Selective sensitive period in chick embryo: toxic effect of exogenous neurotransmitters

The phenomenon of sudden mortality in the chick embryo, induced by the exogenous neurotransmitters norepinephrine, epinephrine, dopamine, serotonin, carbachol, and by the beta-blockers propranolol and atenolol, is described. When introduced singly into eggs (albumen) in doses of 0.1 mg per egg, these substances induced highly significant, sharply increased mortality (60% to 100%) when the introduction was on embryonic (incubation) days 8 to 12 (hereafter called E8 and E12), but none if introduction was earlier. These ages follow the appearance of functional receptors (or receptor-effectors) for these substances. However, the involvement of receptors here has not been proved. Solvent alone (Ringer's solution for chick) had no effect, indicating that the procedures used were in themselves not lethal. Possibly, the above neurotransmitters became lethal to the embryo because they were introduced in excess of the amounts which were produced and needed at that time by the embryo for optimal development.