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

PSA-NCAM is required for activity-induced synaptic plasticity

Hippocampal organotypic slice cultures maintained 10-20 days in vitro express a high level of the polysialylated embryonic form of neural cell adhesion molecule (NCAM) (PSA-NCAM). Treatment of the cultures with endoneuraminidase-N selectively removed polysialic acid (PSA) from NCAM and completely prevented induction of long-term potentiation (LTP) and long-term depression (LTD) without affecting cellular or synaptic parameters. Similarly, slices prepared from transgenic mice lacking the NCAM gene exhibited a decaying LTP. No inhibition of N-methyl-D-aspartic acid receptor-dependent synaptic responses was detected. Washout of the enzyme resulted in reexpression of PSA immunoreactivity which correlated with a complete recovery of LTP and LTD. This reexpression was blocked by TTX and low calcium and enhanced by bicuculline. Taken together, these results indicate that neuronal activity regulates the expression of PSA-NCAM at the synapse and that this expression is required for the induction of synaptic plasticity.

Functional correlation of NMDA receptor epsilon subunits expression with the properties of single-channel and synaptic currents in the developing cerebellum

NMDA receptor (NMDAR) subunits epsilon 1-epsilon 4 are expressed differentially with respect to brain region and ontogenic period, but their functional roles still are unclear. We have compared an epsilon 1 subunit-ablated mutant mouse with the wild-type to characterize the effect of epsilon subunit expression on NMDAR-mediated single-channel currents and synaptic currents of granule cells in cerebellar slices. Single-channel and Western blot analyses indicated that the epsilon 2 subunit disappeared gradually during the first postnatal month in both wild-type and mutant mice. Concomitantly, the voltage-dependent Mg2+ block of NMDAR-mediated EPSCs (NMDA-EPSCs) was decreased. Throughout the developmental period studied, postnatal day 7-24 (P7-P24), the decay time course of NMDA-EPSCs in epsilon 1 mutant (-/-) mice was slower than in wild-type mice. We suggest that the expression of the epsilon 3 subunit late in development is responsible for a reduction in the sensitivity of NMDA-EPSCs to block by extracellular Mg2+ and that receptors containing the epsilon 1 subunit determine the fast kinetics of the NMDA-EPSCs.

Increased neuronal excitability during depolarization-induced suppression of inhibition in rat hippocampus

1. Depolarization-induced suppression of inhibition (DSI) is a form of plasticity of gamma-amino-butyric acid (GABAA)-mediated (henceforth 'GABAergic') responses in the CNS. We made whole-cell recordings from CA1 pyramidal neurons to investigate the effects of DSI on excitatory synaptic transmission in the hippocampal slice preparation. 2. Significant enhancement of the voltage-clamped excitatory postsynaptic current (EPSC) occurs during DSI of the temporally overlapping inhibitory postsynaptic current. With high levels of calcium chelators in the pipette solution, or bath application of bicuculline, EPSC enhancement is blocked, suggesting that it results from DSI and that the DSI process selectively affects GABAergic, but not glutamatergic, transmission. 3. The probability of synaptically evoked action potential firing is increased during DSI under current clamp. DSI could influence other excitatory phenomena as well.

Muscarinic activation of a voltage-dependent cation nonselective current in rat association cortex

The ionic mechanism underlying the acetylcholine-induced depolarization of layer V pyramidal neurons of rat prefrontal cortex was examined using whole-cell recording in in vitro rat brain slices. Consistent with previous results, pressure application of acetylcholine to layer V pyramidal neurons elicited a strong depolarization. Pharmacological analysis of this response indicated that it was mediated by the stimulation of muscarinic receptors as it was mimicked by muscarinic agonists, but not by nicotine, and was blocked by atropine. The inward current responsible for the depolarization resulted from the activation of a voltage-dependent, cation nonselective current. Thus, the amplitude of the current was critically dependent on extracellular sodium concentration but not on extracellular potassium or chloride concentration. Examination of the I-V relationship for the muscarinic current using voltage clamp revealed that the current reversed near -15 mV and exhibited a strong voltage dependence, turning off rapidly in the subthreshold range. The voltage dependence of the current led to the appearance of a current associated with a conductance decrease when examined using steady-state voltage- or current-clamp measurements. This might have led to earlier misidentification of this response as mediated by a decrease in potassium conductance. These results question the traditional interpretation that muscarinic depolarization in cortex is mediated by a decrease in potassium conductance. They indicate that the fundamental mechanism responsible for muscarinic depolarization in prefrontal cortex involves the activation of a voltage-dependent, cation nonselective current. This current might represent a previously unsuspected mechanism capable of mediating slow depolarization in the central nervous system.

Multimodal distribution of amplitudes of miniature and spontaneous EPSPs recorded in rat trigeminal motoneurones

1. The whole-cell variant of the patch recording method has been used to obtain voltage recordings from trigeminal motoneurones in tissue slices (500 microns thick) taken from rats aged 8 days. Membrane properties (input resistance, membrane time constant and rheobase, i.e. threshold current required to elicit an action potential) of the motoneurones were determined and recordings made of the (untriggered) EPSP activity. 2. Untriggered EPSP activity was recorded in standard artificial cerebrospinal fluid (ACSF), ACSF with added tetrodotoxin (TTX) and in nominally Ca(2+)-free ACSF with added TTX. In each case the amplitude distributions of single EPSPs were peaky and could be fitted by a model consisting of the sum of equidistant Gaussians (n = 7/9 cells). In contrast, the amplitude distribution of the noise was always unimodal. 3. All EPSP activity recorded in the presence of TTX was abolished by addition of 6-cyano-7-nitroquinoxaline-2-3-dione (CNQX; 10 microM), suggesting the activity was all mediated by glutamate acting primarily at AMPA/kainate receptors. 4. In the majority of cases, there was no correlation between the amplitude of EPSPs underlying each Gaussian and the EPSP rise time but there was a positive correlation between the EPSP half-width and EPSP rise time. The rise times of EPSPs underlying the first, and all, fitted Gaussians were similar to that for the total sample of EPSPs in each motoneurone. Taken together, this suggests that the EPSPs underlying each Gaussian arise from inputs to different dendritic compartments, and that the range of compartments is similar for EPSPs underlying successive Gaussians. 5. Two conclusions are drawn. First, EPSPs of different dendritic origin have similar amplitudes at the soma. Second, the multimodal distribution of EPSP amplitudes recorded in the presence of TTX raises the possibility that individual boutons may contain multiple release sites, with each perhaps operating on a separate functional group of postsynaptic receptors.

Whole-cell patch-clamp recording reveals subthreshold sound-evoked postsynaptic currents in the inferior colliculus of awake bats

The inferior colliculus receives excitatory and inhibitory input from parallel auditory pathways that differ in discharge patterns, latencies, and binaural properties. Processing in the inferior colliculus may depend on the temporal sequence in which excitatory and inhibitory synaptic inputs are activated and on the resulting balance between excitation and inhibition. To explore this issue at the cellular level, we used the novel approach of whole-cell patch-clamp recording in the midbrain of awake bats (Eptesicus fuscus) to record EPSCs or IPSCs. Sound-evoked EPSCs were recorded in most neurons. These EPSCs were frequently preceded by an IPSC, followed by an IPSC, or both. These findings help explain the large latency range and transient responses that characterize inferior colliculus neurons. The EPSC was sometimes followed by long-lasting oscillatory currents, suggesting that a single brief sound sets up a pattern of altered excitability that persists far beyond the duration of the initial sound. In three binaural neurons, ipsilateral sound evoked a large IPSC that partially or totally canceled the EPSC evoked by contralateral sound. In one binaural neuron with ipsilaterally evoked IPSCs, contralaterally evoked IPSCs occurred in response to frequencies above and below the neuron's best frequency. Thus, both monaural and binaural interactions can occur at single inferior colliculus neurons. These results show that whole-cell patch-clamp recording offers a powerful means of understanding how subthreshold processes determine the responses of auditory neurons.

Is activation of metabotropic glutamate receptors responsible for acute hyposic changes in hippocampal neurons?

In whole-cell recordings from CA1 neurons in slices from rats, the mGLUR agonist (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (ACPD; 10 microM) had a depolarizing action on most cells, associated with an increase in input resistance and suppression of afterhyperpolarizations. Under voltage-clamp, there were corresponding changes in membrane current and conductance; in the presence of ACPD, the slow voltage-dependent outward current recorded at approximately -25 mV was smaller and was more clearly depressed by hypoxia. Neither ACPD nor mGLUR antagonists, L(+)-2-amino-3-phosphonoproprionic acid (L-AP3; 1 mM) and (+)-alpha-methyl-4-carboxyphenyl-glycine (MCPG; 0.5 mM), reduced the hyperpolarization or outward current (or the associated changes in input resistance or conductance) induced by 2 min of hypoxia. Early inward currents, corresponding to the early, transient depolarizing effect of hypoxia, wer also not significantly depressed by either MCPG or L-AP3. The hypoxic responses of CA1 neurons in slices are therefore unlikely to be caused mainly be glutamate release and activation of mGLURs.

Activation of metabotropic glutamate receptor type 2/3 suppresses transmission at rat hippocampal mossy fibre synapses

1. The effects of metabotropic glutamate receptor (mGluR) agonists on excitatory transmission at mossy fibre-CA3 synapses were studied in rat hippocampal slice preparations using both extracellular and whole-cell clamp recording techniques. 2. Application of a novel and potent mGluR2/mGluR3-specific agonist (2S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV, 0.1 microM) reversibly suppressed field excitatory postsynaptic potentials evoked by mossy fibre stimulation. DCG-IV at the same concentration did not affect other glutamatergic excitatory transmissions at the commissural/associational input to CA3 or at the Schaffer collateral/commissural input to CA1 regions. 3. This suppressing effect of DCG-IV on mossy fibre transmission was dose dependent and partly antagonized by a competitive mGluR antagonist (+)-methyl-4-carboxylphenylglycine (1 mM). 4. The field potential changes induced by pressure application of glutamate (0.1 mM) to the stratum lucidum of the CA3 region was unaffected by 0.1 microM DCG-IV. 5. In whole-cell clamp experiments, 0.1 microM DCG-IV suppressed excitatory postsynaptic currents evoked by mossy fibre stimulation without inducing detectable inward current in CA3 neurons, and paired-pulse facilitation was enhanced by DCG-IV application. 6. These results suggest that mGluR2/mGluR3 are specifically expressed at mossy fibre synapses in the hippocampal CA3 region, and activation of the receptor suppresses synaptic transmission by an action on a presynaptic site.

Diversity of postsynaptic amplitude and failure probability of unitary excitatory synapses between CA3 and CA1 cells in the rat hippocampus

Different CA3 cells may have dissimilar effects on a CA1 pyramidal cell. In order to test this idea, we studied the amplitude distribution of excitatory postsynaptic currents (EPSCs) in response to weak electrical stimulation of presynaptic axons in the rat hippocampal slice. We accepted the response populations as representative for the effect of, in most cases, a single axon when the EPSCs appeared at a certain threshold stimulation strength, with the subsequent lack of increase in amplitude with further stimulation increase. By comparing the EPSC amplitude distributions obtained from different synaptic inputs to the same CA1 cell, we found differences in the failure probability and the EPSC amplitude, each of which contributed to differences in the mean response amplitude. We conclude that not only the number but also the specific subset of active CA3 cells is important for the synaptically driven discharge of a given CA1 cell.

A cholecystokinin-B receptor antagonist potentiates GABAergic and glycinergic inhibition in the nucleus of the solitary tract of the rat

In both rodent and primate in vivo models, cholecystokininB (CCKB) antagonists such as PD134,308 have anxiolytic effects that may involve the potentiation of GABAergic transmission. We have investigated this interaction using exogenous application of GABA and whole cell patch recording techniques in neurons of the nucleus of the solitary tract (NTS) in brainstem slice preparations. In the presence of PD143,308 the magnitude of the GABA-evoked decrease in membrane input resistance was enhanced by 41.2 +/- 3.1% and the duration of the response was prolonged by 34.8 +/- 2.2%. Also, PD134, 308 potentiated glycine-evoked decreases in membrane input resistance, increasing the amplitude of the response by 62.8 +/- 4. 85 and prolonging the duration of the response by 23.5 +/- 3.6%. The effect of PD134,308 persisted in the presence of tetrodotoxin, after reversal of the transmembrane gradient of chloride ions and under conditions of exaggerated GABAA receptor desensitization. Our results demonstrate that at least part of the functional link between PD134,308 and the GABAA response occurs postsynaptically.

Acamprosate (calcium acetylhomotaurinate) enhances the N-methyl-D-aspartate component of excitatory neurotransmission in rat hippocampal CA1 neurons in vitro

The taurinate analog acamprosate (calcium acetylhomotaurinate) has received considerable attention in Europe for its ability to prevent relapse in abstained alcoholics. To determine the mechanism of acamprosate actions in the CNS, we superfused acamprosate onto rat hippocampal CA1 pyramidal neurons using an in vitro slice preparation. In current-and voltage-clamp recordings, acamprosate (100 to 100 microM) superfusion had little effect on resting membrane potential or input slope resistance. Acamprosate had no effect on Ca(2+)-dependent action potentials when tetrodotoxin was used to block Na+ spikes. In whole-cell voltage-clamp recordings, and in the presence of tetraethylammonium and Cs+ to block K+ channels, acamprosate had little effect on a Cd(2+)-sensitive inward current likely to be a high voltage-activated Ca2+ current. However, in both current- and voltage-clamp recordings, acamprosate significantly increased the N-methyl-D-aspartate (NMDA) component of excitatory postsynaptic potentials evoked by stimulation of Schaffer collaterals in the stratum radiatum, in the presence of the selective non-NMDA (R,S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid kainate) glutamate receptor antagonist 6-cyano-7-nitro-quinoxaline-2,3-dione and the GABAA receptor antagonist bicuculline. Acamprosate had inconsistent or no effects on the stratum radiatum-evoked non-NMDA component of the excitatory postsynaptic potentials, in the presence of bicuculline and the NMDA antagonist DL-2-amino-5-phosphonovalerate. Acamprosate, on average, had little effect on the late inhibitory postsynaptic potentials thought to be mediated by GABAB receptors. In the presence of tetrodotoxin to block synaptic transmission, acamprosate dramatically increased inward current responses in most CA1 neurons to exogenous NMDA applied by pressure or superfusion, with reversal on washout of acamprosate. These data suggest that acamprosate may act postsynaptically to increase the NMDA component of excitatory transmission to hippocampal CA1 pyramidal neurons. Considering the known interaction of ethanol with NMDA receptors, this acamprosate modulation of NMDA receptor-mediated neurotransmission could provide a mechanism of action underlying the clinical efficacy of acamprosate.

Differential effects of histamine on the N-methyl-D-aspartate channel in hippocampal slices and cultures

The effect of histamine on N-methyl-D-aspartate currents was investigated in pyramidal neurons in the CA1 region of acute hippocampal slices from juvenile rats. The objective was to compare histamine effects in the slice with those previously reported in acutely dissociated and cultured hippocampal neurons. Micromolar concentrations of histamine had no effect on N-methyl-D-aspartate mediated excitatory postsynaptic currents in the slice, in contrast to the large enhancement seen in culture under identical conditions. However, millimolar concentrations of histamine blocked these currents both in the slice and in culture. Possible reasons for the lack of enhancement in the slice were explored as follows. (1) Histamine could not penetrate the slice or was already present at high concentrations inside the slice. This was tested by recording N-methyl-D-aspartate currents elicited in outside-out patches pulled from the somas of CA1 slice neurons. Histamine still had no effect in patches, whereas the corresponding experiment for cultured neurons showed robust enhancement. (2) Slices release an endogenous ligand that binds with high affinity to the histamine site on the N-methyl-D-aspartate receptor, blocking its activation. This was tested by superfusing cultures with supernatant from homogenized slice tissue. Histamine enhancement was maintained in these cultures. (3) CA1 slices and cultures express different N-methyl-D-aspartate receptor subtypes. The reverse transcription-polymerase chain reaction technique was used to examine the expression of messenger RNA encoding N-methyl-D-aspartate receptor subunits in the two systems. No difference was found in the whole-tissue expression of messenger RNA for the NR2A, 2B or 2C subunits or for the eight known splice variants of the NR1 subunit. It is hypothesized that the differential enhancing effect of histamine in slices and culture involves posttranslational modifications or other factors that modulate the N-methyl-D-aspartate receptor/ion channel according to its environment.

Slow inactivation of Na+ current and slow cumulative spike adaptation in mouse and guinea-pig neocortical neurones in slices

1. Spike adaptation of neocortical pyramidal neurones was studied with sharp electrode recordings in slices of guinea-pig parietal cortex and whole-cell patch recordings of mouse somatosensory cortex. Repetitive intracellular stimulation with 1 s depolarizing pulses delivered at intervals of < 5 s caused slow, cumulative adaptation of spike firing, which was not associated with a change in resting conductance, and which persisted when Co2+ replaced Ca2+ in the bathing medium. 2. Development of slow cumulative adaptation was associated with a gradual decrease in maximal rates of rise of action potentials, a slowing in the post-spike depolarization towards threshold, and a positive shift in the threshold voltage for the next spike in the train; maximal spike repolarization rates and after-hyperpolarizations were unchanged. 3. The data suggested that slow adaptation reflects use-dependent removal of Na+ channels from the available pool by an inactivation process which is much slower than fast, Hodgkin-Huxley-type inactivation. 4. We therefore studied the properties of Na+ channels in layer II-III mouse neocortical cells using the cell-attached configuration of the patch-in-slice technique. These had a slope conductance of 18 +/- 1 pS and an extrapolated reversal potential of 127 +/- 6 mV above resting potential (Vr) (mean +/- S.E.M.; n = 5). Vr was estimated at -72 +/- 3 mV (n = 8), based on the voltage dependence of the steady-state inactivation (h infinity) curve. 5. Slow inactivation (SI) of Na+ channels had a mono-exponential onset with tau on between 0.86 and 2.33 s (n = 3). Steady-state SI was half-maximal at -43.8 mV and had a slope of 14.4 mV (e-fold)-1. Recovery from a 2 s conditioning pulse was bi-exponential and voltage dependent; the slow time constant ranged between 0.45 and 2.5 s at voltages between-128 and -68 mV. 6. The experimentally determined parameters of SI were adequate to simulate slow cumulative adaptation of spike firing in a single-compartment computer model. 7. Persistent Na+ current, which was recorded in whole-cell configuration during slow voltage ramps (35 mV s-1), also underwent pronounced SI, which was apparent when the ramp was preceded by a prolonged depolarizing pulse.

Axotomized neonatal motoneurons overexpressing the bcl2 proto-oncogene retain functional electrophysiological properties

Bcl2 overexpression prevents axotomy-induced neuronal death of neonatal facial motoneurons, as defined by morphological criteria. However, the functional properties of these surviving lesioned transgenic neurons are unknown. Using transgenic mice overexpressing the protein Bcl2, we have investigated the bioelectrical properties of transgenic facial motoneurons from 7 to 20 days after neonatal unilateral axotomy using brain-stem slices and whole cell patch-clamp recording. Nonaxotomized facial motoneurons from wild-type and transgenic mice had similar properties; they had an input resistance of 38 +/- 6 M omega and fired repetitively after injection of positive current pulses. When cells were voltage-clamped at or near their resting membrane potential, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), N-methyl-D-aspartic acid (NMDA), or vasopressin generated sustained inward currents. In transgenic axotomized mice, facial motoneurons could be found located ipsilaterally to the lesion; they had an input resistance of 150 +/- 30 M omega, indicating that they were smaller in size, fired repetitively, and were also responsive to AMPA, NMDA, and vasopressin. Morphological measurements achieved 1 week after the lesion have shown that application of brain-derived neurotrophic factor prevented the reduction in size of axotomized transgenic motoneurons. These data indicate that Bcl2 not only prevents morphological apoptotic death of axotomized neonatal transgenic motoneurons but also permits motoneurons to conserve functional electrophysiological properties.

'Intact' dopaminergic midbrain neurons of the rat display unclamped dendritic Ca2+ currents

Calcium channels play an important role in generating the complex electrophysiology of dopaminergic (DAergic) neurons in the substantia nigra pars compacta (SNc). To directly study these calcium currents in dendritically intact neurons, two issues needed to be addressed: the identity of the neuron (DAergic versus non-DAergic) and control of putative dendritic calcium conductances. A measure of the anomalous rectifier (Ih) was used to identify DAergic neurons. Both groups of neurons produced large (approximately 1 nA), sustained inward currents that persisted as 'plateau currents' for hundreds of milliseconds after the termination of the depolarizing voltage clamp step. We conclude that intact DAergic neurons can be identified under conditions optimal for recording calcium currents (i.e. Cs+ internal solution), but powerful dendritic currents limit rigorous biophysical analysis.

Convergence of magno- and parvocellular pathways in layer 4B of macaque primary visual cortex

Early visual processing is characterized by two independent parallel pathways: the magnocellular stream, which carries information useful for motion analysis, and the parvocellular stream, which carries information useful for analyses of shape and colour. Although increasing anatomical and physiological evidence indicates some degree of convergence of the two streams, the pathway through layer 4B of primary visual cortex (VI) and on to higher cortical areas is usually considered to carry only magnocellular input. This is inferred from anatomical descriptions of local circuitry in V1, and functional studies of area MT, which receives input from layer 4B. We have directly measured the sources of local functional input to individual layer 4B neurons by combining intracellular recording and biocytin labelling with laser-scanning photostimulation. We found that most layer 4B neurons receive strong input from both magnocellular-stream-recipient layer 4Calpha neurons and parvocellular-stream-recipient layer 4Cbeta neurons. Thus higher cortical areas that receive input either directly or indirectly from layer 4B are likely to be more strongly influenced by the parvocellular pathway than previously believed.

Cellular mechanisms for neuronal thermosensitivity in the rat hypothalamus

1. To study the basic mechanisms of neuronal thermosensitivity, rat hypothalamic tissue slices were used to record and compare intracellular activity of temperature-sensitive and -insensitive neurones. This study tested the hypothesis that different neuronal types have thermally dependent differences in the transient potentials that determine the interspike interval. 2. Most spontaneously firing neurones displayed depolarizing prepotentials that preceded each action potential. In warm-sensitive neurones, warming increased the rate of rise of the depolarizing prepotential which, in turn, decreased the interspike interval and increased the firing rate. In contrast, temperature had little or no effect on the rate of rise in prepotentials of temperature-insensitive neurones. 3. Prepotential depolarization can be due to increasing depolarizing conductances or decreasing hyperpolarizing conductances. These are differences in the ionic conductances responsible for prepotentials in temperature-sensitive and -insensitive neurones. In warm-sensitive neurones, the net ionic conductance decreased as the prepotential depolarized towards threshold, suggesting that the prepotential is primarily determined by a decrease in outward potassium conductances. In contrast, in low-slope temperature-insensitive neurones, the net conductance remained constant during the interspike interval, suggesting a more balanced combination of both depolarizing and hyperpolarizing conductances. 4. Transient outward potassium currents, including A-currents, are important determinants of neuronal firing rates. These currents were identified in all warm-sensitive neurones tested, as well as in many temperature-insensitive and silent neurones. Since warming increased the rates of inactivation of these currents, transient K+ currents may contribute to the temperature-dependent prepotentials of some hypothalamic neurones.

Cyclic AMP mediates a presynaptic form of LTP at cerebellar parallel fiber synapses

The N-methyl-D-aspartate receptor-independent form of long-term potentiation (LTP) at hippocampal mossy fiber synapses requires presynaptic Ca(2+)-dependent activation of adenylyl cyclase. To determine whether this form of LTP might occur at other synapses, we examined cerebellar parallel fibers that, like hippocampal mossy fiber synapses, express high levels of the Ca2+/calmodulin-sensitive adenylyl cyclase I. Repetitive stimulation of parallel fibers caused a long-lasting increase in synaptic strength that was associated with a decrease in paired-pulse facilitation. Blockade of glutamate receptors did not prevent LTP induction, nor did loading of Purkinje cells with a Ca2+ chelator. LTP was occluded by forskolin-induced potentiation and blocked by the protein kinase A inhibitor Rp-8-CPT-cAMPS. These findings suggest that parallel fiber synapses express a form of LTP that is dependent on the activation of a presynaptic adenylyl cyclase and is indistinguishable from LTP at hippocampal mossy fiber synapses.

Electrophysiological actions of hemoglobin on rat hippocampal CA1 pyramidal neurons

Hemoglobin, the oxygen-carrying component of red blood cells, can be released from erythrocytes in hemorrhagic stroke and intracranial bleeding associated with head injuries. Therefore, neurons may be exposed to this agent. In addition, hemoglobin can chelate nitric oxide (NO) and has been used in studying the role of NO in synaptic plasticity and excitotoxicity. However, the electrophysiological actions of hemoglobin on central neurons are not well characterized. In the present investigation, the electrophysiological actions of hemoglobin on CA1 pyramidal neurons in rat hippocampal slices were studied with conventional intracellular pointed microelectrode- as well as perforated patch-recordings. Superfusion of rat hippocampal slices with hemoglobin (0.05 or 0.1 mM for 10-15 min) induced a depolarization of CA1 neurons and suppressed the stratum radiatum stimulation-induced excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs). The hemoglobin-induced depolarization as well as the suppression of the synaptic transients were present in slices pretreated with 0.1 or 0.5 mM of N omega-nitro-L-arginine, a nitric oxide synthase inhibitor, suggesting that hemoglobin has electrophysiological actions on hippocampal CA1 neurons that are independent of its NO scavenging property.

Mice deficient for prion protein exhibit normal neuronal excitability and synaptic transmission in the hippocampus

We recorded in the CA1 region from hippocampal slices of prion protein (PrP) gene knockout mice to investigate whether the loss of the normal form of prion protein (PrPC) affects neuronal excitability as well as synaptic transmission in the central nervous system. No deficit in synaptic inhibition was found using field potential recordings because (i) responses induced by stimulation in stratum radiatum consisted of a single population spike in PrP gene knockout mice similar to that recorded from control mice and (ii) the plot of field excitatory postsynaptic potential slope versus the population spike amplitude showed no difference between the two groups of mice. Intracellular recordings also failed to detect any difference in cell excitability and the reversal potential for inhibitory postsynaptic potentials. Analysis of the kinetics of inhibitory postsynaptic current revealed no modification. Finally, we examined whether synaptic plasticity was altered and found no difference in long-term potentiation between control and PrP gene knockout mice. On the basis of our findings, we propose that the loss of the normal form of prion protein does not alter the physiology of the CA1 region of the hippocampus.

Heterogeneous axonal arborizations of rat thalamic reticular neurons in the ventrobasal nucleus

The gamma-aminobutyric acid (GABA)-containing neurons of the thalamic reticular nucleus (nRt) are a major source of inhibitory innervation in dorsal thalamic nuclei. Individual nRt neurons were intracellularly recorded and labelled in an in vitro rat thalamic slice preparation to investigate their projection into ventrobasal thalamic nuclei (VB). Camera lucida reconstructions of 37 neurons indicated that nRt innervation ranges from a compact, focal projection to a widespread, diffuse projection encompassing large areas of VB. The main axons of 65% of the cells gave rise to intra-nRt collaterals prior to leaving the nucleus and, once within VB, ramified into one of three branching patterns: cluster, intermediate, and diffuse. The cluster arborization encompassed a focal region averaging approximately 25,000 mu m2 and contained a high density of axonal swellings, indicative of a topographic projection. The intermediate structure extended across an area approximately fourfold greater and also contained numerous axonal swellings. The diffuse arborization of nRt neurons covered a large region of VB and contained a relatively low density of axonal swellings. Analysis of somatic size and shape revealed that diffuse arborizations arose from significantly smaller, fusiform-shaped somata. Cytochrome oxidase reactivity or parvalbumin immunoreactivity was used to delineate a discontinuous staining pattern representing thalamic barreloids. The size of a cluster arborization closely approximated that of an individual barreloid. The heterogeneous arborizations from nRt neurons may reflect a dynamic range of inhibitory influences of nRt on dorsal thalamic activity.

Postsynaptic levels of [Ca2+]i needed to trigger LTD and LTP

Long-term potentiation (LTP) and long-term depression (LTD) in CA1 pyramidal neurons are both triggered by a postsynaptic rise in intracellular Ca2+ concentration ([Ca2+]i). We used photolysis of postsynaptic caged Ca2+ compounds to search for differential thresholds for activation of these processes. Long-lasting potentiation (LLP) resembling LTP, and long-lasting depression (LLD) resembling LTD, were evoked by [Ca2+]i elevations of comparable magnitude and duration in different cells. No distinctions in threshold for these processes were detectable. LLP was occluded by tetanically induced LTP and blocked by calmodulin inhibition, and LLD was occluded by electrically induced LTD and blocked by phosphatase inhibition.

Bidirectional control of quantal size by synaptic activity in the hippocampus

Analysis of strontium-induced asynchronous release of quanta from stimulated synapses revealed that long-term potentiation and long-term depression in the CA1 region of the mammalian hippocampus are associated with an increase and a decrease, respectively, in quantal size. At a single set of synapses, the increase in quantal size seen with long-term potentiation was completely reversed by depotentiating stimuli. Long-term potentiation and depression are also associated with an increase and decrease, respectively, in the frequency of quantal events, consistent with an all-or-none regulation (up or down) of clusters of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, a change in the release of transmitter, or both.

GABAB-activated gK+ in thalamic neurons in the lethargic (lh/lh) mouse model of generalized absence seizures

Whole-cell voltage-clamp recordings were made from thalamic ventrobasal (VB) neurons of age-matched lethargic (lh/lh) and wildtype (+/+) mice. Hyperpolarizing voltage commands (40 mV) from a holding potential of -60 mV were delivered to the cell and the resulting K+ conductance (gK+) activated by the GABAB receptor agonist baclofen was measured and compared between the two groups. VB cells from +/+ and lh/lh displayed no significant differences in resting conductance (gIN) or gK+ activated by baclofen. In addition to this, isolated, evoked GABAB-mediated currents were recorded in VB cells. There was no significant difference in peak amplitude or latency to peak noted between the two groups. These data suggest that postsynaptic GABAB receptor-mediated function is not altered in VB thalamic neurones in this model of absence seizures.

Ischemia and lesion induced imbalances in cortical function

Cortical structures are often critically affected by ischemic and traumatic lesions which may cause transient or permanent functional disturbances. These disorders consist of changes in the membrane properties of single cells and alterations in synaptic network interactions within and between cortical areas including large-scale reorganizations in the representation of the peripheral input. Prominent functional modifications consisting of massive membrane depolarizations, suppression of intracortical inhibitory synaptic mechanisms and enhancement of excitatory synaptic transmission can be observed within a few minutes following the onset of cortical hypoxia or ischemia and probably represent the trigger signals for the induction of neuronal hyperexcitability, irreversible cellular dysfunction and cell death. Pharmacological manipulation of these early events may therefore be the most effective approach to control ischemia and lesion induced disturbances and to attenuate long-term neurological deficits. The complexity of secondary structural and functional alterations in cortical and subcortical structures demands an early and powerful intervention before neuronal damage expands to intact regions. The unsatisfactory clinical experience with calcium and N-methyl-D-aspartate antagonists suggests that this result might be achieved with compounds that show a broad spectrum of actions at different ligand-activated receptors, voltage-dependent channels and that also act at the vascular system. Whether the same therapy strategies developed for the treatment of ischemic injury in the adult brain may be applied for the immature cortex is questionable, since young cortical networks with a high degree of synaptic plasticity reveal a different response pattern to hypoxic and ischemic insults. Age-dependent molecular biological, morphological and physiological parameters contribute to an enhanced susceptibility of the immature brain to these noxae during early ontogenesis and have to be investigated in more detail for the development of adequate clinical therapy.

Evidence that heterosynaptic depolarization underlies associativity of long-term potentiation in rat hippocampus

1. Whole-cell patch-clamp recording has been used to study the effect of heterosynaptic depolarization on pure N-methyl-D-aspartate (NMDA) receptor-mediated synaptic transmission in the CA1 region of rat hippocampal slices. 2. In neurones voltage clamped at -60 mV, paired-pulse stimulation of one set of Schaffer collateral-commissural fibres resulted in homosynaptic paired-pulse facilitation of the NMDA receptor-mediated excitatory postsynaptic current (EPSCN). In contrast, stimulation of one set of fibres prior to stimulation of a second set of fibres (i.e. heterosynaptic paired-pulse stimulation) did not result in any heterosynaptic interactions. 3. However, under current-clamp conditions, heterosynaptic paired-pulse stimulation resulted in heterosynaptic 'paired-pulse facilitation' of the NMDA receptor-mediated excitatory postsynaptic potential (EPSPN). 4. In neurones held at -50 or -40 mV, perfusion of nominally Mg(2+)-free medium converted the response to heterosynaptic paired-pulse stimulation from 'heterosynaptic facilitation' to 'heterosynaptic depression' of EPSPN. 5. When neurones were held at potentials of between -30 and +40 mV then heterosynaptic paired-pulse stimulation, in normal Mg(2+)-containing medium, resulted in 'paired-pulse depression' of EPSPN. Under voltage-clamp conditions (tested at +40 mV) no heterosynaptic interactions were seen. 6. The time course of 'heterosynaptic facilitation' at -60 mV and of 'heterosynaptic depression' at +40 mV of EPSPN was similar to the time course of EPSCN. 7. We conclude, firstly, that the voltage clamp is able to prevent any voltage breakthrough associated with the synaptic activation of NMDA receptors from influencing neighbouring synapses. Secondly, when the neurone is not voltage clamped these same synapses are strongly influenced by the spreading depolarization generated by the synaptic activation of their neighbours. The time course and direction of this influence are compatible with the hypothesis that spreading synaptic depolarization, leading to a reduction of the voltage-dependent Mg2+ block of synaptic NMDA receptor channels, underlies the property of associativity.

Intracellular acidification reduced gap junction coupling between immature rat neocortical pyramidal neurones

1. Developmental changes in electrophysiological properties of pyramidal neurones correlated with the developmental decline in gap junction-dependent dye coupling were investigated in coronal slices of rat prefrontal and sensorimotor cortex. Effects of intracellular acidification induced by application of weak organic acids on neuronal dye coupling, electrotonic parameters as well as synaptic potentials were examined using the patch clamp technique. Optical monitoring of intracellular pH revealed an acidic shift of 0.4-0.5 pH units following sodium propionate application. 2. Dye coupling between layer II-III neurones was prominent during the first two postnatal weeks. During this period, pre-incubation of slices with 30 mM of the sodium salts of weak organic acids reduced the number of cells coupled to the injected neurones by 64%. 3. Between postnatal days 1 and 18, the mean neuronal input resistance decreased significantly (by 81.0%). Both the membrane time constant (tau 0) and the first equalizing time constant (tau 1) also showed a significant developmental decline of 25.8 and 65.8%, respectively. Electrotonic length decreased by 34.9%. The electrophysiological properties of neurones displayed a pronounced intercellular variability which decreased with on-going development. 4. During the first two postnatal weeks, intracellular acidification led to a mean increase in neuronal input resistance of 55.9% and a mean decreae in electrotonic length of 22.2%. The membrane time constant was reduced by approximately 25% in the majority of neurones tested. Significant electrophysiological effects induced by intracellular acidification were not detected in uncoupled neurones from 18-day-old rats. 5. EPSP width at half-maximal amplitude showed a substantial reduction of approximately 50%, while rise times of the non-NMDA receptor-mediated EPSP components displayed no significant change during development. Both weak organic acids, as well as the gap junction blocker 1-octanol, reduced excitatory synaptic transmission independent of developmental age. 6. We conclude that gap junction permeability is regulated by intracellular pH in developing layer II-III pyramidal cells in the rat neocortex. The prominent correlation between pH-induced reduction in dye coupling and changes in electrophysiological cell properties suggests a significant influence of gap junctions on synaptic integration and information transfer in the immature neocortex.

Orthodromic synaptic activation of rat olfactory bulb mitral cells in isolated slices

Axons of olfactory receptor neurons terminate in the glomerular layer of the olfactory bulb, where they synapse with the apical dendrites of mitral cells. Although the mitral cell and its excitation by the olfactory nerve have been the subject of numerous experimental investigations, in vitro studies of these neurons have primarily used nonmammalian preparations. We have recorded the responses of rat olfactory bulb mitral cells to stimulation of the olfactory nerve layer in vitro using extracellular and whole cell patch techniques. Olfactory bulbs were cut into 400-microns thick slices in approximately horizontal section and submerged in a recording chamber. Patch clamp electrodes were guided into the mitral cell layer, which was visible under a dissecting microscope. A stimulating electrode was placed onto the olfactory nerve layer (ONL) rostral to the recording electrode. In extracellular recordings, mitral cells typically responded to ONL stimulation with a prolonged excitation lasting 1 s or longer. With whole cell patch recordings, membrane resistances (mean 272 M omega) were substantially higher than those reported in previous intracellular studies that used sharp electrodes. Small spontaneous excitatory potentials were present in some mitral cells. ONL stimulation caused a prolonged depolarization comparable to the duration of the period of excitation observed in extracellular recordings. At membrane potentials near -55 mV, ONL stimulation evoked a train of spikes. All but the first of these spikes were blocked by hyperpolarization of the membrane to -65 mV.

Whole-cell voltage-clamp investigation of the role of PKC in muscarinic inhibition of IAHP in rat CA1 hippocampal neurons

Muscarinic, cholinergic inputs, largely from the medial septum, have pronounced effects on hippocampal cell excitability. A major effect of synaptically released ACh is block of the slow Ca(2+)-dependent potassium current, called IAHP. Protein kinase C exists in the hippocampus in high concentrations, its activation blocks IAHP, and it has been suggested as a mediator of the muscarinic-receptor-(mAChR)-mediated actions. Using conditions that produce a stable postspike afterhyperpolarizing current (IAHP) in whole-cell recordings from CA1 hippocampal pyramidal neurons in the slice preparation, we have investigated the role of PKC in the cholinergic inhibition of IAHP mediated by mACHRs. Bath application of the general kinase inhibitor, H7, had no effect on inhibition of IAHP by carbachol, although H7 dramatically reduced inhibition of IAHP by the phorbol ester, phorbol-12, 13-diacetate (PDA). Another muscarinic response thought to be mediated by PKC-inhibition of GABAB-mediated hyperpolarization-was reduced by extracellular H7 treatment, suggesting that the coupling between mAChRs and protein kinase activity was maintained in whole-cell recordings. We also discovered that PDA does not mediate its effects on IAHP directly. Intracellular perfusion of high concentrations of H7 (10 mM) or the specific PKC inhibitor, PKCI(19-31) (1 mM), did not prevent inhibition of IAHP by PDA. These results are consistent with an indirect, presynaptic action of phorbol esters on IAHP, possibly mediated through enhanced release of neurotransmitter from surrounding cells.

Only 'de novo' long-term depression (LTD) in the rat hippocampus in vitro is blocked by the same low concentration of FK506 that blocks LTD in the visual cortex

It has been proposed that the long-term depression (LTD) seen following low frequency stimulation (LFS) in the rat hippocampus involves calcineurin. We have tested this by examining the effect of FK506, a macrolide which blocks calcineurin at nanomolar concentrations, on synaptic transmission in the rat hippocampal slice at a concentration of 1 microM which has been shown to block LTD in the visual cortex. The effect of FK506 on long-term potentiation (LTP) and spontaneous transmitter release was also studied. The magnitude of LTD induced by LFS was 16.7 +/- 2.4% in control which was not significantly different from the 22.3 +/- 3.0% seen in the same preparations after exposure to FK506 for 25-30 min. In contrast the magnitude of LTD induced 'de novo' in preparations exposed to FK506 was significantly reduced. FK506 had no significant effect on LTP, miniature EPSP frequency, miniature EPSP amplitude, resting membrane potential or input resistance. These results, therefore, support the hypothesis that calcineurin is involved in 'de novo' LTD but it appears that an event is triggered by LFS whereby FK506-insensitive LTD can subsequently be activated by a second episode of LFS.

In vitro study of afferent synaptic transmission in the rostral gustatory zone of the rat nucleus of the solitary tract

The synaptic responses of rostral nucleus of the solitary tract (rNST) neurons to electrical stimulation of the solitary tract (ST) fibers were investigated using whole-cell recordings in brain slices of adult rat medulla. Most neurons of the rNST (47%) responded to stimulation of the ST with excitatory postsynaptic potentials (EPSPs), 28% responded with mixed excitatory and inhibitory postsynaptic potentials (PSPs) and 25% responded with inhibitory postsynaptic potentials (IPSPs). The estimated reversal potentials for the EPSPs (EEPSP) was -7 mV and for the IPSPs (EIPSP) was -69 mV. The glutamate antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) acting at the alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA)/kainate receptor, either reduced or blocked all EPSPs tested. D-2-Amino-5-phosphonovalerate (APV), a selective N-methyl-D-aspartate (NMDA) receptor antagonist, also reduced the amplitude of the EPSPs. These results suggest that glutamate is released following stimulation of afferent fibers in the ST and acts on both AMPA/kainate and NMDA glutamate receptors. The IPSPs result from release of gamma-aminobutyric acid (GABA) since superfusion of the GABAA receptor antagonist, bicuculline reversibly blocked the IPSPs. The GABAB receptor antagonist, phaclofen, also reduced the IPSP components in some neurons, indicating that both GABAA and GABAB receptors are involved in inhibitory transmission in the rNST. When the morphology of the recorded neurons was examined by filling the neurons with biocytin and reconstructing the neurons, each morphological type of rNST neuron responded with excitatory and inhibitory PSPs following stimulation of the ST.

Metabotropic glutamate receptors inhibiting excitatory synapses in the CA1 area of rat hippocampus

In the CA1 region of hippocampal slices prepared from young adult rats, we studied the ability of several specific agonists of metabotropic glutamate receptors (mGluRs) to depress excitatory synaptic transmission at the CA3-CA1 pyramidal cell synapses. Three groups of mGluRs have been described: group 1 (mGluR1 and 5) receptors are positively coupled to phospholipase C whereas group 2 (mGluR2 and 3) and group 3 (mGluR4, 6, 7 and 8) receptors are negatively coupled to adenylate cyclase. We found that the broad-spectrum agonist (1S,3R)-1-aminocyclopentyl-1,3-dicarboxylate and the group 1-specific agonist (R,S)-dihydroxyphenylglycine both reversibly inhibited evoked field excitatory postsynaptic potentials, indicating the involvement of group 1 mGluRs. (R,S)-3,5-dihydroxyphenylglycine presumably inhibited transmission via a presynaptic mechanism, as whole-cell voltage-clamp recordings revealed that inhibition of the synaptic transmission was always accompanied with an increase in paired-pulse facilitation. Treatment with a specific blocker of mGluR1 receptors, the phenylglycine derivative (S)-4-carboxyphenylglycine, was without effect on the (1S,3R)-1-amino-cyclopentyl-1,3-dicarboxylate-induced depression of the field excitatory postsynaptic potentials, strongly suggesting that mGluR5 receptors are responsible for the (1S,3R)-1-aminocyclopentyl-1,3-dicarboxylate effect. Two selective agonists of group 2 mGluRs, (2S,1's,2's)-2-(2'-carboxycyclopropyl)glycine and 4-carboxy-3-hydroxyphenylglycine, were totally ineffective in blocking CA3-CA1-evoked synaptic transmission, excluding the involvement of mGluR2/3 subtypes at this developmental stage.

GABA and glutamate depolarize cortical progenitor cells and inhibit DNA synthesis

We have found that, during the early stages of cortical neurogenesis, both GABA and glutamate depolarize cells in the ventricular zone of rat embryonic neocortex. In the ventricular zone, glutamate acts on AMPA/kainate receptors, while GABA acts on GABAA receptors. GABA induces an inward current at resting membrane potentials, presumably owing to a high intracellular Cl- concentration maintained by furosemide-sensitive Cl- transport. GABA and glutamate also produce increases in intracellular Ca2+ in ventricular zone cells, in part through activation of voltage-gated Ca2+ channels. Furthermore, GABA and glutamate decrease the number of embryonic cortical cells synthesizing DNA. Depolarization with K+ similarly decreases DNA synthesis, suggesting that the neurotransmitters act via membrane depolarization. Applied alone, GABAA and AMPA/kainate receptor antagonists increase DNA synthesis, indicating that endogenously released amino acids influence neocortical progenitors in the cell cycle. These results demonstrate a novel role for amino acid neurotransmitters in regulating neocortical neurogenesis.

Postsynaptic cAMP pathway gates early LTP in hippocampal CA1 region

The role of the cAMP pathway in LTP was studied in the CA1 region of hippocampus. Widely spaced trains of high frequency stimulation generated cAMP postsynaptically via NMDA receptors and calmodulin, consistent with the Ca2+/calmodulin-mediated stimulation of postsynaptic adenylyl cyclase. The early phase of LTP produced by the same pattern of high frequency stimulation was dependent on postsynaptic cAMP. However, synaptic transmission was not increased by postsynaptic application of cAMP. Early LTP became cAMP-independent when protein phosphatase inhibitors were injected postsynaptically. These observations indicate that in early LTP the cAMP signaling pathway, instead of transmitting signals for the generation of LTP, gates LTP through postsynaptic protein phosphatases.

GABAergic suppression prevents the appearance and subsequent fatigue of an NMDA receptor-mediated potential in neocortex

We have investigated the regulation of an N-methyl-D-aspartate (NMDA) receptor-mediated synaptic potential by gamma-aminobutyric acid (GABA)-mediated inhibition using extracellular and whole-cell voltage clamp recordings in rat auditory cortex in vitro. Single afferent stimulus pulses at low intensity elicited a slow extracellular negativity (Component C) that was mediated by NMDA receptors. At higher intensities, Component C was suppressed by recruitment of GABAergic inhibition. To understand the actions of GABAergic inhibition on Component C, we determined the effects of: (i) paired-pulse stimulation, which depresses GABAergic inhibition; (ii) pharmacological antagonism of GABA receptors; and (iii) afferent stimulation in slices from neonatal rats prior to the development of cortical inhibition. The results indicate that GABAergic inhibition prevents Component C from occurring, thereby preventing its reduction upon repeated stimulation. Whole-cell voltage clamp recordings were used to test the hypothesis that GABAergic suppression occurred by way of membrane hyperpolarization. At hyperpolarized holding potentials no NMDA receptor-mediated synaptic current was elicited, even with paired-pulse stimulation. At depolarized holding potentials a significant NMDA synaptic current was elicited despite the presence of GABAergic synaptic currents. We conclude that membrane hyperpolarization by GABAergic inhibition prevents the appearance and subsequent fatigue of an NMDA receptor-mediated synaptic potential. Reduction of inhibition can act as a 'switch' to fully release the NMDA potential as frequently as once every 10-20 s.

Voltage-gated potassium currents in stratum oriens-alveus inhibitory neurones of the rat CA1 hippocampus

1. Voltage-activated K+ currents were recorded from visually identified inhibitory interneurones of the CA1 stratum oriens-alveus region in neonatal rat hippocampal slices using outside-out patch and whole-cell voltage clamp techniques. 2. Outward currents comprised both a transient and a sustained component when elicited from a holding potential of -90 mV. Tail current analysis of current reversal potentials showed that outward currents were carried by potassium ions. 3. The transient current, IA, was activated with a time to peak within 5 ms, inactivated with a time constant of approximately 15 ms at 0 mV and possessed half-activation at -14 mV. Half-inactivation of the transient current occurred at -71 mV. At -90 mV, the transient current recovered from inactivation with a time constant of 142 ms. 4. Activation of currents from a holding potential of -50 mV permitted isolation of the sustained current, IK. In Ca(2+)-free conditions the sustained current showed rapid activation, reaching about 80% of its maximum within 1.5 ms, and showed little inactivation during 1 s depolarizing steps. The majority of sustained outward currents showed no voltage-dependent inactivation. In approximately 20% of cells, a slow time-dependent inactivation of the sustained current was observed, suggesting the presence of a second type of sustained current in these cells. 5. A Ca(2+)-dependent K+ current comprised a significant portion of the total sustained current; this current was activated at voltages positive to -30 mV and showed no time-dependent inactivation over a 1 s depolarizing step. This current component was removed in Ca(2+)-free conditions or by iberiotoxin. 6. Low concentrations of 4-AP (50 microM) attenuated both the transient and sustained current components recorded in a Ca(2+)-free solution. Higher concentrations of 4-AP (< 10 mM) were without further effect on the sustained current but completely blocked the transient current with an IC50 of 1.8 mM. TEA blocked the sustained current with an IC50 of 7.9 mM without significantly reducing the transient current. Both current components were resistant to dendrotoxin (500 nM).

Hippocampal long-term depression: arachidonic acid as a potential retrograde messenger

Long-term depression (LTD) was studied in hippocampal slices from neonatal rats at the synapse between CA3 and CA1 pyramidal neurons. The induction of LTD requires the pairing of Ca2+ influx into the postsynaptic CA1 neuron through voltage-gated calcium channels with activation of metabotropic glutamate receptors. The expression of this LTD is at least partly presynaptic, implying the need for a retrograde messenger. We present evidence that arachidonic acid might serve such a function. Thus, arachidonic acid applications simulate LTD whereas blockade of arachidonic acid release inhibits LTD.

Adapters for combined intrapipette pressure pulses and patch pipette step movements during 'blind' cell search in brain slices

A procedure is described for 'blind' cell search in brain slices based on pressure pulses instead of steady-state pressure applied to the patch pipette during its stepwise movements. For reproducibility of the pressure/movement pattern during the cell search, we have developed two adapters, one for electrically and the other for hydraulically driven micromanipulators which generate pressure pulses synchronized with patch-pipette step movements. Both adapters increase the intrapipette pressure prior to a step movement of the pipette, maintain the pressure during the pipette movement, and release it between steps, thus minimizing the possibility of 'blowing-away' the cells during the search. The hydraulic micromanipulator adapter converts this into a stepping one. Both adapters also allow simultaneous recording of pipette step movements and of intrapipette pressure. The use of these adapters allows standardization of the 'blind' cell search and greatly increases the success rate of cells detection.

Activation of adenosine A1 receptors initiates short-term synaptic depression in rat striatum

High-frequency stimulation (HFS) of afferent fibers produced short-term depression (STD) and long-term depression (LTD) of corticostriatal synaptic transmission. Application of the non-selective adenosine receptor antagonist 1,3-dipropyl-8-p-sulfophenylxanthine (DPSPX) or the selective A1 antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) blocked the induction of STD but not LTD. Application of adenosine or the selective A1 receptor agonist R(-)N6-(2-phenylisopropyl)adenosine (R-PIA) induced synaptic depression, while the A2a receptor agonist CGS 21680 did not consistently alter synaptic transmission. Depression induced by adenosine or R-PIA was not accompanied by changes in postsynaptic input resistance and appeared to involve a presynaptic depressant effect previously characterized at this synapse. These observations indicate that HFS leads to the production of endogenous adenosine that acts on presynaptic A1 receptors to initiate STD at corticostriatal synapses. Initiation and maintenance of LTD appear to be independent of A1 receptor activation.

Confocal imaging and local photolysis of caged compounds: dual probes of synaptic function

Chemical signals generated at synapses are highly limited in both spatial range and time course, so that experiments studying such signals must measure and manipulate them in both these dimensions. We describe an optical system that combines confocal laser scanning microscopy, to measure such signals, with focal photolysis of caged compounds. This system can elevate neurotransmitter and second messenger levels in femtoliter volumes of single dendrites within a millisecond. The method is readily combined with whole-cell patch-clamp measurements of electrical signals in brain slices. In cerebellar Purkinje cells, photolysis of caged IP3 causes spatially restricted intracellular release of Ca2+, and photolysis of a caged Ca2+ compound locally opens Ca(2+)-dependent K+ channels. Furthermore, localized photolysis of the caged neurotransmitter GABA transiently activates GABA receptors. The use of focal uncaging can yield new information about the spatial range of signaling actions at synapses.

Pharmacological characterization of pre- and postsynaptic GABAB receptors in the deep nuclei of rat cerebellar slices

Whole-cell current-and voltage-clamp recordings were made from deep nuclear neurons in cerebellar slices from seven- to nine-day-old rats. Baclofen, a GABAB agonist, produced a slow postsynaptic hyperpolarization associated with a decrease in input resistance. The hyperpolarization was G-protein-dependent, blocked by intracellular Cs+ and antagonized by CGP 35348, a GABAB antagonist. In dialysed neurons recorded with Cs+ -containing pipettes, baclofen suppressed deep nuclear neuronal inhibitory postsynaptic potentials and inhibitory postsynaptic currents evoked by electrical stimulations of the Purkinje cell axons. This effect was blocked by CGP 35348, indicating that the suppressions were mediated by presynaptic GABAB receptors. The inability of CGP 35348 or uptake inhibitors (nipecotic acid and NO-711) to alter the decay of inhibitory postsynaptic currents evoked by maximal stimulation suggested that GABAB receptors are not activated by the stimulation of the GABAergic input. Paired-pulse depression of inhibitory postsynaptic currents was not blocked by CGP 35348. Moreover, neither uptake inhibitors nor CGP 35348 produced any significant changes to the whole-cell current produced by a tetanic stimulation of Purkinje cell axons, suggesting that GABAB autoreceptors were also not activated by endogenous GABA release. Our findings indicate that while pre- and postsynaptic GABAB receptors are present in the deep nuclei of the rat cerebellum, they are not activated by electrical stimulation of the Purkinje cell axons.

Regulation of hippocampal transmitter release during development and long-term potentiation

Developmental changes in rat hippocampal transmitter release and synaptic plasticity were investigated. Recordings from pairs of pyramidal neurons in slices showed that an action potential in a CA3 neuron released only a single quantum of transmitter onto a CA1 neuron. Failures of synaptic transmission reflected probabilistic transmitter release. The probability of release (Pr) was 0.9 in 4- to 8-day-old rats and decreased to less than 0.5 at 2 to 3 weeks. Long-term potentiation (LTP) in 2- to 3-week-old rats was associated with an increase in Pr from a single synaptic site. The high initial Pr in 4- to 8-day-old rats normally occludes the expression of LTP at this stage.

Extracellular activation of unitary excitatory synapses between hippocampal CA3 and CA1 pyramidal cells

The possibility of regular activation of unitary excitatory synapses on hippocampal CA1 cells by electrical stimulation of Schaffer collaterals was explored in the rat. The amplitude of the excitatory postsynaptic currents (EPSCs) and failures in response to a range of stimulation intensities around the threshold for the smallest detectable EPSC were analysed. After an abrupt appearance of EPSCs in response to increasing stimulation strength, both EPSC amplitude and failure rate could reach a plateau where increasing stimulation intensity did not cause additional responses. This was interpreted as a regular activation of mainly a single axon. Statistical methods showed, however, that only 12 out of approximately 50 experiments using threshold stimulation were without significant contamination from additional fibres. In this subset of experiments, upper limits for contamination from other fibres were estimated by using bootstrapping methods. More than 90% of the responses were probably due to faithful activation of a single axon, assuming that the density of axons connecting to one target cells is relatively homogeneous. This result makes the described method suitable for examining some aspects of the transmission between individual hippocampal cells.

Patterns of excitation and inhibition evoked by horizontal connections in visual cortex share a common relationship to orientation columns

Combined optical imaging and electrophysiological techniques were used to assess directly the functional nature of long-range excitatory and inhibitory synaptic interactions between orientation columns in area 17 of ferret visual cortex. A significant correlation was found between the layout of iso-orientation columns and the pattern of evoked synaptic inputs between cortical sites: the largest-amplitude inhibitory and excitatory synaptic responses were evoked in single neurons when stimulation and recording electrodes were located in orientation columns sharing the same angle preference. Both excitatory and inhibitory synaptic responses decreased in amplitude when stimulation and recording electrodes were located in orientation columns with orthogonal angle preferences. Changing the stimulus intensity altered the balance of evoked excitation and inhibition without changing the columnar specificity of inputs. These results directly demonstrate that horizontal connections modulate both excitatory and inhibitory synaptic interactions between iso-orientation columns.

Channels underlying the slow afterhyperpolarization in hippocampal pyramidal neurons: neurotransmitters modulate the open probability

The slow afterhyperpolarization in hippocampal pyramidal neurons is mediated by a calcium-activated potassium current (IAHP) and is a target for variety of different neurotransmitters. The characteristics of the channels underlying IAHP and how they are modulated by neurotransmitters are, however, unknown. In this study, we have examined the properties of the channels underlying IAHP using fluctuation analysis of the macroscopic current. Our results indicate that this channel has a unitary conductance of 2-5 pS and a mean open time of about 2 ms. When the peak amplitude of IAHP was maximal, these channels have an open probability of 0.4. Noradrenaline and carbachol reduced IAHP amplitude by lowering open channel probability. These result indicate that a novel calcium-activated potassium channel underlies IAHP. This channel is modulated in a similar fashion by two different transmitter systems that utilize distinct protein kinases.

1S,3R-ACPD-preferring inward current in rat dorsolateral septal neurons is mediated by a novel excitatory amino acid receptor

Metabotropic glutamate receptors (mGluRs) form a receptor family that consists of diverse receptor subtypes; now, numbering 8--exclusive of splice variants. (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) has been suggested to be a selective agonist for the mGluRs. We have recently reported that, in rat dorsolateral septal nucleus (DLSN) neurones, a 1S,3R-ACPD-preferring inward current (ACPDi) persists in pertussis toxin-treated rats. We now report that this ACPDi-current: (1) persists in DLSN neurones dialyzed with a stable analog of GTP, namely, GTP gamma S; (2) exhibits a negative slope region with inward rectification in its I-V relationship; (3) persists in neurones superfused with tetrodotoxin or low calcium solutions; (4) is dependent upon both sodium and calcium ions; and (5) is independent of a reduction in temperature. Furthermore, pharmacological data suggest that this current may be activated by a unique type of excitatory amino acid (EAA) receptor, i.e. a receptor which prefers "metabotropic" EAA agonists and is insensitive to AP5 or CNQX. Activation by ACPD of inward currents associated with a conductance increase have also been reported at cultured mouse cerebellar Purkinje neurones; in slices of rat hippocampal CA1 neurones and slice cultures of hippocampal CA3 neurones. We suggest that this ACPDi current may play an important role within the CNS in the induction of long-term potentiation and other neurological processes; processes attributed previously to currents associated with NMDA receptor activation.

Evidence for silent synapses: implications for the expression of LTP

Recent work has suggested that some proportion of excitatory synapses on hippocampal CA1 pyramidal cells that express NMDA receptors (NMDARs) may not express functional AMPA receptors (AMPARs), thus making these synapses silent at the resting membrane potential. In agreement with this hypothesis, we demonstrate here that it is possible to stimulate synapses that yield no detectable excitatory postsynaptic currents (EPSCs) when the cell is held at -60 mV; yet at positive holding potentials (+30 to +60 mV), EPSCs can be elicited that are completely blocked by the NMDAR antagonist, D-APV. When these functionally silent synapses are subjected to an LTP induction protocol, EPSCs mediated by AMPARs appear and remain for the duration of the experiment. This conversion of silent synapses to functional synapses is blocked by D-APV. These results suggest that LTP may involve modification of AMPARs that, prior to LTP, were either not present in the postsynaptic membrane or electrophysiologically silent. This mechanism may account for several experimental results previously attributed to presynaptic changes in quantal content.