Unitary Excitatory Postsynaptic Potentials Research Articles

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

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

Unitary excitatory synaptic currents in preganglionic neurons mediated by two distinct groups of interneurons in neonatal rat sacral parasympathetic nucleus.

1. Excitatory postsynaptic currents (EPSCs) in parasympathetic preganglionic neurons (PGNs) were examined by the use of the whole cell patch-clamp recording technique in slice preparations of the neonatal rat lumbosacral spinal cord. Synaptic responses were evoked in PGNs by extracellular stimulation of a neighboring interneuron. 2. Stimulation of interneurons medial to the sacral parasympathetic nucleus (SPN) elicited EPSCs or inhibitory postsynaptic currents in 58 and 11%, respectively, of PGNs. Stimulation of interneurons dorsal to the SPN evoked EPSCs in 70% of PGNs. 3. EPSCs occurred at short latency (2.1 ms) and were usually elicited in an all-or-none manner, indicating that they were monosynaptic and mediated by a single interneuron (i.e., unitary). 4. EPSCs were mediated by both non-N-methyl-D-aspartate (non-NMDA) and NMDA receptors. 5. Unitary excitatory postsynaptic potentials evoked by single stimuli did not induce action potentials in PGNs, but repetitive stimulation (> 20 Hz) of the single interneurons could evoke firing of PGNs. 2-Amino-5-phosphonovalerate, an NMDA receptor antagonist, reduced the synaptic depolarization induced in PGNs by high-frequency interneuronal impulses. 6. EPSCs mediated by dorsal interneurons were smaller in amplitude (36.3 +/- 15.7 pA, mean +/- SD) than EPSCs mediated by medial interneurons (88.4 +/- 45.7 pA). 7. Paired-pulse facilitation of EPSCs was observed in PGNs (147.2 +/- 26.2%). The degree of facilitation was higher in dorsal (174.6 +/- 10.3%) than in medial interneuronal pathways (120.9 +/- 3.6%). Within each of interneuronal pathways the degree of facilitation was independent of the magnitude of the unitary EPSC. 8. The results show that PGNs receive monosynaptic glutamatergic excitatory inputs from two distinct populations of interneurons in the dorsal and medial regions of the SPN. These two populations of interneurons are likely to have different functions in the regulation of the preganglionic outflow to the pelvic organs.

Neuron as time coherence discriminator

Neuronal excitability under stimuli with a complex time course is investigated on the basis of the numerical solution of the Hodgkin-Huxley equations. Each stimulus is composed of 100-1000 unitary excitatory postsynaptic potentials (uEPSP) that start randomly within a definite time window. Probability of initiating a spike [firing probability, FP(W)] as a function of the window width W is calculated by the Monte Carlo method. FP(W) has a step-like shape: it becomes equal to 1 for small W and almost vanishes as W exceeds some value Ws. The role of long-lasting somatic inhibition is analysed. Ws depends on the inhibition potential, but the step-like shape of FP is preserved. It is concluded that the capability of multisynaptic stimulation to cause a spike can be expressed in terms of temporal coherence between the synaptic inputs. Namely, the spike is initiated if the temporal coherence between active inputs is above a definite threshold. The threshold value can be effectively regulated by varying the inhibition potential.

Cholinergic and electrical synapses between synergistic spinal motoneurones in the Xenopus laevis embryo.

1. To investigate central motoneurone synapses within the spinal cord of a simple vertebrate, the Xenopus embryo, simultaneous intracellular recordings were made from fifty-five pairs of spinal motoneurones. 2. Chemical synapses were found between seventeen out of thirty-five pairs on the same side of the spinal cord. Current-evoked spikes in the presynaptic neurone led to fast depolarizing postsynaptic potentials (PSPs) in the postsynaptic neurone at latencies of 0.5-1.5 ms. The PSPs had an average amplitude of 7 mV, a rise time of 8 ms and a half-fall time of 18 ms. 3. The presynaptic motoneurone was always the more rostral of the pair. No excitatory connections were found which crossed the cord. The fast PSPs were blocked by 10 microM mecamylamine but not by 1 mM kynurenic acid, so were mediated by nicotinic acetylcholine receptors (nAChRs). These are the first unitary excitatory postsynaptic potentials (EPSPs) mediated by nAChRs recorded intracellularly within the vertebrate central nervous system. 4. Bidirectional electrical synapses were found between five pairs of motoneurones. All these pairs were on the same side of the spinal cord and less than 70 microns apart. Each neurone responded in a graded manner to either hyperpolarizing or depolarizing current injected into the other. 5. Since motoneurones are rhythmically active during swimming, both chemical and electrical synapses will add to the fast on-cycle excitation underlying spiking activity in other motoneurones. This may increase the reliability and local synchrony of synergistic motoneurone firing during locomotion.

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

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

Evidence from simultaneous intracellular recordings in rat hippocampal slices that area CA3 pyramidal cells innervate dentate hilar mossy cells

1. Simultaneous intracellular recordings of area CA3 pyramidal cells and dentate hilar "mossy" cells were made in rat hippocampal slices to test the hypothesis that area CA3 pyramidal cells excite mossy cells monosynaptically. Mossy cells and pyramidal cells were differentiated by location and electrophysiological characteristics. When cells were impaled near the border of area CA3 and the hilus, their identity was confirmed morphologically after injection of the marker Neurobiotin. 2. Evidence for monosynaptic excitation of a mossy cell by a pyramidal cell was obtained in 7 of 481 (1.4%) paired recordings. In these cases, a pyramidal cell action potential was followed immediately by a 0.40 to 6.75 (mean, 2.26) mV depolarization in the simultaneously recorded mossy cell (mossy cell membrane potentials, -60 to -70 mV). Given that pyramidal cells used an excitatory amino acid as a neurotransmitter (Cotman and Nadler 1987; Ottersen and Storm-Mathisen 1987) and recordings were made in the presence of the GABAA receptor antagonist bicuculline (25 microM), it is likely that the depolarizations were unitary excitatory postsynaptic potentials (EPSPs). 3. Unitary EPSPs of mossy cells were prone to apparent "failure." The probability of failure was extremely high (up to 0.72; mean = 0.48) if the effects of all presynaptic action potentials were examined, including action potentials triggered inadvertently during other spontaneous EPSPs of the mossy cell. Probability of failure was relatively low (as low as 0; mean = 0.24) if action potentials that occurred during spontaneous activity of the mossy cell were excluded. These data suggest that unitary EPSPs produced by pyramidal cells are strongly affected by concurrent synaptic inputs to the mossy cell. 4. Unitary EPSPs were not clearly affected by manipulation of the mossy cell's membrane potential. This is consistent with the recent report that area CA3 pyramidal cells innervate distal dendrites of mossy cells (Kunkel et al. 1993). Such a distal location also may contribute to the high incidence of apparent failures. 5. Characteristics of unitary EPSPs generated by pyramidal cells were compared with the properties of the unitary EPSPs produced by granule cells. In two slices, pyramidal cell and granule cell inputs to the same mossy cell were compared. In other slices, inputs to different mossy cells were compared. In all experiments, unitary EPSPs produced by granule cells were larger in amplitude but similar in time course to unitary EPSPs produced by pyramidal cells. Probability of failure was lower and paired-pulse facilitation more common among EPSPs triggered by granule cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Quantal analysis of modulating action of adenosine on the mossy fiber synapse in hippocampal slices.

Although adenosine has been considered to depress release of neurotransmitter in central synapses, evidence for this notion is circumstantial. To obtain more direct and quantitative evidence, the suppressing action of adenosine on transmission through the mossy fiber synapse was examined by a quantal analysis in thin transverse sections of the guinea pig hippocampus. Adenosine suppressed excitatory postsynaptic potentials (EPSPs) induced in CA3 neurons by activation of single mossy fibers to 57 +/- 19% at 10 microM (6 neurons). The input resistance of the postsynaptic neurons was reduced to 84 +/- 5%. The suppression of the unitary EPSPs was accompanied by a reduction of the mean quantal content (m) to 51 +/- 13% with no significant decrease in mean quantal amplitude (q). These findings indicate that adenosine at 10 microM suppresses transmission through mossy fiber synapses mainly by depressing release of transmitter. This study provides direct evidence for the presynaptic action of adenosine on central synapses for the first time.

Depression of postsynaptic potentials by high-frequency stimulation in embryonic motoneurons grown in spinal cord slice cultures

1. In embryonic cocultures of spinal cord, dorsal root ganglia, and muscle, excitatory postsynaptic potentials (EPSPs) were recorded in motoneurons during focal electrical stimulation of the dorsal root ganglia or the spinal cord. 2. EPSPs were depressed in amplitude at high-frequency stimulation relative to a control frequency of 0.5 Hz by 47 and 75% at 5 and 10 Hz, respectively. This was true for composite EPSPs and unitary EPSPs. 3. The depression showed a wide range of variability between individual experiments. The degree of depression at 5 Hz was negatively correlated to the rate of spontaneous excitatory input the motoneurons received. There was no correlation to the soma size, the average amplitude of the EPSPs, the rheobase, or the input resistance of the motoneurons. 4. An increase in latency of EPSPs was observed concomitant with or preceding the synaptic depression in most experiments. Total transmission failures, which were absent at low-frequency stimulation, appeared during depression. 5. Large incremental steps in amplitude could be seen during depression, suggesting that several release sites were switched off and on together. 6. Decreasing the extracellular calcium concentration from 5 to 1 mM led to a decrease in the frequency sensitivity of the synaptic efficacy and to a decrease of the EPSP amplitude and latency. 7. Measurements of the antidromic conduction of action potentials evoked in the axons and recorded in the somata of dorsal root ganglion cells revealed an increase in latency and the appearance of conduction failures at stimulation frequencies of 1-10 Hz. The frequency modulation of conduction was decreased in 1 mM compared with 5 mM external calcium. 8. Together these findings suggest that conduction failures in the presynaptic axons contribute to the synaptic depression of EPSPs in embryonic motoneurons.

The integrative properties of spiny distal dendrites

The dendritic spines of many central neurons are generally thought to modulate the ability of individual synaptic conductances to depolarize the dendritic shaft. A compartmental analysis using typical spine dimensions shows that spine neck resistances are probably far too low to support such a function, because low conductance synapses act as time-varying current sources. However, the collective presence of all spines on a dendrite significantly modifies the electrical properties of the branch in ways which have previously been overlooked. In particular, they lower its input impedance and length constant, reducing the amplitude of the unitary excitatory postsynaptic potential as well as the strength of spatial summation. This enables a dendrite to integrate large numbers of synaptic inputs while occupying minimal volume. In this way, dendritic spines are analogous to axonal myelin, which also alters transcellular impedance in order to maximize neurite function and minimize volume. Unlike membrane resistance changes, spines have little effect on the membrane time-constant so they maintain a long window for temporal summation. Though spine shape and neck resistance do not significantly affect dendritic potentials, spine area does. Therefore, while changes in spine morphology probably do not directly potentiate the strength of individual synapses, changes in spine density can regulate the synaptic excitability of an entire dendrite. The shortened length-constant of the spiny dendrite requires excitable membranes to be located in distal dendrites. These, in turn, eliminate many of the electrotonic nonlinearities associated with summation in long, thin processes, and make all distal synapses equipotent. The short length-constant also enhances the sensitivity of dendritic spikes to local impedance changes while decreasing the sensitivity to distant impedance changes. This would enable a neuron to effectively use inhibitory synapses or branch points to regulate propagation through its spiny dendritic tree. A model neuron is developed in which dendritic spines, excitable membranes, and dendritic branching combine to form a two-stage filter, which serves as a synaptic input coincidence detector with adjustable gain. Gain is regulated by potassium conductances which modulate branch point safety factor. The model is consistent with the notion of functional independence of distal dendrites and demonstrates that certain aspects of dendritic spiking which have previously been thought to require membrane hot-spots can also result from geometrical properties. It is suggested that the activation of spiny neurons may depend as much on the density as on the number of active synapses, and that spiny neurons may tend to have discrete output states whereas nonspiny neurons may be more continuous.

Quantal analysis of suppressing action of baclofen on mossy fiber synapses in guinea pig hippocampus

Baclofen, a selective agonist at the γ-aminobutyric acid B (GABA B) receptor, has been considered to reduce the release of transmitter from nerve terminals by acting on presynaptic GABA B receptors, in addition to its postsynaptic action. The purpose of this study has been to re-examine quantitatively the action of baclofen in the hippocampus by a rigorous quantal analysis. (±)-Baclofen suppressed field potentials and intracellularly-recorded synaptic potentials induced in the subfield CA3 by mossy fiber stimulation in thin transverse sections of the guinea pig hippocampus. The amplitude distribution of excitatory postsynaptic potentials (EPSPs) induced monosynaptically by a granule cell could be described by the Pascal statistics. Suppression of the unitary EPSPs by baclofen (1 and 5 μM) was accompanied by decreases both in mean quantal content ( m) and by mean quantal amplitude ( q). The reduction in q was smaller than expected from a decrease in the input resistance of the postsynaptic neuron. It was suggested that the presynaptic and postsynaptic actions of baclofen contribute almost equally to suppression of the transmission at 1 μM, whereas the presynaptic action predominates at 5 μM.

The role of GABAA and GABAB receptors in presynaptic inhibition of Ia EPSPs in cat spinal motoneurones.

1. The role of GABAA and GABAB receptors in presynaptic inhibition was studied by examining the effect of local application of antagonists by ionophoresis during intracellular recording of presynaptic inhibition of compound and unitary group Ia afferent excitatory postsynaptic potentials (EPSPs) in gastrocnemius motoneurones. 2. Ionophoresis of the GABAA antagonist bicuculline methochloride (BMC) was found to block presynaptic inhibition of both compound and unitary EPSPs by up to 85%. BMC also substantially reduced, and occasionally abolished, the late part of the inhibitory postsynaptic potential (IPSP) evoked in motoneurones by the conditioning stimulation. The early part of this IPSP was found to be sensitive to ionophoresis of strychnine hydrochloride. 3. Ionophoresis of 2-OH-saclofen caused a reduction in presynaptic inhibition of compound EPSPs by 5-25% but had no effect on the IPSP evoked in motoneurones by the conditioning stimulation. 4. Ionophoresis of the GABAB antagonist (-)-baclofen reduced the amplitude of unconditioned EPSPs; however it had little effect on presynaptic inhibition. 5. It was concluded that at the Ia afferent-motoneurone synapse presynaptic inhibition is mediated primarily through the activation of GABAA receptors. The activation of GABAB receptors appears to play only a minor role in presynaptic inhibition at this synapse. This contrasts with the relative ease with which (-)-baclofen can reduce transmitter release from Ia afferent terminals and suggests that the receptors activated by (-)-baclofen are predominantly extrasynaptic.

Connections of the forewing tegulae in the locust flight system and their modification following partial deafferentation.

The flight motor pattern of the adult locust (Locusta migratoria L.) is able to recover from the loss of the hindwing tegulae. This recovery is due to a functional substitution of the hindwing tegulae by the forewing tegulae (Büschges, Ramirez, and Pearson, 1992). To assess changes in the pathways from the forewing tegulae in the flight system, we investigated the pathways of the forewing tegula in intact locusts and in animals 2 weeks after hindwing tegula removal. The following physiological alterations in these pathways were found to be associated with the recovery: (1) In the intact locusts, the connections of forewing tegula afferents to flight interneurons are variable but this variability did not occur in recovered animals, and (2) larger numbers of forewing tegula afferents connect to interneurons that excite elevator motoneurons (interneurons 566 and 567) and to an interneuron that inhibits depressor motoneurons (interneuron 511). The size of unitary excitatory postsynaptic potentials (EPSPs) evoked by signal forewing tegula afferents was found not to be altered in recovered animals. The changes in connectivity of forewing tegula afferents are correlated with morphological alterations in the structure of the terminal processes of the afferents and with sprouting of some branches of interneurons receiving input from these afferents.

The Whole-Body Withdrawal Response of Lymnaea Stagnalis II. Activation of Central Motoneurones and Muscles by Sensory Input

The role of centrally located motoneurones in producing the whole-body withdrawal response of Lymnaea stagnalis (L.) was investigated. The motoneurones innervating the muscles used during whole-body withdrawal, the columellar muscle (CM) and the dorsal longitudinal muscle (DLM) were cells with a high resting potential (-60 to -70 mV) and thus a high threshold for spike initiation. In both semi-intact and isolated brain preparations these motoneurones showed very little spontaneous spike activity. When spontaneous firing was seen it could be correlated with the occurrence of two types of spontaneous excitatory postsynaptic potential (EPSP). One was a unitary EPSP that occasionally caused the initiation of single action potentials. The second was a larger-amplitude, long-duration (presumably compound) EPSP that caused the motoneurones to fire a burst of high-frequency action potentials. This second type of EPSP activity was associated with spontaneous longitudinal contractions of the body in semi-intact preparations. Tactile stimulation of the skin of Lymnaea evoked EPSPs in the CM and DLM motoneurones and in some other identified cells. These EPSPs summated and usually caused the motoneurone to fire action potentials, thus activating the withdrawal response muscles and causing longitudinal contraction of the semi-intact animal. Stimulating different areas of the body wall demonstrated that there was considerable sensory convergence on the side of the body ipsilateral to stimulation, but less on the contralateral side. Photic (light off) stimulation of the skin of Lymnaea also initiated EPSPs in CM and DLM motoneurones and in some other identified cells in the central nervous system (CNS). Cutting central nerves demonstrated that the reception of this sensory input was mediated by dermal photoreceptors distributed throughout the epidermis. The activation of the CM and DLM motoneurones by sensory input of the modalities that normally cause the whole-body withdrawal of the intact animal demonstrates that these motoneurones have the appropriate electrophysiological properties for the role of mediating whole-body withdrawal.

Mormyromast electroreceptor organs and their afferent fibers in mormyrid fish. II. Intra-axonal recordings show initial stages of central processing

1. Physiologically and morphologically identified primary afferent fibers from mormyromast electroreceptor organs were recorded intracellularly. The fiber recordings were made from the nerve root of the posterior lateral line nerve, where the fibers enter the brain, and from the electrosensory lateral line lobe (ELL), near the central terminals of the fibers. 2. The intracellular recordings reveal a variety of potentials, synaptic and nonsynaptic, in addition to the large orthodromic action potentials from the periphery. The goal of the present study was to describe and interpret these various potentials in mormyromast afferent fibers as a first step in understanding the processing of electrosensory information in ELL. 3. Three types of synaptic potentials were recorded inside mormyromast afferent fibers: 1) electric organ corollary discharge (EOCD) excitatory postsynaptic potentials (EPSPs), driven by the motor command that elicits the electric organ discharge (EOD); 2) EPSPs evoked by electrosensory stimulation of electroreceptors in the skin near the electroreceptor from which the recorded fiber originates or by direct stimulation of an electrosensory nerve; and 3) inhibitory postsynaptic potentials (IPSPs) evoked by electrosensory stimulation of more distant electroreceptors. These synaptic potentials can be attributed to synaptic input to postsynaptic cells in ELL that is observed inside the afferent fibers because of electrical synapses between the fibers and the postsynaptic cells. 4. The peripherally evoked EPSPs could frequently be shown to be unitary. The unitary EPSPs were identical to the orthodromic spikes in originating from a single electroreceptor, in threshold, and in latency shift with increasing stimulus intensity. These similarities suggest that the unitary EPSPs are electrotonic EPSPs caused by impulses in other mormyromast afferent fibers that terminate on some of the same postsynaptic cells as the recorded fiber. The peripherally evoked IPSPs had a longer latency than the EPSPs or orthodromic spikes, requiring the presence of an inhibitory interneuron. 5. The peripherally evoked EPSPs, both unitary and nonunitary, show absolute refractory periods of 3-8 ms, followed by relative refractory periods of approximately 8 ms, when tested with two identical stimuli to a nerve. These refractory periods are interpreted as because of refractoriness in the fine preterminal branches of the axonal arbor. 6. A depolarizing afterpotential is commonly associated with the orthodromic spike and probably results from the successful propagation of the spike into the entire terminal arbor. The depolarizing afterpotential has a refractory period that is similar to that of the peripherally evoked EPSPs and that is also interpreted as refractoriness in the fine preterminal branches.(ABSTRACT TRUNCATED AT 400 WORDS)

Simultaneous recording of presynaptic spikes and excitatory postsynaptic potentials from monosynaptically connected hippocampal neurons

A technique has been devised to activate single granule cells in the hippocampus, and to record simultaneously spikes from the particular granule cell and excitatory postsynaptic potentials from a monosynaptically connected CA3 neuron. The unitary excitatory postsynaptic potentials (EPSPs) sustained for long observation periods, and increased in size with increases in stimulus frequency and in external Ca 2+ concentration. This technique may be useful for quantal analysis of transmission through the synapse between mossy fibers and CA3 neurons.

Identification of excitatory interneurons contributing to generation of locomotion in lamprey: structure, pharmacology, and function.

1. In the in vitro preparation of the lamprey spinal cord, paired intracellular recordings of membrane potential were used to identify interneurons producing excitatory postsynaptic potentials (EPSPs) on myotomal motoneurons. 2. Seventy-nine interneurons (8.4% of all neuron-motoneuron pairs tested) elicited unitary EPSPs that followed one-for-one at short, constant latencies and were therefore considered monosynaptic according to conventional criteria. Evidence was obtained for selectivity and divergence of excitatory interneuron (EIN) outputs and for convergence of EIN input to motoneurons. 3. The neurotransmitter released by EINs may be an excitatory amino acid such as glutamate, because the EPSPs were depressed by antagonists of excitatory amino acids. 4. Intracellular dye injection revealed that EINs have small cell bodies (average 11 x 27 microns), transversely oriented dendrites, and thin (less than 3 microns) slowly conducting axons (0.7 m/s) that project caudally and ipsilaterally. One EIN exhibited a system of thin multi-branching axon collaterals with periodic swellings. Ultrastructurally, these swellings contained clear spherical vesicles, and they apposed postsynaptic membrane specializations. 5. During fictive locomotion, the membrane-potential oscillations of EINs were greater in amplitude than, but similar in shape and timing to, those of their postsynaptic motoneurons. EINs fired action potentials during fictive locomotion and contributed to the depolarization of motoneurons. 6. These interneurons are proposed to be a source of excitation to motoneurons and interneurons in the lamprey spinal cord, participating in motor activity including locomotion.

Activation of GABAA receptors causes presynaptic and postsynaptic inhibition at synapses between muscle spindle afferents and motoneurons in the spinal cord of bullfrogs

The functional role of GABAA receptors in inhibition of synaptic transmission between muscle spindle afferents and spinal motoneurons was studied in the isolated spinal cord of bullfrogs. Extracellular recording from the ventral root showed that activation of GABAA receptors by muscimol (primarily a GABAA receptor agonist) at 50 microM produced a 38% reduction in the amplitude of the excitatory postsynaptic potential (EPSP) evoked by stimulation of triceps muscle sensory afferents and a 66% reduction in the EPSP half-width, suggesting a large increase in the conductance of the motoneuronal membrane. Quantal analysis of unitary triceps EPSPs recorded intracellularly from motoneurons showed that muscimol reduced the quantal content of release (presynaptic inhibition). In addition, muscimol decreased the quantal size (postsynaptic inhibition) when the postsynaptic conductance change was large. Because the effects of muscimol were completely and reversibly blocked by 100 microM bicuculline (a specific GABAA receptor antagonist), both the pre- and the postsynaptic inhibition caused by muscimol were due to activation of GABAA receptors. Activation of GABAA receptors thus causes both pre- and postsynaptic inhibition of synaptic transmission between muscle afferents and spinal cord motoneurons in the frog.

Activation of GABAB receptors causes presynaptic inhibition at synapses between muscle spindle afferents and motoneurons in the spinal cord of bullfrogs

Baclofen, a specific GABAB receptor agonist, was used to study the functional role of activation of GABAB receptors in synaptic transmission between muscle spindle afferents and motoneurons in the isolated spinal cord of bullfrogs. (+/-)-Baclofen (5 microM) reversibly reduced the amplitude of the excitatory postsynaptic potential (EPSP) evoked by simulation of various brachial muscle nerves and recorded extracellularly from the ventral root by 47% without shortening the falling phase of the EPSP. Neither the time course nor the amplitude of the action potentials in the sensory afferents was affected. Thus, baclofen caused synaptic inhibition without reducing either the potential change occurring in the muscle sensory afferents or the motoneuronal membrane resistance. Quantal analysis, performed using a deconvolution technique, of the monosynaptic EPSPs in brachial motoneurons evoked by activity in single triceps muscle spindle afferents showed that transmission at these synapses was quantal, and baclofen lowered the quantal content without altering the quantal size. Furthermore, quantal analysis of the electrical component of these unitary EPSPs showed that it did not fluctuate in amplitude, either in normal saline or with baclofen. The inhibition produced by activation of GABAB receptors is therefore presynaptic but is not likely to be caused by conduction failures in the sensory terminals.

Electrophysiology of the mammalian cerebellar cortex in organ culture

A direct comparison was made between the electrical properties of rat Purkinje cells in cerebellar organotype cultures and those in acute slices from age-matched animals. Cultures were prepared from 9–11-day-old animals. Intracellular recordings were made 5–12 days later, at which time the folia architecture of the cerebellum was still well preserved. The resting membrane potentials and input resistances of Purkinje cells in cultured and acute slice preparations from young animals were comparable to those of mature Purkinje cells in slices. Neurons from animals younger than 14 days differed from mature Purkinje cells in that they fired at low frequencies in response to outward current pulses. The latter property was found in all cultured neurons studied, independent of their time in culture. These action potentials were generated by Na + and Ca 2+ conductances as shown by the application of selective channel blockers. Cultured or acute slice preparations from animals younger than 11 days shared other immature electroresponsive features. In both groups, Na +-dependent plateau depolarizations were observed in less than 10% of Purkinje cells unless K-conductances were blocked, and considerable membrane depolarization was often required to elicit Ca 2+-dependent action potentials. These findings are compatible with the relative prominence of voltage-dependent outward currents in immature Purkinje cells, a property which may be enhanced in culture. The injection of hyperpolarizing current pulses revealed a marked time-dependent anomalous rectification in all Purkinje cells. At the breaks of such pulses, several events were observed. In all cells, a rebound conductance was identified which could generate post-anodal spike bursts. In cultured neurons, however, hyperpolarizing pulses were also followed by a slow return to resting potential. This membrane potential profile was similar to that produced by the activation of an A conductance. Experiments on acute slices from animals of different ages (P9–P17) showed that this A-like conductance was expressed only during a brief period in Purkinje cell development. A higher level of spontaneous synaptic activity was observed in cultured than in acute slice preparations. Both unitary excitatory postsynaptic potentials and inhibitory postsynaptic potentials could be elicited in the former by parallel fiber stimulation, and could be fully reversed by outward or inward transmembrane current injections, respectively. The inhibitory postsynaptic potentials were blocked by the GABA antagonist picrotoxin, but their reversal properties differed from those of inhibitory postsynaptic potentials recorded in vivo. Inhibitory postsynaptic potentials were also followed by oscillations in the membrane potential compatible with the presence of rebound inward currents. Parallel fiber stimulation generated prolonged plateau-like depolarizations when cultures were exposed to picrotoxin. These plateaus were shown to be caused by a dendritic, voltage-dependent Ca 2+-conductance similar to that which underlies the climbing fiber response in vivo. We conclude that the firing properties of immature Purkinje cells differ from those in adult animals. Further, the electrical properties of these neurons did not develop in slice culture despite evidence that cell growth and synaptogenesis continue in culture.

Quantal analysis of potentiating action of phorbol ester on synaptic transmission in the hippocampus

The mechanism of the potentiating action of phorbol diacetate on synaptic transmission in the hippocampus was studied by the quantal analysis technique. Thin transverse sections were prepared from guinea pig hippocampus and intracellular potentials were recorded from CA 3 neurons. Unitary excitatory postsynaptic potentials (EPSPs) were induced in the impaled neurons by brief glutamate pulses administered to granule cells. The amplitude of the unitary EPSPs fluctuated according to Poisson distribution. From the mean and variance of the amplitude of the unitary EPSPs, the mean quantal content ( m) and the mean quantal amplitude ( q) were calculated. Before phorbol diacetate administration, the values of m and q were 9.7 ± 1.4 and 1.1 ± 0.28 mV (mean ± S.D.), respectively. Potentiation of synaptic transmission by phorbol diacetate was accompanied by increases in the value of m. The value of q remained unchanged in most neurons and decreased in some. These results indicate that the phorbol ester causes an increase in release of neurotransmitter and thereby potentiates synaptic transmission.