Postnatal loss of brainstem serotonin neurons compromises the ability of neonatal rats to survive episodic severe hypoxia

Department of Biomedical Sciences, University of Missouri-Columbia, Columbia, MO 65211, USA.
The Journal of Physiology (Impact Factor: 5.04). 09/2011; 589(Pt 21):5247-56. DOI: 10.1113/jphysiol.2011.214445
Source: PubMed


Pet-1(-/-) mice with a prenatal, genetically induced loss of 5-hydroxytryptamine (5-HT, serotonin) neurones are compromised in their ability to withstand episodic environmental anoxia via autoresuscitation. Given the prenatal role of 5-HT neurones in the development of neural networks, here we ask if a postnatal loss of 5-HT neurones also compromises autoresuscitation. We treated neonatal rat pups at postnatal day (P)2-3 with an intra-cisternal injection of 5,7-dihydroxytryptamine (5,7-DHT; ~40 μg; n = 8) to pharmacologically lesion the 5-HT system, or vehicle (control; n = 14). At P7-10 we exposed unanaesthetized treated and control pups to 15 episodes of environmental anoxia (97% N(2), 3% CO(2)). Medullary 5-HT content was reduced 80% by 5,7-DHT treatment (P < 0.001). Baseline ventilation (V(E)), metabolic rate (V(O(2))), ventilatory equivalent (V(E)/V(O(2))), heart rate (HR), heart rate variability (HRV) and arterial haemoglobin saturation (S(aO(2))) were no different in 5-HT-deficient pups compared to controls. However, only 25% of 5-HT-deficient pups survived all 15 episodes of environmental anoxia, compared to 79% of control littermates (P = 0.007). High mortality of 5,7-DHT-treated pups was associated with delayed onset of gasping (P < 0.001), delayed recovery of HR from hypoxic-induced bradycardia (P < 0.001), and delayed recovery of eupnoea from hypoxic-induced apnoea (P < 0.001). Treatment with 5,7-DHT affected neither the gasping pattern once initiated, nor HR, V(E)/V(O(2)) or S(aO(2)) during the intervening episodes of room air. A significant increase in HRV occurred in all animals with repeated exposure, and in 5-HT-deficient pups this increase occurred immediately prior to death. We conclude that a postnatal loss of brainstem 5-HT content compromises autoresuscitation in response to environmental anoxia. This report provides new evidence in rat pups that 5-HT neurones serve a physiological role in autoresuscitation. Our data may be relevant to understanding the aetiology of the sudden infant death syndrome (SIDS), in which there is medullary 5-HT deficiency and in some cases evidence of severe hypoxia and failed autoresuscitation.

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Available from: Kevin James Cummings
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    • "We have shown that neuroventilatory rhythmogenesis and burst pattern formation in the in situ perfused brainstem preparation do not depend on 5-HT neuron activity (Toppin et al. 2007) but that 5-HT activity is critical to central chemosensitivity (Corcoran et al. 2013). Rodents display perturbed ventilation when 5-HT function is altered genetically (Li and Nattie 2008; Hodges et al. 2008, 2009, 2011; Buchanan and Richerson 2010; Cummings et al. 2011b; Penatti et al. 2011a; Barrett et al. 2012) or disrupted by targeted pharmacalogical lesion (Nattie et al. 2004; Dias et al. 2007; Cummings et al. 2011a), by selective silencing (Ray et al. 2011), or by dietary tryptophan restriction (Penatti et al. 2011b). Intrinsically CO -stimulated 5-HT neurons occur in the brainstem raphé nuclei in vitro, and 5-HT neurons occur in highly vascular regions connected to major homeostatic integration and respiratory control centers (Wang et al. 2001; Severson et al. 2003; Ptak et al. 2009). "
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    ABSTRACT: Brainstem central chemoreceptors are critical to the hypercapnic ventilatory response, but their location and identity are poorly understood. When studied in vitro, serotonin synthesizing (5-HT) neurons within the rat medullary raphe are intrinsically stimulated by CO2/acidosis. The contributions of these neurons to central chemosensitivity in vivo, however, are controversial. Lacking is documentation of CO2-sensitive 5-HT neurons in intact experimental preparations and understanding of their spatial and proportional distribution. Here we test the hypothesis that 5-HT neurons in the rat medullary raphe are sensitive to arterial hypercapnia. We use extracellular recording and hypercapnic challenge of spontaneously active medullary raphe neurons in the unanesthetized in situ perfused decerebrate brainstem preparation to assess chemosensitivity of individual cells. Juxtacellular labeling of a subset of recorded neurons and subsequent immunohistochemistry for the 5-HT synthesizing enzyme tryptophan hydroxylase identify or exclude this neurotransmitter phenotype in electrophysiologically characterized chemosensitive and insensitive cells. We show that the medullary raphe houses a heterogeneous population, including chemosensitive and insensitive 5-HT neurons. Of 124 recorded cells, 16 cells were juxtacellularly filled, visualized, and immunohistochemically identified as 5-HT-synthesizing, based on TPH immunoreactivity (TPH-ir). 44% of 5-HT cells were CO2-stimulated (increased firing rate with hypercapnia), while 56% were unstimulated. Our results demonstrate that medullary raphe neurons are heterogeneous and clearly include a subset of 5-HT neurons that are excited by arterial hypercapnia. Together with data identifying intrinsically CO2-sensitive 5-HT neurons in vitro, these results support a role for such cells as central chemoreceptors in the intact system.
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    • "Interestingly, while Ppt1-KO mice do not show increases in norepinephrine and dopamine levels in brown adipose tissue, they show transient increases in serotonin tissue levels after 0.5 h of cold exposure; a change that was not observed in wild-type animals. Researchers have also shown that during acute cold exposure, serotonergic neurons have a role in thermogenesis possibly by increasing sympathetic nerve activity in brown adipose tissue [32], [33]. Taken together, our findings suggest that PPT1 deficiency might be associated with perturbations in serotonergic neurons and sympathetic nervous system functions which in turn might impair temperature regulations during cold exposure. "
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    ABSTRACT: Infantile neuronal ceroid lipofuscinosis (INCL) is a fatal neurodegenerative disorder caused by a deficiency of palmitoyl-protein thioesterase-1 (PPT1). We have previously shown that children with INCL have increased risk of hypothermia during anesthesia and that PPT1-deficiency in mice is associated with disruption of adaptive energy metabolism, downregulation of peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α), and mitochondrial dysfunction. Here we hypothesized that Ppt1-knockout mice, a well-studied model of INCL that shows many of the neurologic manifestations of the disease, would recapitulate the thermoregulation impairment observed in children with INCL. We also hypothesized that when exposed to cold, Ppt1-knockout mice would be unable to maintain body temperature as in mice thermogenesis requires upregulation of Pgc-1α and uncoupling protein 1 (Ucp-1) in brown adipose tissue. We found that the Ppt1-KO mice had lower basal body temperature as they aged and developed hypothermia during cold exposure. Surprisingly, this inability to maintain body temperature during cold exposure in Ppt1-KO mice was associated with an adequate upregulation of Pgc-1α and Ucp-1 but with lower levels of sympathetic neurotransmitters in brown adipose tissue. In addition, during baseline conditions, brown adipose tissue of Ppt1-KO mice had less vacuolization (lipid droplets) compared to wild-type animals. After cold stress, wild-type animals had significant decreases whereas Ppt1-KO had insignificant changes in lipid droplets compared with baseline measurements, thus suggesting that Ppt1-KO had less lipolysis in response to cold stress. These results uncover a previously unknown phenotype associated with PPT1 deficiency, that of altered thermoregulation, which is associated with impaired lipolysis and neurotransmitter release to brown adipose tissue during cold exposure. These findings suggest that INCL should be added to the list of neurodegenerative diseases that are linked to alterations in peripheral metabolic processes. In addition, extrapolating these findings clinically, impaired thermoregulation and hypothermia are potential risks in patients with INCL.
    Full-text · Article · Nov 2012 · PLoS ONE
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    • "Another mouse line, Lmx1bf/f/p, in which nearly all serotonergic neurons are genetically deleted, also exhibits compromised autonomic functions, shows high mortality [5] and lacks arousal response to inhalation of CO2 [6]. In addition, pharmacological lesion of serotonergic neurons in neonatal rat pups, which reduced serotonin content by ∼ 80%, increased their mortality in response to repeated environmental anoxia, when tested at P7-10, suggesting a physiological role of serotonergic neurons in autoresuscitation [7]. "
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    ABSTRACT: Serotonergic system participates in a wide range of physiological processes and behaviors, but its role is generally considered as modulatory and noncrucial, especially concerning life-sustaining functions. We recently created a transgenic mouse line in which a functional deficit in serotonin homeostasis due to excessive serotonin autoinhibition was produced by inducing serotonin 1A receptor (Htr1a) overexpression selectively in serotonergic neurons (Htr1a raphe-overexpressing or Htr1a(RO) mice). Htr1a(RO) mice exhibit episodes of autonomic dysregulation, cardiovascular crises and death, resembling those of sudden infant death syndrome (SIDS) and revealing a life-supporting role of serotonergic system in autonomic control. Since midbrain serotonergic neurons are chemosensitive and are implicated in arousal we hypothesized that their chemosensitivity might be impaired in Htr1a(RO) mice. Loose-seal cell-attached recordings in brainstem slices revealed that serotonergic neurons in dorsal raphe nucleus of Htr1a(RO) mice have dramatically reduced responses to hypercapnic challenge as compared with control littermates. In control mice, application of 9% CO(2) produced an increase in firing rate of serotonergic neurons (0.260±0.041 Hz, n = 20, p = 0.0001) and application of 3% CO(2) decreased their firing rate (-0.142±0.025 Hz, n = 17, p = 0.0008). In contrast, in Htr1a(RO) mice, firing rate of serotonergic neurons was not significantly changed by 9% CO(2) (0.021±0.034 Hz, n = 16, p = 0.49) and by 3% CO(2) (0.012±0.046 Hz, n = 12, p = 0.97). Our findings support the hypothesis that chemosensitivity of midbrain serotonergic neurons provides a physiological mechanism for arousal responses to life-threatening episodes of hypercapnia and that functional impairment, such as excessive autoinhibition, of midbrain serotonergic neuron responses to hypercapnia may contribute to sudden death.
    Full-text · Article · Sep 2012 · PLoS ONE
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