Direct current stimulation as a non-invasive therapeutic alternative for treating autonomic or non-autonomic neurological disorders affecting breathing.

Direct current stimulation (DCS) is a non-invasive approach to stimulate the nervous system that is now considered a powerful tool for treating neurological diseases such as those affecting cognitive or locomotor functions. DCS, as applied clinically today, is an approach built on early uses in antiquity and knowledge gained over time. Its current use makes use of specific devices and takes into account knowledge of the mechanisms by which this approach modulates functioning of the nervous system at the cellular level. Over the last 20 years, although there are few studies, it has been shown that DCS can also modulate the breathing autonomic function. In this narrative review, after briefly providing the historical perspective and describing the principles and the main cellular and molecular effects, we summarize the currently available data regarding the modulation of ventilation, and propose that DCS could be used to treat autonomic or non-autonomic neurological disorders affecting breathing.

Current Perspectives for the use of Gonane Progesteronergic Drugs in the Treatment of Central Hypoventilation Syndromes.

BACKGROUND: Central alveolar hypoventilation syndromes (CHS) encompass neurorespiratory diseases resulting from congenital or acquired neurological disorders. Hypercapnia, acidosis, and hypoxemia resulting from CHS negatively affect physiological functions and can be lifethreatening. To date, the absence of pharmacological treatment implies that the patients must receive assisted ventilation throughout their lives. OBJECTIVE: To highlight the relevance of determining conditions in which using gonane synthetic progestins could be of potential clinical interest for the treatment of CHS. METHODS: The mechanisms by which gonanes modulate the respiratory drive were put into the context of those established for natural progesterone and other synthetic progestins. RESULTS: The clinical benefits of synthetic progestins to treat respiratory diseases are mixed with either positive outcomes or no improvement. A benefit for CHS patients has only recently been proposed. We incidentally observed restoration of CO2 chemosensitivity, the functional deficit of this disease, in two adult CHS women by desogestrel, a gonane progestin, used for contraception. This effect was not observed by another group, studying a single patient. These contradictory findings are probably due to the complex nature of the action of desogestrel on breathing and led us to carry out mechanistic studies in rodents. Our results show that desogestrel influences the respiratory command by modulating the GABAA and NMDA signaling in the respiratory network, medullary serotoninergic systems, and supramedullary areas. CONCLUSION: Gonanes show promise for improving ventilation of CHS patients, although the conditions of their use need to be better understood.

Key Brainstem Structures Activated during Hypoxic Exposure in One-day-old Mice Highlight Characteristics for Modeling Breathing Network in Premature Infants.

We mapped and characterized changes in the activity of brainstem cell groups under hypoxia in one-day-old newborn mice, an animal model in which the central nervous system at birth is particularly immature. The classical biphasic respiratory response characterized by transient hyperventilation, followed by severe ventilation decline, was associated with increased c-FOS immunoreactivity in brainstem cell groups: the nucleus of the solitary tract, ventral reticular nucleus of the medulla, retrotrapezoid/parafacial region, parapyramidal group, raphe magnus nucleus, lateral, and medial parabrachial nucleus, and dorsal subcoeruleus nucleus. In contrast, the hypoglossal nucleus displayed decreased c-FOS immunoreactivity. There were fewer or no activated catecholaminergic cells activated in the medulla oblongata, whereas ~45% of the c-FOS-positive cells in the dorsal subcoeruleus were co-labeled. Approximately 30% of the c-FOS-positive cells in the parapyramidal group were serotoninergic, whereas only a small portion were labeled for serotonin in the raphe magnus nucleus. None of the c-FOS-positive cells in the retrotrapezoid/parafacial region were co-labeled for PHOX2B. Thus, the hypoxia-activated brainstem neuronal network of one-day-old mice is characterized by (i) the activation of catecholaminergic cells of the dorsal subcoeruleus nucleus, a structure implicated in the strong depressive pontine influence previously reported in the fetus but not in newborns, (ii) the weak activation of catecholaminergic cells of the ventral reticular nucleus of the medulla, an area involved in hypoxic hyperventilation, and (iii) the absence of PHOX2B-positive cells activated in the retrotrapezoid/parafacial region. Based on these results, one-day-old mice could highlight characteristics for modeling the breathing network of premature infants.

The c-FOS Protein Immunohistological Detection: A Useful Tool As a Marker of Central Pathways Involved in Specific Physiological Responses In Vivo and Ex Vivo.

Many studies seek to identify and map the brain regions involved in specific physiological regulations. The proto-oncogene c-fos, an immediate early gene, is expressed in neurons in response to various stimuli. The protein product can be readily detected with immunohistochemical techniques leading to the use of c-FOS detection to map groups of neurons that display changes in their activity. In this article, we focused on the identification of brainstem neuronal populations involved in the ventilatory adaptation to hypoxia or hypercapnia. Two approaches were described to identify involved neuronal populations in vivo in animals and ex vivo in deafferented brainstem preparations. In vivo, animals were exposed to hypercapnic or hypoxic gas mixtures. Ex vivo, deafferented preparations were superfused with hypoxic or hypercapnic artificial cerebrospinal fluid. In both cases, either control in vivo animals or ex vivo preparations were maintained under normoxic and normocapnic conditions. The comparison of these two approaches allows the determination of the origin of the neuronal activation i.e., peripheral and/or central. In vivo and ex vivo, brainstems were collected, fixed, and sliced into sections. Once sections were prepared, immunohistochemical detection of the c-FOS protein was made in order to identify the brainstem groups of cells activated by hypoxic or hypercapnic stimulations. Labeled cells were counted in brainstem respiratory structures. In comparison to the control condition, hypoxia or hypercapnia increased the number of c-FOS labeled cells in several specific brainstem sites that are thus constitutive of the neuronal pathways involved in the adaptation of the central respiratory drive.

Desogestrel enhances ventilation in ondine patients: Animal data involving serotoninergic systems.

Congenital central hypoventilation syndrome (CCHS) is a neurorespiratory disease characterized by life-threatening sleep-related hypoventilation involving an alteration of CO2/H(+) chemosensitivity. Incidental findings have suggested that desogestrel may allow recovery of the ventilatory response to CO2. The effects of desogestrel on resting ventilation have not been reported. This study was designed to test the hypothesis that desogestrel strengthens baseline ventilation by analyzing the ventilation of CCHS patients. Rodent models were used in order to determine the mechanisms involved. Ventilation in CCHS patients was measured with a pneumotachometer. In mice, ventilatory neural activity was recorded from ex vivo medullary-spinal cord preparations, ventilation was measured by plethysmography and c-fos expression was studied in medullary respiratory nuclei. Desogestrel increased baseline respiratory frequency of CCHS patients leading to a decrease in their PETCO2. In medullary spinal-cord preparations or in vivo mice, the metabolite of desogestrel, etonogestrel, induced an increase in respiratory frequency that necessitated the functioning of serotoninergic systems, and modulated GABAA and NMDA ventilatory regulations. c-FOS analysis showed the involvement of medullary respiratory groups of cell including serotoninergic neurons of the raphe pallidus and raphe obscurus nuclei that seem to play a key role. Thus, desogestrel may improve resting ventilation in CCHS patients by a stimulant effect on baseline respiratory frequency. Our data open up clinical perspectives based on the combination of this progestin with serotoninergic drugs to enhance ventilation in CCHS patients.

Apelin counteracts vasopressin-induced water reabsorption via cross talk between apelin and vasopressin receptor signaling pathways in the rat collecting duct.

Apelin receptors (ApelinRs) are expressed along an increasing cortico-medullary gradient in collecting ducts (CDs). We showed here that iv injection of apelin 17 (K17F) in lactating rats characterized by increases in both synthesis and release of arginine vasopressin (AVP) increased diuresis concomitantly with a significant decrease in urine osmolality and no change in Na(+) and K(+) excretion. Under these conditions, we also observed a significant decrease in apical aquaporin-2 immunolabeling in CD, with a cortico-medullary gradient, suggesting that K17F-induced diuresis could be linked to a direct action of apelin on CD. We then examined the potential cross talk between V1a AVP receptor (V1a-R), V2 AVP receptor (V2-R) and ApelinR signaling pathways in outer medullary CD (OMCD) and inner medullary CD microdissected rat CD. In OMCD, expressing the 3 receptors, K17F inhibited cAMP production and Ca(2+) influx induced by 1-desamino-8-D-arginine vasopressin a V2-R agonist. Similar effects were observed in inner medullary CD expressing only V2-R and ApelinR. In contrast, in OMCD, K17F increased by 51% the Ca(2+) influx induced by the stimulation of V1a-R by AVP in the presence of the V2-R antagonist SR121463B, possibly enhancing the physiological antagonist effect of V1a-R on V2-R. Thus, the diuretic effect of apelin is not only due to a central effect by inhibiting AVP release in the blood circulation as previously shown but also to a direct action of apelin on CD, by counteracting the antidiuretic effect of AVP occurring via V2-R.

Data supporting a new physiological role for brain apelin in the regulation of hypothalamic oxytocin neurons in lactating rats.

Apelin is a bioactive peptide identified as the endogenous ligand of the human orphan G protein-coupled receptor APJ in 1998. The present data show that apelin modulates the activity of magnocellular and parvocellular oxytocin (OXY) neurons in the lactating rat. A combination of in situ hybridization and immunohistochemistry demonstrated the presence of apelin receptor mRNA in hypothalamic OXY neurons. Double immunofluorescence labeling then revealed the colocalization of apelin with OXY in about 20% of the hypothalamic OXY-positive neurons. Intracerebroventricular apelin administration inhibited the activity of magnocellular and parvocellular OXY neurons, as shown by measuring the c-fos expression in OXY neurons or by direct electrophysiological measurements of the electrical activity of these neurons. This effect was correlated with a decrease in the amount of milk ejected. Thus, apelin inhibits the activity of OXY neurons through a direct action on apelin receptors expressed by these neurons in an autocrine and paracrine manner. In conclusion, these findings highlight the inhibitory role of apelin as an autocrine/paracrine peptide acting on OXY neurons during breastfeeding.

Effect of apelin on glomerular hemodynamic function in the rat kidney.

Apelin is a vasoactive peptide identified as the endogenous ligand of an orphan G protein-coupled receptor called APJ. Apelin and its receptor have been found in the brain and the cardiovascular system. Here we show that the apelin receptor mRNA is highly expressed in the glomeruli while its level of expression is lower in all nephron segments including collecting ducts that express vasopressin V2 receptors. Intravenous injection of apelin 17 into lactating rats induced a significant diuresis. Apelin receptor mRNA was also found in endothelial and vascular smooth muscle cells of glomerular arterioles. Apelin administration caused vasorelaxation of angiotensin II-preconstricted efferent and afferent arterioles as shown by an increase in their diameter. Activation of endothelial apelin receptors caused release of nitric oxide which inhibited angiotensin II-induced rise in intracellular calcium. In addition, it appears that apelin had a direct receptor-mediated vasoconstrictive effect on vascular smooth muscle. These results show that apelin has complex effects on the pre- and post glomerular microvasculature regulating renal hemodynamics. Its role on tubular function (if any) remains to be determined.

Orally active aminopeptidase A inhibitors reduce blood pressure: a new strategy for treating hypertension.

Overactivity of the brain renin-angiotensin system has been implicated in the development and maintenance of hypertension. We reported previously that angiotensin II is converted to angiotensin III by aminopeptidase A in the mouse brain. We then used specific and selective aminopeptidase A inhibitors to show that angiotensin III is one of the main effector peptides of the brain renin-angiotensin system, exerting tonic stimulatory control over blood pressure in hypertensive rats. Aminopeptidase A, the enzyme generating brain angiotensin III, thus represents a potential candidate central nervous system target for the treatment of hypertension. Given this possible clinical use of aminopeptidase A inhibitors, it was, therefore, important to investigate their pharmacological activity after oral administration. We investigated RB150, a dimer of the selective aminopeptidase A inhibitor, EC33, generated by creating a disulfide bond. This chemical modification allows prodrug to cross the blood-brain barrier when administered by systemic route. Oral administration of RB150 in conscious DOCA-salt rats inhibited brain aminopeptidase A activity, resulting in values similar to those obtained with the brains of normotensive rats, demonstrating the central bioavailability of RB150. Oral RB150 treatment resulted in a marked dose-dependent reduction in blood pressure in DOCA-salt but not in normotensive rats, with an ED(50) in the 1-mg/kg range, achieved in <2 hours and lasting for several hours. This treatment also significantly decreased plasma arginine-vasopressin levels and increased diuresis, which may participate to the blood pressure decrease by reducing the size of fluid compartment. Thus, RB150 may be the prototype of a new class of centrally active antihypertensive agents.

Aminopeptidase A inhibitors as centrally acting antihypertensive agents.

Among the main bioactive peptides of the brain renin-angiotensin system, angiotensin (Ang) II and AngIII exhibit the same affinity for the type 1 and type 2 Ang receptors. Both peptides, injected intracerebroventricularly, cause similar increase in blood pressure (BP). Because AngII is converted in vivo to AngIII, the identity of the true effector is unknown. This review summarized recent insights into the predominant role of brain AngIII in the central control of BP underlining the fact that brain aminopeptidase A (APA), the enzyme forming central AngIII, could constitute a putative central therapeutic target for the treatment of hypertension. This led to the development of potent, systematically active APA inhibitors, such as RB150, as a prototype of a new class of centrally acting antihypertensive agents for the treatment of certain forms of hypertension.

Caffeine in the milk prevents respiratory disorders caused by in utero caffeine exposure in rats.

Consequences of postnatal caffeine exposure by the milk on ponto-medullary respiratory disturbances observed following an in utero caffeine exposure were analysed. Ponto-medullary-spinal cord preparations from newborn rats exposed to caffeine during gestation but not after the birth display an increase in respiratory frequency and an exaggeration of the hypoxic respiratory depression compared to not treated preparations. These data suggest that tachypneic and apneic episodes encountered in human newborns whose mother consumed caffeine during pregnancy are due in large part to central effect of caffeine at the ponto-medullary level. Both baseline respiratory frequency increase and emphasis of hypoxic respiratory depression are not encountered if rat dams consumed caffeine during nursing. Our hypothesis is that newborn rats exposed to caffeine during gestation but not after the birth would be in withdrawal situation whereas, when caffeine is present in drinking fluid of lactating dams, it goes down the milk and is able to prevent ponto-medullary respiratory disturbances.

Endogenous 5-HT(1/2) systems and the newborn rat respiratory control. A comparative in vivo and in vitro study.

Consequences of 5-HT(1/2) systems blockade by methysergide on newborn rats respiratory drive were evaluated in vivo with unrestrained animals and in vitro using brainstem-spinal cord preparations. A decrease in respiratory frequency until a plateau level was observed under both in vivo (82.8 +/- 0.6% of control values) and in vitro (76.8 +/- 0.8% of control values) conditions whereas an increase in inspiratory amplitude (135.1 +/- 2.1% of control values) was only retrieved in vivo. By the use of the c-fos expression analysis, we correlated these effects with neuronal activity changes, particularly, in vivo in two key structures between the respiratory ponto-medullary network and the peripheral or suprapontine afferences, namely the commissural subnucleus of the nucleus of the solitary tract and the lateral parabrachial nucleus. Thus, peripheral and suprapontine inputs seem to be of a primeval importance in the respiratory influence of endogenous 5-HT. Besides, as 5-HT is involved in the respiratory perturbations that occur in sudden infant death syndrome (SIDS), our results suggest a participation of peripheral and suprapontine inputs in these disorders.

Consequences of in utero caffeine exposure on respiratory output in normoxic and hypoxic conditions and related changes of Fos expression: a study on brainstem-spinal cord preparations isolated from newborn rats.

Several aspects of the central regulation of respiratory control have been investigated on brainstem-spinal cord preparations isolated from newborn rats whose dam was given 0.02% caffeine in water as drinking fluid during the whole period of pregnancy. Analysis of the central respiratory drive estimated by the recording of C4 ventral root activity was correlated to Fos ponto-medullary expression. Under normoxic conditions, preparations obtained from the caffeine-treated group of animals displayed a higher respiratory frequency than observed in the control group (9.2 +/- 0.5 versus 7.2 +/- 0.6 burst/min). A parallel Fos detection tends to indicate that the changes of the respiratory rhythm may be due to a decrease in neuronal activity of medullary structures such as the ventrolateral subdivision of the solitary tract, the area postrema, and the nucleus raphe obscurus. Under hypoxic conditions, the preparations displayed a typical hypoxic respiratory depression associated with changes in the medullary Fos expression pattern. In addition, the hypoxic respiratory depression is clearly emphasized after in utero exposure to caffeine and coincides with an increased Fos expression in the area postrema and nucleus raphe obscurus, two structures in which it is not increased in the absence of caffeine. Taken together, these results support the idea that in utero caffeine exposure could affect central respiratory control.