Role of 5-HT2A, 5-HT4 and 5-HT7 receptors in the antigen-induced airway hyperresponsiveness in guinea-pigs.
BACKGROUND
A possible role of 5-hydroxytryptamine (5-HT) in the origin of antigen-induced airway hyperresponsiveness (AI-AHR) has been scarcely investigated.


OBJECTIVE
To explore the participation of different 5-HT receptors in the development of AI-AHR in guinea-pigs.


METHODS
Lung resistance was measured in anaesthetized guinea-pigs sensitized to ovalbumin (OVA). Dose-response curves to intravenous (i.v.) acetylcholine (ACh) were performed before and 1 h after antigenic challenge and expressed as the 200% provocative dose (PD(200)). Organ bath experiments, confocal microscopy and RT-PCR were additionally used. The 5-HT content in lung homogenates was measured by HPLC.


RESULTS
Antigenic challenge significantly decreased PD(200), indicating the development of AI-AHR. This hyperresponsiveness was abolished by a combination of methiothepin (5-HT(1)/5-HT(2)/5-HT(5)/5-HT(6)/5-HT(7) receptors antagonist) and tropisetron (5-HT(3)/5-HT(4) antagonist). Other 5-HT receptor antagonists showed three different patterns of response. Firstly, WAY100135 (5-HT(1A) antagonist) and ondansetron (5-HT(3) antagonist) did not modify the AI-AHR. Secondly, SB269970 (5-HT(7) antagonist), GR113808 (5-HT(4) antagonist), tropisetron or methiothepin abolished the AI-AHR. Thirdly, ketanserin (5-HT(2A) antagonist) produced airway hyporresponsiveness. Animals with bilateral vagotomy did not develop AI-AHR. Experiments in tracheal rings showed that pre-incubation with LP44 or cisapride (agonists of 5-HT(7) and 5-HT(4) receptors, respectively) induced a significant increase of the cholinergic contractile response to the electrical field stimulation. In sensitized lung parenchyma strips, ketanserin diminished the contractile responses to ACh. Sensitization was associated with a ninefold increase in the 5-HT content of lung homogenates. Confocal microscopy showed that sensitization enhanced the immunolabelling and co-localization of nicotinic receptor and 5-HT in airway epithelium, probably located in pulmonary neuroendocrine cells (PNECs). RT-PCR demonstrated that neither sensitization nor antigen challenge modified the 5-HT(2A) receptor mRNA levels.


CONCLUSIONS
Our results suggested that 5-HT was involved in the development of AI-AHR to ACh in guinea-pigs. Specifically, 5-HT(2A), 5-HT(4) and 5-HT(7) receptors seem to be particularly involved in this phenomenon. Participation of 5-HT might probably be favoured by the enhancement of the PNECs 5-HT content observed after sensitization.
Segura
P
P
Departamento de Investigación en Hiperreactividad Bronquial, Instituto Nacional de Enfermedades Respiratorias, México DF, México.
Vargas
M H
MH
Córdoba-Rodríguez
G
G
Chávez
J
J
Arreola
J L
JL
Campos-Bedolla
P
P
Ruiz
V
V
García-Hernández
L M
LM
Méndez
C
C
Montaño
L M
LM
D000818:Animals; D016535:Bronchial Hyperreactivity / Q000139:chemically induced / Q000378:metabolism / Q000503:physiopathology*; D002851:Chromatography, High Pressure Liquid; D004195:Disease Models, Animal; D004305:Dose-Response Relationship, Drug; D006168:Guinea Pigs; D008297:Male; D010047:Ovalbumin / Q000276:immunology; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D044402:Receptor, Serotonin, 5-HT2A / Q000235:genetics / Q000378:metabolism*; D011985:Receptors, Serotonin / Q000235:genetics / Q000378:metabolism*; D044422:Receptors, Serotonin, 5-HT4 / Q000235:genetics / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012680:Sensitivity and Specificity; D012701:Serotonin / Q000032:analysis / Q000378:metabolism; D058830:Serotonin 5-HT2 Receptor Antagonists; D058832:Serotonin 5-HT4 Receptor Antagonists
D012333:RNA, Messenger; D044402:Receptor, Serotonin, 5-HT2A; D011985:Receptors, Serotonin; D058830:Serotonin 5-HT2 Receptor Antagonists; D058832:Serotonin 5-HT4 Receptor Antagonists; C104425:serotonin 7 receptor; D044422:Receptors, Serotonin, 5-HT4; D012701:Serotonin; D010047:Ovalbumin