Olivier Pascual

Ventilatory and central neurochemical reorganisation of O2 chemoreflex after carotid sinus nerve transection in rat

The first step of this study was to determine the early time course and pattern of hypoxic ventilatory response (HVR) recovery following irreversible bilateral carotid sinus nerve transection (CSNT). The second step was to find out if HVR recovery was associated with changes in the neurochemical activity of the medullary catecholaminergic cell groups involved in the O2 chemoreflex pathway. The breathing response to acute hypoxia (10% O2) was measured in awake rats 2, 6, 10, 45 and 90 days after CSNT. In a control group of sham‐operated rats, the ventilatory response to hypoxia was principally due to increased respiratory frequency. There was a large reduction in HVR in the CSNT compared to the sham‐operated rats (−65%, 2 days after surgery). Within the weeks following denervation, the CSNT rats progressively recovered a HVR level similar to the sham‐operated rats (‐37% at 6 days, −27% at 10 days, and no difference at 45 or 90 days). After recovery, the CSNT rats exhibited a higher tidal volume (+38%) than the sham‐operated rats in response to hypoxia, but not a complete recovery of respiratory frequency. Fifteen days after CSNT, in vivo tyrosine hydroxylase (TH) activity had decreased in caudal A2C2 (−35%) and A6 cells (−35%). After 90 days, the CSNT rats displayed higher TH activity than the sham‐operated animals in caudal A1C1 (+51%), caudal A2C2 (+129%), A5 (+216%) and A6 cells (+79%). It is concluded that HVR following CSNT is associated with a profound functional reorganisation of the central O2 chemoreflex pathway, including changes in ventilatory pattern and medullary catecholaminergic activity.

O2-sensing after carotid chemodenervation: hypoxic ventilatory responsiveness and upregulation of tyrosine hydroxylase mRNA in brainstem catecholaminergic cells

Ventilatory responses to acute and long‐term hypoxia are classically triggered by carotid chemoreceptors. The chemosensory inputs are carried within the carotid sinus nerve to the nucleus tractus solitarius and the brainstem respiratory centres. To investigate whether hypoxia acts directly on brainstem neurons or secondarily via carotid body inputs, we tested the ventilatory responses to acute and long‐term hypoxia in rats with bilaterally transected carotid sinus nerves and in sham‐operated rats. Because brainstem catecholaminergic neurons are part of the chemoreflex pathway, the ventilatory response to hypoxia was studied in association with the expression of tyrosine hydroxylase (TH). TH mRNA levels were assessed in the brainstem by in situ hybridization and hypoxic ventilatory responses were measured in vivo by plethysmography. After long‐term hypoxia, TH mRNA levels in the nucleus tractus solitarius and ventrolateral medulla increased similarly in chemodenervated and sham‐operated rats. Ventilatory acclimatization to hypoxia developed in chemodenervated rats, but to a lesser extent than in sham‐operated rats. Ventilatory response to acute hypoxia, which was initially low in chemodenervated rats, was fully restored within 21 days in long‐term hypoxic rats, as well as in normoxic animals which do not overexpress TH. Therefore, activation of brainstem catecholaminergic neurons and ventilatory adjustments to hypoxia occurred independently of carotid chemosensory inputs. O2‐sensing mechanisms unmasked by carotid chemodenervation triggered two ventilatory adjustments: (i) a partial acclimatization to long‐term hypoxia associated with TH upregulation; (ii) a complete restoration of acute hypoxic responsivity independent of TH upregulation.

Selective cardiorespiratory and catecholaminergic areas express the hypoxia-inducible factor-1α (HIF-1α) under in vivo hypoxia in rat brainstem

Under severe oxygen deprivation, all cells are able to express the transcription factor HIF‐1, which activates a wide range of genes. Under tolerable hypoxia, chemosensory inputs are integrated in brainstem areas, which control cardiorespiratory responses. However, the molecular mechanisms of this functional acclimatization are unknown. We investigated when and where the inducible HIF‐1α subunit is expressed in the rat brainstem in vivo, under physiological hypoxia. The regional localization of HIF‐1α mRNA and protein was determined by in situ hybridization and immunocytochemistry in adult male rats exposed to moderate hypoxia (10% O2) for 1–6 h. HIF‐1α protein was found in cell types identified by immunocytochemistry as catecholaminergic neurons. Hypoxia induced HIF‐1α mRNA and protein in only some parts of the brainstem located dorsomedially and ventrolaterally, which are those involved in the cardiorespiratory control. No labelling was detected under normoxia. The protein was detected in glia and neurons after 1 and 6 h of hypoxia, respectively. A subset of A2C2 and A1C1 catecholaminergic neurons colocalized tyrosine hydroxylase and HIF‐1α proteins under hypoxia, but no HIF‐1α was detected in more rostral catecholaminergic areas. In contrast to cardiorespiratory areas, HIF‐1α protein was already present under normoxia in glial cells of brainstem tracts but was not overexpressed under hypoxia, although HIF‐1α mRNA was up‐regulated. In conclusion, there appear to be two regulatory mechanisms for HIF‐1α expression in the brainstem: hypoxic induction of HIF‐1α protein in cardiorespiratory‐related areas and constitutive protein expression unaffected by hypoxia in brainstem tracts.

Progesterone reverses the neuronal responses to hypoxia in rat nucleus tractus solitarius in vitro.

The nucleus tractus solitarius (NTS) is a relay nucleus that integrates peripheral chemoreceptor input in response to hypoxia and hence influences the generation of respiratory rhythm. Several studies have shown that administration of progesterone stimulates ventilatory responses to hypoxia. There is some evidence that this steroid hormone can act at the level of the arterial peripheral chemoreceptors, whereas its action in the central nervous system remains unclear. To investigate a possible central involvement during hypoxia, we studied the effect of progesterone on neuronal activities recorded extra‐ and intracellularly in the NTS using brainstem slices. Central chemosensitivity was tested by comparing synaptic activity and intrinsic electro‐responsiveness of 38 neurones during normoxia and hypoxia. In more than two‐thirds of neurones recorded, hypoxia elicited a hyperpolarisation, a decrease in the input resistance and a decrease in spontaneous activity. In the remaining neurones (n= 12) hypoxia elicited a depolarisation and an increase in spontaneous activity. In all neurones tested, synaptic potentials evoked by stimulation of the tractus solitarius were decreased by hypoxia. While progesterone (1 μM) had no effect under normoxic conditions, it partially reversed all hypoxic neuronal responses. This effect developed over 2‐3 min and reversed within 5 min suggesting a non‐genomic mechanism of action. Taken together these results suggest that progesterone interacts with the hypoxia‐induced cellular signalling. We conclude that in the NTS, transmission of afferent signals is reduced by hypoxia and restored by progesterone administration. Such a mechanism may contribute to the stimulation of breathing in response to hypoxia observed following progesterone administration in vivo.

Sonic hedgehog signalling in neurons of adult ventrolateral nucleus tractus solitarius

The transmembrane receptor Patched (Ptc) mediates the action of the diffusing factor Sonic hedgehog (Shh), which is implicated in establishing morphogenetic gradients during embryonic development. Whereas alteration of Ptc function is associated with developmental abnormalities and brain tumors, its functional activity and roles in the adult brain have yet to be elucidated. Here we describe the complementary pattern of Shh and Ptc expression in the rat dorsal vagal motor nucleus and the ventrolateral nucleus tractus solitarius (vNTS), respectively. Those two interconnected structures regulate the cardiorespiratory function during hypoxia. Bath application of a subnanomolar concentration of aminoterminal Shh protein (ShhN) to a slice preparation of the vNTS induces a rapid decrease in neuronal firing followed by a bursting activity that propagates in the neuronal network. Intracellular current injections show that bursts result from an action on the neuronal membrane electro‐responsiveness. Both inhibiting and bursting effects are blocked by the monoclonal Shh antibody 5E1 and may require the Ptc binding site of ShhN. Thus, ShhN acting on specific neuronal sites controls electrophysiological properties of differentiated neurons of the vNTS. We speculate on a retrocontrol of cardiorespiratory signals in the vNTS, by Shh generated in dorsal vagal motoneurons.

Chemosensory Inputs and Neural Remodeling in Carotid Body and Brainstem Catecholaminergic Cells

Exposure to hypoxia elicits an immediate increase in ventilation in order to face the tissue oxygen deficit. The acute response to hypoxia develops gradually over several days despite a constant level of isocapnic hypoxia, before reaching a steady state level which has been termed ventilatory acclimatization to hypoxia (VAH). The functional acclimatization to hypoxia reveals a striking plasticity of the chemoreflex, which takes place within the first days of exposure and can be prolonged for weeks, months or years. There is clearcut evidence that the peripheral arterial chemoreceptors play a major role in initiating the ventilatory acclimatization to hypoxia. However, this does not preclude a role for central structures involved in the translation of chemosensory inputs and modulating the integration of carotid chemo-afferent inputs. Early and recent studies have shown that the ventilatory plasticity induced by sustained hypoxia is associated with changes in the morphology and phenotype of the carotid chemoreceptors, increases in neurotransmitter biosynthesis and release, modulation of neuroreceptor expression in the carotid body and increased firing rate of the carotid chemo-afferent neurons. More recent studies demonstrated that the neuroplasticity also takes place during long-term hypoxia in restricted areas of the central nervous system, which have been involved in respiratory and sympathetic responses to hypoxia. This short review is devoted to the neurochemical plasticity induced by sustained hypoxia in the carotid body and in brainstem structures involved in translation of the peripheral chemosensory inputs, and their possible role in triggering or modulating ventilatory acclimatization to hypoxia.

Autoimmune encephalitis in psychiatric institutions: current perspectives

Autoimmune encephalitis is a rare and newly described group of diseases involving autoantibodies directed against synaptic and neuronal cell surface antigens. It comprises a wide range of neuropsychiatric symptoms. Sensitive and specific diagnostic tests such as cell-based assay are primordial for the detection of neuronal cell surface antibodies in patients’ cerebrospinal fluid or serum and determine the treatment and follow-up of the patients. As neurological symptoms are fairly well described in the literature, this review focuses on the nature of psychiatric symptoms occurring at the onset or during the course of the diseases. In order to help the diagnosis, the main neurological symptoms of the most representative synaptic and neuronal cell surface autoantibodies were detailed. Finally, the exploration of these autoantibodies for almost a decade allowed us to present an overview of autoimmune encephalitis incidence in psychiatric disease and the general guidelines for the management of psychiatric manifestations. For the majority of autoimmune encephalitis, the prognosis depends on the rapidity of the detection, identification, and the management of the disease. Because the presence of pronounced psychiatric symptoms drives patients to psychiatric institutions and can hinder the diagnosis, the aim of this work is to provide clues to help earlier detection by physicians and thus provide better medical care to patients.

CSF IgA NMDAR antibodies are potential biomarkers for teratomas in anti-NMDAR encephalitis

Objective: To evaluate the presence of immunoglobulin A (IgA) subtype of anti-NMDA receptor (NMDAR) antibodies (IgA-NMDAR-Abs) in the CSF of patients with immunoglobulin G (IgG)-NMDAR-Ab encephalitis and to describe the potential association with a specific clinical pattern. Methods: The retrospective analysis for the presence of IgA-NMDAR-Abs in 94 CSF samples from patients with anti-NMDAR encephalitis diagnosed between October 2007 and February 2014 was conducted at the French Reference Centre on Paraneoplastic Neurological Syndrome. This observational study compared 39 patients with both IgA- and IgG-NMDAR-Abs to 55 patients with only IgG-NMDAR-Abs. Results: In the retrospective cohort, 41% of the patients with NMDAR-Ab encephalitis had both CSF IgG- and IgA-NMDAR-Abs. Approximately half of the IgA-NMDAR-Ab-positive patients (18/38, 49%) definitively possessed associated tumors, primarily ovarian teratomas (17/18, 94%), compared with only 5% (3/55) of the patients in the IgA-NMDAR-Ab-negative group (p < 0.001). In the adult female population at risk for ovarian teratoma, the detection of CSF IgA-NMDAR-Ab positivity showed 85% sensitivity, 70% specificity, a 57% positive predictive value, and a 90% negative predictive value for the diagnosis of ovarian teratoma. No other specific clinical features or clinical outcome were associated with CSF IgA-NMDAR-Ab positivity. Conclusion: These results suggest that in patients with IgG-NMDAR-Ab encephalitis, CSF IgA-NMDAR-Abs could be used as a biological marker for the presence of an ovarian teratoma.

Carotid chemodenervation approach to study oxygen sensing in brain stem catecholaminergic cells.

Publisher Summary This chapter discusses a carotid chemodenervation approach to study oxygen sensing in brain stem catecholaminergic cells. The physiological conditions of hypoxia known as “moderate” or “tolerable” hypoxia allows a long-term exposure lasting for weeks. The exposure to tolerable hypoxia enables the definition of the morphofunctional mechanisms involved in the ventilatory acclimatization of intact or carotid chemodenervated animals. Exposure to moderate hypoxia induces the activation of brain stem catecholaminergic cells located in cardiorespiratory areas and/or involved in the modulation of cardiorespiratory output. The chapter describes the in vivo and in vitro preparations used to test the effect of a moderate hypoxia on brain stem respiratory areas and then the methods used to evaluate the consequences of hypoxia on ventilation, catecholamine neurochemistry, and gene expression. Methods used to test the physiological effect of hypoxia are discussed.

Contactin-associated protein-like 2, a protein of the neurexin family involved in several human diseases

Contactin‐associated protein‐like 2 (CASPR2) is a cell adhesion protein of the neurexin family. Proteins of this family have been shown to play a role in the development of the nervous system, in synaptic functions, and in neurological diseases. Over recent years, CASPR2 function has gained an increasing interest as demonstrated by the growing number of publications. Here, we gather published data to comprehensively review CASPR2 functions within the nervous system in relation to CASPR2‐related diseases in humans. On the one hand, studies on Cntnap2 (coding for CASPR2) knockout mice revealed its role during development, especially, in setting‐up the inhibitory network. Consistent with this result, mutations in the CNTNAP2 gene coding for CASPR2 in human have been identified in neurodevelopmental disorders such as autism, intellectual disability, and epilepsy. On the other hand, CASPR2 was shown to play a role beyond development, in the localization of voltage‐gated potassium channel (VGKC) complex that is composed of TAG‐1, Kv1.1, and Kv1.2. This complex was found in several subcellular compartments essential for action potential propagation: the node of Ranvier, the axon initial segment, and the synapse. In line with a role of CASPR2 in the mature nervous system, neurological autoimmune diseases have been described in patients without neurodevelopmental disorders but with antibodies directed against CASPR2. These autoimmune diseases were of two types: central with memory disorders and temporal lobe seizures, or peripheral with muscular hyperactivity. Overall, we review the up‐to‐date knowledge on CASPR2 function and pinpoint confused or lacking information that will need further investigation.