Zebrafish M2 muscarinic acetylcholine receptor: cloning, pharmacological characterization, expression patterns and roles in embryonic bradycardia.
1. A zebrafish M2 muscarinic acetylcholine receptor (mAChR) gene was cloned. It encodes 495 amino acids in a single exon. The derived amino acid sequence is 73.5% identical to its human homologue. 2. Competitive binding studies of the zebrafish M2 receptor and [(3)H]-NMS gave negative log dissociation constants (pK(i)) for each antagonist as follows: atropine (9.16)>himbacine (8.05)>/=4-DAMP (7.83)>AF-DX 116 (7.26)>/=pirenzepine (7.18)>/=tropicamide (6.97)>/=methoctramine (6.82)>/=p-F-HHSiD (6.67)>carbachol (5.20). The antagonist affinity profile correlated with the profile of the human M2 receptor, except for pirenzepine. 3. Reverse transcription polymerase chain reaction and Southern blotting analysis demonstrated that the M2 mAChR mRNA levels increased during the segmentation period (12 h post-fertilization; h.p.f.) in zebrafish. By whole-mount in situ hybridization, the M2 mAChR was first detectable in the heart, vagus motor ganglion, and vagus sensory ganglion at 30, 48 and 60 h.p.f., respectively. 4. The muscarinic receptor that mediates carbachol (CCh)-induced bradycardia was functionally mature at 72 h.p.f. The effect of CCh-induced bradycardia was antagonized by several muscarinic receptor antagonists with the order of potency (pIC(50) values): atropine (6.76)>methoctramine (6.47)>himbacine (6.10)>4-DAMP (5.72)>AF-DX 116 (4.77), however, not by pirenzepine, p-F-HHSiD, or tropicamide (<10 micro M). 5. The effect of CCh-induced bradycardia was abolished completely before 56 h.p.f. by M2 RNA interference, and the bradycardia effect gradually recovered after 72 h.p.f. The basal heart rate was increased in embryos injected with M2 mAChR morpholino antisense oligonucleotide (M2 MO) and the effect of CCh-induced bradycardia was abolished by M2 MO in a dose-dependent manner. In conclusion, the results suggest that the M2 mAChR inhibit basal heart rate in zebrafish embryo and the M2 mAChR mediates the CCh-induced bradycardia.
Hsieh
Dennis Jine-Yuan
DJ
Graduate Institue of Life Sciences, National Defense Medical Center, Taipei, Taiwan, Republic of China.
Liao
Ching-Fong
CF
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D001667:Binding, Competitive; D001919:Bradycardia / Q000139:chemically induced / Q000503:physiopathology; D002217:Carbachol; D003001:Cloning, Molecular; D004247:DNA / Q000737:chemistry / Q000235:genetics; D004305:Dose-Response Relationship, Drug; D004625:Embryo, Nonmammalian / Q000187:drug effects / Q000378:metabolism / Q000503:physiopathology*; D018507:Gene Expression Regulation, Developmental / Q000187:drug effects; D006339:Heart Rate / Q000187:drug effects / Q000502:physiology; D007739:L Cells; D051379:Mice; D008845:Microinjections; D008969:Molecular Sequence Data; D018727:Muscarinic Antagonists / Q000378:metabolism / Q000494:pharmacology; D016376:Oligonucleotides, Antisense / Q000494:pharmacology; D012330:RNA, Double-Stranded / Q000008:administration & dosage / Q000235:genetics / Q000502:physiology; D043585:Receptor, Muscarinic M2; D011976:Receptors, Muscarinic / Q000187:drug effects / Q000235:genetics* / Q000502:physiology; D016415:Sequence Alignment; D017422:Sequence Analysis, DNA; D017386:Sequence Homology, Amino Acid; D015027:Zebrafish / Q000196:embryology / Q000235:genetics* / Q000502:physiology
D018727:Muscarinic Antagonists; D016376:Oligonucleotides, Antisense; D012330:RNA, Double-Stranded; D043585:Receptor, Muscarinic M2; D011976:Receptors, Muscarinic; D002217:Carbachol; D004247:DNA