Monosynaptic and disynaptic reticulospinal actions on lumbar motoneurons of the rat

Characterization of inducible models of Tay-Sachs and related disease

Tay-Sachs and Sandhoff diseases are lethal inborn errors of acid β-N-acetylhexosaminidase activity, characterized by lysosomal storage of GM2 ganglioside and related glycoconjugates in the nervous system. The molecular events that lead to irreversible neuronal injury accompanied by gliosis are unknown; but gene transfer, when undertaken before neurological signs are manifest, effectively rescues the acute neurodegenerative illness in Hexb−/− (Sandhoff) mice that lack β-hexosaminidases A and B. To define determinants of therapeutic efficacy and establish a dynamic experimental platform to systematically investigate cellular pathogenesis of GM2 gangliosidosis, we generated two inducible experimental models. Reversible transgenic expression of β-hexosaminidase directed by two promoters, mouse Hexb and human Synapsin 1 promoters, permitted progression of GM2 gangliosidosis in Sandhoff mice to be modified at pre-defined ages. A single auto-regulatory tetracycline-sensitive expression cassette controlled expression of transgenic Hexb in the brain of Hexb−/− mice and provided long-term rescue from the acute neuronopathic disorder, as well as the accompanying pathological storage of glycoconjugates and gliosis in most parts of the brain. Ultimately, late-onset brainstem and ventral spinal cord pathology occurred and was associated with increased tone in the limbs. Silencing transgenic Hexb expression in five-week-old mice induced stereotypic signs and progression of Sandhoff disease, including tremor, bradykinesia, and hind-limb paralysis. As in germline Hexb−/− mice, these neurodegenerative manifestations advanced rapidly, indicating that the pathogenesis and progression of GM2 gangliosidosis is not influenced by developmental events in the maturing nervous system.

Sandhoff and Tay-Sachs disease are devastating neurological diseases associated with developmental regression, blindness, seizures, and death in infants and young children. These disorders are caused by mutations in β-hexosaminidase genes, which result in neuronal accumulation of certain lipids, glycosphingolipids, inside the lysosomes of neurons. It is not yet known how accumulation of lipids affects neuronal function, and although promising treatments such as gene therapy are in development, currently none has been clinically approved. We aimed to develop genetic models that allow manipulation of β-hexosaminidase expression over time. Two inducible strains of mice were created in which acute Sandhoff disease could be “turned on” by the addition of doxycycline in the diet. Once induced in the adult mouse, the disease progressed relentlessly and was apparently independent of the rapid developmental processes that occur in the fetal and neonatal brain, resembling disease course in the germline Hexb−/− mouse. These transgenic inducible strains of Sandhoff disease mice provide a dynamic platform with which to explore the pathophysiological sequelae immediately after loss of neuronal lysosomal β-hexosaminidase activity.

Concepts and methods for the study of axonal regeneration in the CNS

Progress in the field of axonal regeneration research has been like the process of axonal growth itself: there is steady progress toward reaching the target, but there are episodes of mistargeting, misguidance along false routes, and connections that must later be withdrawn. This primer will address issues in the study of axonal growth after central nervous system injury in an attempt to provide guidance toward the goal of progress in the field. We address definitions of axonal growth, sprouting and regeneration after injury, and the research tools to assess growth.

Erythropoietin effect on sensorimotor recovery after contusive spinal cord injury: an electrophysiological study in rats

Spinal cord injury (SCI) is a debilitating clinical condition, characterized by a complex of neurological dysfunctions. It has been shown in rats that the acute administration of recombinant human erythropoietin (rhEPO) following a contusive SCI improves the recovery of hindlimb motor function, as measured with the locomotor BBB (Basso, Beattie, Bresnahan) scale. This scale evaluates overall locomotor activity, without testing whether the rhEPO-induced motor recovery is due to a parallel recovery of sensory and/or motor pathways. Aim of the present study was to utilize an electrophysiological test to evaluate, in a rat model of contusive SCI, the transmission of both ascending and descending pathways across the damaged cord at 2, 5, 7, 11, and 30 days after lesion, in animals treated with rhEPO (n=25) vs saline solution (n=25). Motor potentials evoked by epicortical stimulation were recorded in the spinal cord, and sensory-evoked potentials evoked by spinal stimulation were recorded at the cortical level. In the same animals BBB score and immunocytochemical evaluation of the spinal segments caudal to the lesion were performed. In rhEPO-treated animals results show a better general improvement both in sensory and motor transmission through spared spinal pathways, supposedly via the reticulo-spinal system, with respect to saline controls. This improvement is most prominent at relatively early times. Overall these features show a parallel time course to the changes observed in BBB score, suggesting that EPO-mediated spared spinal cord pathways might contribute to the improvement in transmission which, in turn, might be responsible for the recovery of locomotor function.

Formation of descending pathways mediating cortical command to forelimb motoneurons in neonatally hemidecorticated rats

Neonatally hemidecorticated rats show fairly normal reaching and grasping behaviors of the forelimb contralateral to the lesion at the adult stage. Previous experiments using an anterograde tracer showed that the corticospinal fibers originating from the sensorimotor cortex of the intact side projected aberrant collaterals to the spinal gray matter on the ipsilateral side. The present study used electrophysiological methods to investigate whether the aberrant projections of the corticospinal tract mediated the pyramidal excitation to the ipsilateral forelimb motoneurons and, if so, which pathways mediate the effect in the hemidecorticated rats. Electrical stimulation to the intact medullary pyramid elicited bilateral negative field potentials in the dorsal horn of the spinal cord. In intracellular recordings of forelimb motoneurons, oligosynaptic pyramidal excitation was detected on both sides of the spinal cord in the hemidecorticated rats, whereas pyramidal excitation of motoneurons on the side ipsilateral to the stimulation was much smaller in normal rats. By lesioning the dorsal funiculus at the upper cervical level, we clarified that the excitation was transmitted to the ipsilateral motoneurons by at least two pathways: one via the corticospinal tract and spinal interneurons and the other via the cortico-reticulo-spinal pathways. These results suggested that in the neonatally hemidecorticated rats, the forelimb movements on the side contralateral to the lesion were modulated by motor commands through the indirect ipsilateral descending pathways from the sensorimotor cortex of the intact side either via the spinal interneurons or reticulospinal neurons.

Direct and indirect connections with upper limb motoneurons from the primate reticulospinal tract

Although the reticulospinal tract is a major descending motor pathway in mammals, its contribution to upper limb control in primates has received relatively little attention. Reticulospinal connections are widely assumed to be responsible for coordinated gross movements primarily of proximal muscles, whereas the corticospinal tract mediates fine movements, particularly of the hand. In this study, we used intracellular recording in anesthetized monkeys to examine the synaptic connections between the reticulospinal tract and antidromically identified cervical ventral horn motoneurons, focusing in particular on motoneurons projecting distally to wrist and digit muscles. We found that motoneurons receive monosynaptic and disynaptic reticulospinal inputs, including monosynaptic excitatory connections to motoneurons that innervate intrinsic hand muscles, a connection not previously known to exist. We show that excitatory reticulomotoneuronal connections are as common and as strong in hand motoneuron groups as in forearm or upper arm motoneurons. These data suggest that the primate reticulospinal system may form a parallel pathway to distal muscles, alongside the corticospinal tract. Reticulospinal neurons are therefore in a position to influence upper limb muscle activity after damage to the corticospinal system as may occur in stroke or spinal cord injury, and may be a target site for therapeutic interventions.

Simple measurement of spinal cord evoked potential: a valuable data source in the rat spinal cord injury model

Measurement of spinal cord evoked potentials (SCEPs) is proposed as a means of predicting locomotion outcome in the rat spinal cord injury (SCI) model. Using 55 rats, three reproducible peak waves (waves I, II and III) were observed during stimulation at the C7 level with recording at the L1 epidural space. Hemisection at the T13 level showed three wave loss patterns: wave III loss only, loss of both wave II and III, and loss of all three waves. Defining an ideal SCI model as establishment of stable monoparesis or paraparesis, all animals in the wave II-III loss group showed favorable results. Histological data and electrophysiological properties allowed reasonable assumptions of wave origin: wave I from extrapyramidal tracts, wave II from the ventral corticospinal tract, and wave III from the dorsal corticospinal tract. Complete destruction of pyramidal tracts in both dorsal and ventral fibers was essential for long-term impairment of locomotion.

How can corticospinal tract neurons contribute to ipsilateral movements? A question with implications for recovery of motor functions

In this review, the authors discuss some recent findings that bear on the issue of recovery of function after corticospinal tract lesions. Conventionally the corticospinal tract is considered to be a crossed pathway, in keeping with the clinical findings that damage to one hemisphere, for example, in stroke, leads to a contralateral paresis and, if the lesion is large, a paralysis. However, there has been great interest in the possibility of compensatory recovery of function using the undamaged hemisphere. There are several substrates for this including ipsilaterally descending corticospinal fibers and bilaterally operating neuronal networks. Recent studies provide important evidence bearing on both of these issues. In particular, they reveal networks of neurons interconnecting two sides of the gray matter at both brainstem and spinal levels, as well as intrahemispheric transcallosal connections. These may form "detour circuits" for recovery of function, and here the authors will consider some possibilities for exploiting these networks for motor control, even though their analysis is still at an early stage.

Functional and electrophysiological changes after graded traumatic spinal cord injury in adult rat

A graded contusion spinal cord injury (SCI) was created in the adult rat spinal cord using the Infinite Horizons (IH) impactor to study the correlation between injury severity and anatomical, behavioral, and electrophysiological outcomes. Adult Fisher rats were equally divided into five groups and received contusion injuries at the ninth thoracic level (T9) with 100, 125, 150, 175, or 200 kdyn impact forces, respectively. Transcranial magnetic motor-evoked potentials (tcMMEPs) and BBB open-field locomotor analyses were performed weekly for 4 weeks postinjury. Our results demonstrated that hindlimb locomotor function decreased in accordance with an increase in injury severity. The locomotor deficits were proportional to the amount of damage to the ventral and lateral white matter (WM). Locomotor function was strongly correlated to the amount of spared WM, which contains the reticulospinal and propriospinal tracts. Normal tcMMEP latencies were recorded in control, all of 100-kdyn-injured and half of 125-kdyn-injured animals. Delayed latency responses were recorded in some of 125-kdyn-injured and all of 150-kdyn-injured animals. No tcMMEP responses were recorded in 175- and 200-kdyn-injured animals. Comparison of tcMMEP responses with areas of WM loss or demyelination identified the medial ventrolateral funiculus (VLF) as the location of the tcMMEP pathway. Immunohistochemical and electromicroscopic (EM) analyses showed the presence of demyelinated axons in WM tracts surrounding the lesion cavities at 28 days postinjury. These data support the notion that widespread WM damage in the ventral and lateral funiculi may be a major cause for locomotor deficits and lack of tcMMEP responses after SCI.

Conduction pathways of motor evoked potentials following transcranial magnetic stimulation: a rodent study using a "figure-8" coil

We have examined the conduction pathways of motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation, and their correlation with locomotor function in rats. MEPs were concomitantly recorded from the spinal cord (sMEPs) and the limb muscles (mMEPs) before and after various spinal tract ablations. Motor function was also examined using an inclined plane test. sMEPs were composed of four negative peaks (N1-N4) and mMEPs of high-voltage, biphasic waves. Ventral funiculus transection reduced the N1-N3 peaks and abolished mMEPs. Contrarily, dorsal funiculus transection including the pyramidal tract did not alter these MEPs. Motor performance on an inclined plane was worse after ventral funiculus transection than after other transections. These findings indicate that, in rats, the N1-N3 peaks of magnetic sMEPs conduct ventral funiculus activity, and that magnetic mMEPs mainly reflect extrapyramidal activities and are correlated with locomotor function.

New assessment techniques for evaluation of posttraumatic spinal cord function in the rat

To evaluate new pharmacologic agents with potentially beneficial effects on posttraumatic spinal cord function, we used a modified weight drop (WD) technique to induce spinal cord injuries. These contusive spinal cord injuries in the rat closely mimic the human clinicopathologic situation. Especially for drug screening purposes, the moderate and mild injuries are of interest, as both the beneficial and potentially harmful effects of experimental treatment can be detected. In this study, we describe two new functional tests that were particularly designed to detect small differences in spinal cord function after moderate and mild injuries. First, for examination of locomotion, a computer analysis of the thoracolumbar height (TLH) was designed. Second, for investigation of the conduction properties of the injured rat spinal cord, we measured rubrospinal motor evoked potentials (MEP). The efficacy of the new assessment techniques to monitor spinal cord function was compared to Tarlov scores and to morphometric analysis of preserved white matter at the injury site. The results of this study indicated that for behavioral analysis, TLH measurements as compared with Tarlov rating appeared to be more sensitive for exact and objective discrimination between small differences in motor function. Amplitudes of the rubrospinal MEP, but not latencies or the number of peaks, proved to be most sensitive to determine subtle differences in posttraumatic spinal cord function. A significant linear correlation was found between TLH and amplitude of the rubrospinal MEP. We conclude that for objective assessment of the spinal cord after moderate and mild contusive injury, TLH and rubrospinal MEP amplitudes are very valuable measures to demonstrate small functional differences.

The medullary reticular formation is a site of muscle relaxant action of diazepam on deep back and neck muscles in the female rat

We tested the hypothesis that the effect of systemic injections of diazepam (DZ, 125 mg/kg) to reduce the quality of the reproductive behavior, lordosis, and to reduce the EMG of lumbar back muscles involved in lordosis (Schwartz-Giblin et al., 1984) is exerted through a reticulospinal pathway with cells of origin in the nucleus gigantocellularis that excites lumbar motoneurons indirectly (Robbins et al., 1990, Robbins et al., 1992). In contrast, DZ facilitates lordosis behavior when infused into the midbrain central gray (McCarthy et al., 1995). Direct deposits of crystalline mixtures of DZ (20-80 ng) in dextrose were delivered to the medullary reticular formation (MRF) by diffusion from a cannula inserted through a guide to which a bipolar stimulating electrode was attached. The multiunit EMG response evoked by 20 (300 ms long) stimulus trains was recorded in back and neck muscles, lateral longissimus and splenius before and 5, 15, 30 and 60 min after local DZ deposits. There was a significant reduction in EMG response over this time period when stimulus intensities were within the range of 1.2-1.5 times threshold (Friedman two-way non-parametric test, P < 0.002). Large amplitude motor units that provide large tensions were the most sensitive to DZ-induced inhibition. Control deposits of dextrose had no significant effect. Systemic injections of progesterone (1 mg, i.p.) 60 min after DZ deposits, but not after dextrose deposits, further reduced the MRF-evoked EMG responses over the course of 1 h. As predicted, DZ infusions into the midbrain central gray did not reduce the reticulospinal-evoked axial muscle response, consistent with the facilitatory effect of midbrain central gray infusions of DZ on the lordosis quotient. The results suggest that benzodiazepine agonists (if endogenous) acting at sites in the MRF would be effective muscle relaxants during pregnancy, prior to the fall in progesterone that precedes labor.

Structural changes of anterior horn neurons and their synaptic input caudal to a low thoracic spinal cord hemisection in the adult rat: a light and electron microscopic study

Structural changes in lumbosacral ventral horn neurons and their synaptic input were studied at 3, 10, 21, 42, and 90 days following low thoracic cord hemisection in adult rats by light microscopic examination of synaptophysin immunoreactivity (SYN-IR) and by electron microscopy. There was an ipsilateral transient decrease in SYN-IR at the somal and proximal dendritic surfaces of anterior horn neurons which extended caudally from the site of injury over a postoperative (p.o.) period of 42 days. Concomitantly, at 21 days p.o., perineuronal SYN-IR started to recover in upper lumbar segments. By 90 days p.o., a normal staining pattern of SYN was noted in upper and mid lumbar segments, but the perineuronal SYN-IR was still slightly below normal levels in low lumbar and sacral segments. Electron microscopy revealed ultrastructural changes coincident with the alterations in SYN-IR. At 3 days p.o., phagocytosis of degenerating axon terminals by activated microglial cells was observed at the somal and proximal dendritic surfaces of ventral horn neurons. These changes were most prominent up to two segments caudal to the lesion. At 10 days p.o., advanced stages of bouton phagocytosis were still detectable in all lumbosacral motor nuclei. Additionally, abnormal axon terminals, with a few dispersed synaptic vesicles and accumulations of large mitochondria, appeared at the scalloped somal surfaces of anterior horn neurons. At 21 days p.o., several large lumbosacral motoneurons had developed chromatolysis-like ultrastructural alterations and motoneuronal cell bodies had become partially covered by astrocytic lamellae. At 42 days p.o., there was a transient appearance of polyribosomes in some M-type boutons. In addition, at 42 and 90 days p.o., a few degenerating motoneurons were detected in all lumbosacral segments, but most displayed normal neuronal cell bodies contacted by numerous intact synapses as well as by astrocytic processes. In contrast to these striking alterations of synaptic input at somal and proximal dendritic surfaces of motoneurons, relatively few degenerating boutons were detected in the neuropil of motor nuclei at all the p.o. times studied. We suggest that the preferential disturbance of the predominantly inhibitory axosomatic synapses on ventral horn neurons may be involved in the mechanisms which influence the well-established increase in motoneuronal excitability after spinal cord injury.

The acoustic startle reflex: neurons and connections

The startle reflex protects animals from blows or predatory attacks by quickly stiffening the limbs, body wall and dorsal neck in the brief time period before directed evasive or defensive action can be performed. The acoustic startle reflex in rats and cats is mediated primarily by a small cluster of giant neurons in the ventrocaudal part of the nucleus reticularis pontis caudalis (RPC) of the reticular formation. Activation of these RPC neurons occurs 3-8 ms after the acoustic stimulus reaches the ear. Undetermined neurons of the cochlear nuclei activate RPC via weak monosynaptic and strong disynaptic connections. The strong disynaptic input occurs via neurons of the contralateral ventrolateral pons, including large neurons of the ventrolateral tegmental nucleus that integrate auditory, tactile and vestibular information. RPC giant neurons, in turn, activate hundreds of motoneurons in the brain stem and the length of the spinal cord via large reticulospinal axons near the medial longitudinal fasciculus. To hindlimb motoneurons, monosynaptic connections from the reticulospinal tract are weak, but disynaptic connections via spinal cord interneurons are stronger and show temporal facilitation, like the startle response itself.

Reticulospinal and reticuloreticular pathways for activating the lumbar back muscles in the rat

These experiments tested hypotheses about the logic of reticulospinal and reticuloreticular controls over deep back muscles by examining descending efferent and contralateral projections of the sites within the medullary reticular formation (MRF) that evoke EMG responses in lumbar axial muscles upon electrical stimulation. In the first series of experiments, retrograde tracers were deposited at gigantocellular reticular nucleus (Gi) sites that excited the back muscles and in the contralateral lumbar spinal cord. The medullary reticular formation contralateral to the Gi stimulation/deposition site was examined for the presence of single- and double-labeled cells from these injections. Tracer depositions into Gi produced labeled cells in the contralateral Gi and Parvocellular reticular nucleus (PCRt) whereas the lumbar injections retrogradely labeled cells only in the ventral MRF, indicating that separate populations of medullary reticular cells project to the opposite MRF and the lumbar cord. In the second series of experiments the precise relationships between the location of neurons retrogradely labeled from lumbar spinal cord depositions of the retrograde trace, Fluoro-Gold (FG) and effective stimulation tracks through the MRF were examined. The results indicate that the Gi sites that are most effective for activation of the back muscles are dorsal to the location of retrogradely labeled lumbar reticulospinal cells. To verify that cell bodies and not fibers of passage were stimulated, crystals of the excitatory amino acid agonist, N-methyl-D-aspartate (NMDA) were deposited at effective stimulation sites in the Gi. NMDA decreased the ability of electrical stimulation to activate back muscles at 5 min postdeposition, indicating a local interaction of NMDA with cell bodies at the stimulation site. In the third series of experiments, electrical thresholds for EMG activation along a track through the MRF were compared to cells retrogradely labeled from FG deposited into the cervical spinal cord. In some experiments, Fast Blue was also deposited into the contralateral lumbar cord. Neurons at low threshold points on the electrode track were labeled following cervical depositions, indicating a direct projection to the cervical spinal cord. The lumbar depositions, again, labeled cells in MRF areas that were ventral to the locations of effective stimulation sites, primarily on the opposite side of the medulla. In addition, the lumbar depositions back-filled cells in the same cervical segments to which the Gi neurons project. These results suggest that one efferent projection from effective stimulation sites for back muscle activation is onto propriospinal neurons in the cervical cord, which in turn project to lumbar cord levels.(ABSTRACT TRUNCATED AT 400 WORDS)

Participation of the medial reticular formation of the medulla oblongata in the supraspinal control of locomotor and postural activities in the guinea pig

The influence of microinjections of novocaine (1.5 microliters of a 2% solution) into the ventromedial areas of the reticular formation of the medulla oblongata on the effects of electrical stimulation of the mesencephalic locomotor region, and of adequate stimulation of the vestibular apparatus, was investigated in experiments on decerebrate guinea pigs. It was demonstrated that such injections are accompanied by a reversible increase in the threshold of stimulation of the region indicated, necessary for the initiation of locomotor rhythmicity, as well as by the suppression of vestibular influences on the activity of the flexor muscles of the extremities. The role of the reticular formation of the brainstem in the supraspinal control of posture and locomotion is discussed.

The effect of procaine injection into the medullary reticular formation on forelimb muscle activity evoked by mesencephalic locomotor region and vestibular stimulation in the decerebrated guinea-pig

The effect of procaine microinjection into the ventromedial portion of the medullary reticular formation on forelimb muscle activity evoked by electrical mesencephalic locomotor region and natural vestibular stimulation has been investigated in the decerebrated guinea-pig. This injection is followed by a reversible increase of the threshold of mesencephalic locomotor region stimulation necessary for the activation of muscle rhythmic activity. Procaine injection is accompanied by reduction of vestibular influence on flexor muscle activity evoked by electrical cutaneous and mesencephalic locomotor region stimulation. Vestibular influence on extensor muscle activity remains unchanged after the injection. The results indicate that medial medullary reticular formation is the site of the convergence of mesencephalic locomotor region and vestibular activity. It is suggested that the vestibular system contributes to the modulation of reticulospinal activity coupled with the initiation and control of locomotion.

Neural organization in the brainstem circuit mediating the primary acoustic head startle: an electrophysiological study in the rat

In awake rats the latency of auditory startle recorded electromyographically in the neck is about 5 ms, suggesting that the primary component of this brainstem reflex is mediated by a neural circuit with only a few synapses. In the present work, neural relays on acoustic head startle circuit are studied in alpha-chloralose-anesthetized rats by means of precise measurements, at putative brainstem relays, of the click-evoked potential latency and of the latency of nuchal EMG startle-like response elicited electrically from each recorded site using the same bipolar electrode. Allowing 0.6 ms for total synaptic transmission time in every relay nuclei, systematic comparisons of the shortest latencies of evoked potentials and shock-elicited startle, as well as estimations of conduction velocities in pathways from cochlea to C1-C5 spinal cord, suggest that one primary acoustic head startle circuit consists of ventral cochlear nucleus (postsynaptic evoked potential: 1.4 ms; startle: 3.6-4.0 ms), ventral nucleus of the lateral lemniscus (evoked potential: 2.3 ms; startle: 2.7-2.8 ms), medial bulbar reticular formation (evoked potential: 3.2-3.6 ms; startle: 2.1 ms), spinal interneuron and motoneuron. The nucleus reticularis pontis caudalis (NRPC) cannot be considered as an head startle relay intercalated between the ventral nucleus of the lateral lemniscus (VNLL) and the medial bulbar reticular formation (MBRF) because mean latencies of field potentials in the pontine RF and the LL nucleus are the same (2.3 ms). Moreover, startle-like responses in neck muscles are elicited through the three brainstem regions with latency differentials which exclude the possibility of a classical synaptic delay either between VNLL (2.8 ms) and NRPC (2.5 ms) or between NRPC and bulbar RF (2.1 ms). Nevertheless, NRPC probably remains a main primary relay on the acoustic startle circuitry; very short latency auditory responses (2.3 ms) are evoked in NRPC by clicks, and low current stimulations of this reticular region produce startle-like activity in neck muscles with a latency of only 2.5 ms. Two other alternative paths consisting of the VCN and NRPC which then would project directly, or through an unknown bulbar site, upon the spinal motor center are hypothetically proposed in conclusion.

Supraspinal projections to the ventromedial lumbar spinal cord in adult male rats

In the present study, the fluorescent tract tracing compound Fluorogold was used to study the afferents of the SNB (spinal nucleus of the bulbocavernosus), which is found in the ventromedial spinal grey and innervates penile muscles of the male rat. Fluorogold was iontophoretically injected into the SNB, which was located by recording antidromic activation of the motoneurons after stimulating the bulbocavernosus muscle. Retrogradely labeled cells were found in laminae I, V-IX, and area X of the lumbar spinal cord, suggesting segmental input to the SNB. Supraspinally, the greatest number of labeled cells were in the medulla oblongata, particularly in the lateral vestibular nucleus, gigantocellular reticular nucleus, and ventral and alpha divisions of the gigantocellular reticular nucleus. Labeled cells were also observed in the medullary raphe nuclei, the ventral medullary nucleus, and the spinal vestibular nucleus. In the pons, labeled cells were observed in the nucleus locus coeruleus, nucleus subcoeruleus, and caudal pontine reticular nucleus. No labeled cells were present in the cerebellum, rostral pons, mesencephalon, and cerebral cortex. The most rostral occurrence of labeled cells was in the medial parvicellular division of the hypothalamic paraventricular nucleus. These potential afferents to the SNB identified in male rats imply that the inputs to motoneurons that innervate sex-specific muscles involved in male reproductive behavior may be similar to the inputs to lumbar motoneurons described in the female rat that innervate muscles involved in female sexual behavior.

Inhibition of spinal reflexes by paramedian reticular nucleus

The inhibitory actions of the paramedian reticular nucleus (PRN), and its neighbouring structures, i.e., midline raphe nuclei (MRN) and dorsal medullary depressor area (DMD) on the knee jerk (KnJ) and crossed extension movement (CEM) induced by central sciatic stimulation and on the L5 ventral root response (EVRR) evoked by central tibial stimulation, were studied in cats under urethane (400 mg/kg) and alpha-chloralose (40 mg/kg) anesthesia alone, IP or further paralyzed with atracurium besylate (0.5 mg/kg/30 min), IV. Electrical stimulation of the above areas with rectangular pulses (80 Hz, 1.0 msec, 100-200 microA) decreased systemic arterial blood pressure (SAP) in an average value of: 36 +/- 3 mmHg for PRN; 19 +/- 2 mmHg for MRN; and 23 +/- 3 mmHg for DMD. The KnJ and CEM were almost completely suppressed by simultaneous PRN stimulation. The EVRR, including mono- and polysynaptic spinal reflexes with transmission velocity from 10 to 60 m/sec or above, were also suppressed. MRN stimulation only inhibited the KnJ, CEM and polysynaptic spinal reflexes with transmission velocities between 25 and 60 m/sec, but facilitated spinal reflexes with conduction velocities below 10 m/sec. On the other hand, DMD stimulation resulted in small suppression of KnJ, CEM and inhibition of polysynaptic spinal reflexes with conduction velocities between 25 and 60 m/sec. Even though MRN and DMD partially inhibited polysynaptic spinal reflexes, the magnitude of such inhibition was much smaller than that produced by PRN (-20% and -22% vs. -48%). The above-mentioned PRN effects on SAP and EVRR persisted in chronic animals decerebellated 9-12 days before.(ABSTRACT TRUNCATED AT 250 WORDS)

Ascending and descending projections to medullary reticular formation sites which activate deep lumbar back muscles in the rat

The purpose of this study was to determine ascending and descending afferents to a medullary reticular formation (MRF) site that, when electrically stimulated, evoked EMG activity in lumbar deep back muscles. In anesthetized female rats, the MRF was explored with electrical stimulation, using currents less than 50 microA, while EMG activity was recorded from the ipsilateral lateral longissimus (LL) and medial longissimus (ML). MRF sites that evoked muscle activity were located in the gigantocellular nucleus (Gi). At the effective stimulation site, the retrograde fluorescent tracer, Fluoro-Gold (FG), was deposited via a cannula attached to the stimulating electrode. In matched-pair control experiments, FG was deposited at MRF sites that were ineffective in producing EMG activity in LL and ML, for comparison of afferent projections to effective versus ineffective sites. Labeled cells rostral to FG deposition at effective MRF sites were located in the preoptic area, hypothalamus, limbic forebrain and midbrain, with particularly high numbers in the ipsilateral midbrain central gray, tegmentum, paraventricular nucleus and amygdala. At medullary levels, there was a heavy projection from the contralateral Gi. FG labeled cells were also located in the contralateral parvocellular reticular nucleus, and lateral, medial and spinal vestibular nuclei. Labeled cells with ascending projections were observed in greatest number in the rostral cervical spinal cord, with fewer cells at mid cervical levels and even fewer in the lumbar spinal cord. These labeled cells were located primarily in lamina V, VII, VIII and X. Locations of labeled cells following FG deposition at ineffective MRF sites were similar. However, there was a striking difference in the number of cells retrogradely labeled from the effective MRF sites compared to ineffective MRF sites. Significantly greater numbers of labeled cells were observed in the contralateral MRF, the midbrain, and the cervical spinal cord from the FG deposition at effective stimulation sites. These results suggest that one characteristic of MRF sites that activate epaxial muscles is a larger amount of afferent input, from the midbrain central gray and from contralateral Gi, compared to ineffective MRF sites. Ascending and descending inputs converge at the effective MRF sites, and the larger number of descending projections suggests a more powerful contribution of these afferents to deep lumbar back muscle activation.

Corticospinal responses to electrical stimulation of motor cortex in the rat

Direct and indirect corticospinal responses to electrical stimulation of motor cortex were identified in urethane-anesthetized rats. 'Killed-end' recordings were taken from the corticospinal tract at the level of the cervical cord (C1-C2) and from the medullary pyramid. The identities of direct (D) and indirect (I) corticospinal responses were confirmed by: (1) removing motor cortex to eliminate I activity, and (2) pharmacologically increasing neocortical excitability, prior to any lesions, to increase I activity. Our data indicate that the conduction velocity of the fastest corticospinal fibers is approximately 18 m/s. Our identification of the components of the corticospinal response will permit the interpretation of the more complicated surface or 'non-killed-end' depth recordings which have shown particular utility in evaluating spinal cord damage.

Amplitude and latency characteristics of spinal cord motor evoked potentials in the rat

The motor evoked potential (MEP) has become a valuable component of neurophysiological monitoring. A better understanding of the characteristics of the normal MEP is needed before one can fully appreciate the effects of injury on the MEP. We describe characteristic patterns of spinal cord MEPs, recorded epidurally, in response to transcranial (dura-to-palate) brain stimulation in a rat model. Series of signal averaged MEP responses at a duration of 100 microseconds were recorded at T10/11, T12/13, and L1/2 in 8 normal rats. We used a much greater range of current intensities (0.5-65 mA) than has been studied previously. Also, we studied the gradual development of the MEP wave form using smaller increments of current strength than have been reported previously. We confirmed in rats our earlier report in cats that long latency peaks appear first at low intensities while short latency peaks appear with higher intensities (Konrad et al. 1988). We also report average peak latencies over the range of stimulus intensities used for each recording level in each rat. In some rats, conduction velocities of several MEP peaks were calculated, and they range from 35 to 42 m/sec. These velocities are consistent with values reported in the literature for extrapyramidal pathways. Our rat model provides a method of measuring spinal cord potentials at three levels with no trauma to the spinal cord. Therefore, it can be used to repeatedly test motor function in chronic studies of spinal cord injury.

Effects of selective spinal cord lesions on the spinal motor evoked potential (MEP) in the rat

The effects of selective spinal cord lesions on the motor evoked potential (MEP) in 21 rats were investigated. No significant change in peak amplitude was observed following lesions of the pyramidal tract. There was a significant decrease in peaks 1 and 2 with ventral funiculi lesions. All 4 peaks of the MEP were significantly reduced following lesions of the lateral funiculus. The most marked decrease in peak amplitude followed lateral funiculi lesions that involved the lateral grey of the spinal cord. In one animal where the lesion was confined to the grey matter in the cord there was a marked decrease in all peaks of the MEP. In 3 additional animals interruption of the descending tracts of the spinal cord via bilateral hemisections of the spinal cord failed to completely abolish the MEP. Increases in peak latency were also noted following spinal lesions. In some animals the increase in latency occurred in the absence of significant peak amplitude changes. The findings in this study refute the previously held position that the MEP in the rat arises from pyramidal tract activation. The role of the reticulospinal and propriospinal tracts in the generation and propagation of the MEP are discussed.

The influence of adequate vestibular stimulation on evoked locomotor muscle activity in the decerebrated guinea-pig

The influence of adequate vestibular stimulation on locomotor muscle activity has been investigated in the decerebrated guinea-pig. Locomotor activity was evoked by electrical stimulation of the mesencephalic locomotor region, whose location has been ascertained in this animal. Vestibular stimulation was performed by cyclic tiltings about the longitudinal and transverse axes and swinging along the vertical axis. The translation frequency was in the range of 0.02-0.8 Hz with an amplitude of +/-20% for tilting and 40 mm for swinging. Vestibular stimulation was accompanied by distinct changes in locomotor electromyographic activity of fore- and hindlimb antagonist muscles. During stimulation the intensity of discharges in extensor and flexor muscles corresponding to the stance and swing phases of the locomotor cycle was modulated; the alternation of antagonist muscle activity was not as a rule disturbed. The changes in muscle activity had the same pattern and similar phase-frequency properties to those observed under analogous vestibular stimulation during the maintenance of steady posture. It is suggested that the vestibular system is of considerable importance for the regulation of locomotor muscle activity. During locomotion the vestibular system influences mainly spinal motor output but does not act on the locomotor generator.

Vestibulospinal and reticulospinal interactions in the activation of back muscle EMG in the rat

The effects of electrical stimulation of the lateral vestibular nucleus (LVN) and medullary reticular formation (RF) on electromyographic activity in axial muscles medial longissimus (ML) and lateral longissimus (LL) in the rat were studied. Long trains (150-500 ms) at 200-330 Hz and 20-100 microA were sufficient to activate ML and LL at latencies of 20-100 ms from the beginning of the train. Results of stimulation at 200-330 Hz to RF or LVN showed that muscle units were activated at a fixed latency from any effective pulse in the stimulus train. Using high frequency (1 kHz) trains of 3-6 pulses to LVN, EMG activity was detected at minimum latencies of 3.5-6 ms. When conduction times from the medulla to the spinal cord, and the spinal cord to the muscle are subtracted, this latency range is consistent with monosynaptic activation. In many cases, muscle units were recruited in order of size, with both RF and LVN stimulation. Combined stimulation of LVN and RF sites in n. gigantocellularis led to EMG activity in ML and LL at currents which were insufficient to evoke activity when presented singly. When stimulation of one site (300-400 ms train) was just sufficient to evoke a response, a shorter, overlapping train (100-150 ms) to the other site led to a higher rate of muscle activity that continued through the end of the long train, even after the short train had ended. In all cases, the effect of RF facilitating LVN was similar to the effect of LVN facilitating RF. The evidence for convergence between these two systems in the medulla and the spinal cord is discussed.

Electrical stimulation of the midbrain central gray facilitates reticulospinal activation of axial muscle EMG

EMG responses were recorded from axial muscles transversospinalis, medial longissimus, and lateral longissimus in urethane-anesthetized rats during combined electrical stimulation of the reticular formation and midbrain central gray. Central gray stimulation facilitated reticular formation-evoked EMG activity in the back muscles of the rat. Electrical stimulation of the central gray lowered the threshold for reticulospinal activation of axial muscles and could maintain firing in these muscles after the end of a reticular formation train. Units were recruited in order of size from small to large. In only one case, central gray stimulation activated axial muscles directly without reticular formation stimulation. The central gray may be important in relaying hypothalamic influences to the reticular formation, which has direct access to the axial muscles responsible for lordosis behavior.

Facilitation of lumbar monosynaptic reflexes by locus coeruleus in the rat

The present study was initiated to delineate whether species difference exists between cats and rats in the descending influence of locus coeruleus (LC) on spinal motoneuronal activity. In male Sprague-Dawley rats anesthetized with chloral hydrate (400 mg/kg, i.p.), localized activation of LC promoted an exclusive facilitation of lumbar spinal extensor and flexor monosynaptic reflexes (MSRs). Such LC-evoked potentiations may vary in degree (37.5-147.4%), duration (70.6-72.9 ms) and latency (3.0-5.5 ms) among different animals. While minimally affecting the control MSRs, the alpha 1-adrenoceptor blocker prazosin (20 micrograms/kg, i.v.) significantly antagonized the enhancing effect of the LC on MSRs, suggesting the participation of noradrenergic neurotransmission in the process. Since these results are in general agreement with previous observations from our laboratory on the cat, we conclude that the LC exerts similar facilitatory actions on both extensor and flexor motoneuron activity of the hindlimb in at least two animal species, rat and cat.

Conduction velocities of corticospinal axons in the rat studied by recording cortical antidromic responses

The rat corticospinal tract was stimulated at the medullary pyramid and at different levels in the spinal cord (segments C2/3, T2, T12) and responses were recorded from the surface of the cerebral cortex and extracellularly from individual cortical neurones. Irrespective of the site stimulated, the earliest surface and single unit responses had frequency-following and other characteristics which indicated they resulted from antidromic invasion of corticospinal neurones. Synaptically mediated discharges with longer latency were also evoked in cortical neurones other than corticospinal neurones. At least in part these discharges probably resulted from stimulus spread to the dorsal column-medial lemniscus pathway. Corticospinal neurones were almost all between 1.0 and 1.5 mm beneath the cortical surface while synaptically excited units were at all depths greater than 0.4 mm. By stimulating at two sites, estimates of conduction velocity were obtained for single corticospinal axons. For those reaching at least as far as T12, velocities caudal to the pyramid ranged from 5 to 19 m/s (mean 11.4 +/- 2.9 m/s; S.D.). Slow axons in the pyramid (antidromic latency greater than 2.5 ms) could rarely be excited from T12. By stimulating at three sites (pyramid, T2, T12) most axons reaching T12 were found to have similar conduction velocities in the 'cervical' (pyramid-T2) and 'thoracic' (T2-T12) cord. However, in 15% of the axons the 'thoracic' velocity was at least 25% less than the cervical. The results are discussed and related to those from previous investigations.

Spinal projections from the medullary reticular formation of the North American opossum: heterogeneity

Retrograde and orthograde transport techniques show that the nucleus reticularis gigantocellularis pars ventralis and the nucleus reticularis gigantocellularis project the entire length of the spinal cord. Double-labelling methods show that some of the neurons in each area innervate both cervical and lumbar levels. There is evidence, however, that neurons in the lateral part of the nucleus reticularis gigantocellularis pars ventralis and the dorsal extreme of the nucleus reticularis gigantocellularis project mainly to cervical and thoracic levels. The autoradiographic method shows that the above nuclei supply direct innervation to somatic and autonomic motor columns as well as to laminae V-VIII and X. The nucleus reticularis gigantocellularis pars ventralis provides additional projections to lamina I and the outer part of lamina II. Several areas of the medullary reticular formation project mainly, and in some cases exclusively, to cervical and thoracic levels. These areas include the nucleus reticularis parvocellularis, the nucleus reticularis lateralis, the nucleus retrofacialis, the nucleus ambiguus, the nucleus lateralis reticularis, caudal parts of the nuclei reticularis medullae oblongatae dorsalis and ventralis, and the nucleus supraspinalis. Autoradiographic experiments reveal that neurons in the ventrolateral medulla, particularly rostrally (the nucleus reticularis lateralis and neurons related to the nucleus lateralis reticularis), innervate sympathetic nuclei. Our results indicate that spinal projections from bulbar areas of the reticular formation are more complicated than previously supposed. Axons from separate areas project to different spinal levels and in some cases to different nuclear targets. These data are in conformity with the evolving concept of reticular heterogeneity.

Spinal projections from the mesencephalic and pontine reticular formation in the North American Opossum: a study using axonal transport techniques

The results from several experimental approaches lead to the following conclusions. The nucleus cuneiformis projects to at least lumbar levels of the spinal cord. Its axons course through the ipsilateral sulcomarginal and ventral funiculi to distribute within lamina VIII and adjacent portions of lamina VII. Neurons within the nucleus reticularis pontis (RP), particularly within more medial parts of the nucleus, project through comparable routes to the same laminae. In addition, however, neurons within the lateral and dorsolateral RP relay through the lateral and dorsolateral funiculi, ipsilaterally, and the dorsolateral funiculus, contralaterally. Axons could be traced from the dorsolateral tracts to laminae IV through VII, lamina X and, in some instances, to laminae I and II. Injections of the dorsolateral pons also label the intermediolateral cell column and an area presumed to be the sacral parasympathetic nucleus. Many of the neurons which contribute to the contralateral bundle are located adjacent to the ventral nucleus of the lateral lemniscus. The nucleus reticularis gigantocellularis projects mainly via the sulcomarginal, ventral and lateral funiculi to laminae VIII and adjacent portions of lamina VII. The nucleus reticularis gigantocellularis pars ventralis innervates the same laminae; but, in addition, projects heavily to laminae I and II, to lateral portions of laminae IV through VII; to laminae IX and X and to the intermediolateral cell column. Axons destined for laminae I and II, as well as IV through VII and X, traverse the dorsolateral funiculi as described for the cat by Basbaum et al. ('78). Neurons within the nucleus reticularis parvocellularis project to cervical levels, mainly through the ventral funiculi. In general our results show that reticulospinal projections are more complex than suggested by degeneration methods and that laminae I, II. lateral parts of laminae IV-VII, laminae IX and X, as well as the intermediolateral cell column and sacral parasympathetic nucleus are targets of axons from specific areas.

Effects of spinal cord transections on lordosis reflex in female rats

(1) Female rats with complete transections of spinal cord at low thoracic levels were not able to perform the lordosis reflex. (2) Females with bilateral transections of dorsal columns, dorsolateral columns or entire dorsal half of spinal cord performed lordosis in a normal way. (3) Bilateral transection of fibers in the ventromedial columns, added to transection of dorsal columns or entire dorsal half of the cord still allowed lordosis to be performed. (4) Large bilateral transections of the anterolateran columns caused severe loss in the strength and frequency of the lordosis reflex. Effective transections often included some ventromedial or dorsolateral column fibers, and were accompanied by abnormalities of locomotion. (5) We conclude that supraspinal control is required for the normal lordosis reflex, and that fibers necessary and sufficient for lordosis run in the anterolateral columns. These fibers are likely to be dispersed throughout the anterolateral columns, since large transections were required to eliminate lordosis, and also to be involved in (or interspersed with other fibers involved in) other aspects of motor control. Candidate ascending systems are the anterolateral fibers of Mehler. Candidate descending systems are the lateral vestibulospinal and reticulospinal tracts.