Reversal potential for Ia excitatory post synaptic potentials in spinal motoneurones of cats

A technique permitting the introduction of two separate microelectrodes into spinal motoneurones of cats has been used to study the effect of injections of large depolarizing currents on membrane properties and on synaptic potentials. By using one electrode for passage of current, and the other for recording, continuous measurement of membrane potentials was possible. The Ia excitatory postsynaptic potential has been reversed in twelve motoneurones, and the reversal level in most of-these has been measured at between 0 and +10mV. During continuous injection of strong depolarizing currents, large variations in membrane potential and input resistance, sometimes of a repetitive nature, were observed.The results indicate that the characteristics of the Ia excitatory postsynaptic potential are explainable by a chemically mediated increase in ionic conductance, a concept which has been challenged by other recent studies in which reversal of these potentials could not be obtained.     

Action of baclofen on mammalian synaptic transmission.

Microiontophoretic and systemic injections were used to investigate the mechanism of baclofen's powerful depressant action on transmission at primary afferent synapses in the cat. Iontophoretic applications depressed the spontaneous and evoked activity of cuneate cells and reduced the excitability and input resistance of spinal motoneurones. These effects, which were quick to reverse, resemble those of γ-aminobutyrate and may be due to activation of γ-aminobutyrate receptors by high concentrations of baclofen. Systemic doses of baclofen (0.1–5 mg/kg i.V.), which are known to give only a very low tissue concentration (<10^?7M), induced a very prolonged depression of synaptic responses in the spinal cord (motoneuronal excitatory postsynaptic potentials) and the cuneate nucleus (medial lemniscal potentials); but there was no increase in motoneuronal conductance, and responses of cuneate neurones to direct stimulation by electrical pulses, glutamate, or substance P were not diminished. On the other hand, there was some reduction in the excitability of primary afferent fibres, and the dorsal column reflex and primary afferent depolarization (as revealed by tests of terminal excitability) were nearly abolished.These observations are most simply explained if systemic baclofen blocks primary afferent synapses by a presynaptic action, which leads to a depression of transmitter release; this would be in keeping with evidence that, in cortical slices, baclofen selectively inhibits the release of excitatory amino acids.     

Computer simulations indicate that electrical field effects contribute to the shape of the epileptiform field potential

In the presence of convulsant drugs such as picrotoxin, neurons in the hippocampal-slice preparation generate synchronized depolarizing bursts. This synchrony occurs on a time scale of tens of milliseconds and is produced by excitatory synaptic interactions between neurons. The synaptic interactions themselves occur on a time scale of tens of milliseconds. The "epileptiform" local-field potential during such synchronized bursts is comb-shaped ("ringing"), whereas the field potential expected if action potentials in neighboring neurons were uncorrelated is noisy and not comb-shaped. This suggests that individual action potentials are locally synchronized on a time scale of 1 ms. We have previously shown, using computer simulations, that electrical interactions--mediated by currents flowing in the extracellular medium--can plausibly explain action-potential synchronization in experiments where chemical synapses are blocked. The present simulations demonstrate that electrical interactions can also account for action-potential synchronization--and thus the "ringing" shape of the field potential--during epileptiform bursts, where excitatory synapses are functional. The field potential is thus a modulating influence on, as well as a reflection of, underlying neuronal transmembrane events.     

Electrophysiological demonstration of a dentato-rubrospinal projection in cats

Electrophysiological experiments were performed in cats anesthetized with alpha-chloralose to demonstrate the existence of a dentato-rubrospinal projection. In one series of experiments lesions of the red nucleus were found to eliminate a response recorded at C-5 in the spinal cord following stimulation in the dentate nucleus. This response was unaffected by lesions placed in the interposed nuclei adjacent to the region of the dentate nucleus in which the stimulating electrode was located. Other experiments demonstrated that this response was evoked only when the stimulating electrode was located at the edge of or within the dentate nucleus. Together these studies show that stimuli applied in the dentate nucleus evoke a short latency response mediated by the red nucleus which does not result from current spread to the interposed nuclei.The effect of dentate stimulation on identified rubrospinal neurons was evaluated using extracellular and intracellular recordings. Rubrospinal neurons were identified by their antidromic activation from the spinal cord. In several of these neurons, stimulating in the dentate nucleus evoked short-latency synaptically mediated responses. Intracellular recording revealed that dentate stimulation evoked graded depolarizations in rubrospinal neurons with a mean latency of 1.4 ms. These findings indicate that the output of the dentate nucleus directly activates a component of the rubrospinal projection.     

Patterns of functional synaptic connections in organized cultures of cerebellum

Organized cultures of mouse cerebellum with separated regions containing cortical, deep nuclear neurons and brain stem neurons from the peduncular zone were used for electrophysiological studies of axonal projections and synaptic interactions. Responses of single neurons of each of the regions were recorded extracellularly and intracellularly during localized electrical stimulation of other parts of the explant, and indicated extensive synaptic interactions. Cortical stimulation inhibited deep nuclear neurons, apparently monosynaptically, and frequently caused antidromic activation of these cells. Synaptic responses of brain stem neurons to cortical stimulation were usually excitatory, but these were often succeeded by inhibitory potentials. Since brain stem cells were often antidromically activated, the excitatory synaptic responses may be mediated by collaterals of antidromically stimulated brain stem axons. Stimulation of the deep nuclear region could evoke inhibitory or excitatory potentials in cortical neurons, the most frequent response being an excitatory postsynaptic potential which was followed in about 2 ms by an inhibitory potential. Most excitatory and some inhibitory postsynaptic potentials followed high frequency stimulation with constant latencies.The results indicate that within these cultures there are rich synaptic interconnections, many of which follow patterns resembling those seen in the intact brain. The monosynaptic inhibitory projection from the cortex to the deep nuclei and collateral inhibition by Purkinje cell axons appear to be features of cerebellar function that are reproduced in this culture model. In addition, a projection from the deep nuclei to the cortex recently described in the intact cerebellum is also present in the cultures and gives postsynaptic potential responses typical of excitatory afferents to the cerebellar cortex. Such cultures appear useful as an experimental model for the study of synaptic mechanisms or the effects of drugs in the mammalian CNS.     

The effect of transection and cold block of the spinal cord on synaptic transmission between Ia afferents and motoneurones

Composite excitatory postsynaptic potentials were elicited in lumbar motoneurones by Ia afferents from muscles of the triceps surae group. These excitatory postsynaptic potentials were examined in the same cell before, during and after interruption of descending spinal pathways. After transection or cold block of the spinal cord at T12-T13, the amplitude of composite excitatory postsynaptic potentials showed no significant change for a period of up to seven hours after transection. However, there was a reduction in amplitude of the monosynaptic reflex in the extensor motoneurones which may be due to an observed hyperpolarization and reduction in membrane time constant in these neurones. The reduction in amplitude of the monosynaptic reflex observed in spinal shock can be attributed to the effects of these changes, rather than to a decrease in the size of the monosynaptic excitatory postsynaptic potential.     

Neuromuscular transmission in the visceral muscle of locust oviduct

Innervation of the locust oviduct has been investigated with morphological and electrophysiological methods. Using Co2+ and Ni2+ labelling technique, it was found that G7 N2B1 and B2a nerves innervate the oviduct musculature. Ultrastructurally two different terminals could be distinguished: (a) nerve endings containing mainly clear vesicles forming neuromuscular junctions with the muscle fibers; and (b) nerve terminals containing electron-dense granules which showed only "synaptoid" structures, but failed to form junctions with the muscle cells. The neuromuscular junctions proved to be functioning, since it was possible to record intracellularly miniature excitatory postsynaptic potentials and excitatory postsynaptic potentials from the muscle cells. The distribution of the amplitudes of the miniature excitatory postsynaptic potentials suggests a multiterminal innervation. Following electrical stimulation of N2B nerve, excitatory postsynaptic potentials similar to those appearing spontaneously could be evoked. After repetitive stimulation, facilitation or summation of excitatory postsynaptic potentials was observed. The results obtained show that locust oviduct muscle has a double, motor and modulatory innervation.     

Dual transmission at morphologically mixed synapses: Evidence from postsynaptic cobalt injections

Two experimental approaches have been utilized to test the possibility that morphologically mixed synaptic terminals of the eighth nerve fibers mediate both electrotonic and chemical excitation of the goldfish Mauthner cell. First, the spatial distributions of electrotonic and chemical postsynaptic potentials, evoked by stimulation of the eighth nerve, have been determined with intracellular recordings from the Mauthner cell soma and several locations along the lateral dendrite. In some instances, both synaptic components were maximal at distal dendritic recording sites. In that region, it appears that the only presynaptic terminals with morphological characteristics consistent with excitatory chemical transmission are the large myelinated club endings, which actually establish mixed synapses with the lateral dendrite. Second, we have analyzed the effects of postsynaptic Co²⁺ injections on these synaptic responses. With high iontophoretic currents, there was a rapid uncoupling of the electrotonic component. However, with smaller current intensities, uncoupling is accompanied, or preceded, by a transient reduction in the later chemically mediated postsynaptic potentials. This latter effect on chemical transmission is only observed if the postsynaptic potentials are associated with electrotonic synaptic inputs. We speculate that Co²⁺ diffuses across the gap junctions and into the presynaptic terminals, acting there to reduce evoked transmitter release.The results of these two experimental approaches support the hypothesis that mixed synapses on the lateral dendrite of the Mauthner cell do actually mediate transmission by both chemical and electrical modes.     

Disynaptic brainstem—propriospinal projections to mammalian motoneurones

In experiments carried out on cats simultaneous recording from hindlimb motoneurones and propriospinal interneurones receiving monosynaptic inputs from the brain stem was accomplished. The recording of the unitary postsynaptic potentials produced in motoneurones by direct stimulation of individual propriospinal cells has shown that supraspinal projections can govern spinal motor centres via propriospinal cells that establish direct monosynaptic contacts with alpha-motoneurones.     

Effects of ammonium salts on synaptic transmission to hippocampal CA1 and CA3 pyramidal cells in vivo

The effects of ammonium acetate or chloride, perfused through the lateral ventricle, were studied on the hippocampal formation of the rat. During perfusion with ammonia, the population spikes, evoked by stimuli delivered to the fimbria, were first increased and then reduced. On the other hand, the late positive wave gradually decreased throughout the application of ammonia. The inhibition, studied by the paired-pulse test, was found to be reduced when the population spike was transiently enhanced, indicating that disinhibition could be responsible for the enhancement of synaptically evoked responses. Neither antidromically evoked population spikes nor the typical effects of iontophoretically applied glutamate, aspartate or gamma-aminobutyrate were changed by ammonia. These findings can be accounted for by a single action of ammonia, a depression of excitatory synaptic transmission, the excitatory synapses on inhibitory interneurons being more readily depressed than those on the pyramidal cells. Both effects, early hyperexcitability and late depression, are probably due to a reduction in the release of the excitatory neurotransmitter, glutamate and/or aspartate. We tentatively suggest that these mechanisms are responsible for some of the symptoms observed during the development of hyperammonemic encephalopathies.     

Influence of the medial septal nucleus on the excitability of the commissural path-CA1 pyramidal cell synapse in the hippocampus of freely moving mice

Changes in the excitability of the commissural path-CA1 pyramidal cell synapse were studied by varying the interval between the application of a conditioning pulse in the medial septal nucleus and a test pulse in the contralateral hippocampus in freely moving mice. The results showed that septal prestimulation results in marked changes in the excitability of pyramidal cells (population spikes) without any associated changes in the averaged evoked excitatory post synaptic potentials. Thus, as a result of septal stimulation, population spikes were first potentiated for interpulse intervals ranging from 10 to 30 ms; this was followed by inhibition at intervals between 40 and 60 ms and then by another phase of hyperexcitability between 100 and 160 ms.Possible mechanisms underlying this phenomenon are discussed and these successive changes in excitability are compared to the phase-locked ones which occur spontaneously during theta waves.     

Effects of folic and kainic acids on synaptic responses of hippocampal neurones

The actions of the neurotoxic amino acids folate and kainate have been compared on ortho-and antidromic responses evoked in CA1, CA3 and the dentate gyrus of slices of rat hippocampus maintained in vitro. Both in CA1 and the dentate gyrus superfusion of these acids caused an increase in amplitude of the population spike discharging from an excitatory postsynaptic potential which either remained unaffected or was reduced. In the CA3 region kainate and folate had broadly similar actions to enhance the probability of cell firing to synaptic excitation, and also caused epileptiform discharges to occur spontaneously or in response to electrical stimulation. Spontaneous and evoked population bursts in CA3 did not persist in low calcium/high magnesium medium indicating their dependence on intact synaptic transmission; spontaneously occurring bursts in CA1 were eliminated with the latter treatment or when the axonal connections between it and CA3 were cut. Following folate superfusion the commissural-evoked response in CA3 showed large and variable shifts of the latency which were dependent on the stimulus intensity and its timing after a spontaneous population discharge. Although all of the effects of folate were reproduced by bicuculline, no evidence for a decreased recurrent inhibition in CA1 was obtained although this was observed with kainate. The finding that folate and kainate produced their effects in the absence of a detectable effect on the antidromic population spike suggests a mechanism of action other than neuronal depolarization. The implications of these data for the neurotoxic mechanism(s) and the receptor homologies of folate and kainate are discussed.     

Investigation of the cortico-reticulo-spinal connections in cats

Stimulation of the motor cortex evoked postsynaptic potentials in bulbar reticulo-spinal neurons of anaesthetized cats. In 72.4% of reticulo-spinal neurons they were identified as monosynaptic. According to the time characteristics they were classified into two groups (‘fast’ and ‘slow’). Corticobulbar fibres projecting to reticulospinal neurons could be also differentiated as ‘fast’ (conduction velocities above 20 m/s) and ‘slow’ (condition velocities below 20 m/s). Evidence was obtained that ‘fast’ cortico-fugal excitatory postsynaptic potentials are mediated by ‘fast’ cortico-bulbar fibres, and ‘slow’ excitatory postsynaptic potentials by the ‘slow’ ones. It was found that reticulo-spinal neurons with axon conduction velocities of 10.8–65 m/s can be activated by both ‘fast’ and ‘slow’ cortico-bulbar fibres, and the neurons with conduction velocities above 65 m/s only by the ‘slow’ ones. The background activity of reticulo-spinal neurons with slow conducting axons can be characterized as ‘tonic’ (with relatively uniform interspike interval distribution) while the activity of ‘fast’ neurons as ‘phasic’ (with significant grouping of discharges).The possible functional role of the differentiated cortico-reticulo-spinal connections is discussed.     

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 in chains of swellings. Most swellings are located in dorsal horn and in the intermediate zone approximately 100–200 μm from the somata of motoneurones. A few varicosities are located on the cell bodies of the motoneurones.     

Naloxone antagonizes striatally-induced suppression of globus pallidus unit activity

This study was designed to examine the ability of naloxone to antagonize the inhibition of neurons in the globus pallidus produced by electrical stimulation of the caudate nucleus. In rats anesthetized with chloral hydrate, bipolar stimulating electrodes were placed in the head of the caudate and seven-barrel micropipettes were utilized for recording extracellular activity of globus pallidus cells as well as for microiontophoretic application of experimental drugs. In most globus pallidus cells which were inhibited by caudate stimulation, application of naloxone (at currents which antagonized morphine-elicited depression) attenuated the caudate-induced effect. Naloxone did not antagonize depression of globus pallidus neurons induced by gamma-aminobutyrate. The results indicate that caudate stimulation causes the release of endogenous enkephalins which act to suppress neuronal activity in the globus pallidus. Thus, this study supports biochemical and histochemical studies which provide evidence for an enkephalinergic component in the striatopallidal projection.     

Cortically induced postsynaptic potentials in hypoglossal motoneurons after axotomy

Cortically induced postsynaptic potentials were studied in normal and axotomized cat hypoglossal motoneurons. In normal protruder motoneurons innervating tongue protruder muscles, we have demonstrated that stimulation of the orbital gyrus, at the point optimum for inducing lapping movements of the tongue by repetitive stimuli, produced inhibitory postsynaptic potentials or excitatory postsynaptic potentials followed by predominant inhibitory postsynaptic potentials. The cortically induced excitatory postsynaptic potential in normal protruder motoneurons was composed of only the short-latency component. In protruder motoneurons 30, 40, 60 and 80 days after axotomy, we have demonstrated that the number of protruder motoneurons responding with two components of excitatory postsynaptic potentials (the short- and the long-latency component) to cortical stimulation increased in correspondence with the lapse of days after axotomy and that the amplitude of cortically induced inhibitory postsynaptic potentials in axotomized protruder motoneurons was reduced in size as compared with normal protruder motoneurons.     

Postsynaptic factors controlling the shape of potentials at the squid giant synapse

The roles of rectification and cable properties of the squid giant axon in determining the shape of synaptic potentials generated at the giant synapse were investigated. Excitatory postsynaptic potentials were recorded in response to selective stimulation of the main presynaptic axon at various temperatures. Excitatory postsynaptic potentials elicited at low temperatures (less than 18 degrees C) exhibited a marked after-hyperpolarization or undershoot, while those recorded at higher temperatures did not. The postsynaptic current, recorded under voltage clamp conditions, did not show an undershoot. Furthermore, intracellular injection of tetraethylammonium chloride, to block the voltage-dependent rise in potassium conductance, also eliminated the undershoot of the excitatory postsynaptic potential. These results indicate that the duration of synaptic potentials at the squid giant synapse is reduced by rectification due to a delayed rise in potassium conductance. Computer simulations of these synaptic potentials suggested that the effects of rectification will be more prominent in spherical (isopotential) cells than in cells with more complicated geometries.     

Pharmacology of the corticocaudate excitatory postsynaptic potential in the cat: evidence for its mediation by quisqualate- or kainate-receptors

Caudate neurons were recorded with intracellular electrodes in halothane anaesthetized cats during microiontophoretic application of drugs and simultaneous stimulation of the corticocaudate pathway. Application of 2-amino-7-phosphonoheptanoic acid inhibited excitations induced by the N-methyl-D-aspartic acid receptor agonists N-methyl-D-aspartic acid and quinolinic acid, but not those elicited by quisqualic acid or activation of the cortico-caudate pathway. Selective inhibition of N-methyl-D-aspartic acid induced excitations was also found in vitro in the frog hemisected spinal cord preparation where a pA2-value of 5.5 against N-methyl-D-aspartic acid was determined. The endogenous tryptophan metabolite, kynurenic acid, antagonized excitations induced by N-methyl-D-aspartic, quisqualic, L-glutamic and kainic acid as well as the excitatory postsynaptic potential (EPSP) evoked in caudate cells by stimulation of the corticocaudate pathway, while action potentials elicited by an intracellularly applied depolarizing current were only slightly affected. In vitro experiments with the frog hemisected spinal cord preparation suggested that kynurenic acid might be a competitive antagonist of both N-methyl-D-aspartate and quisqualate receptors, with pA2-values of about 4.8 and 4.0, respectively. From these results it is concluded that the three-receptor concept for excitatory amino acids are proposed by Watkins and colleagues is probably applicable to the cat caudate nucleus and that the cortically evoked monosynaptic EPSP is mediated by a non-N-methyl-D-aspartate quisqualate- or kainate-receptor.     

An electrophysiological study of the accessory olfactory bulb in the rabbit--II. Input-output relations as assessed from analysis of intra- and extracellular unit recordings

The input-output relations of the rabbit accessory olfactory bulb were studied by intra- and extracellular single unit recordings following electrical stimulation of the vomeronasal nerves, the lateral olfactory tract and the corticomedial amygdala. Cellular activity of accessory bulb mitral cells evoked by stimulation of the vomeronasal nerves consisted of a brief excitation with a latency of 16 ms. This initial response was followed by a period of reduced firing probability which was due to an inhibitory postsynaptic potential. In many cases this secondary response was followed by a second excitatory postsynaptic potential on which action potentials were generated at higher stimulus intensities. Deeper cells in the granule cell layer responded with a long latency, long duration, excitation, often consisting of bursts of 2-3 spikes. The majority of mitral cells were antidromically invaded by amygdala stimulation. The latencies of the antidromic spikes showed a wide range of variation (12-80 ms). Due to this great variation in antidromic latency the inhibitory postsynaptic potential following the antidromic action potential was rather modest but prolonged in duration. In many cases the onset of the inhibitory postsynaptic potential preceded the antidromic response. The majority of cells did not respond to lateral olfactory tract stimulation. Only 10% of the tested cells were invaded antidromically by stimulation at this site. These neurons were also driven antidromically by amygdala stimulation. We conclude that, although the physiological characteristics of mitral cells of the main and accessory olfactory bulb are very similar, there are important differences. The efferent fibres of the accessory bulb conduct at very slow and variable rates and project directly to the corticomedial amygdala.