Spinal interneurons infected by renal injection of pseudorabies virus in the rat

Attenuating Neurogenic Sympathetic Hyperreflexia Robustly Improves Antibacterial Immunity After Chronic Spinal Cord Injury

Spinal cord injury (SCI) disrupts critical physiological systems, including the cardiovascular and immune system. Plasticity of spinal circuits below the injury results in abnormal, heightened sympathetic responses, such as extreme, sudden hypertension that hallmarks life-threatening autonomic dysreflexia. Moreover, such sympathetic hyperreflexia detrimentally impacts other effector organs, including the spleen, resulting in spinal cord injury-induced immunodeficiency. Consequently, infection is a leading cause of mortality after SCI. Unfortunately, there are no current treatments that prophylactically limit sympathetic hyperreflexia to prevent subsequent effector organ dysfunction. The cytokine soluble tumor necrosis factor α (sTNFα) is upregulated in the CNS within minutes after SCI and remains elevated. Here, we report that commencing intrathecal administration of XPro1595, an inhibitor of sTNFα, at a clinically feasible, postinjury time point (i.e., 3 d after complete SCI) sufficiently diminishes maladaptive plasticity within the spinal sympathetic reflex circuit. This results in less severe autonomic dysreflexia, a real-time gauge of sympathetic hyperreflexia, for months postinjury. Remarkably, delayed delivery of the sTNFα inhibitor prevents sympathetic hyperreflexia-associated splenic atrophy and loss of leukocytes to dramatically improve the endogenous ability of chronic SCI rats to fight off pneumonia, a common cause of hospitalization after injury. The improved immune function with XPro1595 correlates with less noradrenergic fiber sprouting and normalized norepinephrine levels in the spleen, indicating that heightened, central sTNFα signaling drives peripheral, norepinephrine-mediated organ dysfunction, a novel mechanism of action. Thus, our preclinical study supports intrathecally targeting sTNFα as a viable strategy to broadly attenuate sympathetic dysregulation, thereby improving cardiovascular regulation and immunity long after SCI.SIGNIFICANCE STATEMENT Spinal cord injury (SCI) significantly disrupts immunity, thus increasing susceptibility to infection, a leading cause of morbidity in those living with SCI. Here, we report that commencing intrathecal administration of an inhibitor of the proinflammatory cytokine soluble tumor necrosis factor α days after an injury sufficiently diminishes autonomic dysreflexia, a real time gauge of sympathetic hyperreflexia, to prevent associated splenic atrophy. This dramatically improves the endogenous ability of chronically injured rats to fight off pneumonia, a common cause of hospitalization. This preclinical study could have a significant impact for broadly improving quality of life of SCI individuals.

Intraspinal Plasticity Associated With the Development of Autonomic Dysreflexia After Complete Spinal Cord Injury

Traumatic spinal cord injury (SCI) leads to disruption of sensory, motor and autonomic function, and triggers structural, physiological and biochemical changes that cause reorganization of existing circuits that affect functional recovery. Propriospinal neurons (PN) appear to be very plastic within the inhibitory microenvironment of the injured spinal cord by forming compensatory circuits that aid in relaying information across the lesion site and, thus, are being investigated for their potential to promote locomotor recovery after experimental SCI. Yet the role of PN plasticity in autonomic dysfunction is not well characterized, notably, the disruption of supraspinal modulatory signals to spinal sympathetic neurons after SCI at the sixth thoracic spinal segment or above resulting in autonomic dysreflexia (AD). This condition is characterized by unmodulated sympathetic reflexes triggering sporadic hypertension associated with baroreflex mediated bradycardia in response to noxious yet unperceived stimuli below the injury to reduce blood pressure. AD is frequently triggered by pelvic visceral distension (bowel and bladder), and there are documented structural relationships between injury-induced sprouting of pelvic visceral afferent C-fibers. Their excitation of lumbosacral PN, in turn, sprout and relay noxious visceral sensory stimuli to rostral disinhibited thoracic sympathetic preganglionic neurons (SPN) that manifest hypertension. Herein, we review evidence for maladaptive plasticity of PN in neural circuits mediating heightened sympathetic reflexes after complete high thoracic SCI that manifest cardiovascular dysfunction, as well as contemporary research methodologies being employed to unveil the precise contribution of PN plasticity to the pathophysiology underlying AD development.

Soluble TNFα Signaling within the Spinal Cord Contributes to the Development of Autonomic Dysreflexia and Ensuing Vascular and Immune Dysfunction after Spinal Cord Injury

Cardiovascular disease and susceptibility to infection are leading causes of morbidity and mortality for individuals with spinal cord injury (SCI). A major contributor to these is autonomic dysreflexia (AD), an amplified reaction of the autonomic nervous system (hallmarked by severe hypertension) in response to sensory stimuli below the injury. Maladaptive plasticity of the spinal sympathetic reflex circuit below the SCI results in AD intensification over time. Mechanisms underlying this maladaptive plasticity are poorly understood, restricting the identification of treatments. Thus, no preventative treatments are currently available. Neuroinflammation has been implicated in other pathologies associated with hyperexcitable neural circuits. Specifically, the soluble form of TNFα (sTNFα) is known to play a role in neuroplasticity. We hypothesize that persistent expression of sTNFα in spinal cord underlies AD exacerbation. To test this, we intrathecally administered XPro1595, a biologic that renders sTNFα nonfunctional, after complete, high-level SCI in female rats. This dramatically attenuated the intensification of colorectal distension-induced and naturally occurring AD events. This improvement is mediated via decreased sprouting of nociceptive primary afferents and activation of the spinal sympathetic reflex circuit. We also examined peripheral vascular function using ex vivo pressurized arterial preparations and immune function via flow cytometric analysis of splenocytes. Diminishing AD via pharmacological inhibition of sTNFα mitigated ensuing vascular hypersensitivity and immune dysfunction. This is the first demonstration that neuroinflammation-induced sTNFα is critical for altering the spinal sympathetic reflex circuit, elucidating a novel mechanism for AD. Importantly, we identify the first potential pharmacological, prophylactic treatment for this life-threatening syndrome.SIGNIFICANCE STATEMENT Autonomic dysreflexia (AD), a disorder that develops after spinal cord injury (SCI) and is hallmarked by sudden, extreme hypertension, contributes to cardiovascular disease and susceptibility to infection, respectively, two leading causes of mortality and morbidity in SCI patients. We demonstrate that neuroinflammation-induced expression of soluble TNFα plays a critical role in AD, elucidating a novel underlying mechanism. We found that intrathecal administration after SCI of a biologic that inhibits soluble TNFα signaling dramatically attenuates AD and significantly reduces AD-associated peripheral vascular and immune dysfunction. We identified mechanisms behind diminished plasticity of neuronal populations within the spinal sympathetic reflex circuit. This study is the first to pinpoint a potential pharmacological, prophylactic strategy to attenuate AD and ensuing cardiovascular and immune dysfunction.

Autonomic dysreflexia after spinal cord injury: Systemic pathophysiology and methods of management

Traumatic spinal cord injury (SCI) has widespread physiological effects beyond the disruption of sensory and motor function, notably the loss of normal autonomic and cardiovascular control. Injury at or above the sixth thoracic spinal cord segment segregates critical spinal sympathetic neurons from supraspinal modulation which can result in a syndrome known as autonomic dysreflexia (AD). AD is defined as episodic hypertension and concomitant baroreflex-mediated bradycardia initiated by unmodulated sympathetic reflexes in the decentralized cord. This condition is often triggered by noxious yet unperceived visceral or somatic stimuli below the injury level and if severe enough can require immediate medical attention. Herein, we review the pathophysiological mechanisms germane to the development of AD, including maladaptive plasticity of neural circuits mediating abnormal sympathetic reflexes and hypersensitization of peripheral vasculature that collectively contribute to abnormal hemodynamics after SCI. Further, we discuss the systemic effects of recurrent AD and pharmacological treatments used to manage such episodes. Contemporary research avenues are then presented to better understand the relative contributions of underlying mechanisms and to elucidate the effects of recurring AD on cardiovascular and immune functions for developing more targeted and effective treatments to attenuate the development of this insidious syndrome following high-level SCI.

Sympathetic preganglionic neurons: properties and inputs

The sympathetic nervous system comprises one half of the autonomic nervous system and participates in maintaining homeostasis and enabling organisms to respond in an appropriate manner to perturbations in their environment, either internal or external. The sympathetic preganglionic neurons (SPNs) lie within the spinal cord and their axons traverse the ventral horn to exit in ventral roots where they form synapses onto postganglionic neurons. Thus, these neurons are the last point at which the central nervous system can exert an effect to enable changes in sympathetic outflow. This review considers the degree of complexity of sympathetic control occurring at the level of the spinal cord. The morphology and targets of SPNs illustrate the diversity within this group, as do their diverse intrinsic properties which reveal some functional significance of these properties. SPNs show high degrees of coupled activity, mediated through gap junctions, that enables rapid and coordinated responses; these gap junctions contribute to the rhythmic activity so critical to sympathetic outflow. The main inputs onto SPNs are considered; these comprise afferent, descending, and interneuronal influences that themselves enable functionally appropriate changes in SPN activity. The complexity of inputs is further demonstrated by the plethora of receptors that mediate the different responses in SPNs; their origins and effects are plentiful and diverse. Together these different inputs and the intrinsic and coupled activity of SPNs result in the rhythmic nature of sympathetic outflow from the spinal cord, which has a variety of frequencies that can be altered in different conditions.

Spinally projecting preproglucagon axons preferentially innervate sympathetic preganglionic neurons

•

Spinal GLP-1 axons target primarily sympathetic preganglionic neurons.

•

Spinal GLP-1 axons innervate interneurons that may regulate sympathetic outflow.

•

Many GLP-1 neurons in the medulla are spinally-projecting.

•

The lumbar cord contains YFP-expressing neurons that do not innervate the brain.

Glucagon-like peptide-1 (GLP-1) affects central autonomic neurons, including those controlling the cardiovascular system, thermogenesis, and energy balance. Preproglucagon (PPG) neurons, located mainly in the nucleus tractus solitarius (NTS) and medullary reticular formation, produce GLP-1. In transgenic mice expressing glucagon promoter-driven yellow fluorescent protein (YFP), these brainstem PPG neurons project to many central autonomic regions where GLP-1 receptors are expressed. The spinal cord also contains GLP-1 receptor mRNA but the distribution of spinal PPG axons is unknown.

Here, we used two-color immunoperoxidase labeling to examine PPG innervation of spinal segments T1–S4 in YFP-PPG mice. Immunoreactivity for YFP identified spinal PPG axons and perikarya. We classified spinal neurons receiving PPG input by immunoreactivity for choline acetyltransferase (ChAT), nitric oxide synthase (NOS) and/or Fluorogold (FG) retrogradely transported from the peritoneal cavity. FG microinjected at T9 defined cell bodies that supplied spinal PPG innervation.

The deep dorsal horn of lower lumbar cord contained YFP-immunoreactive neurons. Non-varicose, YFP-immunoreactive axons were prominent in the lateral funiculus, ventral white commissure and around the ventral median fissure. In T1–L2, varicose, YFP-containing axons closely apposed many ChAT-immunoreactive sympathetic preganglionic neurons (SPN) in the intermediolateral cell column (IML) and dorsal lamina X. In the sacral parasympathetic nucleus, about 10% of ChAT-immunoreactive preganglionic neurons received YFP appositions, as did occasional ChAT-positive motor neurons throughout the rostrocaudal extent of the ventral horn. YFP appositions also occurred on NOS-immunoreactive spinal interneurons and on spinal YFP-immunoreactive neurons. Injecting FG at T9 retrogradely labeled many YFP-PPG cell bodies in the medulla but none of the spinal YFP-immunoreactive neurons.

These results show that brainstem PPG neurons innervate spinal autonomic and somatic motor neurons. The distributions of spinal PPG axons and spinal GLP-1 receptors correlate well. SPN receive the densest PPG innervation. Brainstem PPG neurons could directly modulate sympathetic outflow through their spinal inputs to SPN or interneurons.

Disordered cardiovascular control after spinal cord injury

Damage to the spinal cord disrupts autonomic pathways, perturbing cardiovascular homeostasis. Cardiovascular dysfunction increases with higher levels of injury and greater severity. Disordered blood pressure control after spinal cord injury (SCI) has significant ramifications as cord-injured people have an increased risk of developing heart disease and stroke; cardiovascular dysfunction is currently a leading cause of death among those with SCI. Despite the clinical significance of abnormal cardiovascular control following SCI, this problem has been generally neglected by both the clinical and research community. Both autonomic dysreflexia and orthostatic hypotension are known to prevent and delay rehabilitation, and significantly impair the overall quality of life after SCI. Starting with neurogenic shock immediately after a higher SCI, ensuing cardiovascular dysfunctions include orthostatic hypotension, autonomic dysreflexia and cardiac arrhythmias. Disordered temperature regulation accompanies these autonomic dysfunctions. This chapter reviews the human and animal studies that have furthered our understanding of the pathophysiology and mechanisms of orthostatic hypotension, autonomic dysreflexia and cardiac arrhythmias. The cardiovascular dysfunction that occurs during sexual function and exercise is elaborated. New awareness of cardiovascular dysfunction after SCI has led to progress toward inclusion of this important autonomic problem in the overall assessment of the neurological condition of cord-injured people.

Identification of the spinal pathways involved in the recovery of baroreflex control after spinal lesion in the rat using pseudorabies virus

Neurons in the rostroventrolateral medulla (RVLM) mediate baroreflex regulation (BR) of spinal sympathetic preganglionic neurons. Previously, our laboratory has shown that recovery of BR occurs in the rat after spinal hemisection. (Zahner MR, Kulikowicz E, and Schramm LP. Am J Physiol Regul Integr Comp Physiol 301: R1584-R1590, 2011). The goal of these experiments was to determine whether the observed recovery of BR is mediated by the reorganization of ipsilateral pathways or by compensation by spared contralateral pathways. To determine this, we infected the left kidney in rats with the retrograde transynaptic tracer, pseudorabies virus (PRV), either 1 or 8 wk after left spinal hemisection at either T(3) or T(8), or after a sham lesion. In sham-lesioned rats, PRV infection of RVLM neurons was bilateral. In all rats with a left hemisection, regardless of the location of the lesion (T(3) or T(8)) or postlesion recovery time (1 or 8 wk), PRV infection of left RVLM neurons was significantly reduced compared with sham-lesioned rats (P < 0.05). In a separate group of rats, we performed BR tests by measuring responses of left renal sympathetic nerve activity to pharmacologically induced decreases and increases in arterial pressure. In rats with T(8) left hemisection and 8-wk recovery, BR was robust, and acute right upper thoracic hemisection abolished all BR of left renal sympathetic nerve activity. Collectively, these data suggest that the recovery of BR is not mediated by reorganization of ipsilateral bulbospinal connections, but instead by improved efficacy of existing contralateral pathways.

The motor cortex communicates with the kidney

We used retrograde transneuronal transport of rabies virus from the rat kidney to identify the areas of the cerebral cortex that are potential sources of central commands for the neural regulation of this organ. Our results indicate that multiple motor and nonmotor areas of the cerebral cortex contain output neurons that indirectly influence kidney function. These cortical areas include the primary motor cortex (M1), the rostromedial motor area (M2), the primary somatosensory cortex, the insula and other regions surrounding the rhinal fissure, and the medial prefrontal cortex. The vast majority of the output neurons from the cerebral cortex were located in two cortical areas, M1 (68%) and M2 (15%). If the visceromotor functions of M1 and M2 reflect their skeletomotor functions, then the output to the kidney from each cortical area could make a unique contribution to autonomic control. The output from M1 could add precision and organ-specific regulation to descending visceromotor commands, whereas the output from M2 could add anticipatory processing which is essential for allostatic regulation. We also found that the output from M1 and M2 to the kidney originates predominantly from the trunk representations of these two cortical areas. Thus, a map of visceromotor representation appears to be embedded within the classic somatotopic map of skeletomotor representation.

Role of the rostral ventrolateral medulla (RVLM) in the patterning of vestibular system influences on sympathetic nervous system outflow to the upper and lower body

Research on animal models as well as human subjects has demonstrated that the vestibular system contributes to regulating the distribution of blood in the body through effects on the sympathetic nervous system. Elimination of vestibular inputs results in increased blood flow to the hindlimbs during vestibular stimulation, because it attenuates the increase in vascular resistance that ordinarily occurs in the lower body during head-up tilts. Additionally, the changes in vascular resistance produced by vestibular stimulation differ between body regions. Electrical stimulation of vestibular afferents produces an inhibition of most hindlimb vasoconstrictor fibers and a decrease in hindlimb vascular resistance, but an initial excitation of most upper body vasoconstrictor fibers accompanied by an increase in upper body vascular resistance. The present study tested the hypothesis that neurons in the principal vasomotor region of the brainstem, the rostral ventrolateral medulla (RVLM), whose projections extended past the T10 segment, to spinal levels containing sympathetic preganglionic neurons regulating lower body blood flow, respond differently to electrical stimulation of the vestibular nerve than RVLM neurons whose axons terminate rostral to T10. Contrary to our hypothesis, the majority of RVLM neurons were excited by vestibular stimulation, despite their level of projection in the spinal cord. These findings indicate that the RVLM is not solely responsible for establishing the patterning of vestibular-sympathetic responses. This patterning apparently requires the integration by spinal circuitry of labyrinthine signals transmitted from the brainstem, likely from regions in addition to the RVLM.

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

The retrograde transsynaptic tracer pseudorabies virus (PRV) has been widely used as a marker for synaptic connectivity in the spinal cord. Notably, the PRV-152 construct expresses enhanced green fluorescent protein (EGFP). We recently reported a significant attenuation of PRV-152 labeling of the intermediolateral cell column (IML) and celiac ganglia after complete T4 spinal cord transection versus sham injury in rats at 96 h after PRV-152 inoculation of the left kidney. Here we found a significant increase in noxious colorectal distention (CRD)-evoked c-Fos expression in spinal cords of injured versus sham rats without PRV infection. In order to assess whether enhancing neuronal activity in spinalized rats might increase PRV-152 labeling, we subjected awake spinalized rats to 1.5 h of intermittent noxious CRD either: (1) just prior to inoculation, or (2) 96 h after inoculation (n = 3/group). Equal numbers of spinalized rats in both groups received PRV-152 inoculations without CRD (non-stimulated; n = 3/group). At 96 h post-inoculation fixed spinal cords and left celiac ganglionic tissues were assessed for the distribution and quantification of EGFP-labeled cells. The injured cohort that received CRD just prior to PRV injection showed a significant reduction in EGFP-labeled cells in both the IML and left celiac ganglion compared to non-stimulated injured rats. In contrast, the injured cohort that received CRD 96 h after PRV-152 inoculation showed no differences in EGFP-labeled cell numbers in the IML or celiac ganglia versus non-stimulated injured rats. Interestingly, microglia near c-Fos-positive cells after acute CRD appeared more reactive compared to non-stimulated spinalized rats, and activated microglial cells markedly reduce viral transduction and progression following PRV inoculation of the CNS. Hence our results imply that increased CRD-induced c-Fos expression in the injured paradigm, prior to but not after PRV injection, further attenuates PRV-152 uptake, perhaps through changes in neuronal activity and/or innate neuro-immune responses.

Sialidase enhances recovery from spinal cord contusion injury

Axons fail to regenerate in the injured spinal cord, limiting motor and autonomic recovery and contributing to long-term morbidity. Endogenous inhibitors, including those on residual myelin, contribute to regeneration failure. One inhibitor, myelin-associated glycoprotein (MAG), binds to sialoglycans and other receptors on axons. MAG inhibition of axon outgrowth in some neurons is reversed by treatment with sialidase, an enzyme that hydrolyzes sialic acids and eliminates MAG-sialoglycan binding. We delivered recombinant sialidase intrathecally to rats following a spinal cord contusive injury. Sialidase (or saline solution) was infused to the injury site continuously for 2 wk and then motor behavior, autonomic physiology, and anatomic outcomes were determined 3 wk later. Sialidase treatment significantly enhanced hindlimb motor function, improved bulbospinally mediated autonomic reflexes, and increased axon sprouting. These findings validate sialoglycans as therapeutic targets and sialidase as a candidate therapy for spinal cord injury.

Electrophysiological characteristics of identified kidney-related neurons in adult rat spinal cord slices

Whole-cell patch-clamp recordings were made from kidney-related neurons in the intermediolateral cell column (IML) in horizontal slices of thoracolumbar spinal cord from adult rats. Kidney-related neurons were identified in vitro subsequent to inoculation of the kidney with a fluorescent, retrograde, transynaptic pseudorabies viral label (i.e., PRV-152). Kidney-related neurons detected in the IML expressed choline acetyltransferase, characteristic of spinal preganglionic motor neurons. Their mean resting potential was -51+/-4 mV and input resistance was 448+/-39 MOmega. Both spontaneous inhibitory and excitatory post-synaptic currents (i.e., sIPSCs and sEPSCs) were observed in all neurons. The mean frequency for sEPSCs (3.1+/-1 Hz) was approximately 2.5 times that for sIPSCs (1.4+/-0.3 Hz). Application of the glycine and GABA(A) receptor-linked Cl(-) channel blocker, picrotoxin (100 microM) blocked sIPSCs, while the ionotropic glutamate receptor antagonist, kynurenic acid (1 mM) blocked all sEPSCs, indicating they were mediated by GABA/glycine and glutamate receptors, respectively. Thus, using PRV-152 labeling allowed whole-cell patch-clamp recording of neurons in the adult spinal cord, which were kidney-related. Excitatory glutamatergic input dominated synaptic responses in these cells, the membrane characteristics of which resembled those of immature IML neurons. Combined PRV-152 pre-labeling and whole-cell patch-clamp recordings may allow more effective analysis of synaptic plasticity seen in adult models of injury or chronic disease.

Sympathetic-correlated c-Fos expression in the neonatal rat spinal cord in vitro

An isolated thoracic spinal cord of the neonatal rat in vitro spontaneously generates sympathetic nerve discharge (SND) at ~25 degrees C, but it fails in SND genesis at < or = 10 degrees C. Basal levels of the c-Fos expression in the spinal cords incubated at < or = 10 degrees C and ~25 degrees C were compared to determine the anatomical substrates that might participate in SND genesis. Cells that exhibited c-Fos immunoreactivity were virtually absent in the spinal cords incubated at < or = 10 degrees C. However, in the spinal cords incubated at ~25 degrees C, c-Fos-positive cells were found in the dorsal laminae, the white matter, lamina X, and the intermediolateral cell column (IML). Cell identities were verified by double labeling of c-Fos with neuron-specific nuclear protein (NeuN), glial fibrillary acidic protein (GFAP), or choline acetyltransferase (ChAT). The c-Fos-positive cells distributed in the white matter and lamina X were NeuN-negative or GFAP-positive and were glial cells. Endogenously active neurons showing c-Fos and NeuN double labeling were scattered in the dorsal laminae and concentrated in the IML. Double labeling of c-Fos and ChAT confirmed the presence of active sympathetic preganglionic neurons (SPNs) in the IML. Suppression of SND genesis by tetrodotoxin (TTX) or mecamylamine (MECA, nicotinic receptor blocker) almost abolished c-Fos expression in dorsal laminae, but only mildly affected c-Fos expression in the SPNs. Therefore, c-Fos expression in some SPNs does not require synaptic activation. Our results suggest that spinal SND genesis is initiated from some spontaneously active SPNs, which are capable of TTX- or MECA-resistant c-Fos expression.

Responses of thoracic spinal interneurons to vestibular stimulation

Vestibular influences on outflow from the spinal cord are largely mediated via spinal interneurons, although few studies have recorded interneuronal activity during labyrinthine stimulation. The present study determined the responses of upper thoracic interneurons of decerebrate cats to electrical stimulation of the vestibular nerve or natural stimulation of otolith organs and the anterior and posterior semicircular canals using rotations in vertical planes. A majority of thoracic interneurons (74/102) responded to vestibular nerve stimulation at median latencies of 6.5 ms (minimum of approximately 3 ms), suggesting that labyrinthine inputs were relayed to these neurons through trisynaptic and longer pathways. Thoracic interneuronal responses to vertical rotations were similar to those of graviceptors such as otolith organs, and a wide array of tilt directions preferentially activated different cells. Such responses were distinct from those of cells in the cervical and lumbar enlargements, which are mainly elicited by ear-down tilts and are synchronous with stimulus position when low rotational frequencies are delivered, but tend to be in phase with stimulus velocity when high frequencies are employed. The dynamic properties of thoracic interneuronal responses to tilts were instead similar to those of thoracic motoneurons and sympathetic preganglionic neurons. However, the preferred tilt directions of the interneurons were more heterogeneous than thoracic spinal outputs, showing that the outputs do not simply reflect an addition of local interneuronal activity.

Spinal cord injury reduces the efficacy of pseudorabies virus labeling of sympathetic preganglionic neurons

The retrograde transsynaptic tracer pseudorabies virus (PRV) is used as a marker for synaptic connectivity in the spinal cord. Using PRV, we sought to document putative synaptic plasticity below a high thoracic (T) spinal cord transection. This lesion has been linked to the development of a number of debilitating conditions, including autonomic dysreflexia. Two weeks after injury, complete T4-transected and/or T4-hemisected and sham rats were injected with PRV-expressing enhanced green fluorescent protein (EGFP) or monomeric red fluorescent protein (mRFP1) into the kidneys. We expected greater PRV labeling after injury because of the plasticity of spinal circuitry, but 96 hours post-PRV-EGFP inoculation, we found fewer EGFP+ cells in the thoracolumbar gray matter of T4-transected compared with sham rats (p < 0.01); Western blot analysis corroborated decreased EGFP protein levels (p < 0.01). Moreover, viral glycoproteins that are critical for cell adsorption and entry were also reduced in the thoracolumbar spinal cord of injured versus sham rats (p < 0.01). Pseudorabies virus labeling of sympathetic postganglionic neurons in the celiac ganglia innervating the kidneys was also significantly reduced in injured versus sham rats (p < 0.01). By contrast, the numbers and distribution of Fluoro-Gold-labeled (intraperitoneal injection) sympathetic preganglionic neurons throughout the sampled regions appeared similar in injured and sham rats. These results question whether spinal cord injury exclusively retards PRV expression and/or transport or whether this injury broadly affects host cell-viral interactions.

Leg sympathetic response to noxious skin stimuli is similar in high and low level human spinal cord injury

OBJECTIVE

To determine if sympathetically mediated vasoconstriction in the lower extremities is injury level dependent. Although sympathetic responses have been measured in the limbs of people with high and low level SCI using blood flow measurements, including Doppler ultrasound and venous plethysmography, a direct comparison between injury levels has not been made.

METHODS

Volunteers with chronic SCI were grouped according to injury level. Above T6: high level (HL, n=7), and T6 and below: low level (LL, n=6). All subjects had complete motor and sensory loss. Leg arterial flows were recorded by venous occlusion plethysmography, and continuous heart rate and mean arterial pressure (MAP) were measured. The conditioning stimulus consisted of transcutaneous stimulation to the arch of the contralateral foot.

RESULTS

HL and LL subjects demonstrated a significant decrease in arterial conductance during stimulation with no significant difference found between groups. As expected, only group HL demonstrated a significant increase in MAP.

CONCLUSIONS

These results support our hypothesis that local (leg) sympathetic responses are similar for both high and low level SCI.

SIGNIFICANCE

While low level SCI does not typically present with autonomic dysreflexia, bouts of increased reflex sympathetic activity could have ramifications for metabolism as well as renal and motor system function.

Elimination of rat spinal substance P receptor bearing neurons dissociates cardiovascular and nocifensive responses to nicotinic agonists

Intrathecal (IT) delivery of nicotinic agonists evokes dose dependent nocifensive behavior and cardiovascular responses. Previous studies suggested that these effects may be attenuated by the loss of substance P positive (sP(+)) primary afferents. To further characterize these cell systems, we examined the effect of selectively destroying neurokinin 1 receptor bearing (NK1-r(+)) dorsal horn neurons on IT nicotinic agonist evoked responses. In the dorsal spinal cord, confocal immunohistochemical microscopy revealed that nAChR subunits (alpha3, alpha4, alpha5, beta2 and beta4), NeuN B (neuronal marker) and NK1-r were all co-expressed in the superficial dorsal horn; however alpha3, alpha5, beta2 and beta4 exhibited the highest degree of colocalization with NK1-r expressing neurons. After intrathecal substance P-saporin (sP-SAP), NK1-r(+) cell bodies and dendrites in the superficial dorsal horn were largely abolished. The greatest loss in co-expression of nAChR subunits with NK1-r was observed with alpha3, alpha5, beta2 and beta4 subunits. Following intrathecal sP-SAP, the nocifensive responses to all nicotinic agonists were reduced; however, in contrast, while cardiovascular responses evoked by IT nicotine were unaltered, IT cytisine and epibatidine exhibited enhanced tachycardia and pressor responses. These results indicate subunit-specific relationships between the NK1-r and nicotinic receptor systems. The loss of nocifensive activity after destruction of the NK1-r bearing cells in spite of the persistence of nicotinic subunits on other cells, emphasizes the importance of the superficial marginal neuron in mediating these nicotinic effects. Further, the exaggerated cardiovascular responses to cytisine following loss of NK1-r bearing cells suggest the presence of a nicotinic receptor-mediated stimulation of inhibitory circuits at the spinal level.

VGLUT1 and VGLUT2 innervation in autonomic regions of intact and transected rat spinal cord

Fast excitatory neurotransmission to sympathetic and parasympathetic preganglionic neurons (SPN and PPN) is glutamatergic. To characterize this innervation in spinal autonomic regions, we localized immunoreactivity for vesicular glutamate transporters (VGLUTs) 1 and 2 in intact cords and after upper thoracic complete transections. Preganglionic neurons were retrogradely labeled by intraperitoneal Fluoro-Gold or with cholera toxin B (CTB) from superior cervical, celiac, or major pelvic ganglia or adrenal medulla. Glutamatergic somata were localized with in situ hybridization for VGLUT mRNA. In intact cords, all autonomic areas contained abundant VGLUT2-immunoreactive axons and synapses. CTB-immunoreactive SPN and PPN received many close appositions from VGLUT2-immunoreactive axons. VGLUT2-immunoreactive synapses occurred on Fluoro-Gold-labeled SPN. Somata with VGLUT2 mRNA occurred throughout the spinal gray matter. VGLUT2 immunoreactivity was not noticeably affected caudal to a transection. In contrast, in intact cords, VGLUT1-immunoreactive axons were sparse in the intermediolateral cell column (IML) and lumbosacral parasympathetic nucleus but moderately dense above the central canal. VGLUT1-immunoreactive close appositions were rare on SPN in the IML and the central autonomic area and on PPN. Transection reduced the density of VGLUT1-immunoreactive axons in sympathetic subnuclei but increased their density in the parasympathetic nucleus. Neuronal cell bodies with VGLUT1 mRNA occurred only in Clarke's column. These data indicate that SPN and PPN are densely innervated by VGLUT2-immunoreactive axons, some of which arise from spinal neurons. In contrast, the VGLUT1-immunoreactive innervation of spinal preganglionic neurons is sparse, and some may arise from supraspinal sources. Increased VGLUT1 immunoreactivity after transection may correlate with increased glutamatergic transmission to PPN.

Multi-tasking in the spinal cord--do 'sympathetic' interneurones work harder than we give them credit for?

The role of interneurones in the control of sympathetic activity has been somewhat of a mystery since, for many years, it was difficult to target these cells for study. Recently scientists have started to unravel the action potential properties of these neurones, where they receive their inputs from and where they project to. This review looks at the information known to date about sympathetic interneurones. The locations of these neurones and their local axonal ramifications suggest that they play a more widespread function than previously thought. Therefore the data to support such a theory are also examined.

Immunoreactivity for cocaine- and amphetamine-regulated transcript in rat sympathetic preganglionic neurons projecting to sympathetic ganglia and the adrenal medulla

Many sympathetic preganglionic neurons (SPN) in the intermediolateral cell column (IML) contain cocaine- and amphetamine-regulated transcript (CART), but the function of these CART-immunoreactive (IR) neurons is unknown. To test the possibility that CART might mark SPN involved in cardiovascular regulation, we first established whether all CART neurons in the spinal cord were SPN by double-immunofluorescent labelling for CART and choline acetyltransferase (ChAT). All autonomic subnuclei contained SPN immunoreactive for ChAT plus CART. Occasional ChAT-negative, CART-positive neurons occurred adjacent to the IML, indicating the existence of CART-IR interneurons. We then retrogradely labelled SPN with cholera toxin subunit B (CTB) from a variety of targets and used double immunofluorescence to detect CTB and CART. Among SPN in the IML, 43% projecting to the coeliac ganglion, 34% projecting to the major pelvic ganglion, and about 15% projecting to the superior cervical ganglion or adrenal medulla contained CART. CART also occurred in most SPN projecting to the major pelvic ganglion from either the central autonomic area (63%) or the intercalated nucleus (58%). Finally, we used drug-induced hypotension in conscious rats to evoke Fos immunoreactivity in barosensitive SPN and immunostained to reveal Fos and CART. CART immunoreactivity was present in 41% of the Fos-IR barosensitive neurons, which were concentrated in the IML of segments T5-T13. CART-positive, Fos-negative neurons also occurred in the same segments. These results indicate that CART occurs in barosensitive SPN, nonbarosensitive SPN, and interneurons. Thus, CART is not an exclusive marker for cardiovascular SPN but is likely to influence many autonomic activities.

Increased close appositions between corticospinal tract axons and spinal sympathetic neurons after spinal cord injury in rats

Treatments for spinal cord injury may promote new spinal cord synapses. However, the potential for new synapses between descending somatomotor and spinal sympathetic neurons has not been investigated. We studied rats with intact spinal cords and rats after a chronic, bilateral, dorsal spinal hemisection. We identified sympathetically related spinal neurons by transynaptic, retrograde transport of renally injected pseudorabies virus. We counted retrogradely labeled sympathetic preganglionic neurons (SPN) and putative sympathetic interneurons (IN) that, under light microscopy, appeared closely apposed by anterogradely labeled axons of the corticospinal tract (CST) and by axons descending from the well-established sympathetic regulatory region in the rostral ventrolateral medulla (RVLM). Spinal sympathetic neurons that were closely apposed by CST axons were significantly more numerous in lesioned rats than in unlesioned rats. CST axons closely apposed 5.4% of SPN and 10.3% of IN in rats with intact spinal cords, and 38.0% of SPN and 37.3% of IN in rats with chronically lesioned spinal cords. Further, CST appositions in SCI rats consisted of many more varicosities than those in uninjured rats. SPN and IN closely apposed by axons from the RVLM were not more numerous in lesioned rats. However, RVLM axons apposed many more SPN than IN in both control and lesioned rats. Therefore, RVLM sympathoexcitation may be mediated largely by direct synapses on SPN. Although we have not determined the functional significance of close appositions between the CST and spinal sympathetic neurons, we suggest that future studies of spinal cord repair and regeneration include an evaluation of potential, new, somatic-autonomic interactions.

Effects of spinal cord injury on synaptic inputs to sympathetic preganglionic neurons

Spinal cord injuries often lead to disorders in the control of autonomic function, including problems with blood pressure regulation, voiding, defecation and reproduction. The root cause of all these problems is the destruction of brain pathways that control spinal autonomic neurons lying caudal to the lesion. Changes induced by spinal cord injuries have been most extensively studied in sympathetic preganglionic neurons, cholinergic autonomic neurons with cell bodies in the lateral horn of thoracic and upper lumbar spinal cord that are the sources of sympathetic outflow. After an injury, sympathetic preganglionic neurons in mid-thoracic cord show plastic changes in their morphology. There is also extensive loss of synaptic input from the brain, leaving these neurons profoundly denervated in the acute phase of injury. Our recent studies on sympathetic preganglionic neurons in lower thoracic and upper lumbar cord that regulate the pelvic viscera suggest that these neurons are not so severely affected by spinal cord injury. Spinal interneurons appear to contribute most of the synaptic input to these neurons so that injury does not result in extensive denervation. Since intraspinal circuitry remains intact after injury, drug treatments targeting these neurons should help to normalize sympathetically mediated pelvic visceral reflexes. Furthermore, sympathetic pelvic visceral control may be more easily restored after an injury because it is less dependent on the re-establishment of direct synaptic input from regrowing brain axons.

Immunohistochemical localisation of the voltage gated potassium ion channel subunit Kv3.3 in the rat medulla oblongata and thoracic spinal cord

Voltage gated K+ channels (Kv) are a diverse group of channels important in determining neuronal excitability. The Kv superfamily is divided into 12 subfamilies (Kv1-12) and members of the Kv3 subfamily are highly abundant in the CNS, with each Kv3 gene (Kv3.1-Kv3.4) exhibiting a unique expression pattern. Since the localisation of Kv subunits is important in defining the roles they play in neuronal function, we have used immunohistochemistry to determine the distribution of the Kv3.3 subunit in the medulla oblongata and spinal cord of rats. Kv3.3 subunit immunoreactivity (Kv3.3-IR) was widespread but present only in specific cell populations where it could be detected in somata, dendrites and synaptic terminals. Labelled neurones were observed in the spinal cord in laminae IV and V, in the region of the central canal and in the ventral horn. In the medulla oblongata, labelled cell bodies were numerous in the spinal trigeminal, cuneate and gracilis nuclei whilst rarer in the lateral reticular nucleus, hypoglossal nucleus and raphe nucleus. Regions containing autonomic efferent neurones were predominantly devoid of labelling with only occasional labelled neurones being observed. Dual immunohistochemistry revealed that some Kv3.3-IR neurones in the ventral medullary reticular nucleus, spinal trigeminal nucleus, dorsal horn, ventral horn and central canal region were also immunoreactive for the Kv3.1b subunit. The presence of Kv3.3 subunits in terminals was confirmed by co-localisation of Kv3.3-IR with the synaptic vesicle protein SV2, the vesicular glutamate transporter VGluT2 and the glycine transporter GlyT2. Co-localisation of Kv3.3-IR was not observed with VGluT1, tyrosine hydroxylase, serotonin or choline acetyl transferase. Electron microscopy confirmed the presence of Kv3.3-IR in terminals and somatic membranes in ventral horn neurones, but not motoneurones. This study provides evidence supporting a role for Kv3.3 subunits in regulating neuronal excitability and in the modulation of excitatory and inhibitory synaptic transmission in the medulla oblongata and spinal cord.

Spinal sympathetic interneurons: Their identification and roles after spinal cord injury

Primary afferent neurons rarely, if ever, synapse on the sympathetic preganglionic neurons that regulate the cardiovascular system, nor do sympathetic preganglionic neurons normally exhibit spontaneous activity in the absence of excitatory inputs. Therefore, after serious spinal cord injury "spinal sympathetic interneurons" provide the sole excitatory and inhibitory inputs to sympathetic preganglionic neurons. Few studies have addressed the anatomy and physiology of spinal sympathetic interneurons, to a great extent because they are difficult to identify. Therefore, this chapter begins with descriptions of both neurophysiological and neuroanatomical criteria for identifying spinal sympathetic interneurons, and it discusses the advantages and disadvantages of each. Spinal sympathetic interneurons also have been little studied because their importance in intact animals has been unknown, whereas the roles of direct projections from the brain to sympathetic preganglionic neurons are better known. This chapter presents evidence that spinal sympathetic interneurons play only a minor role in sympathetic regulation when the spinal cord is intact. However, they play an important role after spinal cord injury, both in generating ongoing activity in sympathetic nerves and in mediating segmental and intersegmental sympathetic reflexes. The spinal sympathetic interneurons that most directly influence the activity of sympathetic preganglionic neurons after spinal cord injury are located close to their associated sympathetic preganglionic neurons, and the inputs from distant segments that mediate multisegmental reflexes are relayed to sympathetic preganglionic neurons multisynaptically via spinal sympathetic interneurons. Finally, spinal sympathetic interneurons are more likely to be excited and less likely to be inhibited by both noxious and innocuous somatic stimuli after chronic spinal transection. The onset of this hyperexcitability corresponds to morphological changes in both sympathetic preganglionic neurons and primary afferents, and it may reflect the pathophysiological processes that lead to autonomic dysreflexia and the hypertensive crises that may occur with it in people after chronic spinal injury.

Enkephalin-immunoreactive interneurons extensively innervate sympathetic preganglionic neurons regulating the pelvic viscera

Enkephalin (ENK)-immunoreactive (IR) axons occur in regions containing spinal autonomic neurons and endogenous opiates contribute to spinal regulation of bladder function. To identify possible spinal sites of opiate action, we used immunocytochemistry for ENK with retrograde tracing from the major pelvic ganglion (MPG), a key location for postganglionic neurons controlling pelvic viscera, with cholera toxin B subunit (CTB) or CTB-horseradish peroxidase (CTB-HRP). We compared the relationship of ENK-IR axons with sympathetic preganglionic neurons (SPNs) projecting to the MPG between intact spinal cords and cords with 2- or 11-week complete transections between thoracic segments 4 and 5. By light microscopy, sections of intact cord showed dense networks of ENK-IR axons surrounding CTB-IR SPNs in the intermediolateral cell column (IML), intercalated nucleus, and central autonomic area of lower thoracic and upper lumbar cord. This staining pattern was similar in rats with 2- or 11-week transections. Ultrastructurally, ENK-IR axons formed synapses on SPNs in all three autonomic subnuclei of intact cord. In the IML, ENK-IR varicosities contributed 52% of the synapses on the somata of MPG-projecting SPNs. In 2-week transected cord, synapses from ENK-IR axons persisted on SPNs and the proportion of input to IML SPNs had increased to 67%, probably reflecting loss of supraspinal input. These results suggest that endogenous opioids could play a major role in controlling sympathetic outflow to the bladder through a direct action on SPNs. The persistence of the dense ENK innervation after complete cord transection indicates that the ENK-IR input to SPNs arises predominantly from intraspinal sources.

GABAergic neurons in the central region of the spinal cord: a novel substrate for sympathetic inhibition

Homeostatic maintenance of widespread functions is critically dependent on the activity of the sympathetic nervous system. This activity is generated by the CNS acting on the sole output cells in the spinal cord, sympathetic preganglionic neurons (SPNs). SPNs are subject to control from both supraspinal and spinal inputs that exert effects through activation of direct or indirect pathways. A high proportion of indirect control is attributable to activation of spinal interneurons in a number of locations. However, little is known about the different groups of interneurons with respect to their neurochemistry or function. In this study, we report on a novel group of GABAergic interneurons located in the spinal central autonomic area (CAA) that directly inhibit SPN activity. In situ hybridization studies demonstrated a group of neurons that contained mRNA for glutamic acid decarboxylase (GAD)65 and GAD67 within the CAA. Combining in situ hybridization with trans-synaptic labeling from the adrenal gland using pseudorabies virus identified presympathetic GABAergic neurons in the CAA. Electrical stimulation of the CAA elicited monosynaptic IPSPs in SPNs located laterally in the intermediolateral cell column. IPSPs were GABAergic, because they reversed at the chloride reversal potential and were blocked by bicuculline. Chemical activation of neurons in the CAA hyperpolarized SPNs, an effect that was also bicuculline sensitive. We conclude that the CAA contains GABAergic interneurons that impinge directly onto SPNs to inhibit their activity and suggest that these newly identified interneurons may play an essential role in the regulation of sympathetic activity and thus homeostasis.

GABAergic and glycinergic presympathetic neurons of rat medulla oblongata identified by retrograde transport of pseudorabies virus and in situ hybridization

Electron microscopy suggests that up to half the synaptic input to sympathetic preganglionic neurons (SPGNs) is GABAergic or glycinergic. A proportion of this input is suspected to originate from neurons located within the medulla oblongata. The present study provides definitive evidence for the existence of these supraspinal presympathetic (PS) neurons with inhibitory phenotypes. PS neurons were identified by retrograde trans-synaptic migration of pseudorabies virus (PRV) injected into the adrenal gland. GABAergic or glycinergic cell bodies were identified by the presence of glutamate decarboxylase (GAD)-67 mRNA or glycine transporter (GlyT)-2 mRNA detected with in situ hybridization (ISH). Neither GABAergic nor glycinergic PS neurons were tyrosine hydroxylase (TH)-immunoreactive (ir). GABAergic PS neurons were located within the ventral gigantocellular nucleus, gigantocellular nucleus alpha, and medial reticular formation, mostly medial to the TH-ir PS neurons. About 30% of GABAergic PS neurons were serotonergic cells located in the raphe pallidus (RPa) and parapyramidal region (PPyr). Glycinergic PS neurons had the same general distribution as the GABAergic cells, except that no glycinergic neurons were located in the RPa or PPyr and none were serotonergic. PRV immunohistochemistry combined with ISH for both GlyT2 and GAD-67 mRNAs showed that at least 63% of midline medulla GABAergic PS neurons were also glycinergic and 76% of glycinergic PS neurons were GABAergic. In conclusion, the rostral ventromedial medulla contains large numbers of GABAergic and glycinergic neurons that innervate adrenal gland SPGNs. Over half of these PS neurons may release both transmitters. The physiological role of this medullary inhibitory input remains to be explored.