest. Photostimulation is achieved by bathing brain slices in a molecularly caged form of the neurotransmitter glutamate [L-glutamic acid alpha-(4,5-dimethoxy-2-nitrobenzyl) ester], which is then converted to the active form by brief pulses (< 1 ms in duration) of ultraviolet irradiation. Direct activation of receptors on recorded neurons in rat hippocampus and ferret visual cortex demonstrates that photostimulation is reliable and reproducible and can be repeated at the same site at least 30 times without obvious decrement in neuronal responsiveness. Photostimulation of presynaptic neurons at sites distant to the recorded neuron evoked synaptic responses in hippocampal and cortical cells at distances of up to several millimeters from the recorded neuron. Stimulation of 25-100 distinct presynaptic sites while recording from a single postsynaptic neuron was easily achieved. Caged glutamate-based photostimulation eliminates artifacts and limitations inherent in conventional stimulation methods, including stimulation of axons of passage, desensitization, and poor temporal resolution of "puffer" pipettes, and current artifacts of iontophoretic application. This approach allows detailed physiological investigation and manipulation of the complex intrinsic circuitry of the mammalian brain.

Substance P excites neurons in the gustatory zone of the rat nucleus tractus solitarius

Whole-cell patch recordings of neurons in the rostral (gustatory) nucleus tractus solitarius (rNTS) were performed in a brain slice preparation from rat medulla. Neural responses to brief applications (10-45 s) of substance P (SP), via a constant superfusion apparatus, were recorded. SP transiently depolarized 80 of 117 (68%) rNTS neurons in a dose-dependent manner. Sub-micromolar concentrations of SP had potent excitatory effects, and the half maximal response occurred at 0.6 microM. The depolarizing effect of SP was accompanied by an increase in input resistance in 81% of the responsive neurons. The excitatory effects of SP persisted in low Ca2+ (0.2 mM) and high Mg2+ (12 mM) saline as well as in the presence of 2 microM TTX (n = 5 for each), suggesting direct postsynaptic action on the recorded neurons. SP also hyperpolarized 4 neurons (4%) and had no effect on 33 neurons (28%). Each of the 4 neurons which were hyperpolarized by SP showed a decrease in input resistance. A more detailed assessment of the types of neurons in the rNTS which respond to SP was also conducted. Neurons were separated into 4 electrophysiological groups on the basis of their repetitive firing pattern induced by a hyperpolarizing and depolarizing current injection paradigm. Neurons belonging to each of the 4 electrophysiological groups responded to SP. Eighteen neurons, which were filled with 1% biocytin during recording, were categorized as ovoid, multipolar or fusiform based on their morphological characteristics. SP excited all 3 morphological types of neurons in similar proportion. These results suggest that SP is an excitatory neurotransmitter in the rNTS. The effects of SP are not restricted to a particular neuron type defined either biophysically or morphologically. The implications of these results on the possible role of SP in processing gustatory and somatosensory information within the rNTS are discussed.

Influence of GABA on neurons of the gustatory zone of the rat nucleus of the solitary tract

The role of gamma-aminobutyric acid (GABA) as an inhibitory neurotransmitter in the rostral, gustatory zone of the nucleus of the solitary tract (rNST) was examined using whole cell recordings in brain slices of the adult rat medulla. Superfusion of GABA resulted in a concentration-dependent reduction in input resistance in 68% of the neurons in rNST. The change in input resistance was often accompanied by membrane hyperpolarization. The effect of GABA was a direct action on the postsynaptic membrane since it could be elicited when synaptic transmission was blocked by tetrodotoxin or in a low Ca2+ and high Mg2+ perfusing solution. The mean reversal potential of the GABA effect was about -60 mV, determined by applying GABA at different holding potentials, or from the intersection of current-voltage curves measured in control saline and saline containing GABA. When neurons were separated into groups based on intrinsic membrane properties, some neurons in each group responded to GABA. Superfusion of the slices with either the GABAA agonist, muscimol, or the GABAB agonist, baclofen, caused a decrease in input resistance accompanied by membrane hyperpolarization. The GABAA antagonist bicuculline either totally or partially blocked the neuronal response to GABA and blocked the response to muscimol but did not antagonize responses to baclofen. Superfusion of the GABAB antagonist phaclofen depressed the membrane responses to GABA. The use of the GABAA and GABAB agonists and antagonists demonstrates that some neurons in rNST have both GABAA and GABAB receptors. Since most rNST neurons studied respond to GABA, inhibition probably plays a major role in sensory processing by the rNST.

Solutions for transients in arbitrarily branching cables: III. Voltage clamp problems

Branched cable voltage recording and voltage clamp analytical solutions derived in two previous papers are used to explore practical issues concerning voltage clamp. Single exponentials can be fitted reasonably well to the decay phase of clamped synaptic currents, although they contain many underlying components. The effective time constant depends on the fit interval. The smoothing effects on synaptic clamp currents of dendritic cables and series resistance are explored with a single cylinder + soma model, for inputs with different time courses. "Soma" and "cable" charging currents cannot be separated easily when the soma is much smaller than the dendrites. Subtractive soma capacitance compensation and series resistance compensation are discussed. In a hippocampal CA1 pyramidal neurone model, voltage control at most dendritic sites is extremely poor. Parameter dependencies are illustrated. The effects of series resistance compound those of dendritic cables and depend on the "effective capacitance" of the cell. Plausible combinations of parameters can cause order-of-magnitude distortions to clamp current waveform measures of simulated Schaeffer collateral inputs. These voltage clamp problems are unlikely to be solved by the use of switch clamp methods.

Patch-clamp recordings from the soma and dendrites of neurons in brain slices using infrared video microscopy

A description is given of the implementation of infrared differential interference contrast (IR-DIC) video microscopy to an upright compound microscope. Using the improved resolution offered by IR-DIC a procedure is described for making patch-pipette recordings from visually identified neuronal somata and dendrites in brain slices. As an example of the application of this technique to electrophysiological recordings from small neuronal processes in brain slices we describe whole-cell current-clamp and cell-attached and excised patch-clamp recordings from the apical dendrites of layer V pyramidal neurons in slices of rat neocortex.

A simple method for internal perfusion of mammalian central nervous system neurones in brain slices with multiple solution changes

A simple system for internal perfusion of whole-cell patch-clamped neurones in brain slices allowing multiple, fast solution exchanges is described. With this system, perfusing solutions can be loaded immediately before use during the recording. At perfusion rates of 5-10 microliters/min, complete replacement of solutions in the patch pipette tip, as determined by changes in the pipette resistance and liquid junction potential, was achieved in less than 20 s. This occurred after a 1-min latency that was due to solution flow through the infusion tube. The effectiveness of the system was tested on rat hippocampal CA1 neurones in the slice preparation. The effects of replacement of internal 150 mM K+ by 150 mM Cs+ ions on voltage-activated K+ currents and of changing internal [Cl-] between 20 mM and 150 mM on evoked GABAA-mediated inhibitory postsynaptic currents (IPSCs) were studied. The blockade of K+ currents by Cs+ ions and the changes of IPSCs by altered internal [Cl-] ion concentration were achieved within 3.2 and 1.5 min, respectively, including the 'flow latency' of about 1 min, and recovery following solution change occurred within 5.2 and 1.5 min, respectively. More than 10 effective internal solution replacements could be performed within 1 h in a single neurone without affecting the recording stability.

Induction of LTP in the hippocampus needs synaptic activation of glutamate metabotropic receptors

Understanding the mechanisms of long-term potentiation (LTP) should provide insights into the molecular basis of learning and memory in vertebrates. Ionotropic glutamate receptors play a central role in LTP; AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate) receptors and NMDA (N-methyl-D-aspartate) receptors mediate synaptic responses that are enhanced in LTP and, in addition, NMDA receptors are necessary for the induction of LTP in most pathways. There is also circumstantial evidence that metabotropic glutamate receptors (mGluRs) may be involved in LTP because the specific mGluR agonist aminocyclopentane dicarboxylate can augment tetanus-induced LTP2 and, under certain circumstances, can itself induce a slow-onset potentiation. But the absence of any effective mGluR antagonist has prevented the determination of whether mGluRs are involved in the induction of tetanus-induced LTP. We report here that (RS)-alpha-methyl-4-carboxyphenylglycine is a specific mGluR antagonist in the hippocampus and have used this compound to examine the nature of the involvement of mGluRs in LTP. We show that synaptic activation of mGluRs is necessary for the induction of both NMDA receptor-dependent and NMDA receptor-independent forms of LTP in the hippocampus.

Glutamate iontophoresis induces long-term potentiation in the absence of evoked presynaptic activity

Protocols that induce long-term potentiation (LTP) typically involve afferent stimulation. We tested the hypothesis that LTP induction does not require presynaptic activity. The significance of this hypothesis is underscored by results suggesting that LTP expression may involve activity-dependent presynaptic changes. An induction protocol using glutamate iontophoresis was developed that reliably induced LTP in hippocampal slices without afferent stimulation. Iontophoresis LTP was Ca2+ dependent, was blocked by MK-801, and occluded tetanus-induced LTP. Iontophoresis LTP was induced when excitatory postsynaptic potentials were completely blocked by adenosine plus tetrodotoxin. Our results suggest constraints on the involvement of presynaptic mechanisms and putative retrograde messengers in LTP induction and expression; namely, these processes must function without many forms of activity-dependent presynaptic processes.

Calcium/calmodulin-dependent protein kinase II regulates hippocampal synaptic transmission

Extracellular application of protein kinase inhibitors was used to examine the role of calcium/calmodulin-dependent protein kinase II (CaM-KII) in synaptic transmission in the CA1 region of rat hippocampus. Bath application of the broad spectrum, membrane permeable kinase inhibitor H7 (250 microM) decreased excitatory synaptic responses elicited in hippocampal slices. Whereas H7 inhibits several protein kinases and has non-specific effects, several synthetic peptides have been developed as specific inhibitors of CaM-KII. Using in situ phosphorylation in hippocampal slices, we demonstrate that extracellular application of synthetic peptide inhibitors of CaM-KII preferentially suppresses the phosphorylation of synapsin I at the CaM-KII specific site. This suppression was not reversed by the application of a calcium ionophore indicating the decrease in phosphorylation does not result only from blockade of presynaptic calcium influx. Thus, it appears the peptides gain access to intracellular compartments and retain their inhibitory properties. Further, we found that extracellular application of these peptide inhibitors decreased excitatory synaptic responses elicited in the CA1 region of hippocampal slices with relative potencies consistent with their ability to block CaM-KII activity in vitro. Peptide application did not alter the input resistance of postsynaptic cells nor responses elicited by glutamate iontophoresis. These results suggest that CaM-KII activity, possibly through phosphorylation of presynaptic synapsin I, is required for sustained synaptic transmission at mammalian synapses.

Glutamate metabotropic receptor mediated depression of synaptic inputs to lamprey reticulospinal neurones

The transmission of vestibular inputs to reticulospinal (RS) neurones of the posterior rhombencephalic nucleus (PRRN) has been shown to be depressed by the bath application of N-methyl-D-aspartate (NMDA). The aim of this study was to investigate the pharmacological mechanism involved using patch clamp recordings of reticulospinal neurones. It is demonstrated that the chemical component of vestibular inputs to the PRRN is mediated by glutamatergic synapses utilising alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptors on the PRRN neurones. Monosynaptic excitatory postsynaptic currents (EPSCs) from octavomotorius relay cells to RS neurones are markedly depressed by the application of NMDA, a depression which was insensitive to competitive and non-competitive NMDA receptor antagonists. The effect of NMDA was eliminated by inactivation of G proteins. A similar depressive effect was observed following application of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) to the superfusate. It is concluded that NMDA acts at a metabotropic receptor located most likely presynaptically to reticulospinal neurones on terminals of octavomotorius relay cells.

Long-term potentiation during whole-cell recording in rat hippocampal slices

Factors involved in the production of long-term potentiation in the CA1 region of rat hippocampal slices were examined using whole-cell voltage clamp recordings. The pairing of postsynaptic membrane depolarization with tetanic stimulation produced a reliable long-lasting enhancement of synaptic currents provided that the pairing was performed within 15 min after establishing intracellular contact. This time could be extended to 30 min by including adenosine triphosphate and guanosine triphosphate in the recording pipette. Once established, the potentiation persisted for 3 h or more. The washout of long-term potentiation generating ability was not correlated with a rundown in baseline synaptic currents or in the N-methyl-D-aspartate receptor-mediated component of synaptic responses, but followed a time course similar to the loss of calcium spikes. Long-term potentiation could be reliably produced by depolarizing the postsynaptic membrane to -40 or -20 mV during the tetanus, but decreased when the membrane was held at membrane potentials greater than 0 mV. At -20 mV, 50 microM 2-amino-5-phosphonovalerate blocked the potentiation but this agent was ineffective at +40 mV. In contrast, 50 microM verapamil, a calcium channel blocker, failed to alter long-term potentiation at -20 mV but blocked the enhancement at +40 mV. These results suggest that whole-cell recording causes a washout of postsynaptic factors important in the initiation of long-term potentiation. However, these factors are less important in maintaining the potentiation. Furthermore, depending on the postsynaptic membrane potential during tetanic stimulation, voltage-gated calcium channels contribute to CA1 long-term potentiation.

Blind patch-clamp recordings from substantia gelatinosa neurons in adult rat spinal cord slices: pharmacological properties of synaptic currents

Whole cell patch-clamp recordings were made from substantia gelatinosa neurons in the thick slice of the adult rat spinal cord, which retained an attached dorsal root to study the pharmacological properties of spontaneous and primary afferent fibre-evoked synaptic currents. The majority of substantia gelatinosa neurons tested exhibited miniature excitatory postsynaptic currents in the presence of tetrodotoxin (0.5 microM). Stimulation of primary afferent A delta fibres evoked monosynaptic and/or polysynaptic excitatory postsynaptic currents. In Mg(2+)-containing solution, 6-cyano-7-nitroquinoxaline-2,3-dione (10 microM) abolished the evoked excitatory postsynaptic currents and the miniature excitatory postsynaptic currents. 2-Amino-5-phosphonovaleric acid (50-100 microM) had little effect on the miniature excitatory postsynaptic current. In Mg(2+)-free solution, however, 6-cyano-7-nitroquinoxaline-2,3-dione reduced but did not abolish the miniature excitatory postsynaptic currents, leaving the miniature excitatory postsynaptic currents with a small amplitude and a slow time course, which were abolished by 2-amino-5-phosphonovaleric acid. At holding potentials more positive than -60 mV, stimulation of A delta fibres evoked outward postsynaptic currents in 11 out of 28 substantia gelatinosa neurons. The evoked inhibitory postsynaptic currents were abolished in seven out of 11 neurons by either strychnine (0.5 microM) or bicuculline (10 microM), and in the remaining four neurons by the combination of both antagonists.(ABSTRACT TRUNCATED AT 250 WORDS)

Different proportions of N-methyl-D-aspartate and non-N-methyl-D-aspartate receptor currents at the mossy fibre-granule cell synapse of developing rat cerebellum

The mossy fibre-granule cell synapse undergoes major developmental changes during the second and third weeks after birth. We investigated synaptic transmission during postnatal days 10-22 by means of whole-cell patch-clamp recordings from granule cells in situ. Parasagittal slices were cut from rat cerebellar vermis, and excitatory postsynaptic currents were evoked in granule cells by mossy fibre stimulation with 1.2 mM Mg++ in the extracellular solution. In the majority of granule cells recorded at postnatal days 16-22, excitatory currents were characterized by a fast initial peak followed by a slower component, while in many of the cells recorded at more immature stages, the fast peak was virtually absent. Pharmacological and kinetic data indicated that the fast and slow components were mediated by non-N-methyl-D-aspartate and N-methyl-D-aspartate receptor activation, respectively. The magnitude of the non-N-methyl-D-aspartate current increased with developmental age, while the magnitude of the NMDA current did not change markedly. The age-dependent change of the non-N-methyl-D-aspartate currents could not be accounted for by changes in recording conditions or granule cell electrotonic properties. Furthermore, from postnatal day 11 to 16 the extent of Mg++ block on the N-methyl-D-aspartate receptor did not change, and could not explain the increasing non-N-methyl-D-aspartate/N-methyl-D-aspartate current ratio. We concluded therefore that the age-dependent increase of the non-N-methyl-D-aspartate current was the main cause of the different postsynaptic current waveforms observed at different ages. The developmental change in the proportion of N-methyl-D-aspartate and non-N-methyl-D-aspartate currents may be relevant to the processes regulating granule cell maturation and excitability.

Local and diffuse synaptic actions of GABA in the hippocampus

In the CNS, gamma-aminobutyric acid (GABA) acts as an inhibitory transmitter via ligand-gated GABAA receptor channels and G protein-coupled GABAB receptors. Both of these receptor types mediate inhibitory postsynaptic transmission in the hippocampus. In addition to these direct postsynaptic actions, GABAB receptor agonists inhibit excitatory transmission through presynaptic receptors on excitatory afferent terminals. However, a physiological role for the GABAB receptors on excitatory nerve endings has not been established. In this study, we have found a brief, heterosynaptic depression of excitatory synaptic transmission in the CA1 region of the hippocampal slice following short-lasting repetitive stimulation and determined that this inhibition is mediated by presynaptic GABAB receptors. The inhibition of GABA uptake greatly enhanced both the presynaptic action of GABA and the slow GABAB-mediated inhibitory postsynaptic current. Transmitter uptake was also found to regulate the "spill-over" of GABA at conventional GABAA synapses. These results suggest that uptake mechanisms restrict the spatial range of both point-to-point synaptic transmission mediated by GABA and its action at a distance.

Electrical and integrative properties of rabbit sympathetic neurones re-evaluated by patch clamping non-dissociated cells

1. Voltage recordings were performed on non-dissociated sympathetic neurones from rabbit coeliac ganglia using the whole-cell configuration of the patch clamp technique. 2. Cells were classified depending on their firing pattern as silent cells (63%) producing either phasic (24%) or tonic (76%) spike discharge in response to depolarizing currents, and pacemaker cells (37%). 3. All the cells produced large overshooting spikes and prolonged postspike after-hyperpolarization. The peak-to-peak spike amplitude was 113.8 +/- 1 mV. Spikes were shortened and the after-hyperpolarization was suppressed when calcium channel blockers (Cd2+ and La3+) were added. 4. Silent cells have a resting potential of -58.8 +/- 1.5 mV. At potentials ranging from -50 to -90 mV, the input impedance was 490 +/- 27 M omega at 22-24 degrees C and 426 +/- 47 M omega at 35-36 degrees C. The time constant at voltages corresponding to the high input impedance region was 126 +/- 7 ms at 22-24 degrees C and 86 +/- 7 ms at 35-36 degrees C. 5. The firing frequency of the pacemaker cells was 3.2 +/- 0.5 Hz at 35-36 degrees C in the presence of nicotinic blockers. Evidence is given that the firing did not result from cell injury but was induced by an intrinsic pacemaker mechanism. Input impedance of pacemaker neurones was 580 +/- 47 M omega at 22-24 degrees C and 473 +/- 56 M omega at 35-36 degrees C. 6. Most of the pacemaker cells (63%) were motoneurones, since they were antidromically fired by stimulating post-ganglionic nerves. In addition, they received synaptic inputs from both preganglionic fibres (splanchnic nerves) and the periphery (postganglionic nerves). Long-lasting depolarizations were induced in either silent or pacemaker cells by single shocks applied to pre- and postganglionic nerves. 7. Slowly rising voltage ramps revealed the presence of an N-shaped current-voltage relationship in voltage clamped pacemaker cells. The negative slope was located in a subthreshold voltage range, between -83.4 +/- 1.4 and -59.0 +/- 1.8 mV. It was induced by the activation of a low threshold persistent inward current. Although it was tiny (22 +/- 3 pA at its peak level) this current brought the null-current voltage up to -41.0 +/- 1.4 mV, which resulted in continuous firing. 8. Due to the instability introduced by the N-shaped I-V relationship, pacemaker cells can display bistable behaviour characterized by hyperpolarizing responses.(ABSTRACT TRUNCATED AT 400 WORDS)

On the sites of presynaptic inhibition by neuropeptide Y in rat hippocampus in vitro

Neuropeptide Y (NPY) reduces excitatory synaptic transmission between stratum radiatum and CA1 pyramidal cells in rat hippocampal slice in vitro by a presynaptic action. To understand NPY's role in the control of excitability in hippocampus, its actions on excitatory and inhibitory synaptic transmission were examined, using intracellular, sharp microelectrode, and tight-seal, whole cell recordings from principal neurons in areas CA1, CA3, and dentate. Bath application of 1 microM NPY reversibly inhibited excitatory postsynaptic potentials (EPSPs) evoked in CA1 pyramidal cells from either stratum radiatum or stratum oriens by about 50%. Neuropeptide Y also inhibited EPSPs at mossy fiber-CA3, stratum oriens-CA3, and CA3-CA3 synapses by between 45% and 55%. As in CA1, the action of NPY was presynaptic. By contrast, NPY did not inhibit EPSPs evoked in dentate granule cells from either perforant path or commissural inputs. Neuropeptide Y did not alter postsynaptic membrane properties in any cell type. Although NPY attenuated the orthodromically evoked (stratum radiatum) inhibitory postsynaptic potentials in CA1 pyramidal cells by about the same amount as it inhibited the EPSPs, it did not affect the IPSPs evoked in the same cells by antidromic stimulation from alveus. Inhibitory postsynaptic potentials evoked in pharmacological isolation in CA1, CA3, or dentate were also not significantly affected by NPY. The evidence supports the hypothesis that NPY acts at feedforward excitatory synapses to presynaptically reduce the amplitude of excitation as it travels through hippocampal circuits. By contrast, synaptically mediated inhibition is not directly affected by NPY.(ABSTRACT TRUNCATED AT 250 WORDS)

Cholinergic excitation of GABAergic interneurons in the rat hippocampal slice

1. Intracellular recordings were made from CA1 pyramidal cells in the rat hippocampal slice to study the cholinergic modulation of GABAergic inhibition. The cholinergic receptor agonist, carbamylcholine (carbachol), depressed evoked excitatory postsynaptic potentials (EPSPs) and evoked inhibitory postsynaptic potentials (IPSPs), but enhanced small spontaneously occurring membrane potential fluctuations that resembled IPSPs. Both atropine (1 microM) and picrotoxin (25-60 microM) abolished the small fluctuations. 2. Recording from cells with potassium or caesium chloride (KCl or CsCl)-filled microelectrodes enhanced and inverted spontaneous Cl(-)-dependent GABAA-mediated IPSPs. These events appeared to result from the spontaneous firing of GABAergic interneurons since they could be inhibited by picrotoxin or bicuculline and nearly eliminated by tetrodotoxin. 3. Muscarinic acetylcholine (ACh) receptor activation significantly increased the frequency of spontaneous-activity-dependent IPSPs from 1.7 +/- 0.4 s (mean +/- S.E.M.) in control saline to 7.0 +/- 1.1 s in carbachol (10-50 microM)-containing saline, although evoked IPSPs were inhibited. All effects of carbachol were completely reversed by atropine. 4. The increase in frequency of spontaneous IPSPs observed in carbachol was not secondary to changes in the postsynaptic cell and was not blocked by high doses of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 5-10 microM) and 2-amino-5-phosphonovaleric acid (APV, 10-20 microM), which abolished evoked excitatory transmission. Amplitude histograms showed an increase in mean size as well as of frequency of spontaneous IPSCs in carbachol. 5. Stimulation of cholinergic afferents in stratum oriens in the presence of the acetylcholinesterase inhibitor eserine (1 microM) also increased spontaneous IPSP frequency, and the time course of this response was similar to that of the muscarinic slow EPSP. Postsynaptic factors or the activation of glutamatergic excitatory pathways could not account for this effect. 6. Evoked monosynaptic IPSCs in CNQX and APV were diminished by carbachol. 7. We conclude that GABAergic inhibitory interneurons possess muscarinic receptors, that activation of these receptors increases the excitability of the interneurons and that synaptically released ACh increases interneuronal activity. Cholinergic reduction of the monosynaptic IPSC may point to additional complexity in cholinergic regulation of the GABA system.

In vitro responses of caudal hypothalamic neurons to hypoxia and hypercapnia

Results from previous studies have suggested that the hypothalamus modulates cardiorespiratory responses to hypoxia and/or hypercapnia. Many neurons in the caudal hypothalamus are stimulated by hypercapnia and hypoxia in vivo; however, it is not known if these responses are dependent upon input from other areas. Whole-cell patch and extracellular recordings from a brain slice preparation were used in the present study to determine the direct effects of hypoxia (5% CO2/95% N2 or 10% O2/5% CO2/85% N2) and hypercapnia (7% CO2/93% O2) on caudal hypothalamic neurons in vitro. Coronal sections (400-500 microns) were obtained from young Sprague-Dawley rats and placed in a recording chamber that was perfused with nutrient media equilibrated with 95% O2/5% CO2. Extracellular recordings demonstrated that hypoxia stimulated over 80% of the neurons tested; the magnitude of the response was dependent upon the degree of hypoxia. In addition, over 80% of cells that were excited by hypoxia retained this response during synaptic blockade. Hypercapnia increased the discharge frequency of 22% of the caudal hypothalamic neurons that were studied. A second set of caudal hypothalamic neurons were studied with whole-cell patch recordings; the mean resting membrane potential of these neurons was -51.8 +/- 1.0 mV with an average input resistance of 399 +/- 49 M omega. Hypoxia produced a depolarization in 76% of these neurons; a poststimulus hyperpolarization often occurred. A depolarization and/or increase in discharge rate during hypercapnia was observed in 35% of the neurons tested. Only 10% of all neurons studied were excited by both hypoxia and hypercapnia. These findings suggest that separate subpopulations of caudal hypothalamic neurons are sensitive to hypoxia and hypercapnia. Thus, this hypothalamic area may be a site of central hypoxic and hypercapnic chemoreception.

Ca2+ Entry via postsynaptic voltage-sensitive Ca2+ channels can transiently potentiate excitatory synaptic transmission in the hippocampus

We have studied the role of Ca2+ entry via voltage-sensitive Ca2+ channels in long-term potentiation (LTP) in the CA1 region of the hippocampus. Repeated depolarizing pulses, in the presence of the NMDA receptor antagonist D-APV and without synaptic stimulation, resulted in a potentiation of excitatory postsynaptic potentials (EPSPs) or currents (EPSCs). This depolarization-induced potentiation was augmented in raised extracellular Ca2+ and was blocked by intracellular BAPTA, a Ca2+ chelator, or by nifedipine, a Ca2+ channel antagonist, indicating that the effect was mediated by Ca2+ entry via voltage-sensitive Ca2+ channels. Although the peak potentiation could be as large as 3-fold, the EPSP(C)s decayed back to baseline values within approximately 30 min. However, synaptic activation paired with depolarizing pulses in the presence of D-APV converted the transient potentiation into a sustained form. These results indicate that a rise in postsynaptic Ca2+ via voltage-sensitive Ca2+ channels can transiently potentiate synaptic transmission, but that another factor associated with synaptic transmission may be required for LTP.

Postsynaptic action of endogenous GABA released by nipecotic acid in the hippocampus

Intracellular and whole-cell recording from CA1 pyramidal cells and dentate granule cells was used to study the release of endogenous GABA by nipecotic acid. Local application of nipecotic acid produced responses that could be entirely blocked by a combination of the GABAA receptor antagonist picrotoxin and the GABAB receptor antagonist CGP 35348. These responses were due to the heteroexchange release of endogenous GABA because they were blocked by low Na+ which blocks the GABA transporter and by SKF 89976 which is a competitive antagonist of the GABA transporter. Local application of nipecotic acid could, depending on the location, evoke pure GABAA or pure GABAB responses supporting proposals that GABAA and GABAB receptors can be segregated at separate inhibitory synapses.

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

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

Activation and desensitization of glutamate-activated channels mediating fast excitatory synaptic currents in the visual cortex

Brief glutamate applications to membrane patches, excised from neurons in the rat visual cortex, were used to assess the role of desensitization in determining the AMPA/kainate receptor-mediated excitatory postsynaptic current (EPSC) time course. A brief (1 ms) application of glutamate (1-10 mM) produced a response that mimicked the time course of miniature EPSCs (mEPSCs). Direct evidence is presented that the rate of onset of desensitization is much slower than the decay rate of the response to a brief application of glutamate, implying that the decay of mEPSCs reflects channel closure into a state readily available for reactivation. Rapid application of glutamate combined with nonstationary variance analysis provided an estimate of the single-channel conductance and open probability, allowing an approximation of the number of available channels at a single synaptic site.

Mechanisms underlying induction of homosynaptic long-term depression in area CA1 of the hippocampus

The mechanisms responsible for long-lasting, activity-dependent decreases in synaptic efficacy are not well understood. We have examined the initial steps required for the induction of long-term depression (LTD) in CA1 pyramidal cells by repetitive low frequency (1 Hz) synaptic stimulation. This form of LTD was synapse specific, was saturable, and required activation of post-synaptic NMDA receptors. Loading CA1 cells with the Ca2+ chelator BAPTA prevented LTD, whereas lowering extracellular Ca2+ resulted in the induction of LTD by stimulation that previously elicited long-term potentiation. Following LTD, synaptic strength could be increased to its original maximal level, indicating that LTD is reversible and not due to deterioration of individual synapses. Induction of homosynaptic LTD therefore requires an NMDA receptor-dependent change in postsynaptic Ca2+ which may be distinct from that required for long-term potentiation.

Putative Single Quantum and Single Fibre Excitatory Postsynaptic Currents Show Similar Amplitude Range and Variability in Rat Hippocampal Slices

Transmission at excitatory synapses in the mammalian brain is thought to depend on the release of transmitter quanta through exocytosis of presynaptic vesicles (Katz, 1969). The number of vesicles released by a single presynaptic action potential is important for understanding the impact of a single synapse, and the variability in transmission from one impulse to the next. In addition, the number of vesicles released may be an important factor for synaptic regulation and plasticity, such as facilitation, post-tetanic potentiation and long-term potentiation (LTP). Three recent studies suggest that an increase in the number of transmitter quanta underlies hippocampal LTP (Malinow and Tsien, 1990; Bekkers and Stevens 1990; Malinow, 1991), whereas other reports suggest a postsynaptic mechanism (Kauer et al., 1988; Muller et al., 1988; Foster and McNaughton, 1989). We have used the whole-cell recording technique to compare putative quantal and single fibre responses at excitatory synapses in rat hippocampal slices, and find (i) a surprisingly large variability in single fibre excitatory postsynaptic currents (sfEPSCs); (ii) an equally large variability of putative quantal (pq) EPSCs elicited by hyperosmolar media or ruthenium red; (iii) the observed amplitude ranges for the sfEPSCs and the pqEPSCs overlap almost completely; and (iv) in neither case can the variability be attributed to a scatter in electrotonic distance from the soma of the engaged synapses. Thus, the data are compatible with the hypothesis that a presynaptic action potential usually releases only a single quantum. Other possibilities are also discussed.

Dendritic morphology of pyramidal neurones of the visual cortex of the rat. IV: Electrical geometry

Features of the dendritic morphology of pyramidal neurones of the visual cortex of the rat that are relevant to the development of models of their passive electrical geometry were investigated. The sample of 39 neurones that was used came from layers 2/3 and 5. They had been recorded from and injected intracellularly with horseradish peroxidase (HRP) in vitro as part of a previous study (Larkman and Mason, J. Neurosci 10:1407, 1990). These cells had been reconstructed and measured previously by light microscopy. The relationship between the diameters of parent and daughter dendrites during branching was examined. It was found that most dendrites did not closely obey the "3/2 branch power relationship" required for representation of the dendrites as single equivalent cylinders. Estimates of total neuronal membrane area ranged from 27,100 +/- 7,900 microns2 for layer 2/3 cells to 52,200 +/- 11,800 microns2 for thick layer 5 cells. Dendritic spines contributed approximately half the total membrane area. Both neuronal input resistance and the ratio of membrane time constant to input resistance were correlated with neuronal membrane area as measured anatomically. The relative electrical lengths of the different dendrites of individual neurones were investigated, by using simple transformations to take account of the differences in diameter and spine density between dendritic segments. A novel "morphotonic" transformation is described that represents the purely morphological component of electrotonic length. Morphotonic lengths can be converted into electrotonic lengths by division by a "morphoelectric factor" ([Rm/Ri]1/2). This procedure has the advantage of separating the steps involving anatomical and electrical parameters. These transformations indicated that the dendrites of the apical terminal arbor were much longer electrically than the basal or apical oblique dendrites. In relative electrical terms, most apical oblique trees arose extremely close to the soma, and terminated at similar distances to the basals. These results indicate that the dendrites of these pyramidal cells cannot be represented as single equivalent cylinders. The electrotonic lengths of the dendrites were calculated by using the electrical parameters specific membrane capacitance (Cm), intracellular resistivity (Ri), and specific membrane resistivity (Rm). Conventional values were assumed for Cm (1.0 muFcm-2) and Ri (100 omega cm), but three different Rm values were used for each cell. Two of these were within the conventionally accepted range (10,000-20,000 omega cm2), while the third value was an order of magnitude higher, in line with some recent evidence from modeling and whole-cell recording studies.(ABSTRACT TRUNCATED AT 400 WORDS)

A comparison of the electrophysiological properties of morphologically identified cells in layers 5B and 6 of the rat neocortex

In vitro studies performed in mammalian brain slices have shown that cortical neurons differ in their intrinsic membrane properties. In the rodent cortex these properties are related to a specific cell morphology and synaptic connectivity in some cells but not in others. Due to their small size, little is known about the intrinsic membrane properties of layer 6 cells, however, and it is not clear whether cell morphology is related to electrophysiological properties in this layer. We used a combination of intracellular recording and dye-filling to study the electrophysiological and morphological characteristics of layer 6 cells of the rat sensorimotor cortex in vitro and compared their properties to those of large layer 5B pyramidal cells. Our sample of 24 filled and anatomically reconstructed cells in layer 6 confirms previous Golgi studies that showed them to be a morphologically diverse group consisting of regularly and irregularly oriented pyramidal cells and spiny nonpyramidal cells. Regular layer 6 pyramidal cells differed with respect to the length of their apical dendrites and extent of their axonal arborizations, while irregularly oriented pyramidal cells consisted of sideways or inverted pyramidal cells of variable size and morphology. Spiny nonpyramidal cells included bi-tufted and multi-polar cell types that differed in size and extent of dendritic trees. Many layer 6 cells showed long horizontal axon collaterals in layer 6, and an oblique or vertical projection to layer 4. Stimulation with intracellular constant current pulses revealed that the morphological diversity was mirrored by a similar electrophysiological diversity. Most layer 6 cells were capable of firing trains of action potentials characterized by an initial doublet or triplet followed by a train of single spikes (phasic-tonic mode). The majority of layer 6 cells could fire in either a tonic (single spikes only) mode with low strength current input and a phasic-tonic pattern with higher current strengths. A minority fired either always phasic-tonic or tonic-only spike trains. The size and sequence of spike afterpotentials during low-rate repetitive firing was highly variable in layer 6 cells suggesting that the relative importance of ionic currents responsible for spike repolarization and afterpotentials varied from cell to cell. Subthreshold responses showed prominent inward rectification, while hyperpolarizing "sag" was present in most cells tested. In comparison, large layer 5B pyramidal cells fired either phasic-tonic only or both phasic-tonic and tonic patterns. A minority of cells were capable of firing repetitive bursts, while the remainder fired repetitive single spikes.(ABSTRACT TRUNCATED AT 400 WORDS)

An intracellular medium formulary

Whole-cell patch-clamp recordings allow diffusible intracellular ions and molecules to be replaced by the contents of the recording pipette. In this review, the formulation of intracellular media is considered with a view to improving the stability of recordings and emulating the intracellular environment.