Intermittent Fasting After Spinal Cord Injury Does Not Improve the Recovery of Baroreflex Regulation in the Rat

Modest recovery of somatic function after incomplete spinal cord injury (SCI) has been widely demonstrated. Recently we have shown that spontaneous recovery of baroreflex regulation of sympathetic activity also occurs in rats. Dietary restriction in the form of every other day fasting (EODF) has been shown to have beneficial effects on the recovery of motor function after SCI in rats. The goal of this study was to determine if EODF augments the improvement of baroreflex regulation of sympathetic activity after chronic left thoracic (T8) surgical spinal hemisection. To determine this, we performed baroreflex tests on ad-lib fed or EODF rats 1 week or 7 weeks after left T8 spinal hemisection. One week after T8 left hemisection baroreflex testing revealed that gain of baroreflex responsiveness, as well as the ability to increase renal sympathetic nerve activity (RSNA) at low arterial pressure, was significantly impaired in the ad-lib fed but not the EODF rats compared with sham lesioned control rats. However, baroreflex tests performed 7 weeks after T8 left hemisection revealed the inability of both ad-lib and EODF rats to decrease RSNA at elevated arterial pressures. While there is evidence to suggest that EODF has beneficial effects on the recovery of motor function in rats, EODF did not significantly improve the recovery of baroreflex regulation of sympathetic activity

Neuroanatomical Distribution of Neurons within the Hypothalamic Paraventricular Nucleus that Project to the Brainstem Rostral Ventrolateral Medulla

The sympathetic nervous system plays an important role in maintaining cardiovascular regulation. Elevated cardiovascular-related sympathetic activity can lead to neurogenic hypertension and a host of other serious cardiac related abnormalities. The paraventricular nucleus (PVN) of the hypothalamus plays an important role in sympathetic cardiovascular regulation. Neurons from the PVN project to the rostral ventrolateral medulla (RVLM), which is the main brain stem sympathetic cardiovascular control center. While RVLM-projecting PVN neurons have been well characterized, the topographical organization within the PVN subnuclei are still not fully known. The goal of this neuroanatomical study was to map the topographical distribution of RVLM-projecting PVN neurons. To do this we microinjected four different carboxylate FluoSphere retrograde tracers (blue, 365/415; green, 505/515; red, 565/580; and far red, 660/680) at different rostro-caudal coordinates within the RVLM. The vast majority of RVLM-projecting PVN neurons were ipsilateral and located in the medial parvocellular subnucleus. Whereas most neurons were ipsilateral, there is a small fraction of neurons that crossed the midline. Neurons were also identified within the dorsal, ventral, and posterior parvocellular subnuclei of the PVN and no labeling in the anterior parvocellular or magnocellular subnuclei. We unexpectantly observed different efficiencies of the retrograde tracers with blue (365/415) being the least efficient and red (565/580) being the best. These neuroanatomical data will serve as important preliminary functional and histochemical data for future research studies

GLP-1 agonist liraglutide increases metabolic- and cardiovascular-related sympathetic activity of the central nervous system.

Metabolic syndrome is associated with pathologies that include type 2 diabetes, hypertension, and dyslipidemia, all of which increase the risks of heart disease. Glucagon-like peptide (GLP-1) is a hormone produced by intestinal enteroendocrine L‑cells. GLP-1 increases insulin sensitivity, augments glucose-dependent insulin secretion, and suppresses glucagon release. GLP-1 also works centrally to decrease appetite and increase metabolism. Evidence suggests that the beneficial effect is mediated by metabolically related sympathetic neurons within the hypothalamus. Although the hypothalamus contains neurons that control metabolism, there are also neurons that control cardiovascular activity. Considering that one main goal of obesity and diabetes treatments is to reduce cardiovascular-related comorbidities, any drug‑induced increase in blood pressure is unacceptable. Therefore, a better understanding of GLP-1 agonists on sympathetic activity and the role of the hypothalamus in central GLP‑1 activity is essential. In this study, we tested the hypothesis that the long‑acting FDA approved GLP-1 receptor agonist liraglutide activates both metabolic and cardiovascular‑related hypothalamic neurons and augments reflex cardiovascular sympathetic activity in rats. To test this hypothesis, we administered liraglutide (125 mg/kg, SC, n=10) or vehicle (saline, n=10) to rats for 15 days and measured food intake and body weight. Next, we recorded blood pressure and renal sympathetic nerve activity (RSNA) in the anesthetized rat before and after liraglutide treatment. Finally, to determine the activation of hypothalamic neurons we performed neuroanatomical tracing studies and turned metabolically-related (raphe‑projecting) neurons green, and cardiovascular-related (rostroventrolateral medulla, RVLM) neurons red. After treating rats with liraglutide, (75 mg/kg IV) we performed immunohistochemical (IHC) labeling to identify neurons expressing cFos, a marker of neuronal activation. Daily liraglutide significantly (p \u3c 0.05) reduced both food intake and body weight from the pretreatment baseline. In vehicle-treated rats, the mean baseline food intake was 27.9 ± 0.5g. During vehicle treatment, the mean food intake was 28.6 ± 0.8 g and body weight was 110 ± 1.5% of its baseline. In liraglutide-treated rats, the mean baseline food intake was 29.9 ± 0.7g. During liraglutide treatment, the mean food intake was 22.7 ± 1.4g and body weight was 105 ± 1.1% of its baseline. At the end of liraglutide treatment, food intake and body weight returned to that of the vehicle-treated rats. In the anesthetized rat, liraglutide significantly (p \u3c 0.05) increased basal RSNA and augmented baroreflex and chemoreflex activity. Lastly, our cFos data show that liraglutide activates metabolic, but not cardiovascular hypothalamic neurons. Collectively, these data suggest that although liraglutide elevates sympathetic activity, it is not by activation of pre-sympathetic hypothalamic neurons

Spinal regions involved in baroreflex control of renal sympathetic nerve activity in the rat

Spinal cord injury causes debilitating cardiovascular disturbances. The etiology of these disturbances remains obscure, partly because the locations of spinal cord pathways important for sympathetic control of cardiovascular function have not been thoroughly studied. To elucidate these pathways, we examined regions of the thoracic spinal cord important for reflex sympathetic control of arterial pressure (AP). In anesthetized rats, baroreceptor relationships between pharmacologically induced changes in AP and changes in left renal sympathetic nerve activity (RSNA) were generated in spinally intact rats and after acute surgical hemisection of either the dorsal, left, or right T8 spinal cord. None of these individual spinal lesions prevented the baroreceptor-mediated increases in RSNA caused by decreases in AP. Thus, baroreceptor-mediated increases in RSNA in rats are mediated by relatively diffuse, bilateral, descending, excitatory projections. The ability to reduce RSNA at increased AP was impaired after both dorsal and left hemisections, and baroreceptor gain was significantly decreased. Baroreceptor-induced maximum decreases in RSNA were not affected by right hemisections. However, baroreflex gain was impaired. Because both dorsal and left hemisections, but not right hemisections, attenuated the decrease in RSNA at elevated AP, we conclude that pathways involved in the tonic inhibition of spinal sources of sympathetic activity descend ipsilaterally in the dorsal spinal cord. Our results show that many lesions that do not fully transect the spinal cord spare portions of both descending excitatory pathways that may prevent orthostatic hypotension and descending inhibitory pathways that reduce the incidence of autonomic dysreflexia

Cardiac vanilloid receptor 1-expressing afferent nerves and their role in the cardiogenic sympathetic reflex in rats

Myocardial ischaemia causes the release of metabolites such as bradykinin, which stimulates cardiac sensory receptors to evoke a sympathoexcitatory reflex. However, the molecular identity of the afferent neurons and fibres mediating this reflex response is not clear. In this study, we tested the hypothesis that the cardiogenic sympathoexcitatory reflex is mediated by capsaicin-sensitive afferent fibres. Enhanced immunofluorescence labelling revealed that vanilloid receptor 1 (VR1)-containing afferent nerve fibres were present on the epicardial surface of the rat heart. Resiniferatoxin (RTX), a potent analogue of capsaicin, was used to deplete capsaicin-sensitive afferent fibres in rats. Depletion of these fibres was confirmed by a substantial reduction of VR1 immunoreactivity in the epicardium and dorsal root ganglia. The thermal sensitivity was also diminished in RTX-treated rats. Renal sympathetic nerve activity (RSNA) and blood pressure were recorded in anaesthetized rats during epicardial application of bradykinin or capsaicin. In vehicle-treated rats, epicardial bradykinin (10 μg ml−1) or capsaicin (10 μg ml−1) application produced a significant increase in RSNA and arterial blood pressure. The RSNA and blood pressure responses caused by bradykinin and capsaicin were completely abolished in RTX-treated rats. Furthermore, epicardial application of iodo-RTX, a highly specific antagonist of VR1 receptors, blocked capsaicin- but not bradykinin-induced sympathoexcitatory responses. Thus, these data provide important histological and functional evidence that the heart is innervated by VR1-expressing afferent nerves and these afferent nerves are essential for the cardiogenic sympathoexcitatory reflex during myocardial ischaemia

Sialidase, Chondroitinase ABC, and Combination Therapy after Spinal Cord Contusion Injury

Axon regeneration in the central nervous system is severely hampered, limiting functional recovery. This is in part because of endogenous axon regeneration inhibitors that accumulate at the injury site. Therapeutic targeting of these inhibitors and their receptors may facilitate axon outgrowth and enhance recovery. A rat model of spinal cord contusion injury was used to test the effects of two bacterial enzyme therapies that target independent axon regeneration inhibitors, sialidase (Vibrio cholerae) and chondroitinase ABC (ChABC, Proteus vulgaris). The two enzymes, individually and in combination, were infused for 2 weeks via implanted osmotic pumps to the site of a moderate thoracic spinal cord contusion injury. Sialidase was completely stable, whereas ChABC retained>30% of its activity in vivo over the 2 week infusion period. Immunohistochemistry revealed that infused sialidase acted robustly throughout the spinal cord gray and white matter, whereas ChABC activity was more intense superficially. Sialidase treatment alone resulted in improved behavioral and anatomical outcomes. Rats treated exclusively with sialidase showed significantly increased hindlimb motor function, evidenced by higher Basso Beattie and Bresnahan (BBB) and BBB subscores, and fewer stepping errors on a horizontal ladder. Sialidase-treated rats also had increased serotonergic axons caudal to the injury. ChABC treatment, in contrast, did not enhance functional recovery or alter axon numbers after moderate spinal cord contusion injury, and dampened the response of sialidase in the dual enzyme treatment group. We conclude that sialidase infusion enhanced recovery from spinal cord contusion injury, and that combining sialidase with ChABC failed to improve outcomes