Reciprocal functional interactions between the brainstem and the lower spinal cord

Immature functional development of lumbar locomotor networks in adult Irf8-/- mice

To date, research on the role of the brainstem and spinal cord in motor behavior has relied on in vitro preparations of the neonatal rodent spinal cord, with or without the brainstem; their spatial and temporal scope are subject to technical limitations imposed by low oxygen tension in deep tissues. Therefore, we created an arterially perfused in situ preparation that allowed us to investigate functional interactions in the CNS from the neonatal to adult period. Decerebrated rodents were kept alive via total artificial cardiopulmonary bypass for extracorporeal circulation; the plasma oxygen and ion components needed for survival were supplied through the blood vessels. Interferon regulatory factor 8 (IRF8) is a transcription factor that promotes myeloid cell development and stimulates innate immune responses. In the brain, IRF8 is expressed only in microglia and directs the expression of many genes that serve microglial functions. Recent evidence indicates that IRF8 affects behavior and modulates Alzheimer's disease progression in a mouse model. However, whether this immune deficiency arising from the absence of IRF8 influences the development of the neuronal network in the spinal cord is unknown. We applied the above methodology to mice of all ages and electrophysiologically explored whether the absence of IRF8 influences the development of lumbar central pattern generator (CPG) networks. In mice of all ages, bilateral neuronal discharges by the normal CPG networks activated by the modulated sympathetic tone via descending pathways at high flow rates became organized into discharge episodes punctuated by periods of quiescence. Similar discharge episodes were generated by the adult CPG networks (≥P14 days) activated by drug application. However, discharge episodes elicited by activating the neonatal-juvenile CPG networks ( Collapse

Key Words

• adult interferon regulatory factor 8 (IRF8)-deficient mice
• decerebrate and arterially perfused in situ preparations
• discharge episodes
• immature adult lumbar CPG
• rhythmic discharge

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Affiliation(s)

Itaru Yazawa Department of Food and Nutrition, Kyushu Nutrition Welfare University, Kitakyushu, Japan Laboratory of Neural Control, National Institute of Neurological Disorders and Stroke (NINDS), National Institutes of Health (NIH), Bethesda, MD, United States
Yuko Yoshida Division of Developmental Biology, National Institute of Child Health and Human Development (NICHD), NIH, Bethesda, MD, United States
Ryusuke Yoshimi Division of Developmental Biology, National Institute of Child Health and Human Development (NICHD), NIH, Bethesda, MD, United States Department of Stem Cell and Immune Regulation, Graduate School of Medicine, Yokohama City University, Yokohama, Japan
Keiko Ozato Division of Developmental Biology, National Institute of Child Health and Human Development (NICHD), NIH, Bethesda, MD, United States

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Passive limb training modulates respiratory rhythmic bursts

Exercise modifies respiratory functions mainly through the afferent feedback provided by exercising limbs and the descending input from suprapontine areas, two contributions that are still underestimated in vitro. To better characterize the role of limb afferents in modulating respiration during physical activity, we designed a novel experimental in vitro platform. The whole central nervous system was isolated from neonatal rodents and kept with hindlimbs attached to an ad-hoc robot (Bipedal Induced Kinetic Exercise, BIKE) driving passive pedaling at calibrated speeds. This setting allowed extracellular recordings of a stable spontaneous respiratory rhythm for more than 4 h, from all cervical ventral roots. BIKE reversibly reduced the duration of single respiratory bursts even at lower pedaling speeds (2 Hz), though only an intense exercise (3.5 Hz) modulated the frequency of breathing. Moreover, brief sessions (5 min) of BIKE at 3.5 Hz augmented the respiratory rate of preparations with slow bursting in control (slower breathers) but did not change the speed of faster breathers. When spontaneous breathing was accelerated by high concentrations of potassium, BIKE reduced bursting frequency. Regardless of the baseline respiratory rhythm, BIKE at 3.5 Hz always decreased duration of single bursts. Surgical ablation of suprapontine structures completely prevented modulation of breathing after intense training. Albeit the variability in baseline breathing rates, intense passive cyclic movement tuned fictive respiration toward a common frequency range and shortened all respiratory events through the involvement of suprapontine areas. These observations contribute to better define how the respiratory system integrates sensory input from moving limbs during development, opening new rehabilitation perspectives.

Role of Propriospinal Neurons in Control of Respiratory Muscles and Recovery of Breathing Following Injury

Respiratory motor failure is the leading cause of death in spinal cord injury (SCI). Cervical injuries disrupt connections between brainstem neurons that are the primary source of excitatory drive to respiratory motor neurons in the spinal cord and their targets. In addition to direct connections from bulbospinal neurons, respiratory motor neurons also receive excitatory and inhibitory inputs from propriospinal neurons, yet their role in the control of breathing is often overlooked. In this review, we will present evidence that propriospinal neurons play important roles in patterning muscle activity for breathing. These roles likely include shaping the pattern of respiratory motor output, processing and transmitting sensory afferent information, coordinating ventilation with motor activity, and regulating accessory and respiratory muscle activity. In addition, we discuss recent studies that have highlighted the importance of propriospinal neurons for recovery of respiratory muscle function following SCI. We propose that molecular genetic approaches to target specific developmental neuron classes in the spinal cord would help investigators resolve the many roles of propriospinal neurons in the control of breathing. A better understanding of how spinal circuits pattern breathing could lead to new treatments to improve breathing following injury or disease.

An aromatic substance, eugenol induces distinct depressant effects on respiratory activity in different postnatal developmental stages of the rat

Eugenol modulates neuronal activity through actions on voltage-gated ionic channels and/or transient receptor potential channels. We previously suggested that eugenol inhibited cellular (and/or network) mechanisms essential for the maintenance of the respiratory burst activity in a brainstem-spinal cord preparation from newborn rat (postnatal day 0-3). Study of the distinct effects of eugenol in neonatal and later developmental stage rats may offer new information about postnatal developmental changes of respiratory neuron networks. In the present study, therefore, we compared effects of eugenol in an in vitro newborn rat preparation with those in an arterially perfused in situ preparation from juvenile rat (postnatal day 12-15). In the former preparation, application of 1 mM eugenol decreased respiratory rate and inspiratory burst duration. In contrast, in the latter preparation, 1 mM eugenol induced a gradual decrease in the amplitude of integrated phrenic nerve activity. Phrenic nerve activity gradually recovered at 25-30 min after washout with a burst duration similar to control values. We hypothesized that the depressant effects of eugenol were caused by inhibition of cell excitability in the neonatal rat in vitro preparation but by a reduction of synaptic interactions in the juvenile rat in situ preparation.

Modulation of Respiratory System by Limb Muscle Afferents in Intact and Injured Spinal Cord

Breathing constantly adapts to environmental, metabolic or behavioral changes by responding to different sensory information, including afferent feedback from muscles. Importantly, not just respiratory muscle feedback influences respiratory activity. Afferent sensory information from rhythmically moving limbs has also been shown to play an essential role in the breathing. The present review will discuss the neuronal mechanisms of respiratory modulation by activation of peripheral muscles that usually occurs during locomotion or exercise. An understanding of these mechanisms and finding the most effective approaches to regulate respiratory motor output by stimulation of limb muscles could be extremely beneficial for people with respiratory dysfunctions. Specific attention in the present review is given to the muscle stimulation to treat respiratory deficits following cervical spinal cord injury.

Sudomotor and vasomotor activity during the menstrual cycle with global heating

Many studies have reported that there are changes in sympathetic activity throughout the menstrual cycle as there are oestrogen receptor in the hypothalamus and all other parts of the sympathetic nervous system. The purpose of this study was to see whether there were variations in sympathetic activity, skin vasomotor and sweat gland sudomotor rhythms during the menstrual cycle. Eight young female subjects with a regular menstrual cycle participated in the study. Subjects were tested once during the follicular phase and once during the luteal phase. Skin blood flow and sweat rate were significantly higher in the luteal phase compared with the follicular phase (p < .05), but the frequency and magnitude of sudomotor and vasomotor rhythms were significantly greater in the follicular phase (p < .05). In contrast, spectral data showed less sympathetic activity in the luteal phase. A significant finding here is that the sudomotor rhythm of sweat glands is altered by the menstrual cycle.

Long-lasting facilitation of respiratory rhythm by treatment with TRPA1 agonist, cinnamaldehyde

The transient receptor potential (TRP) channels are widely distributed in the central nervous system (CNS) and peripheral nervous system. We examined the effects of TRP ankyrin 1 (TRPA1) agonists (cinnamaldehyde and allyl isothiocyanate) on respiratory rhythm generation in brainstem-spinal cord preparations from newborn rats [postnatal days 0-3 (P0-P3)] and in in situ-perfused preparations from juvenile rats (P11-P13). Preparations were superfused with modified Krebs solution at 25-26°C, and activity of inspiratory C4 ventral root (or phrenic nerve) was monitored. In the newborn rat, an in vitro preparation of cinnamaldehyde (0.5 mM) induced typically biphasic responses in C4 rate: an initial short increase and subsequent decrease, then a gradual recovery of rhythm during 15 min of bath application. After washout, the respiratory rhythm rate further increased, remaining 200% of control for >120 min, indicating long-lasting facilitation. Allyl isothiocyanate induced effects similar to those of cinnamaldehyde. The long-lasting facilitation of respiratory rhythm was partially antagonized by the TRPA1 antagonist HC-030031 (10 μM). We obtained similar long-lasting facilitation in an in situ-perfused reparation from P11-P13 rats. On the basis of results from transection experiments of the rostral medulla and whole-cell recordings from preinspiratory neurons in the parafacial respiratory group (pFRG), we suggest that the rostral medulla, including the pFRG, is important to the induction of long-lasting facilitation. A histochemical analysis demonstrated a wide distribution of TRPA1 channel-positive cells in the reticular formation of the medulla, including the pFRG. Our findings suggest that TRPA1 channel activation could induce long-lasting facilitation of respiratory rhythm and provide grounds for future study on the roles of TRPA1 channels in the CNS.

Reciprocal functional interactions between the respiration/circulation center, the upper spinal cord, and the trigeminal system

The interplay of neural discharge patterns involved in "respiration", "circulation", "opening movements in the mandible", and "locomotion" was investigated electrophysiologically in a decerebrate and arterially perfused in situ rat preparation. Sympathetic tone increased with increases in perfusion flow rate. All nerve discharges became clearly organized into discharge episodes of increasing frequency and duration punctuated by quiescent periods as the perfusion flow rate increased at 26°C. The modulated sympathetic tone at 10× total blood volume/ min activated the forelimb pattern generator and spontaneously generated fictive forelimb movement during discharge episodes. The coupling rhythm of respiration and locomotion during motion occurred at frequency ratios ranges of 1:2 and 1:3. Small increases in systemic pressure were always generated after the initiation of motion. Opening movements in the mandible, occurring during the inspiratory phase at all tested flow rates, were generated in both the inspiratory and expiratory phases during motion. Although the central mechanism for the entrainment of respiratory and locomotor rhythms has not been identified, a spinal-feedback mechanism generating fictive locomotion in the upper spinal cord contributed to generating the opening movement in the mandible in the expiratory phase during motion. The existence of this mechanism implies that there is a reciprocal functional interaction between the brainstem and the spinal cord, whereby the intake and output of air by the lungs is efficiently improved during movement by both nasal and mouth breathing. These results suggest that this reciprocal functional interaction plays an important role in increasing oxygenated blood flow during locomotion.

Effects of ouabain on respiratory rhythm generation in brainstem-spinal cord preparation from newborn rats and in decerebrate and arterially perfused in situ preparation from juvenile rats

The significance of Na/K-ATPase on respiratory rhythm generation is not well understood. We investigated the effects of the Na/K-ATPase blocker, ouabain, on respiratory rhythm. Experiments were performed with brainstem-spinal cord preparation from 0 to 3-day-old Wistar rats and with decerebrate and arterially perfused in situ preparation from juvenile rats (postnatal day 11-13). Newborn rat preparations were superfused at a rate of 3.0 ml/min with artificial cerebrospinal fluid, equilibrated with 95% O2 and 5% CO2, pH 7.4, at 26-27 °C. Inspiratory activity was monitored from the fourth cervical ventral root (C4). Application of ouabain (15-20 min) resulted in a dose-dependent increase in the burst rate of C4 inspiratory activity. After washout, the burst rate further increased to reach quasi-maximum values under each condition (e.g. 183% of control in 1 μM, 253% in 10 μM, and 303% in 20 μM at 30 min washout). Inspiratory or pre-inspiratory neurons in the rostral ventrolateral medulla were depolarized. We obtained similar results (i.e. increased phrenic burst rate) in an in situ perfused preparation of juvenile rats. Genes encoding the Na/K-ATPase α subunit were expressed in the region of the parafacial respiratory group (pFRG) in neonatal rats, suggesting that cells (neurons and/or glias) in the pFRG were one of the targets of ouabain. We concluded that Na/K-ATPase activity could be an important factor in respiratory rhythm modulation.