Modifications to synaptic transmission at group Ia synapses on cat spinal motoneurones by 4-aminopyridine

4-aminopyridine improves evoked potentials and ambulation in the taiep rat: A model of hypomyelination with atrophy of basal ganglia and cerebellum

The taiep rat is a tubulin mutant with an early hypomyelination followed by progressive demyelination of the central nervous system due to a point mutation in the Tubb4a gene. It shows clinical, radiological, and pathological signs like those of the human leukodystrophy hypomyelination with atrophy of the basal ganglia and cerebellum (H-ABC). Taiep rats had tremor, ataxia, immobility episodes, epilepsy, and paralysis; the acronym of these signs given the name to this autosomal recessive trait. The aim of this study was to analyze the characteristics of somatosensory evoked potentials (SSEPs) and motor evoked potentials (MEPs) in adult taiep rats and in a patient suffering from H-ABC. Additionally, we evaluated the effects of 4-aminopyridine (4-AP) on sensory responses and locomotion and finally, we compared myelin loss in the spinal cord of adult taiep and wild type (WT) rats using immunostaining. Our results showed delayed SSEPs in the upper and the absence of them in the lower extremities in a human patient. In taiep rats SSEPs had a delayed second negative evoked responses and were more susceptible to delayed responses with iterative stimulation with respect to WT. MEPs were produced by bipolar stimulation of the primary motor cortex generating a direct wave in WT rats followed by several indirect waves, but taiep rats had fused MEPs. Importantly, taiep SSEPs improved after systemic administration of 4-AP, a potassium channel blocker, and this drug induced an increase in the horizontal displacement measured in a novelty-induced locomotor test. In taiep subjects have a significant decrease in the immunostaining of myelin in the anterior and ventral funiculi of the lumbar spinal cord with respect to WT rats. In conclusion, evoked potentials are useful to evaluate myelin alterations in a leukodystrophy, which improved after systemic administration of 4-AP. Our results have a translational value because our findings have implications in future medical trials for H-ABC patients or with other leukodystrophies.

Differential modulation of repetitive firing and synchronous network activity in neocortical interneurons by inhibition of A-type K(+) channels and Ih

GABAergic interneurons provide the main source of inhibition in the neocortex and are important in regulating neocortical network activity. In the presence 4-aminopyridine (4-AP), CNQX, and D-APV, large amplitude GABAA-receptor mediated depolarizing responses were observed in the neocortex. GABAergic networks are comprised of several types of interneurons, each with its own protein expression pattern, firing properties, and inhibitory role in network activity. Voltage-gated ion channels, especially A-type K(+) channels, differentially regulate passive membrane properties, action potential (AP) waveform, and repetitive firing properties in interneurons depending on their composition and localization. HCN channels are known modulators of pyramidal cell intrinsic excitability and excitatory network activity. Little information is available regarding how HCN channels functionally modulate excitability of individual interneurons and inhibitory networks. In this study, we examined the effect of 4-AP on intrinsic excitability of fast-spiking basket cells (FS-BCs) and Martinotti cells (MCs). 4-AP increased the duration of APs in both FS-BCs and MCs. The repetitive firing properties of MCs were differentially affected compared to FS-BCs. We also examined the effect of Ih inhibition on synchronous GABAergic depolarizations and synaptic integration of depolarizing IPSPs. ZD 7288 enhanced the amplitude and area of evoked GABAergic responses in both cell types. Similarly, the frequency and area of spontaneous GABAergic depolarizations in both FS-BCs and MCs were increased in presence of ZD 7288. Synaptic integration of IPSPs in MCs was significantly enhanced, but remained unaltered in FS-BCs. These results indicate that 4-AP differentially alters the firing properties of interneurons, suggesting MCs and FS-BCs may have unique roles in GABAergic network synchronization. Enhancement of GABAergic network synchronization by ZD 7288 suggests that HCN channels attenuate inhibitory network activity.

Rapid activation of dormant presynaptic terminals by phorbol esters

Presynaptic stimulation stochastically recruits transmission according to the release probability (P(r)) of synapses. The majority of central synapses have relatively low P(r), which includes synapses that are completely quiescent presynaptically. The presence of presynaptically dormant versus active terminals presumably increases synaptic malleability when conditions demand synaptic strengthening or weakening, perhaps by triggering second messenger signals. However, whether modulator-mediated potentiation involves recruitment of transmission from dormant terminals remains unclear. Here, by combining electrophysiological and fluorescence imaging approaches, we uncovered rapid presynaptic awakening by select synaptic modulators. A phorbol ester phorbol 12,13-dibutyrate (PDBu) (a diacylglycerol analog), but not forskolin (an adenylyl cyclase activator) or elevated extracellular calcium, recruited neurotransmission from presynaptically dormant synapses. This effect was not dependent on protein kinase C activation. After PDBu-induced awakening, these previously dormant terminals had a synaptic P(r) spectrum similar to basally active synapses naive to PDBu treatment. Dormant terminals did not seem to have properties of nascent or immature synapses, judged by NR2B NMDAR (NMDA receptor) receptor subunit contribution after PDBu-stimulated awakening. Strikingly, synapses rendered inactive by prolonged depolarization, unlike basally dormant synapses, were not awakened by PDBu. These results suggest that the initial release competence of synapses can dictate the acute response to second messenger modulation, and the results suggest multiple pathways to presynaptic dormancy and awakening.

Effect of 4-aminopyridine on synaptic transmission in rat hippocampal slices

Extracellular field excitatory postsynaptic potentials (fEPSPs) were recorded in area CA1 of rat hippocampal slices in vitro. The responses evoked by spontaneously released glutamate and GABA were recorded from area CA1 pyramidal neurons in rat hippocampal slices in whole-cell mode. The glutamate and GABA receptor-associated ligand-gated currents were obtained from dissociated single hippocampal pyramidal cells. The results showed that 4-aminopyridine (4-AP) had obvious effects on both presynaptic and postsynaptic events. Applications of 4-AP in micromolar concentration resulted in persistent enhancement of the initial slope of fEPSPs with the half-maximal enhancement concentration (EC(50)) of 46.7+/-2.68 microM. At the concentration of 200 microM, 4-AP increased the initial slopes of the total fEPSPs, NMDA- and AMPA-mediated fEPSPs components to 225.6+/-23.8%, 177.4+/-20.1% and 142.3+/-18.9%, respectively, but had no effect on the fiber volley. The half-maximal stimulus intensity to induce responses was reduced from 5.14+/-0.27 to 3.58+/-0.23 V. The frequencies of mEPSCs and mIPSCs were increased to 324.2+/-25.4% and 287.3+/-36.3% by 200 microM 4-AP. The amplitude histograms of mEPSCs and mIPSCs were fitted with Gaussian distributions. After 200 microM 4-AP application, the first and second peaks in Gaussian distributions of mEPSCs were shifted from 8.73+/-0.94 and 17.78+/-2.13pA to 10.48+/-0.82 and 21.14+/-2.45 pA, while those of mIPSCs were shifted from 13.65+/-0.96 and 25.51+/-2.95 pA to 11.21+/-1.04 and 23.08+/-2.37 pA. At 200 microM, 4-AP reduced paired-pulse facilitation and accelerated synaptic fatigue induced by stimulation at 10 Hz (for 1 s) and the ratio of fEPSPs(10)/fEPSPs(1) was decreased from 1.62+/-0.16 to 0.61+/-0.15. At 200 microM, 4-AP inhibited postsynaptic GABA currents induced by 5 microM GABA to 68.2+/-15.5%: by countering the effect of enhanced release of GABA from presynaptic terminals, this could depress the inhibitory pathway. Also at 200 microM, 4-AP increased NMDA currents to 155.3+/-17.8%, but had no significant effect on AMPA currents (94.2+/-15.6%). Our experimental results thus show that 4-AP-induced changes of synaptic transmission in area CA1 of rat hippocampus may be attributed to 4-AP's effects on both presynaptic terminals and postsynaptic receptors.

Low micromolar concentrations of 4-aminopyridine facilitate fictive locomotion expressed by the rat spinal cord in vitro

Upregulating the operation of spinal locomotor networks is one mechanism to restore, at least partially, lesion-impaired locomotion. We investigated if the K+ channel blocker 4-aminopyridine (4-AP) could facilitate spinal locomotor networks in addition to its well-known effect on motor nerve conduction. Fictive locomotor patterns were recorded from ventral roots (VRs) of the isolated spinal cord of the neonatal rat. 4-AP (0.1-50 microM) produced synchronous VR oscillations which did not develop into fictive locomotion. These oscillations had network origin, required intact glutamatergic transmission and were probably amplified via electrotonic coupling because of their depression by the selective gap junction blocker carbenoxolone. 4-AP (5 microM) slightly increased input resistance of lumbar motoneurons without affecting their action or resting potentials. Dorsal root (DR) evoked synaptic responses were enhanced (217 +/- 65%) by 5 microM 4-AP without changes in axon conduction. 4-AP (5 microM) accelerated fictive locomotion induced by N-methyl-d-aspartate (NMDA) and serotonin (5-HT) without altering cycle amplitude and facilitated the onset of fictive locomotion in the presence of sub-threshold concentrations of NMDA and 5-HT. Furthermore, in the presence of 4-AP, weak DR stimuli, previously insufficient to activate locomotor patterns, generated alternating VR discharges. Thus, although 4-AP per se could not directly activate the locomotor network of the spinal cord, it could strongly facilitate the locomotor program initiated by neurochemicals or electrical stimuli. These data suggest that the reported improvement by 4-AP in locomotor activity of spinal-injury patients may include activation of locomotor networks when low concentrations of this drug are administered in coincidence with appropriate stimuli.

Silent synapses in the developing hippocampus: lack of functional AMPA receptors or low probability of glutamate release?

At early developmental stages, silent synapses have been commonly found in different brain areas. These synapses are called silent because they do not respond at rest but are functional at positive membrane potentials. A widely accepted interpretation is that N-methyl-d-aspartate (NMDA) but not alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are functionally expressed on the subsynaptic membrane. Here we show that, in both CA3 and CA1 hippocampal regions, AMPA-mediated synaptic responses can be detected already at early stages of postnatal development. However, some synapses appear silent because of a very low probability of glutamate release. They can be converted into functional ones by factors that enhance release probability such as paired-pulse stimulation, increasing the temperature or cyclothiazide (CTZ), a drug that blocks AMPA receptor desensitization and increases transmitter release. Conversely, conducting synapses can be switched off by increasing the frequency of stimulation. Although we cannot exclude that "latent AMPA receptors" can become functional after activity-dependent processes, our results clearly indicate that, in the neonatal hippocampus, a proportion of glutamatergic synaptic connections are presynaptically rather than postsynaptically silent.

Presynaptic selection of afferent inflow in the spinal cord

The synaptic effectiveness of sensory fibers ending in the spinal cord of vertebrates can be centrally controlled by means of specific sets of GABAergic interneurons that make axo-axonic synapses with the terminal arborizations of the afferent fibers. In the steady state, the intracellular concentration of chloride ions in these terminals is higher than that predicted from a passive distribution, because of an active transport mechanism. Following the release of GABA by spinal interneurons and activation of GABA(A) receptors in the afferent terminals, there is an outwardly directed efflux of chloride ions that produces primary afferent depolarization (PAD) and reduces transmitter release (presynaptic inhibition). Studies made by intrafiber recording of PAD, or by measuring changes in the intraspinal threshold of single afferent terminals (which is reduced during PAD), have further indicated that muscle and cutaneous afferents have distinctive, but modifiable PAD patterns in response to segmental and descending stimuli. This has suggested that PAD and presynaptic inhibition in the various types of afferents is mediated by separate sets of last-order GABAergic interneurons. Direct activation, by means of intraspinal microstimulation, of single or small groups of last-order PAD-mediating interneurons shows that the monosynaptic PAD elicited in Ia and Ib afferents can remain confined to some sets of the intraspinal collaterals and not spread to nearby collaterals. The local character of PAD allows cutaneous and descending inputs to selectively inhibit the PAD of segmental and ascending intraspinal collaterals of individual muscle spindle afferents. It thus seems that the intraspinal branches of the sensory fibers are not hard wired routes that diverge excitation to spinal neurons, but are instead dynamic pathways that can be centrally controlled to address information to selected neuronal targets. This feature appears to play an important role in the selection of information flow in muscle spindles that occurs at the onset of voluntary contractions in humans.

Ionic mechanism of 4-aminopyridine action on leech neuropile glial cells

Extracellular 4-aminopyridine (4-AP), tetraethylammonium chloride (TEA) and quinine depolarized the neuropile glial cell membrane and decreased its input resistance. As 4-AP induced the most pronounced effects, we focused on the action of 4-AP and clarified the ionic mechanisms involved. 4-AP did not only block glial K+ channels, but also induced Na+ and Ca2+ influx via other than voltage-gated channels. The reversal potential of the 4-AP-induced current was -5 mV. Application of 5 mM Ni2+ or 0.1 mM d-tubocurarine reduced the 4-AP-induced depolarization and the associated decrease in input resistance. We therefore suggest that 4-AP mediates neuronal acetylcholine release, apparently by a presynaptic mechanism. Activation of glial nicotinic acetylcholine receptors contributes to the depolarization, the decrease in input resistance, and the 4-AP-induced inward current. Furthermore, the 4-AP-induced depolarization activates additional voltage-sensitive K+ and Cl- channels and 4-AP-induced Ca2+ influx could activate Ca2+-sensitive K+ and Cl- channels. Together these effects compensate and even exceed the 4-AP-mediated reduction in K+ conductance. Therefore, the 4-AP-induced depolarization was paralleled by a decreasing input resistance.

4-Aminopyridine enhances motor evoked potentials following graded spinal cord compression injury in rats

Although several experimental and clinical studies have demonstrated the ability of 4-aminopyridine (4-AP) to restore electrophysiological and/or behavioral function following chronic spinal cord injury, the mechanism by which this occurs remains unclear. Demonstration of efficacy in rat spinal cord injury has not been reported, evidently because even relatively mild spinal cord contusions that produce only minor permanent locomotor disturbances abolish hind limb myoelectric motor evoked potentials (mMEPs). In this study, mMEPs were recorded acutely 25 days following graded thoracic spinal cord compression in rats. mMEP amplitudes were significantly enhanced by a single, 2 mg/kg i.v. dose of 4-AP. mMEPs were increased in all rats showing some evoked responses initially, and also in some animals which had no responses prior to treatment. 4-AP was further found to increase the maximum following frequency of mMEPs in both normal and injured rats from about 0.1 Hz to between 1 and 10 Hz. These data suggest that 4-AP might act by enhancing synaptic efficacy, as well as enhancing conduction in spinal axons whose myelination has been rendered dysfunctional by trauma.

Release properties of isolated neuromuscular boutons of the garter snake

1. Motor nerve terminals innervating fibres in the transversus abdominis muscle of the garter snake comprise discrete boutons. Using a combination of enzymatic digestion and mechanical manipulation, individual boutons were removed from living terminals for study in isolation. 2. Boutons freed from terminals were usually allowed to remain in their original location on the endplate ('attached' one-bouton synapse). Alternatively, they were removed from the endplate, and then placed on the same or another vacant endplate site to form a 'reconstructed' one-bouton synapse. When removed from the endplate, boutons were 2-4 microns in diameter and nearly spherical in shape, in contrast to the variety of complex shapes seen among boutons still in contact with muscle fibre endplates. 3. Transmitter release was assessed by intracellular recording from the postsynaptic fibre. Boutons produced spontaneous miniature endplate potentials (MEPPs) of nearly normal amplitude; extracellular stimulation elicited endplate potentials (EPPs) which resembled MEPPs. Typical EPP amplitudes fluctuated between zero and five quanta per stimulus. For low-frequency stimulation under normal physiological conditions, mean quantal content, m, averaged 1.4; the binomial number of release sites, n, averaged 2.4; and the binomial probability of release, p, averaged 0.57. Statistics of the quantal fluctuations recorded from single boutons agreed only approximately with predictions of simple binomial theory, the discrepancy being that the apparent number of quanta released exceeded n in 5% of the events. 4. In separate experiments, activity-dependent probes were used to locate rare naturally occurring nerve terminals comprising a single bouton. Activation of these small synapses evoked quantal responses similar to those of attached and reconstructed one-bouton synapses described above.

Physiological adjustments in a reflex pathway following partial loss of target neurons

This investigation was undertaken to study plasticity in a reflex pathway following partial elimination of target neurons. Adult cats were subjected to unilateral avulsion of the L7 spinal ventral root, which induces retrograde cell death among the motoneurons of the L7 segment. At 1, 3, 6 and 12 weeks after the lesion, the monosynaptic reflexes were recorded in the L6 and S1 ventral roots during stimulation of the L6, L7 and S1 dorsal roots. Since the group Ia muscle spindle afferents passing through these dorsal roots were deprived of their target motoneurons in the L7 segment, compensatory reflex changes were searched for in the remaining monosynaptic contacts with the intact target motoneurons of the L6 and S1 segments. The results indicate that a partial loss of target motoneurons triggers changes leading to increased monosynaptic reflexes of the remaining intact target motoneurons. On average, the reflexes had more than doubled their size at 12 weeks postoperatively. Possible mechanisms for this reflex potentiation are discussed.

"Latent" inhibitory connections become functional during activity-dependent plasticity

Simultaneous pre- and postsynaptic recordings from identified glycinergic inhibitory interneurons and the Mauthner cell showed that 25% of the afferents produced no or extremely small postsynaptic responses. Morphological determination of the number of contacts made by these cells on the Mauthner cell revealed a connectivity similar to that of functional neurons which always produce clear inhibitory postsynaptic potentials, suggesting that most of the endings, made by weak interneurons are silent. Intraaxonal injection of 4-aminopyridine or Ca2+ greatly enhanced transmission at functional connections but did not modify those which were ineffective. However, after eighth nerve tetanic stimuli, transmission at the weak connections was unmasked or enhanced for prolonged periods and was twice as likely to be potentiated, with a 6-fold greater mean enhancement than the potent ones. This result provides additional support for long-term potentiation of inhibitory synapses. Furthermore, weakly functional junctions represent a "reserve" pool which can be critical for the expression of plasticity within a network, and, consequently, for setting the threshold of reflex activities such as the escape reaction mediated by the Mauthner cell.

Direct patch recording from identified presynaptic terminals mediating glutamatergic EPSCs in the rat CNS, in vitro

1. An in vitro brainstem slice preparation of the superior olivary complex has been developed permitting patch recording from a presynaptic terminal (calyx of Held) and from its postsynaptic target--the principal neurone of the medial nucleus of the trapezoid body (MNTB). 2. The fluorescent stain DiI (1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate) was used in fixed tissue and Lucifer Yellow in living slices, to identify calices enclosing single MNTB neuronal somata. 3. Whole-cell recording from the MNTB neurone shows evoked EPSCs preceded by a prespike, corresponding to the presynaptic action potential (AP). In some cases one patch pipette recorded from both pre- and postsynaptic elements, but confirmation of exclusively presynaptic recording was obtained using pipettes containing Lucifer Yellow in a further eleven cases. 4. Under current clamp, the pre- and postsynaptic sites could be distinguished by their response to step depolarizations; presynaptic terminals generated a train of APs at frequencies up to 200 Hz, while MNTB neurones gave a single AP. Each presynaptic AP had an after-hyperpolarization lasting less than 2 ms. 5. Under voltage clamp, step depolarizations of presynaptic terminals generated a tetrodotoxin-sensitive inward current followed by rapidly activating outward potassium currents at potentials more positive than -60 mV. The outward current exhibited little inactivation over the 150 ms steps and 4-aminopyridine (200 microM) blocked 63.0 +/- 14.5% (mean +/- S.D., n = 3) of the sustained current at 0 mV. Like the squid giant synapse, mammalian terminals express rapidly activating 'delayed rectifier'-type potassium currents.

Differential modulation of chemical and electrical components of mixed synapses in the lobster stomatogastric ganglion

1. Two pairs of neurons in the pyloric network of the spiny lobster, Panulirus interruptus, communicate through mixed graded chemical and rectifying electrical synapses. The anterior burster (AB) chemically inhibits and is electrically coupled to the ventricular dilator (VD); the lateral pyloric (LP) and pyloric (PY) neurons show reciprocal chemical inhibition and electrical coupling. We examined the effects of dopamine (DA), serotonin (5HT) and octopamine (Oct) on these mixed synapses to determine the plasticity possible with opposing modes of synaptic interaction. 2. Dopamine increased net inhibition at all three pyloric mixed synapses by both reducing electrical coupling and increasing chemical inhibition. This reversed the sign of the net synaptic interaction when electrotonic coupling dominated some mixed synapses, and activated silent chemical components of other mixed synapses. 3. Serotonin weakly enhanced LP-->PY net inhibition, by reducing electrical coupling without altering chemical inhibition. Serotonin reduced AB-->VD electrical coupling, but variability in its effect on the chemical component made the net effect non-significant. 4. Octopamine enhanced LP-->PY and PY-->LP net inhibition by enhancing the chemical inhibitory component without altering electrical coupling. 5. Differential modulation of chemical and electrical components of mixed synapses markedly changes the net synaptic interactions. This contributes to the flexible outputs that modulators evoke from anatomically defined neural networks.

Rate of quantal transmitter release at the mammalian rod synapse

Under scotopic conditions, the mammalian rod encodes either one photon or none within its integration time. Consequently the signal presented to its synaptic terminal is binary. The synapse has a single active zone that releases neurotransmitter quanta tonically in darkness and pauses briefly in response to a rhodopsin isomerization by a photon. We asked: what minimum tonic rate would allow the postsynaptic bipolar cell to distinguish this pause from an extra-long interval between quanta due to the stochastic timing of release? The answer required a model of the circuit that included the rod convergence onto the bipolar cell and the bipolar cell's signal-to-noise ratio. Calculations from the model suggest that tonic release must be at least 40 quanta/s. This tonic rate is much higher than at conventional synapses where reliability is achieved by employing multiple active zones. The rod's synaptic mechanism makes efficient use of space, which in the retina is at a premium.

An ultrastructural size principle

Recent ultrastructural descriptions of synaptic contacts suggest that potential synaptic efficacy may be directly correlated with bouton size. The characteristics of a synaptic bouton which presumably underlie its potential physiological strength (such as vesicle number, active zone number and area, and mitochondrial volume) are all linearly related to the volume of the bouton. Furthermore, at synapses which contact dendritic spines in both the hippocampus and cerebellum, the volume of the spine is linearly related to bouton volume. The existence of these scaling relationships has widespread implications for interpreting synaptic anatomy and variability, and for examining synaptic plasticity. We review evidence in support of the "ultrastructural size principle" outlined above and its potential generality.

The probability of transmitter release at a mammalian central synapse

When an action potential reaches a synaptic terminal, fusion of a transmitter-containing vesicle with the presynaptic membrane occurs with a probability (pr) of less than one. Despite the fundamental importance of this parameter, pr has not been directly measured in the central nervous system. Here we describe a novel approach to determine pr, monitoring the decrement of NMDA (N-methyl-D-aspartate)-receptor mediated synaptic currents in the presence of the use-dependent channel blocker MK-801 (ref. 2). On a single postsynaptic CA1 hippocampal slice neuron, two classes of synapses with a sixfold difference in pr are resolved. Synapses with low pr contribute to over half of transmission and are more sensitive to drugs enhancing transmitter release. Switching between these two classes of synapses provides the potential for large changes in synaptic efficacy and could underlie forms of activity-dependent plasticity.

Responses to sympathetic nerve stimulation of the sinus venosus of the toad

1. The changes in membrane potential produced by sympathetic nerve stimulation were recorded from sinus venosus preparations of the toad, Bufo marinus, in which beating had been prevented by the dihydropyridine calcium antagonist, nifedipine. 2. Supramaximal sympathetic stimuli initiated long-lasting excitatory junction potentials which started with the same latencies, some 1 to 2 s, as did sympathetic tachycardias recorded from beating preparations. 3. Brief trains of stimuli increased the amplitude of excitatory junction potentials and shortened their latency of onset. Similarly when excitatory junction potentials were facilitated their latency of onset was shortened. 4. The time courses of excitatory junction potentials were prolonged by cooling the preparation but unchanged when the neuronal uptake of catecholamines was inhibited. 5. In arrested preparations, beta-adrenoceptor activation causes a hyperpolarization, as did the inhibition of phosphodiesterases or the activation of adenylate cyclase. This contrasts with the depolarization produced by sympathetic nerve stimulation which could be mimicked by the rapid application of either adrenaline or noradrenaline but not by beta-adrenoceptor activation, phosphodiesterase inhibition or by adenylate cyclase activation. 6. The results are discussed in relation to the idea that neuronally released adrenaline activates a set of adrenoceptors which are linked to a set of channels by a pathway that does not involve cyclic AMP.

4-Aminopyridine in chronic spinal cord injury: a controlled, double-blind, crossover study in eight patients

The potassium channel blocking drug 4-aminopyridine (4-AP) was administered to eight patients with chronic spinal cord injury, in a therapeutic trial based on the ability of the drug to restore conduction of impulses in demyelinated nerve fibers. The study was performed using a randomized, double-blind, crossover design, so that each patient received the drug and a vehicle placebo on different occasions, separated by 2 weeks. Drug and placebo were delivered by infusion over 2 h. An escalating total dose from 18.0 to 33.5 mg was used over the course of the study. Subjects were evaluated neurologically before and after the infusion. Two subjects returned for a second trial after 4 months and were examined daily for 3 to 4 days following drug infusion. Side effects were consistent with previous reports. Administration of the drug was associated with significant temporary neurologic improvement in five of six patients with incomplete spinal cord injury. No effect was detected in two cases of complete paraplegia and one of two severe incomplete cases (Frankel class B). Improvements in neurologic status following drug administration included increased motor control and sensory ability below the injury, and reduction in chronic pain and spasticity. The effects persisted up to 48 h after infusion of the drug, and patients largely returned to preinfusion status by 3 days. Compared with the more rapid elimination of the drug, these prolonged neurologic effects appear to involve a secondary response and are probably not a direct expression of potassium channel blockade.

Miniature excitatory postsynaptic potentials in embryonic motoneurons grown in slice cultures of spinal cord, dorsal root ganglia and skeletal muscle

Miniature excitatory postsynaptic potentials (mEPSPs) were recorded in motoneurons grown in organotypic cocultures of embryonic rat spinal cord, dorsal root ganglia and muscle in the presence of TTX. The motoneurons were electrically compact with a mean electrotonic length of 0.6. Spontaneous EPSPs were found in most of these motoneurons. With TTX the large EPSPs disappeared, whereas in more than half of the experiments mEPSPs persisted with a range in size of 1 to 4 mV (mean: 2.1 mV), probably originating from the spontaneous release of single vesicles. The net inward charge transfer at the soma ranged from 0.12 to 0.34 pC. The mEPSPs were heterogeneous in size even within pools of potentials that were homogeneous in shape. They had similar shapes and amplitudes as the smallest spontaneous unitary EPSPs mediated by presynaptic impulses, suggesting that for the smallest afferents not more than one vesicle was released per afferent impulse. Both the miniature and the TTX-sensitive EPSPs were readily blocked by the glutamate antagonist DNQX.

Synapses on motoneuron dendrites in the brachial section of the frog spinal cord: a computer-aided electron microscopic study of cobalt-filled cells

Cobalt-labelled motoneuron dendrites of the frog spinal cord at the level of the second spinal nerve were photographed in the electron microscope from long series of ultrathin sections. Three-dimensional computer reconstructions of 120 dendrite segments were analysed. The samples were taken from two locations: proximal to cell body and distal, as defined in a transverse plane of the spinal cord. The dendrites showed highly irregular outlines with many 1-2 microns-long 'thorns' (on average 8.5 thorns per 100 microns 2 of dendritic area). Taken together, the reconstructed dendrite segments from the proximal sites had a total length of about 250 microns; those from the distal locations, 180 microns. On all segments together there were 699 synapses. Nine percent of the synapses were on thorns, and many more close to their base on the dendritic shaft. The synapses were classified in four groups. One third of the synapses were asymmetric with spherical vesicles; one half were symmetric with spherical vesicles; and one tenth were symmetric with flattened vesicles. A fourth, small class of asymmetric synapses had dense-core vesicles. The area of the active zones was large for the asymmetric synapses (median value 0.20 microns 2), and small for the symmetric ones (median value 0.10 microns 2), and the difference was significant. On average, the areas of the active zones of the synapses on thin dendrites were larger than those of synapses on large calibre dendrites. About every 4 microns 2 of dendritic area received one contact. There was a significant difference between the areas of the active zones of the synapses at the two locations. Moreover, the number per unit dendritic length was correlated with dendrite calibre. On average, the active zones covered more than 4% of the dendritic area; this value for thin dendrites was about twice as large as that of large calibre dendrites. We suggest that the larger active zones and the larger synaptic coverage of the thin dendrites compensate for the longer electrotonic distance of these synapses from the soma.

Modulation of the transient potassium current in rat hippocampal neurones by cholecystokinin

Cholecystokinin (CCK) affects neuronal excitability in a variety of in vivo and in vitro preparations, apparently by modulating a resting potassium conductance. The data presented here show that CCK (applied as CCK8-S) also affects the transient potassium current in hippocampal neurones, by changing the voltage dependence of the inactivation and activation of the current. The way in which the voltage dependence is changed can lead to either an enhancement of the current or an attenuation, depending upon the voltage protocol used. This effect of CCK does not desensitise over a time period of minutes, and may therefore be important in controlling neuronal excitability in the CNS.

Activity-dependent recruitment of silent synapses

At the crayfish neuromuscular junction, a long-lasting enhancement of synaptic transmission can be induced by tetanic stimulation of 10-20 Hz for several minutes. The long-lasting enhancement is presynaptic in origin, because quantal content increases but not quantal size, and is not dependent upon broadening or enlargement of the presynaptic action potential. The enhancement can be selectively blocked by presynaptic injection of agents that inhibit adenylate cyclase or the cyclic AMP-dependent protein kinase. Entry of calcium may not be sufficient in itself to produce the enhancement. Analyses of quantal events using both a simple binomial statistical method, and a more refined method that takes into account the possibility of unequal probabilities of responding units, have shown that the number of responding units increases during the long-lasting enhancement. In addition, there is an increase in the probability of transmitter release at preexisting units. In contrast, during short-term facilitation accompanying repetitive stimulation, response probability increases greatly whereas the number of responding units increases only moderately with frequencies of activation up to 20 Hz, which increase quantal output severalfold. These results indicate that responding units, hypothesized to be transmitting synapses, can be recruited to active transmission from an unresponsive pool by tetanic activity, and that protein phosphorylation is required for long-lasting changes to occur. The existence of an excess of synapses on crustacean nerve terminals is indicated by ultrastructural studies, which invariably show many synapses on the terminals. The number of morphologically defined synapses is always greater than the number of responding units seen in statistical analyses of quantal release for the same recording location.

Physiological properties of newly formed synapses between sympathetic preganglionic neurons and sympathetic ganglion neurons

We have examined the physiological properties of transmission at newly formed synapses between sympathetic preganglionic neurons and sympathetic ganglion neurons in vitro. Chick neurons were labeled with fluorescent carbocyanine dyes before they were placed into culture (Honig and Hume, 1986), and were studied by making intracellular recordings during the first 2 weeks of coculture. Evoked monosynaptic excitatory postsynaptic potentials (EPSPs) were not observed until 48 h of coculture. Beyond this time, the frequency with which connected pairs could be found did not vary greatly with time. With repetitive stimulation, the evoked monosynaptic EPSPs fluctuated in amplitude from trial to trial and showed depression at frequencies as low as 1 Hz. To gain further information about the quantitative properties of transmission at newly formed synapses, we analyzed the pattern of fluctuations of delayed release EPSPs. In mature systems, delayed release EPSPs are known to represent responses to single quanta, or to the synchronous release of a small number of quanta. For more than half of the connections we studied, the histograms of delayed release EPSPs were extremely broad. This result suggested that either quantal responses are drawn from a continuous distribution that has a large coefficient of variation or that there are several distinct size classes of quantal responses. The pattern of fluctuations of monosynaptic EPSPs was consistent with both of these possibilities, and was inconsistent with the possibility that monosynaptic EPSPs are composed of quantal subunits with very little intrinsic variation. Although variation in the size of responses to single quanta might arise in a number of ways, one attractive explanation for our results is that the density and type of acetylcholine receptors varies among the different synaptic sites on the surface of developing sympathetic ganglion neurons.

The effects of 4-aminopyridine on neurological deficits in chronic cases of traumatic spinal cord injury in dogs: a phase I clinical trial

A Phase I trial of 4-aminopyridine (4-AP) was carried out in 39 dogs referred to the veterinary teaching hospital with naturally occurring traumatic paraplegia or paraparesis. The rationale for the study was provided by the observation that 4-AP restores conduction in demyelinated nerve fibers in experimental spinal cord injury. Most injuries (77%) resulted from degenerative disk disease, occurring at or near the thoracolumbar junction, and producing chronic, complete paraplegia. Neurological examination of each dog was recorded on videotape before and at intervals after administration of 4-AP. The drug was administered systemically in total doses between 0.5 and 1 mg/kg body weight. Three areas of neurological status changed significantly at 15-45 minutes following administration of 4-AP: (a) striking improvements in hindlimb placing occurred in 18 animals; (b) increased awareness of painful stimuli to the hindlimb in 10 animals; (c) partial recovery of the cutaneus trunci muscle reflex of the back skin in 9 animals. These effects reversed within a few hours of administration. Other animals (36%) showed no change in neurological signs except a slight enhancement of hindlimb reflex tone. Significant side effects were seen in 6 dogs receiving higher intravenous doses, with elevation of body temperature and apparent anxiety, leading to mild seizures in 3 of the animals. These seizures were controlled with diazepam. The results indicate that conduction block may contribute significantly to functional deficits in closed-cord injuries and that potassium channel blockade may prove to be a valid, if limited approach to therapeutic intervention in chronic paraplegia and paraparesis.

Simulation of action potential propagation in complex terminal arborizations

Action potential propagation in complex terminal arborizations was simulated using SPICE, a general purpose circuit simulation program. The Hodgkin-Huxley equations were used to simulate excitable membrane compartments. Conduction failure was common at branch points and regularly spaced boutons en passant. More complex arborizations had proportionally more inactive synapses than less complex arborizations. At lower temperature the safety factor for impulse propagation increased, reducing the number of silent synapses in a particular arborization. Small structural differences as well as minute changes in the discharge frequency of the action potential resulted in very different activation patterns of the arborization and terminal boutons. The results suggest that the structural diversity of terminal arborizations allows a wide range of presynaptic information processing. The results from this simulation study are discussed in the context of experimental results on the modulation of synaptic transmission.

"Upstream" regulation of the release probability in sympathetic nerve varicosities

The results appear to support the following tentative working hypothesis. (1) Nerve impulse-induced transmitter release from sympathetic nerve varicosities is monoquantal and highly intermittent (probability range: 0-0.03). (2) Nerve impulses invade varicosities as all-or-none, Na+ channel-dependent action potentials; invasion failure may be rare. (3) The release probability is not controlled by properties (amplitude or duration) of the invading action potential or the resulting Ca2+ current, but by the availability of an as yet unidentified permissive factor. (4) The permissive factor is actively transported intra-axonally, probably in association with organelles (LDVs?). (5) The activation and/or transport of the permissive factor are controlled "upstream" of the varicosity; they depend on Ca2+ influx through channels insensitive to nifedipine (hence, not of L-type) but blocked by Cd2+ and apparently opened by slight depolarization of the resting membrane, in this respect behaving more as T- than N-type channels. (6) A high resting K+ efflux "upstream" of the varicosity restricts the availability of the permissive factor; it is the main mechanism maintaining the (economically necessary) low release probability. (7) Prejunctional agonists do not inhibit transmitter secretion by causing a conduction block or by reducing the action potential-induced Ca2+ influx into the varicosity itself, but by depressing the Ca2(+)-dependent activation and/or transport of the permissive factor; they act at least in part via receptors "upstream" of the varicosity. (8) This hypothesis for regulation of the release probability in sympathetic nerves may apply, at least in part, to other neurons as well.

A statistical test for demonstrating a presynaptic site of action for a modulator of synaptic amplitude

A statistical technique for demonstrating a presynaptic site of action for a modulator of synaptic amplitude was developed and tested. It requires that multiple measurements of peak synaptic amplitude be made under control and test conditions. The ratio of the coefficients of variation (CV) obtained under test and control conditions is calculated. A method was developed for determining the confidence interval for the CV ratio (CVR) statistic based on the null hypothesis that the synaptic modulation is purely postsynaptic. If the measured CVR falls outside the confidence interval, this implies that the modulator of synaptic amplitude is, at least in part, acting at a presynaptic site. The effectiveness of the technique and its limitations were investigated using Monte Carlo simulations. It was found to be sensitive and reliable under a variety of realistic recording conditions. The test was effective even in the presence of simulated presynaptic rundown of the synaptic response. Conventional deconvolution analysis was also applied to the Monte Carlo simulations and was found to be an inadequate indicator of the site of synaptic modulation when the discrete amplitude components were not well resolved. The CVR technique was applied to excitatory postsynaptic currents (epsc) recorded between pairs of cultured hippocampal neurones in control and test media containing 1 mM Ca2+ and 2 mM Ca2+, respectively. Test conditions increased the average synaptic amplitude, and the statistical analysis indicated that this modulation was produced by an action at a presynaptic site.

A physiological study of the monosynaptic reflex responses of cat spinal alpha-motoneurons after partial lumbosacral deafferentation

In adult cats the whole S1 and rostral half of the L7 dorsal roots were cut on the left side of the spinal cord to produce a partial monosynaptic deafferentation of the ipsilateral alpha-motoneurons. Three, 6 or 12 weeks later, monosynaptic reflexes (MSRs) were recorded from the L6, L7 and S1 ventral roots or from various peripheral nerves during stimulation of the L6 and remaining parts of the L7 dorsal roots. Also, monosynaptic excitatory postsynaptic potentials (EPSPs) were recorded intracellularly in different types of medial gastrocnemius alpha-motoneurons of the L7 segment during stimulation of various hind limb muscle nerves. The right side with an identical acute deafferentation served as control. On the chronically lesioned side the MSRs were increased in size, also during post-tetanic potentiation. The monosynaptic EPSPs had increased amplitudes in all motoneuron types, but the relation in EPSP size between different motoneuron types as well as between different synergistic inputs remained largely unchanged. EPSP rise times were not changed, and aberrant monosynaptic connections from non-synergist muscles were not observed. It is concluded that the extent of reactive reflex changes may be related to both the number of vacant synaptic sites and the degree of functional synergism between the eliminated and remaining monosynaptic pathways. Possible underlying mechanisms are discussed.

Augmentation by 4-aminopyridine of vestibulospinal free fall responses in chronic spinal-injured cats

This study examines the effect of the potassium channel blocker 4-aminopyridine (4-AP) on free fall responses (FFR) in the hindlimb muscles of chronically spinal injured cats. The thoracic spinal cord of 7 adult female cats was injured by a standardized contusion method. At 3-7 months post-injury the FFR in 6 hindlimb muscles was recorded electromyographically in each animal, under ketamine sedation. The normal short-latency response to a sudden drop was severely attenuated in all injured animals and practically undetectable in 2 cases. Within 15 min following intravenous administration of 1 mg/kg 4-AP, there was profound augmentation of the amplitude of the FFR and a tendency toward normalization of latency in all animals, though the normal amplitude range was not attained. The same 4-AP dose produced a relatively small increase of FFR amplitude in only 2 of 4 normal, uninjured animals tested. The data are consistent with previous observations that low doses of 4-AP restore conduction in some critically demyelinated axons, and provide support for the hypothesis that conduction block in surviving axons is responsible for a proportion of the dysfunction in chronic spinal injury. Augmentation of FFR in injured animals may also result partly from increased transmitter release in both spinal cord and periphery, due to the presynaptic effects of 4-AP.

Presynaptic inhibition of synaptic potentials evoked in cat spinal motoneurones by impulses in single group Ia axons

1. Single-fibre group Ia excitatory post-synaptic potentials (e.p.s.p.s) were evoked in triceps surae motoneurones. These e.p.s.p.s were reduced by conditioning stimulation of group I axons in posterior biceps-semitendinosus nerves. 2. The investigation concentrated on e.p.s.p.s of somatic origin, because the amplitude of these e.p.s.p.s is not reduced by post-synaptic conductance increases. Any reduction in these e.p.s.p.s could therefore be attributed to presynaptic inhibition. 3. The reduction in somatic e.p.s.p. amplitude was greatest when the conditioning stimulus preceded the e.p.s.p. by 30 ms, and was negligible when the conditioning interval was extended to 200-300 ms. 4. The percentage reduction of somatic e.p.s.p.s was independent of their unconditioned peak amplitude. 5. E.p.s.p.s of somatic origin were reduced by the same amount, on average, as e.p.s.p.s of dendritic origin. 6. E.p.s.p.s evoked in the same motoneurone by impulses in different Ia axons were reduced by different amounts and e.p.s.p.s evoked in different motoneurones by impulses in the same Ia axon were also reduced by different amounts. 7. Analysis of fluctuations in e.p.s.p.s before and after conditioning indicated that after conditioning, larger discrete amplitudes became less probable, while smaller discrete amplitudes became more probable. The average increment between discrete amplitudes did not alter; nor were the discrete amplitudes reduced. 8. The probabilities of transmitter release at synaptic boutons were calculated before and during presynaptic inhibition. The maximum decrease in release probability was 0.64, suggesting a reduction in calcium influx of 10-15%.

Effects of calcium and magnesium on transmitter release at Ia synapses of rat spinal motoneurones in vitro

The lumbar spinal cord excised from neonatal rats was superfused with an oxygenated saline, and monosynaptic excitatory post-synaptic potentials (e.p.s.p.s) were recorded from the lumbar motoneurones following stimulation of muscle nerves of the hind leg. The amplitude of monosynaptic e.p.s.p.s increased with an increase in the external Ca2+ concentration ([Ca2+]o). In [Ca2+]o of 0.5-2 mM, the relation between the e.p.s.p. amplitude and [Ca2+]o was linear with a slope of 1.6 on double logarithmic co-ordinates. An increase in [Mg2+]o in a range of 1-5 mM reduced the e.p.s.p. amplitude without affecting the slope of the relation between log e.p.s.p. amplitude and log [Ca2+]o, suggesting competitive interaction between Ca2+ and Mg2+. When [Ca2+]o was increased to 4-12 mM, the latency of monosynaptic e.p.s.p.s was prolonged, and the antidromic action potential failed to invade the cell body or initial segment of motoneurones. Under these conditions, the monosynaptic e.p.s.p.s were still present. It is concluded that at near-normal levels of [Ca2+]o the effects of Ca2+ and Mg2+ on the e.p.s.p. amplitude are accounted for entirely by their competitive interaction for the probability of transmitter release without altering the mode of impulse propagation at central terminals of the Group Ia sensory fibres. It is suggested that impulses of Ia sensory fibres normally invade their terminals without intermittent blocks at their terminal branch points.

Distal dendrites of frog motor neurons: a computer-aided electron microscopic study of cobalt-filled cells

With the aid of the cobalt labelling technique, frog spinal cord motor neuron dendrites of the subpial dendritic plexus have been identified in serial electron micrographs. Computer reconstructions of various lengths (2.5-9.8 micron) of dendritic segments showed the contours of these dendrites to be highly irregular, and to present many thorn-like projections 0.4-1.8 micron long. Number, size and distribution of synaptic contacts were also determined. Almost half of the synapses occurred at the origins of the thorns and these synapses had the largest contact areas. Only 8 out of 54 synapses analysed were found on thorns and these were the smallest. For the total length of reconstructed dendrites there was, on average, one synapse per 1.2 micron, while 4.4% of the total dendritic surface was covered with synaptic contacts. The functional significance of these distal dendrites and their capacity to influence the soma membrane potential is discussed.

4-Aminopyridine induces expansion of cutaneous receptive fields of dorsal horn cells

Systemic administration of 4-aminopyridine (4-AP) increased the size of the cutaneous receptive fields of 9 of the 15 dorsal horn cells tested. These receptive fields were on the feet and toes of the hind limbs of cats. Receptive field sizes increased with increasing doses of 4-AP. However, 4-AP administration did not change the responses of dorsal horn cells to graded mechanical stimuli administered near the centers of their receptive fields.

Cutaneous receptive field and morphological properties of hamstring flexor alpha-motoneurones in the rat

Intracellular recordings have been made from twenty antidromically identified posterior biceps femoris/semitendinosus (p.b.s.t.) hamstring flexor alpha-motoneurones in the decerebrate-spinal rat. The hamstring motoneurones had either low or no spontaneous background activity. In nineteen of the twenty cells high-frequency phasic responses could be elicited by stimulation of the ipsilateral hind paw with firm pressure or pinch. There was no response to light touch or brush. Contralateral cutaneous mechanoreceptive fields with higher thresholds and weaker responses were present in 70% of the motoneurones. Noxious heating of the ipsilateral hind paw produced excitatory responses in six of eight cells tested and two of these cells also responded to heating of the contralateral hind paw. Stimulation of the ipsilateral sural nerve at graded strengths that successively activated A beta, A delta and C afferents produced excitatory post-synaptic potentials (e.p.s.p.s) at progressively longer latencies in the motoneurones. The C-fibre induced e.p.s.p. lasted up to 200 ms. Horseradish peroxidase was injected into ten motoneurones and in seven cases full reconstructions of dendritic field, cell body and axon could be made. In agreement with previous reports from studies in the cat, the dendritic fields of rat motoneurones are very extensive in the rostrocaudal, mediolateral and dorsoventral planes. The general pattern of dendritic branching for each motoneurone in this functionally homogeneous population was uniformly organized. Three major spatial orientations were always present: a rostrocaudally restricted series of dendrites emerging from the cell body and directed dorsolaterally towards the dorsolateral funiculus with branches in the lateral dorsal horn, a laterally, and a ventromedially directed series of branches arranged obliquely in the ventral horn, both of which were distributed rostrocaudally for equal distances from the cell body. Many of these dendritic branches terminated within the lateral and ventral white columns. Although the sizes of the rat flexor motoneurones' somas (51 +/- 4.9 micron, S.E., n = 10) were similar to those of cat lumbosacral alpha-motoneurones, the tip-to-tip rostrocaudal extent of their dendritic fields (1130 +/- 34 micron, S.E., n = 7) was half that reported in the cat. These results are discussed in terms of the organization of the cutaneous flexor withdrawal reflex in the rat.

Structural and topographical influences on functional connectivity in spinal monosynaptic reflex arcs in the cat

A greatly expanded version of spike-triggered averaging (Mendell & Henneman, 1971), performed off-line on tape-recorded signals, was utilized to determine the presence or absence of functional connexions between stretch-afferent fibres and homonymous motoneurones. As many as 264 possible connexions between eleven Ia or spindle group II fibres and twenty-four motoneurones were studied in each single, acute experiment. Morphological and topographical factors influencing functional connectivity were analysed with the aid of wiring diagrams and connectivity matrices. In all experiments the greater the conduction velocity (i.e. diameter) of a Ia or group II fibre, the higher was the probability of its having functional connexions with homonymous motoneurones. The greater the longitudinal distance between the spinal entry points of Ia fibres and the location of a motoneurone, the less was the same probability. The influence of axonal conduction velocity of motoneurones on functional connectivity was apparent in some experiments, but not in others. In pooled data large motoneurones received functional connexions from a higher percentage of group II fibres than did small cells. The projection percentage reached 100 only when both Ia fibres and motoneurones were large, suggesting that motoneurone size influences the probability of functional connexions from group Ia as well as group II fibres. On a cell-to-cell level, connectivity apparently does not follow strict, deterministic rules. The results raise the question of how probabilistic connexions between afferent fibres and motoneurones give rise to deterministic outputs from the whole pool.

Post-natal development of ganglia in the lower lumbar sympathetic chain of the rat

The initial stages in the development of functional synapses have been examined in ganglia of the lower lumbar sympathetic chain of the rat using intracellular recording techniques. In animals of age up to 7 days post-natal, many impaled cells were inexcitable and possessed no synaptic input. The proportion of excitable cells impaled increased with the age of the animal. Two types of action potential could be identified. Initially the synaptic input consisted of one or a few subthreshold synaptic potentials. The number of preganglionic inputs to each cell increased over the first 1-2 weeks after birth. The quantal content of each input was initially very low. At least some inputs showed an increase in quantal content during development; eventually one or occasionally two inputs became suprathreshold. Voltage-clamp studies indicated that the time course of excitatory synaptic currents did not change during development. The amplitudes of miniature excitatory synaptic currents in animals aged less than 10 days were some three to five times the size of those recorded from mature animals.

Amplitude fluctuations in synaptic potentials evoked in cat spinal motoneurones at identified group Ia synapses

Excitatory post-synaptic potentials (e.p.s.p.s) were evoked in spinal motoneurones (of anaesthetized cats) by impulses in single group Ia axons. The morphological details of the Ia axon-motoneurone connexion involved in generating each e.p.s.p. were subsequently reconstructed, as described by Redman & Walmsley (1983). The fluctuation pattern of the peak amplitude of each e.p.s.p. was determined using a deconvolution method, taking into account the recording noise. Four e.p.s.p.s were analysed. One did not fluctuate in amplitude; the others fluctuated between discrete amplitudes which were separated by quantal increments. The number of increments which must be added to produce the largest peak amplitude of each e.p.s.p. was always less than, or equal to, the number of synaptic boutons in the connexion at which the e.p.s.p. was generated. The results are consistent with the hypothesis that transmission occurs in an all-or-none manner at each synaptic bouton. Different boutons in the termination of a Ia axon with a motoneurone have different probabilities of releasing transmitter, and this probability is sometimes zero at low stimulation rates. The results support the idea that the discrete amplitudes of an evoked e.p.s.p. result from intermittent transmission, in an all-or-none manner, at some or all of the boutons in the termination.

The time course of synaptic potentials evoked in cat spinal motoneurones at identified group Ia synapses

Excitatory post-synaptic potentials (e.p.s.p.s) were evoked in motoneurones by impulses in single group Ia axons. After recording the e.p.s.p., the motoneurone and the group Ia axon were injected with horseradish peroxidase. The morphological details of the connexion formed by each group Ia axon with a motoneurone were subsequently reconstructed. Four Ia axon-motoneurone pairs were obtained. The electrotonic distance from the soma to each synaptic bouton in the connexion was calculated. The electrotonic lengths of those dendrites on which synaptic connexions were found were also calculated. The shape indices of each recorded e.p.s.p. and the standard cable model of the motoneurone were used to calculate the electrotonic distance from the soma to the point on the equivalent dendritic cable at which the e.p.s.p. originated. This distance was compared with the distance obtained from the reconstruction of the synaptic connexion at which the e.p.s.p. was generated. For two of the four connexions, the locations calculated by both methods agreed to within 0.1 lambda. Similar agreement could only be obtained for the other two connexions if synaptic transmission did not occur at some of the boutons in the termination. Evidence that some boutons were not involved in transmission is presented in the following paper (Redman & Walmsley, 1983).

The synaptic current evoked in cat spinal motoneurones by impulses in single group 1a axons

Excitatory post-synaptic potentials (e.p.s.p.s) were evoked in motoneurones of anaesthetized cats by impulses in single group 1 a axons. E.p.s.p.s with a time course which indicated a somatic site of origin were voltage-clamped using a single micro-electrode clamp. Excitatory post-synaptic currents (e.p.s.c.s) were found to peak in less than 0.2 ms, and to decay with an exponential time course. The time constant of decay was usually in the range 0.3-0.4 ms (at 37 degrees C). At the resting membrane potential, an e.p.s.p. with a peak of 100 microV was generated by an average peak e.p.s.c. of 330 pA. This corresponded to an average peak conductance increase of 5 nS. The e.p.s.c. decreased with membrane depolarization, and reversed to become an outward current at a null potential of +4.6 +/- 2 mV (+/- S.E. of mean; n = 7). Membrane hyperpolarization caused the peak e.p.s.c. to increase and the time constant of decay of the e.p.s.c. to decrease. The total charge in the synaptic current did not increase with hyperpolarization. This observation can explain earlier observations which showed that the peak amplitude of the e.p.s.p. did not increase with hyperpolarization. The number of ion channels opened by transmitter release at a single somatic bouton was estimated to be in the range 40-240.

4-Aminopyridine and fiber potentials in rat and human hippocampal slices

Compound fiber action potentials of stratum radiatum afferents in slices from human and rat hippocampus are shown to be prolonged by 4-aminopyridine (4-AP). This action can explain the enormous increase in synaptic transmitter release caused by 4-AP.

Selective inhibition by methysergide of the monosynaptic reflex discharge in the isolated spinal cord of the newborn rat

In the isolated spinal cord of the newborn rat, methysergide and LSD-25 depressed the monosynaptic reflex discharge selectively. Cyproheptadine and dimethothiazine did not inhibit the monosynaptic reflex. The selective inhibitory effect of methysergide on the monosynaptic reflex was not due to a presumptive low safety factor of this reflex. The inhibition was restored under a condition such as the compound action potential in the dorsal root was enhanced by 4-aminopyridine. Methysergide did not decrease the sensitivity of the motoneuron to substance P and L-glutamic acid. It is suggested that methysergide acts at the presynaptic terminal of Ia afferent fibers and depresses evoked transmitter release.