Antidromic Action Potentials Research Articles

Baclofen inhibits more effectively C-afferent than Aδ-afferent glutamatergic transmission in substantia gelatinosa neurons of adult rat spinal cord slices

Although intrathecal administration of baclofen, a selective GABA B-receptor agonist, is known to have an antinociceptive effect on various pain models, the role of presynaptic GABA B receptors in antinociception is not well characterized. In the present study, the action of baclofen on primary afferent-evoked glutamatergic excitatory transmission was examined in substantia gelatinosa (SG) neurons of an adult rat spinal cord slice with an attached dorsal root, prepared from the lumbar segment, by use of the blind whole-cell patch-clamp technique. Under the condition where a postsynaptic action of baclofen was inhibited, baclofen (1 μM) reduced the amplitudes of excitatory postsynaptic currents (EPSCs; V H=−70 mV) which were monosynaptically evoked by stimulating primary-afferent C- and/or Aδ-fibers and which were remarkably depressed by CNQX (10 μM). The identification of the C-fiber or Aδ-fiber EPSC was based on antidromic action potentials recorded from neurons of isolated dorsal root ganglia. The C-fiber EPSC was depressed in peak amplitude by baclofen (1 μM) to a larger extent than the Aδ-fiber EPSC (20 and 45% of control, respectively). Each of the baclofen actions was suppressed by a selective GABA B-receptor antagonist, CGP 35348 (50 μM). Baclofen (1 μM) did not affect a response of SG neurons to bath-applied AMPA (10 μM). These results indicate that baclofen inhibits the release of L-glutamate from Aδ and C primary-afferent terminals in the SG through the activation of GABA B receptor; this action is more effective to C-fiber than Aδ-fiber transmission. Considering that the SG is the main part of termination of Aδ- and C-fibers transmitting nociceptive information, the present finding would account for at least a part of the inhibitory action of baclofen on pain transmission.

Blocking the trigeminal EPSP in rat abducens motoneurons in vivo with the AMPA antagonists NBQX and GYKI 53655

In pentobarbitone-anaesthetized rats, the effects of two AMPA receptor antagonists, the competitive antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo-(F)-quinoxaline (NBQX) and the non-competitive 2,3-benzodiazepine GYKI 53655, were compared on excitatory synaptic transmission of trigeminal origin in intracellularly-recorded abducens motoneurons. The effects of both antagonists were also investigated on the α-amino-3-hydroxy-5-methyl isoxazole-4-propionic acid (AMPA)-, kainate-, and N-methyl-D-aspartate (NMDA)-induced depression of extracellular antidromic field potentials in the abducens motor nucleus. Microiontophoretic application (≤100 nA) or intravenous injection of NBQX (≤5 mg/kg) affected both AMPA- and kainate-induced depressions whereas GYKI 53655 (≤100 nA; ≤4 mg/kg) blocked only the AMPA-induced depression. Neither NBQX or GYKI 53655 affected NMDA-induced depressions of antidromic field potentials. Using low intravenous (i.v.) doses of the antagonists NBQX or GYKI 53655 (2–2.5 mg/kg), a complete blockade of the composite disynaptic trigeminal excitatory post-synaptic potential (EPSP) was obtained without any changes in membrane potential, input resistance and antidromic action potentials in abducens motoneurons. GYKI 53655 was more potent at low i.v. doses (0.5–1.8 mg/kg) but NBQX had longer-lasting effects. The results show the existence of differences between the blocking action of NBQX and GYKI 53655 on AMPA-mediated receptor EPSP in abducens motoneurons.

Evidence for functional compartmentalization of trigeminal muscle spindle afferents during fictive mastication in the rabbit.

Primary afferent neurons innervating muscle spindles in jaw-closing muscles have cell bodies in the trigeminal mesencephalic nucleus (NVmes) that are electrically coupled and receive synapses. Each stem axon gives rise to a peripheral branch and a descending central branch. It was previously shown that some spikes generated by constant muscle stretch fail to enter the soma during fictive mastication. The present study examines whether the central axon is similarly controlled. These axons were functionally identified in anaesthetized and paralysed rabbits, and tonic afferent firing was elicited by muscle stretch. For the purpose of comparison, responses were recorded extracellularly both from the somatic region and from the central axon in the lateral brainstem. Two types of fictive masticatory movement patterns were induced by repetitive stimulation of the masticatory cortex and monitored from the trigeminal motor nucleus. Field potentials generated by spike-triggered averaging of action potentials from the spindle afferents were employed to determine their postsynaptic effects on jaw-closing motoneurons. Tonic firing of 32% NVmes units was inhibited during the jaw-opening phase, but spike frequency during closing was almost equal to the control rate during both types of fictive mastication. A similar inhibition occurred during opening in 83% of the units recorded along the central branch. However, firing frequency in these was significantly increased during closing in 94%, probably because of the addition of antidromic action potentials generated by presynaptic depolarization of terminals of the central branch. These additional spikes do not reach the soma, but do appear to excite motoneurons. The data also show that the duration and/or frequency of firing during the bursts varied from one pattern of fictive mastication to another. We conclude that the central axons of trigeminal muscle spindle afferents are functionally decoupled from their stem axons during the jaw-closing phase of mastication. During this phase, it appears that antidromic impulses in the central axons provide one of the inputs from the masticatory central pattern generator (CPG) to trigeminal motoneurons.

Functional characteristics of the input-output correlation in the vestibular nuclear complex of the frog.

Experiments on perfused frog brains were used to record focal and intracellular potentials of neurons in the vestibular nuclear complex produced in response to stimulation of the anterior branch of the ipsilateral vestibular nerve and the spinal cord. Stimulation of the vestibular nerve evoked mono- and polysynaptic EPSP with orthodromic action potentials. These were accompanied by recordings of antidromic activation (with a mean latent period of 0.75 sec) of neurons which send their axons into the labyrinth. Antidromic action potentials from vestibular neurons arose with latent periods of the order of 1.43 msec in response to stimulation of the cervical thickening and 2.19 msec in response to stimulation of the lumbar thickening of the spinal cord. Bursts from the spinal cord often evoked EPSP with orthodromic action potentials in vestibular neurons. The characteristics of the functional correlation between the vestibular input and the vestibulospinal system are discussed.

Spontaneous and locomotor-related GABAergic input onto primary afferents in the neonatal rat.

The in vitro brain stem-spinal cord preparation of neonatal rats (0-5 days old) was used to examine the contribution of GABAA (gamma-aminobutyric acid) receptors to the spontaneous and locomotor-related antidromic firing in the dorsal roots of neonatal rats. Spontaneous bursts of antidromic discharges were generated by the underlying afferent terminal depolarizations reaching spiking threshold. The number of antidromic action potentials increased significantly in saline solution with Cl- concentration reduced to 50% of control. Bath application of the GABAA receptor antagonist bicuculline, at low concentrations (1-2 microM), or picrotoxin blocked the antidromic discharges in the dorsal roots almost completely. The increase in Cl- conductance was therefore mediated by an activation of GABAA receptors. Increasing the concentration of bicuculline to 10-20 microM never blocked these discharges further. On the contrary, in half of the preparations, the number of antidromic action potentials was higher in the presence of high concentrations of bicuculline (10-20 microM) than in the presence of picrotoxin or low concentrations of bicuculline. This suggests that bicuculline, at high concentrations, may have other effects, in addition to blocking GABAA receptors. Dorsal root firing was observed during fictive locomotion induced by bath application of excitatory amino acids and serotonin. A rhythmical pattern was often demonstrated. Bicuculline at low concentrations caused a decrease of the antidromic discharge whereas, at high concentrations, bursts of discharges appeared. A double-bath with a barrier built at the L3 level was then used to separate the mechanisms which generate locomotion from those mediating primary afferent depolarizations. Excitatory amino acids and serotonin were perfused in the rostral pool only. Decreasing the concentration of chloride in the caudal bath caused a sharp increase in the number of antidromic action potentials recorded from the L5 dorsal root. These discharges, which were modulated in phase with the locomotor rhythm, were blocked by bicuculline. These data demonstrate the existence of a locomotor-related GABAergic input onto primary afferent terminals in the neonatal rat.

Bidirectional Neurones in the Cervical Enlargement of the Cat Spinal Cord with Axons Descending to Sacral Segments and Ascending to the Cerebellum and the Lateral Reticular Nucleus

Neurones located in cervical segments (C6/C7) of the spinal cord were investigated electrophysiologically in cats deeply anaesthetized with alpha-chloralose. Extracellular recordings of antidromic action potentials were performed in order to establish whether long descending propriospinal neurones projecting to sacral segments could have collateral axonal branches ascending to supraspinal centres. The effects of stimulation of the lateral reticular nucleus (LRN) and the inferior cerebellar peduncle (restiform body, RB), as well as the thirteenth thoracic (Th13) and sacral (S1/S2) segments of the spinal cord were tested in 93 cells. Two main groups of cells were identified: 54 % of the total sample were classified as purely propriospinal and 46 % as bidirectional neurones. Various patterns of projections, as well as the ipsi-, contra- or bilateral courses of axons in the lateral funiculi of the spinal cord, enabled several types of neurones to be distinguished within the above groups. Comparison between particular types showed no significant difference with respect to location in the grey matter (predominantly Rexed's laminae VII-VIII) and the conduction velocities of descending axons. However, the mean axonal conduction velocities of branches ascending to LRN and/or RB were significantly lower in comparison to those measured for spinal collaterals. The hypothetical function of the neurones examined is discussed. Since the same information can be conveyed simultaneously by these branching neurones to lower spinal segments and supraspinal centres, an integrative role in the system of motor control is suggested.

The continuing search for outer hair cell afferents in the guinea pig spiral ganglion

Antidromic stimulation of the stump of the VIIIth nerve was combined with microelectrode recording in the spiral ganglion of the guinea pig cochlea in an attempt to identify a sub-population of neurons with long-latency antidromic action potentials that might correspond to the thin unmyelinated afferent neurons emanating from the outer hair cells. The techniques used were similar but not identical to those employed in an earlier study by Brown (1994). By far the largest population of cells contacted had short antidromic latencies (0.58±0.12 ms, 76 units) and also responded to acoustic stimulation. These were assumed to be type I afferents emanating from inner hair cells. Eight cells had antidromic latencies larger than 1 ms, all but one of which had a zero spontaneous rate. All eight of these longer-latency cells were unresponsive to acoustic stimulation despite the fact that short-latency neurons in the same cochleas showed robust responses to sound before and after they were contacted. Four of these longer-latency cells had their antidromic thresholds accurately measured and two had significantly higher thresholds to electrical stimulation (0.1 ms duration) than type I cells in the same animal while two had similar electrical thresholds. Attempts to trace the eight long-latency neurons to the outer hair cells using intracellular injection of horseradish peroxidase were unsuccessful. On the basis of present evidence, we cannot conclude definitively that the long-latency neurons found in the spiral ganglion belong to the outer hair cell afferent population.

Presynaptic inhibition and antidromic discharges in crayfish primary afferents

The mechanisms of presynaptic inhibition have been studied in sensory afferents of a stretch receptor in an in vitro preparation of the crayfish. Axon terminals of these sensory afferents display primary afferent depolarisations (PADs) mediated by the activation of GABA receptors that open chloride channels. Intracellular labeling of sensory axons by Lucifer yellow combined with GABA immunohistochemistry revealed the presence of close appositions between GABA-immunoreactive boutons and sensory axons close to their first branching point within the ganglion. Electrophysiological studies showed that GABA inputs mediating PADs appear to occur around the first axonal branching point, which corresponds to the area of transition between active and passive propagation of spikes. Moreover, this study demonstrated that whilst shunting appeared to be the sole mechanism involved during small amplitude PADs, sodium channel inactivation occurred with larger amplitude PADs. However, when the largest PADs (>25 mV) are produced, the threshold for spike generation is reached and antidromic action potentials are elicited. The mechanisms involved in the initiation of antidromic discharges were analyzed by combining electrophysiological and simulation studies. Three mechanisms act together to ensure that PAD-mediated spikes are not conveyed distally: 1) the lack of active propagation in distal regions of the sensory axons; 2) the inactivation of the sodium channels around the site where PADs are produced; and 3) a massive shunting through the opening of chloride channels associated with the activation of GABA receptors. The centrally generated spikes are, however, conveyed antidromically in the sensory nerve up to the proprioceptive organ, where they inhibit the activity of the sensory neurons for several hundreds of milliseconds.

Antidromic discharges of dorsal root afferents in the neonatal rat

Presynaptic inhibition of primary afferents can be evoked from at least three sources in the adult animal: 1) by stimulation of several supraspinal structures; 2) by spinal reflex action from sensory inputs; or 3) by the activity of spinal locomotor networks.The depolarisation in the intraspinal afferent terminals which is due, at least partly, to the activation of GABA A receptors may be large enough to reach firing threshold and evoke action potentials that are antidromically conducted into peripheral nerves. Little is known about the development of presynaptic inhibition and its supraspinal control during ontogeny. This article, reviewing recent experiments performed on the in vitro brainstem/spinal cord preparation of the neonatal rat, demonstrates that a similar organisation is present,to some extent, in the new-born rat. A spontaneous activity consisting of antidromic discharges can be recorded from lumbar dorsal roots. The discharges are generated by the underlying afferent terminal depolarizations reaching firing threshold. The number of antidromic action potentials increases significantly in saline solution with chloride concentration reduced to 50% of control. Bath application of the GABA A receptor antagonist, bicuculline (5–10 μM) blocks the antidromic discharges almost completely. Dorsal root discharges are therefore triggered by chloride-dependent GABA A receptor-mediated mechanisms; 1) activation of descending pathways by stimulation delivered to the ventral funiculus (VF) of the spinal cord at the C1 level; 2) activation of sensory inputs by stimulation of a neighbouring dorsal root; or 3) pharmacological activation of the central pattern generators for locomotion evokes antidromic discharges in dorsal roots. VF stimulation also inhibited the response to dorsal root stimulation. The time course of this inhibition overlapped with that of the dorsal root discharge suggesting that part of the inhibition of the monosynaptic reflex may be exerted at a presynaptic level. The existence of GABA A receptor-independent mechanisms and the roles of the antidromic discharges in the neonatal rat are discussed.

Distinct temporal profiles of activity-dependent calcium increase in pyramidal neurons of the rat visual cortex.

1. Using fluo-3-based fluorometry, we studied variation in depolarization-induced calcium increases in the proximal dendrites or soma of pyramidal neurons in layer II/III of the rat visual cortex. 2. Depolarization for all durations tested (0.1-2 s; 0.5 nA) evoked a train of action potentials and a small increase in calcium signal (mean 26 %) which peaked within 1 s of the onset of depolarization. With depolarization for longer than 1 s, this small increase was often followed by a larger increase (73 %). This later phase of calcium increase occurred without sudden changes in action potential firing. 3. Application of ryanodine, which suppresses intracellular calcium release, abolished the second phase without affecting the early phase in a use-dependent manner. Meanwhile, no major changes were observed in the pattern of action potential firing. 4. In calcium-free medium, both the early and late phases were almost undetectable, although action potential firing was still evoked by injection of depolarizing currents. Since the late phase depended on intracellular calcium release, this effect of calcium-free medium on the late phase is likely to be indirect through an influence on the early phase. 5. This two-phase profile was observed with somatic depolarization or with antidromic action potentials induced by tetanization. Neocortical pyramidal neurons can thus recruit calcium from different sources, even without chemical sensitization, generating temporally diverse profiles of intracellular calcium signal in response to action potential firing. 6. Such variety in the mechanisms of calcium increase may be relevant to the role of calcium as a versatile second messenger for various types of synaptic plasticity.

Specific distribution of sodium channels in axons of rat embryo spinal motoneurones.

1. The distribution of Na+ channels and development of excitability were investigated in vitro in purified spinal motoneurones obtained from rat embryos at E14, using electrophysiological, immunocytochemical and autoradiographical methods. 2. One hour after plating the motoneurones (DIV0), only somas were present. They expressed a robust delayed rectifier K+ current (IDR) and a fast-inactivating A-type K+ current (IA). The rapid neuritic outgrowth was paralleled by the emergence of a fast-activating TTX-sensitive sodium current (INa), and by an increase in both K+ currents. 3. The change in the three currents was measured daily, up to DIV8. The large increase in INa observed after DIV2 was accompanied by the onset of excitability. Spontaneous activity was observed as from DIV6. 4. The occurrence of axonal differentiation was confirmed by the fact that (i) only one neurite per motoneurone generated antidromic action potentials; and (ii) 125I-alpha-scorpion toxin binding, a specific marker of Na+ channels, labelled only one neurite and the greatest density was observed in the initial segment. Na+ channels therefore selectively targeted the axon and were absent from the dendrites and somas. 5. The specific distribution of Na+ channels was detectable as soon as the neurites began to grow. When the neuritic outgrowth was blocked by nocodazole, no INa developed. 6. It was concluded that, in spinal embryonic motoneurone in cell culture, Na+ channels, the expression of which starts with neuritic differentiation, are selectively addressed to the axonal process, whereas K+ channels are present in the soma prior to the neuritic outgrowth.

Divergence of lamina VII and VIII neurones of S1 and S2 segments of the cat's spinal cord to the cerebellum and the reticular formation

Cerebellar and reticular projections of neurones located in sacral segments of the spinal cord were electrophysiologically investigated in alpha-chloralose anaesthetized cats. Antidromic action potentials were recorded following stimulation of the contralateral restiform body (coRB), contralateral gigantocellular nucleus (coGRN) as well as ipsi- and contralateral lateral funiculus of the 13th thoracic segment (iTh13 and coTh13). Eighty seven neurones were found in the medial lamina VII and lamina VIII of the gray matter of S1 and S2 segments. Their axons ascended in lateral funiculi on the contralateral side and in 46 cases also on the ipsilateral side of the spinal cord. A projection to coRB was found in 20 neurones, to coGRN in 10 and dual projections to both coRB and coGRN in 20 neurones. Axons of the remaining 37 cells were found to ascend to the level of Th13 only. Conduction velocities of neurones investigated were comprised in the range 35–83 m/s and no significant differences were found between particular groups. However, an evident decrease in conduction was observed in most neurones when comparing proximal to distal parts of their axons, suggesting the possibility of more extensive divergence than indicated in this study. The pattern of projections revealed that the information from the periphery is conveyed in parallel to various supraspinal and possibly also spinal centres.

Projection to the lateral reticular nucleus from neurons located in C6-C7 segments of the spinal cord. An electrophysiological study in the cat

Spinoreticular neurons projecting to the lateral reticular nucleus (LRN) were investigated electrophysiologically in the cervical enlargement of the cat spinal cord. Experiments were performed on α-chloralose anaesthetized animals. Antidromic action potentials were recorded extracellularly from cells located in C6 and C7 segments following stimulation of the ipsilateral LRN. The total sample included 50 neurons. Their cell bodies were found to be distributed in Rexed's laminae VI-VIII of the gray matter. Axonal conduction velocities ranged from 14.7 to 89.7 ms -1. Considerable differences between particular cases enabled two separate groups of slower and faster conduction to be distinguished. Values for these two groups were 14.7–44.3 ms -1 and 52.2–89.7 ms -1, respectively. Discrete differences with regard to the location of these groups were also pointed out. Such differentiation suggests that a proportion of axons from the slower conducting pool may be in fact collaterals of neurons that project to other brainstem centers or to lower levels of the spinal cord.

Optical mapping of neural network activity in chick spinal cord at an intermediate stage of embryonic development.

We have applied multiple-site optical recording of transmembrane potential changes to recording of neuronal pathway/network activity from embryonic chick spinal cord slice preparations. Spinal cord preparations were dissected from 8-day-old chick embryos at Hamburger-Hamilton stage 33, and transverse slice preparations were prepared with the 13th cervical spinal nerve or with the 2nd or 5th lumbosacral spinal nerve intact. The slice preparations were stained with a voltage-sensitive merocyanine-rhodanine dye (NK2761). Transmembrane voltage-related optical (dye-absorbance) changes evoked by spinal nerve stimulation with positive square-current pulses using a suction electrode were recorded simultaneously from many loci in the preparation, using a 128- or 1,020-element photodiode array. Optical responses were detected from dorsal and ventral regions corresponding to the posterior (dorsal) and anterior (ventral) gray horns. The optical signals were composed of two components, fast spike-like and slow signals. In the dorsal region, the fast spike-like signal was identified as the presynaptic action potential in the sensory nerve and the slow signal as the postsynaptic potential. In the ventral region, the fast spike-like signal reflects the antidromic action potential in motoneurons, and the slow signal is related to the postsynaptic potential evoked in the motoneuron. In preparations in which the ventral root was cut microsurgically, the antidromic action potential-related optical signals were eliminated. The areas of the maximal amplitude of the evoked signals in the dorsal and ventral regions were located near the dorsal root entry zone and the ventral root outlet zone, respectively. Quasiconcentric contour-line maps were obtained in the dorsal and ventral regions, suggesting the functional arrangement of the dorsal and ventral synaptic connections. Synaptic fatigue induced by repetitive stimuli in the ventral synapses was more rapid than in the dorsal synapses. The distribution patterns of the signals were essentially similar among C13, LS2, and LS5 preparations, suggesting that there is no difference in the spatiotemporal pattern of the neural responses along the rostrocaudal axis of the spinal cord at this developmental stage. In the ventral root-cut preparations, comparing the delay times between the ventral slow optical signals, we have been able to demonstrate that neural network-related synaptic connections are generated functionally in the embryonic spinal cord at Hamburger-Hamilton stage 33.

Reorganization of the primary afferent termination in the rat spinal dorsal horn during post-natal development

To study the reorganization of the primary afferent input in the spinal dorsal horn during post-natal development, synaptic responses evoked by large Aβ and fine Aδ afferents were recorded from substantia gelatinosa (SG) neurons in slices obtained from immature (post-natal days 21–23) and mature rats (post-natal days 56–60). Threshold stimulus intensities and conduction velocities (CVs) of Aβ and Aδ afferents were determined by intracellular recordings of the antidromic action potentials from dorsal root ganglion (DRG) neurons isolated from immature and mature rats. In immature rats, excitatory postsynaptic currents (EPSCs) were elicited by stimulation sufficient to activate Aβ afferents in the majority of SG neurons (64.9%, 24 of 37 neurons), while most EPSCs observed in mature rats were elicited by stimulation of Aδ afferents (62.5%, 25 of 40 neurons). These observations suggest that the primary afferents innervating SG neurons were reorganized following maturation; Aβ afferents were the predominant inputs to the SG neurons in the immature state, thereafter Aδ afferents were substituted for the Aβ afferents to convey sensory information to the SG neurons. This relatively slow reorganization of the sensory circuitry may correlate with slow maturation of the SG neurons and with a delay in the functional connections of C afferents to the SG neurons.

Long-term effects of synaptic activation at low frequency on excitability of myenteric AH neurons

Intracellular microelectrodes were used to record the effects of extended periods (1–30 min) of synaptic activation on AH neurons in the myenteric ganglia of the guinea-pig ileum. Low-frequency (1 Hz) stimulation gave rise to a slowly developing, sustained increase in excitability of the neurons associated with depolarization and increased input resistance. The increased excitability lasted for up to 3.5 h following the stimulus period. Successive stimulus trains (1–4 min) elicited successively greater increases in excitability. The neurons went through stages of excitation. Before stimulation, 500-ms depolarizing pulses evoked up to three action potentials (phasic response) and anode break action potentials were not observed. As excitability increased, more action potentials were evoked by depolarization (the responses became tonic), anode break action potentials were observed, prolonged afterhyperpolarizing potentials that follow multiple action potentials were diminished and, with substantial depolarization of the neurons, invasion by antidromic action potentials was suppressed. It is concluded that a state of elevated excitability is induced in myenteric AH neurons by synaptic activation at low frequency and that changes in excitability can outlast stimulation by several hours.

Triggering action potentials in an axo-dendritic inhibitory synaptic terminal.

Intracellular activity of the slow adapting neurone (SAN) was recorded together with extracellular action potentials (APs) of SAN, fast adapting neurone and inhibitory fibre in the isolated stretch receptor of the moulting crayfish. Following strychnine application, spontaneous APs and seizure-like AP trains were observed. A delay of the inhibitory fibre APs relative to SAN inhibitory postsynaptic potential, their waveforms and their initiation by intracellular SAN hyperpolarization indicate that their generation was close to the inhibitory nerve terminal. Such antidromic (ectopic) APs assert their postsynaptic action on both receptor neurones via the axon reflex. The findings provide first evidence of antidromic APs generated in the vicinity of inhibitory terminals, their direct and axon reflex-mediated action on target neurones and support a hypothesis that increased terminal excitability contributes to seizure activity.

Antidromic discharges of dorsal root afferents and inhibition of the lumbar monosynaptic reflex in the neonatal rat

The in vitro brain stem–spinal cord preparation of neonatal (0- to five-day-old) rats was used to establish whether pathways descending from the brain stem are capable of modulating synaptic transmission from primary afferents to lumbar motoneurons within the first few days after birth. We stimulated the ventral funiculus of the spinal cord at the cervical (C1–C2) level. Single-pulse stimulations evoked both excitatory and inhibitory postsynaptic potentials in ipsilateral lumbar (L2–L5) motoneurons which were recorded intracellularly. Twin-pulse stimulations evoked bursts of action potentials in ventral roots. The amplitude of the monosynaptic dorsal root-evoked excitatory postsynaptic potential decreased when a conditioning stimulation was applied to the ventral funiculus 50–300 ms prior to the stimulation of the ipsilateral dorsal root. A decreased input resistance of the motoneurons during the early part (25–100 ms after the artifact) of the ventral funiculus-evoked postsynaptic potentials could account, at least partly, for the decreased amplitude of the dorsal root-evoked response. However, the duration of the inhibition of the dorsal root-evoked excitatory postsynaptic potential was longer than that of the decrease in input resistance. Ventral funiculus stimulation evoked antidromic discharges in dorsal roots. Recordings of dorsal root potentials showed that these discharges were generated by the underlying afferent terminal depolarizations reaching firing threshold. The dorsal root discharge overlapped with most of the time-course of the ventral funiculus-evoked inhibition of the response to dorsal root stimulation, suggesting that part of this inhibition may be exerted at a presynaptic level. The number of antidromic action potentials evoked in dorsal roots by ventral funiculus stimulation increased significantly in saline solution with chloride concentration reduced to 50% of control. Bursts of action potentials disappeared when chloride was removed completely. Antidromic discharges were therefore due to chloride conductance. The number of action potentials evoked in ventral roots was increased in low-chloride saline solutions. Removing chloride from the bathing solution resulted in an unstable ventral root activity. Bath application of the GABA A receptor antagonist, bicuculline (5–10 μM), blocked the ventral funiculus-evoked antidromic discharges in the dorsal roots. The increase in chloride conductance which generated the depolarizations underlying the dorsal root discharges was therefore mediated by an activation of GABA A receptors. In contrast, bursts of action potentials in the ventral roots were increased in both amplitude and duration under bicuculline. Our data demonstrate that pathways running in the ventral funiculus of the spinal cord exert a control on interneurons mediating presynaptic inhibition at birth.

The electrophysiological study of axonal branching of the C6-C7 neurones of the cat's spinal cord projecting to the lateral reticular nucleus and the cerebellum.

Ascending projections of the cervical spinal cord neurones to the lateral reticular nucleus (LRN) and the restiform body (RB) were electrophysiologically investigated in alpha-chloralose anaesthetized cats. The main purpose of the study was to demonstrate whether the some of C6/C7 segments neurones gave off collateral branches that reached both the LRN and RB. Antidromic action potentials were recorded from 50 neurones located in laminae IVVIII of the grey matter of C6/C7 segments following stimulation of the LRN and RB with tungsten electrodes. Analysis of the antidromic responses allowed to distinguish three populations of neurones according to their projection patterns, 41% cells ascended exclusively to either one of the two structures investigated (26% to LRN and 15% to RB), while in the remaining cases (59%) collateral projections ascended to both LRN and RB. The axonal conduction velocities were comprised in the range of 14-90 m/s and no differences were found when compared mean values for particular groups of neurones. In conclusion, the results obtained indicate that in a great number of C6/C7 neurones both spinoreticular and spinocerebellar projections are present. It is likely that afferent input from forelimbs may be directly of indirectly transmitted to the cerebellum through the same neurones.

Triple projections of neurones located in S1 and S2 segments of the cat spinal cord to the C6 segment, the cerebellum and the reticular formation.

Electrophysiological investigation of neurones in sacral segments of the spinal cord was performed in alpha-chloralose-anaesthetized cats in order to establish whether at least some ascending tract neurones could diverge to three different centres located in the brainstem, the cerebellum or the spinal cord. Recordings of antidromic action potentials from cells in S1 and S2 segments were taken following stimulation of the contralateral gigantocellular nucleus, contralateral restiform body and ipsi- and contralateral grey matter of the C6 spinal segment. Antidromic responses allowed identification of several types of neurones that differed in their pattern of supraspinal or propriospinal projections. In eighteen out of a total of sixty-three neurones triple projections to all the above structures were found. In the majority of cells investigated their axons divided into two branches ascending both ipsi- and contralaterally in the lateral funiculi of the spinal cord. Their cell bodies were distributed in laminae VII-VIII except for a minor group of neurones that projected to the C6 segment only, which were located in laminae V-VI. The latter group also displayed lower values of axonal conduction velocities. Comparison of conduction velocities in proximal and distal parts of axons revealed significant slowing in most, raising the possibility that additional collaterals were present to other spinal or supraspinal centres. Dual and triple projections from most cells in this study suggest that such a divergence may be a more common feature of ascending tract neurones than has been reported before.