Preliminary evidence suggesting delayed development in the hypoglossal and vagal nuclei of SIDS infants: a necropsy study

Nitric oxide controls excitatory/inhibitory balance in the hypoglossal nucleus during early postnatal development

Synaptic remodeling during early postnatal development lies behind neuronal networks refinement and nervous system maturation. In particular, the respiratory system is immature at birth and is subjected to significant postnatal development. In this context, the excitatory/inhibitory balance dramatically changes in the respiratory-related hypoglossal nucleus (HN) during the 3 perinatal weeks. Since, development abnormalities of hypoglossal motor neurons (HMNs) are associated with sudden infant death syndrome and obstructive sleep apnea, deciphering molecular partners behind synaptic remodeling in the HN is of basic and clinical relevance. Interestingly, a transient expression of the neuronal isoform of nitric oxide (NO) synthase (NOS) occurs in HMNs at neonatal stage that disappears before postnatal day 21 (P21). NO, in turn, is a determining factor for synaptic refinement in several physiopathological conditions. Here, intracerebroventricular chronic administration (P7–P21) of the broad spectrum NOS inhibitor l-NAME (N(ω)-nitro-l-arginine methyl ester) differentially affected excitatory and inhibitory rearrangement during this neonatal interval in the rat. Whilst l-NAME led to a reduction in the number of excitatory structures, inhibitory synaptic puncta were increased at P21 in comparison to administration of the inactive stereoisomer d-NAME. Finally, l-NAME decreased levels of the phosphorylated form of myosin light chain in the nucleus, which is known to regulate the actomyosin contraction apparatus. These outcomes indicate that physiologically synthesized NO modulates excitatory/inhibitory balance during early postnatal development by acting as an anti-synaptotrophic and/or synaptotoxic factor for inhibitory synapses, and as a synaptotrophin for excitatory ones. The mechanism of action could rely on the modulation of the actomyosin contraction apparatus.

Role of fast inhibitory synaptic transmission in neonatal respiratory rhythmogenesis and pattern formation

Several studies have investigated the general role of chloride-based neurotransmission (GABA_A and glycinergic signaling) in respiratory rhythmogenesis and pattern formation. In several brain regions, developmental alterations in these signaling pathways have been shown to be mediated by changes in cation-chloride cotransporter (CC) expression. For instance, CC expression changes during the course of neonatal development in medullary respiratory nuclei and other brain/spinal cord regions in a manner which decreases the cellular import, and increases the export, of chloride ions, shifting reversal potentials for chloride to progressively more negative values with maturation. In slice preparations of the same, this is related to an excitatory-to-inhibitory shift of GABA_A- and glycinergic signaling. In medullary slices, GABA_A-/glycinergic signaling in the early neonatal period is excitatory, becoming inhibitory over time. Additionally, blockade of the Na⁺/K⁺/2Cl^- cotransporter, which imports these ions via secondary active transport, converts excitatory response to inhibitory ones. These effects have not yet been demonstrated at the individual respiratory-related neuron level to occur in intact (in vivo or in situ) animal preparations, which in contrast to slices, possess normal network connectivity and natural sources of tonic drive. Developmental changes in respiratory rhythm generating and pattern forming pontomedullary respiratory circuitry may contribute to critical periods, during which there exist increased risk for perinatal respiratory disturbances of central, obstructive, or hypoxia/hypercapnia-induced origin, including the sudden infant death syndrome. Thus, better characterizing the neurochemical maturation of the central respiratory network will enhance our understanding of these conditions, which will facilitate development of targeted therapies for respiratory disturbances in neonates and infants.

Reduced levels of brain-derived neurotrophic factor contribute to synaptic imbalance during the critical period of respiratory development in rats

Previously, our electrophysiological studies revealed a transient imbalance between suppressed excitation and enhanced inhibition in hypoglossal motoneurons of rats on postnatal days (P) 12-13, a critical period when abrupt neurochemical, metabolic, ventilatory and physiological changes occur in the respiratory system. The mechanism underlying the imbalance is poorly understood. We hypothesised that the imbalance was contributed by a reduced expression of brain-derived neurotrophic factor (BDNF), which normally enhances excitation and suppresses inhibition. We also hypothesised that exogenous BDNF would partially reverse this synaptic imbalance. Immunohistochemistry/single-neuron optical densitometry, real-time quantitative PCR (RT-qPCR) and whole-cell patch-clamp recordings were done on hypoglossal motoneurons in brainstem slices of rats during the first three postnatal weeks. Our results indicated that: (1) the levels of BDNF and its high-affinity tyrosine receptor kinase B (TrkB) receptor mRNAs and proteins were relatively high during the first 1-1.5 postnatal weeks, but dropped precipitously at P12-13 before rising again afterwards; (2) exogenous BDNF significantly increased the normally lowered frequency of spontaneous excitatory postsynaptic currents but decreased the normally heightened amplitude and frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) during the critical period; (3) exogenous BDNF also decreased the normally heightened frequency of miniature IPSCs at P12-13; and (4) the effect of exogenous BDNF was partially blocked by K252a, a TrkB receptor antagonist. Thus, our results are consistent with our hypothesis that BDNF and TrkB play an important role in the synaptic imbalance during the critical period. This may have significant implications for the mechanism underlying sudden infant death syndrome.

Intact numbers of cerebellar purkinje and granule cells in sudden infant death syndrome: a stereologic analysis and critical review of neuropathologic evidence

Despite much research during recent decades, the etiology and pathogenesis of sudden infant death syndrome (SIDS) remain unknown. Because of the role of the cerebellum in respiratory and cardiovascular control, it has been proposed that it plays an important role in the pathogenesis of SIDS. To date, 5 postmortem studies on the cerebellum of SIDS cases have yielded conflicting results. Using a rigorous design-based stereologic approach, we investigated postmortem cerebella from 9 SIDS patients who died between 2 and 10 months of age and from 9 age- and sex-matched control children. Neither the volumes of the cerebellar external granule cell layer, molecular layer, internal granule cell layer (including the Purkinje cell layer), and white matter nor the total numbers of Purkinje cells, granule cells in the internal granule cell layer, and the number of granule cells per Purkinje cell showed statistically significant differences between the SIDS cases and the controls. Based on these observations, we conclude that structural alterations in cerebellar development are not involved in the etiology and pathogenesis of SIDS.

Loss of motoneurons in the ventral compartment of the rat hypoglossal nucleus following early postnatal exposure to alcohol

Perinatal alcohol exposure (AE) has multiple detrimental effects on cognitive and various behavioral outcomes, but little is known about its impact on the autonomic functions. In a rat model of fetal alcohol spectrum disorders (FASD), we investigated neurochemical and neuroanatomical alterations in two brainstem nuclei, the hypoglossal nucleus (XIIn) and the dorsal nucleus of the vagus nerve (Xdn). One group of male Sprague-Dawley rats (n=6) received 2.625 g/kg ethanol intragastrically twice daily on postnatal days (PD) 4-9, a period equivalent to the third trimester of human pregnancy, and another group (n=6) was sham-intubated. On PD 18-19, the rats were perfused and medullary sections were immunohistochemically processed for choline acetyltransferase (ChAT) or two aminergic receptors that mediate excitatory drive to motoneurons, α₁-adrenergic (α₁-R) and serotonin 2A (5-HT(2A)-R), and c-Fos. Based on ChAT labeling, AE rats had reduced numbers of motoneurons in the ventral XIIn (XIIn-v; 35.4±1.3 motoneurons per side and section vs. 40.0±1.2, p=0.022), but not in the dorsal XIIn or Xdn. Consistent with ChAT data, both the numbers of α₁-R-labeled motoneurons in the XIIn-v and the area of the XIIn-v measured using 5-HT(2A)-R staining were significantly smaller in AE rats (19.7±1.5 vs. 25.0±1.4, p=0.031 and 0.063 mm² ±0.002 vs. 0.074±0.002, p=0.002, respectively). Concurrently, both 5-HT(2A)-R and c-Fos staining tended to be higher in AE rats, suggesting an increased activation. Thus, postnatal AE causes motoneuronal loss in the XIIn-v. This may compromise upper airway control and contribute to increased risk of upper airway obstructions and sudden infant death in FASD victims.

The dorsal motor nucleus of the vagus (DMNV) in sudden infant death syndrome (SIDS): pathways leading to apoptosis

Sudden infant death syndrome (SIDS) remains the commonest cause of death in the post-neonatal period in the developed world. A leading hypothesis is that an abnormality in the brainstem of infants who succumb to SIDS, either causes or predisposes to failure to respond appropriately to an exogenous stressor. Neuronal apoptosis can lead to loss of cardiorespiratory reflexes, compromise of the infant's ability to respond to stressors such as hypoxia, and ultimately a sleep-related death. The dorsal motor nucleus of the vagus (DMNV) is a medullary autonomic nucleus where abnormalities have regularly been identified in SIDS research. This review collates neurochemical findings documented over the last 30 years, including data from our laboratory focusing on neuronal apoptosis and the DMNV, and provides potential therapeutic interventions targeting neurotransmitters, growth factors and/or genes.

Excitatory-inhibitory imbalance in hypoglossal neurons during the critical period of postnatal development in the rat

Hypoglossal motoneurons (HMs) innervate tongue muscles and are critical in maintaining patency of the upper airway during respiration. Abnormalities in HMs have been implicated in sudden infant death syndrome (SIDS) and obstructive sleep apnoea. Previously, we found a critical period in respiratory network development in rats around postnatal day (P) 12-13, when abrupt neurochemical, metabolic and physiological changes occurred. To test our hypothesis that an imbalance between inhibitory and excitatory synaptic transmission exists during the critical period, whole-cell patch-clamp recordings of HMs were done in brainstem slices of rats daily from P0 to P16. The results indicated that: (1) the amplitude and charge transfer of miniature excitatory postsynaptic currents (mEPSCs) were significantly reduced at P12-13; (2) the amplitude, mean frequency and charge transfer of miniature inhibitory postsynaptic currents (mIPSCs) were significantly increased at P12-13; (3) the kinetics (rise time and decay time) of both mEPSCs and mIPSCs accelerated with age; (4) the amplitude and frequency of spontaneous EPSCs were significantly reduced at P12-13, whereas those of spontaneous IPSCs were significantly increased at P12-13; and (5) both glycine and GABA contributed to mIPSCs. However, GABAergic currents fluctuated within a narrow range during the first three postnatal weeks, whereas glycinergic ones exhibited age-dependent changes comparable to those of total mIPSCs, indicating a reversal in dominance from GABA to glycine with development. Thus, our results provide strong electrophysiological evidence for an excitatory-inhibitory imbalance in HMs during the critical period of postnatal development in rats that may have significant implications for SIDS.

Study of the human hypoglossal nucleus: normal development and morpho-functional alterations in sudden unexplained late fetal and infant death

This study evaluated the development and the involvement in sudden perinatal and infant death of the medullary hypoglossal nucleus, a nucleus that, besides to coordinate swallowing, chewing and vocalization, takes part in inspiration. Through histological, morphometrical and immunohistochemical methods in 65 cases of perinatal and infant victims (29 stillbirths, 7 newborns and 29 infants), who died of both unknown and known cause, the authors observed developmental anomalies of the hypoglossal nucleus (HGN) in high percentage of sudden unexplained fetal and infant deaths. In particular, HGN hypoplasia, hyperplasia, positive expression of somatostatin and absence of interneurons were frequently found particularly in infant deaths, with a significant correlation with maternal smoking.

Active caspase-3 in the sudden infant death syndrome (SIDS) brainstem

In a retrospective postmortem study, we examined the neuronal expression of active caspase-3, a specific apoptotic marker, in the brainstem of 67 infants dying from sudden infant death syndrome (SIDS), and 25 age-matched control infants (non-SIDS). Neuronal immunostaining for active caspase-3 was semi-quantitatively scored in nuclei from five brainstem levels: rostral, mid and caudal pons, and rostral and caudal medulla. Regardless of the cause of death (SIDS vs. non-SIDS), age-related differences in active caspase-3 expression were identified, predominantly in the medulla. No gender-related differences were identified. Comparing SIDS to non-SIDS cases, increased active caspase-3 expression was restricted to four nuclei in the caudal pons (abducens, facial, superior olivary, and pontine nuclei) and two nuclei in the rostral medulla (hypoglossal and dorsal motor nucleus of the vagus). We conclude that neuronal apoptosis is increased in the brainstem of SIDS compared to non-SIDS infants.

Postnatal hypokinesia and the delayed time frame of sudden infant death syndrome

The sudden infant death syndrome peaks in the second and third month of life. This is the period of the "two-month transformation of the central nervous system" in the human infant. Studies of 120 days of imposed hypokinesia in man demonstrated that the maximum period of autonomic dysfunction was delayed until the beginning of the second month through to the fourth month of the experiment. Hypokinesia also impaired sleep mechanisms and induced polymorphic changes in almost all systems of the human body. These studies suggest that prolonged postnatal hypokinesia in infants may induce autonomic dysfunction in the CNS, especially during the "two-month transformation period" of major postnatal neural development.

Glial and neuronal alterations in the nucleus tractus solitarii of sudden infant death syndrome victims

The factors underlying the sudden infant death syndrome (SIDS) are still unknown, but in recent years much attention has been focused on the central cardiorespiratory control system. In the present work we analyzed the nucleus tractus solitarii (nTS) of 23 SIDS victims and 17 age-matched control cases. We studied the functional and morphological alterations of neurons and glial cells to evaluate the results of possible hypoxic-ischemic injury that could have led to sudden death. Morphometric and immunohistochemical analyses were performed on medullary sections. In the nTS of SIDS victims we observed modifications of both neuronal and glial cells. Brain injury triggers the activation of both astrocytes and microglia, which respond to neuronal damage by characteristic changes that could explain our observations in the nTS of SIDS victims. In our investigation of the nTS of SIDS victims we found a significant increase of reactive astrocytes density, a significantly higher percentage of necrotic cells, an increase of reactive microglial cells density, a significantly higher expression of substance P and the presence of NMDA receptors immunoreactivity. Our results support the hypothesis that there is injury of the nTS neurons in SIDS victims, even if the causes of this damage are still unknown. This neuronal damage may explain why adequate ventilation is often not maintained during hypoxia. Such histological findings have never been thought sufficient to explain SIDS, but the tissue findings could be an indication of the impairment of several pathophysiological mechanisms which may underlie brainstem dysfunction, affecting cardiorespiratory control.

Monolateral hypoplasia of the motor vagal nuclei in a case of sudden infant death syndrome

During the development of motor vagal nuclei (MVN), the neuroblasts of the myeloencephalic basal plate migrate in the dorsolateral direction to form the dorsal motor vagal nucleus (DMVN) and ventrolaterally to form the ventral motor vagal nucleus (VMVN). Those neuroblasts that remain close to the median sulcus will form the hypoglossal nucleus. In support of the congenital origin of the alteration of the MVN in sudden infant death syndrome (SIDS), we report the case of an 8-month-old female child who was found dead in her cot. The neuropathological assessment revealed that the medullary triangle of the 4th ventricle floor was asymmetric, owing to the presence of three prominences to the left side of the median sulcus. The medial prominence corresponded to the hypoglossal nucleus, which showed a marked increase in the number of large neurons; the intermediate prominence corresponded to the DMVN whose large neurons were reduced and were recognizable mainly at the level of the medial fringe; the lateral prominence corresponded to the solitary nucleus. The left solitary tract showed a reduction of the transverse diameter. Also, the left VMVN showed marked reduction in the number of neurons. Inflammatory and astrocytic reactions were absent. We suggest that in SIDS cases the hypocellularity of the MVN and the increased number of neurons of the hypoglossal nucleus are intimately related, indicating a congenital alteration due to incomplete migration of the vagal neuroblasts with abnormality of the autonomic cardio-respiratory control.

The nonuniform distribution of the GABA(A) receptor alpha 1 subunit influences inhibitory synaptic transmission to motoneurons within a motor nucleus

Using immunohistochemistry we studied the distribution of GABA(A) and glycine receptor alpha1 subunits in the rat hypoglossal nucleus during postnatal development. In the neonate [postnatal day (P) 1-3] and adult nucleus (P28-30), GABA(A) receptor alpha1 subunit labeling was relatively modest. However, in the juvenile nucleus (P9-13), labeling was strong in the ventrolateral region and moderate in the dorsal region. Glycine receptor alpha1 subunit labeling was strong and uniform in the juvenile and adult nucleus and absent in the neonate nucleus. GABA and glycine neurotransmitter labeling was uniform throughout the neonatal and juvenile nucleus. To study the functional consequences of this regional differential GABA(A) receptor alpha1 subunit distribution, we voltage clamped juvenile hypoglossal motoneurons (HMs) from the ventrolateral and dorsal regions and recorded spontaneous miniature IPSCs (mIPSCs). Pure GABAergic events had slower decay times than glycinergic events. Although pure GABAergic and glycinergic decay times did not differ depending on HM location, the decays of mixed mIPSCs from ventrolateral HMs, recorded without GABA(A) and glycine receptor antagonists, had significantly slower decays than mIPSCs from dorsal HMs. Focally applied GABA and glycine onto outside-out patches revealed that the GABAergic to glycinergic peak current amplitude ratio was larger for patches from ventrolateral HMs compared with dorsal HMs. Dual component mIPSCs, presumably caused by co-release of GABA and glycine, were recorded more frequently in the ventrolateral nucleus. These data suggest that the number of synapses using GABA(A) receptor-mediated transmission is greater on ventrolateral HMs than dorsal HMs, demonstrating a nonuniformity of synaptic function within a defined motor nucleus.

Tracing cranial nerve pathways (glossopharyngeal, vagus, and hypoglossal) in SIDS and control infants: a DiI study

It has been proposed that Sudden Infant Death Syndrome (SIDS) might occur as a consequence of a developmental deficit associated with the cardiorespiratory and arousal control centers located within the brainstem. In this study 1.1' dioctadecyl-3,3,3',3-tetramethylindocarbocyanine perchlorate (DiI) was used to investigate the trajectories of the glossopharyngeal and vagus nerves which carry essential afferent and efferent fiber tracts associated with cardiac and respiratory control and of the hypoglossal nerve which innervates the tongue, in SIDS (n = 14) and control (n = 7) infants. The postnatal development of the trajectories of these nerves was examined in non-SIDS brains and comparisons were then made with age-matched SIDS brains. The mean profile area of hypoglossal and dorsal motor neurons were also assessed. In controls, no major alterations were observed in the trajectories of axon bundles with increasing age (7 wk to 2 yr) in each of the nerves investigated although axon bundles appeared to increase in thickness with age. In SIDS cases (2 wk to 44 wk), the trajectories of the cranial nerves were not different from those seen in age-matched control cases. The mean profile area of hypoglossal and dorsal motor neurons was not significantly different between control and SIDS infants. We conclude that the DiI tracing technique can be used successfully to trace the pathways of cranial nerves in human infant fixed-tissue. Furthermore, if functional differences exist between SIDS and non-SIDS brains in the control of respiration, circulation, or arousal they do not appear to be related to markedly reduced or aberrant projections of the glossopharyngeal, vagus, or hypoglossal nerves.

Metabotropic glutamate receptors and blockade of glial Krebs cycle depress glycinergic synaptic currents of mouse hypoglossal motoneurons

Metabotropic glutamate receptors are known to depress synaptic transmission by inhibiting transmitter release from presynaptic nerve terminals. This study reports the effects of presynaptic metabotropic glutamate receptor activation on inhibitory synaptic transmission in hypoglossal motoneurons in brainstem slice preparations of neonatal mice. Whole-cell patch-clamp recordings were performed on hypoglossal motoneurons of 2-6-day-old mice. Monosynaptic glycinergic currents were elicited by electrical stimulation of the nucleus of Roller. Application of the specific metabotropic glutamate receptor agonists (+/-)-1-aminocyclopentane-trans-1,3,dicarboxylic acid (t-ACPD), (2S, 2'R,3'R)-2-(2',3'-dicarboxylcyclopropyl)-glycine (DCG-IV) or L-2-amino-4-phosphonobutyric acid (L-AP4) depressed stimulus-evoked glycinergic inhibitory postsynaptic currents (IPSCs) by an average of 39.5, 59.4 and 39.2%, respectively. In the presence of t-ACPD, glycinergic miniature IPSCs were reduced in frequency but not in amplitude, which is indicative of a presynaptic mechanism. A similar reduction of IPSC amplitude was observed in the presence of elevated extracellular glutamate or during application of D, L-threo-hydroxyaspartate (THA), a blocker of glutamate transport, respectively. The data suggest that uptake of glutamate, which is predominately carried out by glial cells, can prevent spill-over of glutamate and activation of metabotropic glutamate receptors. A reduction of IPSCs was also observed following application of monofluoroacetic acid, a substance acting specifically on glial cells. Our results suggest that glial regulation of extracellular glutamate uptake can prevent spill-over of glutamate, and glutamatergic depression of glycinergic inhibition in hypoglossal motoneurons.

Developmental modulation of mouse hypoglossal nerve inspiratory output in vitro by noradrenergic receptor agonists

The ontogeny of the noradrenergic receptor subtypes modulating hypoglossal (XII) nerve inspiratory output was characterized. Noradrenergic agents were locally applied over the XII nucleus of rhythmically active medullary slice preparations isolated from mice between zero and 13 days of age (P0-P13) and the effects on XII inspiratory burst amplitude quantified. The alpha1 receptor agonist phenylephrine (PE, 0.1-10 microM) produced a dose-dependent, prazosin-sensitive (0.1-10 microM) increase in XII nerve inspiratory burst amplitude. The magnitude of this potentiation increased steadily from a maximum of 15+/-8% in P0 mice to 134+/-4% in P12-P13 mice. The beta receptor agonist isoproterenol (0.01-1.0 mM) produced a prazosin-insensitive, propranolol-sensitive potentiation of XII nerve burst amplitude. The isoproterenol-mediated potentiation increased with development from 27+/-5% in P0-P1 slices, to 37+/-3% in P3 slices and 45+/-4% in P9-P10 slices. The alpha2 receptor agonist clonidine (1 mM) reduced XII nerve inspiratory burst amplitude in P0-P3 slices by 29+/-5%, but had no effect on output from P12-P13 slices. An alpha2 receptor-mediated inhibition of inspiratory activity in neonates (P0-P3) was further supported by a 19+/-3% reduction in XII nerve burst amplitude when norepinephrine (NE, 100 microM) was applied in the presence of prazosin (10 microM) and propranolol (100 microM). Results indicate that developmental increases in potentiating alpha1 and, to a lesser extent, beta receptor mechanisms combine with a developmentally decreasing inhibitory mechanism, most likely mediated by alpha2 receptors, to determine the ontogenetic time course by which NE modulates XII MN inspiratory activity.

Sudden infant death syndrome and the modulation of neuropeptides released during shock

It is generally accepted that sudden infant death syndrome (SIDS) victims fail to survive relatively minor stress in infancy. My hypothesis is that failure to orchestrate the endocrine response in stress leads to excessive release of neutral-endopeptidase-sensitive peptide substrates that enhance lethality. The 'quick zinc' response Reid recorded in livestock with circulatory shock is described. It is concluded that the failure to mount an endocrine response leads to SIDS.

Phrenic nerves and diaphragms in sudden infant death syndrome

Disturbances of the respiratory system may be an important factor in the cascade of events leading to sudden infant death syndrome (SIDS). Even though the diaphragm is the major respiratory muscle in infants, little is known about alterations of this muscle and of the phrenic nerve in SIDS. In the present study, diaphragms and phrenic nerves of 24 SIDS infants and seven controls were analyzed. Morphometric analysis revealed only slightly larger cross sectional areas of phrenic nerve axons but no increase in myelin sheath thickness in SIDS cases. However, in one SIDS case, myelinated nerve fibre density was severely reduced. Using electron microscopy, several nerve fibres of SIDS infants showed focal accumulations of neurofilaments. Muscle fibre diameters in SIDS diaphragms were significantly larger compared to controls (P < 0.0001). However, in almost all SIDS and control cases, axons and myelin sheaths were artificially swollen, and acute segmental muscle fibre ruptures and contracture bands were found. These prominent nonspecific ultrastructural alterations should advise caution in the interpretation of morphometric data. Thus, in some cases exemplified by one case of the present series, decreased density of phrenic nerve myelinated axons might contribute to SIDS. Still, the present results indicate that development of phrenic nerves and diaphragms is not delayed in most SIDS infants.

The enkephalinergic innervation of the genioglossus musculature in the rat: implications for the respiratory control of the tongue

This study sought to determine if the enkephalinergic (ENK) innervation of the hypoglossal nucleus (nXII) in the rat was organized differentially for the control of the genioglossus musculature whose activity is essential in maintaining the patency of the upper airway. Immunocytochemical results revealed that the genioglossus motoneuron pool, comprising the ventrolateral subcompartment of the nXII, was consistently and heavily labeled throughout its rostrocaudal dimension. Labeling was characterized by dense focal clustering throughout the neuropil, and by the appearance of numerous perisomatic-like profiles. Similarly, the ventromedial subcompartment mainly rostrally, and the dorsal compartment caudally, whose motoneurons control the caudal intrinsic protrusor and rostral retrusor muscles, respectively, were also consistently labeled. While these results demonstrate that the genioglossus musculature is targeted by ENK inputs, they also suggest that other selected musculature of the tongue is controlled by ENK. It is argued that the innervation pattern identified in the present study is consistent with a functional role for ENK in the respiratory control of the tongue.

Developmental modulation of glutamatergic inspiratory drive to hypoglossal motoneurons

Proper function of hypoglossal motoneurons (XII MNs) innervating tongue muscles is critical for respiratory control of the airway. Morphological and electrophysiological properties of XII MNs change during postnatal development, as do modulatory systems. Despite these changes, the system producing respiratory movements must remain fully functional throughout life. Modulatory systems have therefore received considerable attention since coordination of their development with a developing neuromuscular system may be critical for maintenance of continuous, efficient breathing. Developmental modulation of XII inspiratory activity by three transmitter systems is examined. Thyrotropin-releasing hormone (TRH) mediates an increase in MN input resistance (RN) in juvenile but not neonate MNs, and this likely underlies the developmental increase in TRH potentiation of inspiratory activity. Norepinephrine (NE) potentiation of inspiratory activity, which in the neonate is produced in part by an alpha 1-mediated increase in RN, also increases postnatally. Effects of purinergic transmission on XII inspiratory activity remain constant during the first 2 weeks of postnatal development. Adenosine-triphosphate (ATP) produces tonic excitation and inspiratory potentiation that likely result from activation of postsynaptic P2 receptors. A secondary inhibitory effect likely results from hydrolysis of ATP to adenosine and activation of presynaptic A1 adenosine receptors. The functional relevance of these postnatal changes is discussed.

Chronic placental insufficiency in the fetal guinea pig affects neurochemical and neuroglial development but not neuronal numbers in the brainstem: a new method for combined stereology and immunohistochemistry

This study has examined the development of the brainstem in a suboptimal intrauterine environment induced via chronic placental insufficiency in the fetal guinea pig. Placental insufficiency was produced by unilateral ligation of the maternal uterine artery at mid-gestation (term = 66-68 days) resulting in the production of growth-retarded fetuses that are chronically hypoxic and malnourished. The structural and neurochemical development of brainstem nuclei either directly or indirectly related to cardiorespiratory control were analysed by using new stereological methods and immunohistochemistry. A technique was devised to enable the procedures to be performed on alternate frozen sections. There were no significant differences between control and growth-retarded fetuses in the total number of neurons, area of neuronal somata or volume of the hypoglossal nucleus. Quantitative densitometry was used to measure immunohistochemical staining in the brainstem of growth-retarded fetuses compared to controls and revealed a significant (P < 0.02) decrease in substance P(SP)-immunoreactivity in the spinal trigeminal nucleus and a significant (P < 0.05) increase in met-enkephalin-immunoreactivity in the hypoglossal nucleus. Counts of stained neurons demonstrated a significant increase in the density of SP-positive neurons in the nucleus tractus solitarius (P < 0.05) and of met-enkephalin-positive neurons in the ventral medullary reticular formation (P < 0.05). There was also a proliferation of astrocytes, as determined by immunoreactivity to glial fibrillary acidic protein in the dorsal motor nucleus of the vagus, nucleus tractus solitarius and more generally around blood vessels throughout the brainstem. Thus, these results have been shown that although chronic intrauterine deprivation does not alter neuronal numbers, at least in the hypoglossal nucleus, there is a proliferation of astrocytes, and the expression of neurotransmitters/neuromodulators is markedly effected in some of the nuclei involved with cardiorespiratory control.

Morphological study of the brainstem in Fukuyama type congenital muscular dystrophy

We have observed sudden clinical death due to Fukuyama-type congenital muscular dystrophy (FCMD). In FCMD, brain abnormalities, such as polymicrogyria, leptomeningeal neuroglial heterotopia and abnormal course of the corticospinal tracts, are well known. We investigated the brainstem of 10 FCMD and 7 control cases. Among the control cases, 5 with Duchenne type muscular dystrophy died of heart failure and 2 died accidental death. In the brainstem, the catecholaminergic neurons characterized by reaction with antiserum to tyrosin hydroxylase showed notable reduction in the reticular formation, vagal nuclei, and nucleus tractus solitarius. Delays or aberrations of neural control may contribute to the pathogenesis of sudden infant death syndrome, and medullary gliosis occurs in the reticular formation of sudden infant death syndrome. The pathogenesis of neurons in the brainstem in FCMD may be similar to that in sudden infant death syndrome. These findings suggest neuronal dysfunction in the brainstem and may be related to respiratory, circulatory, or sleep-wake regulation disorders.

Brain stem nuclei in sudden infant death syndrome (SIDS): volumes, neuronal numbers and positions

It has been suggested that the defect underlying the sudden infant death syndrome (SIDS) lies in brain stem nuclei involved in cardiac and respiratory function. However, most studies have not used rigorous quantitative techniques to assess brain stem nuclear volumes and neuronal numbers. We have measured the volume, neuronal numbers and position of brain stem nuclei in 11 SIDS and 11 aged-matched control infants. Using serial sagittal sections, nuclei involved in maintaining airway patency (hypoglossal, ambiguus and retroambiguus), heart rate (dorsal vagal) and generation of respiratory rhythm (ambiguus and dorsal vagal) were studied. No significant differences were found in nuclear volume increase with age, total neuronal number or nuclear position between SIDS and control cases. These findings support the hypothesis that the nervous system in SIDS may be normal until the final event that kills these infants.