Synaptic actions of peripheral nerve impulses upon Deiters neurones via the climbing fibre afferents

1. The cerebellar integration of sensory inputs to Deiters neurones was studied in cats under Nembutal anaesthesia.2. Stimulation of peripheral nerves produced in the Deiters neurones a sequence of an initial excitatory post-synaptic potential (e.p.s.p.) and a later inhibitory post-synaptic potential (i.p.s.p.), or a relatively small e.p.s.p.3. The Deiters neurones were classified as forelimb (FL)- or hind limb (HL)-type cells according to the location of the most effective peripheral nerve. In the FL cells stimulation of the forelimb nerves produced the e.p.s.p.-i.p.s.p. sequence (dominant response), while stimulation of the hind limb nerves was ineffective or produced the small e.p.s.p. (non-dominant response). In contrast, in the HL cells the non-dominant response was evoked from the forelimb nerves, and the dominant response from the hind limb nerves.4. The stimulus intensity-response relation indicates that Group I and II muscle afferents and low and high threshold cutaneous afferents contribute to the dominant and non-dominant responses.5. Antidromic identification of these Deiters neurones revealed that 90% of the HL cells and 85% of the FL cells project to the lumbo-sacral and cervico-thoracic segments of the spinal cord, respectively, while 10% of the HL cells and 15% of the FL cells innervate the cervico-thoracic and lumbo-sacral segments, respectively.6. The mean latency of the e.p.s.p. evoked from the forelimb nerves was 14 msec in the FL cells and 13 msec in the HL cells, and the latency of the e.p.s.p. evoked from the hind limb nerves was 17 msec in the FL cells and 18 msec in the HL cells. The later i.p.s.p. regularly followed the onset of the e.p.s.p. with a delay of 3-5 msec.7. The dominant and non-dominant responses in both types of cells exhibited the following three characteristic features: (i) a strong depression after conditioning stimulation of the inferior olive, (ii) an increase of the inferior olivary excitability during the responses, and (iii) a striking frequency depression with stimulation at relatively low frequency (5-10/sec).8. Consequently it was concluded that all of the responses were produced through the climbing fibres originating from the inferior olive, the i.p.s.p.s due to inhibition from Purkyne cells activated by the climbing fibres and the e.p.s.p.s due to excitation from the collaterals of the climbing fibres.     

Synaptic action of the fastigiobulbar impulses upon neurones in the medullary reticular formation and vestibular nuclei

Summary In anaesthetized cats, the fastigial nucleus of cerebellum was stimulated with electric pulse currents, and the effects thereby induced were investigated by recording intracellularly from cells in the medullary reticular formation, the nucleus of Deiters and the descending vestibular nucleus. The early effect commonly seen in these cells was initiation of excitatory postsynaptic potentials (EPSPs) with monosynaptic latencies from both sides of the fastigial nuclei. These EPSPs appeared to be produced in part by a kind of axon reflex through cerebellar afferent fibres, but a certain portion of them was ascribable to the crossed fastigiobulbar axons, as they were influenced by stimulation of the cerebellar cortex in the manner to be expected from the previous study on cerebellar nuclei. These EPSPs were followed by a sequence of a prolonged disfacilitatory hyperpolarization and a late facilitatory depolarization, which apparently reflected the inhibition and disinhibition, respectively, produced in fastigial neurones via Purkinje cell axons of the corticonuclear projection. Either EPSPs or IPSPs were also induced in both reticular and vestibular neurones through polysynaptic pathways in which the fastigiobulbar projection might have been involved.     

Contribution of mossy fiber-granule cell pathway to the cerebellar-induced delayed inhibition in Deiters neurones

Summary In anaesthetized cats, electric pulse stimuli were applied at various lateralities to the anterior lobe of the cerebellum. In dorsal Deiters neurones delayed IPSPs with latencies of 3–6 msec were evoked from the entire area of the culmen including the paravermis bilaterally. The delayed IPSPs had a summit time of about 2 msec and a duration of about 7 msec. They showed a marked temporal facilitation and subsequent depression with double shock stimulation. Corticovestibular fibers were penetrated within the nucleus of Deiters and showed delayed, labile responses to cortical stimulation, corresponding to the delayed IPSPs in Deiters neurones. During stimulation of the anterior lobe at any laterality, field potentials recorded in the cerebellar cortex further revealed that there was activation, presumably through axon collaterals of mossy fibers, of granule cells and subsequently of Purkinje cells in the vermal cortex. Cortical events exhibited a prominent temporal facilitation and subsequent depression, in parallel with that observed for the delayed IPSPs in Deiters neurones. The delayed IPSPs in Deiters neurones arising from a wide area of the cerebellar cortex thus were attributed to activation through mossy fiber-granule cell pathway of Purkinje cells of the corticovestibular projection.     

Axon reflex activation of Deiters neurones from the cerebellar cortex through collaterals of the cerebellar afferents

Summary When recording intracellularly from cat's Deiters neurones, stimulation of the anterior lobe of the cerebellar cortex produced excitatory postsynaptic potentials (EPSPs) monosynaptically, in addition to the inhibitory ones (IPSPs) that were identified previously as being produced via Purkinje cell axons. The EPSPs were induced bilaterally from a wide area of the anterior and posterior lobes of the cerebellum, in contrast to the IPSPs that were evoked only ipsilaterally, mainly from the vermal cortex. The latency of the EPSPs was slightly, but significantly, shorter than that of the IPSPs. The presynaptic impulses responsible for these EPSPs were represented by the discrete field potentials and also by unit spikes of individual fibres. The pathway for these EPSPs and presynaptic impulses was pursued by testing their interference, in the manner of impulse collision and refractoriness, with those induced from various spots within or outside the cerebellum. It is found that the excitatory fibres for Deiters neurones extend transversely, and probably longitudinally too, over the culmen and pass out of the cerebellum through cerebellar peduncles. The major portion of them appears to originate from the medulla and a minority from the spinal cord. It is postulated that cerebellar afferents from these structures have synapses with Deiters neurones via their collateral branches, through which a kind of axon reflex occurs to Deiters neurones during stimulation of the cerebellar cortex.     

Electrophysiological analysis of the vestibulospinal reflex pathway of rabbit. II. Synaptic actions upon spinal neurones

Summary In anaesthetized rabbits, the effects of VIIIth nerve stimulation were studied in the cervicothoracic segments of the spinal cord by recording spike field potentials representing the secondary vestibular volleys, postsynaptic potentials in spinal neurones and corresponding field potentials. It was revealed that an excitatory action was conveyed by the medial vestibulospinal tract (MVST) fibers bilaterally and also by the lateral vestibulospinal tract (LVST) fibers ipsilaterally. Both of these two excitatory fiber groups conducted the secondary vestibular volleys with relatively fast velocities. An inhibitory action was effected bilateraly by those MVST fibers which conducted secondary vestibular volleys with a relatively slow velocity, as previously described in oats. By direct stimulation of the medulla it was demonstrated that both excitatory and inhibitory MVST fibers arise from the middle portion of the vestibular nuclear complex in and around Deiters' nucleus.     

Synaptic actions of the ventral root afferents on cat hindlimb motoneurons

Postsynaptic potentials (PSPs) with long latencies were evoked in cat hindlimb motoneurons by stimulation of the distal stump of a cut ventral root. Measurements of their latency and the threshold in the responsible afferent fibers showed that they were produced mainly by the activity of the unmyelinated fibers in the ventral root that enter the spinal cord through the dorsal root. Patterns of PSPs evoked in flexor and extensor motoneurons by ventral root stimulation were similar to those observed in the flexion reflex.     

Synaptic actions of individual vestibular neurones on cat neck motoneurones

1. Unitary synaptic potentials evoked by the activity of single vestibulocollic neurones were recorded by means of spike-triggered signal averaging in neck extensor motoneurones of decerebrate cats. Properties of the vestibulocollic neurones which produced the potentials were examined.2. Vestibulocollic neurones were first identified as projecting to the C3 grey matter by antidromic microstimulation within the C3 extensor motoneurone pool. The spontaneous or glutamate-driven activity of the vestibulocollic neurones was then used to trigger the averaging computer. In this way ten inhibitory and two excitatory neurones were identified (20% of neurones tested).3. Action potentials in local branches of vestibulocollic neurones were usually recorded in the vicinity of motoneurones. Mean orthodromic conduction time from the foot of the extracellular spike, recorded in the vestibular nuclei, that triggered the averager was 0.72 msec. Mean synaptic delay was 0.4 msec.4. I.p.s.p.s had a mean time to peak of 0.81 msec and were readily reversed by injection of hyperpolarizing current. These data, together with the shape indices of i.p.s.p.s indicate that they are generated proximally on motoneurones.5. All vestibulocollic neurones making synapses with motoneurones were monosynaptically driven by stimulation of the ipsilateral vestibular nerve. Four out of seven tested were inhibited by stimulation of the contralateral vestibular nerve (commissural inhibition).6. Two excitatory neurones were located in Deiters' nucleus or on the Deiters'-descending border. Inhibitory neurones were found relatively medially in the vestibular complex in the medial, descending and Deiters' nuclei.7. Vestibulocollic neurones acting on motoneurones were tested for axon branching to more caudal levels of the spinal cord with electrodes placed at C5-7. Both of the excitatory and two out of nine inhibitory neurones branched.     

The pharmacology of the inhibition of dorsal horn neurones by impulses in myelinated cutaneous afferents in the cat

The effects of microelectrophoretic strychnine and bicuculline methochloride were studied on the time course of synaptic inhibitions of single dorsal horn neurones in the lumbar spinal cord of cats anaesthetized with pentobarbitone. The inhibitions, evoked by volleys in mixed myelinated cutaneous afferents, varied in latency and duration. In general, early inhibitions (latency less than 12msec; duration less than 36 msec) were reduced by microelectrophoretic strychnine whereas late inhibitions (latency more than 16 msec and more prolonged in duration) were usually sensitive to bicuculline. These results can be interpreted in terms of glycine and GABA as the inh ibitory transmitters of early and late inhibitions respectively.     

Synaptic organization of eighth nerve afferents to cat dorsal cochlear nucleus

The synaptic organization of eighth nerve afferents to the dorsal cochlear nucleus (DCN) of cats was studied using extracellular field potential analyses. The eighth nerve was electrically stimulated and potentials produced in the DCN characterized using single shocks, paired shocks, repetitive stimulation, and one-dimensional current source-density (CSD) analysis. Field potentials and CSD profiles were correlated with the laminated cytoarchitecture of the DCN. At least four major temporally discrete components can be identified in field potentials evoked by a single shock to the eighth nerve. The amplitude and polarity of these events depends on the layers in which they are recorded. A brief positive-negative deflection (the P1-N1) is present in all layers but is maximal in the deeper layers 3 and 4. The N1 has a peak latency of approximately 0.8 ms in these layers. The N1 in the deep layers is followed by a large negative potential, termed the N2, with a peak latency of about 1.6 ms. In the superficial layers (1 and 2), the N1 is followed by a small positive potential (the P2) occurring nearly simultaneously with the N2, Immediately following the N2 is another negative potential that is most clearly observed in layer 2. The layer 2 negative wave is termed N3 and is also identifiable on the repolarizing phase of the N2 in layers 3 and 4. The N3 in layer 2 can be followed by a positive potential, the P4. Simultaneous with the P4 is a small negative wave ion layer 1, termed the N4. The P4-N4 complex is observed in about half the recordings. Frequency-following tests indicate that both the N1 and N2 waves can follow shock trains up to 333 Hz. The N1 remains nearly constant in amplitude up to about 300 Hz and decreases as the stimulus frequency is raised to 500 Hz. The N2 decrements more rapidly than the N1 at frequencies above about 250 Hz and is considerably reduced at 500 Hz. The N2 sometimes shows an increased amplitude (by about 20-40%) between 100 and 250 Hz. A paired-shock paradigm was used to characterize further potentials. The N1 was little affected by prior stimulation for intervals greater than 5 ms. The N2 and N3 generally showed a small facilitation for shock intervals from about 7 to 30 ms, with a return to base line at longer intervals. The N4 and P4 (when present) were profoundly depressed for intervals from 7 to about 30 ms, with recovery to control values by 50 ms.(ABSTRACT TRUNCATED AT 400 WORDS)     

Nociceptive afferents selectively modulate the cardiac component of the peripheral chemoreceptor reflex via actions within the solitary tract nucleus

Our previous findings showed that the nucleus of the solitary tract (NTS) mediated part of the tachycardia evoked during somatic noxious stimulation. Here, we investigated the interaction between somatic nociceptor- and peripheral chemoreceptor-evoked cardiac changes. We sought to determine whether this interaction occurred within the NTS, the primary site of termination of chemoreceptor afferents. In a working heart-brainstem preparation of rat, mechanical noxious activation of a forelimb evoked a tachycardia of 17.5+/-3 (mean+/-S.E.M.) b.p.m., whereas sodium cyanide (7-30 microg) stimulation of peripheral chemoreceptors produced a sub-maximal bradycardia of -140+/-15 b.p.m. During nociceptor stimulation the sodium cyanide-evoked bradycardia was attenuated to -42.6+/-12 b.p.m. but could be prevented by a multiple bilateral NTS microinjection of bicuculline (i.e. -173+/-18 b.p.m.). Furthermore, the activity of NTS neurones responding to peripheral chemoreceptor stimulation increased from 2.8+/-1.3 to 9.4+/-1.9 Hz during sodium cyanide injection (n=7; P<0.01). The latter response was attenuated reversibly to 2.9+/-0.9 Hz during simultaneous stimulation of the brachial nerve. Pressure ejection of bicuculline abolished this inhibitory action of brachial-nerve stimulation on the chemoreceptor-evoked excitatory synaptic response. We conclude that somatic noxious stimulation attenuates the chemoreceptor reflex-evoked bradycardia via a GABA(A)ergic mechanism in the NTS.     

The effects of depolarization of motor nerve terminals upon the release of transmitter by nerve impulses

1. Depolarizing currents were applied to motor nerve terminals in the rat phrenic nerve-diaphragm muscle preparation in vitro.2. During the passage of depolarizing currents the amplitude of the presynaptic nerve action potentials and of end-plate potentials (e.p.p.s) was reduced in proportion to the current strength.3. The reduction in e.p.p. amplitudes was shown to be due to a reduction in the number of quanta released.4. An excess of Mg ions or the previous application of a hyperpolarizing current could prevent the reduction of e.p.p. amplitudes and quantal contents by depolarizing current.5. Depolarizing current application prevented later hyperpolarizing currents affecting e.p.p. amplitudes.     

Further study on pharmacological properties of the cerebellar-induced inhibition of Deiters neurones

Summary The pharmacological properties of Deiters neurones were studied in anaesthetized cats, together with their inhibition by cerebellar Purkinje cells. Purkinje cell inhibition, as detected by depression of antidromic field potentials of Deiters neurones, was blocked by relatively large doses of picrotoxin administered intravenously (5–10 mg/kg). The cerebellar inhibition was also detected as a depression of the discharge of Deiters neurones excited by electrophoretic administration of DL-homocysteic acid. This inhibition was abolished or reduced by picrotoxin, but was not altered by strychnine, administered either systemically or electrophoretically. -Aminobutyric acid (GABA), glycine, -alanine and imidazole acetic acid, when administered electrophoretically, also depressed the antidromic field potentials and the spike discharges of Deiters neurones. Picrotoxin antagonized the effect of GABA but not of glycine, while strychnine blocked only the action of glycine. Intracellular recording from Deiters neurones revealed that both GABA and glycine, ejected extracellularly, hyperpolarized the membrane and increased the membrane conductance. The reversal of the GABA-induced hyperpolarization was at the same potential level as that of the inhibitory postsynaptic potentials induced by cerebellar stimulation. These results are consistent with the hypothesis that the inhibitory transmitter released from Purkinje cell axons is GABA or a related substance.     

Synaptic activation of N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors in the mossy fibre pathway in adult and immature rat cerebellar slices

The participation of excitatory amino acid receptors in mossy fibre-granule cell synapses in lobule VIa of adult and immature rat cerebellar slices was investigated using an extracellular grease-gap technique. For the immature slices, the age selected (14 days after birth) was one at which the sensitivity of granule cells to exogenous N-methyl-D-aspartate is much higher than in the adult. The principal synaptic potentials observed after low-frequency electrical stimulation of the white matter resembled closely those found to be centred in the granule cell layer in field potential studies in the cat in vivo. They comprised a short latency negative potential, a slow negative wave and, in the adult, a further late negative wave. In the adult, with 1.2 mM Mg2+ in the perfusing solution, none of these potentials was significantly affected by the N-methyl-D-aspartate antagonist, 2-amino-5-phosphonovalerate, but they were all markedly inhibited by the broad spectrum antagonist, kynurenate, and, more potently, by the selective non-N-methyl-D-aspartate receptor blocker, 6-cyano-2,3-dihydroxy-7-nitro-quinoxaline. After removal of Mg2+, which has a blocking action on the ion channels associated with N-methyl-D-aspartate receptors, the size of all the potentials increased. The increase in the short latency potential was insensitive to 2-amino-5-phosphonovalerate but a component of the slow negative wave (and of the late negative wave) was reduced back to control levels by the antagonist. Application of 6-cyano-2,3-dihydroxy-7-nitro-quinoxaline (10 microM) in Mg2+-free solution revealed, in near isolation, a slow wave (latency to peak, 28 ms) which could be abolished by 2-amino-5-phosphonovalerate. In the immature slices, bathed in normal (Mg2+-containing) medium, 2-amino-5-phosphonovalerate caused a small reduction in the short latency potential and inhibited a component of the slow negative wave which could, again, be observed in relative isolation after perfusion of 6-cyano-2,3-dihydroxy-7-nitro-quinoxaline. Removal of Mg2+ increased the amplitudes of the short latency potential and the slow negative wave in a manner which was sensitive to 2-amino-5-phosphonovalerate and increased the size of the slow, 6-cyano-2,3-dihydroxy-7-nitro-quinoxaline-resistant wave. It is concluded that glutamate is likely to be the transmitter released by mossy fibres, at least those innervating lobule VIa.(ABSTRACT TRUNCATED AT 400 WORDS)     

Synaptic extensions from the mossy fibers of the fascia dentata

Summary In the hilar region of the rat hippocampus, filamentous extensions have been observed to originate from the en passant synaptic expansions on the so-called mossy fibers, which are the axons of the dentate granule cells. These extensions range in length from about 1 m to 30 m, are often branched, and appear to contact the processes of various cell types in the hilar region. In 28-day-old rats, there are between 4 and 9 such extensions from most mossy fiber expansions, and the total length of the extensions from any one expansion is on the order of 75 m. Analysis of serial electron micrographs through normal and Golgi-impregnated mossy fibers has confirmed that these extensions are, indeed, presynaptic processes. Each contains one or more vesicle-rich foci along its length, and is associated with asymmetric membrane specializations. At these sites, the extensions are in synaptic contact with dendrites and dendritic spines of, as yet, unknown origin. A quantitative analysis of these extensions in Golgi material from rats at different ages indicates that they reach their greatest length around 14 days and then decline to adult values by 28 days.This work was supported in part by grant NS-10943 and was carried out while the author was in receipt of a U.S. Public Health Service post-doctoral fellowship (F32 NSO 5765-02)