Caudal Medulla Research Articles

Absence of the transcription factor Nfib delays the formation of the basilar pontine and other mossy fiber nuclei

Transcription factors of the Nuclear Factor I (Nfi) family are important for the development of specific neuronal and glial populations in the nervous system. One such population, the neurons of the basilar pontine nuclei, expresses high levels of Nfi proteins, and the pontine nuclei are greatly reduced in mice lacking a functional Nfib gene. Pontine neurons, along with other precerebellar neurons that populate the hindbrain, arise from precursors in the lower rhombic lip and migrate anteroventrally to reach their final location. Using immunohistochemistry, we find that NFI-B expression is specific for mossy fiber populations of the precerebellar system. Analysis of the Nfib(-/-) hindbrain indicates that the development of the basilar pontine nuclei is delayed, with pontine neurons migrating 1-2 days later than in control animals, and that significantly fewer pontine neurons are produced. While the mossy fiber nuclei of the caudal medulla do form, they also exhibit a developmental delay. Nfia and Nfix null mice exhibit no apparent pontine phenotype, implying specificity in the action of NFI family members. Collectively, these data demonstrate that Nfib plays an important role in the generation of precerebellar mossy fiber neurons, and may do so at least in part by regulating neurogenesis.

Central Fos expression and conditioned flavor avoidance in rats following intragastric administration of bitter taste receptor ligands

G protein-coupled receptors that signal bitter taste (T2Rs) are expressed in the mucosal lining of the oral cavity and gastrointestinal (GI) tract. In mice, intragastric infusion of T2R ligands activates Fos expression within the caudal viscerosensory portion of the nucleus of the solitary tract (NTS) through a vagal pathway (Hao S, Sternini C, Raybould HE. Am J Physiol Regul Integr Comp Physiol 294: R33-R38, 2008). The present study was performed in rats to further characterize the distribution and chemical phenotypes of brain stem and forebrain neurons activated to express Fos after intragastric gavage of T2R ligands, and to determine a potential behavioral correlate of this central neural activation. Compared with relatively low brain stem and forebrain Fos expression in control rats gavaged intragastrically with water, rats gavaged intragastrically with T2R ligands displayed significantly increased activation of neurons within the caudal medial (visceral) NTS and caudal ventrolateral medulla, including noradrenergic neurons, and within the lateral parabrachial nucleus, central nucleus of the amygdala, and paraventricular nucleus of the hypothalamus. A behavioral correlate of this Fos activation was evidenced when rats avoided consuming flavors that previously were paired with intragastric gavage of T2R ligands. While unconditioned aversive responses to bitter tastants in the oral cavity are often sufficient to inhibit further consumption, a second line of defense may be provided postingestively by ligand-induced signaling at GI T2Rs that signal the brain via vagal sensory inputs to the caudal medulla.

Partial Infraorbital Nerve Ligation as a Model of Trigeminal Nerve Injury in the Mouse: Behavioral, Neural, and Glial Reactions

Trigeminal nerve damage often leads to chronic pain syndromes including trigeminal neuralgia, a severely debilitating chronic orofacial pain syndrome. Options for treatment of neuropathic pain are limited in effectiveness and new approaches based on a better understanding of the underlying pathologies are required. Partial ligation has been shown to effectively mimic many of the qualities of human neuropathic pain syndromes. We have devised a mouse model of trigeminal neuralgia using a partial infraorbital nerve ligation (pIONL) that induces persistent pain behaviors and morphological changes in the brainstem. We found that the pIONL effectively induced mechanical allodynia lasting for more than 3 weeks. Cell proliferation (bromodeoxyuridine), activation of astrocytes and microglia in the ipsilateral caudal medulla, and persistent satellite cell reaction in the ipsilateral ganglion were observed. Neurochemical markers calcitonin gene-related peptide, substance P were decreased in medullary dorsal horn ipsilateral to the injury side, whereas substance P receptor NK1 expression was increased after 8 days. Nerve injury marker ATF3 was markedly increased in ipsilateral trigeminal ganglion neurons at 8 days after pIONL. The data indicate that partial trigeminal injury in mice produces many persistent anatomical changes in neuropathic pain, as well as mechanical allodynia. This study describes the development of a new mouse model of trigeminal neuropathic pain. Our goal is to devise better treatments of trigeminal pain, and this will be facilitated by characterization of the underlying cellular and molecular neuropathological mechanisms in genetically designed mice.

Intrinsic and synaptic long-term depression of NTS relay of nociceptin- and capsaicin-sensitive cardiopulmonary afferents hyperactivity

The nucleus tractus solitarius (NTS) in the caudal medulla is a gateway for a variety of cardiopulmonary afferents important for homeostatic regulation and defense against airway and cardiovascular insults and is a key central target potentially mediating the response habituation to these inputs. Here, whole-cell and field population action potential recordings and infrared imaging in rat brainstem slices in vitro revealed a compartmental pain-pathway-like organization of capsaicin-facilitated vs. nocistatin-facilitated/nociceptin-suppressed neuronal clusters in an NTS region, which receives cardiopulmonary A- and C-fiber afferents with differing capsaicin sensitivities. All capsaicin-sensitive neurons and a fraction of nociceptin-sensitive neurons expressed N-methyl-D: -aspartate (NMDA) receptor-dependent synaptic long-term depression (LTD) following afferent stimulation. All neurons also expressed activity-dependent decrease of excitability (intrinsic LTD), which converted to NMDA receptor-dependent intrinsic long-term potentiation after GABA(A) receptor blockade. Thus, distinct intrinsic and synaptic LTD mechanisms in the NTS specific to the relay of A- or C-fiber afferents may underlie the response habituation to persistent afferents hyperactivity that are associated with varying physiologic challenges and cardiopulmonary derangements-including hypertension, chronic cough, asthmatic bronchoconstriction, sustained elevated lung volume in chronic obstructive pulmonary disease or in continuous positive-airway-pressure therapy for sleep apnea, metabolic acidosis, and prolonged exposure to hypoxia at high altitude.

Brain-derived neurotrophic factor (BDNF) and TrkB in the piglet brainstem after post-natal nicotine and intermittent hypercapnic hypoxia

Brain-derived neurotrophic factor (BDNF) and its receptor TrkB play a significant role in the regulation of cell growth, survival and death during central nervous system development. The expression of BDNF and TrkB is affected by noxious insults. Two insults during the early post-natal period that are of interest to our laboratory are exposure to nicotine and to intermittent hypercapnic hypoxia (IHH). Piglet models were used to mimic the conditions associated with the risk factors for the sudden infant death syndrome (SIDS) including post-natal cigarette smoke exposure (nicotine model) and prone sleeping where the infant is subjected to re-breathing of expired gases (IHH model). We aimed to determine the effects of nicotine and IHH, alone or in combination, on pro- and rhBDNF and TrkB expression in the developing piglet brainstem. Four piglet groups were studied, with equal gender ratios in each: control ( n = 14), nicotine ( n = 14), IHH ( n = 10) and nic + IHH ( n = 14). Applying immunohistochemistry, and studying six nuclei of the caudal medulla, we found that compared to controls, TrkB was the only protein significantly decreased after nicotine and nic + IHH exposure regardless of gender. For pro-BDNF and rhBDNF however, observed changes were more evident in males than females exposed to nicotine and nic + IHH. The implications of these findings are that a prior nicotine exposure makes the developing brainstem susceptible to greater changes in the neurotrophic effects of BDNF and its receptor TrkB in the face of a hypoxic insult, and that the effects are greater in males than females.

Cardiovascular responses induced by excitation/lesion of caudal paramedial medulla (CPM)

Previous studies suggested that neurons located in the midline and adjacent regions of the caudal medulla participate in cardiovascular regulation. Presently we sought to determine the effects of chemical excitation or lesion of this area. Male Wistar rats were anesthetized, paralized, and artificially ventilated and prepared for mean arterial pressure (MAP), heart rate (HR), and blood flow (BF) recordings. Hindquarter (HQVC) and renal vascular conductance (RVC) were determined by the ratio MAP/BF. Bilateral microinjection of KA (5 Mm, 50 nL) into CPM (1.6 rostral, 0.5 lat and −2.3 mm to the CS) increased MAP (+43±10.3 mmHg), HR (+48±23.6 bpm) and induced renal and hindquarter vasoconstriction (−72±8.3; −75±3.6% of baseline, respectively). Reversely, bilateral microinjection of ibotenic acid (2.5nM, 50nL) induced hypotension (−41±13.3 mmHg) and bradicardia (−75±43.1 bmp) without significant changing RVC or HVC (2±12.8; 33±29.2% of baseline respectively). Forty minutes after IBO microinjection of KA into the same sites overturned these effects, inducing marked hypertension (+38±13.7 mmHg), tachycardia (43±8.3 bpm) and renal and hindlimb vasoconstriction (−74±5.9,−75±6.4% of baseline, respectively). These results suggested that paramedial medulla contains excitatory and inhibitory cardiovascular pathways that may be differentiated by their sensitivity to glutamate agonists.

Localization of 5‐HT1A/B receptors in the rat brainstem, their distribution in IML‐projecting cells and activation during acute hypoxia

Serotonin (5‐HT) type‐1 receptors contribute to cardiorespiratory (CR) function. Yet, distribution of these receptors in CR nuclei, and specifically in cells projecting to sympathetic preganglionic neurons in the intermediolateral (IML) cell column, is lacking. Whether 5‐HT1 containing cells are activated during the chemoreflex is also not known. We hypothesized acute hypoxia (10% O2, 3 hr) would stimulate IML‐projecting cells containing 5‐HT1 receptors. Immunohistochemistry in rat brainstem slices retrogradely‐labeled with fluorogold (FG, 1%) in the IML (T2 level) was used to test this notion. Fos‐like reactivity was used as a marker of cell stimulation. Fos, FG, 5‐HT1A and 5‐HT1B distribution was examined. 5‐HT1A and 1B labeling was similarly distributed in the following nuclei: nucleus of the solitary tract (NTS), facial, lateral paragigantocellularis, hypoglossal, ambiguus, rostral and caudal ventral lateral medulla (RVLM, CVLM), and occasionally, the Botzinger and pre‐Botzinger. In the RVLM, ~26% of 5‐HT1A and 18% of 5‐HT1B co‐labeled with FG, and a greater percent of 5‐HT1B than 1A co‐labeled with PNMT. In the NTS, 5‐HT1A/B did not co‐label with FG. Hypoxia increased Fos in the NTS, RVLM and CVLM compared to room air. In the NTS, a small percentage of Fos cells contained 5‐HT1A and in RVLM, some co‐labeled with FG and 5‐HT1B. These data suggest 5‐HT1 receptors may play a role in the CR response to hypoxia.

Neural pathways of cardiovascular depressor reflex during gastric distension and its modulation by electroacupuncture

Mild mechanical gastric distension (GD) frequently evokes a decrease in blood pressure and bradycardia. We hypothesized that the GD‐induced cardiovascular depressor reflex is transmitted through both sympathetic and parasympathetic afferent pathways. We also hypothesized that it can be modulated by electroacupuncture (EA) and is mediated by caudal ventral lateral medulla (cVLM) and nucleus ambiguus (NA). Sprague‐Dawley rats were anesthetized, ventilated, and their mean arterial blood pressures (MAP) and heart rates (HR) monitored. Repeated balloon inflation of the stomach every 10 minutes to 15–20 mmHg gastric wall tension induced consistent falls in MAP and HR. This inhibitory response was reversed by chemical denervation of celiac ganglia and intra‐abdominal vagal nerves with 0.1mL of 1% lidocaine, suggesting that the decreased MAP was mediated by both the withdrawal of sympathetic tone and increased parasympathetic tone. Similarly, the reduced HR was reversed after vagotomy and celiac ganglionectomy. Bilateral stimulation with EA at cardiovascular acupoints P5‐6 for 30 min significantly decreased the depressor response. This modulation lasted for 50 min. Unilateral microinjection of atropine into cVLM immediately after termination of EA reversed the inhibition by EA, whereas microinjection of normal saline into cVLM did not reverse it. Microinjection of gabazine into the NA reversed the inhibition by EA. These data suggest that GD activates both sympathetic and vagal afferents to evoke reflex cardiovascular sympatho‐ and parasympatho‐inhibition through the cVLM and NA respectively. EA attenuates the cardiovascular depressor reflex through a cholinergic mechanism in the cVLM and a gabanergic mechanism in the NA.

Hypovolemic shock: Critical involvement of a projection from the ventrolateral periaqueductal gray to the caudal midline medulla

Previous research has suggested that the ventrolateral column of the periaqueductal gray (vlPAG) plays a crucial role in triggering a decompensatory response (sympathoinhibition, hypotension, bradycardia) to severe blood loss. vlPAG excitation triggers also quiescence, decreased vigilance and decreased reactivity, the behavioral response which usually accompanies hypovolemic shock. The aim of this study was to identify, in unanesthetized rats, the main descending pathway(s) via which vlPAG neurons trigger sympathoinhibition and bradycardia in response to severe blood loss. Firstly, immediate early gene (c-Fos) expression was used to identify vlPAG neurons selectively activated by severe blood loss. Subsequently, the specific medullary projections of these vlPAG neurons were defined by combined c-Fos, retrograde tracing (double-label) experiments. It was found that vlPAG neurons selectively activated by severe hemorrhage project overwhelmingly to the vasodepressor portion of the caudal midline medulla (CMM). Previous studies indicate that this CMM region mediates behaviorally-coupled cardiovascular adjustments and the findings described here fit with the idea that CMM neurons are uniquely recruited by salient challenges, the adaptive responses to which require more than reflexive homeostatic cardiovascular adjustments.

Brainstem circuitry of tracheal–bronchial cough: c-fos study in anesthetized cats

The c-fos gene expression method was used to localize brainstem neurons functionally related to the tracheal-bronchial cough on 13 spontaneously breathing, pentobarbitone anesthetized cats. The level of Fos-like immunoreactivity (FLI) in 6 animals with repetitive coughs (170+/-12) induced by mechanical stimulation of the tracheobronchial mucosa was compared to FLI in 7 control non-stimulated cats. Thirty-four nuclei were compared for the number of labeled cells. Enhanced cough FLI was found bilaterally at following brainstem structures, as compared to controls: In the medulla, FLI was increased in the medial, interstitial and ventrolateral subnuclei of the solitary tract (p < 0.02), in the retroambigual nucleus of the caudal medulla (p < 0.05), in the ambigual, paraambigual and retrofacial nuclei of the rostral medulla along with the lateral reticular nuclei, the ventrolateral reticular tegmental field (p < 0.05), and the raphe nuclei (p < 0.05). In pons, increased FLI was detected in the lateral parabrachial and Kölliker-Fuse nuclei (p < 0.01), in the posteroventral cochlear nuclei (p < 0.01), and the raphe midline (p < 0.05). Within the mesencephalon cough-related FLI was enhanced at the rostral midline area (p < 0.05), but a decrease was found at its caudal part in the periaqueductal gray (p < 0.02). Results of this study suggest a large medullary - pontine - mesencephalic neuronal circuit involved in the control of the tracheal-bronchial cough in cats.

Diffusion Tensor Imaging in Joubert Syndrome

Neuropathologic findings and preliminary imaging studies demonstrated the absence of pyramidal tract and superior cerebellar peduncular decussation in individual patients with Joubert syndrome (JS). We hypothesized that functional-structural neuroimaging findings do not differ between the genetic forms of JS. MR imaging was performed with a 3T MR imaging-unit. Multiplanar T2- and T1-weighted imaging was followed by diffusion tensor imaging (DTI). Isotropic diffusion-weighted images, apparent diffusion coefficient maps, and color-coded fractional anisotropy maps, including tractography, were subsequently calculated. In all 6 patients studied, DTI showed that the fibers of the superior cerebellar peduncles did not decussate in the mesencephalon and the corticospinal tract failed to cross in the caudal medulla. The patients represented various genetic forms of JS. In JS, the fibers of the pyramidal tract and the superior cerebellar peduncles do not cross, irrespective of the underlying mutation.

Molecular Profiling of Ovine Prion Diseases by Using Thermolysin-Resistant PrP Sc and Endogenous C2 PrP Fragments

Disease-associated PrP fragments produced upon in vitro or in vivo proteolysis can provide significant insight into the causal strain of prion disease. Here we describe a novel molecular strain typing assay that used thermolysin digestion of caudal medulla samples to produce PrPres signatures on Western blots that readily distinguished experimental sheep bovine spongiform encephalopathy (BSE) from classical scrapie. Furthermore, the accumulation of such PrPres species within the cerebellum also appeared to be dependent upon the transmissible spongiform encephalopathy (TSE) strain, allowing discrimination between two experimental strains of scrapie and grouping of natural scrapie isolates into two profiles. The occurrence of endogenously produced PrP fragments, namely, glycosylated and unglycosylated C2, within different central nervous system (CNS) regions is also described; this is the first detailed description of such scrapie-associated fragments within a natural host. The advent of C2 fragments within defined CNS regions, compared between BSE and scrapie cases and also between two experimental scrapie strains, appeared to be largely dependent upon the TSE strain. The combined analyses of C2 fragments and thermolysin-resistant PrP species within caudal medulla, cerebellum, and spinal cord samples allowed natural scrapie isolates to be separated into four distinct molecular profiles: most isolates produced C2 and PrPres in all CNS regions, a second group lacked detectable cerebellar C2 fragments, one isolate lacked both cerebellar PrPres and C2, and a further isolate lacked detectable C2 within all three CNS regions and also lacked cerebellar PrPres. This CNS region-specific deposition of disease-associated PrP species may reflect the natural heterogeneity of scrapie strains in the sheep population in the United Kingdom.

Melanocortinergic modulation of food intake in the medulla: Evidence for presynaptic MC4-receptors on vagal afferents.

The dorsal vagal complex in the caudal medulla is considered an important integrative area in the distributed neural network controlling food intake with melanocortin receptor-4 (MC4R) signaling playing a crucial role. Here, we used patch-clamp whole cell recording from NTS neurons to investigate the effects of MC4R ligands that have previously been shown to powerfully modulate meal structure and total food intake. Thirty-nine percent (25/64) of NTS neurons responded to perfusion with MTII or alpha-MSH, 28% with an increase and 11% with a decrease in the frequency of spontaneous EPSPs, without affecting their amplitude. MTII-induced effects in the frequency of EPSPs were unaffected by TTX, but all effects were abolished by the MC3/4R antagonist SCHU9119. To test whether the mediating MC4-receptors are located on vagal afferent terminals, we also recorded NTS neurons from rats that had undergone vagal afferent rhizotomies 4 days before slice preparation. Only 1 out of 10 neurons from such rats responded with an increase in the frequency of spontaneous EPSCs following MTII. Together with earlier observations, these results suggest that alpha-MSH released from hypothalamic and local POMC neurons acts on presynaptic MC4-receptors on vagal afferent terminals to predominantly increase glutamate transmission to NTS neurons, but that in a minority of cases the effect can also be inhibitory. Such a mechanism is consistent with the notion that descending melanocortinergic projections from leptin-sensitive areas in the basomedial hypothalamus can suppress food intake by changing the capacity of vagal hindbrain mechanisms of satiation. Supported by NIDDK 47348.

Similarities of the neuronal circuit for the induction of fictive vomiting between ferrets and dogs

Previous studies suggested that the following neuronal circuit participates in the induction of vomiting by afferent vagal stimulation in decerebrated paralyzed dogs: (1) afferent fibers of the vagus nerve, (2) neurons of the solitary nucleus (NTS), (3) neurons of the prodromal sign center near the semicompact part of the nucleus ambiguus (scAMB), (4) neurons of the central pattern generator in the reticular area adjacent to the compact part of nucleus ambiguus (cAMB), (5) respiratory premotor neurons in the caudal medulla, (6) motor neurons of the diaphragm and abdominal muscles. However, the commonality of this neuronal circuit in different species has not yet been clarified. Thus, this study was conducted to clarify this point. This study clarified for the first time that fictive vomiting in decerebrated paralyzed ferrets could be induced by vagal stimulation, and could be identified by centrifugal activity patterns of the phrenic and abdominal muscle nerves. The distributions of c-Fos immunoreactive neurons in the NTS, scAMB and cAMB areas in ferrets that exhibited fictive vomiting were denser than those in ferrets that did not. Application of the nonNMDA receptor antagonist into the 4th ventricle produced the reversible suppression of fictive vomiting. The NK1 receptor immunoreactive puncta were found in the reticular area adjacent to the scAMB. Microinjections of NK1 receptor antagonist into the reticular areas on both sides abolished fictive vomiting. All these results in the ferrets are identical with results previously obtained in dogs and cats. Therefore, this suggests that the above neuronal circuit commonly participates in the induction of emesis in these animal species.

Uncrossed actions of feline corticospinal tract neurones on hindlimb motoneurones evoked via ipsilaterally descending pathways

Despite numerous investigations on the corticospinal system there is only scant information on neuronal networks mediating actions of corticospinal neurones on ipsilateral motoneurones. We have previously demonstrated double crossed pathways through which pyramidal tract neurones can influence ipsilateral motoneurones, via contralaterally descending reticulospinal neurones and spinal commissural interneurones. The aim of the present study was to examine the effects of stimulation of pyramidal tract (PT) fibres mediated via ipsilaterally descending pathways and to find out which neurones relay these effects. This was done by using intracellular recordings from 96 lumbar motoneurones in deeply anaesthetized cats. To eliminate actions of fibres descending on the side contralateral to the location of the motoneurones, the spinal cords were hemisected on this side at a low-thoracic level. Stimuli that selectively activated ipsilateral PT fibres evoked EPSPs and/or IPSPs in 34/47 motoneurones tested. These PSPs were evoked at latencies indicating that the most direct coupling between PT neurones and motoneurones in uncrossed pathways is disynaptic. Occlusion and spatial facilitation between actions evoked by stimulation of ipsilateral PT and of reticulospinal tract fibres in the ipsilateral medial longitudinal fascicle (MLF) indicated that PT actions are mediated by reticulospinal neurones with axons in the MLF. However, after transection of the MLF in the caudal medulla, stimulation of the ipsilateral PT continued to evoke EPSPs and IPSPs with characteristics similar to when the MLF was intact (in 15/49 motoneurones) suggesting the existence of parallel disynaptic pathways via other relay neurones.

Changes in serotoninergic receptors 1A and 2A in the piglet brainstem after intermittent hypercapnic hypoxia (IHH) and nicotine

We studied the effects of intermittent hypercapnic hypoxia (IHH) and/or nicotine on the immunoreactivity of serotoninergic (5-HT) receptors 1A and 2A in the piglet brainstem. These exposures were developed to mimic two common risk factors for Sudden Infant Death Syndrome (SIDS); prone sleeping (IHH) and cigarette smoke exposure (nicotine). Immunoreactivity for 5-HT 1AR and 5-HT 2AR were studied in four nuclei of the caudal medulla. Three exposure groups were compared to controls ( n = 14): IHH ( n = 10), nicotine ( n = 14), and nicotine + IHH ( n = 14). In control piglets, the immunoreactivity of 5-HT 1AR was highest in the hypoglossal nucleus (XII), followed by inferior olivary nucleus (ION), nucleus of the solitary tract (NTS) and dorsal motor nucleus of the vagus (DMNV), whereas for 5-HT 2AR, the immunoreactivity was highest in DMNV/NTS and then ION. Compared to controls, IHH reduced 5-HT 1AR immunoreactivity in all studied nuclei ( p < 0.05) but had no effect on 5-HT 2AR immunoreactivity. Nicotine reduced 5-HT 1AR immunoreactivity in the DMNV, ION and NTS ( p < 0.001), and reduced 5-HT 2AR immunoreactivity in DMNV/NTS ( p < 0.05). Nicotine + IHH reduced 5-HT 1AR in DMNV, ION and NTS ( p < 0.001) but had no effect on 5-HT 2AR immunoreactivity. Effects of nicotine on the DMNV were more significant in males compared to the females. These results show for the first time that IHH and/or nicotine can reduce 5-HT receptor immunoreactivity within functionally important nuclei of the piglet medulla. The findings support our hypothesis that 5-HT receptor abnormalities may be caused by postnatal exposures to clinically-relevant stimuli such as cigarette smoke exposure and/or prone sleeping.

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.

The respiratory drive to thoracic motoneurones in the cat and its relation to the connections from expiratory bulbospinal neurones.

The descending control of respiratory-related motoneurones in the thoracic spinal cord remains the subject of some debate. In this study, direct connections from expiratory bulbospinal neurones to identified motoneurones were investigated using spike-triggered averaging and the strengths of connection revealed were related to the presence and size of central respiratory drive potentials in the same motoneurones. Intracellular recordings were made from motoneurones in segments T5-T9 of the spinal cord of anaesthetized cats. Spike-triggered averaging from expiratory bulbospinal neurones in the caudal medulla revealed monosynaptic EPSPs in all groups of motoneurones, with the strongest connections to expiratory motoneurones with axons in the internal intercostal nerve. In the latter, connection strength was similar irrespective of the target muscle (e.g. external abdominal oblique or internal intercostal) and the EPSP amplitude was positively correlated with the amplitude of the central respiratory drive potential of the motoneurone. For this group, EPSPs were found in 45/83 bulbospinal neurone/motoneurone pairs, with a mean amplitude of 40.5 microV. The overall strength of the connection supports previous measurements made by cross-correlation, but is about 10 times stronger than that reported in the only previous similar survey to use spike-triggered averaging. Calculations are presented to suggest that this input alone is sufficient to account for all the expiratory depolarization seen in the recorded motoneurones. However, extra sources of input, or amplification of this one, are likely to be necessary to produce a useful motoneurone output.

The descending motorcortical pathway to the laryngeal motoneurons in the squirrel monkey

The motor cortex of primates contains an area (“larynx area”) which, when stimulated unilaterally, produces bilateral vocal fold adduction. In order to identify the pathway along which the cortical larynx area exerts its control on the laryngeal motoneurons, we have blocked excitatory neurotransmission in each of the main projection fields of the cortical larynx area and tested for the elicitability of vocal fold movements from this area in the squirrel monkey. Blocking was carried out by injection of the glutamate antagonist kynurenic acid. We found that injection into the dorsal reticular nucleus of the caudal medulla ipsilateral to the stimulation site blocked vocal fold movements bilaterally; injections invading major parts of the nucleus ambiguus blocked vocal fold movements exclusively ipsilateral to the injection site; and injections centered on the parvocellular reticular formation bordering the nucleus ambiguus blocked exclusively contralateral vocal fold movements. We conclude from this that the corticobulbar laryngeal control pathway synapses in the ipsilateral dorsal reticular nucleus and then divides into one component running directly to the ipsilateral nucleus ambiguus and a second component crossing to the contralateral nucleus ambiguus after having synapsed in the ipsilateral peri-ambigual reticular formation.

Mapping the spinal and supraspinal pathways of dynamic mechanical allodynia in the human trigeminal system using cardiac-gated fMRI

Following injury and inflammation, pain to light stroking (dynamic mechanical allodynia) might develop at the damaged site (primary area) or in adjacent normal tissue (secondary area).Using fMRI we mapped changes in the spinal trigeminal nucleus (spV), and supraspinal brainstem nuclei following heat/capsaicin-induced primary and secondary dynamic mechanical allodynia in the human trigeminal system. The role of these structures in dynamic mechanical allodynia has not been clarified yet in humans. During the control session we applied the same mechanical stimuli to the same untreated trigeminal area.Primary and secondary mechanical allodynia showed equal levels of perceived pain intensity, and compared to control mechanical stimulation exhibited similar responses in the ipsilateral spV and contralateral ventrolateral periaqueductal gray (vlPAG). Activity in the spV was significantly higher during both conditions versus the control mechanical stimulation, indicating that central sensitization of second-order neurons is similar for primary and secondary mechanical allodynia. The vlPAG showed decreased activity that inversely correlated with pain ratings during primary allodynia, i.e. the more deactivated the vlPAG the higher the pain intensity (p<0.05, Pearson's correlation).Primary and secondary dynamic mechanical allodynia were also characterized by significant differences involving distinct supraspinal structures mainly involved in pain modulation and including the rostroventromedial medulla, pons reticular formation, dorsolateral PAG, all more active during primary versus secondary allodynia, and the medial reticular formation of the caudal medulla that was more active during secondary versus primary allodynia. These results indicate that the pain modulatory system is involved to a different extent during primary versus secondary mechanical allodynia.