Lack of a bilateral projection of individual spinal neurons to the lateral reticular nucleus in the rat: a retrograde, non-fluorescent, double labeling study

Action of Norepinephrine on Lamina X of the Spinal Cord

Lamina X is localized in the spinal cord within the region surrounding the central canal and receives descending projections from the supraspinal nuclei. Norepinephrine (NE) is a neurotransmitter in descending pathways emanating from the brain stem; NE-containing fibers terminate in the spinal dorsal cord, particularly in the substantia gelatinosa (SG). NE enhances inhibitory synaptic transmission in SG neurons by activating presynaptic α1-receptors and hyperpolarizes the membranes of SG neurons by acting on α2-receptors; NE may thus act directly on SG neurons of the dorsal spinal cord and inhibit nociceptive transmission at the spinal level. NE-containing fibers also reportedly terminate in lamina X, suggesting that NE also modulates synaptic transmission in lamina X. However, the cellular mechanisms underlying such action have not been investigated. We hypothesized that NE might directly act on lamina X and enhance inhibitory synaptic transmission therein. Using rat spinal cord slices and in vitro whole-cell patch-clamps, we found that the bath-application of NE to lamina X does not affect the excitatory interneurons but enhances GABAergic and glycinergic miniature inhibitory postsynaptic currents (mIPSCs) and induces an outward current. NE-induced enhancement of mIPSCs was blocked by α1A-receptor antagonists, and NE-induced outward current was blocked by α2-receptor antagonists. NE did not affect GABA- or glycine- induced outward currents. These findings are similar to those obtained from SG neurons: NE may act at presynaptic terminals of GABAergic and glycinergic interneurons on lamina X to facilitate inhibitory-transmitter release through α1A-receptor activation and directly induce inhibitory interneuron membrane hyperpolarization through α2-receptors activation.

The spino-bulbar-cerebellar pathway: organization and neurochemical properties of spinal cells that project to the lateral reticular nucleus in the rat

In addition to classical spinocerebellar pathways, the cerebellum receives information from the spinal cord indirectly via spino-bulbar-cerebellar systems. One of the structures in this pathway is the lateral reticular nucleus (LRt). We performed series of experiments to investigate the organization and neurotransmitter content of spinoreticular tract (SRT) neurons in the lumbar spinal cord that project to the LRt. Three rats received injections of the b subunit of Cholera toxin (CTb) or Fluorogold (FG) within the left and right LRt. The majority of SRT cells (56–61%) were found within the contralateral medial intermediate gray matter where small numbers (7–10%) of double-labeled cells were also present on both sides of the cord. Six rats received unilateral spinal injections of CTb to label spinal projections to the LRt. Injections of FG were made also into the anterior lobe of the cerebellum to label LRt pre-cerebellar neurons. Terminals were found mainly ipsilateral to spinal injection sites within the central and ventrolateral regions of the LRt. Immunocytochemical analysis of SRT terminals revealed that the majority (75%) were contained vesicular glutamate transporter 2 but a minority (20%) contained the vesicular GABA transporter. The inhibitory subpopulation was found to be GABAergic, glycinergic, or contained both transmitters. Inhibitory and excitatory terminals were present within overlapping regions of the nucleus. Most CTb terminals contacting LRt pre-cerebellar neurons were excitatory (80%) whereas a minority were inhibitory and most cells (88%) received contacts from both inhibitory and excitatory terminals. This study shows that SRT axons in the LRt have the capacity to exert direct excitatory and inhibitory actions on LRt pre-cerebellar neurons. Thus spinal cord input has the capacity to facilitate or depress the activity of individual LRt cells which in turn adjust activity in the cerebellum to produce coordinated motor behaviors.

Motor-circuit communication matrix from spinal cord to brainstem neurons revealed by developmental origin

Accurate motor-task execution relies on continuous comparison of planned and performed actions. Motor-output pathways establish internal circuit collaterals for this purpose. Here we focus on motor collateral organization between spinal cord and upstream neurons in the brainstem. We used a newly developed mouse genetic tool intersectionally with viruses to uncover the connectivity rules of these ascending pathways by capturing the transient expression of neuronal subpopulation determinants. We reveal a widespread and diverse network of spinal dual-axon neurons, with coincident input to forelimb motor neurons and the lateral reticular nucleus (LRN) in the brainstem. Spinal information to the LRN is not segregated by motor pool or neurotransmitter identity. Instead, it is organized according to the developmental domain origin of the progenitor cells. Thus, excerpts of most spinal information destined for action are relayed to supraspinal centers through exquisitely organized ascending connectivity modules, enabling precise communication between command and execution centers of movement.

Collateralization of cerebellar output to functionally distinct brainstem areas. A retrograde, non-fluorescent tracing study in the rat

The organization of the cerebellum is characterized by a number of longitudinally organized connection patterns that consist of matching olivo-cortico-nuclear zones. These entities, referred to as modules, have been suggested to act as functional units. The various parts of the cerebellar nuclei (CN) constitute the output of these modules. We have studied to what extent divergent and convergent patterns in the output of the modules to four, functionally distinct brain areas can be recognized. Two retrograde tracers were injected in various combinations of the following nuclei: the red nucleus (RN), as a main premotor nucleus; the prerubral area, as a main supplier of afferents to the inferior olive (IO); the nucleus reticularis tegmenti pontis (NRTP), as a main source of cerebellar mossy fibers; and the IO, as the source of climbing fibers. For all six potential combinations three cases were examined. All nine cases with combinations that involved the IO did not, or hardly, resulted in double labeled neurons. In contrast, all other combinations resulted in at least 10% and up to 67% of double labeled neurons in cerebellar nuclear areas where both tracers were found. These results show that the cerebellar nuclear neurons that terminate within the studied areas represent basically two intermingled populations of projection cells. One population corresponds to the small nucleo-olivary neurons whereas the other consists of medium- to large-sized neurons which are likely to distribute their axons to several other areas. Despite some consistent differences between the output patterns of individual modules we propose that modular cerebellar output to premotor areas such as the RN provides simultaneous feedback to both the mossy fiber and the climbing fiber system and acts in concert with a designated GABAergic nucleo-olivary circuit. These features seem to form a basic characteristic of cerebellar operation.

A light microscope-based double retrograde tracer strategy to chart central neuronal connections

This protocol describes a double retrograde tracing method to chart divergent projections in the CNS using light microscope techniques. It is based on immunohistochemical visualization of retrograde transport of cholera toxin b-subunit (CTb) and silver enhancement of a gold-lectin conjugate. Production of the gold-lectin is explained in detail, and a technique is offered to record through the injection pipettes, to help guide accurate placement of injections. Visualization of the two tracers results in light brown staining of CTb-labeled neurons and labeling by black particles of gold-lectin-containing neurons. Both types of label are easily recognized in the same neuron. The labeling is permanent and is well suited for studies in which large areas of the brain need to be surveyed. The whole procedure (excluding survival time) takes approximately 5-7 d to complete.

Correlation of noxious evoked c-fos expression in areas of the somatosensory system during chronic pain: Involvement of spino-medullary and intra-medullary connections

Chronic pain induces functional alterations of the endogenous pain control system namely in the modulation of nociceptive transmission at the spinal cord. We used the c-fos expression as a tool to study correlated neuronal activation, induced by bending the inflamed paw of monoarthritic animals, between the spinal dorsal horn and medullary centers belonging to the endogenous pain control system, namely the lateralmost reticular formation of the ventrolateral medulla (VLMlat), the lateral reticular nucleus (LRt), the dorsal reticular nucleus (DRt), the nucleus tractus solitarius (Sol) and the rostroventromedial medulla (RVM). Awake monoarthritic rats were subjected to 4 min of paw bending followed by anaesthesia and perfusion either immediately or 2h later. The numbers of Fos immunoreactive neurons in the spinal dorsal horn and in the medulla oblongata were significantly correlated mainly immediately after stimulation: lamina I correlated with the VLMlat, LRt, Sol and RVM; lamina II correlated with the VLMlat, LRt and Sol; and laminae IV-V correlated with the VLMlat and LRt. Between medullary pain control centers significant correlations occurred immediately and 2h after bending at the VLMlat-Sol and LRt-Sol, at the VLMlat-LRt and VLMlat-RVM in animals perfused immediately, and at the VLMlat-DRt and LRt-RVM in animals perfused 2h later. These data demonstrate that the mobilization of a chronically inflamed paw triggers intense correlated neuronal activity in several areas of the somatosensory system, indicating functional relevant links in pain control.

Brain afferents to the lateral caudal ventrolateral medulla: a retrograde and anterograde tracing study in the rat

The ventrolateral medulla (VLM) modulates autonomic functions, motor reactions and pain responses. The lateralmost part of the caudal VLM (VLMlat) was recently shown to be the VLM area responsible for pain modulation. In the present study, the brain sources of VLMlat afferent fibers were determined by tract-tracing techniques. Following injection of cholera toxin subunit B into the VLMlat, retrogradely labeled neurons in the forebrain occurred at the somatosensory, insular, motor, limbic and infralimbic cortices, and at the central amygdaloid nucleus. Retrogradely labeled neurons in diencephalic regions were observed in the lateral hypothalamus, posterior hypothalamus and paraventricular nucleus. In the brainstem, retrograde labeling occurred at the periaqueductal gray, red nucleus, parabrachial area, nucleus raphe magnus, nucleus tractus solitarii, lateral reticular nucleus and dorsal and ventral medullary reticular formation. In the cerebellum, retrogradely labeled neurons occurred at the lateral nucleus. Following injections of the anterograde tracer biotinylated dextran amine (BDA) into the lateral hypothalamus or paraventricular nucleus, anterogradely labeled fibers were mainly observed in the VLMlat. Injections of BDA into the periaqueductal gray, red nucleus or lateral nucleus of the cerebellum resulted in anterograde labeling in the VLMlat and lateral reticular nucleus. The present study gives an account of the brain regions putatively involved in triggering the modulatory actions elicited from the VLMlat. These include areas committed to somatosensory processing, autonomic control, somatic and visceral motor activity and affective reactions. The findings suggest that the VLMlat may play a major homeostatic role in the integration of nociception with other brain functions.

Neurons labeled from locomotor-related ventrolateral funiculus stimulus sites in the neonatal rat spinal cord

Spinal cord/brainstem preparations from 5- to 8-day-old rats, maintained in vitro, were used to determine the cells of origin and regions of termination of fibers in the superficial ventrolateral funiculus (VLF) at a site from which rhythmic locomotor-like activity can be induced. Rhythmic locomotor-like activity was recorded from lumbar ventral roots after short trains of stimuli (50 Hz for 0.5-2 seconds) delivered to the VLF. Field potential mapping revealed that single VLF stimuli elicited responses in the ipsilateral ventrolateral medulla. Tract-tracing experiments by using biocytin, pressure-injected into the VLF, showed that only a small number of brainstem neurons were labeled and these were scattered bilaterally in the ventrolateral and lateral medulla. Dense concentrations of nerve terminals were found in the lateral reticular nucleus ipsilateral to the stimulation site. Labeled spinal cord neurons included a primary population of large cells distributed bilaterally in lamina VII from T13 to L4, with peak numbers in L2 ipsilaterally and in L3 contralaterally. Intracellular recordings revealed that some L2 and L3 neurons with rhythmic responses to VLF stimulation could be activated antidromically from the VLF, with latencies of less than 1.0 msec. These observations led us to speculate that the superficial VLF carries a locomotor-related tract originating bilaterally in lumbar lamina VII and terminating in the ipsilateral medulla, including the lateral reticular nucleus. This pathway may be part of the spinoreticular or spinoreticulotectal pathway that has been described in many species, the function of which has only loosely been ascribed.

Projection to the lateral reticular nucleus from neurons located in C6-C7 segments of the spinal cord. An electrophysiological study in the cat

Spinoreticular neurons projecting to the lateral reticular nucleus (LRN) were investigated electrophysiologically in the cervical enlargement of the cat spinal cord. Experiments were performed on alpha-chloralose anaesthetized animals. Antidromic action potentials were recorded extracellularly from cells located in C6 and C7 segments following stimulation of the ipsilateral LRN. The total sample included 50 neurons. Their cell bodies were found to be distributed in Rexed's laminae VI-VIII of the gray matter. Axonal conduction velocities ranged from 14.7 to 89.7 ms-1. Considerable differences between particular cases enabled two separate groups of slower and faster conduction to be distinguished. Values for these two groups were 14.7-44.3 ms-1 and 52.2-89.7 ms-1, respectively. Discrete differences with regard to the location of these groups were also pointed out. Such differentiation suggests that a proportion of axons from the slower conducting pool may be in fact collaterals of neurons that project to other brainstem centers or to lower levels of the spinal cord.

Single Purkinje cell can innervate multiple classes of projection neurons in the cerebellar nuclei of the rat: a light microscopic and ultrastructural triple-tracer study in the rat

Two different populations of projection neurons are intermingled in the cerebellar nuclei. One group consists of small, gamma-aminobutyric acid-containing (GABAergic) neurons that project to the inferior olive, and the other group consists of larger, non-GABAergic neurons that provide an input to one or more, usually premotor, centers in the brainstem, such as the red nucleus, the thalamus, and the superior colliculus. All cerebellar nuclear neurons are innervated by GABAergic Purkinje cells. In this study, we investigated whether individual Purkinje cells of the C1 zone of the paramedian lobe of the rat innervate both groups of projection neurons in the anterior interposed nucleus. Two different, retrogradely transported tracers, either cholera toxin beta subunit (CTb) or wheat germ agglutinin coupled to horseradish peroxidase (WGA-HRP) and a gold lectin tracer were injected into the red nucleus and the inferior olive, respectively, whereas Purkinje cell axons were anterogradely labeled with biotinylated dextran amine (BDA) injected into the paramedian lobule. Cerebellar nuclear sections studied with the light microscope demonstrated a close relation of varicosities from BDA-labeled Purkinje cell axons with both gold lectin- and CTb-labeled neurons. Branches of individual axons could be traced to both retrogradely labeled cell populations. At the ultrastructural level, synapses of labeled Purkinje cell terminals with profiles of WGA-HRP-labeled projection neurons predominated over contacts with gold lectin-containing neurons. Nine out of 367 investigated BDA-labeled terminals were observed to be presynaptic to a WGA-HRP-labeled profile as well as to a gold lectin-labeled profile. This indicates that nuclear cells that project to the inferior olive as well as those that project to premotor centers are under the influence of the same Purkinje cells. Such an arrangement would suggest an in-phase cortical modulation of the activation patterns of the inhibitory cells that project to the inferior olive and excitatory cells that project to premotor nuclei, which could explain why olivary neurons, especially those of the rostral part of the dorsal accessory olive, appear to be unresponsive to stimuli generated during active movement.

Convergence of central respiratory and locomotor rhythms onto single neurons of the lateral reticular nucleus

We have analyzed the behavior of neurons of the lateral reticular nucleus (LRN) during fictive respiration and locomotion and found that some LRN neurons have both central respiratory and locomotor. rhythms. Experiments were conducted on decerebrate, decerebellate, immobilized, and artificially ventilated cats, with the spinal cord transected at the lower thoracic cord. Fictive respiration and fictive forelimb locomotion were ascertained by monitoring activities from the phrenic nerve and forelimb extensor and flexor nerves, respectively. Fictive locomotion was evoked by electrical stimulation of the mesencephalic locomotor region (MLR) or sometimes occurred spontaneously. During fictive locomotion many LRN neurons fired in certain phases of the locomotion cycle; i.e., with respect to the nerve discharge of the ipsilateral forelimb they fired in either the extensor, flexor, extensor-flexor, or flexor-extensor phase. Firing of some LRN neurons was modulated synchronously with central respiratory rhythm. Neurons with inspiratory activity and those with expiratory activity were both found. More than half of these respiration-related LRN neurons had locomotor rhythm as well. The majority of the three types of LRN neurons, i.e., neurons with only locomotor rhythm, those with only respiratory rhythm, and those with both respiratory and locomotor rhythms, were antidromically activated by electrical stimulation of the ipsilateral inferior cerebellar peduncle. Electrical stimulation of the upper cervical cord showed that these LRN neurons, not only locomotion-related but also respiration-related neurons, received short latency inputs from the spinal cord. The LRN neurons studied were distributed widely in the LRN, relatively densely in the caudal two-thirds of the nucleus. No particular differences were detected between the three types of LRN neurons with respect to their location in the nucleus. These results indicate that the information about central respiratory and locomotor rhythms that is necessary for cerebellar control of the coordination between respiration and locomotion converges, at least partly, at the level of the LRN.