Bulbospinal Neurons Research Articles

Physical (in)activity‐dependent structural plasticity in bulbospinal catecholaminergic neurons of rat rostral ventrolateral medulla

ABSTRACTIncreased activity of the sympathetic nervous system is thought to play a role in the development and progression of cardiovascular disease. Recent work has shown that physical inactivity versus activity alters neuronal structure in brain regions associated with cardiovascular regulation. Our physiological studies suggest that neurons in the rostral ventrolateral medulla (RVLM) are more responsive to excitation in sedentary versus physically active animals. We hypothesized that enhanced functional responses in the RVLM may be due, in part, to changes in the structure of RVLM neurons that control sympathetic activity. We used retrograde tracing and immunohistochemistry for tyrosine hydroxylase (TH) to identify bulbospinal catecholaminergic (C1) neurons in sedentary and active rats after chronic voluntary wheel‐running exercise. We then digitally reconstructed their cell bodies and dendrites at different rostrocaudal levels. The dendritic arbors of spinally projecting TH neurons from sedentary rats were more branched than those of physically active rats (P < 0.05). In sedentary rats, dendritic branching was greater in more rostral versus more caudal bulbospinal C1 neurons, whereas, in physically active rats, dendritic branching was consistent throughout the RVLM. In contrast, cell body size and the number of primary dendrites did not differ between active and inactive animals. We suggest that these structural changes provide an anatomical underpinning for the functional differences observed in our in vivo studies. These inactivity‐related structural and functional changes may enhance the overall sensitivity of RVLM neurons to excitatory stimuli and contribute to an increased risk of cardiovascular disease in sedentary individuals. J. Comp. Neurol. 522:499–513, 2014. © 2013 Wiley Periodicals, Inc.

Abstract 444: Presence and Functions of B1- and B2-adrenergic Receptors on Bulbospinal Neurons in the Rostral Ventrolateral Medulla

The presence and effects of β-adrenergic receptors (β-ARs) on bulbospinal RVLM neurons are little examined. To directly investigate whether RVLM neurons have sensitivity to propranolol (a non-selective β-AR antagonist), metoprolol (a β1-AR antagonist), dobutamine (a β1-AR agonist), butoxamine (a β2-AR antagonist), and salbutamol (a β2-AR agonist), we examined changes in the membrane potentials (MPs) of bulbospinal RVLM neurons using whole-cell patch-clamp technique, during superfusion with these drugs. During propranolol superfusion eight of 17 RVLM neurons were depolarized, and nine of 17 RVLM neurons were hyperpolarized. In contrast, during metoprolol superfusion sixteen of 20 RVLM neurons were hyperpolarized, and during dobutamine superfusion five of 6 RVLM neurons were depolarized. During butoxamine superfusion eleven of 16 RVLM neurons were depolarized, and during salbutamol uperfusion all of 8 RVLM neurons were hyperpolarized. To clarify the character of RVLM neurons by themselves, superfusion with metoprolol or butoxamine, dissolved in a low-Ca 2+ , high-Mg 2+ solution, was performed. During metoprolol superfusion nine of 12 RVLM neurons were again hyperpolarized, and during butoxamine superfusion seven of 8 RVLM neurons were depolarized. These results suggest that β1- and β2-ARs are actually present on the RVLM neurons. β1-AR antagonists decreased the activity of bulbospinal RVLM neurons, and β2-AR antagonists increased it. To determine the presence of β1- and β2-ARs histologically, immunofluorescent examination including double staining was performed. All of 5 metoprolol-hyperpolarized neurons showed β1-AR immunoreactivity, and all of 3 butoxamine-depolarized neurons showed β2-AR immunoreactivity. Furthermore, two RVLM neurons, which were depolarized during metoprolol superfusion and were hyperpolarized during followed butoxamine superfusion, were examined for β1- and β2-ARs immunoreactivity. These neurons showed immunoreactivity for both β1- and β2-ARs, and most β1-ARs immunoreactive neurons in the RVLM had immunoreactivity for β2-ARs.Our findings suggest that β1-AR antagonists or β2-AR agonists may decrease blood pressure through decreasing activity of bulbospinal RVLM neurons.

Specificity in monosynaptic and disynaptic bulbospinal connections to thoracic motoneurones in the rat

The respiratory activity in the intercostal nerves of the rat is unusual, in that motoneurones of both branches of the intercostal nerves, internal and external, are activated during expiration. Here, the pathways involved in that activation were investigated in anaesthetised and in decerebrate rats by cross-correlation and by intracellular spike-triggered averaging from expiratory bulbospinal neurones (EBSNs), with a view to revealing specific connections that could be used in studies of experimental spinal cord injury. Decerebrate preparations, which showed the strongest expiratory activity, were found to be the most suitable for these measurements. Cross-correlations in these preparations showed monosynaptic connections from 16/19 (84%) of EBSNs, but only to internal intercostal nerve motoneurones (24/37, 65% of EBSN/nerve pairs), whereas disynaptic connections were seen for external intercostal nerve motoneurones (4/19, 21% of EBSNs or 7/25, 28% of EBSN/nerve pairs). There was evidence for additional disynaptic connections to internal intercostal nerve motoneurones. Intracellular spike-triggered averaging revealed excitatory postsynaptic potentials, which confirmed these connections. This is believed to be the first report of single descending fibres that participate in two different pathways to two different groups of motoneurones. It is of interest compared with the cat, where only one group of motoneurones is activated during expiration and only one of the pathways has been detected. The specificity of the connections could be valuable in studies of plasticity in pathological situations, but care will be needed in studying connections in such situations, because their strength was found here to be relatively weak.

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.

Immunoreactivity for the NMDA NR1 subunit in bulbospinal catecholamine and serotonin neurons of rat ventral medulla

Bulbospinal neurons in the ventral medulla play important roles in the regulation of sympathetic outflow. Physiological evidence suggests that these neurons are activated by N-methyl-D-aspartate (NMDA) and non-NMDA subtypes of glutamate receptors. In this study, we examined bulbospinal neurons in the ventral medulla for the presence of immunoreactivity for the NMDA NR1 subunit, which is essential for NMDA receptor function. Rats received bilateral injections of cholera toxin B into the tenth thoracic spinal segment to label bulbospinal neurons. Triple immunofluorescent labeling was used to detect cholera toxin B with a blue fluorophore, NR1 with a red fluorophore, and either tyrosine hydroxylase or tryptophan hydroxylase with a green fluorophore. In the rostral ventrolateral medulla, NR1 occurred in all bulbospinal tyrosine hydroxylase-positive neurons and 96% of bulbospinal tyrosine hydroxylase-negative neurons, which were more common in sections containing the facial nucleus. In the raphe pallidus, the parapyramidal region, and the marginal layer, 98% of bulbospinal tryptophan hydroxylase-positive neurons contained NR1 immunoreactivity. NR1 was also present in all of the bulbospinal tryptophan hydroxylase-negative neurons, which comprised 20% of bulbospinal neurons in raphe pallidus and the parapyramidal region. These results show that virtually all bulbospinal tyrosine hydroxylase and non-tyrosine hydroxylase neurons in the rostral ventrolateral medulla and virtually all bulbospinal serotonin and non-serotonin neurons in raphe pallidus and the parapyramidal region express NR1, the obligatory subunit of the NMDA receptor. NMDA receptors on bulbospinal neurons in the rostral ventral medulla likely influence sympathoexcitation in normal and pathological conditions.

Gene expression of NMDA receptor subunits using laser capture microdissection in the rostral ventrolateral medulla (RVLM) of sedentary versus physically active rats.

The RVLM is important in tonic and reflex control of blood pressure and sympathetic nerve activity (SNA). We have observed enhanced SNA responses in sedentary versus physically active rats after microinjection of glutamate in the RVLM. Changes in glutamate receptor gene expression in the RVLM may contribute to enhanced responses in sedentary rats. In order to test this hypothesis, active (voluntary running for 15–18 wks) and sedentary (no running) rats were injected with Fluorogold (5%) at spinal levels T9‐T10 (n=5 ea). Brainstems were cryosectioned at 8μm for laser capture microdissection of bulbospinal RVLM neurons. RNA was isolated to analyze changes in gene expression of NMDA receptor subunits (NR2A‐D) using RT‐PCR. Gene expression was normalized to a housekeeping gene (HPRT). Preliminary results suggest that expression of NR2A‐D is not different between groups. However, within active rats, total running distance was significantly correlated to NR2C gene expression (r2=0.87, p<0.05). These findings suggest that physical activity contributes to NR2C neuroplasticity in the RVLM. However further studies are needed to address whether NMDA receptor protein expression contributes to enhanced responses in sedentary rats. (R01HL096787; DC007733)

Location of cat brain stem neurons that drive sweating

The brain stem premotor pathways controlling most noncardiovascular sympathetic outflows are unknown. Here, we mapped the brain stem neurons that drive sweating, by microinjecting excitant amino acid (L-glutamate or D,L-homocysteate: 0.4-3 nmol) into 420 sites over the pons and medulla of eight chloralose-anesthetized cats (70 mg/kg iv). Sweating was recorded by the electrodermal potential at the ipsilateral forepaw pad. Responses were classified as immediate (<5 s latency) or delayed (>10 s latency). Immediate responses were obtained from 16 sites (1-3 per animal) and were accompanied by no change in blood pressure. Those sites were clustered between the facial nucleus and the pyramidal tract in the rostral ventromedial medulla (RVMM). Microinjections into 33 surrounding sites caused delayed electrodermal responses of lesser amplitude, while the remaining 371 sites evoked none. To retrogradely label bulbospinal neurons that may mediate electrodermal responses, fluorescent latex microspheres were injected into the region of the intermediolateral cell column in the fourth thoracic segment in an earlier preparatory procedure on six of the animals. A cluster of retrogradely labeled neurons was identified between the facial nucleus and the pyramidal tract. Neurons in this discrete region of the RVMM, thus, drive sweating in the cat's paw and may do so via direct spinal projections.

Arterial chemoreceptor activation reduces the activity of parapyramidal serotonergic neurons in rats

The parapyramidal (ppy) region targets primarily the intermediolateral cell column and is probably involved in breathing and thermoregulation. In the present study, we tested whether ppy serotonergic neurons respond to activation of central and peripheral chemoreceptors. Bulbospinal ppy neurons (n=30) were recorded extracellularly along with the phrenic nerve activity in urethane/α-chloralose-anesthetized, paralyzed, intact (n=7) or carotid body denervated (n=6) male Wistar rats. In intact animals, most of the ppy neurons were inhibited by hypoxia (n=14 of 19) (8% O2, 30s) (1.5±0.03 vs. control: 2.4±0.2Hz) or hypercapnia (n=15 of 19) (10% CO2) (1.7±0.1 vs. control: 2.2±0.2Hz), although some neurons were insensitive to hypoxia (n=3 of 19) or hypercapnia (n=4 of 19). Very few neurons (n=2 of 19) were activated after hypoxia, but not after hypercapnia. In carotid body denervated rats, all the 5HT-ppy neurons (n=11) were insensitive to hypercapnia (2.1±0.1 vs. control: 2.3±0.09Hz). Biotinamide-labeled cells that were recovered after histochemistry were located in the ppy region. Most labeled cells (90%) showed strong tryptophan hydroxylase immunocytochemical reactivity, indicating that they were serotonergic. The present data reveal that peripheral chemoreceptors reduce the activity of the serotonergic premotor neurons located in the ppy region. It is plausible that the serotonergic neurons of the ppy region could conceivably regulate breathing automaticity and be involved in autonomic regulation.

Aldosterone is synthesized in and activates bulbospinal neurons through mineralocorticoid receptors and ENaCs in the RVLM

The effects of aldosterone and mineralocorticoid receptor (MR) blockers on presympathetic neurons in the rostral ventrolateral medulla (RVLM) are well studied. To directly investigate whether aldosterone, eplerenone (an MR blocker), FAD286 (an aldosterone synthase inhibitor) and benzamil (an epithelial sodium channel (ENaC) blocker) affect RVLM neurons, we examined changes in the membrane potentials (MPs) of bulbospinal RVLM neurons using the whole-cell patch-clamp technique during superfusion with these drugs to brainstem-spinal cord preparations. Aldosterone superfusion (0.1 μmol/l) depolarized the RVLM neurons. In contrast, eplerenone superfusion (1 μmol/l) hyperpolarized them. To evaluate the existence of aldosterone, FAD286 superfusion (10 μmol/l) was performed, and the RVLM neurons became hyperpolarized during FAD superfusion. These data suggest that MRs exist and that aldosterone is synthesized in the brainstem. Benzamil superfusion (1 μmol/l) hyperpolarized the RVLM neurons. To clarify whether aldosterone, eplerenone, FAD286 and benzamil acted directly on the RVLM neurons, a low-Ca(2+), high-Mg(2+) solution was used to block the synaptic input to the RVLM neurons, and the above-mentioned drugs were added during the low-Ca(2+) superfusion. During the aldosterone superfusion, the RVLM neurons became depolarized, and they became hyperpolarized during eplerenone, FAD286 or benzamil superfusion. Importantly, when aldosterone was superfused after the benzamil solution, the MPs of the RVLM neurons did not depolarize. These results suggest that MRs are present in the RVLM neurons and that aldosterone is synthesized in the RVLM. The RVLM neurons themselves possess ENaCs, and ENaCs are the underlying mechanism by which aldosterone activates RVLM neurons.

Connections between expiratory bulbospinal neurons and expiratory motoneurons in thoracic and upper lumbar segments of the spinal cord

Cross-correlation of neural discharges was used to investigate the connections between expiratory bulbospinal neurons (EBSNs) in the caudal medulla and expiratory motoneurons innervating thoracic and abdominal muscles in anesthetized cats. Peaks were seen in the cross-correlation histograms for around half of the EBSN-nerve pairs for the following: at T8, the nerve branches innervating internal intercostal muscle and external abdominal oblique muscle and a more distal branch of the internal intercostal nerve; and at L1, a nerve branch innervating internal abdominal oblique muscle and a more distal branch of the ventral ramus. Fewer peaks were seen for the L1 nerve innervating external abdominal oblique, but a paucity of presumed α-motoneuron discharges could explain the rarity of the peaks in this instance. Taking into account individual EBSN conduction times to T8 and to L1, as well as peripheral conduction times, nearly all of the peaks were interpreted as representing monosynaptic connections. Individual EBSNs showed connections at both T8 and L1, but without any discernible pattern. The overall strength of the monosynaptic connection from EBSNs at L1 was found to be very similar to that at T8, which was previously argued to be substantial and responsible for the temporal patterns of expiratory motoneuron discharges. However, we argue that other inputs are required to create the stereotyped spatial patterns of discharges in the thoracic and abdominal musculature.

Modulation of Bulbospinal Rostral Ventral Lateral Medulla Neurons by Hypoxia/Hypercapnia but Not Medullary Respiratory Activity

Although sympathetic vasomotor discharge has respiratory modulation, the site(s) responsible for this cardiorespiratory interaction is unknown. One likely source for this coupling is the rostral ventral lateral medulla (RVLM), where presympathetic neurons originate in close apposition to respiratory neurons. The current study tested the hypothesis that RVLM bulbospinal neurons are modulated by medullary respiratory network activity using whole-cell patch-clamp electrophysiological recordings of RVLM neurons while simultaneously recording fictive respiratory bursting activity from the hypoglossal rootlet. Additionally, we examined whether challenges to cardiorespiratory function, mainly hypoxia/hypercapnia, alter the activity of bulbospinal neurons and, secondarily, whether changes in synaptic input mediate these responses. Surprisingly, our results indicate that inspiratory-related activity did not modulate glutamatergic, γ-aminobutyric acid-ergic, or glycinergic synaptic events or spontaneous action potential firing in these RVLM neurons. However, hypoxia/hypercapnia reversibly decreased the frequency of γ-aminobutyric acid and glycine inhibitory postsynaptic currents. Glycinergic inhibitory postsynaptic current frequency was depressed from the fifth through the 10th minute, whereas the depression of γ-aminobutyric acid-ergic events became significant only at the 10th minute of hypoxia/hypercapnia. On the basis of spontaneous firing activity, there were 2 populations of RVLM bulbospinal neurons. The firing frequency of low-discharging RVLM neurons was facilitated by hypoxia/hypercapnia, and this increase depended on reduced inhibitory neurotransmission. The firing frequency in RVLM neurons with high-discharge rates was inhibited, independent of synaptic input, by hypoxia/hypercapnia. This article demonstrates that sympathetic-respiratory coupling is not active in the neonatal brain stem slice, and reductions in inhibitory neurotransmission to low spontaneously active bulbospinal RVLM neurons are responsible for hypoxia/hypercapnia-elicited increases in activity.

C1 neurons excite locus coeruleus and A5 noradrenergic neurons along with sympathetic outflow in rats

C1 neurons activate sympathetic tone and stimulate the hypothalamic–pituitary–adrenal axis in circumstances such as pain, hypoxia or hypotension. They also innervate pontine noradrenergic cell groups, including the locus coeruleus (LC) and A5. Activation of C1 neurons reportedly inhibits LC neurons; however, because these neurons are glutamatergic and have excitatory effects elsewhere, we re-examined the effect of C1 activation on pontine noradrenergic neurons (LC and A5) using a more selective method. Using a lentivirus that expresses channelrhodopsin2 (ChR2) under the control of the artificial promoter PRSx8, we restricted ChR2 expression to C1 neurons (67%), retrotrapezoid nucleus neurons (20%) and cholinergic neurons (13%). The LC contained ChR2-positive terminals that formed asymmetric synapses and were immunoreactive for vesicular glutamate transporter type 2. Low-frequency photostimulation of ChR2-expressing neurons activated LC (38 of 65; 58%) and A5 neurons (11 of 16; 69%) and sympathetic nerve discharge. Locus coeruleus and A5 inhibition was not seen unless preceded by excitation. Locus coeruleus activation was eliminated by intracerebroventricular kynurenic acid. Stimulation of ChR2-expressing neurons at 20 Hz produced modest increases in LC and A5 neuronal discharge. In additional rats, the retrotrapezoid nucleus region was destroyed with substance P–saporin prior to lentivirus injection into the rostral ventrolateral medulla, increasing the proportion of C1 ChR2-expressing neurons (83%). Photostimulation in these rats activated the same proportion of LC and A5 neurons as in control rats but produced no effect on sympathetic nerve discharge owing to the destruction of bulbospinal C1 neurons. In conclusion, low-frequency stimulation of C1 neurons activates pontine noradrenergic neurons and sympathetic nerve discharge, possibly via the release of glutamate from monosynaptic C1 inputs.

Extensive respiratory plasticity after cervical spinal cord injury in rats: Axonal sprouting and rerouting of ventrolateral bulbospinal pathways

Spinal cord injury (SCI) causes an interruption of descending motor and autonomic nervous tracts. However, a partial injury, and particularly a unilateral section, is generally followed by spontaneous locomotor and respiratory recovery. Although locomotor functional recovery has been correlated to spontaneous anatomical plasticity of the corticospinal tract, the remodeling of the bulbospinal tract that sustains respiratory improvement is unknown and has therefore been investigated here after chronic lateral cervical injury in rats (90days post-lesion by comparison to 7days post-lesion).We show that chronic lateral C2 SCI leads both to a decreased thickness of the ipsilateral ventrolateral funiculus at sus and sub-lesional levels and to an opposite effect on the contralateral side. At C1 level, the number of ventrolateral bulbospinal fibers, stained with anterograde tracer was reduced within the ipsilateral ventrolateral funiculi while collateral arborization toward the gray matter and growth associated protein-43 levels was increased. At C2 lesional level, fibers rerouting toward the gray matter were also identified for 5% of the axotomized axon terminals. Despite these chronic sprouting processes respiratory bulbospinal projections to ipsilateral phrenic nucleus remained poor (less than 10% compared to non-injured conditions). Retrograde labeling of projections onto the phrenic nucleus revealed, after chronic injury, an increased recruitment of C1 propriospinal interneurons which moreover received more contacts from bulbospinal collaterals. This chronic remodeling was correlated with chronic diaphragm recovery under conditions of respiratory stress. Thus, despite extensive axonal loss and absence of direct phrenic reinnervation by bulbospinal respiratory neurons, sprouting processes toward cervical propriospinal neurons may contribute to the observed partial respiratory recovery.

Immunoreactivity for the NR1 subunit of the NMDA receptor occurs in spinally‐projecting catecholamine and serotonin neurons of the rat ventral medulla

Bulbospinal neurons in the ventral medulla play important roles in sympathetic nervous regulation. Current evidence suggests that these neurons are activated by NMDA and non‐NMDA subtypes of glutamate receptors. In this study, we examined bulbospinal neurons for the NR1 subunit of the NMDA receptor. Rats received bilateral injections of cholera toxin B (CTB) into spinal segment T9 to label bulbospinal neurons. Triple immunofluorescence was used to detect CTB (blue, AMCA), NR1 (red, Cy3) and either tyrosine hydroxylase (TH) or tryptophan hydroxylase (TpTH) (green, FITC). In the rostral ventrolateral medulla, NR1 occurred in both CTB+/TH+ neurons and CTB+ neurons. CTB+/TpTH+ neurons in raphé pallidus and the parapyramidal region also contained NR1. In addition, NR1 was present in occasional CTB+ neurons in raphé pallidus. These results show that the obligatory subunit of the NMDA receptor is expressed by spinally‐projecting C1 and non‐C1 neurons as well as spinally‐projecting serotonin neurons in raphé pallidus and the parapyramidal region. These receptors likely mediate at least some of the neuronal excitation in these regions in normal and pathological conditions. (R01‐ HL096787, NHMRC #480414).

Sympathetic premotor neurones project to and are influenced by neurones in the contralateral rostral ventrolateral medulla of the rat in vivo

The tonic activity of bulbospinal neurones in the rostral ventrolateral medulla (RVLM) is thought to underlie basal sympathetic nerve activity. A key research objective is to delineate the mechanisms that contribute to the firing of these neurones. In the current study we investigate the hypothesis that inputs arising in the contralateral RVLM converge on barosensitive bulbospinal neurones and contribute to their discharge pattern. Extracellular recordings were made from 24 barosensitive bulbospinal neurones in urethane anaesthetised, vagotomised and artificially ventilated rats during activation (glutamate or D,L-homocysteic acid microinjection, 50nl, 50mM, or monopolar electrical stimulation) or inhibition (microinjection of GABA receptor agonists muscimol or isoguvacine, 50nl, 10mM) of the contralateral RVLM. Chemical RVLM activation strongly increased (10/17) or inhibited (6/17) the spontaneous activity of neurones recorded in the contralateral RVLM. Electrical RVLM stimulation evoked a combination of short latency (median 6ms) inhibitory and longer latency (median 9.1ms, P<0.01) excitatory orthodromic responses in contralateral sympathetic premotor neurones and in some cases evoked antidromic action potentials that collided with spontaneous spikes. RVLM inhibition increased the discharge rate of sympathetic premotor neurones in the contralateral brainstem by 21±13% (P<0.05) and reduced the variability of unit firing by 37±12% (n=5, p<0.05). These findings indicate that sympathetic premotor neurones receive inhibitory and excitatory input from the contralateral RVLM, that inhibitory inputs predominate under baseline conditions, and that a population of sympathetic premotor neurones project to the contralateral RVLM in addition to their spinal targets.

Aspects of destructive and degenerative diseases of early life

The chronology of fetal and neonatal HIE In comparison to the relatively stereotypical consequences of noxious events in the mature brain, the pathology resulting from any insult, and particularly hypoxia-ischaemia, during the extended perinatal period is extraordinarily variable. This is a critical period for rapid developmental change and brain growth, with an 18-fold increase in brain weight and a multiplicity of organisational events, such as neuronal differentiation, gliogenesis and myelination occurring alongside maturational changes in tissue responses, including an optimal capacity for plasticity and repair. Against this rapidly changing background, the effects of any hypoxic-ischaemic insult are somewhat time-locked. In this overview, aspects of these pathological lesions will be considered in relation to developmental changes influencing their pathogenesis. Spinal muscular atrophy: more than the motor neuron Proximal recessively inherited spinal muscular atrophy (SMA), the second most frequently lethal genetic disorder in childhood, is characterised by progressive degeneration of bulbo-spinal lower motor neurons and concomitant skeletal muscle atrophy. The three clinical forms of childhood onset SMA [acute early onset severe type 1 Werdnig-Hoffman disease (WH), chronic intermediate type 2, and chronic mild type 3 or Kugelberg-Welander] are caused by abnormally low levels of the ubiquitous protein ‘survival of motor neuron’ (SMN) resulting from a combination of homozygous deletions or mutations of the telomeric copy of the gene SMN1 on chromosome 5q and the presence of one or more copies of SMN2, an almost identical but partially functional centromeric copy, unique to man, which may modulate the clinical phenotype. Now that we know the genetic basis, and have a genetically engineered mouse model, it is time to re-evaluate the pathology. Available morphological data in genetically proven cases is limited, but suggests that earlier pathological reports were accurate and show a wider spectrum of pathology than has generally been appreciated, particularly involving the thalamus and dorsal root ganglia. In the most severe fetal onset cases which have the lowest copy number of SMN2, neurodegenerative changes encompass many other regions suggesting differential thresholds to reduction in SMN protein level among different cell populations.

Rhythmic activity of neurons in the rostral ventrolateral medulla of conscious cats: effect of removal of vestibular inputs.

Although it is well established that bulbospinal neurons located in the rostral ventrolateral medulla (RVLM) play a pivotal role in regulating sympathetic nerve activity and blood pressure, virtually all neurophysiological studies of this region have been conducted in anesthetized or decerebrate animals. In the present study, we used time- and frequency-domain analyses to characterize the naturally occurring discharges of RVLM neurons in conscious cats. Specifically, we compared their activity to fluctuations in carotid artery blood flow to identify neurons with cardiac-related (CR) activity; we then considered whether neurons with CR activity also had a higher-frequency rhythmic firing pattern. In addition, we ascertained whether the surgical removal of vestibular inputs altered the rhythmic discharge properties of RVLM neurons. Less than 10% of RVLM neurons expressed CR activity, although the likelihood of observing a neuron with CR activity in the RVLM varied between recording sessions, even when tracking occurred in a very limited area and was higher after vestibular inputs were surgically removed. Either a 10-Hz or a 20- to 30-Hz rhythmic discharge pattern coexisted with the CR discharges in some of the RVLM neurons. Additionally, the firing rate of RVLM neurons, including those with CR activity, decreased after vestibular lesions. These findings raise the prospect that RVLM neurons may or may not express rhythmic firing patterns at a particular time due to a variety of influences, including descending projections from higher brain centers and sensory inputs, such as those from the vestibular system.

Distribution of electrical activation to the external intercostal muscles during high frequency spinal cord stimulation in dogs

In contrast to previous methods of electrical stimulation of the inspiratory muscles, high frequency spinal cord stimulation (HF-SCS) results in more physiological activation of these muscles. The spatial distribution of activation to the external intercostal muscles by this method is unknown. In anaesthetized dogs, multiunit and single motor unit (SMU) EMG activity was monitored in the dorsal portion of the 3rd, 5th and 7th interspaces and ventral portion of the 3rd interspace during spontaneous breathing and HF-SCS following C2 spinal section. Stimulus amplitude during HF-SCS was adjusted such that inspired volumes matched spontaneous breathing (Protocol 1). During HF-SCS, mean peak SMU firing frequency was highest in the 3rd interspace (dorsal) (18.8 ± 0.3 Hz) and significantly lower in the 3rd interspace (ventral) (12.2 ± 0.2 Hz) and 5th interspace (dorsal) (15.3 ± 0.3 Hz) (P <0.05 for each comparison). Similar rostrocaudal and dorsoventral gradients of activity were observed during spontaneous breathing prior to C2 section. No significant activity was observed in the 7th interspace during either spontaneous breathing or HF-SCS. Since peak discharge frequencies of the SMUs were higher and rib cage movement greater during HF-SCS compared to spontaneous breathing, stimulus amplitude during HF-SCS was adjusted such that rib cage movement matched (Protocol 2). Under these conditions, mean peak SMU frequencies and rostrocaudal and dorsoventral gradients of activity during HF-SCS were not significantly different compared to spontaneous breathing. These results indicate that (a) the topographic pattern of electrical activation of the external intercostal muscles during HF-SCS is similar to that occurring during spontaneous breathing and (b) differential descending synaptic input from supraspinal centres is not a required component of the differential spatial distribution of external intercostal muscle activation. HF-SCS may provide a more physiological method of inspiratory muscle pacing.

Commentaries on Viewpoint: Initiating inspiration outside the medulla does produce eupneic breathing

Commentaries on Viewpoint: Initiating inspiration outside the medulla does produce eupneic breathing

Regulation of visceral sympathetic tone by A5 noradrenergic neurons in rodents

The ventrolateral pons contains the A5 group of noradrenergic neurons which regulate the circulation and probably breathing. The present experiments were designed to identify these neurons definitively in vivo, to examine their response to chemoreceptor stimuli (carotid body stimulation and changes in brain pH) and to determine their effects on sympathetic outflow. Bulbospinal A5 neurons, identified by juxtacellular labelling in anaesthetized rats, had a slow regular discharge, were vigorously activated by peripheral chemoreceptor stimulation with cyanide, but only mildly activated by hyperoxic hypercapnia (central chemoreceptor stimulation). The caudal end of the A5 region also contained neurons with properties reminiscent of retrotrapezoid neurons. These cells lacked a spinal axon and were characterized by a robust response to CO2. The pH sensitivity of A5 neurons, examined in brain slices from neonatal (postnatal days 6–10) tyrosine hydroxylase (TH)-GFP transgenic mice, was about 10 times smaller than that of similarly recorded retrotrapezoid neurons. Selective stimulation of the A5 neurons in rats using channelrhodopsin optogenetics (A5 TH neurons represented 66% of transfected cells) produced fivefold greater activation of the renal nerve than the lumbar sympathetic chain. In summary, adult A5 noradrenergic neurons are vigorously activated by carotid body stimulation. This effect presumably contributes to the increase in visceral sympathetic nerve activity elicited by acute hypoxia. A5 neurons respond weakly to hypercapnia in vivo or to changes in pH in slices suggesting that their ability to sense local variations in brain pH or Pco₂ is limited.