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Constitutive overexpression of muscarinic receptors leads to vagal hyperreactivity.

Livolsi A, Niederhoffer N, Dali-Youcef N, Mokni W, Olexa-Zorn C, Gies JP, Marcellin L, Feldman J, Bousquet P - PLoS ONE (2010)

Bottom Line: Sequencing of the coding fragment of the M(2) receptor gene revealed a single nucleotide mutation in 83% of hyperreactive animals, possibly contributing for the transcript overexpression.This phenomenon might represent a compensatory consequence of muscarinic receptors overexpression.The results suggest that cardiac muscarinic receptor overexpression plays a critical role in the development of vagal hyperreactivity, whereas AchE hyperactivity appears as a compensatory consequence of it.

View Article: PubMed Central - PubMed

Affiliation: Laboratoire de Neurobiologie et Pharmacologie Cardiovasculaire (EA 4438), Université de Strasbourg, Strasbourg, France.

ABSTRACT

Background: Alterations in muscarinic receptor expression and acetylcholinesterase (AchE) activity have been observed in tissues from Sudden Infant Death Syndrome (SIDS). Vagal overactivity has been proposed as a possible cause of SIDS as well as of vasovagal syncopes. The aim of the present study was to seek whether muscarinic receptor overexpression may be the underlying mechanism of vagal hyperreactivity. Rabbits with marked vagal pauses following injection of phenylephrine were selected and crossed to obtain a vagal hyperreactive strain. The density of cardiac muscarinic receptors and acetylcholinesterase (AchE) gene expression were assessed. Blood markers of the observed cardiac abnormalities were also sought.

Methodology/principal findings: Cardiac muscarinic M(2) and M(3) receptors were overexpressed in hyperreactive rabbits compared to control animals (2.3-fold and 2.5-fold, respectively) and the severity of the phenylephrine-induced bradycardia was correlated with their densities. A similar overexpression of M(2) receptors was observed in peripheral mononuclear white blood cells, suggesting that cardiac M(2) receptor expression can be inferred with high confidence from measurements in blood cells. Sequencing of the coding fragment of the M(2) receptor gene revealed a single nucleotide mutation in 83% of hyperreactive animals, possibly contributing for the transcript overexpression. Significant increases in AchE expression and activity were also assessed (AchE mRNA amplification ratio of 3.6 versus normal rabbits). This phenomenon might represent a compensatory consequence of muscarinic receptors overexpression. Alterations in M(2) receptor and AchE expression occurred between the 5th and the 7th week of age, a critical period also characterized by a higher mortality rate of hyperreactive rabbits (52% in H rabbits versus 13% in normal rabbits) and preceeded the appearance of functional disorders.

Conclusions/significance: The results suggest that cardiac muscarinic receptor overexpression plays a critical role in the development of vagal hyperreactivity, whereas AchE hyperactivity appears as a compensatory consequence of it. Since similar vagal disorders were observed recently by us in SIDS, muscarinic receptor overexpression could become a marker of risk of vasovagal syncopes and SIDS.

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Correlations between R-R interval and total, M2 and M3 muscarinic receptor density in hearts from normal (N) and vagal hyperreactive (H) rabbits.R-R intervals were measured in conscious rabbits challenged with PNE 500 µg kg−1 following the procedure described in Material and Methods. Total, M2 and M3 muscarinic receptor densities in hearts (Bmax; fmol mg−1 protein) were determined from Scatchard analysis of the saturation data using [3H]NMS, [3H]AF-DX 384 and [3H]4-DAMP, respectively, as radioligands. Binding conditions were as described in Material and Methods. Each symbol represents one animal. (a) Total muscarinic receptors; n = 9 H (full symbols) and 6 N (open symbols). (b) M2 muscarinic receptors; n = 10 H (full symbols) and 7 N (open symbols). (c) M3 muscarinic receptors; n = 10 H (full symbols) and 6 N (open symbols).
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pone-0015618-g001: Correlations between R-R interval and total, M2 and M3 muscarinic receptor density in hearts from normal (N) and vagal hyperreactive (H) rabbits.R-R intervals were measured in conscious rabbits challenged with PNE 500 µg kg−1 following the procedure described in Material and Methods. Total, M2 and M3 muscarinic receptor densities in hearts (Bmax; fmol mg−1 protein) were determined from Scatchard analysis of the saturation data using [3H]NMS, [3H]AF-DX 384 and [3H]4-DAMP, respectively, as radioligands. Binding conditions were as described in Material and Methods. Each symbol represents one animal. (a) Total muscarinic receptors; n = 9 H (full symbols) and 6 N (open symbols). (b) M2 muscarinic receptors; n = 10 H (full symbols) and 7 N (open symbols). (c) M3 muscarinic receptors; n = 10 H (full symbols) and 6 N (open symbols).

Mentions: The R-R interval duration was taken as a measure of the bradycardic response to PNE. In H rabbits used for [3H]NMS binding experiments, the R-R interval durations, after injection of PNE, ranged from 6500 to 15200 ms (mean 10644±2633) compared to N rabbits (1200 to 2800 ms; mean 1867±615). The 2.5- to 3-fold increase in the density of muscarinic receptors was accompanied by a 4.5 to 5-fold increase in R-R duration (Table 1). Within this group of animals, a highly significant positive correlation was found between the R-R interval and the total muscarinic receptor density (Fig. 1a); compared to the control group, all values for receptor density (and consequently for R-R interval) were shifted to higher levels.


Constitutive overexpression of muscarinic receptors leads to vagal hyperreactivity.

Livolsi A, Niederhoffer N, Dali-Youcef N, Mokni W, Olexa-Zorn C, Gies JP, Marcellin L, Feldman J, Bousquet P - PLoS ONE (2010)

Correlations between R-R interval and total, M2 and M3 muscarinic receptor density in hearts from normal (N) and vagal hyperreactive (H) rabbits.R-R intervals were measured in conscious rabbits challenged with PNE 500 µg kg−1 following the procedure described in Material and Methods. Total, M2 and M3 muscarinic receptor densities in hearts (Bmax; fmol mg−1 protein) were determined from Scatchard analysis of the saturation data using [3H]NMS, [3H]AF-DX 384 and [3H]4-DAMP, respectively, as radioligands. Binding conditions were as described in Material and Methods. Each symbol represents one animal. (a) Total muscarinic receptors; n = 9 H (full symbols) and 6 N (open symbols). (b) M2 muscarinic receptors; n = 10 H (full symbols) and 7 N (open symbols). (c) M3 muscarinic receptors; n = 10 H (full symbols) and 6 N (open symbols).
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3008725&req=5

pone-0015618-g001: Correlations between R-R interval and total, M2 and M3 muscarinic receptor density in hearts from normal (N) and vagal hyperreactive (H) rabbits.R-R intervals were measured in conscious rabbits challenged with PNE 500 µg kg−1 following the procedure described in Material and Methods. Total, M2 and M3 muscarinic receptor densities in hearts (Bmax; fmol mg−1 protein) were determined from Scatchard analysis of the saturation data using [3H]NMS, [3H]AF-DX 384 and [3H]4-DAMP, respectively, as radioligands. Binding conditions were as described in Material and Methods. Each symbol represents one animal. (a) Total muscarinic receptors; n = 9 H (full symbols) and 6 N (open symbols). (b) M2 muscarinic receptors; n = 10 H (full symbols) and 7 N (open symbols). (c) M3 muscarinic receptors; n = 10 H (full symbols) and 6 N (open symbols).
Mentions: The R-R interval duration was taken as a measure of the bradycardic response to PNE. In H rabbits used for [3H]NMS binding experiments, the R-R interval durations, after injection of PNE, ranged from 6500 to 15200 ms (mean 10644±2633) compared to N rabbits (1200 to 2800 ms; mean 1867±615). The 2.5- to 3-fold increase in the density of muscarinic receptors was accompanied by a 4.5 to 5-fold increase in R-R duration (Table 1). Within this group of animals, a highly significant positive correlation was found between the R-R interval and the total muscarinic receptor density (Fig. 1a); compared to the control group, all values for receptor density (and consequently for R-R interval) were shifted to higher levels.

Bottom Line: Sequencing of the coding fragment of the M(2) receptor gene revealed a single nucleotide mutation in 83% of hyperreactive animals, possibly contributing for the transcript overexpression.This phenomenon might represent a compensatory consequence of muscarinic receptors overexpression.The results suggest that cardiac muscarinic receptor overexpression plays a critical role in the development of vagal hyperreactivity, whereas AchE hyperactivity appears as a compensatory consequence of it.

View Article: PubMed Central - PubMed

Affiliation: Laboratoire de Neurobiologie et Pharmacologie Cardiovasculaire (EA 4438), Université de Strasbourg, Strasbourg, France.

ABSTRACT

Background: Alterations in muscarinic receptor expression and acetylcholinesterase (AchE) activity have been observed in tissues from Sudden Infant Death Syndrome (SIDS). Vagal overactivity has been proposed as a possible cause of SIDS as well as of vasovagal syncopes. The aim of the present study was to seek whether muscarinic receptor overexpression may be the underlying mechanism of vagal hyperreactivity. Rabbits with marked vagal pauses following injection of phenylephrine were selected and crossed to obtain a vagal hyperreactive strain. The density of cardiac muscarinic receptors and acetylcholinesterase (AchE) gene expression were assessed. Blood markers of the observed cardiac abnormalities were also sought.

Methodology/principal findings: Cardiac muscarinic M(2) and M(3) receptors were overexpressed in hyperreactive rabbits compared to control animals (2.3-fold and 2.5-fold, respectively) and the severity of the phenylephrine-induced bradycardia was correlated with their densities. A similar overexpression of M(2) receptors was observed in peripheral mononuclear white blood cells, suggesting that cardiac M(2) receptor expression can be inferred with high confidence from measurements in blood cells. Sequencing of the coding fragment of the M(2) receptor gene revealed a single nucleotide mutation in 83% of hyperreactive animals, possibly contributing for the transcript overexpression. Significant increases in AchE expression and activity were also assessed (AchE mRNA amplification ratio of 3.6 versus normal rabbits). This phenomenon might represent a compensatory consequence of muscarinic receptors overexpression. Alterations in M(2) receptor and AchE expression occurred between the 5th and the 7th week of age, a critical period also characterized by a higher mortality rate of hyperreactive rabbits (52% in H rabbits versus 13% in normal rabbits) and preceeded the appearance of functional disorders.

Conclusions/significance: The results suggest that cardiac muscarinic receptor overexpression plays a critical role in the development of vagal hyperreactivity, whereas AchE hyperactivity appears as a compensatory consequence of it. Since similar vagal disorders were observed recently by us in SIDS, muscarinic receptor overexpression could become a marker of risk of vasovagal syncopes and SIDS.

Show MeSH
Related in: MedlinePlus