Noxious colorectal distention in spinalized rats reduces pseudorabies virus labeling of sympathetic neurons

The Neuroplastic and Therapeutic Potential of Spinal Interneurons in the Injured Spinal Cord

The central nervous system is not a static, hard-wired organ. Examples of neuroplasticity, whether at the level of the synapse, the cell, or within and between circuits, can be found during development, throughout the progression of disease, or after injury. One essential component of the molecular, anatomical, and functional changes associated with neuroplasticity is the spinal interneuron (SpIN). Here, we draw on recent multidisciplinary studies to identify and interrogate subsets of SpINs and their roles in locomotor and respiratory circuits. We highlight some of the recent progress that elucidates the importance of SpINs in circuits affected by spinal cord injury (SCI), especially those within respiratory networks; we also discuss potential ways that spinal neuroplasticity can be therapeutically harnessed for recovery.

Neuroanatomical autonomic substrates of brainstem-gut circuitry identified using transsynaptic tract-tracing with pseudorabies virus recombinants

To investigate autonomic substrates of brainstem-gut circuitry identified using trans-synaptic tracing with pseudorabies virus (PRV)-152, a strain that expresses enhanced green fluorescent protein, and PRV-614, a strain that expresses enhanced red fluorescent protein, injecting into the rat rectum wall. 3-7 days after PRV-152 injection, spinal cord and brainstem were removed and sectioned, and processed for PRV-152 visualization using immunofluorescence labeling against PRV-152. 6 days after PRV-614 injection, brainstem was sectioned and the neurochemical phenotype of PRV-614-positive neurons was identified using double immunocytochemical labeling against PRV-614 and TPH. We observed that the largest number of PRV-152- or PRV-614-positive neurons was located in the gigantocellular reticular nucleus (Gi), lateral paragigantocellular (LPGi), rostral ventrolateral reticular nucleus (RVL), solitary tract nucleus (Sol), locus coeruleus (LC), raphe magnus nucleus (RMg), subcoeruleus nucleus (SubCD). Double-labeled PRV-614/tryptophan hydroxylase (TPH) neurons were concentrated in the RMg, LPGi and Sol. These brainstem neurons are candidates for relaying autonomic command signals to the gut. The autonomic substrate of brainstem-gut circuitry likely plays an important role in mediating different aspects of stress behaviors.

Silencing spinal interneurons inhibits immune suppressive autonomic reflexes caused by spinal cord injury

Spinal cord injury (SCI) at high spinal levels (e.g., above thoracic level 5) causes systemic immune suppression; however, the underlying mechanisms are unknown. Here, we show that profound plasticity develops within spinal autonomic circuitry below the injury, creating a sympathetic anti-inflammatory reflex, and that chemogenetic silencing of this reflex circuitry blocks post-SCI immune suppression. These data provide new insights and potential therapeutic options for limiting the devastating consequences of post-traumatic autonomic hyperreflexia and post-injury immune suppression.

Spinal interneurons and forelimb plasticity after incomplete cervical spinal cord injury in adult rats

Cervical spinal cord injury (cSCI) disrupts bulbospinal projections to motoneurons controlling the upper limbs, resulting in significant functional impairments. Ongoing clinical and experimental research has revealed several lines of evidence for functional neuroplasticity and recovery of upper extremity function after SCI. The underlying neural substrates, however, have not been thoroughly characterized. The goals of the present study were to map the intraspinal motor circuitry associated with a defined upper extremity muscle, and evaluate chronic changes in the distribution of this circuit following incomplete cSCI. Injured animals received a high cervical (C2) lateral hemisection (Hx), which compromises supraspinal input to ipsilateral spinal motoneurons controlling the upper extremities (forelimb) in the adult rat. A battery of behavioral tests was used to characterize the time course and extent of forelimb motor recovery over a 16 week period post-injury. A retrograde transneuronal tracer - pseudorabies virus - was used to define the motor and pre-motor circuitry controlling the extensor carpi radialis longus (ECRL) muscle in spinal intact and injured animals. In the spinal intact rat, labeling was observed unilaterally within the ECRL motoneuron pool and within spinal interneurons bilaterally distributed within the dorsal horn and intermediate gray matter. No changes in labeling were observed 16 weeks post-injury, despite a moderate degree of recovery of forelimb motor function. These results suggest that recovery of the forelimb function assessed following C2Hx injury does not involve recruitment of new interneurons into the ipsilateral ECRL motor pathway. However, the functional significance of these existing interneurons to motor recovery requires further exploration.

Central circuits regulating the sympathetic outflow to lumbar muscles in spinally transected mice by retrograde transsynaptic transport

Despite considerable interest in the mechanisms that control the hyperalgesia associated with muscle inflammation, the CNS descending pathways that coordinate autonomic circuits regulating lumbar muscles are not adequately understood. Here we used both pseudorabies virus (PRV)-614 retrograde transsynaptic tracing and spinally transected method in 33 C57BL/6J mice to map the polysynaptic pathways between lumbar muscle and CNS. Tissues were processed for dual-label immunocytochemical detection between PRV-614 and tryptophan hydroxylase (TPH) or tyrosine hydroxylase (TH)-expressing neurons in CNS. In intact mice, PRV-614 was transported to the intermediolateral column (IML) and ventral horn (VH) of spinal cord, with subsequent transport to many brain regions, including the medullary raphe nuclei, rostral ventrolateral medulla (RVLM), A5 cell group regions (A5), locus coeruleus (LC), the medullary and pontine reticular formation nucleus (MRN and PRN), paraventricular nucleus of the hypothalamus (PVN), and other central sites. However, PRV-614 in spinally transected mice produced retrograde infection of IML, with subsequent transport to main brain regions that have been shown to contribute to regulating sympathetic circuits, including RVLM, Lateral paragigantocellular reticular nucleus (LPGi), A5, LC, and PVN, whereas PRV-614 labeling in VH and MRN was eliminated in almost every case. In above five brain regions, dual-labeling immunocytochemistry showed coexpression of PRV-614/TPH and PRV-614/TH immunoreactive (IR) neurons involved in these regulatory circuits. Our results reveal a hierarchical organization of central autonomic circuits controlling the lumbar muscles, thus providing neuroanatomical substrates for the central catecholaminergic and serotonergic system to regulate the lumbar muscles.

Efficient retrograde transport of pseudorabies virus within neurons requires local protein synthesis in axons

After replicating in epithelial cells, alphaherpesviruses such as pseudorabies virus (PRV) invade axons of peripheral nervous system neurons and undergo retrograde transport toward the distant cell bodies. Although several viral proteins engage molecular motors to facilitate transport, the initial steps and neuronal responses to infection are poorly understood. Using compartmented neuron cultures to physically separate axon infection from cell bodies, we found that PRV infection induces local protein synthesis in axons, including proteins involved in cytoskeletal remodeling, intracellular trafficking, signaling, and metabolism. This rapid translation of axonal mRNAs is required for efficient PRV retrograde transport and infection of cell bodies. Furthermore, induction of axonal damage, which also induces local protein synthesis, prior to infection reduces virion trafficking, suggesting that host damage signals and virus particles compete for retrograde transport. Thus, similar to axonal damage, virus infection induces local protein translation in axons, and viruses likely exploit this response for invasion.

Translational spinal cord injury research: preclinical guidelines and challenges

Advances in the neurobiology of spinal cord injury (SCI) have prompted increasing attention to opportunities for moving experimental strategies towards clinical applications. Preclinical studies are the centerpiece of the translational process. A major challenge is to establish strategies for achieving optimal translational progression while minimizing potential repetition of previous disappointments associated with clinical trials. This chapter reviews and expands upon views pertaining to preclinical design reported in recently published opinion surveys. Subsequent discussion addresses other preclinical considerations more specifically related to current and potentially imminent cellular and pharmacological approaches to acute/subacute and chronic SCI. Lastly, a retrospective and prospective analysis examines how guidelines currently under discussion relate to select examples of past, current, and future clinical translations. Although achieving definition of the "perfect" preclinical scenario is difficult to envision, this review identifies therapeutic robustness and independent replication of promising experimental findings as absolutely critical prerequisites for clinical translation. Unfortunately, neither has been fully embraced thus far. Accordingly, this review challenges the notion "everything works in animals and nothing in humans", since more rigor must first be incorporated into the bench-to-bedside translational process by all concerned, whether in academia, clinical medicine, or corporate circles.

Respiratory function following bilateral mid-cervical contusion injury in the adult rat

The consequences of spinal cord injury (SCI) are often viewed as the result of white matter damage. However, injuries occurring at any spinal level, especially in cervical and lumbar enlargement regions, also entail segmental neuronal loss. Yet, the contributions of gray matter injury and plasticity to functional outcomes are poorly understood. The present study addressed this issue by investigating changes in respiratory function following bilateral C(3)/C(4) contusion injuries at the level of the phrenic motoneuron (PhMN) pool which in the adult rat extends from C(3) to C(5/6) and provides innervation to the diaphragm. Despite extensive white and gray matter pathology associated with two magnitudes of injury severity, ventilation was relatively unaffected during both quiet breathing and respiratory challenge (hypercapnia). On the other hand, bilateral diaphragm EMG recordings revealed that the ability to increase diaphragm activity during respiratory challenge was substantially, and chronically, impaired. This deficit has not been seen following predominantly white matter lesions at higher cervical levels. Thus, the impact of gray matter damage relative to PhMNs and/or interneurons becomes evident during conditions associated with increased respiratory drive. Unaltered ventilatory behavior, despite significant deficits in diaphragm function, suggests compensatory neuroplasticity involving recruitment of other spinal respiratory networks which may entail remodeling of connections. Transynaptic tracing, using pseudorabies virus (PRV), revealed changes in PhMN-related interneuronal labeling rostral to the site of injury, thus offering insight into the potential anatomical reorganization and spinal plasticity following cervical contusion.