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

A comparison of the effects of selective metabotropic glutamate receptor agonists on synaptically evoked whole cell currents of rat spinal ventral horn neurones in vitro

1. Whole cell synaptic currents were recorded under voltage clamp from a total of 54 ventral horn neurones held near to their resting potential by the patch clamp technique in immature rat spinal cord preparations in vitro. Twenty eight neurones were identified, by antidromic invasion from ventral roots, as motoneurones. Excitatory postsynaptic currents (e.p.s.cs) of peak amplitude -480 pA +/- 66 s.e. mean and -829 +/- 124 pA were evoked respectively from the unidentified ventral horn neurones and the motoneurones in response to maximal activation of the segmental dorsal root. 2. The e.p.s.cs were depressed reversibly by the metabotropic glutamate agonists 1S3S-1-aminocyclopentane-1,3-dicarboxylate (1S3S-ACPD) (EC50 17.1 microM +/- 0.3 s.e. mean, n = 14) and L-2-amino-4-phosphonobutanoate (L-AP4) (EC50 = 2.19 +/- 0.19 microM, n = 15). Since both agonists independently produced more than 90% depression it is likely that the receptors that mediate their effects are present on the same presynaptic terminals. 3. When the Mg2+ concentration was raised from 0.75 mM to 2.75 mM together with the addition of 50 microM D-2-amino-5-phosphonopentanoate (AP5), a treatment which would increase the proportion of monosynaptic component in the e.p.s.c. the concentration-effect plots for both 1S3S-ACPD (EC50 1.95 +/- 0.4 microM, n = 8) and L-AP4 (EC50 0.55 +/- 0.20 microM, n = 7) were shifted to the left, suggesting that monosynaptic e.p.cs of primary afferents to ventral horn neurones are more susceptible to L-AP4 and 1S3S-ACPD than are other synapses in polysynaptic pathways. 4. lS3S-ACPD (20 and 50 microM) also caused mean sustained inward currents of 95 +/- 31 pA (n = 6) and248 +/- 49 pA (n = 10) respectively. In the combined presence of AP5 (50 microM) and Mg2+ (2.75 mM) themean response to 50 microM lS3S-ACPD was reduced to 106+/- 18 pA (n = 4). In the presence of tetrodotoxin(1 microM) the corresponding value was 48 +/- 6 pA (n = 4). Similar sustained inward currents produced by N-methyl-D-aspartate (NMDA) were almost abolished to < 10 pA in the presence of AP5 and 2.75 mMMg2+. In the presence of tetrodotoxin the maximum inward current produced by NMDA was undiminished. Thus a large component of the excitatory action of lS3S-ACPD was mediated at non-NMDA receptors both directly at the patch-clamped neurones and indirectly by synaptic relay.

Fiber-type composition of hindlimb muscles in the turtle, Pseudemys (Trachemys) scripta elegans

A description is provided of the fiber-type composition of several hindlimb muscles of the adult turtle, Pseudemys (Trachemys) scripta elegans. In addition, cross-section areas of each fiber type and an estimation of the relative (weighted) cross-section area (wCSA) occupied by the different fiber types are also provided. Seven muscles were selected for study, based on their suitability for future neurophysiological analysis as components of the segmental motor system, and on their homologies with muscles in other vertebrates. The test muscles were iliofibularis (ILF), ambiens (AMB), external gastrocnemius (EG), extensor digitorum communis (EDC), flexor digitorum longus (FDL), tibialis anterior (TA), and peroneus anterior (PA). Serial sections of these muscles were stained for myosin adenosine triphosphatase (ATPase), NADH-diaphorase, and alpha-glycerophosphate dehydrogenase (alpha-GPDH), thereby enabling fiber-type classification on the basis of indirect markers for contraction speed and oxidative (aerobic) vs. glycolytic (anaerobic) metabolism. All muscles contained three fiber types: slow oxidative (SO; possibly including some non-twitch tonic fibers); fast oxidative glycolytic (FOG); and fast glycolytic (Fg). There were at least 30% FOG and 50% FOG + Fg fibers in the seven muscles, the extreme distributions being the predominantly glycolytic ILF vs. the predominantly oxidative FDL muscle (ILF--15.5% SO, 35.2% FOG, 49.3% Fg vs. FDL--49.1% SO, 41.1% FOG, 9.8% Fg). As in other species, the test muscles exhibited varying degrees of regional concentration (compartmentalization) of the different fiber types. This feature was most striking in ILF. Pronounced compartmentalization was also observed in AMB, EG, PA, TA, and EDC, whereas the distribution of fiber types in the highly oxidative FDL was homogeneous. In five of the seven muscles, fiber size was ranked with Fg > FOG > SO. In terms of wCSA, which provides a coarse-grain measure of the different fiber types' potential contribution to whole muscle peak force, all muscles exhibited a higher Fg and lower SO contribution to cross-section area than suggested by their corresponding fiber-type composition. The largest relative increase in wCSA vs. fiber-type composition were in the ILF and AMB muscles. We conclude that the turtle hindlimb provides some interesting possibilities for testing for a division of labor among different muscles during different movements (e.g., sustained vs. ballistic), and for study of the behavior of the different fiber (and motor unit) types under normal and perturbed conditions. The relationships between the present results and previous findings on homologous muscles of the mammalian (cat, rat) and reptilian (lizard) hindlimb are discussed.

Modulation of N-methyl-D-aspartic acid receptors by extracellular calcium in immature and adult hippocampal slices: whole cell recordings in CA3 pyramidal cells

Lowering extracellular calcium concentration [Ca2+]o in rat hippocampal slices can lead to an induction of epileptiform activity. It has been shown that this effect is more pronounced in slices of neonatal rats (postnatal day, PND 8-19) than in mature slices (> PND 40) and it has been suggested that unique N-methyl-D-aspartic acid (NMDA) receptor properties of immature rat hippocampal pyramidal cells contribute to this developmental effect. In a voltage clamp experiment we tested NMDA receptor properties in hippocampal pyramidal cells by measuring NMDA receptor mediated currents evoked by iontophoretic applied NMDA in the basal dendrites of CA3 pyramidal neurons. We found that lowering extracellular calcium from 2 to 1 mM, increases NMDA evoked inward current in pyramidal cells around the resting membrane potential. However, this effect is observed in slices of neonatal as well as in slices of mature rats, suggesting that there is no difference in NMDA receptor sensitivity to extracellular Ca2+ between these two age groups. The modulation of the NMDA receptor by extracellular calcium at physiological concentrations can have important consequences in pathological conditions during which extracellular calcium reaches low levels. Because this 'hypocalcemic' condition induces a larger current influx via the NMDA receptor channel at resting membrane potentials, it can further enhance cellular excitability and contribute to sustain epileptiform activity.

Passive propagation of LTD to stratum oriens-alveus inhibitory neurons modulates the temporoammonic input to the hippocampal CA1 region

Excitatory synaptic activity in horizontal stratum oriens-alveus interneurons (OAIs) is driven by the recurrent collaterals of CA1 pyramidal cells and is strongly influenced by protocols that elicit synaptic plasticity in these principal neurons. Induction of LTD in the Schaffer collateral-CA1 pyramidal neuron synapse causes a passive down-regulation of stratum radiatum-evoked excitatory synaptic responses onto OAIs. In addition, we show that the strength of the temporoammonic input to the CA1 pyramidal neuron distal dendrites is regulated by OAI activity. The passive propagation of LTD to OAIs consequently disinhibits the direct entorhinal cortex-CA1 input, resulting in an enhanced excitation of CA1 pyramidal neurons by a mechanism not requiring activation of the trisynaptic pathway.

Dendritic spines as basic functional units of neuronal integration

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

Tonic inhibition originates from synapses close to the soma

Central neurons are subject to a tonic barrage of randomly occurring spontaneous inhibitory events (mIP-SCs) resulting from the action potential-independent release of gamma-aminobutyric acid (GABA). Do the terminals making synapses onto somatic versus dendritic sites, which arise from specific populations of interneurons, differ in their ability to generate mIPSCs? We have combined the techniques of whole-cell patch-clamp recording and computational simulation to demonstrate that in granule cells of the dentate gyrus, most of the action potential-independent inhibition taking place as mIPSCs originates from proximal sites. Indeed, removal of the bulk (> 50%) of the dendritic tree did not change the characteristics of mIPSCs. These results are consistent with a functional segregation of GABAergic terminals synapsing at proximal versus distal portions of central neurons. Thus, proximal GABAergic terminals are responsible for tonic inhibition targeted at the soma.

Use of the whole-cell patch-clamp method in studies on the role of cAMP in regulating the spontaneous firing of locus coeruleus neurons

The whole-cell patch-clamp technique represents a major advance over conventional intracellular recordings in the study of the modulation of ion channels by intracellular messengers. This report illustrates how application of the whole-cell technique to noradrenergic neurons of the rat locus coeruleus in brain slices has led to the finding that cAMP via its phosphorylation pathway modulates tonic pacemaking in these neurons. In the studies to be described, the particular advantage of the whole-cell technique was that it allowed introduction of macromolecules related to the cAMP pathway (e.g., protein kinase inhibitor and protein kinase A) directly into cells. Furthermore, these studies were carried out in situ, in thick brain slices allowing a direct comparison with a large body of existing extracellular and intracellular data obtained under similar conditions.

Electrophysiology of the neurons in the area of the enkephalinergic magnocellular dorsal nucleus of the guinea-pig hypothalamus, studied by intracellular and whole-cell recordings

The electrophysiological characteristics of 103 hypothalamic neurons in the area of the guinea-pig enkephalinergic magnocellular dorsal nucleus were studied in a thick slice preparation with sharp microelectrodes (63 neurons) and patch pipettes for whole-cell recordings (40 neurons). Of the sampled cells, 79.6% displayed tetrodotoxin-resistant, calcium-dependent slow-depolarizing potentials when the membrane potential was hyperpolarized to approximately -70 mV (type I neurons). Half of them showed robust slow depolarizing potentials, generating bursts of fast action potentials. In the remaining neurons, the slow-depolarizing potentials did not cause burst-firing action potentials but triggered single action potentials. The other class of neurons (20.4% of the sample: type II neurons) did not exhibit calcium-dependent slow-depolarizing potentials. Resting potential, input resistance and the membrane time constant did not distinguish among the two classes of neurons. Current-voltage relationships were heterogeneous. A transient outward rectification was observed in the two classes. This was not totally blocked by 2 mM 4-aminopyridine but was abolished when using perfusion with cobalt instead of calcium. Input resistance and the time constant were higher when measured in the whole-cell mode but the other electrical parameters and the sampling of the recorded neurons were strikingly similar between the two methods of recording. Intracellular staining of 22 neurons retrogradely labelled from the lateral septum allowed confirmation of their location within the magnocellular dorsal nucleus. The study indicates that the electrical properties of these neurons did not differ from those of neurons found throughout the area explored. It also indicates the presence of distinct electrophysiological types of cells in the magnocellular dorsal nucleus, although the nucleus is composed of a single type of enkephalinergic neuron. It provides a basis for the study of the regulation of activity of the neurons at the origin of an enkephalinergic tractus which is involved in neuroendocrine, psychoneuroendocrine and immune processes.

Development of intrinsic electrophysiological properties in neurons from the gustatory region of rat nucleus of solitary tract

There is no current understanding of the nature or time course of maturation of intrinsic electrophysiological properties for neurons in the gustatory region of the nucleus of the solitary tract (NST). Therefore, we used whole cell recordings in an in vitro slice preparation of the rat brainstem to characterize development of resting membrane, action potential and repetitive discharge properties of cells in gustatory NST at postnatal days 5, 10, 15, 20, and 30, and adult ages. Neurons were filled with Biocytin to verify location and characterize morphology. Membranes from younger neurons demonstrated a steeper current-voltage relation or higher input resistance, and a longer time constant than mature cells. Action potentials in younger cells had a slower rate of rise and were longer in duration. The afterhyperpolarization that typically follows the spike discharge usually had one phase in younger neurons, but was characterized by two or more phases in an increasing proportion of older cells. The repetitive discharge frequency in response to a range of depolarizing current pulses increased during development, and frequency/current plots were steeper in older compared with younger neurons. However, in all age groups there was clear accommodation of the discharge frequency. The greatest changes in resting membrane, action potential, and discharge properties were observed between P5 and P15, and mature values were generally reached by P20. At each postnatal age, neurons could be categorized in four neuron groups, based on the discharge pattern in response to a hyperpolarizing/depolarizing current protocol. Anatomical reconstructions indicated that although cells increased in overall dendritic expanse during development, neurons became less complex as illustrated by decreases in number of dendritic branch points, and in number and density of spines. The timing of major developmental differences in intrinsic electrical characteristics observed here is associated with a period of previously reported maturational changes in extracellular taste responses to number and concentration of chemical stimuli. However, further alterations in extracellular taste responses proceed after apparent maturation of intrinsic neural properties.

Antisense oligonucleotide-induced block of individual GABAA receptor alpha subunits in cultured visual cortex slices reduces amplitude of evoked inhibitory postsynaptic currents

Whole cell patch clamp recordings were made in layer II-IV from organotypic slices of rat primary visual cortex, explanted at postnatal day 6 and maintained in a serum-free medium. Neurons evinced current clamp characteristics typical for stellate cells. Between 7 and 21 days in culture, both glutamate- and GABA-mediated postsynaptic currents were observed. Long-term culturing in the presence of a degenerate 15-mer antisense oligonucleotide directed against the transcripts of all alpha subunits genes of the GABAA receptor resulted in a dose dependent reduction of evoked GABA synaptic currents. This reduction was maximal (80%) at 20 microM. A randomized control oligo had no effect. Evoked glutamatergic excitatory postsynaptic currents were unaffected following oligo treatment. A 15-mer antisense oligo directed against the alpha 1 subunit gave variable effects: in some cells the amplitude of evoked GABAergic inhibitory postsynaptic currents (IPSCs) was reduced by 50-75%, while in other cells recorded from the same slices, there was little or no effect. An antisense oligo, directed against the alpha 2 subunit, however, gave a consistent and robust 80% reduction of the amplitude of evoked IPSCs. A 15-mer 3-base mismatch oligo against alpha 2 had no effect. We conclude that the alpha 2 subunit functions in postsynaptic GABAA receptors located on or close to the cell bodies of stellate cells. The role of the alpha 1 subunit is less clear, but this subunit seems spatially differentiated. The in situ antisense oligo technique should provide further insight into the biophysical and pharmacological consequences of the subunit composition of ligand gated channels at functional synapses.

A new interface chamber for the study of mammalian nervous tissue slices

We describe a new interface-type chamber for electrophysiological studies in mammalian brain slices. Thermoregulation of the inner recording chamber is achieved using the Peltier effect and a feedback control unit. Between 15 and 40 degrees C, and for perfusion rates from 1 to 5 ml/min, the temperature can be maintained within +/- 0.1 degrees C of the command value; it can also be rapidly and reliably changed. An external bath, heated by a coiled resistor, generates a humidified, oxygenated atmosphere diffusing above the slices. Survival of neuronal tissue is excellent and stable intracellular recordings can be obtained using either sharp or patch-clamp micropipettes. Perfusion solutions can be readily exchanged, rendering this chamber suitable for the study of bath-applied neuroactive compounds.

Methods for studying neurotransmitter transduction mechanisms

This review describes the methodologies used to study the transduction mechanisms that are activated in excitable cells by G-protein-coupled agonists. In view of the complexity of second-messenger systems, it is no longer relevant to ask, "What is the transduction mechanism involved in the action of a given neuromodulator?" because, in many cases, a variety of transduction mechanisms and physiological responses are invoked following receptor activation. This means that a single aspect of the physiological response must be selected for study in order to address the question of transduction mechanism. This review is therefore concerned with a description the use of patch- and voltage-clamp procedures to study transduction mechanism because they are designed to isolate one aspect of the physiological response: the change in activity of a single type of membrane ion channel.

Epileptogenesis in immature neocortical slices induced by 4-aminopyridine

The laminar site of onset of 4-aminopyridine (4AP)-induced epileptiform discharges in immature neocortical brain slices was localized to layer V using conventional extracellular field electrodes, current source-density analysis (CSD), and subdivided slices. Intracellular patch-electrode recordings in immature layer V neurons confirmed that intrinsically bursting (IB) neurons were not present at the ages studied or with bath application of 50-200 microM 4AP. IB properties were not being masked in the younger animals by the patch electrodes because typical IB neurons in layer V were seen in older rats when the same intracellular techniques were used. These results demonstrate that epileptiform activity can be initiated in the absence of IB neurons, and suggest that other factors are responsible for the preferential onset in layer V.

In vitro brainstem-gastric preparation with intact vagi for study of primary visceral afferent input to dorsal vagal complex in caudal medulla

An in vitro neonatal rat preparation, consisting of the isolated caudal brainstem and stomach joined by the intact vagi, was developed using Sprague-Dawley rats. The animals were 0 to 4 days of age. This preparation provided an opportunity to investigate the extracellular and intracellular responses of neurons in the nucleus tractus solitarius (NTS) of the brainstem to electrical stimulation of subdiaphragmatic vagal fibers. The dorsal and ventral vagal branches were electrically stimulated at the point of the common subdiaphragmatic vagal trunk. The isolated preparation was superfused in a recording chamber at 28 degrees C with a modified Krebs solution, equilibrated with 95% O2 and 5% CO2. Suction microelectrodes, for electrical stimulation, were positioned on the common vagal trunk just below the diaphragm to evaluate extracellular and intracellular evoked responses in NTS. A total of 204 subdiaphragmatic vagally-evoked (SDVe) brainstem unitary responses in the NTS were recorded. The mean latency of the extracellular SDVe brainstem responses was 89 +/- 12.9 ms (mean +/- SD). The peripheral gastric effects of CCK-8 on SDVe unitary responses in NTS neurons were evaluated. The peptide caused a significant increase in the excitability of these NTS neurons which was blocked by the CCKA receptor antagonist L-364,718. Neurons in the NTS and the dorsal motor nucleus of the vagus which showed excitatory responses to vagal stimulation were filled with Lucifer Yellow to evaluate their morphology.

The neuroactive steroid 5 alpha-tetrahydrodeoxycorticosterone increases GABAergic postsynaptic inhibition in rat neocortical neurons in vitro

The neuroactive steroid 5 alpha-pregnane-3 alpha, 21-diol-20-one (5 alpha-tetrahydrodeoxycorticosterone; 5 alpha-THDOC) has been shown to potentiate GABA-induced chloride currents in cell cultures and subcellular preparations. In this study, we recorded from pyramidal neurons in an in vitro slice preparation of the adult rat frontal neocortex using intracellular microelectrodes. 5 alpha-THDOC (10 microM) increased and prolonged the inhibitory postsynaptic potential (IPSP). The mean maximal synaptic conductance of the early, GABAA receptor-mediated, IPSP was enhanced to more than 700%, the one at the maximum of the late, partially GABAB receptor-mediated, IPSP to approximately 400%. The progesterone/glucocorticoid receptor antagonist RU 38486 did not prevent the IPSP increase. At a concentration of 1 microM 5 alpha-THDOC increased only the early IPSP to about 125%. Responses to the iontophoretically applied specific GABAA receptor agonist muscimol but not to the specific GABAB receptor agonist L-baclofen were enhanced by 5 alpha-THDOC (10 microM). In the giga-seal whole-cell configuration when the GABAB receptor-mediated IPSP component was absent due to intracellular perfusion, 5 alpha-THDOC (10 microM) increased IPSPs to a similar extent as in the conventional microelectrode recordings. Excitatory postsynaptic potentials, resting membrane potential, input resistance and action potential amplitude were not affected by 5 alpha-THDOC (10 microM). These data demonstrate that in neocortical tissue of the rat 5 alpha-THDOC enhances GABAergic inhibition by interacting with postsynaptic GABAA receptors while synaptic excitation and parameters of electric excitability remain unchanged.

Synaptic integration in layer IV of the ferret striate cortex

1. Whole-cell patch recording were made with dye-filled electrodes from layer IV in slices of the ferret striate cortex. Projections from the thalamus and layer VI provide most of the extralaminar input to layer IV. Interactions between these two pathways are thought to play a role in the generation of suppressive non-linearities such as end-inhibition. Thus, synaptic responses evoked by stimulating each pathway individually were compared with those produced by activating both projections together. 2. Spiny stellate cells are the majority population in layer IV and were the most frequently patched neurons (n = 23); all fired adapting trains of large, fast action potentials. About half of those tested (n = 13) were progressively inhibited by strengthening the stimulus to layer VI, while the rest became more excited. For the former, the response evoked by stimulating both pathways in coincidence was often more hyperpolarizing than would have been predicted by summing the responses to either projection alone (n = 4). Hence, the inputs from the thalamus and layer VI are integrated by circuits that can produce strong and non-linear inhibition. 3. Recordings from various basket cells, which are inhibitory, have provided a first view of the suppressive circuits in layer IV (n = 5). Two cells were excited by stimulation of both pathways. The remaining three cells were mainly excited by activation of thalamic afferents but were largely inhibited by stimulation of layer VI. Thus, inhibition seen at the level of the spiny stellate cells could result from two mechanisms operating via presynaptic smooth cells: convergent excitation provided by both ascending pathways on the one hand, and a push-pull relationship between pathways on the other.

Activity-dependent depression of monosynaptic fast IPSCs in hippocampus: contributions from reductions in chloride driving force and conductance

Whole-cell recordings techniques were used to record pharmacologically isolated fast inhibitory postsynaptic currents (IPSCs) in CA1 pyramidal neurons from rat hippocampal slices. Repetitive extracellular stimulation up to 10 Hz progressively reduced steady-state fast IPSC amplitude. At low stimulation frequencies (up to 1 Hz), this attenuation was characterized by a positive shift of IPSC reversal potential with no change in IPSC conductance. Above 1 Hz stimulation, fast IPSC depression was associated with changes in both reversal potential and IPSC conductance. Use-dependent depression at low frequencies was prevented when cells were chloride-loaded using cesium chloride based intracellular solutions. These findings suggest that activity-dependent depression of fast IPSCs at low stimulus frequencies results entirely from a reduction in chloride driving force, stemming from intracellular chloride accumulation. Activity-dependent changes in fast IPSC conductance occur only at stimulation rates above 1 Hz.

Reduced hippocampal LTP and spatial learning in mice lacking NMDA receptor epsilon 1 subunit

The NMDA (N-methyl-D-aspartate) receptor channel is important for synaptic plasticity, which is thought to underlie learning, memory and development. The NMDA receptor channel is formed by at least two members of the glutamate receptor (GluR) channel subunit families, the GluR epsilon (NR2) and GluR zeta (NR1) subunit families. The four epsilon subunits are distinct in distribution, properties and regulation. On the basis of the Mg2+ sensitivity and expression patterns, we have proposed that the epsilon 1 (NR2A) and epsilon 2 (NR2B) subunits play a role in synaptic plasticity. Here we show that targeted disruption of the mouse epsilon 1 subunit gene resulted in significant reduction of the NMDA receptor channel current and long-term potentiation at the hippocampal CA1 synapses. The mutant mice also showed a moderate deficiency in spatial learning. These results support the notion that the NMDA receptor channel-dependent synaptic plasticity is the cellular basis of certain forms of learning.

Spatial synaptic integration in Purkinje cell dendrites

Synaptic integration occurs within a framework of synaptic connections, and cell type-specific, intrinsic and transmitter-gated ion channels. These components are differentially distributed over the somato-dendritic membrane. Recent results from Purkinje cells and pyramidal cells exemplify some of these mechanisms of spatial synaptic integration. This paper focusses on the cerebellar Purkinje cell. In these neurons, the amplitude and distribution of single climbing fibre and parallel fibre EPSP-evoked Ca2+ influx were regulated by the transient outward, IA-like current in the distal (spiny) dendrites. The synaptically evoked Ca2+ influx was graded from a local response involving only a few terminal spiny dendrites to a propagated Ca2+ spike. The climbing fibre-evoked Ca2+ influx in the spiny dendrites was finely graded by parallel fibre-induced depolarization. Climbing fibre and parallel fibre-evoked Ca2+ influx elicited a short lasting afterhyperpolarization that affected subsequent dendritic Ca2+ influx. In addition, inhibitory synaptic input controlled dendritic Ca2+ influx. Interaction between information from different sources along the dendrites is thus controlled by intrinsic potassium conductances and IPSPs. Different electrophysiological properties are found in the cerebellar neurons. Thus, Golgi cells, stellate cells and granule cells seem to integrate on a shorter intrinsic timescale than do Purkinje cells, the output neuron of the cerebellar cortex. The specific mechanisms by which different types of presynaptic neurons specifically innervate a given dendritic compartment remain to be elucidated, but recent results provide some experimental evidence of a differential distribution of cell adhesion molecules between the axonal and the somato-dendritic membrane, suggesting one mechanism contributing to the ordered distribution of synapses during synaptogenesis.

Synaptic potentiation of dual-component excitatory postsynaptic currents in the rat hippocampus

1. Whole-cell patch-clamp recording has been used to study tetanus-induced synaptic potentiation of dual-component excitatory postsynaptic currents (EPSCs) in the CA1 region of rat hippocampal slices, following blockade of GABAA and GABAB receptor-mediated synaptic inhibition. 2. At a holding potential of -60 mV, the initial slope of the EPSC (between 10 and 60% of maximum amplitude) provided an accurate measurement of the AMPA receptor-mediated component, and the amplitude of the EPSC at a latency of 100 ms provided the best estimate of the size of the NMDA receptor-mediated component. 3. Neurons were voltage clamped for at least 45 min prior to delivery of a tetanus (test intensity, 100 Hz, 1 s). Measurements at 10 and 30 min following the tetanus were used as indications of short-term potentiation (STP) and long-term potentiation (LTP), respectively. One set of neurons were voltage clamped at -60 mV throughout. These neurons could be subdivided into two populations on the basis of whether or not there was LTP (n = 9), or only STP (n = 6), of the AMPA receptor-mediated component. A second set of neurons were voltage clamped at -60 mV for 30 min and then at -50 mV for 15 min before, during and for 30 min following tetanization. In these experiments there was STP but not LTP (n = 8). 4. In all neurons (n = 23), the time course of the potentiation of the NMDA receptor-mediated component paralleled that of the AMPA receptor-mediated component. In addition, potentiation of the NMDA and AMPA receptor-mediated components were of a similar magnitude. 5. These data demonstrate that it is possible to induce LTP by high frequency stimulation after 45 min of whole-cell recording. Under these conditions, there is a parallel potentiation of the AMPA and NMDA receptor-mediated components of dual-component EPSCs. This constitutes the first evidence, from studies of dual-component synaptic responses, which is consistent with a presynaptic locus of expression of tetanus-induced STP and LTP in the hippocampus.

Hippocampal long-term potentiation and neural cell adhesion molecules L1 and NCAM

Synaptic membranes express cell adhesion molecules. Here we investigate the role of the neural cell adhesion molecules L1 and NCAM in hippocampal long-term potentiation (LTP), a sustained-use-dependent increase in synaptic efficacy that has been implicated in learning and memory. L1 and NCAM mediate cell interactions during neural development and are strongly expressed in the hippocampus. They cooperate to strengthen L1-dependent cell adhesion and are coupled to second messenger pathways. We show that LTP in CA1 neurons of rat hippocampal slices was reduced by application of various L1 and NCAM antibodies, recombinant L1 fragments, and upon dissociation of the L1/NCAM complex through oligomannosidic carbohydrates and NCAM peptides. Neither the activation of NMDA (N-methyl-D-aspartate) receptors nor the maintenance of LTP was affected. These results suggest that L1 and NCAM modulate the development or the stabilization of LTP.

Developmental alterations in the sensitivity of hippocampal NMDA receptors to AP5

Changes in the properties of the N-methyl-D-aspartate (NMDA) receptor are proposed as a factor in the decrease in synaptic plasticity during maturation of the brain. Alterations in the hippocampal NMDA receptor population were studied during development by comparing competitive antagonist efficacy during a window characterized by hyperexcitability and active synaptogenesis to that seen in a more mature period. A developmental change in the sensitivity of the N-methyl-D-aspartate (NMDA) evoked response to the competitive antagonist D(-)2-amino-5- phosphonopentanoic acid (D-AP5) was observed by whole-cell mode voltage-clamp, intracellular and extracellular recordings in hippocampal slices. These differences were observed in the portions of the hippocampus that contain the terminal axon arbors of the CA3 pyramidal neurons, the synaptic fields of the Schaffer collateral, associational and commissural pathways. The apparent antagonist efficacy was 2-3 times greater in immature slices obtained on postnatal day (PND) 10-16 than that observed in more mature tissue (> PND38). Given the possible role of the NMDA receptor in network plasticity, these observations could indicate a role for the molecular diversity of this important receptor subtype family in the maturation of hippocampal pathways.

Facilitation of an NMDA receptor-mediated EPSP by paired-pulse stimulation in rat neocortex via depression of GABAergic IPSPs

1. Tight seal, whole-cell recordings from auditory cortex in vivo and in vitro were obtained to investigate modification of N-methyl-D-aspartate (NMDA) receptor-mediated synaptic activity by paired-pulse afferent stimulation. 2. In recordings from urethane-anaesthetized rats (at 37 degrees C), or from cortical slices maintained in vitro (32 degrees C), afferent stimulation elicited a monosynaptic early EPSP and polysynaptic early and late IPSPs. In addition, a late EPSP could be elicited when the stimulus was preceded by an identical priming stimulus (interval approximately 200 ms). The late EPSP was attenuated by the NMDA receptor antagonist DL-2-amino-5-phosphonovalerate (APV, 50 microM). 3. Bath application of the gamma-aminobutyric acid-B (GABAB) receptor antagonist 3-amino-2-(4-chlorophenyl)-2-hydroxy-propylsulphonic acid (2-OH-saclofen; 50 microM) attenuated the late IPSP and clearly revealed a late EPSP. However, 2-OH-saclofen had lesser effects on the second late EPSP elicited during paired-pulse stimulation. Membrane depolarization in 2-OH-saclofen increased the magnitude of the early IPSP, which suppressed the late EPSP once again. Since pharmacological blockade of EPSPs revealed paired-pulse depression of monosynaptically elicited early and late IPSPs, these data indicate that (1) both early and late IPSPs were capable of suppressing the late EPSP, and (2) these effects were reduced during paired-pulse stimulation. 4. Pharmacological isolation of the late EPSP allowed testing of the direct effect of paired-pulse stimulation. Application of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 20 microM), picrotoxin (10 microM) and 2-OH-saclofen (50 microM) isolated the late EPSP (onset, 3 ms; peak latency, 28 ms; peak amplitude, 7 mV; duration, 240 ms), which grew in magnitude with membrane depolarization and was largely (> 90%) blocked by APV. Paired-pulse stimulation depressed the isolated late EPSP by 30%. 5. Thus, apparent paired-pulse facilitation of the late EPSP is attributable to release from GABAergic inhibition, and not to direct facilitation. Facilitation of the late EPSP is a functional consequence of IPSP depression. The results indicate the importance of inhibition in regulating synaptic activity mediated by NMDA receptors.

Properties of isolated GABAB-mediated inhibitory postsynaptic currents in hippocampal pyramidal cells

Whole-cell recording techniques were used to record isolated slow inhibitory postsynaptic currents in CA1 pyramidal neurons from rat hippocampal slices. Application of 6-cyano-7-nitroquinoxaline-2,3-dione and 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid eliminated excitatory synaptic transmission, resulting in a 38% reduction in slow inhibitory postsynaptic current magnitude. Subsequent addition of the GABAA antagonist picrotoxin caused a further decrease in slow inhibitory postsynaptic current amplitude. The remaining, isolated slow inhibitory postsynaptic current was blocked by the GABAB antagonist 2-hydroxysaclofen and when cesium was substituted for intracellular potassium. The kinetics of isolated slow inhibitory postsynaptic currents were characterized by single exponential, fourth power activation, and double exponential inactivation. These slow inhibitory postsynaptic currents had a reversal potential of -85.7 +/- 1.6 mV, and a slope conductance of 935 +/- 277 pS. Single slow inhibitory postsynaptic currents carried a total charge flux of 13.4 +/- 7.6 pC. Repetitive stimulation up to 1 Hz progressively reduced steady-state slow inhibitory postsynaptic current amplitude. This attenuation was characterized by a decrease in slope conductance, but slow inhibitory postsynaptic current reversal potential remained unchanged, as did slow inhibitory postsynaptic current kinetics. These results indicate that, under physiological conditions, both ionotropic glutamate- and GABAA-mediated transmission contribute to slow inhibitory postsynaptic current recruitment. Given this finding, activity-dependent decreases in GABAA transmission could contribute to slow inhibitory postsynaptic current depression, though not exclusively, since isolated slow inhibitory postsynaptic currents also demonstrated this property. The use-dependent depression of isolated slow inhibitory postsynaptic currents may be a consequence of a reduction in transmitter release.

Depolarization-induced suppression of GABAergic inhibition in rat hippocampal pyramidal cells: G protein involvement in a presynaptic mechanism

Following postsynaptic activation of a pyramidal cell, the degree of GABAergic synaptic inhibition that the cell receives is reduced dramatically for many seconds. Previously, we found that induction of depolarization-induced suppression of inhibition (DSI) required post-synaptic increases in intracellular [Ca2+], but absence of a decrease in responsiveness to iontophoretically applied GABA left the mechanism of DSI expression uncertain. We investigated DSI with whole-cell voltage-clamp recordings in rat hippocampal slices. Bath-applied carbachol was ordinarily used to increase the spontaneous action potential-induced IPSCs (sIPSCs) and enhance detectability of DSI; synaptically released ACh has the same effects. TTX-sensitive sIPSCs are markedly reduced by DSI, whereas TTX-insensitive miniature IPSC amplitudes do not change, suggesting that DSI represents a retrograde influence on presynaptic GABA release. A lag (approximately 1 s) prior to maximal DSI and prevention of DSI by pertussis toxin pointed to a G protein-linked second messenger that may be presynaptic, since perturbation of postsynaptic G protein function did not alter DSI.

Bridging the cleft at GABA synapses in the brain

A fragile balance between excitation and inhibition maintains the normal functioning of the CNS. The dominant inhibitory neurotransmitter of the mammalian brain is GABA, which acts mainly through GABAA and GABAB receptors. Small changes in GABA-mediated inhibition can alter neuronal excitability profoundly and, therefore, a wide range of compounds that clearly modify GABAA-receptor function are used clinically as anesthetics or for the treatment of various nervous system disorders. Recent findings have started to unravel the operation of central GABA synapses where inhibitory events appear to result from the synchronous opening of only tens of GABAA receptors activated by a saturating concentration of GABA. Such properties of GABA synapses impose certain constraints on the physiological and pharmacological modulation of inhibition in the brain.

Infrared videomicroscopy: a new look at neuronal structure and function

Brain slices were introduced as a standard preparation for neurophysiological experiments some 20 years ago. A drawback of this preparation compared with cell culture has been the difficulty to visualize individual neurones in standard thick slices. This problem has been overcome by the use of infrared videomicroscopy. Neurones in slices can now be visualized in great detail, and neuronal processes can be patch-clamped under direct visual control. Infrared video-microscopy has also been applied successfully to other fields of neuroscience such as neuronal development and neurotoxicity. A further development of infrared videomicroscopy enables one to visualize the spread of excitation in slices making the technique a tool for the direct investigation of neuronal function.

NMDA receptors in layers II and III of rat cerebral cortex

Glutamate receptors are found in all layers of the cerebral cortex, but NMDA receptors are concentrated in layers II and III in the adult. We investigated the location of these receptors, and their contribution to the responses of cells in layers V and VI, by iontophoresing NMDA at various distances from the cell body along the apical dendrite of the cells, first in artificial CSF, then in TTX to abolish action potentials. Comparison of responses at various distances along the apical dendrite showed that the response generally increases as distance from the cell body decreases. Comparison of responses in layers II and III, before and after TTX, showed that TTX reduced the response considerably. We conclude first that NMDA receptors in layers II and III are located primarily on cells in layers II and III, rather than on the apical dendrites of cells in layers V and VI, and second that the contribution of NMDA receptors to the response of cells in layers V and VI comes primarily from receptors close to the cell body.

Combining patch-clamp and optical methods in brain slices

Combining patch-clamp and optical imaging techniques in brain slices offers several advantages for physiological studies of nerve cells. Numerous practical considerations weigh heavily in this design of an apparatus suitable for such combined measurements. These considerations include the thickness of the slices, the type of microscope to be used for imaging and the kind of optical signal to be measured. A system that combine optical and patch-clamp methods can be modified readily to permit studies of intracellular and extracellular signaling pathways via flash photolysis of caged compounds.

Characteristics of GABAA channels in rat dentate gyrus

Single channel currents were activated by GABA (0.5 to 5 microM) in cell-attached and inside-out patches from cells in the dentate gyrus of rat hippocampal slices. The currents reversed at the chloride equilibrium potential and were blocked by bicuculline (100 microM). Several different kinds of channel were seen: high conductance and low conductance, rectifying and "nonrectifying." Channels had multiple conductance states. The open probability (Po) of channels was greater at depolarized than at hyperpolarized potentials and the relationship between Po and potential could be fitted with a Boltzmann equation with equivalent valency (z) of 1. The combination of outward rectification and potential-dependent open probability gave very little chloride current at hyperpolarized potentials but steeply increasing current with depolarization, useful properties for a tonic inhibitory mechanism.

5-Hydoxytryptamine evokes depolarizations and membrane potential oscillations in rat sympathetic preganglionic neurones

1. Whole-cell recordings were made from seventy-seven identified rat sympathetic preganglionic neurones (SPN) in spinal cord slices. Perfusion of 5-HT (0.5-30 microM) strongly depolarized 90% of neurones. The response was slow in onset, could last over 10 min and was associated with an increase in input resistance. 5-HT could also evoke rhythmical membrane potential oscillations in a population of previously quiescent neurones. 2. The 5-HT response persisted in TTX and also in low-Ca(2+)-high-Mg2+ artificial cerebrospinal fluid (ACSF), suggesting that the receptors are on SPN. The 5-HT uptake inhibitor 6-nitroquipazine potentiated the 5-HT-induced depolarization. 3. The 5-HT-induced depolarization was reduced and then abolished by membrane hyperpolarization to potentials of about -100 mV, but was not reversed in sign by further hyperpolarization. In voltage clamp, 5-HT evoked inward currents associated with the reduction of an outwardly rectifying potassium conductance. 4. The 5-HT2 receptor agonist alpha-methyl-5-HT mimicked the 5-HT response on all neurones, as did the 5-HT1 receptor agonist 5-carboxamidotryptamine (5-CT) on 71% of SPN. The responses to 5-HT, alpha-methyl-5-HT and 5-CT were inhibited by the 5-HT2 antagonists ketanserin and ritanserin. 5. Pressure ejection of 5-HT over the central canal region could evoke a biphasic inhibitory-excitatory response. This response persisted in TTX, suggesting that an inhibitory 5-HT receptor may be located on the medial dendrites. 6. SPN are powerfully depolarized by 5-HT acting at 5-HT2 receptors, via the closure of an outwardly rectifying potassium conductance. The long duration of the response and the ability of 5-HT to induce rhythmical oscillations suggest that 5-HT may have an important role in regulating SPN excitability.

Release of adenosine by activation of NMDA receptors in the hippocampus

Adenosine is present in the mammalian brain in large amounts and has potent effects on neuronal activity, but its role in neural signaling is poorly understood. The glutamate receptor agonist N-methyl-D-aspartate (NMDA) caused a presynaptic depression of excitatory synaptic transmission in the CA1 region of guinea pig hippocampal slices. This depression was blocked by an adenosine A1 receptor antagonist, which suggests that activation of the NMDA subtype of glutamate receptor raises the concentration of extracellular adenosine, which acts on presynaptic inhibitory A1 receptors. Strong tetanic stimulation caused a heterosynaptic inhibition that was blocked by both NMDA and A1 receptor antagonists. Enkephalin, which selectively inhibits interneurons, antagonized the heterosynaptic inhibition. These findings suggest that synaptically released glutamate activates NMDA receptors, which in turn releases adenosine, at least in part from interneurons, that acts at a distance to inhibit presynaptically the release of glutamate from excitatory synapses. Thus, interneurons may mediate a widespread purinergic presynaptic inhibition.

A comparison of paired-pulsed facilitation of AMPA and NMDA receptor-mediated excitatory postsynaptic currents in the hippocampus

Paired-pulse facilitation of excitatory synaptic transmission was investigated in the CA1 region of rat hippocampal slices using whole-cell patch-clamp recording. To optimise the measurement of excitatory synaptic transmission, gamma-amino-butyric acid (GABA)-mediated synaptic inhibition was eliminated using both GABAA and GABAB antagonists. Pure alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) or N-methyl-D-aspartate (NMDA) receptor-mediated excitatory postsynaptic currents (EPSCs) were then isolated pharmacologically. Paired-pulse facilitation of either AMPA or NMDA receptor-mediated EPSCs (EPSCA and EPSCN, respectively) was investigated using two stimuli of identical strength delivered at intervals of between 25 and 1000 ms. The paired-pulse facilitation profiles of both EPSCA and EPSCN were similar. Paired-pulse facilitation of EPSCA was independent of holding potential. In contrast paired-pulse facilitation of EPSCN was markedly voltage-dependent; maximum facilitation was recorded at hyperpolarised membrane potentials. At positive membrane potentials there was little or no paired-pulse facilitation and, in most neurones, paired-pulse depression was observed. Voltage-dependence of paired-pulse facilitation of EPSCN was similar in the presence of nominal absence of Mg2+ in the bathing medium, and was unaffected by extensive dialysis of neurones with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA). These data are consistent with a presynaptic locus for paired-pulse facilitation of EPSCA. However, paired-pulse facilitation of EPSCN involves postsynaptic factors.

Involvement of N- and non-N-type calcium channels in synaptic transmission at corticostriatal synapses

Calcium channels participate in the events linking axon terminal depolarization to neurotransmitter secretion. We wished to evaluate the role of N-type and non-N-type calcium channels in glutamatergic transmission at corticostriatal synapses, since this is a well defined excitatory synapse. In addition, these synapses are subject to a variety of forms of presynaptic modulation, some of which may involve alterations in calcium channel function. Application of the selective N-type channel blocker omega-conotoxin GVIA produced an irreversible depression of excitatory synaptic transmission in rat neostriatal slices shown by a decrease of approximately 50% in the amplitude of the synaptically driven population spike during field potential recording and a similar decrease in the amplitude of excitatory postsynaptic potentials during whole-cell recording. The component of transmission which was resistant to omega-conotoxin GVIA was blocked by omega-conotoxin MVIIC. omega-Agatoxin IVA had little effect on transmission. Activation of a presynaptic metabotropic glutamate receptor depressed transmission to a similar extent before and after omega-conotoxin GVIA treatment. Likewise, protein kinase C-activating phorbol esters potentiated transmission to the same extent before and after omega-conotoxin GVIA treatment. N-type calcium channels appear to be crucial for a component of excitation-secretion coupling at corticostriatal synapses. A component of transmission involves non-N-, non-L-type high-voltage-activated calcium channels. The effects of presynaptic metabotropic receptors and protein kinase C activation cannot be accounted for solely by alterations in the N-type channel function.

A role for protein kinases and phosphatases in the Ca(2+)-induced enhancement of hippocampal AMPA receptor-mediated synaptic responses

We have investigated the effects of inhibitors of protein kinases and protein phosphatases on the NMDA receptor-independent potentiation of evoked and miniature (m) excitatory postsynaptic currents (EPSCs) induced by the entry of Ca2+ via voltage-gated Ca2+ channels in hippocampal CA1 pyramidal neurons. Voltage pulse-induced potentiation was markedly attenuated when evoked in the presence of the protein kinase blockers KN-62, K-252a, or H-7. Bath application of the protein phosphatase inhibitor calyculin A converted the usual transient potentiation of both evoked and spontaneous EPSCs induced by voltage pulses into a more sustained potentiation. Similarly, the introduction of the phosphatase inhibitors microcystin LR or okadaic acid into postsynaptic cells, via patch pipettes, also resulted in a sustained increase in the amplitude of mEPSCs. We propose that entry of Ca2+ into CA1 neurons activates calcium/calmodulin-dependent protein kinase II, which leads to an enhanced responsiveness of synaptic AMPA receptor channels. The enhancement is transient, however, owing to postsynaptic phosphatase activity.

Orientation selectivity of cortical neurons during intracellular blockade of inhibition

Neurons in the primary visual cortex of the cat are selectively activated by stimuli with particular orientations. This selectivity can be disrupted by the application of antagonists of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) to a local region of the cortex. In order to determine whether inhibitory inputs are necessary for a single cortical neuron to show orientation selectivity, GABA receptors were blocked intracellularly during whole cell recording. Although the membrane potential, spontaneous activity, subfield antagonism, and directional selectivity of neurons were altered after they were perfused internally with the blocking solution, 18 out of 18 neurons remained selective for stimulus orientation. These results indicate that excitatory inputs are sufficient to generate orientation selectivity.

Endogenous H+ modulation of NMDA receptor-mediated EPSCs revealed by carbonic anhydrase inhibition in rat hippocampus

1. The occurrence of extracellular alkaline transients during excitatory synaptic transmission suggests that the NMDA receptor H(+)-modulatory site may have a physiological role. Here we amplify these pH shifts using benzolamide (a carbonic anhydrase inhibitor) and describe concomitant effects on EPSCs in whole-cell clamped CA1 neurones in rat hippocampal slices. 2. In CO2-HCO3(-)-buffered media, benzolamide increased the time to 50% decay (t50) of the EPSCs by 78 +/- 14% (P < 0.01, n = 10). This occurred simultaneously with amplification of the extracellular alkaline shift (154 +/- 14%). 3. In CO2-HCO3(-)-buffered media containing DL-2-amino-5-phosphonovalerate (APV), the EPSC t50 was unaltered by benzolamide, while the extracellular alkaline shifts were increased (111 +/- 23%, n = 8). 4. In Hepes-buffered media, neither the EPSC t50 nor the extracellular alkaline shift was altered by benzolamide (n = 9). 5. These data demonstrate that NMDA receptor activity is dependent on the buffering kinetics of the brain extracellular space. The results suggest that endogenous pH shifts can modulate NMDA receptor function in a physiologically relevant time frame.

The 5-HT3 receptor agonist 2-methyl-5-HT reduces postsynaptic potentials in rat CA1 pyramidal neurons of the hippocampus in vitro

The effects of the serotonin (5-HT)3 receptor agonist, 2-methyl-5-hydroxytryptamine (2-methyl-5-HT), were studied in CA1 pyramidal cells of the rat hippocampus in vitro using the whole cell gigaseal technique. 2-Methyl-5-HT (10 and 50 microM) did not change significantly the electrophysiologic properties of the cells but reversibly reduced excitatory and inhibitory postsynaptic potentials evoked by stimulation of the Schaffer collaterals. The onset and termination of this effect was in the order of minutes and no desensitization was observed. The selective 5-HT3 receptor antagonist, granisetron, when applied as a pretreatment completely prevented but did not reverse this action when given after administration of 2-methyl-5-HT while the non-specific 5-HT1,2 receptor antagonist, metergoline, was ineffective. These results suggest that the activation of 5-HT3 receptors reduces the efficacy of glutamatergic synaptic transmission in this area.

Modulation of fast excitatory synaptic transmission by cyclothiazide and GYKI 52466 in the rat hippocampus

The effects of cyclothiazide, a drug which potentiates AMPA receptor-mediated responses and GYKI 52466, a non-competitive AMPA receptor antagonist, were studied on fast glutamatergic transmission in rat hippocampal slices. Cyclothiazide prolonged the decay of AMPA receptor-mediated EPSCs (AMPA-EPSCs) in a concentration-dependent manner. GYKI 52466 reduced the peak amplitude of AMPA-EPSCs and blocked the induction of LTP. When GYKI 52466 was applied in the presence of cyclothiazide it still reduced the peak amplitude of AMPA-EPSCs but was not able to reverse the cyclothiazide induced prolongation of AMPA-EPSC duration. These data suggest that GYKI 52466 and cyclothiazide probably mediate their effects on the AMPA receptor via different binding sites.

The role of Rab3A in neurotransmitter release

The small GTP-binding protein Rab3A is a Rab family member that is abundant in brain synaptic vesicles. Here we show that mice in which the rab3A gene has been mutated by homologous recombination do not express Rab3A but are viable and fertile. Electrophysiological recordings in hippocampal CA1 pyramidal cells indicate that most of their synaptic parameters are also normal, although synaptic depression after short trains of repetitive stimuli (15-30 stimuli at 14 Hz) is significantly increased. Levels of the Rab3A-binding protein rabphilin are decreased by 70%, but expression of more than 20 other synaptic proteins is unchanged. No compensatory changes were detected in other GTP-binding proteins or in proteins that interact with Rab3. Rab3A thus appears not to be essential for synaptic vesicle exocytosis but to play a role in the recruitment of synaptic vesicles for exocytosis during repetitive stimulation.

Relationship between structure and function of neurons in the rat rostral nucleus tractus solitarii

To investigate the relationship between the structure and function of neurons in the rostral (gustatory) nucleus tractus solitarii (rNTS), we analyzed the morphological and biophysical properties of rNTS neurons by performing whole-cell recordings in a brain slice preparation. Overall, neurons (n = 58) had a mean somal diameter of 16 microns, an average dendritic length of 598 microns, an average dendritic thickness of 0.91 microns, and a spine density of 0.037 spines/microns. Neurons were separated into three groups (elongate, multipolar, and ovoid) on the basis of previously established morphological criteria. The highest percentage (49%) of neurons were classified as ovoid, while 35% were multipolar and only 16% were elongate. The most frequently observed firing pattern, in all three cell types, elicited by a 1,200 ms, 100 pA depolarizing current pulse was a regularly firing spike train. However, the intrinsic firing properties of the remaining neurons were different. Thirty-one percent of the ovoid neurons responded with a short burst of action potentials and 44% of the elongate neurons showed a delay in the onset of the spike train following a hyperpolarizing prepulse. Less than 16% of the multipolar neurons demonstrated either of these firing characteristics. Therefore, rNTS neurons with similar morphology do not have unique biophysical properties. However, the data suggest that there may be subpopulations of the three morphological types, each of which displays a different firing pattern. Since the structure and function of the three morphological groups were not strictly correlated, these subpopulations may represent functional groups.

The cholinergic inhibition of afterhyperpolarization in rat hippocampus is independent of cAMP-dependent protein kinase

The possible involvement of protein kinase A (PKA) in the muscarinic inhibition of the slow afterhyperpolarizing current (IAHP) was investigated in rat hippocampal pyramidal cells. IAHP was recorded using the whole cell method in hippocampal slices, and Rp-cAMPS, a PKA antagonist, was applied intracellularly. The inhibition of IAHP by carbachol was not affected by Rp-cAMPS. In contrast, Rp-cAMPS reduced the cAMP-dependent inhibition of IAHP by norepinephrine. The results show that phosphorylation by PKA does not contribute to the muscarinic effect on IAHP.

Muscarinic modulation of synaptic transmission in slices of the rat ventral striatum is dependent on the frequency of afferent stimulation

Extracellular, intracellular and tight-seal patch-clamp recordings in ventral striatal slices were used to investigate whether the effectiveness of muscarinic neuromodulation of fast synaptic transmission may be dependent on the frequency of afferent stimulation. In all neurons tested, EPSPs were reversibly attenuated by muscarine or carbachol. This action was completely antagonized by atropine or pirenzepine. Several observations indicated a presynaptic site of action. In extracellular recordings, carbachol reduced the monosynaptic population spike but not the non-synaptic compound action potential. The acetylcholinesterase inhibitors eserine and pyridostigmine also induced an atropine-sensitive reduction of the EPSP. When the rate of afferent stimulation was increased, control EPSPs or EPSCs exhibited a decline in peak amplitude until reaching a steady-state value. Muscarinic modulation of steady-state EPSPs/EPSCs was significantly stronger in the range of lower frequencies (0.25-4 Hz) than at higher frequencies (8 and 12 Hz). The GABAA and GABAB-receptor/channel antagonists picrotoxin and 2-hydroxy-saclofen, the opiate receptor antagonist naloxone and atropine failed to alter the shape of the frequency-response curve. These results show that both exogenous and endogenous muscarinic receptor agonists are capable of activating a presynaptic mechanism by which fast excitatory inputs to the ventral striatum are depressed. The depressive effect is clearly stronger at lower rates of afferent stimulation than at high rates. This frequency-dependent attenuation of excitatory synaptic inputs exemplifies a new type of activity-dependent neuromodulation in central neural circuits.

Whole-cell patch-clamp recordings from visualized bulbospinal neurons in the brainstem slices

The purpose of this study was to develop a method for electrophysiological characterization of retrogradely labeled bulbospinal neurons in the specific cytoarchitectonic regions in the brainstem slices. Several days after the spinal cord was injected with the carbocyanine dye, DiI, retrogradely labeled bulbospinal neurons were visualized by epifluorescence optics in the brainstem slices with the aid of a silicon intensifier tube (SIT) camera. Labeled somata were routinely seen in the caudal raphe nuclei, rostroventral medial and lateral portions of the medulla, the A5 group and in other medullary sites known to project to the spinal cord. Electrophysiological properties of the DiI-labeled neurons were assessed by whole-cell recordings using micropipettes filled with biocytin. The slices were subsequently processed for dual visualization of biocytin and serotonin or a marker for noradrenergic neurons, tyrosine hydroxylase (TH). The electrophysiological properties of bulbospinal neurons were correlated with their morphology and neurochemical content. This technique may be useful in other areas of CNS for studying morphology, neurochemical content and physiology of retrogradely labeled neurons.

Voltage-dependent kinetics of N-methyl-D-aspartate synaptic currents in rat cerebellar granule cells

Decay kinetics of N-methyl-D-aspartate excitatory postsynaptic currents (NMDA-EPSCs) have been voltage-dependent in some, but not all neurons studied so far, and almost no information has been available on the voltage-dependence of the rising phase. In this work we investigated the effect of membrane potential on rising and decay kinetics of the NMDA-EPSC in cerebellar granule cells using the tight-seal whole-cell recording technique. NMDA-EPSCs were evoked by electrical mossy fibre stimulation in the presence of 10 microM 6-cyano-7-nitroquinoxaline-2,3-dione, 1.2 mM Mg2+ and 5 microM glycine. The rate of rise of NMDA-EPSCs remained substantially unchanged when the cell was depolarized, indicating that the limiting step of channel opening was voltage-insensitive. The NMDA-EPSC, however, flattened around the peak and the time-to-peak increased. This observation was explained by the influence of decay. Decay was biphasic and slowed down with membrane depolarization. Moreover, the fast component of decay increased less than the slow component. This complex voltage-dependence may extend the integrative role of the NMDA current during synaptic transmission.