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Development of cholinergic nerve transmission in the chick oesophagus
H Miyazaki1, T Taneike, A Ohga
1Department of Veterinary Pharmacology, Faculty of Dairy Science, Rakuno Gakuen University, Ebetsu, Japan.
Insights
Cholinergic nerve stimulation and acetylcholine response in chick esophagus smooth muscle begin during incubation and mature progressively. Functional and biochemical receptor development precedes cholinergic innervation in developing chick esophagus.
Area of Science:
- Developmental Biology
- Neurogastroenterology
- Pharmacology
Background:
- Cholinergic mechanisms are crucial for smooth muscle function.
- Understanding the development of these mechanisms in the avian esophagus provides insights into neuromuscular development.
Purpose of the Study:
- To investigate the onset and maturation of cholinergic neurotransmission in chick esophagus smooth muscle.
- To correlate functional responses with biochemical changes in acetylcholine synthesis and degradation.
Main Methods:
- Electrophysiological recordings and mechanical responses to transmural and vagal nerve stimulation.
- Pharmacological characterization using atropine, physostigmine, and hexamethonium.
- Assays for choline acetyltransferase (ChAT) and cholinesterase activity.
- Measurement of contractile responses to acetylcholine and muscarinic receptor binding using [3H]-quinuclidinyl benzilate ([3H]-QNB).
Main Results:
- Nerve-evoked contractions initiated at 10-11 days of incubation, sensitive to atropine and physostigmine.
- Choline acetyltransferase activity increased with age, correlating with nerve-mediated contractions.
- Muscarinic receptor sensitivity increased up to 15 days of incubation, while receptor numbers remained stable.
Conclusions:
- Extrinsic and intrinsic cholinergic innervation of the chick esophagus becomes functional during embryonic development.
- Functional and biochemical maturation of muscarinic receptors precedes the full development of cholinergic innervation.
- This study elucidates the developmental timeline of cholinergic control in avian esophageal smooth muscle.
Abstract:
1. The onset and development of cholinergic mechanisms in the smooth muscle of the chick oesophagus were studied by estimating the changes in mechanical response and biochemical parameters between 9 days of incubation and 7 days after hatching. 2. Transmural and vagal nerve stimulation first evoked contraction in the oesophagus at 10 days and 11 days of incubation, respectively. These contractions were inhibited by atropine (1-2 microM) and potentiated by physostigmine (0.2 microM). On the other hand, hexamethonium (200 microM) had an inhibitory effect on vagal nerve stimulation but not on transmural nerve stimulation. 3. The relative amplitude of contraction induced by both vagal nerve and transmural stimulations compared to high K+ (80 mM)-induced contractions, progressively increased with age in embryos up to 19 days of incubation. 4. The activity of choline acetyltransferase (ChAT), an enzyme synthesizing acetylcholine (ACh), also gradually increased in the oesophagus during the period from 9 days to 19 days of incubation, which was similar to the change in the nerve-mediated contraction. On the other hand, the cholinesterase activity reached a maximum at 13 days of incubation and decreased until 7 days after hatching. 5. The contractile response to ACh and binding sites of [3H]-quinuclidinyl benzilate ([3H]-QNB) were observed in the oesophagus at 9 days of incubation. The maximum response produced by ACh (300 microM) tended to be greater in early stages (9-13 days of incubation) than in later stages. The sensitivity estimated from pD2 values increased up to 15 days of incubation. The maximum response produced by ACh (300 microM) tended to be greater in early stages (9-13 days of incubation) than in later stages. The sensitivity estimated from pD2 values increased up to 15 days of incubation. During the embryonic period, the number of muscarinic receptors estimated from the binding of [3H]-QNB changed very little. 6. These results suggest that in the chick oesophagus, extrinsic and intrinsic cholinergic innervation start to function at 10 days and 11 days of incubation, respectively and continue to develop progressively up to the time of hatching. It seems likely that the functional and biochemical maturation of receptive mechanisms on the smooth muscle precede those of cholinergic innervation.