Evidence of disynaptic projections from the rostral ventrolateral medulla to the thoracic spinal cord

Role of Descending Serotonergic Fibers in the Development of Pathophysiology after Spinal Cord Injury (SCI): Contribution to Chronic Pain, Spasticity, and Autonomic Dysreflexia

As the nervous system develops, nerve fibers from the brain form descending tracts that regulate the execution of motor behavior within the spinal cord, incoming sensory signals, and capacity to change (plasticity). How these fibers affect function depends upon the transmitter released, the receptor system engaged, and the pattern of neural innervation. The current review focuses upon the neurotransmitter serotonin (5-HT) and its capacity to dampen (inhibit) neural excitation. A brief review of key anatomical details, receptor types, and pharmacology is provided. The paper then considers how damage to descending serotonergic fibers contributes to pathophysiology after spinal cord injury (SCI). The loss of serotonergic fibers removes an inhibitory brake that enables plasticity and neural excitation. In this state, noxious stimulation can induce a form of over-excitation that sensitizes pain (nociceptive) circuits, a modification that can contribute to the development of chronic pain. Over time, the loss of serotonergic fibers allows prolonged motor drive (spasticity) to develop and removes a regulatory brake on autonomic function, which enables bouts of unregulated sympathetic activity (autonomic dysreflexia). Recent research has shown that the loss of descending serotonergic activity is accompanied by a shift in how the neurotransmitter GABA affects neural activity, reducing its inhibitory effect. Treatments that target the loss of inhibition could have therapeutic benefit.

Sedentary conditions and enhanced responses to GABA in the RVLM: role of the contralateral RVLM

A sedentary lifestyle is a major risk factor for cardiovascular disease, and both conditions are associated with overactivity of the sympathetic nervous system. Ongoing discharge of sympathetic nerves is regulated by the rostral ventrolateral medulla (RVLM), which in turn is modulated by the primary excitatory and inhibitory neurotransmitters glutamate and γ-amino-butyric acid (GABA), respectively. We reported previously that sedentary conditions enhance GABAergic modulation of sympathoexcitation in the RVLM, despite overall increased sympathoexcitation. Thus the purpose of this study was to test the hypothesis that sedentary conditions increase responsiveness to GABA in RVLM. Male Sprague-Dawley rats performed either chronic wheeling running or remained sedentary for 12-15 wk. Animals were instrumented to perform RVLM microinjections under Inactin anesthesia while mean arterial pressure (MAP) and splanchnic sympathetic nerve activity (SSNA) were recorded. Unilateral microinjections of GABA (30 nl, 0.3-600 mM) into the RVLM produced dose-dependent decreases in MAP and SSNA; however, no group differences were observed. Inhibition of the contralateral RVLM (muscimol, 2 mM, 90 nl) caused decreases in MAP and SSNA that were not different between groups but enhanced decreases in SSNA to GABA in sedentary rats only. In sinoaortic denervated rats, GABA microinjections before or after inhibition of the contralateral RVLM caused decreases in MAP and SSNA that were not different between groups. Our results suggest that the contralateral RVLM plays an important role in buffering responses to inhibition of the ipsilateral RVLM under sedentary but not physically active conditions. Based on these studies and others, sedentary conditions appear to enhance both sympathoinhibitory and sympathoexcitatory mechanisms in the RVLM. Enhanced sympathoinhibition may act to reduce already elevated sympathetic nervous system activity following sedentary conditions.

Autonomic consequences of spinal cord injury

Spinal cord injury (SCI) results not only in motor and sensory deficits but also in autonomic dysfunctions. The disruption of connections between higher brain centers and the spinal cord, or the impaired autonomic nervous system itself, manifests a broad range of autonomic abnormalities. This includes compromised cardiovascular, respiratory, urinary, gastrointestinal, thermoregulatory, and sexual activities. These disabilities evoke potentially life-threatening symptoms that severely interfere with the daily living of those with SCI. In particular, high thoracic or cervical SCI often causes disordered hemodynamics due to deregulated sympathetic outflow. Episodic hypertension associated with autonomic dysreflexia develops as a result of massive sympathetic discharge often triggered by unpleasant visceral or sensory stimuli below the injury level. In the pelvic floor, bladder and urethral dysfunctions are classified according to upper motor neuron versus lower motor neuron injuries; this is dependent on the level of lesion. Most impairments of the lower urinary tract manifest in two interrelated complications: bladder storage and emptying. Inadequate or excessive detrusor and sphincter functions as well as detrusor-sphincter dyssynergia are examples of micturition abnormalities stemming from SCI. Gastrointestinal motility disorders in spinal cord injured-individuals are comprised of gastric dilation, delayed gastric emptying, and diminished propulsive transit along the entire gastrointestinal tract. As a critical consequence of SCI, neurogenic bowel dysfunction exhibits constipation and/or incontinence. Thus, it is essential to recognize neural mechanisms and pathophysiology underlying various complications of autonomic dysfunctions after SCI. This overview provides both vital information for better understanding these disorders and guides to pursue novel therapeutic approaches to alleviate secondary complications.

Rostroventrolateral medulla neurons with commissural projections provide input to sympathetic premotor neurons: anatomical and functional evidence

The activity of neurons in the rostral ventrolateral medulla (RVLM) is critical for the generation of vasomotor sympathetic tone. Multiple pre-sympathetic pathways converge on spinally projecting RVLM neurons, but the origin and circumstances in which such inputs are active are poorly understood. We have previously shown that input from the contralateral brainstem contributes to the baseline activity of this population: in the current study we investigate the distribution, phenotype and functional properties of RVLM neurons with commissural projections in the rat. We firstly used retrograde transport of fluorescent microspheres to identify neurons that project to the contralateral RVLM. Labelled neurons were prominent in a longitudinal column that extended over 1 mm caudal from the facial nucleus and contained hybridisation products indicating enkephalin (27%), GABA (15%) and adrenaline (3%) synthesis and included 6% of bulbospinal neurons identified by transport of cholera toxin B. Anterograde transport of fluorescent dextran-conjugate from the contralateral RVLM revealed extensive inputs throughout the RVLM that frequently terminated in close apposition with catecholaminergic and bulbospinal neurons. In urethane-anaesthetised rats we verified that 28/37 neurons antidromically activated by electrical stimulation of the contralateral pressor region were spontaneously active, of which 13 had activity locked to central respiratory drive and 15 displayed ongoing tonic discharge. In six tonically active neurons sympathoexcitatory roles were indicated by spike-triggered averages of splanchnic sympathetic nerve activity. We conclude that neurons in the RVLM project to the contralateral brainstem, form synapses with sympathetic premotor neurons, and have functional properties consistent with sympthoexcitatory function.

Abnormal baroreflex function is dissociated from central angiotensin II receptor expression in chronic heart failure

Neurohumoral disturbances characterize chronic heart failure (CHF) and are reflected, in part, as impairment of baroreflex sensitivity (BRS) and sympathetic function. However, the mechanisms that trigger these neurohumoral abnormalities in CHF are not clear. We hypothesized that the BRS is blunted early in CHF and that the humoral effects occur later and contribute to progressive loss of cardiovascular control in CHF. We assessed the BRS (beats/min per mmHg) and recorded renal sympathetic nerve activity (RSNA) in four groups of conscious rabbits at varying time intervals: control, 1-week CHF, 2-week CHF, and 3-week CHF. Chronic heart failure was induced by ventricular pacing at 360 beats/min and was assessed by echocardiography. Arterial blood pressure and heart rate were recorded by an implanted telemetric device and RSNA through an implanted electrode. A significant fall in the ejection fraction, fractional shortening, and an increase in left ventricular end-systolic diameter and left ventricular end-diastolic diameter were observed in all CHF groups. The BRS was significantly reduced in all the CHF groups with no significant change in the basal RSNA (% of maximum) after 1 week of pacing; a small but insignificant rise in RSNA was seen at 2 weeks, and a significant rise in RSNA was observed at 3 weeks. Angiotensin II type 1 (AT-1) receptor protein (Western Blot) and mRNA (reverse transcriptase-polymerase chain reaction) expression in the rostral ventrolateral medulla exhibited a progressive increase with the duration of CHF, reaching significance after 3 weeks, the same time point in which RSNA was significantly elevated. These data are the first to examine early changes in central AT-1 receptors in CHF and suggest that the fall in BRS and hemodynamic changes occur early in the development of CHF followed by sympathoexcitation and overexpression of AT-1 receptors with the progression of CHF, causing further impairment of cardiovascular control.

Response of NADPH-diaphorase-exhibiting neurons in the medullar reticular formation to high spinal cord injury

1. The effect of hemisection of the cervical spinal cord on NADPH-diaphorase staining in the reticular nuclei of the rabbit medulla was investigated using histochemical technique. 2. A quantitative assessment of somal and neuropil NADPH-diaphorase staining was made by an image analyzer in a selected area of each reticular nucleus of the rabbit medulla. 3. On the 7th postsurgery day, the highest up-regulation of somatic NADPH-diaphorase staining was observed in regions regulating cardiorespiratory processes; however, the highest increase of neuropil NADPH-diaphorase staining was found in the reticular nuclei modulating the tonus of postural muscles. 4. The degeneration of non-NADPH-diaphorase-stained neurons was detected throughout the reticular formation of the medulla, but the extent of neuronal death did not correlate with the up-regulation of the NADPH-diaphorase staining in the reticular nuclei of the medulla. 5. The findings provide evidence that NADPH-diaphorase-exhibiting neurons are refractory to the hemisection of the cervical spinal cord and that the neuronal up-regulation of NADPH-diaphorase at the medullar level is probably not a causative factor leading to the death of the reticulospinal neurons.

Modulatory inputs on sympathetic neurons in the rostral ventrolateral medulla in the rat

1. The first part of this study looks at spontaneously active neurons located in the rostral ventrolateral medulla (RVLM) with projections to the thoracic spinal cord. Sixteen neurons were intracellularly recorded in vivo. Four out of 16 neurons were antidromically activated from the thoracic spinal cord (axonal conduction velocities varied from 1.8 m/s to 9.5 m/s). 2. The simultaneous averages of the neuronal membrane potential and arterial blood pressure triggered by the pulsatile arterial wave or the EKG-R wave demonstrated changes in membrane potential (hyperpolarization or depolarization) locked to the cardiac cycle in four neurons in this group. These neurons (three of them bulbospinal) were further tested for barosensitivity by characterizing the responses to electrical stimulation of the aortic depressor nerve. Four neurons responded with inhibitory hyperpolarizing responses characterized as inhibitory postsynaptic potentials (IPSP) to aortic nerve stimulation (onset latency: 32.3 +/- 5.0 ms; mean +/- SEM). 3. In two neurons in the RVLM, one of them characterized as barosensitive, electrical stimulation of the opposite RVLM (0.5 Hz, 1.0 ms pulse duration, 25-100 microA) elicited excitatory postsynaptic potentials (EPSPs) with latencies of 9.07 and 10.5 ms. At resting membrane potential, the onset latency of the evoked EPSPs did not change with increasing stimulus intensities. Some of the recorded neurons were intracellularly labelled with biocytin for visualization. They were found in the RVLM. 4. These experiments in vivo would support the idea of a functional commissural pathway between the RVLM of both sides. 5. Anatomical data have shown that some of those commissural bundle fibers originate in the C1 adrenergic neuronal group in the RVLM. In the second part of this study, we used an intracellular recording technique in vitro to investigate the effects of the indirect adrenergic agonist tyramine on neurons in the RVLM with electrophysiological properties similar to premotor sympathetic neurons in vivo. 6. Tyramine (0.5-1 mM) produced a pronounced inhibitory effect with hyperpolarization and increase in membrane input resistance on two neurons characterized as regularly firing (R), and on one neuron characterized as irregularly firing (1). This effect was preceded by a transient depolarization with increases in firing rate. 7. These results would indicate that neurons in the RVLM recorded in vitro and with properties similar to premotor sympathetic neurons can be modulated by catecholamines released from terminals probably making synaptic contacts.

Lateralisation of projections from the rostral ventrolateral medulla to sympathetic preganglionic neurons in the rat

Spinally projecting sympathoexcitatory neurons in the rostral ventrolateral medulla (RVLM), synapse with sympathetic preganglionic neurons (SPN) and regulate the activity of sympathetic nerves that control the heart, blood pressure and the adrenal medulla (AM). However, the degree of lateralization of the bulbospinal projections to SPN innervating specific targets is poorly understood. Three approaches were used in this study. Anterograde tracer was iontophoresed into a pressor site in the RVLM (left or right) and retrograde tracer injected into the superior cervical ganglion (SCG, right) and the AM (left). Close appositions between anterogradely labelled axons and retrogradely labelled SCG- or AM-SPN were counted. Projections to the SCG were bilateral. Projections to the AM were markedly ipsilateral. In the second part, retrograde tracers were injected unilaterally into the region of the intermediolateral cell column at spinal segment T2 or T8 on one side and the number of labelled neurons in the RVLM counted. The results from each level of injection were similar showing that approximately 63-64% of the neurons were ipsilateral. Responses to glutamate microinjection into the RVLM on adrenal nerve (left) and superior cervical nerve (left) activity were measured. The ratio of the nerve responses was the same even when different sides of the RVLM were injected. The anterograde data strongly suggest that the RVLM projections to AM-SPN are predominantly ipsilateral. Although other experimental approaches also attempted to investigate lateralization, the retrograde data target different and functionally heterogeneous pools of SPN that may mask the ipsilateral projection to the AM. Similarly, chemical stimulation of the RVLM will excite not only monosynaptic projections but also polysynaptic projections that may also mask the predominant ipsilateral monosynaptic projection to AM.

Microinjection of bombesin into the ventrolateral reticular formation inhibits peripherally stimulated gastric acid secretion through spinal pathways in rats

Bombesin injected into the cisterna magna potently inhibits gastric acid secretion stimulated by intravenous infusion of pentagastrin. Sites in the medulla oblongata where bombesin acts to suppress gastric acid secretion were investigated in urethane-anesthetized rats with gastric cannula. Bombesin or vehicle was injected into the medullary parenchyma or intracisternally (i.c.) 60 min after the start of an intravenous pentagastrin infusion; gastric acid secretion was monitored every 10 min for 20 min before and 150 min after the start of pentagastrin. Bombesin (0.2, 0.6 or 6.2 pmol) microinjected into the ventrolateral reticular formation (VLRF) inhibited dose-dependently the net acid response to pentagastrin by 40.8+/-11.1, 75.4+/-12.8 and 96.7+/-19.4%, respectively, at the 40-50 min period after microinjection compared with the vehicle group. Bombesin action in the VLRF was long lasting (96% inhibition still observed at 90 min after 6.2 pmol), and completely abolished by cervical spinal cord transection at the C6 level. By contrast, bombesin injected i.c. at 0.2 or 0.6 pmol had no effect while at 6.2 pmol, there was a 79.0+/-3.9% peak inhibition of pentagastrin-stimulated acid secretion. Bombesin (6.2 pmol) injected into the dorsal motor nucleus reduced the acid response to pentagastrin by 29%. The parvicellular and gigantocellular reticular nuclei were not responsive to bombesin. These results indicate that bombesin acts in the VLRF to inhibit pentagastrin-stimulated gastric acid secretion through spinal pathways, suggesting a potential role of medullary VLRF area in the sympathetic control of gastric acid secretion.

Three types of putative presympathetic neurons in the rostral ventrolateral medulla studied with rat brainstem-spinal cord preparation

To study the electrophysiological properties of presympathetic neurons in the rostral ventrolateral medulla (RVLM), intracellular recordings were performed by the whole-cell patch-clamp technique. We utilized the neonatal rat brainstem-spinal cord preparation, in which the sympathetic neuronal network is thought to be preserved, unlike in slice preparation. In response to stimulation in the ipsilateral Th2 spinal segment including intermediolateral cell column (IML), 33 of 151 non-respiratory RVLM neurons showed antidromic action potentials with a constant latency of 45 ms, and can be considered as presympathetic neurons. We classified and characterized the RVLM presympathetic neurons into three types: 'regularly firing neurons (n=7)', which showed ramp depolarization and frequent action potentials (4.2+/-0.9 spikes/s) with rare excitatory postsynaptic potentials (EPSPs); 'irregularly firing neurons (n=21)', which exhibited many EPSPs that modulated the firing rate; and 'silent-type neurons (n=5)', which discharged action potentials only during current-induced depolarization. Lucifer-Yellow staining showed that the irregularly firing neurons were significantly larger and had more dendrites than the regularly firing neurons. All regularly firing neurons retained their discharges during low-Ca2+ -high-Mg2+ superfusion that blocks synaptic input, whereas the discharges in 11 of 16 irregularly firing neurons were abolished, suggesting that the regularly firing neurons discharged independently of synaptic input. Seven of 31 RVLM neurons were hyperpolarized by stimulation of vagal afferent nerves. In summary, three types of RVLM presympathetic neurons were characterized by the patch-clamp technique in the brainstem-spinal cord preparation, in which the connection was preserved from vagal afferent to the Th2 spinal segment through the RVLM. Since antidromic action potentials were demonstrated by stimulation in the Th2 spinal segment in 33 neurons of all three types, all types of RVLM neurons constitute a part of the sympathetic neuronal network.

The human nucleus of the solitary tract: visceral pathways revealed with an "in vitro" postmortem tracing method

Visceral relay neurons in the nucleus of the solitary tract (NTS) regulate behavior and autonomic reflex functions. NTS projections have been extensively characterized in animal studies but not in humans. For the first time, NTS fiber trajectories in the human medulla oblongata were revealed with an "in vitro" postmortem tracing method. Local intramedullary pathways were labeled by direct pressure injections of free horseradish peroxidase centered on the medial subnucleus at a level adjacent to true obex. Labeled elements were resolved by peroxidase histochemistry as a dark brown intracellular reaction product. A prominent transtegmental system of axons emerged from the NTS injection sites and entered the intermediate reticular zone, a region corresponding to an autonomic reflex center in other mammals. A medial system of axons arched across the dorsomedial reticular formation toward the dorsal medullary raphe and projected ventrally toward the nucleus gigantocellularis. A small lateral fiber trajectory coursed towards the dorsomedial zone of spinal trigeminal nucleus caudalis. Presumptive terminals appeared as dustings of fine punctate processes within the NTS, dorsomotor nucleus and reticular formation. NTS projections in humans resemble those identified in other mammals including primates. Axonal tracing studies predict that visceral impulses in humans may transmit over evolutionarily conserved pathways involved in autonomic feedback control and stress adaptation.

Expression of Fos protein in adrenal preganglionic neurons following chemical stimulation of the rostral ventrolateral medulla of the rat

The ventrolateral medulla is known to be involved in the regulation of arterial blood pressure, especially via its connections with sympathetic preganglionic neurons (SPNs) mainly located in the intermediolateral nucleus of the spinal cord. It has been shown that stimulation of the rostral part of the ventrolateral medulla (RVLM) elicits a release of catecholamines from the adrenal medulla. The aim of the present study was to demonstrate the existence of a functional pathway between the RVLM and adrenal SPNs using the combination of a retrograde tract tracing technique (cholera toxin B subunit) with the immunohistochemical detection of Fos protein following the chemical stimulation of RVLM. The data obtained showed that: (1) chemical stimulation of the RVLM induced Fos immunoreactivity in the intermediolateral nucleus and particularly in SPNs projecting to the adrenal medulla; (2) along the thoracic segments T2-T12, 26.1% of retrogradely identified adrenal SPNs were Fos-immunoreactive with the greatest percentage (30.9%) in the T8 segment. These results favored a functional control of the RVLM on adrenal SPNs which may contribute to a substantial activation of the cardiovascular system via the release of adrenal catecholamines.