Eike D. Schomburg

Spinal sensorimotor systems and their supraspinal control.

Abstract Spinal motor systems receive a convergent input from descending tracts and peripheral afferents. As a rule, there is multiform descending convergence from different supraspinal centers and wide multisensorial convergence from different peripheral afferents. The balance between the descending and segmental input may vary between different systems and different cells within a system. The different spinal motor systems do not consist of homogeneous interneuronal populations, but of different interneuronal subgroups with different patterns of descending and peripheral afferent convergence and with different patterns of projection to different target motoneurons. The basic function of spinal motor systems in motor control is to initiate and organize the course of movements. Three components of this function may be discerned: (a) Transmission and coordination of descending information for the performance of centrally induced movements and their adaptation to peripheral conditions. Different subgroups of interneurons may be selected for different motor tasks. (b) Transformation of afferent sensory information to motor patterns. These ‘spinal reflexes’ are not stereotyped responses to a given sensory input. Their amplitude and direction depend on the descending and segmental conditions and, accordingly, the utilization of different available pathways. (c) Endogenous generation and/or participation in the performance of complex movement patterns (e.g. spinal locomotion, scratching and possibly breathing). The three functions are given in the order of their presumed importance for motor control in higher vertebrates. Under natural conditions they are interwoven and complement each other.

Neuronal LRP1 Functionally Associates with Postsynaptic Proteins and Is Required for Normal Motor Function in Mice

ABSTRACT The LDL receptor-related protein 1 (LRP1) is a multifunctional cell surface receptor that is highly expressed on neurons. Neuronal LRP1 in vitro can mediate ligand endocytosis, as well as modulate signal transduction processes. However, little is known about its role in the intact nervous system. Here, we report that mice that lack LRP1 selectively in differentiated neurons develop severe behavioral and motor abnormalities, including hyperactivity, tremor, and dystonia. Since their central nervous systems appear histoanatomically normal, we suggest that this phenotype is likely attributable to abnormal neurotransmission. This conclusion is supported by studies of primary cultured neurons that show that LRP1 is present in close proximity to the N-methyl-d-aspartate (NMDA) receptor in dendritic synapses and can be coprecipitated with NMDA receptor subunits and the postsynaptic density protein PSD-95 from neuronal cell lysates. Moreover, treatment with NMDA, but not dopamine, reduces the interaction of LRP1 with PSD-95, indicating that LRP1 participates in transmitter-dependent postsynaptic responses. Together, these findings suggest that LRP1, like other ApoE receptors, can modulate synaptic transmission in the brain.

NO mediates microglial response to acute spinal cord injury under ATP control in vivo

To understand the pathomechanisms of spinal cord injuries will be a prerequisite to develop efficient therapies. By investigating acute lesions of spinal cord white matter in anesthetized mice with fluorescently labeled microglia and axons using in vivo two‐photon laser‐scanning microscopy (2P‐LSM), we identified the messenger nitric oxide (NO) as a modulator of injury‐activated microglia. Local tissue damages evoked by high‐power laser pulses provoked an immediate attraction of microglial processes. Spinal superfusion with NO synthase and guanylate cyclase inhibitors blocked these extensions. Furthermore, local injection of the NO‐donor spermine NONOate (SPNO) or the NO‐dependent second messenger cGMP induced efficient migration of microglial cells toward the injection site. High‐tissue levels of NO, achieved by uniform superfusion with SPNO and mimicking extended tissue damage, resulted in a fast conversion of the microglial shape from ramified to ameboid indicating cellular activation. When the spinal white matter was preconditioned by increased, ambient ATP (known as a microglial chemoattractant) levels, the attraction of microglial processes to local NO release was augmented, whereas it was abolished at low levels of tissue ATP. Because both signaling molecules, NO and ATP, mediate acute microglial reactions, coordinated pharmacological targeting of NO and purinergic pathways will be an effective mean to influence the innate immune processes after spinal cord injury.

Synaptic responses of lumbar α-motoneurones to selective stimulation of cutaneous nociceptors and low threshold mechanoreceptors in the spinal cat

Summary1. The reflex projection from afferents of cutaneous nociceptors and low threshold mechanoreceptors onto intracellularly recorded lumbar α-motoneurones was tested in high spinal unanaesthetized cats. Low threshold mechanoreceptors were activated by light stroking of the hairy skin of the foot, nociceptors by radiant heat and in a few cases, for comparison, by pinching of the skin. In each experiment only one cutaneous nerve (Sur, SPC, Saph or Tib) was left intact. 2. Flexor motoneurones (PBSt) generally showed a depolarization together with an increase of synaptic noise to both types of stimulation, the amplitude during nociceptive stimulation being larger in most cases. 3. In extensor motoneurones (GS, ABSm) the responses to noxious and mechanical skin stimulation were more variable. Only a slight depolarization or no change of the level of the membrane potential, together with an increase of synaptic noise, was observed in most cases. Besides that, minor hyperpolarizations or transitions from light depolarization to hyperpolarization were induced during stimulation. The effects of noxious and mechanoreceptive skin stimulation were not strictly related to the effects of electrical stimulation of flexor reflex afferents. 4. It is assumed that particularly for extensor motoneurones, the excitatory and the inhibitory segmental reflex pathways were activated in parallel during skin stimulation. The simultaneous action of the inhibitory pathway in addition to the excitatory one may serve as a mechanism to neutralize unwanted surplus excitation.

Synaptic effects from chemically activated fine muscle afferents upon α-motoneurones in decerebrate and spinal cats

In spinal and decerebrate cats fine muscle afferents (group III and IV) were selectively activated by intra-arterial injection of bradykinin and KCl into the gastrocnemius-soleus muscle. By this method the synaptic responses induced in lumbar alpha-motoneurones by fine muscle afferents could be examined without interference of effects from large afferents. alpha-Motoneurones receiving EPSPs evoked by electrical stimulation of cutaneous and high threshold muscle afferents (mainly flexor motoneurones) responded to the activation of fine muscle afferents with a depolarization of their membrane and an increase in synaptic noise, while motoneurones in which IPSPs were evoked by electrical stimulation of cutaneous and high threshold muscle afferents (mainly extensor motoneurones), responded with hyperpolarization of their membrane. Cells with mixed excitatory-inhibitory electrically induced response characteristic responded with an increase in synaptic noise without substantial change in the level of their membrane potential to chemical stimulation of fine muscle afferents. The results indicate that one function of group III and IV muscle afferents is to participate in the complex reflex control performed by the flexor reflex system.

Phasic control of the transmission in the excitatory and inhibitory reflex pathways from cutaneous afferents to α-motoneurones during fictive locomotion in cats

Abstract In high spinal paralyzed cats the effect of cutaneous nerve stimulation on lumbar motoneurons was investigated during fictive locomotion. EPSPs evoked from the cutaneous afferents were generally larger during the active phase of the motoneurones, while IPSPs tended to increase during the reciprocal phase. In some cases EPSPs occurred during the active phase, while IPSPs dominated during the reciprocal phase. Apparently, the transmission in the excitatory and inhibitory segmental reflex pathways from cutaneous afferents to α-motoneurones depends on the phase of the step cycle, but there is no general phase dependent alternating switching between these two pathways.

Contribution of group III and IV muscle afferents to multisensorial spinal motor control in cats.

The contribution of group III and IV muscle afferents to multisensorial segmental reflex pathways was investigated by testing for spatial facilitation between these afferents and non-nociceptive segmental afferents from skin, muscles and joints on postsynaptic potentials (PSPs) in alpha-motoneurones recorded in anaemically decapitated high spinal cats. Group III and IV muscle afferents were activated by intraarterial injection of potassium chloride (320 mM) or bradykinin triacetate (81 microM). Skin, joint and group I-II muscle afferents were stimulated by graded electrical stimulation of various nerves. Conditioning by stimulation of group III and IV muscle afferents spatially facilitated the transmission in segmental reflex pathways from low- to medium-threshold cutaneous and joint afferents as well as from lb and group II muscle afferents. Both excitatory and inhibitory pathways from these afferents were facilitated. Monosynaptic excitation and disynaptic antagonistic inhibition from group Ia afferents remained unaffected. It is concluded that the spatial facilitation observed between group III and IV muscle afferents and the other afferents indicate a convergence from group III and IV muscle afferents and the other afferents on common interneurones in segmental flexor reflex pathways. Under physiological conditions they thus contribute to the multisensorial feedback of the flexor reflex pathways. Pathophysiologically, the observed convergence may aggravate muscle weakness and atrophy of muscles induced by group III and IV muscle afferents.

Functional organization of the spinal reflex pathways from forelimb afferents to hindlimb motoneurones in the cat

Abstract The effects of electrically stimulated forelimb afferents on hindlimb motoneurones of high spinal cats were investigated by means of intracellular recordings. The results revealed four essential findings: (1) EPSPs with a minimum latency of 5.6 msec, measured from the incoming volley recorded at C6; (2) IPSPs with a minimum latency of 8 msec, often superimposed on the EPSPs; (3) long lasting, late hyperpolarizations with a latency of 25–60 msec; and (4) early IPSPs with a minimum latency of 3.1 msec evoked exclusively in FDL motoneurones. With the exception of sartorius motoneurones, which received almost pure inhibition, combined excitatory-inhibitory effects were observed in all species of hindlimb motoneurones, although the effects in ankle extensors (GS, P1) were generally larger than those in ankle flexors (DP, SPM). The relation between excitation and inhibition was variable, so that in extreme cases almost pure EPSPs or IPSPs could occur. Except for their time course, the conditions for mediating EPSPs and IPSPs were similar: they were evoked from medium to high threshold afferents of both sides and were easily diminished by increasing the stimulation frequency. It is discussed whether, although the excitatory and inhibitory pathways are activated together, the final action on a motoneurone is dependent on the phase or position of the hindlimb. By comparison, the early IPSP evoked specifically in FDL motoneurones had fundamentally different characteristics: it was mainly evoked from low threshold afferents of more distal cutaneous or mixed nerves from the ipsilateral side only and it followed stimulus frequencies up to about 100 Hz. It is proposed that this inhibition prevents plantar flexion of the toes during the first extension phase of the hindlimb.

Phase-dependent transmission in the excitatory propriospinal reflex pathway from forelimb afferents to lumbar motoneurones during fictive locomotion.

In high spinal paralyzed cats the effect of forelimb nerve stimulation on hindlimb motoneurones was investigated during fictive locomotion, which was induced by injection of nialamide and L-DOPA. The EPSPs which were evoked by forelimb nerve stimulation in almost all species of hindlimb motoneurones showed a distinct dependence on the phase of the step cycle. In motoneurones to extensor they were only observed during the extension phase, in those to flexors only during the flexion phase. It is assumed that the transmission in the descending propriospinal excitatory reflex pathway is cyclically modulated at the lumbar level.

In Vivo Imaging Reveals Distinct Inflammatory Activity of CNS Microglia versus PNS Macrophages in a Mouse Model for ALS

Mutations in the enzyme superoxide dismutase-1 (SOD1) cause hereditary variants of the fatal motor neuronal disease Amyotrophic lateral sclerosis (ALS). Pathophysiology of the disease is non-cell-autonomous: neurotoxicity is derived not only from mutant motor neurons but also from mutant neighbouring non-neuronal cells. In vivo imaging by two-photon laser-scanning microscopy was used to compare the role of microglia/macrophage-related neuroinflammation in the CNS and PNS using ALS-linked transgenic SOD1G93A mice. These mice contained labeled projection neurons and labeled microglia/macrophages. In the affected lateral spinal cord (in contrast to non-affected dorsal columns), different phases of microglia-mediated inflammation were observed: highly reactive microglial cells in preclinical stages (in 60-day-old mice the reaction to axonal transection was ∼180% of control) and morphologically transformed microglia that have lost their function of tissue surveillance and injury-directed response in clinical stages (reaction to axonal transection was lower than 50% of control). Furthermore, unlike CNS microglia, macrophages of the PNS lack any substantial morphological reaction while preclinical degeneration of peripheral motor axons and neuromuscular junctions was observed. We present in vivo evidence for a different inflammatory activity of microglia and macrophages: an aberrant neuroinflammatory response of microglia in the CNS and an apparently mainly neurodegenerative process in the PNS.