Ischemia-induced disturbance of neuronal network function in the rat spinal cord analyzed by voltage-imaging

Structural and functional identification of two distinct inspiratory neuronal populations at the level of the phrenic nucleus in the rat cervical spinal cord

The diaphragm is driven by phrenic motoneurons that are located in the cervical spinal cord. Although the anatomical location of the phrenic nucleus and the function of phrenic motoneurons at a single cellular level have been extensively analyzed, the spatiotemporal dynamics of phrenic motoneuron group activity have not been fully elucidated. In the present study, we analyzed the functional and structural characteristics of respiratory neuron population in the cervical spinal cord at the level of the phrenic nucleus by voltage imaging, together with histological analysis of neuronal and astrocytic distribution in the cervical spinal cord. We found spatially distinct two cellular populations that exhibited synchronized inspiratory activity on the transversely cut plane at C4–C5 levels and on the ventral surface of the mid cervical spinal cord in the isolated brainstem–spinal cord preparation of the neonatal rat. Inspiratory activity of one group emerged in the central portion of the ventral horn that corresponded to the central motor column, and the other appeared in the medial portion of the ventral horn that corresponded to the medial motor column. We identified by retrogradely labeling study that the anatomical distributions of phrenic and scalene motoneurons coincided with optically detected central and medial motor regions, respectively. Furthermore, we anatomically demonstrated closely located features of putative motoneurons, interneurons and astrocytes in these regions. Collectively, we report that phrenic and scalene motoneuron populations show synchronized inspiratory activities with distinct anatomical locations in the mid cervical spinal cord.

Alterations in autonomic cerebrovascular control after spinal cord injury

Among chronic cardiovascular and metabolic sequelae of spinal cord injury (SCI) is an up-to four-fold increase in the risk of ischemic and hemorrhagic stroke, suggesting that individuals with SCI cannot maintain stable cerebral perfusion. In able-bodied individuals, the cerebral vasculature is able to regulate cerebral perfusion in response to swings in arterial pressure (cerebral autoregulation), blood gases (cerebral vasoreactivity), and neural metabolic demand (neurovascular coupling). This ability depends, at least partly, on intact autonomic function, but high thoracic and cervical spinal cord injuries result in disruption of sympathetic and parasympathetic cerebrovascular control. In addition, alterations in autonomic and/or vascular function secondary to paralysis and physical inactivity can impact cerebrovascular function independent of the disruption of autonomic control due to injury. Thus, it is conceivable that SCI results in cerebrovascular dysfunction that may underlie an elevated risk of stroke in this population, and that rehabilitation strategies targeting this dysfunction may alleviate the long-term risk of adverse cerebrovascular events. However, despite this potential direct link between SCI and the risk of stroke, studies exploring this relationship are surprisingly scarce, and the few available studies provide equivocal results. The focus of this review is to provide an integrated overview of the available data on alterations in cerebral vascular function after SCI in humans, and to provide suggestions for future research.

Standardization of size, shape and internal structure of spinal cord images: comparison of three transformation methods

Functional fluorescence imaging has been widely applied to analyze spatio-temporal patterns of cellular dynamics in the brain and spinal cord. However, it is difficult to integrate spatial information obtained from imaging data in specific regions of interest across multiple samples, due to large variability in the size, shape and internal structure of samples. To solve this problem, we attempted to standardize transversely sectioned spinal cord images focusing on the laminar structure in the gray matter. We employed three standardization methods, the affine transformation (AT), the angle-dependent transformation (ADT) and the combination of these two methods (AT+ADT). The ADT is a novel non-linear transformation method developed in this study to adjust an individual image onto the template image in the polar coordinate system. We next compared the accuracy of these three standardization methods. We evaluated two indices, i.e., the spatial distribution of pixels that are not categorized to any layer and the error ratio by the leave-one-out cross validation method. In this study, we used neuron-specific marker (NeuN)-stained histological images of transversely sectioned cervical spinal cord slices (21 images obtained from 4 rats) to create the standard atlas and also to serve for benchmark tests. We found that the AT+ADT outperformed other two methods, though the accuracy of each method varied depending on the layer. This novel image standardization technique would be applicable to optical recording such as voltage-sensitive dye imaging, and will enable statistical evaluations of neural activation across multiple samples.

Spatiotemporal and anatomical analyses of P2X receptor-mediated neuronal and glial processing of sensory signals in the rat dorsal horn

Extracellularly released adenosine triphosphate (ATP) modulates sensory signaling in the spinal cord. We analyzed the spatiotemporal profiles of P2X receptor-mediated neuronal and glial processing of sensory signals and the distribution of P2X receptor subunits in the rat dorsal horn. Voltage imaging of spinal cord slices revealed that extracellularly applied ATP (5-500 μM), which was degraded to adenosine and acting on P1 receptors, inhibited depolarizing signals and that it also enhanced long-lasting slow depolarization, which was potentiated after ATP was washed out. This post-ATP rebound potentiation was mediated by P2X receptors and was more prominent in the deep than in the superficial layer. Patch clamp recording of neurons in the superficial layer revealed long-lasting enhancement of depolarization by ATP through P2X receptors during the slow repolarization phase at a single neuron level. This depolarization pattern was different from that in voltage imaging, which reflects both neuronal and glial activities. By immunohistochemistry, P2X(1) and P2X(3) subunits were detected in neuropils in the superficial layer. The P2X(5) subunit was found in neuronal somata. The P2X(6) subunit was widely expressed in neuropils in the whole gray matter except for the dorsal superficial layer. Astrocytes expressed the P2X(7) subunit. These findings indicate that extracellular ATP is degraded into adenosine and prevents overexcitation of the sensory system, and that ATP acts on pre- and partly on postsynaptic neuronal P2X receptors and enhances synaptic transmission, predominantly in the deep layer. Astrocytes are involved in sensitization of sensory network activity more importantly in the superficial than in the deep layer.

Respiratory and metabolic acidosis differentially affect the respiratory neuronal network in the ventral medulla of neonatal rats

Two respiratory-related areas, the para-facial respiratory group/retrotrapezoid nucleus (pFRG/RTN) and the pre-Bötzinger complex/ventral respiratory group (preBötC/VRG), are thought to play key roles in respiratory rhythm. Because respiratory output patterns in response to respiratory and metabolic acidosis differ, we hypothesized that the responses of the medullary respiratory neuronal network to respiratory and metabolic acidosis are different. To test these hypotheses, we analysed respiratory-related activity in the pFRG/RTN and preBötC/VRG of the neonatal rat brainstem-spinal cord in vitro by optical imaging using a voltage-sensitive dye, and compared the effects of respiratory and metabolic acidosis on these two populations. We found that the spatiotemporal responses of respiratory-related regional activities to respiratory and metabolic acidosis are fundamentally different, although both acidosis similarly augmented respiratory output by increasing respiratory frequency. PreBötC/VRG activity, which is mainly inspiratory, was augmented by respiratory acidosis. Respiratory-modulated pixels increased in the preBötC/VRG area in response to respiratory acidosis. Metabolic acidosis shifted the respiratory phase in the pFRG/RTN; the pre-inspiratory dominant pattern shifted to inspiratory dominant. The responses of the pFRG/RTN activity to respiratory and metabolic acidosis are complex, and involve either augmentation or reduction in the size of respiratory-related areas. Furthermore, the activation pattern in the pFRG/RTN switched bi-directionally between pre-inspiratory/inspiratory and post-inspiratory. Electrophysiological study supported the results of our optical imaging study. We conclude that respiratory and metabolic acidosis differentially affect activities of the pFRG/RTN and preBötC/VRG, inducing switching and shifts of the respiratory phase. We suggest that they differently influence the coupling states between the pFRG/RTN and preBötC/VRG.

Postnatal developmental changes in activation profiles of the respiratory neuronal network in the rat ventral medulla

Two putative respiratory rhythm generators (RRGs), the para-facial respiratory group (pFRG) and the pre-Bötzinger complex (preBötC), have been identified in the neonatal rodent brainstem. To elucidate their functional roles during the neonatal period, we evaluated developmental changes of these RRGs by optical imaging using a voltage-sensitive dye. Optical signals, recorded from the ventral medulla of brainstem-spinal cord preparations of neonatal (P0-P4) rats (n = 44), were analysed by a cross correlation method. With development during the first few postnatal days, the respiratory-related activity in the pFRG reduced and shifted from a preinspiratory (P0-P1) to an inspiratory (P2-P4) pattern, whereas preBötC activity remained unchanged. The mu-opioid agonist [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO) augmented preinspiratory activity in the pFRG, while the mu-opioid antagonist naloxone induced changes in spatiotemporal activation profiles that closely mimicked the developmental changes. These results are consistent with the recently proposed hypothesis by Janczewski and Feldman that the pFRG is activated to compensate for the depression of the preBötC by perinatal opiate surge. We conclude that significant reorganization of the respiratory neuronal network, characterized by a reduction of preinspiratory activity in the pFRG, occurs at P1-P2 in rats. The changes in spatiotemporal activation profiles of the pFRG neurones may reflect changes in the mode of coupling of the two respiratory rhythm generators.