Brainstem galanin-synthesizing neurons are differentially activated by chemoreceptor stimuli and represent a subpopulation of respiratory neurons

Galaninergic and hypercapnia-activated neuronal projections to the ventral respiratory column

In mammals, the ventral respiratory column (VRC) plays a pivotal role in integrating neurochemically diverse inputs from brainstem and forebrain regions to generate respiratory motor patterns. VRC microinjection of the neuropeptide galanin has been reported to dampen carbon dioxide (CO2)-mediated chemoreflex responses. Additionally, we previously demonstrated that galaninergic neurons in the retrotrapezoid nucleus (RTN) are implicated in the adaptive response to hypercapnic stimuli, suggesting a link between RTN neuroplasticity and increased neuronal drive to the VRC. VRC neurons express galanin receptor 1, suggesting potential regulatory action by galanin, however, the precise galaninergic chemoreceptor-VRC circuitry remains to be determined. This study aimed to identify sources of galaninergic input to the VRC that contribute to central respiratory chemoreception. We employed a combination of retrograde neuronal tracing, in situ hybridisation and immunohistochemistry to investigate VRC-projecting neurons that synthesise galanin mRNA. In an additional series of experiments, we used acute hypercapnia exposure (10% CO2, 1 h) and c-Fos immunohistochemistry to ascertain which galaninergic nuclei projecting to the VRC are activated. Our findings reveal that a total of 30 brain nuclei and 51 subnuclei project to the VRC, with 12 of these containing galaninergic neurons, including the RTN. Among these galaninergic populations, only a subset of the RTN neurons (approximately 55%) exhibited activation in response to acute hypercapnia. Our findings highlight that the RTN is the likely source of galaninergic transmission to the VRC in response to hypercapnic stimuli.

Potential therapeutic effect of NK1R antagonist in diabetic non-healing wound and depression

Diabetes is a global disease with huge impacts on patients due to its complications, among which non-healing wounds and depression are common and challenging. The neurokinin 1 receptor (NK1R) inhibitor, aprepitant has been broadly applied for an antidepressant effect in depressive patients. Recent literature has indicated a therapeutic effect of downregulation in NK1R to diabetes-related fracture, cardiomyopathy, gastroparesis, and ocular surface disorders. In this study, differential expression genes in diabetes and depression were analyzed based on several RNA sequencing datasets from the GEO database to confirm NK1R in the overlapping set. Interaction network and gene set enrichment analysis were subsequently conducted. As a result, NK1R-related genes took part in angiogenesis, epithelial-mesenchymal transition (EMT), collagen deposition, and inflammation in diabetes and depression. In vivo, the downregulation of NK1R was proved to promote vascular proliferation and enhance diabetic wound healing, which provides a potential therapeutic target for the management of diabetic non-healing wounds and depression.

Chronic treatment with escitalopram and venlafaxine affects the neuropeptide S pathway differently in adult Wistar rats exposed to maternal separation

Neuropeptide S (NPS), which is a peptide that is involved in the regulation of the stress response, seems to be relevant to the mechanism of action of antidepressants that have anxiolytic properties. However, to date, there have been no reports regarding the effect of long-term treatment with escitalopram or venlafaxine on the NPS system under stress conditions. This study aimed to investigate the effects of the above-mentioned antidepressants on the NPS system in adult male Wistar rats that were exposed to neonatal maternal separation (MS). Animals were exposed to MS for 360 min. on postnatal days (PNDs) 2-15. MS causes long-lasting behavioral, endocrine and neurochemical consequences that mimic anxiety- and depression-related features. MS and non-stressed rats were given escitalopram or venlafaxine (10mg/kg) IP from PND 69 to 89. The NPS system was analyzed in the brainstem, hypothalamus, amygdala and anterior olfactory nucleus using quantitative RT-PCR and immunohistochemical methods. The NPS system was vulnerable to MS in the brainstem and amygdala. In the brainstem, escitalopram down-regulated NPS and NPS mRNA in the MS rats and induced a tendency to reduce the number of NPS-positive cells in the peri-locus coeruleus. In the MS rats, venlafaxine insignificantly decreased the NPSR mRNA levels in the amygdala and a number of NPSR cells in the basolateral amygdala, and increased the NPS mRNA levels in the hypothalamus. Our data show that the studied antidepressants affect the NPS system differently and preliminarily suggest that the NPS system might partially mediate the pharmacological effects that are induced by these drugs.

Altered Retinal Dopamine Levels in a Melatonin-proficient Mouse Model of Form-deprivation Myopia

Reduced levels of retinal dopamine, a key regulator of eye development, are associated with experimental myopia in various species, but are not seen in the myopic eyes of C57BL/6 mice, which are deficient in melatonin, a neurohormone having extensive interactions with dopamine. Here, we examined the relationship between form-deprivation myopia (FDM) and retinal dopamine levels in melatonin-proficient CBA/CaJ mice. We found that these mice exhibited a myopic refractive shift in form-deprived eyes, which was accompanied by altered retinal dopamine levels. When melatonin receptors were pharmacologically blocked, FDM could still be induced, but its magnitude was reduced, and retinal dopamine levels were no longer altered in FDM animals, indicating that melatonin-related changes in retinal dopamine levels contribute to FDM. Thus, FDM is mediated by both dopamine level-independent and melatonin-related dopamine level-dependent mechanisms in CBA/CaJ mice. The previously reported unaltered retinal dopamine levels in myopic C57BL/6 mice may be attributed to melatonin deficiency.

Intrathecally administered substance P activated the spinal defecation center and enhanced colorectal motility in anesthetized rats

Noxious stimuli on the colorectum cause colorectal contractions through activation of descending monoaminergic pathways projecting from the supraspinal defecation center to the spinal defecation center. Since it is known that substance P is involved in the response to peripheral noxious stimuli in the spinal cord, we investigated the effects of intrathecally administered substance P at L6-S1 levels on colorectal motility in rats that were anesthetized with α-chloralose and ketamine. Intrathecally administered substance P enhanced colorectal motility, even after transection of the thoracic spinal cord at the T4 level. Severing the pelvic nerves, but not the colonic nerves, abolished substance P enhanced colorectal motility. In the spinal cord at L6-S1 levels, expression of mRNA coding neurokinin (NK) 1-3 receptors was detected by RT-PCR. Immunohistological experiments revealed that preganglionic neurons of the pelvic nerves express NK1 receptors, whereas expression of NK2 receptors was not found. In addition, substance P-containing fibers densely innervated around the preganglionic neurons expressing NK1 receptors. An intrathecally administered NK1 receptor antagonist (spantide) attenuated capsaicin-induced colorectal contractions. These results suggest that the colokinetic action of substance P is mediated by the NK1 receptor in the spinal defecation center. Our findings indicate that substance P may function as a neurotransmitter in the spinal defecation center.NEW & NOTEWORTHY We found that intrathecally administered substance P enhanced colorectal motility in anesthetized rats. Neurokinin (NK) 1 receptors, but not NK2 receptors, were detected in preganglionic neurons of the pelvic nerves. Blockade of NK1 receptors in the spinal cord attenuated the enhanced colorectal motility in response to intracolonic noxious stimuli. The findings indicate that substance P may function as a neurotransmitter in the spinal reflex pathway controlling defecation.

Review: Neuropathology findings in autonomic brain regions in SUDEP and future research directions

Autonomic dysfunction is implicated from clinical, neuroimaging and experimental studies in sudden and unexpected death in epilepsy (SUDEP). Neuropathological analysis in SUDEP series enable exploration of acquired, seizure-related cellular adaptations in autonomic and brainstem autonomic centres of relevance to dysfunction in the peri-ictal period. Alterations in SUDEP compared to control groups have been identified in the ventrolateral medulla, amygdala, hippocampus and central autonomic regions. These involve neuropeptidergic, serotonergic and adenosine systems, as well as specific regional astroglial and microglial populations, as potential neuronal modulators, orchestrating autonomic dysfunction. Future research studies need to extend to clinically and genetically characterized epilepsies, to explore if common or distinct pathways of autonomic dysfunction mediate SUDEP. The ultimate objective of SUDEP research is the identification of disease biomarkers for at risk patients, to improve post-mortem recognition and disease categorisation, but ultimately, for exposing potential treatment targets of pharmacologically modifiable and reversible cellular alterations.

Hormonal and neural responses to restraint stress in an animal model of perimenopause in female rats

The present study investigated the hormonal and neural responses to stress in a perimenopause animal model induced by 4-vinylcyclohexene diepoxide (VCD), which induces progressive follicular depletion in rodents, allowing studies on the transition to ovarian failure. Female rats, aged 28 days old, were s.c. injected for 15 consecutive days with corn oil or VCD. At 85 ± 5 days after the onset of treatment, the jugular vein was cannulated in the afternoon of metoestrus and in next morning (dioestrus) at 10.00 am, rats were subjected to 30 minutes of restraint stress. Blood samples were withdrawn before (-5 minutes), during (2, 5, 15 and 30 minutes) and after (45, 60 and 90 minutes) stress and plasma prolactin, progesterone and corticosterone levels were measured. Animals were perfused, brains processed for c-Fos/tyrosine hydroxylase (TH) in the locus coeruleus (LC) and c-Fos/corticotrophin-releasing factor (CRF) in the paraventricular nucleus (PVN). In unstressed rats the density of β-endorphin fibres was assessed in LC and PVN. In VCD-treated rats, stress-induced prolactin peak was higher, basal and peak progesterone levels were lower, and both levels of corticosterone were similar to controls. However, the recovery period was longer for both adrenal hormones. In VCD-treated rats the number of c-Fos/TH and c-Fos/CRF-immunoreactive neurones was higher whereas the density of β-endorphin fibres was lower in LC and PVN. We surmise that the hyperactivity of the LC and PVN neurones in VCD-treated rats may be a result of the lower progesterone levels that resulted in the decrease of β-endorphin content in both nuclei, thus impairing the negative-feedback mechanism in the recovery period.

RFamide-Related Peptide Neurons Modulate Reproductive Function and Stress Responses

RF-amide related peptide 3 (RFRP-3) is a neuropeptide thought to inhibit central regulation of fertility. We investigated whether alterations in RFRP neuronal activity led to changes in puberty onset, fertility, and stress responses, including stress and glucocorticoid-induced suppression of pulsatile luteinizing hormone secretion. We first validated a novel RFRP-Cre mouse line, which we then used in combination with Cre-dependent neuronal ablation and DREADD technology to selectively ablate, stimulate, and inhibit RFRP neurons to interrogate their physiological roles in the regulation of fertility and stress responses. Chronic RFRP neuronal activation delayed male puberty onset and female reproductive cycle progression, but RFRP-activated and ablated mice exhibited apparently normal fertility. When subjected to either restraint- or glucocorticoid-induced stress paradigms. However, we observed a critical sex-specific role for RFRP neurons in mediating acute and chronic stress-induced reproductive suppression. Female mice exhibiting RFRP neuron ablation or silencing did not exhibit the stress-induced suppression in pulsatile luteinizing hormone secretion observed in control mice. Furthermore, RFRP neuronal activation markedly stimulated glucocorticoid secretion, demonstrating a feedback loop whereby stressful stimuli activate RFRP neurons, which in turn further activate the stress axis. These data provide evidence for a neuronal link between the stress and reproductive axes.

Adaptation of Respiratory-Related Brain Regions to Long-Term Hypercapnia: Focus on Neuropeptides in the RTN

Long-term hypercapnia is associated with respiratory conditions including obstructive sleep apnea, chronic obstructive pulmonary disease and obesity hypoventilation syndrome. Animal studies have demonstrated an initial (within hours) increase in ventilatory drive followed by a decrease in this response over the long-term (days–weeks) in response hypercapnia. Little is known about whether changes in the central respiratory chemoreflex are involved. Here we investigated whether central respiratory chemoreceptor neurons of the retrotrapezoid nucleus (RTN), which project to the respiratory pattern generator within the ventral respiratory column (VRC) have a role in the mechanism of neuroplasticity associated with long-term hypercapnia. Adult male C57BL/6 mice (n = 5/group) were used. Our aims were (1) to determine if galanin, neuromedin B and gastrin-releasing peptide gene expression is altered in the RTN after long-term hypercapnia. This was achieved using qPCR to measure mRNA expression changes of neuropeptides in the RTN after short-term hypercapnia (6 or 8 h, 5 or 8% CO₂) or long-term hypercapnia exposure (10 day, 5 or 8% CO₂), (2) in the mouse brainstem, to determine the distribution of preprogalanin in chemoreceptors, and the co-occurrence of the galanin receptor 1 (GalR1:Gi-coupled receptor) with inhibitory GlyT2 ventral respiratory column neurons using in situ hybridization (ISH) to better characterize galaninergic RTN-VRC circuitry, (3) to investigate whether long-term hypercapnia causes changes to recruitment (detected by cFos immunohistochemistry) of respiratory related neural populations including the RTN neurons and their galaninergic subset, in vivo. Collectively, we found that hypercapnia decreases neuropeptide expression in the RTN in the short-term and has the opposite effect over the long-term. Following long term hypercapnia, the number of RTN galanin neurons remains unchanged, and their responsiveness to acute chemoreflex is sustained; in contrast, we identified multiple respiratory related sites that exhibit blunted chemoreflex activation. GalR1 was distributed in 11% of preBötC and 30% of BötC glycinergic neurons. Our working hypothesis is that during long-term hypercapnia, galanin co-release from RTN neurons may counterbalance glutamatergic inputs to respiratory centers to downscale energetically wasteful hyperventilation, thereby having a role in neuroplasticity by contributing to a decrease in ventilation, through the inhibitory effects of galanin.

Characterisation of medullary astrocytic populations in respiratory nuclei and alterations in sudden unexpected death in epilepsy

Central failure of respiration during a seizure is one possible mechanism for sudden unexpected death in epilepsy (SUDEP). Neuroimaging studies indicate volume loss in the medulla in SUDEP and a post mortem study has shown reduction in neuromodulatory neuropeptidergic and monoaminergic neurones in medullary respiratory nuclear groups. Specialised glial cells identified in the medulla are considered essential for normal respiratory regulation including astrocytes with pacemaker properties in the pre-Botzinger complex and populations of subpial and perivascular astrocytes, sensitive to increased pCO_2, that excite respiratory neurones. Our aim was to explore niches of medullary astrocytes in SUDEP cases compared to controls. In 48 brainstems from three groups, SUDEP (20), epilepsy controls (10) and non-epilepsy controls (18), sections through the medulla were labelled for GFAP, vimentin and functional markers, astrocytic gap junction protein connexin43 (Cx43) and adenosine A1 receptor (A1R). Regions including the ventro-lateral medulla (VLM; for the pre-Bötzinger complex), Median Raphe (MR) and lateral medullary subpial layer (MSPL) were quantified using image analysis for glial cell populations and compared between groups. Findings included morphologically and regionally distinct vimentin/Cx34-positive glial cells in the VLM and MR in close proximity to neurones. We noted a reduction of vimentin-positive glia in the VLM and MSPL and Cx43 glia in the MR in SUDEP cases compared to control groups (p < 0.05-0.005). In addition, we identified vimentin, Cx43 and A1R positive glial cells in the MSPL region which likely correspond to chemosensory glia identified experimentally. In conclusion, altered medullary glial cell populations could contribute to impaired respiratory regulatory capacity and vulnerability to SUDEP and warrant further investigation.

The ventrolateral medulla and medullary raphe in sudden unexpected death in epilepsy

Sudden unexpected death in epilepsy (SUDEP) is a leading cause of premature death in patients with epilepsy. One hypothesis proposes that sudden death is mediated by post-ictal central respiratory depression, which could relate to underlying pathology in key respiratory nuclei and/or their neuromodulators. Our aim was to investigate neuronal populations in the ventrolateral medulla (which includes the putative human pre-Bötzinger complex) and the medullary raphe. Forty brainstems were studied comprising four groups: 14 SUDEP, six epilepsy controls, seven Dravet syndrome cases and 13 non-epilepsy controls. Serial sections through the medulla (from obex 1 to 10 mm) were stained for Nissl, somatostatin, neurokinin 1 receptor (for pre-Bötzinger complex neurons) and galanin, tryptophan hydroxylase and serotonin transporter (neuromodulatory systems). Using stereology total neuronal number and densities, with respect to obex level, were measured. Whole slide scanning image analysis was used to quantify immunolabelling indices as well as co-localization between markers. Significant findings included reduction in somatostatin neurons and neurokinin 1 receptor labelling in the ventrolateral medulla in sudden death in epilepsy compared to controls (P < 0.05). Galanin and tryptophan hydroxylase labelling was also reduced in sudden death cases and more significantly in the ventrolateral medulla region than the raphe (P < 0.005 and P < 0.05). With serotonin transporter, reduction in labelling in cases of sudden death in epilepsy was noted only in the raphe (P ≤ 0.01); however, co-localization with tryptophan hydroxylase was significantly reduced in the ventrolateral medulla. Epilepsy controls and cases with Dravet syndrome showed less significant alterations with differences from non-epilepsy controls noted only for somatostatin in the ventrolateral medulla (P < 0.05). Variations in labelling with respect to obex level were noted of potential relevance to the rostro-caudal organization of respiratory nuclear groups, including tryptophan hydroxylase, where the greatest statistical difference noted between all epilepsy cases and controls was at obex 9-10 mm (P = 0.034), the putative level of the pre-Bötzinger complex. Furthermore, there was evidence for variation with duration of epilepsy for somatostatin and neurokinin 1 receptor. Our findings suggest alteration to neuronal populations in the medulla in SUDEP with evidence for greater reduction in neuromodulatory neuropeptidergic and mono-aminergic systems, including for galanin, and serotonin. Other nuclei need to be investigated to evaluate if this is part of more widespread brainstem pathology. Our findings could be a result of previous seizures and may represent a pathological risk factor for SUDEP through impaired respiratory homeostasis during a seizure.

The Expression of Galanin in the Parafacial Respiratory Group and its Effects on Respiration in Neonatal Rats

The inhibitory peptide galanin is expressed within the retrotrapezoidal nucleus (RTN) - a key central chemoreceptor site that also contains the active expiratory oscillator. It was previously reported that microinjection of galanin into pre-Bötzinger complex - containing the inspiratory oscillator - exerts inhibitory effects on inspiratory motor output and respiratory rhythm. In neonatal rats, the present study aimed to investigate: (1) expression of galanin within the parafacial respiratory group (pFRG), which overlaps anatomically and functionally with the adult RTN, and; (2) effects of galanin on respiratory rhythm using the in vitro brainstem-spinal cord preparation. We showed that 14 ± 2% of Phox2b-immunoreactive (ir) neurons in the parafacial region were also galanin-ir. Galanin peptide expression was confirmed within 3/9 CO₂-sensitive, Phox2b-ir Pre-Inspiratory neurons (Pre-I) recorded in parafacial region. Bath application of galanin (0.1-0.2 µM): (1) decreased the duration of membrane depolarization in both Pre-I and inspiratory pFRG neurons, and; (2) decreased the number of C4 bursts that were associated with each burst in Pre-I neurons within the pFRG. In preparations showing episodic breathing at baseline, the respiratory patterning reverted to the 'normal' pattern of single, uniformly rhythmic C4 bursts (n = 10). In preparations with normal respiratory patterning at baseline, slowing of C4 rhythm (n = 7) resulted although rhythmic bursting in recorded Pre-I neurons remained unperturbed (n = 6). This study therefore demonstrates that galanin is expressed within the pFRG of neonatal rats, including neurons that are intrinsically chemosensitive. Overall the peptide has an inhibitory effect on inspiratory motor output, as previously shown in adults.

Neurochemistry of neurons in the ventrolateral medulla activated by hypotension: Are the same neurons activated by glucoprivation?

Previous studies have demonstrated that a range of stimuli activate neurons, including catecholaminergic neurons, in the ventrolateral medulla. Not all catecholaminergic neurons are activated and other neurochemical content is largely unknown hence whether stimulus specific populations exist is unclear. Here we determine the neurochemistry (using in situ hybridization) of catecholaminergic and noncatecholaminergic neurons which express c-Fos immunoreactivity throughout the rostrocaudal extent of the ventrolateral medulla, in Sprague Dawley rats treated with hydralazine or saline. Distinct neuronal populations containing PPCART, PPPACAP, and PPNPY mRNAs, which were largely catecholaminergic, were activated by hydralazine but not saline. Both catecholaminergic and noncatecholaminergic neurons containing preprotachykinin and prepro-enkephalin (PPE) mRNAs were also activated, with the noncatecholaminergic population located in the rostral C1 region. Few GlyT2 neurons were activated. A subset of these data was then used to compare the neuronal populations activated by 2-deoxyglucose evoked glucoprivation (Brain Structure and Function (2015) 220:117). Hydralazine activated more neurons than 2-deoxyglucose but similar numbers of catecholaminergic neurons. Commonly activated populations expressing PPNPY and PPE mRNAs were defined. These likely include PPNPY expressing catecholaminergic neurons projecting to vasopressinergic and corticotrophin releasing factor neurons in the paraventricular nucleus, which when activated result in elevated plasma vasopressin and corticosterone. Stimulus specific neurons included noncatecholaminergic neurons and a few PPE positive catecholaminergic neuron but neurochemical codes were largely unidentified. Reasons for the lack of identification of stimulus specific neurons, readily detectable using electrophysiology in anaesthetized preparations and for which neural circuits can be defined, are discussed.

Medullary mediation of the laryngeal adductor reflex: A possible role in sudden infant death syndrome

The laryngeal adductor reflex (LAR) is a laryngeal protective reflex. Vagal afferent polymodal sensory fibres that have cell bodies in the nodose ganglion, originate in the sub-glottal area of the larynx and upper trachea. These polymodal sensory fibres respond to mechanical or chemical stimuli. The central axons of these sensory vagal neurons terminate in the dorsolateral subnuclei of the tractus solitarius in the medulla oblongata. The LAR is a critical, reflex in the pathways that play a protective role in the process of ventilation, and the sychronisation of ventilation with other activities that are undertaken by the oropharyngeal systems including: eating, speaking and singing. Failure of the LAR to operate properly at any time after birth can lead to SIDS, pneumonia or death. Despite the critical nature of this reflex, very little is known about the central pathways and neurotransmitters involved in the management of the LAR and any disorders associated with its failure to act properly. Here, we review current knowledge concerning the medullary nuclei and neurochemicals involved in the LAR and propose a potential neural pathway that may facilitate future SIDS research.

Somatostatin 2a receptors are not expressed on functionally identified respiratory neurons in the ventral respiratory column of the rat

Microinjection of somatostatin (SST) causes site-specific effects on respiratory phase transition, frequency, and amplitude when microinjected into the ventrolateral medulla (VLM) of the anesthetized rat, suggesting selective expression of SST receptors on different functional classes of respiratory neurons. Of the six subtypes of SST receptor, somatostatin 2a (sst2a ) is the most prevalent in the VLM, and other investigators have suggested that glutamatergic neurons in the preBötzinger Complex (preBötC) that coexpress neurokinin-1 receptor (NK1R), SST, and sst2a are critical for the generation of respiratory rhythm. However, quantitative data describing the distribution of sst2a in respiratory compartments other than preBötC, or on functionally identified respiratory neurons, is absent. Here we examine the medullary expression of sst2a with particular reference to glycinergic/expiratory neurons in the Bötzinger Complex (BötC) and NK1R-immunoreactive/inspiratory neurons in the preBötC. We found robust sst2a expression at all rostrocaudal levels of the VLM, including a large proportion of catecholaminergic neurons, but no colocalization of sst2a and glycine transporter 2 mRNA in the BötC. In the preBötC 54% of sst2a -immunoreactive neurons were also positive for NK1R. sst2a was not observed in any of 52 dye-labeled respiratory interneurons, including seven BötC expiratory-decrementing and 11 preBötC preinspiratory neurons. We conclude that sst2a is not expressed on BötC respiratory neurons and that phasic respiratory activity is a poor predictor of sst2a expression in the preBötC. Therefore, sst2a is unlikely to underlie responses to BötC SST injection, and is sparse or absent on respiratory neurons identified by classical functional criteria. J. Comp. Neurol. 524:1384-1398, 2016. © 2015 Wiley Periodicals, Inc.

Defining inhibitory neurone function in respiratory circuits: opportunities with optogenetics?

Pharmacological and mathematical modelling studies support the view that synaptic inhibition in mammalian brainstem respiratory circuits is essential for generating normal and stable breathing movements. GABAergic and glycinergic neurones are known components of these circuits but their precise functional roles have not been established, especially within key microcircuits of the respiratory pre-Bötzinger (pre-BötC) and Bötzinger (BötC) complexes involved in phasic control of respiratory pump and airway muscles. Here, we review briefly current concepts of relevant complexities of inhibitory synapses and the importance of synaptic inhibition in the operation of these microcircuits. We highlight results and limitations of classical pharmacological studies that have suggested critical functions of synaptic inhibition. We then explore the potential opportunities for optogenetic strategies that represent a promising new approach for interrogating function of inhibitory circuits, including a hypothetical wish list for optogenetic approaches to allow expedient application of this technology. We conclude that recent technical advances in optogenetics should provide a means to understand the role of functionally select and regionally confined subsets of inhibitory neurones in key respiratory circuits such as those in the pre-BötC and BötC.

Intrathecal melanin-concentrating hormone reduces sympathetic tone and blocks cardiovascular reflexes

Melanin-concentrating hormone (MCH) is a neuropeptide that acts to increase feeding behavior and decrease energy expenditure. The role of MCH in central cardiorespiratory regulation is still poorly understood. Experiments were conducted on urethane-anesthetized, vagotomized, and artificially ventilated male Sprague-Dawley rats ( n = 22) to ascertain whether MCH modulates sympathetic vasomotor tone, as well as barosympathetic, chemosympathetic, and somatosympathetic reflexes at the level of the spinal cord. Intrathecal injection of 10 μl of MCH produced a dose-dependent hypotension, bradycardia, and sympathoinhibition. Peak response was observed following administration of 1 mM MCH, causing a decrease in mean arterial pressure of 39 ± 2 mmHg ( P < 0.001), splanchnic sympathetic nerve activity of 78 ± 11% ( P < 0.001), and heart rate of 87 ± 11 beats per minute (bpm) ( P < 0.01). The two peaks of the somatosympathetic reflex were decreased by intrathecal MCH, 7 ± 3% ( P < 0.01) and 31 ± 6% ( P < 0.01), respectively, and the spinal component of the reflex was accentuated 96 ± 23% ( P < 0.05), with respect to the baseline for MCH, compared with the two peaks and spinal component of the somatosympathetic reflex elicited following saline injection with respect to the baseline for saline. MCH decreased the sympathetic gain to 120 s of hyperoxic hypercapnea (10% CO2 in 90% O2) and to 10–12 s poikilocapneic anoxia (100% N2) from 0.74 ± 0.14%/s to 0.23 ± 0.04%/s ( P < 0.05) and 16.47 ± 3.2% to 4.35 ± 1.56% ( P < 0.05), respectively. There was a 34% decrease in gain and a 62% decrease in range of the sympathetic baroreflex with intrathecal MCH. These data demonstrate that spinal MCH blunts the central regulation of sympathetic tone and adaptive sympathetic reflexes.

Intrathecal neurotensin is hypotensive, sympathoinhibitory and enhances the baroreflex in anaesthetized rat

BACKGROUND AND PURPOSE

The neuromodulatory effects of the gut-neuropeptide neurotensin on sympathetic vasomotor tone, central respiratory drive and adaptive reflexes in the spinal cord, are not known.

EXPERIMENTAL APPROACH

Neurotensin (0.5 µM-3 mM) was administered into the intrathecal (i.t.) space at the sixth thoracic spinal cord segment in urethane-anaesthetized, paralysed, vagotomized male Sprague-Dawley rats. Pulsatile arterial pressure, splanchnic sympathetic nerve activity (sSNA), phrenic nerve activity, ECG and end-tidal CO(2) were recorded.

KEY RESULTS

Neurotensin caused a dose-related hypotension, sympathoinhibition and bradycardia. The maximum effects were observed at 3000 µM, where the decreases in mean arterial pressure (MAP), heart rate (HR) and sSNA reached -25 mmHg, -26 beats min(-1) and -26% from baseline, respectively. The sympathetic baroreflex was enhanced. Changes in central respiratory drive were characterized by a fall in the amplitude of the phrenic nerve activity. Finally, administration of SR 142948A (5 mM), a potent, selective antagonist at neurotensin receptors, caused a potent hypotension (-35 mmHg), bradycardia (-54 beats min(-1) ) and sympathoinhibition (-44%). A reduction in the amplitude and frequency of the phrenic nerve activity was also observed.

CONCLUSIONS AND IMPLICATIONS

The data demonstrate that neurotensin plays an important role in the regulation of spinal cardiovascular function, affecting both tone and adaptive reflexes.

Cells in the female retrotrapezoid region upregulate c-fos in response to 10%, but not 5%, carbon dioxide

The retrotrapezoid nucleus (RTN) is thought to regulate breathing in response to changes in blood carbon dioxide (CO(2)), and to make a vital contribution to respiratory drive, especially during sleep. However, cells in the female RTN fail to upregulate c-fos in response to low level CO(2) exposure, while cells in the male RTN have a robust upregulation of c-fos in response to low level CO(2) exposure. In this study, we examined the possibility that the female RTN has a higher threshold for c-fos upregulation in response to CO(2). Following exposure of Fos-Tau-LacZ (FTL) transgenic mice to 10% CO(2), c-fos was upregulated in just as many cells in the female as in the male RTN. In addition, the male RTN responded equivalently to 5% and 10% CO(2), consistent with a lack of a dose response to CO(2) in the male RTN. Cells in the nearby facial nucleus upregulated c-fos in the same number of cells regardless of sex or gas exposure, confirming that the sex difference in the RTN is unique to that nucleus. We propose that the male and female RTN upregulate c-fos differently in response to CO(2) due to differences in the transcriptional regulation by estrogens of genes that encode proteins related to neuronal excitability or specifically related to central chemoreception, such as potassium channels. These findings could have clinical relevance to sleep related breathing disorders that disproportionately affect males, including the sudden infant death syndrome and sleep apnea.