Role of the locus coeruleus catecholaminergic neurons in the chemosensory control of breathing in a Parkinson's disease model

doi:10.1016/j.expneurol.2017.04.006

Experimental Neurology

Volume 293, July 2017, Pages 172-180

https://doi.org/10.1016/j.expneurol.2017.04.006 Get rights and content

Highlights

•
Parkinson´s disease (PD) is associated with respiratory dysfunction.
•
a rat model of PD has a reduced chemosensory control of breathing compared with age-matched control rats.
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PD animals have more fos-activated neurons in the LC and a reduced number in the RTN neurons during high levels of CO₂.
•
selective depletion of LC neurons in PD animals further reduced the chemosensory control of breathing in conscious animals.

Abstract

A previous study has demonstrated that in the 6-hydroxydopamine (6-OHDA)-model of Parkinson's disease (PD) there is a reduction in the number of Phox2b neurons in the retrotrapezoid nucleus (RTN) and a decrease in the respiratory response to hypercapnia 40 days after PD-induction. The functional deficiency is restored 60 days after 6-OHDA injection and here we tested the hypothesis that the locus coeruleus (LC) could be a candidate to restore the breathing deficiency. Minute Ventilation (V_E) in response to hypercapnia (7% CO₂) was assessed one day before, and then 40 and 60 days after bilateral 6-OHDA (24 μg/μL) or vehicle injections into the LC in control or PD-induced male Wistar rats. Bilateral injections of 6-OHDA decreased catecholaminergic neurons by 86% and 83% in the substantia nigra pars compacta (SNpc) and LC, respectively. As already described, in animals with lesions to the SNpc (N = 6/group), the reduction in the ventilatory response to hypercapnia was restored 60 days after PD (1257 ± 81 vs. vehicle: 1185 ± 49 mL/kg/min). However, in animals with PD and lesion in the LC, the ventilation was blunted (674 ± 39 mL/kg/min). In another group of PD rats, we observed a reduction in the number of hypercapnia-induced-fos⁺ cells in the RTN region (40 days: 38 ± 3 and 60 days: 8.5 ± 0.9 vs. vehicle 78 ± 3 cells) and an increase in the LC (40 days: 46 ± 4 and 60 days: 94 ± 22 vs. vehicle 1 ± 1 cells). Our data suggest that LC catecholaminergic neurons can be a candidate structure mediating chemoreceptor function in a model of PD.

Introduction

Breathing is continually adjusted by particular mechanisms originating in the central nervous system (CNS) to maintain blood gas homeostasis in the body. The physiological process by which blood gases are detected, called chemoreception, depends on chemical sensors present in the carotid body (peripheral chemoreceptors) and within the CNS (central chemoreceptors) (Feldman et al., 2013, Guyenet, 2014, Loeschcke, 1982).

Detection of an increase in Pa_CO2 by central chemoreception in concert with peripheral chemoreceptors acts as a powerful feedback mechanism that maintains arterial Pa_CO2 within very narrow limits (Feldman et al., 2003, Guyenet and Bayliss, 2015). It is clear that central chemoreception depends on a few specialized cell clusters located within the brainstem (Guyenet and Bayliss, 2015, Guyenet et al., 2010, Nattie and Li, 2009). Only a few types of neurons, such as those in the retrotrapezoid nucleus (RTN), locus coeruleus (LC), medullary raphe, hypothalamic orexinergic neurons and nucleus of the solitary tract (NTS) are involved in central chemoreception (Biancardi et al., 2008, Dean et al., 1989, Deng et al., 2007, Kumar et al., 2015, Li et al., 2008, Mulkey et al., 2004, Nattie et al., 2004, Takakura et al., 2006, Williams et al., 2007).

A recent study from our laboratory showed that the loss of chemoreceptor neurons located in the RTN could contribute to breathing impairment in a Parkinson's disease (PD) model induced by 6-OHDA into the striatum (Tuppy et al., 2015). It was found that the hypercapnia ventilatory response was impaired at 40 and 50 days after PD induction, and the neuroanatomical degeneration was linked with respiratory disturbances. Interestingly, the hypercapnia ventilatory response was restored 60 days after PD induction, suggesting that another brain region could assume chemoreceptor function in this PD model (Tuppy et al., 2015).

Considering that the reduction in respiratory activity in the PD model is restored after 60 days, our hypothesis is that the LC will contribute to maintaining breathing activity during chemoreceptor challenge, such as hypoxia or hypercapnia. To test that possibility, we recorded breathing activity in vivo during exposure to hypoxia or hypercapnia in PD animals with selective depletion of catecholaminergic neurons in the LC. To determine if neurons in the LC region functionally express fos-immunoreactivity as a marker of cell activation, we also performed a set of immunohistochemical experiments to evaluate the number of catecholaminergic neurons in the LC activated by hypercapnia in control and PD model animals.

Section snippets

Animals

Experiments were performed in 52 adult male Wistar rats weighing between 250 and 300 g. The animals had free access to water and food and were housed in a temperature-controlled chamber at 24 °C, with a 12:12 h light/dark cycle. All experimental and surgical procedures conformed to the National Institutes of Health and were approved by the Institutional Animal Care and Use Committee at the University of São Paulo (protocol number: 65/2015).

Surgery and anesthesia: Injections of 6-OHDA

The injections of the 6-OHDA (6-hydroxydopamine

Effect of hypercapnia on fos expression by a population of chemosensitive neurons in a PD model

The first series of experiments was designed to test neuronal activation using the proto-oncogene product fos in different cell populations described as central chemoreceptors, such as RTN and LC (Biancardi et al., 2008, Kumar et al., 2015). Here we used a hypercapnia challenge (7% CO₂) to induce fos expression (Fortuna et al., 2009, Teppema et al., 1997) in control and PD model. Injections of the 6-OHDA neurotoxin is one of the most widely used models for PD in rats and mice (Gee et al., 2016

Discussion

The main findings of the present study are as follow: a) a rat model of PD has reduced chemosensory control of breathing compared with age-matched control rats; b) PD animals have more fos-activated neurons in the locus coeruleus (fos⁺/TH⁺) and a reduced number of RTN chemoreceptor neurons (fos⁺/TH⁻) during high levels of CO₂; and c) selective depletion of catecholaminergic neurons of the LC in PD animals further reduced the chemosensory control of breathing in unrestrained conscious animals.

Conclusion

Levels of CO₂ are tightly controlled by a combination of both central and peripheral chemoreceptors. This is a very sensitive control system to maintain stable breathing and pH homeostasis. The disruption of many individual sites can curtail, but not abolish, the sensitivity. Our study provides evidence that in PD there is a reduction in the ventilatory response to CO₂ due to the reduction in the number of RTN neurons; however, to maintain the sensitivity of the reflex, noradrenergic neurons in

Conflict of interest

No conflicts of interest, financial or otherwise, are declared by the author(s).

Author contributions

TSM and ACT designed the experiments; LMO, MT and ACT collected and analyzed data; LMO, TSM and ACT wrote the paper. All authors approved the final version of the manuscript.

Acknowledgements

We thank Fabiane Cristina Fernandes for expert technical assistance. Supported by public funding from São Paulo Research Foundation (FAPESP) (Grants: 2014/22406-1 and 2016/23281-3 to ACT; 2015/23376-1 to TSM), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) (grants: 471263/2013-3 to ACT) and by funds from FAPESP fellowship (2015/18842-3 to LMO) and CNPq fellowship (301904/2015-4 to TSM and 301219/2016-8 to ACT).

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Research PaperRole of the locus coeruleus catecholaminergic neurons in the chemosensory control of breathing in a Parkinson's disease model

Highlights

Abstract

Introduction

Section snippets

Animals

Surgery and anesthesia: Injections of 6-OHDA

Effect of hypercapnia on fos expression by a population of chemosensitive neurons in a PD model

Discussion

Conclusion

Conflict of interest

Author contributions

Acknowledgements

Respir. Physiol. Neurobiol.

Neuroscience

Respir. Physiol. Neurobiol.

Parkinsonism Relat. Disord.

Prog. Neurobiol.

Parkinsonism Relat. Disord.

Brain Res.

Exp. Neurol.

Neuron

Lancet

Respir. Med.

Neurobiol. Dis.

Neuroscience

Respir. Physiol. Neurobiol.

Neuroscience

Exp. Neurol.

Neuroscience

Respir. Physiol. Neurobiol.

Sleep Med.

Sleep disturbances in Parkinson's disease with emphasis on rapid eye movement sleep behavior disorder

Int. J. Neurosci.

Depletion of ventromedullary NK-1 receptor-immunoreactive neurons in multiple system atrophy

Brain

Nonmotor manifestations in Parkinson disease

Neurologist

Locus coeruleus noradrenergic neurons and CO2 drive to breathing

Pflugers Arch.

CO2 decreases membrane conductance and depolarizes neurons in the nucleus tractus solitarii

Exp. Brain Res.

Contribution of orexin in hypercapnic chemoreflex: evidence from genetic and pharmacological disruption and supplementation studies in mice

J. Appl. Physiol.

Chemosensory responses to CO2 in multiple brain stem nuclei determined using a voltage-sensitive dye in brain slices from rats

J. Neurophysiol.

Breathing: rhythmicity, plasticity, chemosensitivity

Annu. Rev. Neurosci.

Understanding the rhythm of breathing: so near, yet so far

Annu. Rev. Physiol.

Activation of the retrotrapezoid nucleus by posterior hypothalamic stimulation

J. Physiol.

Research Paper
Role of the locus coeruleus catecholaminergic neurons in the chemosensory control of breathing in a Parkinson's disease model

Locus coeruleus noradrenergic neurons and CO₂ drive to breathing

CO₂ decreases membrane conductance and depolarizes neurons in the nucleus tractus solitarii

Chemosensory responses to CO₂ in multiple brain stem nuclei determined using a voltage-sensitive dye in brain slices from rats