Effects of some divalent cations on synaptic transmission in frog spinal neurones

Opposite effects of low and high doses of Abeta42 on electrical network and neuronal excitability in the rat prefrontal cortex

Changes in neuronal synchronization have been found in patients and animal models of Alzheimer's disease (AD). Synchronized behaviors within neuronal networks are important to such complex cognitive processes as working memory. The mechanisms behind these changes are not understood but may involve the action of soluble beta-amyloid (Abeta) on electrical networks. In order to determine if Abeta can induce changes in neuronal synchronization, the activities of pyramidal neurons were recorded in rat prefrontal cortical (PFC) slices under calcium-free conditions using multi-neuron patch clamp technique. Electrical network activities and synchronization among neurons were significantly inhibited by low dose Abeta42 (1 nM) and initially by high dose Abeta42 (500 nM). However, prolonged application of high dose Abeta42 resulted in network activation and tonic firing. Underlying these observations, we discovered that prolonged application of low and high doses of Abeta42 induced opposite changes in action potential (AP)-threshold and after-hyperpolarization (AHP) of neurons. Accordingly, low dose Abeta42 significantly increased the AP-threshold and deepened the AHP, making neurons less excitable. In contrast, high dose Abeta42 significantly reduced the AP-threshold and shallowed the AHP, making neurons more excitable. These results support a model that low dose Abeta42 released into the interstitium has a physiologic feedback role to dampen electrical network activity by reducing neuronal excitability. Higher concentrations of Abeta42 over time promote supra-synchronization between individual neurons by increasing their excitability. The latter may disrupt frontal-based cognitive processing and in some cases lead to epileptiform discharges.

Robust integration of motion information in the fly visual system revealed by single cell photoablation

In the brain, sensory information needs often to be read out from the ensemble activity of presynaptic neurons. In the most basic case, this may be accomplished by an individual postsynaptic neuron. In the visual system of the blowfly, an identified motion-sensitive spiking neuron is known to be postsynaptic to an ensemble of graded-potential presynaptic input elements. Both the presynaptic and postsynaptic neurons were shown previously to be capable of representing the velocity of preferred-direction motion reliably and linearly over a large frequency range of velocity fluctuations. Accordingly, the synaptic transfer properties of the connecting excitatory synapses between individual input elements and the postsynaptic neuron were shown to be linear over a similar range of presynaptic membrane potential fluctuations. It was not known, however, how the postsynaptic neuron integrates and reads out the presynaptic ensemble activity. We were able to compare the response properties of the integrating cell before and after eliminating individual presynaptic elements by a laser ablation technique. For most of the input elements, we found that their elimination strongly affected the activity of the postsynaptic neuron but did not degrade its performance to encode motion with constant and time-varying velocity. Our results suggest that the integration of individual synaptic inputs within the neural circuit operates with some redundancy. This feature might help the postsynaptic neuron to encode in a highly robust way the direction and the velocity of self-motion of the animal.

Multivesicular release at climbing fiber-Purkinje cell synapses

Synapses driven by action potentials are thought to release transmitter in an all-or-none fashion; either one synaptic vesicle undergoes exocytosis, or there is no release. We have estimated the glutamate concentration transient at climbing fiber synapses on Purkinje cells by measuring the inhibition of excitatory postsynaptic currents (EPSCs) produced by a low-affinity competitive antagonist of AMPA receptors, gamma-DGG. The results, together with simulations using a kinetic model of the AMPA receptor, suggest that the peak glutamate concentration at this synapse is dependent on release probability but is not affected by pooling of transmitter released from neighboring synapses. We propose that the mechanism responsible for the elevated glutamate concentration at this synapse is the simultaneous release of multiple vesicles per site.

Antagonism of calcium currents and neurotransmitter release by barium ions at frog motor nerve endings

1. The effects of Ba(2+) (0.1 - 2 mM) on the component of the perineural voltage change associated with nerve terminal calcium currents (prejunctional Ca(2+) currents) were compared with the effects of this ion to antagonize calcium-dependent acetylcholine (ACh) release. These experiments were made on isolated neuromuscular junctions of the frog. 2. In the presence of sufficient concentrations of K(+) channel blockers to eliminate measurable prejunctional K(+) currents, low concentrations of Ba(2+) selectively antagonized prejunctional Ca(2+) currents in normal Ca(2+) solutions. Higher concentrations of Ba(2+) also substantially reduced the Na(+) component of the perineural waveform. 3. Ba(2+) inhibited the prolonged prejunctional Ca(2+) currents that developed in the presence of higher concentrations of K(+) channel blockers. 4. Simultaneous measurements of the prejunctional Ca(2+) currents and the electrophysiological correlates of ACh release (i.e. end-plate potentials, EPPs) were made under conditions of modest K(+) channel blockade. Under these conditions, Ba(2+) generally produced simultaneous decreases in both Ca(2+) currents and EPP amplitudes. In some instances, a prolongation of prejunctional Ca(2+) currents and a transient increase in EPP amplitudes preceded the decreases in both electrophysiological events. 5. These results suggest that Ba(2+) ions can antagonize the entry of calcium into motor nerve endings and this effect is likely to be responsible for the inhibitory effects of Ba(2+) on evoked ACh release.

Bidirectional modulation of GABA-gated chloride channels by divalent cations: inhibition by Ca2+ and enhancement by Mg2+

The effects of the divalent cations Ca2+, Sr2+, Ba2+, Mg2+, Mn2+, and Cd2+ were studied on gamma-aminobutyric acidA (GABAA) responses in rat cerebral cortical synaptoneurosomes. The divalent cations produced bidirectional modulation of muscimol-induced 36Cl- uptake consistent with their ability to permeate and block Ca2+ channels. The order of potency for inhibition of muscimol responses was Ca2+ > Sr2+ > Ba2+, similar to the order for permeation of Ca2+ channels in neurons. The order of potency for enhancement of muscimol responses was Cd2+ > Mn2+ > Mg2+, similar to the order for blockade of Ca2+ channels in neurons. Neither Ca2+ nor Mg2+ caused accumulation of GABA in the extravesicular space due to increased GABA release or decreased reuptake of GABA by the synaptoneurosomes. The inhibition of muscimol responses by Ca2+ was most likely via an intracellular site of action because additional inhibition could be obtained in the presence of the Ca2+ ionophore, A23187. This confirms electrophysiologic findings in cultured neurons from several species. In contrast, the effects of Cd2+, Mn2+, and Mg2+ may be mediated via blockade of Ca2+ channels or by intracellular sites, although the results of these studies do not distinguish between the two loci. The effects of Zn2+ were also studied, because this divalent cation is reported to have widely divergent effects on GABAA responses. In contrast to other studies, we demonstrate that Zn2+ inhibits GABAA responses in an adult neuronal preparation. Zn2+ produced a concentration-dependent inhibition (limited to 40%) of muscimol responses with an EC50 of 60 microM. The inhibition of muscimol-induced 36Cl- uptake by Zn2+ was noncompetitive.(ABSTRACT TRUNCATED AT 250 WORDS)

CO2 chemoreception in the pulmonate snail, Helix aspersa

We have studied the response of the pneumostome to CO2, O2 and combined CO2 and O2 in intact snails. We found that pneumostomal opening increases in response to both hypercapnia and mild hypoxia. We determined which neural structures were essential for the pneumostomal response to CO2 by eliminating parts of the nervous system: the subesophageal ganglia and an intact anal nerve were necessary and sufficient elements for the CO2 response. Within the subesophageal ganglia, we identified a discrete region on the medial margin of the visceral ganglion that was capable of increasing pneumostomal area when focally stimulated with 6% CO2. Ion substitution experiments indicated that pneumostomal responses to hypercapnia were not mediated by the pneumostomal motor neurons themselves, but rather by interneurons connected polysynaptically to the motor neurons controlling pneumostomal function. In conclusion, intact H. aspersa have a ventilatory response to CO2, and this response is mediated by CO2 sensitive cells located in a small area of the central nervous system.

The frog spinal cord: a model to study methanethiol-central nervous system interaction

1. The effects of methanethiol (MT), DL-dithiothreitol (DTT) and N-ethyl-maleimide (NEM) were studied on the electrical activities of the frog cord. 2. Methanethiol depressed spontaneous dorsal and ventral root potentials, whereas no effects were observed on evoked responses. 3. N-ethyl-maleimide, a SH modifying agent, irreversibly depressed electrical cord activities. 4. DL-Dithiothreitol, a SH reducing agent, dramatically increased spontaneous electrical cord pattern. 5. It is suggested that interneuronal membrane sulphydryl groups of dorsal horn cell population are involved in the origin of spontaneous electrical cord activities and that MT interacts with these interneurones, probably oxidizing membrane SH groups.

Large enhancement of excitatory postsynaptic potentials and currents by thyrotropin-releasing hormone (TRH) in frog spinal motoneurones

Frog spinal motoneurones were studied in vitro under voltage clamp conditions to examine the effect of the peptide thyrotropin-releasing hormone (TRH). TRH (50 microM) produced a small membrane depolarization, without obvious changes in the neuronal conductance or sensitivity to exogenously applied glutamate. These effects were seen regardless of the presence of 1 mM Mg2+ in the bathing solution. Low- and high-threshold excitatory postsynaptic potentials, induced by dorsal root stimulation, were enhanced by TRH. Under voltage clamp, TRH did not change the cell leak conductance while producing a considerable enhancement of polysynaptic current amplitude, particularly when recorded at a rather negative potential level. It is suggested that TRH might potentiate excitatory neurotransmission by facilitating presynaptic release of the excitatory neurotransmitter(s) onto motoneurones.

The effect of temperature on electrical interactions between antidromically stimulated frog motoneurons and dorsal root afferent axons

The effect of temperature on electrical interactions between antidromically stimulated motoneurons and dorsal root afferents was studied in the isolated and hemisected spinal cord of the frog, superfused with Ringer in which Ca2+ was equimolarly replaced by Co2+ or Mn2+ to suppress chemical synaptic transmission. Suction electrodes were used for stimulating and/or recording from dorsal and ventral roots from segments IX or X. Intrafibre recordings from sensory fibres were made at their point of entry into the spinal cord. Supramaximal ventral root stimuli elicited two distinct responses in the segmental dorsal root. First a brief short-latency depolarizing potential. Second, at temperatures below 11 degrees C, a second depolarizing root potential appeared following the short-latency depolarizing potential-I. Amplitude and duration of short-latency depolarizing potential-II reversibly increased as the bath temperature was decreased, reaching a maximum at 3 degrees C. Between 8 and 3 degrees C, short-latency depolarizing potential-II increased in amplitude by 20%/degrees C. In contrast short-latency depolarizing potential-I did not show substantial changes with temperature. The short-latency depolarizing potential-II, unlike short-latency depolarizing potential-I showed stepped fluctuations in amplitude, and appeared to be composed of unitary events. Intrafibre records revealed that the unitary events corresponded to action potentials on individual dorsal root fibres. Double shocks applied to the ventral root, at constant bath temperatures (below 11 degrees C), revealed facilitation of the short-latency depolarizing potential-II, which was maximal between 50 and 80 ms and lasted about 200 ms. Neither the antidromic motoneurone field potential nor the short-latency depolarizing potential-I were facilitated.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophysiological study of the effects of cardiodilatin 1-16 on the frog spinal cord in vitro

Using the isolated spinal cord of the frog, hemisected and further divided into two distinct quadrants, we studied electrophysiological changes produced by peptides present in the atrial natriuretic factor (ANF) preprohormone. ANF and related peptides (atriopeptin I and atriopeptin III) did not affect the frog spinal cord. The 1-16 fragment from cardiodilatin (10(-5) M) induced slow depolarization in ventral and dorsal nerve stumps. The depolarization was associated with an increase of the evoked dorsal root potentials and depression of the fast component of the reflex responses. When depolarization approached its maximum value, spontaneous slow potentials appeared progressively similar to the evoked potentials, and became rhythmic until they reached a frequency of one potential every 15-20 seconds. The effects of cardiodilatin 1-16 are localized at dorsal horn level. It is suggested that this substance exerts a modulatory effect on frog cord physiology.

Excitatory postsynaptic currents in response to different synaptic inputs of frog spinal motoneurons

Excitatory postsynaptic currents (EPSCs) evoked by the primary afferents (dorsal root; DR) and the descending lateral column (LC) fibers were studied in frog spinal motoneurons under voltage clamp with two separate electrodes. The average rise time and half-width of the EPSCs were shorter for LC-EPSCs than for DR-EPSCs, though the values of the parameters for LC- and DR-EPSCs were distributed within a similar range. The relation between the amplitudes of the EPSP and EPSC was almost linear. The amount of current required to generate a 1 mV increment in the EPSP was 5.0 +/- 2.3 nA for the DR-EPSC and 3.8 +/- 1.2 nA for the LC-EPSC. The decay time was shortened by hyperpolarization and prolonged by depolarization in DR- and LC-EPSCs and spontaneous EPSCs. The reversal potential ranged from -30 to -5 mV and was almost identical for DR- and LC-EPSCs and spontaneous EPSCs in individual motoneurons. The current-voltage relation was linear from -100 to +50 mV for these EPSCs. Spontaneous EPSCs became more prominent and frequent during a large hyperpolarization or a large depolarization. These results suggest that the ionic mechanisms underlying EPSC are similar for the functionally different excitatory synapses located on motoneurons.

Efficiency of electrical transmission in reticulomotoneuronal synapses of lamprey spinal cord

Synaptic responses in motoneurons in the isolated spinal cord of the lamprey during stimulation of the reticulospinal axons were examined. Chemical transmission in the synapses was partially or completely blocked by temperature reduction of the perfusing solution, pentobarbitone application or substitution of Mn2+ ions for Ca2+ in the perfusate. Excitatory postsynaptic potentials (EPSPs) which were indifferent to the influences mentioned above, had an amplitude of 6-12 mV and were capable of evoking action potentials (APs) in motoneurons due to their high amplitude, the absence of a shunting effect at the postsynaptic membrane and the fast rise-time of the wave front. The suggestion is made that the electrical transmission is involved in functioning of the lamprey nervous system. Its stability and efficiency are likely to ensure functional connection between the brain and spinal cord under such unfavourable conditions when the chemical transmission does not operate and when the ability for locomotion would be prerequisite for the individual to survive.

Synaptic organization of dorsal root projections to lumbar motoneurons in the clawed toad (Xenopus laevis)

Synaptic connexions between dorsal root primary afferents and lumbar motoneurons have been investigated in the isolated spinal cord of the clawed toad. The study of monosynaptic actions evoked in motoneurons by 9th or 10th dorsal root stimulation or by impulses in single primary afferents provided evidence for both electrical and chemical junctional transmission at the sensory-motor synapses. The anterograde filling of the 9th and 10th dorsal roots with horseradish peroxidase (HRP) shows that afferents do project to the motoneuron field of the segments IX and X. Some of the fibres not only reach the dorsally located motoneurons, but also cross the lateral motor column (LMC) and terminate in the marginal zone of ventral horn gray matter. The projections of the 9th and 10th dorsal root fibres are most numerous in the caudal part of segment X. Simultaneous HRP labeling of single motoneurons and the whole 10th dorsal root has revealed that afferent fibres make contacts not only on the distal dendrites of the motor cells, but also on the proximal ones. This latter finding is in a good agreement with the electrophysiological data.

Effects of barium on isolated frog spinal cord

The effects of Ba2+ were studied in vitro on the isolated frog spinal cord. Ba2+ (25 microM-5 mM) caused a concentration-dependent depolarization of ventral (VR) and dorsal (DR) roots. TTX and Mg2+ substantially reduced the depolarization suggesting that interneuronal effects were involved. Ba2+ (25-500 microM) markedly increased the frequency and duration of spontaneous VR and DR potentials and substantially enhanced the duration (and frequently the amplitude) of VR and DR potentials evoked by DR stimulation. Higher concentrations of Ba2+ (1-5 mM) reduced both spontaneous and evoked potentials. Ba2+ (25-500 microM) enhanced the amount of K+ released by a DR volley and by application of L-glutamate and L-aspartate. The cation reduced VR and DR root depolarizations produced by elevated [K+]0. VR potentials induced by L-glutamate, L-aspartate, GABA and glycine and DR depolarizations caused by GABA were reduced by Ba2+. These results show that Ba2+ has complex actions on reflex transmission, interneuronal activity, the postsynaptic actions of excitatory and inhibitory amino acids and the evoked release of K+.

Low-calcium field burst discharges of CA1 pyramidal neurones in rat hippocampal slices

Incubation of rat hippocampal slices in solutions containing low Ca2+ and increased Mg2+ rapidly blocked synaptic responses and increased spontaneous firing of all the principal neurones. More remarkably, a rhythmic and synchronous bursting discharge developed, which was restricted to the CA1 population of pyramidal neurones. These 'field bursts' or 'spreading excitation' were rapidly abolished by restoring the Ca2+ to 2 mM, by increasing the Mg2+ to 6 mM or by decreasing K+ from 6 to 3 mM. The CA1 pyramidal cells depolarized after the change to the low-Ca2+ solution by about 10-20 mV. Individual field bursts were associated with a further depolarization of 10-12 mV surmounted by a burst of action potentials at about 20/s. This transient depolarization shift, recorded extracellularly as a negative field, could be attributed to the increase of [K+]o during the bursts, reaching 9-10 mM as measured by ion-sensitive electrodes. The bursts were followed by a hyperpolarization, seen extracellularly as a small soma-positive field, which was attributed to an electrogenic pump and/or a Ca2+-activated K+ conductance. Stimulation of the tightly packed pyramidal cell axons in the alveus elicited a train of population spikes, instead of the single spike normally seen, and could trigger a full field burst. Recordings of the alvear tract volley suggested that the repeated spikes arose within the pyramidal cells. Multiple recordings from CA1 revealed that field bursts usually, but by no means always, started near the caudal (subicular) end of the area. They spread through the cell layer at 0.04-0.12 m/s. The most rapid propagation was seen when the bursts had an abrupt onset; slower propagation (1-10 mm/s) occurred when the bursts started gradually, which generally was the case near the sites of burst initiation and termination. Usually the action potentials within each burst were synchronized into population spikes which spread across CA1 at 0.04-0.15 m/s. The site of initiation and the extent of the spread of these population spikes varied during each burst, as did their amplitude. The degree of spike synchronization was enhanced by various treatments expected to increase neuronal excitability. Measurements of transmembrane potential during the burst confirmed the role in the generation of population spikes of ephaptic or field interactions between the pyramidal cells. It is proposed that the increased firing of all neurones is due to the block of tonic inhibition, depression of after-hyperpolarization and to increased membrane excitability.(ABSTRACT TRUNCATED AT 400 WORDS)

The absence of specific dye-coupling among frog spinal neurons

A double fluorescence labeling technique was developed to study the specificity of dye-coupling among frog spinal neurons. A pool of motoneurons known to be electrically coupled was prelabeled with a large molecule (rhodamine conjugated to horseradish peroxidase) that was not expected to pass through gap junctions. Then a single sensory or motor neuron within or outside this pool was injected with lucifer yellow to see if the dye spread specifically among neurons that are electrically coupled. We observed almost no examples of specific dye-coupling.

Divalent cations differentially support transmitter release at the squid giant synapse

The ability of Ca, Sr and Ba ions to support transmitter release was studied at the squid giant synapse by examining their respective actions on presynaptic current and post-synaptic responses. Transmitter-induced post-synaptic currents were smaller in Sr- than in Ca- containing solutions, and much smaller in Ba-containing solutions. The time course and amplitude of spontaneous miniature post-synaptic potentials were similar in the presence of all three divalent ions. Sr or Ba substitution has little effect on the resting potential of presynaptic terminals. In Sr-containing solutions, action potentials were similar in amplitude and time course to those recorded in Ca. Ba slightly prolonged action potential duration but had no effect on amplitude. Voltage-clamped presynaptic terminals exhibited inward Ca, Sr or Ba currents which were apparently carried through Ca channels. These currents were similar in amplitude and time course in all three ions, being somewhat larger in Ba. Although presynaptic currents were similar in these ions, transmitter release induced by these currents depended upon the divalent species entering the presynaptic terminal. Release was greatest in response to presynaptic current carried by Ca and smallest in response to current carried by Ba. Transfer curves relating presynaptic current to post-synaptic potential were sigmoidal in all three ions, and exhibited limiting slopes of approximately 2. Divalent cations differentially support transmitter release at the squid giant synapse in the sequence Ca greater than Sr much greater than Ba. The differential efficacy of the divalent cations is not due to post-synaptic alterations, presynaptic potential changes or differences in presynaptic divalent cation conductances. This sequence may reflect the cation selectivity of the exocytotic process responsible for transmitter release.

The effect of some metal ions on the intradental sensory nerves of the cat

Intradental nerve activity from canine teeth of anesthetized cats was recorded during the application of different metal ion solutions in a dentinal cavity. The application of Ag2+, Ca2+, Cd2+, Cr3+, Hg2+, Mg2+, Mn2+, Ni2+, Pb2+, Sn2+, and Zn2+ reduced the nerve activity induced by NaCl 0.76 M and compound 48/80. The application of Cu2+ and Co2+ excited the intradental sensory nerves.

The ionic mechanisms associated with the excitatory response of kainate, L-glutamate, quisqualate, ibotenate, AMPA and methyltetrahydrofolate on leech Retzius cells

Intracellular recordings were made from Retzius cells from segmental ganglia of the leech, Hirudo medicinalis. The ionic mechanisms of the following compounds were examined: L-glutamate, ibotenate, quisqualate, AMPA, kainate, methyltetrahydrofolate and carbachol. All these compounds depolarise and excite Retzius cells. In sodium-free Ringer, the responses to L-glutamate, kainate, ibotenate and AMPA were greatly reduced, the response to quisqualate was reduced, the response to methyltetrahydrofolate was normal while the response to carbachol was abolished. In sodium-free high calcium Ringer the responses to L-glutamate, ibotenate and carbachol were absent, the responses to quisqualate and AMPA greatly reduced, the responses to methyltetrahydrofolate and kainate were normal. The methyltetrahydrofolate and kainate responses in sodium-free high calcium Ringer were greatly reduced on addition of cobalt. All the responses are associated with an increase in conductance, the increase being the largest in the case of kainate. It is concluded that the response to L-glutamate, ibotenate and carbachol are dependent on sodium, the responses to quisqualate and AMPA are mainly sodium dependent, possibly with a small calcium component. The kainate response in normal Ringer is largely sodium dependent but in sodium-free Ringer calcium can completely substitute for sodium. The methyltetrahydrofolate response appears to be sodium independent but at least partly calcium dependent. These studies provide further evidence that L-glutamate and ibotenate act on a common receptor on leech Retzius cells while kainate acts on a separate receptor which can activate a calcium ionophore. It is probable that methyltetrahydrofolate acts on a different ionophore system to kainate. N-Methyl-D-aspartate has no agonist activity on any of these receptors.

Development of sensory-motor synapses in the spinal cord of the frog

The development and specificity of monosynaptic sensory-motor synapses were studied in the brachial spinal cord of bullfrog tadpoles. Intracellular and extracellular recordings were made from motoneurones innervating several different muscles of the forelimb. Excitatory synaptic potentials (e.p.s.p.s) were elicited by stimulation of various peripheral muscle nerves. Sensory and motor axons in the triceps brachii muscle nerves were electrically excitable at stage XIII, the earliest stage studied. Their conduction velocities were 0.2-0.4 m/s. These velocities increased during subsequent development so that by stage XXII they were approximately 5 m/s. Before stage XVII, synaptic potentials evoked in motoneurones by stimulation of the triceps sensory fibres had a long central latency and fatigued easily. These potentials were probably mediated polysynaptically. At stage XVII, the first short-latency triceps synaptic potentials appeared. They had central latencies of less than 3 ms and represented the direct, monosynaptic input from muscle sensory cells on to motoneurones. During subsequent development the percentage of triceps motoneurones innervated by triceps sensory fibres increased, while the number of long-latency polysynaptic inputs decreased. Both the electrical and chemical components, characteristic of these monosynaptic e.p.s.p.s in adult frogs, were prominent from the time the e.p.s.p.s first appeared. The pattern of innervation of brachial motoneurones by triceps sensory afferents was specific from the beginning. Triceps sensory fibres innervated most triceps motoneurones but very few subscapular or pectoralis motoneurones, just as in adult frogs. At no time were there appreciable numbers of 'aberrant' connexions. The developmental time course of several different classes of sensory-motor connexions was similar. Thus the synaptic specificity of this system cannot be explained by a differential timing of synaptogenesis.

Tracing of motoneurones and primary afferent projections after intracellular staining with Lucifer Yellow: dye-coupling

Intracellular injection of the fluorescent dye Lucifer Yellow CH into single motoneurones of the isolated perfused frog spinal cord resulted in backfilling of presynaptic fibres originating from dorsal roots and ventrolateral funiculi. The dye transfer from primary sensory fibres into motoneurones was observed following application of Lucifer Yellow to the central end of the cut dorsal root. The dye-coupling coincides with electrical coupling at sensory-motor synapses presumably through gap junctions. The fluorescent primary afferent fibres were traced from the dorsal roots to the motor nucleus where they terminate the chains of swellings. Most swellings are located in dorsal horn and in the intermediate zone approximately 100-100 micrometers from the somata of motoneurones. A few varicosities are located ion the cell bodies of the motoneurones.

Sites and mechanisms of action of lidocaine upon the isolated spinal cord of the frog

The sites of action of lidocaine on the responses evoked by stimulation of lateral column (LC) and dorsal root (DR) were studied in the isolated, intra-arterially perfused spinal cord of the bullfrog. When the ventral root volley produced by stimulation was abolished by lidocaine, the presynaptic focal potential was almost unchanged. Intracellular recordings from motoneurons clearly demonstrated a marked reduction in amplitude of the EPSPs before the block of conduction of presynaptic fibers and the block of invasion of the neuron soma by antidromic spike potential. At low concentrations of lidocaine, the EPSPs elicited by LC stimulation produced shortening in time to peak, slowing in the decay time, decrease in amplitude and smaller changes in the later EPSPs of a train than the earlier ones. From the observations, it was concluded that the low concentrations of lidocaine affected primarily synaptic transmission in the spinal cord. The possible mechanisms of action of lidocaine were discussed.

The use of low concentrations of divalent cations to demonstrate a role for N-methyl-D-aspartate receptors in synaptic transmission in amphibian spinal cord

1 Synaptic potentials and the responses of frog spinal cord to various acidic amino acids were examined by means of the sucrose gap recording technique. 2 Divalent cations (50-250 microM) specifically antagonized responses evoked at N-methyl-D-aspartate (NMDA) receptors by N-methyl D,L aspartic acid (NMDLA). The rank order of potency was Ni2+ greater than Co2+ greater than Mg2+ greater than Mn2+. Responses to glutamate and aspartate were relatively insensitive to these concentrations of divalent cations. 3 The rank order of potency for divalent ions (1 mM) for antagonism of synaptic transmission in bullfrog sympathetic ganglia was Mn2+ greater than Co2+ greater than Ni2+ greater than Mg2+. Thus synaptic transmission in ganglia was especially sensitive to Mn2+ whereas NMDLA responses were especially sensitive to Co2+ and Mg2+. 4 It was possible to depress selectively the dorsal root-dorsal root potential (DR-DRP) and dorsal root-ventral root potential (DR-VRP) of frog spinal cord using low doses of Co2+ or Mg2+ which did not affect VR-DRP (ventral root-dorsal root potential). It was not possible to produce this selective depression of DR-DRP and DR-VRP with Mn2+, as this cation non-selectively depressed all responses. 5 These results suggest that: (i) divalent cations do not antagonize NMDLA responses by blocking Ca2+ channels which may mediate the response; (ii) postsynaptic NMDA receptors are activated by a neurotransmitter involved in the DR-DRP and DR-VRP pathways but not by any neurotransmitters involved in the VR-DRP pathway; (iii) the neurotransmitter activating NMDA receptors in amphibian spinal cord may be an aspartate-like substance rather than aspartate itself or glutamate.

Separation of temperature sensitive and temperature insensitive components of the postsynaptic potentials in the frog motoneurons

Intracellular measurements were made in the in situ spinal cord of the frog at temperatures below 5 degrees C. Responses to volleys in the sciatic nerve, in the descending fibres and in the motor axons were studied. About 30% of the motoneurons responded to sciatic volleys with 1-3 ms segmental latency, which was short enough to assume electrotonic mediation of these responses. Another group of motoneurons responded with 6-8 ms latency, i.e. with the expected delay at chemical synapses at low temperature. Latency distribution of the sciatic-evoked postsynaptic potentials was clearly bimodal in contrast with that found at higher temperatures. Postsynaptic discharges occurred with rather long latency and they were attributed to chemically-mediated excitation. Some of the postsynaptic potentials to descending volleys also occurred with quite short latency, indicating possible electrotonic transmission from supraspinal centres to motoneurons. Latency distribution of the action potentials evoked from the motor axons was bimodal, corresponding to the different, i.e. antidromic and recurrent facilitatory, mechanism of these spikes. Calculated Q10 ratios for the sciatic-evoked reflex discharges and the afferent fibre volleys were about 2.3 and 1.8, respectively. We concluded that cooling helps to separate postsynaptic potentials according to their electrotonic and chemical mediation and that electrotonic excitation does not seem to have a primary role in the generation of postsynaptic discharges initiated by dorsal root volleys in the frog.

Biochemical and electrophysiological demonstrations of the actions of beta-bungarotoxin on synapses in brain

Homogeneous beta-bungarotoxin interacts irreversibly with rat olfactory cortex and produced permanent inhibition of neurotransmission (half-time of blockade for 230 nM toxin in 25 min). Binding occurs in the absence of divalent cations, but the rate of synaptic blockade is increased by Ca2+, which activates the intrinsic phospholipase A2 activity of the toxin. Other observable actions of the toxin, seen with rat cerebrocortical synaptosomes, are an increase in the release of acetylcholine, glutamate and gamma-aminobutyrate and impairment of transmitter uptake, which are all insensitive to tetrodotoxin. Inactivation of the toxin's phospholipase activity by chemical modification with p-bromophenacyl bromide diminishes the observed concomitant efflux of the neurotransmitters and lactate dehydrogenase. Collectively, the results support the idea that the toxin binds specifically and irreversibly to component(s) on nerve terminals and this together with the resultant phospholipolysis leads eventually to synaptic blockade. Such a proposal would account for the unique toxicity of the protein relative to phospholipase A2 enzymes.

An analysis of the epileptogenic potency of CO2+- its ability to induce acute convulsive activity in the isolated frog spinal cord

The action of Co2+ on the isolated frog spinal cord was studied by extracellular application of the ion in the superfusing solution. A complete and reversible blockade of chemical synaptic transmission by Co2+ (3 mmol/l) could be achieved after a superfusion period of 20-30 min. During continued Co2+ application (greater than 60 min) the following effects upon the motoneuron membrane, dorsal root and ventral root fibres were observed. Motoneurons and ventral root fibers: 1. prolongation of initial segment action potential to a maximum of 30 ms, 2. blockade of the long afterhyperpolarization, 3. abolition of adaptation, 4. increased duration of fibre action potential in the ventral root, 5. backfiring after ventral root stimulation. Dorsal root fibres: 1. prolongation of the extraspinal fibre action potential, 2. marked prolongation of the action potential of the terminal region, 3. backfiring of multiple action potentials after dorsal root stimulation. Even in the presence of Co2+, when synaptic transmission was completely blocked, strong convulsive reactions of the isolated spinal cord were observed. Intracellular injection of Co2+ into motoneurons did not affect the action potential, but led to a shift of the EIPSP towards the membrane potential. The results indicate that the induction of convulsive reactions by Co2+ is mainly due to a prolongation of action potentials. The plateau-like deformation of the action potential of the initial segment membrane and presumably of the terminal region of nerve endings results in retrograde propagation of action potentials and in some cases induces oscillatory discharge of single neurons.

Miniature synaptic potentials absent from motoneurons of intact spinal cord

Intracellular recordings were made from lumbar motoneurons of decerebrate, paralyzed frogs with minimal surgical damage to the spinal cord. Detectable spontaneous synaptic activity was absent in most motoneurons, as compared with published in vitro recordings. Lesions of the thoracic cord increased the incidence of small spontaneously occurring potentials. This suggests that spontaneous quantal release of transmitter observed in isolated preparations is a consequence of presynaptic neuronal damage.

The effect of lesions in the neural crest on the formation of synaptic connexions in the embryonic chick spinal cord

1. The pattern of synaptic activity in lateral gastrocnemius (l.g.) motoneurones in the lumbar spinal cord of chick embryos (Stage 44-45, 19-21 d of incubation) has been examined using intracellular recording. In the motoneurones of normal chick embryos, stimulation of different peripheral, sciatic nerve branches gave rise to characteristic synaptic responses. Stimulation of the lateral gastrocnemius nerve caused a monosynaptic e.p.s.p. which was graded by the intensity of nerve stimulation. Stimulation of synergistic muscle afferents also caused a brief latency e.p.s.p., followed by longer latency excitatory and inhibitory synaptic potentials. Stimulation of antagonistic muscle afferents or cutaneous afferents gave rise to longer latency inhibitory and excitatory synaptic potentials respectively.2. The synaptic activity of l.g. motoneurones was also recorded in embryos in which short segments of the lumbar neural crest had been destroyed by microcautery at 3 d of incubation (Stage 18). The embryos developed without sensory ganglia and dorsal roots in the corresponding region.3. At 19-21 d of incubation, the amplitude of the l.g. e.p.s.p. of l.g. motoneurones in deafferented segments was on the average only a half to a third of the amplitude seen in motoneurones of intact spinal segments. However, both the l.g. and synergist e.p.s.p.s were larger than those seen in acutely deafferented segments of normal embryos.4. In spite of the weak monosynaptic input from l.g. and synergistic afferents, the pattern of synaptic activity evoked by antagonistic muscle afferent or cutaneous afferent stimulation was not different from normal. This was even the case for gastrocnemius motoneurones in which no early e.p.s.p. could be evoked by stimulating the l.g. or synergistic muscle nerves.5. No muscle spindles could be seen in sections of l.g. muscles from embryos with extensive lesions of the lumbosacral neural crest. Incomplete lesions of l.g. segments reduced the number of spindles in the muscle.6. These results suggest that when motoneurones are deprived of part of their normal synaptic input before the formation of peripheral connexions, the identity of the motoneurones (in terms of the origin of their synaptic input) is preserved. Missing synaptic inputs are either replaced by appropriate afferent fibres, if they are available, or not at all. The chick sensory ganglion cells with monosynaptic connexions to motoneurones appear to be unable to compensate significantly for peripheral or central defects in the innervation of the hind limb. They behave as if their developmental possibilities were quite rigidly determined at an early embryonic stage.

Synaptic organization of sensory and motor neurones innervating triceps brachii muscles in the bullfrog

1. The anatomy and physiology of sensory-motor pathways were studied in the brachial spinal cord of adult bullfrogs to characterize the properties and specificity of these connexions.2. Motoneurones innervating a given forelimb muscle are located in discrete and reproducible regions of the lateral motor column. Yet only a fraction of the motoneurones in a particular region innervates any one muscle.3. The central projections of sensory afferent axons from the triceps muscles extend throughout the rostro-caudal length of the brachial spinal cord. Within this region these projections terminate in an area containing many motoneuronal dendrites.4. Within the triceps motor pool sensory neurones from the triceps muscles produce monosynaptic potentials only in triceps motoneurones even though these motoneurones are mingled with motoneurones innervating other muscles.5. Motoneurones innervating each of the three heads of the triceps muscles, medial, internal and external, receive monosynaptic input from their own, homonymous muscle head. Sensory fibres from the medial head also innervate 98% of the heteronymous motoneurones projecting to the internal or external heads, and nearly 90% of the medial triceps motoneurones are innervated by sensory axons from the other two heads.6. Similarly, other brachial motoneurones receive monosynaptic input from sensory axons in their own muscle nerves. However, most of the synaptic potentials evoked in triceps motoneurones by stimulation of muscle nerves other than triceps are of longer latency and probably involve polysynaptic pathways.7. Thus, the pattern of synaptic connexions between muscle sensory afferents and motoneurones in the frog's spinal cord is specific. Furthermore, comparison with homologous pathways in the cat's spinal cord suggests that the strength and pattern of these connexions are similar.

Epileptiform burst afterhyperolarization: calcium-dependent potassium potential in hippocampal CA1 pyramidal cells

Synaptic excitation of hippocampal cells during blockade of synaptic inhibition results in an epileptiform "burst" potential followed by a prolonged afterhyperpolarization. This afterhyperpolarization resembles the one that is seen after the epileptic interictal spike and that is considered of critical importance in preventing seizure development. The afterhyperpolarization produced in the presence of y-aminobutyric acid antagonists is associated with a conductance increase and is inhibitory. It can occur in an all-or-none fashion after a burst, is independent of chloride, and is depressed by barium. The afterhyperpolarization has a reversal potential of (-86) millivolts, and the reversal potential is strongly dependent on the extracellular concentration of potassium. The afterhyperpolarization appears to be an intrinsic, inhibitory potassium potential mediated by calcium. This finding has implications for understanding the cellular mechanisms of epilepsy.

Selective depression of excitatory amino acid induced depolarizations by magnesium ions in isolated spinal cord preparations

1. The depressant actions of Mg2+ and a range of other divalent ions on synaptic excitation and on responses produced by excitatory amino acids and other putative transmitters have been investigated in hemisected isolated spinal cords of frogs and neonatal rats. Some comparative studies were also made using the rat isolated superior cervical ganglion. 2. At concentrations above 10 microM, Mg2+ selectively antagonized N-methyl-D-aspartate (NMDA)-induced motoneurone depolarization as recorded from ventral roots of tetrodotoxin-blocked spinal cords. Depolarization evoked by quisqualate (unaffected by 20 mM-Mg2+) was resistant to the depressant action of these ions, while depolarizations evoked by other excitant amino acids were depressed to intermediate degrees. 3. Mn2+, Co2+ and Ni2+ had qualitatively similar actions to Mg2+; Mn2+ was somewhat less potent and Co2+ and Ni2+ more potent than Mg2+. The alkaline earth metal ions, Ca2+, Sr2+ and Ba2+, had very weak Mg2+-like actions. Ca2+ and Mg2+ acted additively in depressing amino acid-induced responses. 4. Mg2+ also depressed motoneurone responses evoked by noradrenaline, substance P and carbachol in the neonatal rat isolated spinal cord. However, none of these effects were as marked as the depression of NMDA-induced responses by Mg2+ in this preparation. Mg2+ did not depress motoneurone depolarization produced by 5-HT in the rat spinal cord or the depolarizing action of GABA on primary afferent terminals of the isolated frog spinal cord. 5. At concentrations producing marked depression of NMDA-induced responses, Mg2+ also depressed synaptic transmission in spinal cords in the absence of an effect on ganglionic transmission. At the same concentrations, Mn2+, Co2+ and Ni2+ depressed synaptic transmission in both preparations. 6. From the similarity in action between Mg2+ and the D-alpha-aminoadipate group of NMDA antagonists, it is suggested that the central depressant action of low concentrations of Mg2+ involves predominantly a postsynaptically mediated interference with the action of an excitatory amino acid transmitter.

Voltage sensitive calcium entry in frog motoneurones

1. The electrical properties of motoneurone membrane were investigated in the isolated and hemisected spinal cord of frogs, using intracellular recording techniques. 2. TTX (1 x 10(-6) g/ml.) blocked action potentials produced either by intracellular depolarizing current pulses or ventral root stimuli. Voltage--current relations from these cells showed a diminishing slope for depolarizing current pulses of increasing intensity. 3. If TEA (5--10 mM) was added to the media containing TTX, intracellular depolarizing pulses elicited prolonged regenerative depolarizations characterized by a peak of variable amplitude and a repolarizing phase preceded by a prolonged plateau of variable duration. 4. During the plateau of the response, the membrane conductance was increased above its resting value. 5. The response was shortened during repetitive stimulation and could be curtailed by applying a hyperpolarizing pulse during the plateau. 6. The response depended on the presence of external Ca2+ and increased in size and duration with increasing Ca2+ concentration. Sr2+ substituted effectively for Ca2+. Sr2+-dependent responses were considerably longer than the Ca2+-dependent ones. Ca2+ or Sr2+ dependent responses persisted in Na+-free media containing isotonic TEA, and were abolished by addition of Co2+. 7. Ca2+ or Sr2+-dependent regenerative responses were followed by a hyperpolarization which could last several seconds. The current responsible for this after-hyperpolarization was TTX and TEA resistant. 8. It is concluded that the TTX-resistant regenerative response is probably generated in the soma-dendritic membrane, and is due to influx of Ca2+ or Sr2+ through voltage sensitive channels different to those through which Na+ permeates during generation of 'normal' action potentials. In addition it is shown that the hyperpolarization following 'Ca spikes', and which might be due to an increase in K+ conductance can also be triggered by Sr2+.