Polysynaptic Excitatory Postsynaptic Potentials Research Articles

Candidate Interneurons Mediating the Resetting of the Locomotor Rhythm by Extensor Group I Afferents in the Cat

Sensory information arising from limb movements controls the spinal locomotor circuitry to adapt the motor pattern to demands of the environment. Stimulation of extensor group (gr) I afferents during fictive locomotion in decerebrate cats prolongs the ongoing extension, and terminates ongoing flexion with an initiation of the subsequent extension, i. e. “resetting to extension”. Moreover, instead of the classical Ib non-reciprocal inhibition, stimulation of extensor gr I afferents produces a polysynaptic excitation in extensor motoneurons with latencies (∼3.5–4.0 ms) compatible with 3 interposed interneurons. We assume that some interneurons in this pathway actually belong to the rhythm-generating layer of the locomotor Central Pattern Generator (CPG), since their activity was correlated to a resetting of the rhythm. In the present work fictive locomotion was (mostly) induced by i.v. injection of nialamide followed by l-DOPA in paralyzed cats following decerebration and spinalization at C1 level. In some experiments, we extended previous observations during fictive locomotion on the emergence and locomotor state-dependence of polysynaptic excitatory postsynaptic potentials from extensor gr I afferents to ankle extensor motoneurons. However, the main focus was to record location and properties of interneurons (n = 62) that (i) were active during the extensor phase of fictive locomotion and (ii) received short-latency excitation (mono-, di- or polysynaptic) from extensor gr I afferents. We conclude that the interneurons recorded fulfill the characteristics to belong to the neuronal pathway activated by extensor gr I afferents during locomotion, and may contribute to the ‘resetting to extension’ as part of the locomotor CPG.

Low- and high-threshold primary afferent inputs to spinal lamina III antenna-type neurons.

The dorsal horn of the spinal cord (laminae I-VI) processes diverse modalities of nociceptive and nonnociceptive sensory information. Antenna-type neurons with cell bodies located in lamina III and large dendritic trees extending from the superficial lamina I to deep lamina IV are best shaped for the integration of a wide variety of inputs arising from primary afferent fibers and intrinsic spinal circuitries. Although the somatodendritic morphology, the hallmark of antenna neurons, has been well studied, little is still known about the axon structure and basic physiological properties of these cells. Here, we did whole-cell recordings in a rat (P9-P12) spinal cord preparation with attached dorsal roots to examine the axon course, intrinsic firing properties, and primary afferent inputs of antenna cells. Nine antenna cells were identified from a large sample of biocytin-filled lamina III neurons (n = 46). Axon of antenna cells showed intensive branching in laminae III-IV and, in half of the cases, issued dorsally directed collaterals reaching lamina I. Antenna cells exhibited tonic and rhythmic firing patterns; single spikes were followed by hyperpolarization or depolarization. The neurons received monosynaptic inputs from the low-threshold Aβ afferents, Aδ afferents, as well as from the high-threshold Aδ, and C afferents. When selectively activated, C-fiber-driven monosynaptic and polysynaptic excitatory postsynaptic potentials were sufficiently strong to evoke firing in the neurons. Thus, lamina III antenna neurons integrate low-threshold and nociceptive high-threshold primary afferent inputs and can function as wide dynamic range neurons able to directly connect deep dorsal horn with the major nociceptive projection area lamina I.

Histological and electrophysiological analysis of the corticospinal pathway to forelimb motoneurons in common marmosets

Using histological and electrophysiological methods, we identified the neuroanatomical properties of the common marmoset corticospinal tract (CST), which underlies hand/arm motor control. Biotinylated dextran amine (BDA) was injected into the primary motor cortex to anterogradely label CST axons in the cervical segments, revealing that most CST axons descend in the contralateral dorsolateral funiculus (DLF; 85.0%), and some in the ipsilateral DLF (10.7%). Terminal buttons were mainly found in the contralateral lamina VII of the gray matter, but projection to lamina IX, where forelimb motoneurons are located, was rare. Bilateral projections were more abundant than found in the rat CST, resembling the CST organization of other primates. Intracellular recordings were made from 57 forelimb motoneurons on the contralateral side to stimulation, which revealed no monosynaptic excitatory postsynaptic potentials (EPSPs), but di- or polysynaptic EPSPs and inhibitory synaptic potentials were commonly found. Local field potentials showed monosynaptic excitation mainly in laminae VII, where abundant BDA-labeled CST terminals were observed. These results suggest that direct corticomotoneuronal projection is absent in common marmosets but di- or oligosynaptic effects would be mediated by spinal interneurons.

Polysynaptic excitatory postsynaptic potentials that trigger spasms after spinal cord injury in rats are inhibited by 5-HT1B and 5-HT1F receptors

Sensory afferent transmission and associated spinal reflexes are normally inhibited by serotonin (5-HT) derived from the brain stem. Spinal cord injury (SCI) that eliminates this 5-HT innervation leads to a disinhibition of sensory transmission and a consequent emergence of unusually long polysynaptic excitatory postsynaptic potentials (EPSPs) in motoneurons. These EPSPs play a critical role in triggering long polysynaptic reflexes (LPRs) that initiate muscles spasms. In the present study we examined which 5-HT receptors modulate the EPSPs and whether these receptors adapt to a loss of 5-HT after chronic spinal transection in rats. The EPSPs and associated LPRs recorded in vitro in spinal cords from chronic spinal rats were consistently inhibited by 5-HT(1B) or 5-HT(1F) receptor agonists, including zolmitriptan (5-HT(1B/1D/1F)) and LY344864 (5-HT(1F)), with a sigmoidal dose-response relation, from which we computed the 50% inhibition (EC(50)) and potency (-log EC(50)). The potencies of 5-HT receptor agonists were highly correlated with their binding affinity to 5-HT(1B) and 5-HT(1F) receptors, and not to other 5-HT receptors. Zolmitriptan also inhibited the LPRs and general muscle spasms recorded in vivo in the awake chronic spinal rat. The 5-HT(1B) receptor antagonists SB216641 and GR127935 and the inverse agonist SB224289 reduced the inhibition of LPRs by 5-HT(1B) agonists (zolmitriptan). However, when applied alone, SB224289, SB216641, and GR127935 had no effect on the LPRs, indicating that 5-HT(1B) receptors do not adapt to chronic injury, remaining silent, without constitutive activity. The reduction in EPSPs with zolmitriptan unmasked a large glycine-mediated inhibitory postsynaptic current (IPSC) after SCI. This IPSC and associated chloride current reversed at -73 mV, slightly below the resting membrane potential. Zolmitriptan did not change motoneuron properties. Our results demonstrate that 5-HT(1B/1F) agonists, such as zolmitriptan, can restore inhibition of sensory transmission after SCI without affecting general motoneuron function and thus may serve as a novel class of antispastic drugs.

Activity-Dependent Depression of Local Excitatory Connections in the CA1 Region of Mouse Hippocampus

The existence of recurrent excitatory synapses between pyramidal cells in the hippocampal CA1 region has been known for some time yet little is known about activity-dependent forms of plasticity at these synapses. Here we demonstrate that under certain experimental conditions, Schaffer collateral/commissural fiber stimulation can elicit robust polysynaptic excitatory postsynaptic potentials due to recurrent synaptic inputs onto CA1 pyramidal cells. In contrast to CA3 pyramidal cell inputs, recurrent synapses onto CA1 pyramidal cells exhibited robust paired-pulse depression and a sustained, but rapidly reversible, depression in response to low-frequency trains of Schaffer collateral fiber stimulation. Blocking GABA(B) receptors abolished paired-pulse depression but had little effect on low-frequency stimulation (LFS)-induced depression. Instead, LFS-induced depression was significantly attenuated by an inhibitor of A1 type adenosine receptors. Blocking the postsynaptic effects of GABA(B) and A1 receptor activation on CA1 pyramidal cell excitability with an inhibitor of G-protein-activated inwardly rectifying potassium channels had no effect on either paired-pulse depression or LFS-induced depression. Thus activation of presynaptic GABA(B) and adenosine receptors appears to have an important role in activity-dependent depression at recurrent synapses. Together, our results indicate that CA3-CA1 and CA1-CA1 synapses exhibit strikingly different forms of short-term synaptic plasticity and suggest that activity-dependent changes in recurrent synaptic transmission can transform the CA1 region from a sparsely connected recurrent network into a predominantly feedforward circuit.

Cortico-motoneuronal system and dexterous finger movements

A recent paper in the Journal of Neurophysiology by [Sasaki et al. (2004)][1] reports that in macaque monkeys, skilled movements of the digits can survive a large unilateral lesion of the lateral corticospinal tract (LSCT) at the C5 level. This important report forces us to reconsider our views

Polysynaptic slow depolarization and spiking activity elicited after induction of long-term potentiation in rat auditory cortex

Polysynaptic activity was recorded in supragranular pyramidal neurons before and after the induction of long-term potentiation (LTP) in slices obtained from rat auditory cortex. LTP was induced by tetanic stimulation of layer IV. In the pyramidal neurons exhibiting LTP, repetitive stimulation at 50 Hz with 15 pulses triggered a slow 15–35 mV depolarization lasting 0.5–2 s with two to five spike discharges. There was no such response before the induction of LTP or in the neurons that did not exhibit LTP. Slow depolarization with spike discharges was blocked by an NMDA receptor antagonist but not by a metabotropic glutamate receptor antagonist. The reversal potential of the slow depolarization was approximately −7 mV and the membrane resistance decreased during slow depolarization, suggesting that the slow depolarization was produced by polysynaptic excitatory post-synaptic potentials. LTP was also induced by low frequency stimulation paired with a depolarizing current injection. In the pyramidal neurons exhibiting LTP after the paired stimulation, the slow depolarization amplitude was small and repetitive stimulation did not trigger spike discharges. Tetanic stimulation is expected to induce LTP in the polysynaptic neural circuits connecting many pyramidal neurons. The present findings suggest that polysynaptic activity can be generated in the potentiated neural circuits. Such activity might serve to read out the memory stored in polysynaptic neural circuits in the cerebral cortex.

Involvement of intracellular calcium and protein phosphatases in long-term depression of A-fiber-mediated primary afferent neurotransmission

Long-term depression (LTD) of monosynaptic and polysynaptic excitatory postsynaptic potentials (EPSPs) in substantia gelatinosa (SG) neurons can be induced by brief high-frequency electrical stimulation (HFS, 300 pulses at 100 Hz) of primary afferent fibers in dorsal roots [57]. Here we examined the possible cellular mechanism underlying spinal LTD. Conventional intracellular recordings were made from SG neurons in a transverse slice-dorsal root preparation of the young rat lumbar spinal cord. LTD of both monosynaptic and polysynaptic EPSPs was induced in 16 of 24 SG neurons by HFS of dorsal root in either the presence or absence of the GABA A receptor antagonist bicuculline and the glycine receptor antagonist strychnine. Loading the postsynaptic cell with BAPTA, an intracellular Ca 2+ chelator, almost completely blocked the induction of LTD. Induction of LTD was abolished by bath application of calyculin A (100 nM), a potent inhibitor of protein phosphatases 1 and 2A. These results indicate that: (i) a rise in postsynaptic Ca 2+ is necessary for LTD induction, (ii) synaptic activation of protein phosphatases 1 and 2A plays an important role in the induction of LTD of primary afferent A-fiber neurotransmission in the young rat spinal cord, and (iii) the effect of LTD may be physiologically relevant for transmission and integration of sensory information, including nociception.

Dual modulation of excitatory synaptic transmission by agonists at group I metabotropic glutamate receptors in the rat spinal dorsal horn

The effects of group I metabotropic glutamate (mGlu) receptors on excitatory transmission in the rat dorsal horn, but mostly substantia gelatinosa, neurons were investigated using conventional intracellular recording in slices. The broad spectrum mGlu receptor agonist (1 S,3 R)-1-aminocyclopentane-1,3-dicarboxylic acid (1 S,3 R-ACPD), the group I mGlu receptor selective agonist ( S)-3,5-dihydroxyphenylglycine (DHPG), and the selective mGlu subtype 5 agonist ( RS)-2-chloro-5-hydroxyphenylglycine (CHPG), all induce long-lasting depression of A primary afferent fibers-mediated monosynaptic excitatory postsynaptic potential (EPSP), and long-lasting potentiation of polysynaptic EPSP, and EPSP in cells receiving C-afferent fiber input. The DHPG potentiation of polysynaptic EPSP was partially or fully reversed by ( S)-4-carboxyphenylglycine ( S-4CPG), the mGlu subtype 1 preferring antagonist. 2-Methyl-6-(phenylethynyl)-pyridine, the potent and selective mGlu subtype 5 antagonist, partially reversed the CHPG potentiation of polysynaptic EPSP. The effects of DHPG on monosynaptic and polysynaptic EPSPs were reduced, or abolished, by the N-methyl- d-aspartate (NMDA) receptor antagonist d(−)-2-amino-5-phosphonopentanoic acid (AP5). A clear and pronounced facilitation of the expression of DHPG- and CHPG-induced enhancement of polysynaptic EPSP, and EPSP evoked at C-fiber strength, was seen in the absence of gamma-aminobutyric acid subtype A receptor- and glycine-mediated synaptic inhibition. Besides dual modulation of excitatory synaptic transmission, DHPG induces depression of inhibitory postsynaptic potentials evoked by primary afferent stimulation in dorsal horn neurons. In addition, group I mGlu receptor agonists produced a direct persistent excitatory postsynaptic effect consisting of a slow membrane depolarization, an increase in input resistance, and an intense neuronal discharge. Cyclothiazide and ( S)-4-CPG, the mGlu receptor subtype 1 preferring antagonists, significantly attenuated the DHPG-induced depolarization. These results demonstrate that the pharmacological activation of group I metabotropic glutamate receptors induces long-term depression (LTD) and long-term potentiation (LTP) of synaptic transmission in the spinal dorsal horn. These types of long-term synaptic plasticity may play a functional role in the generation of post-injury hypersensitivity (LTP) or antinociception (LTD).

Group II and group III metabotropic glutamate receptor agonists depress synaptic transmission in the rat spinal cord dorsal horn

The effects of group II and group III metabotropic glutamate receptor agonists on synaptic responses evoked by primary afferent stimulation in the dorsal horn, but mostly substantia gelatinosa, neurons were studied in the spinal cord slice preparation using conventional intracellular recording technique. Bath application of a potent metabotropic glutamate receptor 2- and 3-selective agonist (2S,1′R,2′R,3′R)-2-(2′,3′-dicarboxycyclopropyl) glycine reversibly suppressed monosynaptic and polysynaptic excitatory postsynaptic potentials evoked by A primary afferent fibers stimulation, the effect likely mediated by mGlu3 receptor subtype. This suppressing effect of (2S,1′R,2′R,3′R)-2-(2′,3′-dicarboxycyclopropyl) glycine on primary afferent neurotransmission was dose dependent and reduced by (S)-α-ethylglutamate, a group II metabotropic glutamate receptor antagonist. (2S,1′R,2′R,3′R)-2-(2′,3′-dicarboxycyclopropyl) glycine suppressed excitatory postsynaptic potentials without inducing detectable changes of postsynaptic membrane potential and neuronal input resistance in dorsal horn neurons. The paired-pulse depression at excitatory synapses between primary afferent fibers and dorsal horn neurons was reduced by (2S,1′R,2′R,3′R)-2-(2′,3′-dicarboxycyclopropyl) glycine application, suggesting a presynaptic site of action. The selective group III metabotropic glutamate receptor agonist (S)-2-amino-4-phosphonobutanoate also depressed A afferent fibers-evoked monosynaptic and polysynaptic excitatory postsynaptic potentials in a dose-dependent and reversible manner. The concentration-dependence of (S)-2-amino-4-phosphonobutanoate-mediated depression was most consistent with activation of mGlu receptor subtypes 4 and 7. However, on the basis of anatomical distribution of mGlu 4 and 7 subtypes, it is also possible that the (S)-2-amino-4-phosphonobatanoate effect is due to interaction with mGlu 7 receptor alone. (RS)-α-cyclopropyl-4-phosphonophenylglycine a preferential antagonist at group III metabotropic glutamate receptors, completely reversed the depressant effects of (S)-2-amino-4-phosphonobutanoate on both monosynaptic and polysynaptic responses. (S)-2-amino-4-phosphonobutanoate reduced the paired-pulse depression at excitatory synapses between primary afferent fibers and dorsal horn neurons, but did not alter their postsynaptic membrane potential and input resistance. A clear facilitation of the (S)-2-amino-4-phosphonobutanoate-induced depression of monosynaptic and polysynaptic excitatory postsynaptic potentials in the absence of gamma-aminobutyric acid-subtype A receptor- and glycine-mediated synaptic inhibition was shown. Besides the depressant effect on excitatory synaptic transmission, inhibitory actions of group II and III metabotropic glutamate receptor agonists on the inhibitory postsynaptic potentials evoked by primary afferent stimulation in dorsal horn neurons were observed.These results suggest that group II and group III metabotropic glutamate receptors are expressed at primary afferent synapses in the dorsal horn region, and activation of the receptors suppresses synaptic transmission by an action on the presynaptic site.

A spinal cord slice preparation for analyzing synaptic responses to stimulation of pelvic and pudendal nerves in mature rats

The dorsal commissural nucleus (DCN) in the lumbosacral spinal cord (L6–S1) receives primary afferent fibers from both pelvic and pudendal nerves in rats. However, the physiological and pharmacological properties of synaptic responses of the DCN neurons to stimulation of those nerves remain unclear. We have developed a longitudinal spinal cord (L6–S1) slice preparation from mature rats that retained both nerves attached. Blind whole-cell recordings were made from the DCN neurons in this preparation. In most neurons, mono- and/or poly-synaptic fast excitatory postsynaptic potentials (EPSPs) were evoked by electrical stimulation of either the pelvic or pudendal nerve. These EPSPs were mediated by activation of Aβ/Aδ and/or C fibers (conduction velocities, 0.5–17.3 m/s), and were abolished by CNQX. Fast EPSPs elicited by either pelvic or pudendal nerve stimulation were occasionally accompanied by bicuculline- and strychnine-sensitive IPSPs. In one-third of the neurons tested, mono- and/or poly-synaptic EPSPs were elicited by the stimulation of both the pelvic and pudendal nerves, indicating convergence of the visceral and somatic primary afferent inputs from the pelvic region onto the DCN neurons. The preparation is applicable to study the mechanism of the integration of the visceral and somatic inputs in the spinal cord.

Neuropeptide Y reduces epileptiform discharges and excitatory synaptic transmission in rat frontal cortex in vitro

Neuropeptide Y reduced spontaneous and stimulation-evoked epileptiform discharges in rat frontal cortex slices perfused with a magnesium-free solution and with the GABA A receptor antagonist picrotoxin. To investigate the mechanism of that action, effects of neuropeptide Y on intrinsic membrane properties and synaptic responses of layer II/III cortical neurons were studied using intracellular recording. Neuropeptide Y (1 μM) had no detectable effect on the membrane properties of neurons. The evoked synaptic potentials were attenuated by neuropeptide Y. Moreover, the pharmacologically isolated excitatory postsynaptic potentials, mediated by N-methyl- d-aspartate and non- N-methyl- d-aspartate receptors, were reversibly depressed by neuropeptide Y. The most pronounced inhibitory effect of neuropeptide Y was observed on late polysynaptic excitatory postsynaptic potentials. To assess a putative postsynaptic action of neuropeptide Y, N-methyl- d-aspartate was locally applied in the presence of tetrodotoxin. The N-methyl- d-aspartate-evoked depolarizations were unaffected by neuropeptide Y, which suggests that the depression of excitatory postsynaptic potentials was due to an action at sites presynaptic to the recorded neurons. These data show that neuropeptide Y attenuates epileptiform discharges and the glutamate receptor-mediated synaptic transmission in the rat frontal cortex. The above results indicate that neuropeptide Y may regulate neuronal excitability within the cortex, and that neuropeptide Y receptors are potential targets for an anticonvulsant therapy.

Norepinephrine facilitates inhibitory transmission in substantia gelatinosa of adult rat spinal cord (part 2): effects on somatodendritic sites of GABAergic neurons.

It has been reported previously that norepinephrine, when applied to the spinal cord dorsal horn, excites a subpopulation of dorsal horn neurons, presumably inhibitory interneurons. In the current study, the authors tested whether norepinephrine could activate inhibitory interneurons, specifically those that are "GABAergic." A transverse slice was obtained from a segment of the lumbar spinal cord isolated from adult male Sprague-Dawley rats. Whole-cell patch-clamp recordings were made from substantia gelatinosa neurons using the blind patch-clamp technique. The effects of norepinephrine on spontaneous GABAergic inhibitory postsynaptic currents were studied. In the majority of substantia gelatinosa neurons tested, norepinephrine (10-60 microM) significantly increased both the frequency and the amplitude of GABAergic inhibitory postsynaptic currents. These increases were blocked by tetrodotoxin (1 microM). The effects of norepinephrine were mimicked by the alpha1-receptor agonist phenylephrine (10-80 microM) and inhibited by the alpha1-receptor-antagonist WB-4101 (0.5 microM). Primary-afferent-evoked polysynaptic excitatory postsynaptic potentials or excitatory postsynaptic currents in wide-dynamic-range neurons of the deep dorsal horn were also attenuated by phenylephrine (40 microM). The observations suggest that GABAergic interneurons possess somatodendritic alpha1 receptors, and activation of these receptors excites inhibitory interneurons. The alpha1 actions reported herein may contribute to the analgesic action of intrathecally administered phenylephrine.

Electrophysiological properties and synaptic responses of cells in the trigeminal principal sensory nucleus of postnatal rats.

In the rodent brain stem trigeminal complex, select sets of neurons form modular arrays or "barrelettes," that replicate the patterned distribution of whiskers and sinus hairs on the ipsilateral snout. These cells detect the patterned input from the trigeminal axons that innervate the whiskers and sinus hairs. Other brain stem trigeminal cells, interbarrelette neurons, do not form patterns and respond to multiple whiskers. We examined the membrane properties and synaptic responses of morphologically identified barrelette and interbarrelette neurons in the principal sensory nucleus (PrV) of the trigeminal nerve in early postnatal rats shortly after whisker-related patterns are established. Barrelette cell dendritic trees are confined to a single barrelette, whereas the dendrites of interbarrelette cells span wider territories. These two cell types are distinct from smaller GABAergic interneurons. Barrelette cells can be distinguished by a prominent transient A-type K(+) current (I(A)) and higher input resistance. On the other hand, interbarrelette cells display a prominent low-threshold T-type Ca(2+) current (I(T)) and lower input resistance. Both classes of neurons respond differently to electrical stimulation of the trigeminal tract. Barrelette cells show either a monosynaptic excitatory postsynaptic potential (EPSP) followed by a large disynaptic inhibitory postsynaptic potential (IPSP) or just simply a disynaptic IPSP. Increasing stimulus intensity produces little change in EPSP amplitude but leads to a stepwise increase in IPSP amplitude, suggesting that barrelette cells receive more inhibitory input than excitatory input. This pattern of excitation and inhibition indicates that barrelette cells receive both feed-forward and lateral inhibition. Interbarrelette cells show a large monosynaptic EPSP followed by a small disynaptic IPSP. Increasing stimulus intensity leads to a stepwise increase in EPSP amplitude and the appearance of polysynaptic EPSPs, suggesting that interbarrelette cells receive excitatory inputs from multiple sources. Taken together, these results indicate that barrelette and interbarrelette neurons can be identified by their morphological and functional attributes soon after whisker-related pattern formation in the PrV.

Electrophysiological properties of the somatotopic organization of the vestibulospinal system in the frog

In experiments on the preparation of a frog perfused brain ( Rana ridibunda), field and intracellular potentials were recorded from neurons of the vestibular nuclear complex following stimulation of the ipsilateral vestibular nerve and different levels of the spinal cord. Stimulation of the vestibular nerve evoked mono- and polysynaptic excitatory postsynaptic potentials and orthodromic action potentials. In parallel, an antidromic activation of vestibular neurons sending their axons to the labyrinth was recorded. Vestibulospinal neurons sending their axons to the cervical (C neurons) and lumbar (L neurons) enlargements of the spinal cord were identified by their antidromic activation. A rather high conduction velocity along vestibulospinal fibres (mean 15.47 m/s) was observed. A somatotopic arrangement of the vestibulospinal system was established in spite of extremely large overlapping zones for the fore- and hindlimb representations in the vestibular nuclear complex. The hindlimbs were represented more poorly than the forelimbs. Antidromic potentials of C and L neurons were recorded in the medial, descending and with the highest density in the lateral vestibular nuclei (Deiters' nucleus). C neurons were evenly distributed in the other vestibular nuclei studied, while L neurons were located predominantly in the caudal parts of the vestibular nuclear complex. The multiplicity of the origin of the vestibulospinal axons was established. Peculiarities of the functional correlation between the vestibular input and vestibulospinal system are discussed.

Fast excitatory post synaptic potentials and their response to catecholaminergic antagonists in rat sympathetic preganglionic neurones in vitro

In an in vitro slice preparation from neonatal rats intracellular recordings were made from electrophysiologically identified sympathetic preganglionic neurones. Electrical stimulation in the lateral funiculus (>500 μm) from the recording site elicited a mono- or polysynaptic excitatory post synaptic potential. The latter potential was blocked with the dopamine D 2 antagonist haloperidol but not with the dopamine D 1 antagonist SCH 23390. We therefore report the first showing of a functional descending pathway in an in vitro slice preparation describing both the transmitter and the receptor subtype involved and physiologically show that dopamine may exert an indirect excitatory influence on sympathetic preganglionic neurones possibly via interneurones present in the spinal cord.

Synaptic inputs to stellate cells in the ventral cochlear nucleus.

Auditory information is carried from the cochlear nuclei to the inferior colliculi through six parallel ascending pathways, one of which is through stellate cells of the ventral cochlear nuclei (VCN) through the trapezoid body. To characterize and identify the synaptic influences on T stellate cells, intracellular recordings were made from anatomically identified stellate cells in parasagittal slices of murine cochlear nuclei. Shocks to the auditory nerve consistently evoked five types of synaptic responses in T stellate cells, which reflect sources intrinsic to the cochlear nuclear complex. 1) Monosynaptic excitatory postsynaptic potentials (EPSPs) that were blocked by 6,7-dinitroquinoxaline-2,3-dione (DNQX), an antagonist of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors, probably reflected activation by auditory nerve fibers. Electrophysiological estimates indicate that about five auditory nerve fibers converge on one T stellate cell. 2) Disynaptic, glycinergic inhibitory postsynaptic potentials (IPSPs) arise through inhibitory interneurons in the VCN or in the dorsal cochlear nucleus (DCN). 3) Slow depolarizations, the source of which has not been identified, that lasted between 0.2 and 1 s and were blocked by -2-amino-5-phosphonovaleric acid (APV), the N-methyl-D-aspartate (NMDA) receptor antagonist. 4) Rapid, late glutamatergic EPSPs are polysynaptic and may arise from other T stellate cells. 5) Trains of late glycinergic IPSPs after single or repetitive shocks match the responses of D stellate cells, showing that D stellate cells are one source of glycinergic inhibition to T stellate cells. The source of late, polysynaptic EPSPs and IPSPs was assessed electrophysiologically and pharmacologically. Late synaptic responses in T stellate cells were enhanced by repetitive stimulation, indicating that the interneurons from which they arose should fire trains of action potentials in responses to trains of shocks. Late EPSPs and late IPSPs were blocked by APV and enhanced by the removal of Mg2+, indicating that the interneurons were driven at least in part through NMDA receptors. Bicuculline, a gamma-aminobutyric acid-A (GABAA) receptor antagonist, enhanced the late PSPs, indicating that GABAergic inhibition suppresses both the glycinergic interneurons responsible for the trains of IPSPs in T-stellate cells and the interneuron responsible for late EPSPs in T stellate cells. The glycinergic interneurons that mediate the series of IPSPs are intrinsic to the ventral cochlear nucleus because long series of IPSPs were recorded from T stellate cells in slices in which the DCN was removed. These experiments indicate that T stellate cells are a potential source of late EPSPs and that D stellate cells are a potential source for trains of late IPSPs.

Diencephalic projection to reticulospinal neurons involved in the initiation of locomotion in adult lampreys Lampetra fluviatilis

Morphological and electrophysiological techniques were used to characterize a diencephalic projection from the ventral thalamus to reticulospinal neurons and its role in initiating rhythmic locomotor activity in the spinal cord of adult lampreys (Lampetra fluviatilis). Injection of fluorescein-coupled dextran amine (FDA) into the rhombencephalic reticular nuclei labeled neurons in the ventral thalamus region on both the ipsilateral side and the contralateral side. Injection of FDA into the ventral thalamus labeled axonal projections in all reticular nuclei, but no direct projections were found to the spinal cord. Extracellular stimulation of the ventral thalamus elicited monosynaptic excitatory postsynaptic potentials (EPSPs), polysynaptic EPSPs, and inhibitory postsynaptic potentials (IPSPs) in reticulospinal neurons in the posterior (prrn) and middle (mrrn) rhombencephalic reticular nuclei. The monosynaptic EPSPs were blocked by the glutamate antagonist kynurenic acid and can be considered glutamatergic. The monosynaptic EPSPs were potentiated (up to 12 minutes) following a brief high-frequency stimulation. Stimulation of the ventral thalamus induced rhythmic firing of reticulospinal neurons and elicited rhythmic burst activity in the spinal ventral roots. The projections from the ventral thalamus to the reticulospinal neurons in the prrn and mrrn thus provide excitatory inputs to the reticulospinal neurons, which, in turn, can activate the spinal circuits underlying locomotion. Also, the input nuclei to the ventral thalamus were labeled following injection of FDA into this nucleus. Labeled cells were found in the olfactory bulb, pallial areas, striatum, preoptic nucleus, hypothalamus, dorsal thalamus, optic tectum, and dorsal isthmic gray. The ventral thalamus, therefore, receives inputs from several different regions in the brain and controls the level of excitability in reticulospinal neurons. J. Comp. Neurol. 389:603–616, 1997. © 1997 Wiley-Liss, Inc.

Diencephalic projection to reticulospinal neurons involved in the initiation of locomotion in adult lampreysLampetra fluviatilis

Morphological and electrophysiological techniques were used to characterize a diencephalic projection from the ventral thalamus to reticulospinal neurons and its role in initiating rhythmic locomotor activity in the spinal cord of adult lampreys (Lampetra fluviatilis). Injection of fluorescein-coupled dextran amine (FDA) into the rhombencephalic reticular nuclei labeled neurons in the ventral thalamus region on both the ipsilateral side and the contralateral side. Injection of FDA into the ventral thalamus labeled axonal projections in all reticular nuclei, but no direct projections were found to the spinal cord. Extracellular stimulation of the ventral thalamus elicited monosynaptic excitatory postsynaptic potentials (EPSPs), polysynaptic EPSPs, and inhibitory postsynaptic potentials (IPSPs) in reticulospinal neurons in the posterior (prrn) and middle (mrrn) rhombencephalic reticular nuclei. The monosynaptic EPSPs were blocked by the glutamate antagonist kynurenic acid and can be considered glutamatergic. The monosynaptic EPSPs were potentiated (up to 12 minutes) following a brief high-frequency stimulation. Stimulation of the ventral thalamus induced rhythmic firing of reticulospinal neurons and elicited rhythmic burst activity in the spinal ventral roots. The projections from the ventral thalamus to the reticulospinal neurons in the prrn and mrrn thus provide excitatory inputs to the reticulospinal neurons, which, in turn, can activate the spinal circuits underlying locomotion. Also, the input nuclei to the ventral thalamus were labeled following injection of FDA into this nucleus. Labeled cells were found in the olfactory bulb, pallial areas, striatum, preoptic nucleus, hypothalamus, dorsal thalamus, optic tectum, and dorsal isthmic gray. The ventral thalamus, therefore, receives inputs from several different regions in the brain and controls the level of excitability in reticulospinal neurons.

Cerebellar input to corticothalamic neurons in layers V and VI in the motor cortex

To investigate whether corticothalamic (CT) neurons in the motor cortex (Mx) receive cerebellar input via the ventroanterior–ventrolateral nucleus of the thalamus (VA–VL), we recorded intracellular potentials from neurons in the Mx of anesthetized cats and examined effects of stimulation of the VA–VL and the brachium conjunctivum on them. After this electrophysiological identification, horseradish peroxide (HRP) was injected iontophoretically into the recorded neurons for morphological analysis. We identified 34 neurons as CT neurons by their antidromic response to stimulation of the VA–VL, of which 13 were layer VI CT neurons and 21 were layer V CT neurons. A majority of the CT neurons of both layers VI and V received monosynaptic excitatory postsynaptic potentials (EPSPs) from the VA–VL and di- or polysynaptic EPSPs from the cerebellum. The laminar distribution and morphological characteristics of single CT neurons receiving cerebellar input were analyzed on 19 HRP-labeled CT neurons. Eight layer V and six layer VI CT neurons were reconstructed from serial sections. All the reconstructed layer VI CT neurons were modified pyramidal neurons whose apical dendrites ended in layer III or V, and all the stained layer V CT neurons were typical pyramidal neurons, although the laminar and tangential distribution of recurrent collaterals of these neurons varied from neuron to neuron.