Expression of functional neurotransmitter receptors in an uninnervated tissue: avian amnion

The smooth muscle of the avian amnion is unusual because it is normally never innervated. However, as assessed by contractile response, this tissue expressed at least 11 different types of receptor for neurotransmitter substances including acetylcholine, norepinephrine, histamine, 5-hydroxytryptamine, vasoactive intestinal peptide, urotensin II, neurotensin, and somatostatin-28. Three neurotransmitters, histamine, 5-hydroxytryptamine, and norepinephrine, each acted via 2 separate and antagonistic types of receptors. The amnion also responded to prostaglandin E2. On the other hand, the tissue did not respond to substance P or bradykinin, 2 peptides that are known to affect smooth muscle contractility in a variety of other systems. Studies with organ-cultured amnion demonstrated that the smooth muscle can be cultured early in development and will differentiate in vitro. Some, but not all, of the amniotic responses developed in a defined medium. The results indicate that this novel smooth muscle preparation will be useful for identifying epigenetic factors that control the expression of functional receptors.