The enhancement of the reflex reactions of the rat spinal cord after ablation of the cerebellum]

The character of evoked monosynaptic responses of spinal cord 3 weeks after total or partial ablation of cerebellum was studied on white rats. There was a substantial increase of amplitude of this response on the side of hemicerebellectomy and of animals with total cerebellectomy. In acute experiments cutting of spinal cord does not remove hyperreflexia.     

Effect of various neurotropic drugs on chronic hyperreflexia in rats after cordotomy]

Substances modulating calcium permeability of cell membrane (verapamil imidazol, 4-aminopyridine), decreasing motor activity (meprobamate) and blocking adrenoreceptors (clophelin, propranolol, droperidol, aminazine++ have been studied for their action on the monosynaptic discharge of the ventral roots (MD VR) reinforced due to chronic cutting of the spinal cord. It is found that verapamil and meprobamate depress more strongly MD VR of rats with chronic cutting of the spinal cord than of those with the acute one. Imidazol and 4-aminopyridine reinforcing MD VR of rats with acute cutting of the spinal cord have no influence on the analogous index of rats with chronic cutting of the spinal cord. Adrenoblockers do not change the amplitude of MD VR in both groups of the animals.     

Presynaptic inhibition of spinal α motoneurons in humans adapted to different types of motor activity

Changes in the presynaptic inhibition of spinal α motoneurons were studied in athletes during motor activities of different types. In the state where muscles were at relative rest, the presynaptic inhibition of spinal α motoneurons of the m. soleus was stronger in samboists (athletes specializing in the martial art of sambo) and sprinters than in long-distance runners. In samboists performing repeated static efforts, the presynaptic inhibition of spinal α motoneurons became stronger from one trial to the next. Both technique training and strength training enhanced the presynaptic inhibition of spinal α motoneurons, this enhancement being greater after strength training.     

Measuring Spinal Presynaptic Inhibition in Mice By Dorsal Root Potential Recording In Vivo

Presynaptic inhibition is one of the most powerful inhibitory mechanisms in the spinal cord. The underlying physiological mechanism is a depolarization of primary afferent fibers mediated by GABAergic axo-axonal synapses (primary afferent depolarization). The strength of primary afferent depolarization can be measured by recording of volume-conducted potentials at the dorsal root (dorsal root potentials, DRP). Pathological changes of presynaptic inhibition are crucial in the abnormal central processing of certain pain conditions and in some disorders of motor hyperexcitability. Here, we describe a method of recording DRP in vivo in mice. The preparation of spinal cord dorsal roots in the anesthetized animal and the recording procedure using suction electrodes are explained. This method allows measuring GABAergic DRP and thereby estimating spinal presynaptic inhibition in the living mouse. In combination with transgenic mouse models, DRP recording may serve as a powerful tool to investigate disease-associated spinal pathophysiology. In vivo recording has several advantages compared to ex vivo isolated spinal cord preparations, e.g. the possibility of simultaneous recording or manipulation of supraspinal networks and induction of DRP by stimulation of peripheral nerves.     

Impaired Excitatory Drive to Spinal Gabaergic Neurons of Neuropathic Mice

Adequate pain sensitivity requires a delicate balance between excitation and inhibition in the dorsal horn of the spinal cord. This balance is severely impaired in neuropathy leading to enhanced pain sensations (hyperalgesia). The underlying mechanisms remain elusive. Here we explored the hypothesis that the excitatory drive to spinal GABAergic neurons might be impaired in neuropathic animals. Transgenic adult mice expressing EGFP under the promoter for GAD67 underwent either chronic constriction injury of the sciatic nerve or sham surgery. In transverse slices from lumbar spinal cord we performed whole-cell patch-clamp recordings from identified GABAergic neurons in lamina II. In neuropathic animals rates of mEPSC were reduced indicating diminished global excitatory input. This downregulation of excitatory drive required a rise in postsynaptic Ca²⁺. Neither the density and morphology of dendritic spines on GABAergic neurons nor the number of excitatory synapses contacting GABAergic neurons were affected by neuropathy. In contrast, paired-pulse ratio of Aδ- or C-fiber-evoked monosynaptic EPSCs following dorsal root stimulation was increased in neuropathic animals suggesting reduced neurotransmitter release from primary afferents. Our data indicate that peripheral neuropathy triggers Ca²⁺-dependent signaling pathways in spinal GABAergic neurons. This leads to a global downregulation of the excitatory drive to GABAergic neurons. The downregulation involves a presynaptic mechanism and also applies to the excitation of GABAergic neurons by presumably nociceptive Aδ- and C-fibers. This then leads to an inadequately low recruitment of inhibitory interneurons during nociception. We suggest that this previously unrecognized mechanism of impaired spinal inhibition contributes to hyperalgesia in neuropathy.     

The effect of DOPA on the reciprocal inhibition of the innervation centers of antagonistic muscles in newborn rat pups]

In experiments on 5--7- and 16-day rat puppies with acute lesion of the spinal cord, by means of monosynaptic tests, studies have been made of the effect of DOPA on reciprocal inhibition of antagonist muscles. Test stimuli were applied to n. tibialis, conditioned ones--to n. peroneus. It was shown that the pattern of the effects depends on the action of the drug on the magnitude of the initial monosynaptic reflex used as a test. It the latter was initially inhibited, the conditioned stimulation resulted within the first 1-8 msec not in the development of postsynaptic inhibition, but in evident facilitation, which was longer in 5--7-day animals. The level of presynaptic inhibition was somewhat lower than the initial one, but exhibited longer duration. In case of facilitation of a control test after DOPA injection, configuration of reciprocal inhibition curves did not significantly differ from that obtained before administration of the drug.     

In vitro studies of the effects of naloxone on the root potentials in the frog spinal cord: enkephalin-like effect on the recurrent presynaptic inhibition

Specific binding of [3H]naloxone was demonstrated in the frog spinal cord. In isolated and perfused frog spinal cord, naloxone increased the spontaneous discharges of the ventral root. Naloxone decreased the ventral root-dorsal root potential (VR-DRP) in a dose-dependent manner, and inhibited presynaptic inhibition of the ventral root reflex. Methionine-enkephalin also decreased the VR-DRP, and naloxone partially antagonized this effect. These results suggest the existence of enkephalinergic control of spinal motor activities and that naloxone has a partial agonistic effect in the frog spinal cord.     

Effect of Pressure on the Release of Radioactive Glycine and γ-Aminobutyric Acid from Spinal Cord Synaptosomes

Exposure to high hydrostatic pressure produces neurological changes referred to as the high-pressure nervous syndrome (HPNS). Manifestations of HPNS include tremor, EEG changes, and convulsions. These symptoms suggest an alteration in synaptic transmission, particularly with inhibitory neural pathways. Because spinal cord transmission has been implicated in HPNS, this study investigated inhibitory neurotransmitter function in the cord at high pressure. Guinea pig spinal cord synaptosome preparations were used to study the effect of compression to 67.7 atmospheres absolute on [3H]glycine and [3H]gamma-aminobutyric acid ([3H]GABA) release. Pressure was found to exert a significant suppressive effect on the depolarization-induced calcium-dependent release of glycine and GABA by these spinal cord presynaptic nerve terminals. This study suggests that decreased tonic inhibitory regulation at the level of the spinal cord contributes to the hyperexcitability observed in animals with compression to high pressure.     

Inhibition in the lamprey spinal cord]

Glycine and GABA play the role of inhibitory transmitters in the lamprey spinal cord. The mechanisms of action of both amino acids to the membrane receptors producing the postsynaptic inhibition as well as role and mechanism of GABA action producing the presynaptic inhibition are considered in this paper. The data concerned with morphological substrates of both type inhibitions are discussed.     

Alpha adrenergic receptor mediated formation of cyclic AMP in rat spinal cord

Methoxamine and phenylephrine (PE), postsynaptic alpha adrenergic agonists stimulated the accumulation of cyclic AMP in spinal cord tissue slices. Naphazoline, oxymetazoline and clonidine, previously shown to have greater efficacy at presynaptic alpha receptors did not alter accumulation and, in fact, blocked the PE response. The PE-stimulation was completely inhibited by postsynaptic alpha antagonists, incompletely by agents which bl ock presynaptic alpha receptors, and slightly by the beta blocker propranolol. Pe-stimulated accumulation was potentiated by phosphodiesterase inhibition (RO 20-1724). In contrast to previous reports on the requirement of the copresence of adenosine for alpha receptor stimulated accumulation of cyclic AMP in neuronal tissue, the PE-stimulation in spinal cord slices was unchanged by adenosine receptor blockade (theophylline), hydrolysis of endogenous adenosine (adenosine deaminase), inhibition of adenosine deaminase (EHNA) or blockade of adenosine uptake (dipyridamole). Added adenosine increased basal accumulation and produced a marked potentiation of the PE response. From this data it is evident that, in spinal cord tissue slices, there occurs a postsynaptic alpha adrenergic receptor linked to cyclic AMP accumulation which does not require the presence of other neurohumoral agents for activation.     

Functional organization of mechanisms of presynaptic inhibition induced by stimulation of cutaneous afferents

Changes in the N₁-component and P-phase of the dorsal surface potential (DSP) of the spinal cord evoked by test stimulation of the posterior tibial nerve after conditioning stimulation of the sural nerve were investigated in anesthetized cats. The test responses were inhibited if stimulation was applied at short intervals. They then recovered to some extent, but after 1.8–2.2 msec, a further prolonged period of inhibition began. The initial inhibition was connected with occlusion of synaptic action, and the subsequent prolonged inhibition with the development of presynaptic inhibition. The latent periods of prolonged inhibition of the N₁-component and P-phase of the DSP (2 msec) were almost exactly identical, and the curves showing the diminution of the initial occlusion of these components were very similar. The results demonstrate that presynaptic inhibition of the interneurons generating the N₁-component of the DSP and of cells of the substantia gelatinosa which participate in depolarization of the presynaptic terminals of the cutaneous afferents is due to the action of depolarizing systems with similar temporal characteristics.Dnepropetrovsk State University. Translated from Neirofiziologiya, Vol. 4, No. 5, pp. 510–515, September–October, 1972.     

The Beneficial Effects of Treadmill Step Training on Activity-Dependent Synaptic and Cellular Plasticity Markers After Complete Spinal Cord Injury

Several studies have shown that treadmill training improves neurological outcomes and promotes plasticity in lumbar spinal cord of spinal animals. The morphological and biochemical mechanisms underlying these phenomena remain unclear. The purpose of this study was to provide evidence of activity-dependent plasticity in spinal cord segment (L5) below a complete spinal cord transection (SCT) at T8-9 in rats in which the lower spinal cord segments have been fully separated from supraspinal control and that subsequently underwent treadmill step training. Five days after SCT, spinal animals started a step-training program on a treadmill with partial body weight support and manual step help. Hindlimb movements were evaluated over time and scored on the basis of the open-field BBB scale and were significantly improved at post-injury weeks 8 and 10 in trained spinal animals. Treadmill training also showed normalization of withdrawal reflex in trained spinal animals, which was significantly different from the untrained animals at post-injury weeks 8 and 10. Additionally, compared to controls, spinal rats had alpha motoneuronal soma size atrophy and reduced synaptophysin protein expression and Na(+), K(+)-ATPase activity in lumbar spinal cord. Step-trained rats had motoneuronal soma size, synaptophysin expression and Na(+), K(+)-ATPase activity similar to control animals. These findings suggest that treadmill step training can promote activity-dependent neural plasticity in lumbar spinal cord, which may lead to neurological improvements without supraspinal descending control after complete spinal cord injury.     

Presynaptic inhibition of segmental interneurons

In experiments on spinal cats changes in the second negative postsynaptic component (N₂) of the dorsal surface potential (DSP) of the spinal cord recorded in the region of segment L7 was used as the index of inhibition of segmental dorsal horn interneurons. Conditioning and testing stimuli were applied at increasing time intervals to the popliteal and superficial peroneal nerves respectively. Changes in the N₂ component were compared with changes in the N₁ component of the DSP, reflecting mainly activity of nonsegmental ascending dorsal horn interneurons. After an initial short facilitation a conditioning volley of pulses evokes prolonged (over 500 msec) inhibition of the N₂ component, characterized by the presence of two maxima (on the average at the 16th and 80th milliseconds) which indicate that two systems with different latent periods play a role in this inhibition. In its shape and temporal characteristics the curve of inhibition of the N₂ component corresponds to the two-component dorsal root potential (DRP) recorded in spinal animals in response to stimulation of flexor afferents (FRA) [8, 19]. Together with other features, this similarity is evidence of the presynaptic nature of this inhibition. Intravenous injection of hexobarbital has a stronger action on inhibition of the N₂ component, leading to a marked increase in its depth and duration. Suggestions are made regarding the functional organization of systems responsible for presynaptic inhibition of segmental dorsal horn interneurons.Deceased.Dnepropetrovsk State University. Translated from Neirofiziolgiya, Vol. 4, No. 1, pp. 75–82, January–February, 1972.     

Characteristics of the potential of the dorsal surface of the spinal cord in a chronic deficit of afferent influences

The changes of spinal cord dorsal potential (SCDP) has been studied on white rats to the posterior root stimulation at different intervals after sciatic nerve cutting. The increase of threshold, the decrease of amplitude, the growth of duration in some components of SCDP have been revealed on the site of the cutting. These changes were manifested at a less degree on the contralateral cutting site. A conclusion concerning the relative resistance of the spinal cord afferent system to the prolonged absence of peripheral afferent influence has been drawn.     

Possible involvement of GABA in the central actions of benzodiazepines.

The effects of several benzodiazepines on a variety of nervous activities known or presumed to depend on GABA are presented and compared with those of agents that deplete or increase the level of endogenous GABA: antagonism of various convulsant agents in mice, enhancement of presynaptic inhibition in the spinal cord and the cuneate nucleus of cats, decrease of the spontaneous firing rate of cerebellar Purkinje cells in cats and rats, antagonism of bicuculine-induced depression of the strio-nigral-evoked potential in the cat, potentiation of haloperidol-induced catalepsy in rats, GABA-mimetic actions on drug-induced PGO-waves in cats and on eserine-induced circling in guinea pigs. Diazepam slightly increased the GABA level in the cat spinal cord and in the total brain of mice and rats; this increase does not seem to be due to an increase of GABA synthesis. It is concluded that benzodiazepines probably enhance presynaptic inhibition at all levels of the neuraxis and that this effect requires not only the presence of GABA but is also dependent on an activity of GABA-ergic neurons. Benzodiazepines also appear to enhance postsynaptic inhibition where this is mediated by GABA. Many actions of benzodiazepines can be tentatively explained by a stimulus-bound enhancement of GABA effects.     

Neuromimetic model of a neuronal filter

Experimental work in cats has shown that a series of afferent impulses from muscle receptors activated during contractions of an ankle extensor elicit declining inhibitory potentials in homonymous and synergic motoneurones. Inhibitory potentials were ascribed to the action of Ib afferents from Golgi tendon organs that are specific contraction-sensitive mechano-receptors. The decline of inhibition was, at least partly, due to presynaptic inhibition acting as a filter of tendon organ information in the spinal cord. In the present work, a computer model of the simplest spinal pathways from Ib fibres to motoneurones was designed. In order to make the model as realistic as possible, the most pertinent of the known functional properties of the neuronal elements were incorporated. Functions simulating primary afferent depolarizations of Ib terminals, i.e. the electrophysiological correlate of presynaptic inhibition, were introduced in the network. Simulations showed that declining inhibitory potentials were computed in the output stage of the network that represented the motoneurone-like element. These results support the assumption that the filtering out of Ib inputs is to a great extent due to presynaptic inhibition. The model behaved as expected, suggesting that predictions of the behaviour of neural components in the biological network should be possible upon introduction in the model of other, more complex, spinal pathways from Ib fibres to motoneurones.     

Flexible processing of sensory information induced by axo-axonic synapses on afferent fibers.

Recent experiments indicate that afferent information is processed in the intraspinal arborisation of mammalian group I fibres. During muscle contraction, Ib inputs arising from tendon organs are filtered out by presynaptic inhibition after their entry in the spinal cord. This paper reviews the mechanisms by which GABAergic axo-axonic synapses, i.e., the morphological substrate of presynaptic inhibition, exert this filtering effect. Using confocal microscopy, axo-axonic synapses were demonstrated on segmental Ib collaterals. Most synapses were located on short preterminal and terminal branches. Using a simple compartmental model of myelinated axon, the primary afferent depolarisation (PAD), generated by such synapses, was predicted to reduce the amplitude of incoming action potentials by inactivating the sodium current, and this prediction was experimentally verified. A further theoretical work, relying on cable theory, suggests that the electrotonic structure of collaterals and the distribution of axo-axonic synapses allow large PADs (about 10 mV) to develop on some distal branches, which is likely to result in a substantial presynaptic inhibition. In addition, the electrotonic structure of group I collaterals is likely to prevent PAD from spreading to the whole arborisation. Such a non-uniform diffusion of the PAD accounts for differential presynaptic inhibition in intraspinal branches of the same fibre. Altogether, our experimental and theoretical works suggest that axo-axonic synapses can control the selective funnelling of sensory information toward relevant targets specified according to the motor task.     

Disruption of KCC2 reveals an essential role of K-Cl cotransport already in early synaptic inhibition

Synaptic inhibition by GABA(A) and glycine receptors, which are ligand-gated anion channels, depends on the electrochemical potential for chloride. Several potassium-chloride cotransporters can lower the intracellular chloride concentration [Cl(-)](i), including the neuronal isoform KCC2. We show that KCC2 knockout mice died immediately after birth due to severe motor deficits that also abolished respiration. Sciatic nerve recordings revealed abnormal spontaneous electrical activity and altered spinal cord responses to peripheral electrical stimuli. In the spinal cord of wild-type animals, the KCC2 protein was found at inhibitory synapses. Patch-clamp measurements of embryonic day 18.5 spinal cord motoneurons demonstrated an excitatory GABA and glycine action in the absence, but not in the presence, of KCC2, revealing a crucial role of KCC2 for synaptic inhibition.     

Effect of diaphragm small-fiber afferent stimulation on ventilation in dogs

Little is known regarding the role of diaphragm small-fiber afferents (groups III and IV) in the control of breathing. This study was designed to determine whether activation of these afferents with use of capsaicin affects phrenic efferent activity. Capsaicin injections into the phrenic artery were made in 10 alpha-chloralose-anesthetized dogs after each of the following procedures performed in succession: bilateral cervical vagotomy, C7 spinal cord transection, bilateral cervical dorsal rhizotomy. In six of these animals injections were also made after C2 spinal cord transection and removal of the cervical spinal cord. Injections made in the vagotomized animals were associated with apneusis followed by hyperpnea. C7 spinal transection eliminated the hyperpneic response, but the apneusis remained. Cervical dorsal rhizotomy or C2 spinal cord transection failed to abolish the apneusis in response to injection. No diaphragm response was obtained after removal of the cervical spinal cord. Experiments in three additional animals showed that capsaicin does not have a direct excitatory effect on the muscle cells of the crural diaphragm, nor does it potentiate the release of neurotransmitter in the diaphragm. The results of this study indicate that small-fiber afferents in the diaphragm have an excitatory effect on phrenic motoneurons. There is a segmental component to this reflex, since the response is observed after C2 spinal cord transection. The data also suggest that at least some of these afferents enter the spinal cord through the ventral roots.     

Contralateral input modulates the excitability of dorsal horn neurons involved in noxious signal processes. Potential role in neuronal sensitization

Wide Dynamic Range (WDR) neurons in the spinal cord receive inputs from the contralateral side that, under normal conditions, are ineffective in generating an active response. These inputs are effective when the target WDRs change their excitability conditions. To further reveal the mechanisms supporting this effectiveness shift, we investigated the weight of the excitation of the contralateral neurons on the target WDR responses. In the circuit of presynaptic (sending) and postsynaptic (receiving) neurons in crossed spinal connections the fibres that form the presynaptic neurons impinge on postsynaptic neurons can be considered the final relay of this contralateral pathway. The enhancement of the presynaptic neuron excitability may thus modify the efficacy of the contralateral input. Pairs of neurons each on a side of the spinal cord, at the L5-L6 lumbar level were simultaneously recorded in intact, anaesthetized, paralysed rats. The excitatory aminoacid NMDA and strychnine, the antagonist of the inhibitory aminoacid glycine, were iontophoretically administrated to presynaptic neurons to increase their excitability. Before and during the drug administration, spontaneous and noxious-evoked activities of the neurons were analysed. During the iontophoresis of the two substances we found that noxious stimuli applied to the receptive field of presynaptic neurons activated up to 50% of the previously unresponsive postsynaptic neurons on the opposite side. Furthermore, the neurons on both sides of the spinal cord showed significantly increased spontaneous activity and amplified responses to ipsilateral noxious stimulation. These findings indicate that the contralateral input participates in the circuit dynamics of spinal nociceptive transmission, by modulating the excitability of the postsynaptic neurons. A possible functional role of such a nociceptive transmission circuit in neuronal sensitization following unilateral nerve injury is hypothesized.