**8. Muscarinic receptors in avian cardiovascular system**

As mentioned above, initially the only available data on chicken muscarinic receptors was that of the M2 muscarinic receptor subtype. But many ex-vivo experiments have given proof for the existence of avian correlates in atria of chick for muscarinic M2 receptor [77], M3 receptors in atrium and ventricles [23] and M5 receptor subtype in embryonic chick heart and brain [24]. The potent muscarinic cholinergic antagonist 3 -quinuclidinyl benzylate (QNB) has been used to detect and quantify muscarinic receptors as early as day 3 *in ovo* in the developing chick heart [87]. The study found an exponential increase in 3 -quinuclidinyl benzylate (QNB) binding sites, which reduced at day 18 *in ovo.* However, the receptor density and subtype were not investigated in the above study.

Another study determined the effect of exposure of cardiomyocytes from chicken embryos for 3 days to the beta adrenoceptor agonist, isoproterenol. In the study, the 3 days exposure induced an increase in the level of muscarinic acetylcholine receptors by about 30% in chicken cardiomyocytes [88].

Another study found out the inotropic response of muscarinic acetyl choline receptors to the stimulation of isolated chick ventricular myocardium at various developmental stages. The study also pharmacologically characterized the receptor subtype involved in embryonic chick ventricles. Carbachol produced positive inotropy in embryonic chick ventricles at micromolar concentrations, whereas in hatched 1–3-day old chick ventricles it produced negative inotropy at nanomolar concentrations. However, in the 19–21-day-old embryos, neither positive nor negative inotropy was observed. Conclusions were made by comparing the pA2 values that positive inotropy is most likely due to muscarinic M1 receptors and the negative inotropy is most likely mediated by muscarinic M4 receptors [22].
