PACAP-PAC1 Receptor Activation Is Necessary for the Sympathetic Response to Acute Intermittent Hypoxia

Serotonin (5-HT) is a key regulator of spinal respiratory motor plasticity. For example, spinal 5-HT receptor activation is necessary for the induction of phrenic long-term facilitation (pLTF), a form of respiratory motor plasticity triggered by moderate acute intermittent hypoxia (mAIH). mAIH-induced pLTF is blocked by cervical spinal application of the broad-spectrum 5-HT-receptor antagonist, methysergide. However, methysergide does not allow distinctions between the relative contributions of different 5-HT receptor subtypes. Intravenous administration of the Gq protein-coupled 5-HT2A/2C receptor antagonist ketanserin blocks mAIH-induced pLTF when administered before, but not after, mAIH; thus, 5-HT2 receptor activation is necessary for the induction but not maintenance of mAIH-induced pLTF. However, systemic ketanserin administration does not identify the site of the relevant 5-HT2A/2C receptors. Furthermore, this approach does not differentiate between the roles of 5-HT2A versus 5-HT2C receptors, nor does it preclude involvement of other Gq protein-coupled metabotropic 5-HT receptors capable of eliciting long-lasting phrenic motor facilitation, such as 5-HT2B receptors. Here we tested the hypothesis that mAIH-induced pLTF requires cervical spinal 5-HT2 receptor activation and determined which 5-HT2 receptor subtypes are involved. Anesthetized, paralyzed, and ventilated adult male Sprague Dawley rats were pretreated intrathecally with cervical (~C3-C5) spinal injections of subtype selective 5-HT2A/2C, 5-HT2B, or 5-HT2C receptor antagonists before mAIH. Whereas cervical spinal 5-HT2C receptor inhibition had no impact on mAIH-induced pLTF, pLTF was no longer observed after pretreatment with either 5-HT2A/2C or 5-HT2B receptor antagonists. Furthermore, spinal pretreatment with an MEK/ERK MAPK inhibitor blocked phrenic motor facilitation elicited by intrathecal injections of 5-HT2A but not 5-HT2B receptor agonists. Thus, mAIH-induced pLTF requires concurrent cervical spinal activation of both 5-HT2A and 5-HT2B receptors. However, these distinct receptor subtypes contribute to phrenic motor facilitation via distinct downstream signaling cascades that differ in their requirement for ERK MAPK signaling. The demonstration that both 5-HT2A and 5-HT2B receptors make unique contributions to mAIH-induced pLTF advances our understanding of mechanisms that underlie 5-HT-induced phrenic motor plasticity. NEW & NOTEWORTHY Moderate acute intermittent hypoxia (mAIH) triggers a persistent enhancement in phrenic motor output, an effect termed phrenic long-term facilitation (pLTF). mAIH-induced pLTF is blocked by cervical spinal application of the broad-spectrum serotonin (5-HT) receptor antagonist methysergide, demonstrating the need for spinal 5-HT receptor activation. However, the exact type of 5-HT receptors required for initiation of pLTF remains unknown. To the best of our knowledge, the present study is the first to demonstrate that 1) spinal coactivation of two distinct Gq protein-coupled 5-HT2 receptor subtypes is necessary for mAIH-induced pLTF, and 2) these receptors contribute to pLTF via cascades that differ in their requirement for ERK MAPK signaling.

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Chronic intermittent hypoxia alters NMDA and AMPA-evoked currents in NTS neurons receiving carotid body chemoreceptor inputs

AJP Regulatory Integrative and Comparative Physiology ◽

10.1152/ajpregu.00760.2006 ◽

2007 ◽

Vol 292 (6) ◽

pp. R2259-R2265 ◽

Cited By ~ 48

Author(s):

Patricia M. de Paula ◽

Gleb Tolstykh ◽

Steve Mifflin

Keyword(s):

Sleep Apnea ◽

Nmda Receptors ◽

Dose Response ◽

Carotid Body ◽

Intermittent Hypoxia ◽

Receptor Activation ◽

Response Curves ◽

Ampa And Nmda Receptors ◽

Afferent Inputs ◽

Dose Response Curves

Chronic exposure to intermittent hypoxia (CIH) has been used in animals to mimic the arterial hypoxemia that accompanies sleep apnea. Humans with sleep apnea and animals exposed to CIH have elevated blood pressures and augmented sympathetic nervous system responses to acute exposures to hypoxia. To test the hypothesis that exposure to CIH alters neurons within the nucleus of the solitary tract (NTS) that integrate arterial chemoreceptor afferent inputs, we measured whole cell currents induced by activation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and N-methyl-d-aspartate (NMDA) receptors in enzymatically dispersed NTS neurons from normoxic (NORM) and CIH-exposed rats (alternating cycles of 3 min at 10% O2 followed by 3 min at 21% O2 between 8 AM and 4 PM for 7 days). To identify NTS neurons receiving carotid body afferent inputs the anterograde tracer 4- (4-(dihexadecylamino)styryl- N-methylpyridinum iodide (DiA) was placed onto the carotid body 1 wk before exposure to CIH. AMPA dose-response curves had similar EC50 but maximal responses increased in neurons isolated from DiA-labeled CIH (20.1 ± 0.8 μM, n = 9) compared with NORM (6.0 ± 0.3 μM, n = 8) rats. NMDA dose-response curves also had similar EC50 but maximal responses decreased in CIH (8.4 ± 0.4 μM, n = 8) compared with NORM (19.4 ± 0.6 μM, n = 9) rats. These results suggest reciprocal changes in the number and/or conductance characteristics of AMPA and NMDA receptors. Enhanced responses to AMPA receptor activation could contribute to enhanced chemoreflex responses observed in animals exposed to CIH and humans with sleep apnea.

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PAC1 Receptor Activation by PACAP-38 Mediates Ca2+ Release from a cAMP-dependent Pool in Human Fetal Adrenal Gland Chromaffin Cells

Journal of Biological Chemistry ◽

10.1074/jbc.m206470200 ◽

2002 ◽

Vol 278 (3) ◽

pp. 1663-1670 ◽

Cited By ~ 32

Author(s):

Marcel D. Payet ◽

Lyne Bilodeau ◽

Lyne Breault ◽

Alain Fournier ◽

Laurent Yon ◽

...

Keyword(s):

Adrenal Gland ◽

Chromaffin Cells ◽

Receptor Activation ◽

Pac1 Receptor

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Adrenergic α1 receptor activation is sufficient, but not necessary for phrenic long-term facilitation

Journal of Applied Physiology ◽

10.1152/japplphysiol.00904.2013 ◽

2014 ◽

Vol 116 (11) ◽

pp. 1345-1352 ◽

Cited By ~ 19

Author(s):

A. G. Huxtable ◽

P. M. MacFarlane ◽

S. Vinit ◽

N. L. Nichols ◽

E. A. Dale ◽

...

Keyword(s):

Motor Neuron ◽

Receptor Antagonist ◽

Adrenergic Receptors ◽

Intermittent Hypoxia ◽

Receptor Agonist ◽

Receptor Activation ◽

Motor Nucleus ◽

Long Term Facilitation ◽

Dose Dependent

Acute intermittent hypoxia (AIH; three 5-min hypoxic episodes) causes a form of phrenic motor facilitation (pMF) known as phrenic long-term facilitation (pLTF); pLTF is initiated by spinal activation of Gq protein-coupled 5-HT2 receptors. Because α1 adrenergic receptors are expressed in the phrenic motor nucleus and are also Gq protein-coupled, we hypothesized that α1 receptors are sufficient, but not necessary for AIH-induced pLTF. In anesthetized, paralyzed, and ventilated rats, episodic spinal application of the α1 receptor agonist phenylephrine (PE) elicited dose-dependent pMF (10 and 100 μM, P < 0.05; but not 1 μM). PE-induced pMF was blocked by the α1 receptor antagonist prazosin (1 mM; −20 ± 20% at 60 min, −5 ± 21% at 90 min; n = 6). Although α1 receptor activation is sufficient to induce pMF, it was not necessary for AIH-induced pLTF because intrathecal prazosin (1 mM) did not alter AIH-induced pLTF (56 ± 9% at 60 min, 78 ± 12% at 90 min; n = 9). Intravenous (iv) prazosin (150 μg/kg) appeared to reduce pLTF (21 ± 9% at 60 min, 26 ± 8% at 90 min), but this effect was not significant. Hypoglossal long-term facilitation was unaffected by intrathecal prazosin, but was blocked by iv prazosin (−4 ± 14% at 60 min, −13 ± 18% at 90 min), suggesting different LTF mechanisms in different motor neuron pools. In conclusion, Gq protein-coupled α1 adrenergic receptors evoke pMF, but they are not necessary for AIH-induced pLTF.

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PACAP causes long‐term increases in sympathetic nerve activity and is necessary for the sympathetic response to acute intermittent hypoxia

The FASEB Journal ◽

10.1096/fasebj.26.1_supplement.891.6 ◽

2012 ◽

Vol 26 (S1) ◽

Author(s):

Melissa Mary-Jean Farnham ◽

Paul M Pilowsky

Keyword(s):

Sympathetic Nerve ◽

Intermittent Hypoxia ◽

Sympathetic Nerve Activity ◽

Nerve Activity ◽

Sympathetic Response

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Doxycycline and Minocycline Act as Positive Allosteric Modulators of the PAC1 Receptor and Induce Plasminogen Activators in RT4 Schwann Cells

Applied Sciences ◽

10.3390/app11167673 ◽

2021 ◽

Vol 11 (16) ◽

pp. 7673

Author(s):

Sarah Thomas Broome ◽

Giuseppe Musumeci ◽

Alessandro Castorina

Keyword(s):

Plasminogen Activator ◽

Schwann Cells ◽

Receptor Activation ◽

Allosteric Modulators ◽

Pac1 Receptor ◽

Western Blots ◽

Tissue Plasminogen ◽

Positive Allosteric Modulators ◽

Extracellular Environment ◽

Expression And Activity

Regeneration of peripheral nerves depends on the ability of axons to navigate through an altered extracellular environment. It has been suggested that Schwann cells facilitate this process through their secretion of neuropeptides and proteases. Using the RT4-D6P2T Schwann cell line (RT4), we have previously shown that RT4 cultures endogenously express the neuropeptide PACAP, and respond to exogenous stimulation by inducing the expression of tissue plasminogen activator (tPA) and urokinase plasminogen activator (uPA) via PAC1 receptor activation. In this study, based on recent findings showing that doxycycline and minocycline act as positive allosteric modulators (PAMs) of the PAC1 receptor, we tested if treatment with these tetracyclines induced the expression and activity of tPA and uPA in RT4 cells. Using ELISA and zymographic analyses, we demonstrate that doxycycline and minocycline reliably induce the secretion and activity of both tPA and uPA, which is paralleled by increased expression levels, as shown by immunocytochemistry and Western blots. These actions were mediated, at least in part, by the PAC1 receptor, as PACAP6-38 mitigated tetracycline-induced expression and activity of tPA and uPA. We conclude that doxycycline and minocycline act as PAMs of the PAC1 receptor to promote proteolytic activity in RT4 cells.

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Orexin A excites the rat olivary pretectal nucleus via OX2 receptor in a daily manner

10.1101/2021.05.11.443625 ◽

2021 ◽

Author(s):

Lukasz Chrobok ◽

Anna Alwani ◽

Kamil Pradel ◽

Jasmin Daniela Klich ◽

Marian Henryk Lewandowski

Keyword(s):

Environmental Changes ◽

Ex Vivo ◽

Electrode Array ◽

Receptor Activation ◽

Pac1 Receptor ◽

Orexin A ◽

Brain Site ◽

Array Technology ◽

Pretectal Nucleus ◽

Living Organisms

Pronounced environmental changes between the day and night forced living organisms to evolve specialised mechanisms organising their daily physiology, named circadian clocks. Currently, it has become clear that the master clock in the suprachiasmatic nuclei of the hypothalamus is not an exclusive brain site to generate daily rhythms. Indeed, several brain areas, including the subcortical visual system have been recently shown to change their neuronal activity across the daily cycle. Here we focus our investigation on the olivary pretectal nucleus (OPN) - a retinorecipient structure primarily involved in the pupillary light reflex. Using the multi-electrode array technology ex vivo we provide evidence for OPN neurons to elevate their firing during the behaviourally quiescent light phase. Additionally, we report the robust sensitivity to orexin A via the identified OX2 receptor in this pretectal centre, with higher responsiveness noted during the night. Interestingly, we likewise report a daily variation in the response to PAC1 receptor activation, with implications for the convergence of orexinergic and visual input on the same OPN neurons. Altogether, our report is first to suggest a daily modulation of the OPN activity via intrinsic and extrinsic mechanisms, organising its temporal physiology.

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