**7. Muscarinic receptors in avian gastrointestinal system**

Characterization of muscarinic receptors in avian smooth muscles has been done by many researchers, especially in the ileum of chicken using functional and binding studies. Decades back, a contractile response to carbachol antagonized by atropine had been studied in the chick ileum [86]. In the functional studies, the affinities obtained exhibited good agreement with the existence of a functional M3 muscarinic receptor subtype in the ileum of chicken, however the findings were in direct

contrast to the binding experiments, in which the single binding site appeared to be the muscarinic M2 receptor subtype. Hence, even though the contractile response was mostly due to a functional M3 receptor subtype, analysis of the competition binding curves suggested the presence of a uniform population of the muscarinic M2 receptor subtype also even though it did not contribute to the contractile response [28].
