Reflexes from the carotid body have been implicated in cardiorespiratory disorders associated with chronic intermittent hypoxia (CIH). To investigate whether CIH causes functional and͞or structural plasticity in the carotid body, rats were subjected to 10 days of recurrent hypoxia or normoxia. Acute exposures to 10 episodes of hypoxia evoked long-term facilitation (LTF) of carotid body sensory activity in CIH-conditioned but not in control animals. The magnitude of sensory LTF depended on the length of CIH conditioning and was completely reversible and unique to CIH, because conditioning with a comparable duration of sustained hypoxia was ineffective. Histological analysis revealed no differences in carotid body morphology between control and CIH animals. Previous treatment with superoxide anion (O 2•؊ ) scavenger prevented sensory LTF. In the CIH-conditioned animals, carotid body aconitase enzyme activity decreased compared with controls. These observations suggest that increased generation of reactive oxygen species contribute to sensory LTF. In CIH animals, carotid body complex I activity of the mitochondrial electron transport is inhibited, suggesting mitochondria as one source of O 2•؊ generation. These observations demonstrate that CIH induces a previously uncharacterized form of reactive oxygen species-dependent, reversible, functional plasticity in carotid body sensory activity. The sensory LTF may contribute to persistent reflex activation of sympathetic nerve activity and blood pressure in recurrent apnea patients experiencing CIH.H ypoxia is a pervasive physiological stimulus. Changes in arterial blood oxygen levels are monitored continuously by peripheral chemoreceptors, especially the carotid bodies (reviewed in ref. 1). The ensuing reflexes are critical for maintaining homeostasis during hypoxia. Hypoxia is encountered under many different circumstances. For instance, sojourn to high altitude exposes individuals to sustained hypoxia (SH). On the other hand, people living at sea level experience intermittent hypoxia more often in life than SH. Many pathophysiological situations including recurrent apnea syndromes (central or obstructive sleep apneas), apneas in premature infants, and asthmatic attacks are associated with intermittent hypoxia. Decreases in arterial blood oxygen occur in both types of hypoxia. However, cardiorespiratory systems adapt to chronic SH to maintain adequate oxygen delivery to tissues (reviewed in refs. 2 and 3), whereas chronic intermittent hypoxia (CIH) leads to serious pathophysiological consequences such as hypertension, myocardial infarcts, and even stroke (4, 5).Reflexes from the carotid body have been proposed to play a critical role in ventilatory and circulatory adaptations to chronic SH and are associated with structural changes in the carotid body including hypertrophy of the organ, increased vascularization, and increased number of glomus cells (the putative oxygensensing cells; reviewed in refs. 2 and 3). There is evidence that reflexes arising from the carotid ...

To investigate whether the transcriptional activator hypoxiainducible factor 1 (HIF-1) is required for ventilatory responses to hypoxia, we analyzed mice that were either wild type or heterozygous for a loss-of-function (knockout) allele at the Hif1a locus, which encodes the O 2-regulated HIF-1␣ subunit. Although the ventilatory response to acute hypoxia was not impaired in Hif1a ϩ/Ϫ mice, the response was primarily mediated via vagal afferents, whereas in wild-type mice, carotid body chemoreceptors played a predominant role. When carotid bodies isolated from wild-type mice were exposed to either cyanide or hypoxia, a marked increase in sinus nerve activity was recorded, whereas carotid bodies from Hif1a ϩ/Ϫ mice responded to cyanide but not to hypoxia. Histologic analysis revealed no abnormalities of carotid body morphology in Hif1a ϩ/Ϫ mice. Wild-type mice exposed to hypoxia for 3 days manifested an augmented ventilatory response to a subsequent acute hypoxic challenge. In contrast, prior chronic hypoxia resulted in a diminished ventilatory response to acute hypoxia in Hif1a ϩ/Ϫ mice. Thus partial HIF-1␣ deficiency has a dramatic effect on carotid body neural activity and ventilatory adaptation to chronic hypoxia.O xygen homeostasis is mediated by the combined physiologic functioning of the circulatory and respiratory systems. Recent studies have demonstrated that the transcriptional activator hypoxia-inducible factor 1 (HIF-1) is required for both the establishment of the circulatory system during embryonic development and physiologic responses in postnatal life. Analysis of Hif1a Ϫ/Ϫ knockout mice, which are homozygous for a lossof-function mutation in the Hif1a gene encoding the O 2 -regulated HIF-1␣ subunit, revealed that complete HIF-1␣ deficiency results in embryonic lethality at midgestation with major malformations of the heart and vasculature (1-3). Hif1a ϩ/Ϫ heterozygous mice, which are partially HIF-1␣ deficient, develop normally and are indistinguishable from their wild-type littermates under normoxic conditions. However, adult Hif1a ϩ/Ϫ mice manifest impaired physiological responses to chronic hypoxia including significantly reduced rates of erythropoiesis and pulmonary vascular remodeling (4). Thus HIF-1␣ plays essential roles in cardiac, erythroid, and vascular development and physiology, i.e., all three major components of the circulatory system.An essential adaptation to both acute and chronic hypoxia is an increase in ventilation that depends on the activity of peripheral chemoreceptors, particularly those within the carotid body, which detect changes in arterial blood O 2 concentration and relay sensory information to the brainstem neurons that regulate breathing (reviewed in ref. 5). Altered ventilatory responses to hypoxia may play a critical role in asthma, diabetes, Parkinson's disease, sleep-related breathing disorders, and sudden infant death syndrome (6-8). We hypothesized that HIF-1␣ is required for carotid body function and ventilatory adaptation to chronic hypoxia. To test this hyp...

Postnatal deficits in Brain-Derived Neurotrophic Factor (BDNF) are thought to contribute to pathogenesis of Rett syndrome (RTT), a progressive neurodevelopmental disorder caused by mutations in the gene encoding methyl-CpG-binding protein 2 (MeCP2). In Mecp2 null mice, a model of RTT, BDNF deficits are most pronounced in structures important for autonomic and respiratory control, functions that are severely affected in RTT patients. However, relatively little is known about how these deficits affect neuronal function or how they may be linked to specific RTT endophenotypes. To approach these issues we analyzed synaptic function in the brainstem nucleus tractus solitarius (nTS), the principal site for integration of primary visceral afferent inputs to central autonomic pathways and a region in which we found markedly reduced levels of BDNF in Mecp2 mutants. Our results demonstrate that the amplitude of spontaneous miniature and evoked EPSCs in nTS neurons is significantly increased in Mecp2 null mice and, accordingly, that mutant cells are more likely than wildtype to fire action potentials in response to primary afferent stimulation. These changes occur without any increase in intrinsic neuronal excitability and are unaffected by blockade of inhibitory GABA currents. However, this synaptopathy is associated with decreased BDNF availability in the primary afferent pathway and can be rescued by application of exogenous BDNF. On the basis of these findings we hypothesize that altered sensory gating in nTS contributes to cardiorespiratory instability in RTT and that nTS is a site at which restoration of normal BDNF signaling could help reestablish normal homeostatic controls.

The respiratory system is highly pliable in its adaptation to low-oxygen (hypoxic) environments. After chronic intermittent hypoxia (CIH), alterations in the regulation of cardiorespiratory system become persistent because of changes in the peripheral chemoreceptor reflex. We present evidence for the induction of a novel form of homeostatic plasticity in this reflex pathway in the nucleus tractus solitarius (NTS), the site of termination of the chemosensory afferent fibers. CIH induces an increase in NTS postsynaptic cell activity initiated by spontaneous presynaptic transmitter release that is counterbalanced by a reduction in evoked synaptic transmission between sensory afferents and NTS second-order cells. This is accomplished via presynaptic mechanisms involving changes in neurotransmitter release and calcium/calmodulin-dependent kinase II activation.

Peripheral chemoreceptor afferent information is sent to the nucleus tractus solitarii (nTS), integrated, and relayed to other brain regions to alter cardiorespiratory function. The nTS projects to the hypothalamic paraventricular nucleus (PVN), but activation and phenotype of these projections during chemoreflex stimulation is unknown. We hypothesized that activation of PVN-projecting nTS neurons occurs primarily at high intensities of hypoxia. We assessed ventilation and cardiovascular parameters in response to increasing severities of hypoxia. Retrograde tracers were used to label nTS PVN-projecting neurons and, in some rats, rostral ventrolateral medulla (RVLM)-projecting neurons. Immunohistochemistry was performed to identify nTS cells that were activated (Fos-immunoreactive, Fos-IR), catecholaminergic, and GABAergic following hypoxia. Conscious rats underwent 3 h normoxia ( n = 4, 21% O2) or acute hypoxia (12, 10, or 8% O2; n = 5 each). Hypoxia increased ventilation and the number of Fos-IR nTS cells (21%, 13 ± 2; 12%, 58 ± 4; 10%, 166 ± 22; 8%, 186 ± 6). Fos expression after 10% O2was similar whether arterial pressure was allowed to decrease (−13 ± 1 mmHg) or was held constant. The percentage of PVN-projecting cells activated was intensity dependent, but contrary to our hypothesis, PVN-projecting nTS cells exhibiting Fos-IR were found at all hypoxic intensities. Notably, at all intensities of hypoxia, ∼75% of the activated PVN-projecting nTS neurons were catecholaminergic. Compared with RVLM-projecting cells, a greater percentage of PVN-projecting nTS cells was activated by 10% O2. Data suggest that increasing hypoxic intensity activates nTS PVN-projecting cells, especially catecholaminergic, PVN-projecting neurons. The nTS to PVN catecholaminergic pathway may be critical even at lower levels of chemoreflex activation and more important to cardiorespiratory responses than previously considered.

1. The role of endogenous nitric oxide (NO) generated by neuronal nitric oxide synthase (NOS_1) in the control of respiration during hypoxia and hypercapnia was assessed using mutant mice deficient in NOS_1. 2. Experiments were performed on awake and anaesthetized mutant and wild-type control mice. Respiratory responses to varying levels of inspired oxygen (100, 21 and 12% Oµ) and carbon dioxide (3 and 5% COµ balanced oxygen) were analysed. In awake animals, respiration was monitored by body plethysmograph along with oxygen consumption (ýOµ), COµ production (ýCOµ) and body temperature. In anaesthetized, spontaneously breathing mice, integrated efferent phrenic nerve activity was monitored as an index of neural respiration along with arterial blood pressure and blood gases. Cyclic 3',5'-guanosine monophosphate (cGMP) levels in the brainstem were analysed by radioimmunoassay as an index of nitric oxide generation. 3. Unanaesthetized mutant mice exhibited greater respiratory responses during 21 and 12% Oµ than the wild-type controls. Respiratory responses were associated with significant decreases in oxygen consumption in both groups of mice, and the magnitude of change was greater in mutant than wild-type mice. Changes in COµ production and body temperature, however, were comparable between both groups of mice. 4. Similar augmentation of respiratory responses during hypoxia was also observed in anaesthetized mutant mice. In addition, five of the fourteen mutant mice displayed periodic oscillations in respiration (brief episodes of increases in respiratory rate and tidal phrenic nerve activity) while breathing 21 and 12% Oµ, but not during 100% Oµ. The time interval between the episodes decreased by reducing inspired oxygen from 21 to 12% Oµ. 5. Changes in arterial blood pressure and arterial blood gases were comparable at any given level of inspired oxygen between both groups of mice, indicating that changes in these variables do not account for the differences in the response to hypoxia. 6. Respiratory responses to brief hyperoxia (Dejours test) and to cyanide, a potent chemoreceptor stimulant, were more pronounced in mutant mice, suggesting augmented peripheral chemoreceptor sensitivity. 7. cGMP levels were elevated in the brainstem during 21 and 12% Oµ in wild-type but not in mutant mice, indicating decreased formation of nitric oxide in mutant mice. 8. The magnitude of respiratory responses to hypercapnia (3 and 5% COµ balanced oxygen) was comparable in both groups of mice in the awake and anaesthetized conditions. 9. These observations suggest that the hypoxic responses were selectively augmented in mutant mice deficient in NOS-1. Peripheral as well as central mechanisms contributed to the altered responses to hypoxia. These results support the idea that nitric oxide generated by NOS_1 is an important physiological modulator of respiration during hypoxia.

10.1152/jn.00224.2002. Dopamine (DA) modulates the cardiorespiratory reflex by peripheral and central mechanisms. The aim of this study was to examine the role of DA in synaptic transmission of the nucleus tractus solitarius (NTS), the major integration site for cardiopulmonary reflexes. To examine DA's role, we used whole cell, voltage-clamp recordings in a rat horizontal brain stem slice. Solitary tract stimulation evoked excitatory postsynaptic currents (EPSCs) that were reduced to 70 +/- 5% of control by DA (100 microM). The reduction in EPSCs by DA was accompanied by a decrease in the paired pulse depression ratio with little or no change in input resistance or EPSC decay, suggesting a presynaptic mechanism. The D1-like agonist SKF 38393 Br (30 microM) did not alter EPSC amplitude, whereas the D2-like agonist, quinpirole HCl (30 microM), depressed EPSCs to 73 +/- 4% of control. The D2-like receptor antagonist, sulpiride (20 microM), abolished DA modulation of EPSCs. Most importantly, sulpiride alone increased EPSCs to 131 +/- 10% of control, suggesting a tonic D2-like modulation of synaptic transmission in the NTS. Examination of spontaneous EPSCs revealed DA reversibly decreased the frequency of events from 9.4 +/- 2.2 to 6.2 +/- 1.4 Hz. Sulpiride, however, did not alter spontaneous events. Immunohistochemistry of NTS slices demonstrated that D2 receptors colocalized with synaptophysin and substance P, confirming a presynaptic distribution. D2 receptors also localized to cultured petrosal neurons, the soma of presynaptic afferent fibers. In the petrosal neurons, D2 was found in cells that were TH-immunopositive, suggesting they were chemoreceptor afferent fibers. These results demonstrate that DA tonically modulates synaptic activity between afferent sensory fibers and secondary relay neurons in the NTS via a presynaptic D2-like mechanism.

The nucleus tractus solitarius (nTS) of the brainstem receives sensory afferent inputs, processes that information, and sends projections to a variety of brain regions responsible for influencing autonomic and respiratory output. The nTS sends direct projections to the rostral ventrolateral medulla (RVLM), an area important for cardiorespiratory reflexes and homeostasis. Since the net reflex effect of nTS processing ultimately depends on the properties of output neurons, we determined the characteristics of these RVLM-projecting nTS neurons using electrophysiological and immunohistochemical techniques. RVLM-projecting nTS neurons were identified by retrograde tracers. Patch clamp analysis in the horizontal brainstem nTS slice demonstrated that RVLM-projecting nTS cells exhibit constant latency solitary tract evoked EPSCs, suggesting they receive strong monosynaptic contacts from visceral afferents. Three distinct patterns of action potential firing, associated with different underlying potassium currents, were observed in RVLM-projecting cells. Following activation of the chemoreflex in conscious animals by three hours of acute hypoxia, 11.2 ± 1.9% of the RVLM-projecting nTS neurons were activated, as indicated by positive Fos-immunoreactivity. Very few RVLM-projecting nTS cells were catecholaminergic. Taken together, these data suggest that RVLM projecting nTS neurons receive strong monosynaptic inputs from sensory afferents and a subpopulation participates in the chemoreflex pathway.