Differential ontogenesis of D₁ and D₂ dopaminergic receptors in the chick embryo retina

The differentiation of D₁ and D₂ dopamine receptors was investigated during the ontogenesis of the chick embryo retina. Our results reveal an interesting complexity in dopaminergic differentiation, with one major receptor system developing before synapses and another one developing after. The dopamine-dependent increase of chick retina cAMP level differentiates early during retina ontogeny. By the embryonic day 10-11 10^-4 M dopamine and ADTN elicit a 13-fold increase in cAMP content of the retina. However, [³H]spiperone (D₂ ligand) binds very little to crude membrane preparation of retinas from embryos in the same developmental stage (12-13 fmol/mg protein). High specific binding of [³H]spiperone is only detected after the embryonic day 17-18, attaining 80 to 100 fmol of specific spiperone binding sites in the retinas from post-hatched animals. Apomorphine also promotes the accumulation of cAMP of retinas from early embryonic stages. However, it is only 20-30% as effective as ADTN or dopamine. In addition, while the dopamine responsiveness of the tissue decreases sharply during its ontogeny, the apomorphine effect remains practically constant throughout this period. Both dopamine and apomorphine are equally effective in eliciting cAMP accumulation of retinas from post-hatched animals. Moreover, apomorphine is a potent inhibitor of dopamine-induced cAMP level of the embryonic tissue. The results presented here indicate that D₁ and D₂ receptors differentiate independently from each other, and that apomorphine elevates retina cAMP levels via a subclass of D₁ receptors that does not desensitize significantly during retina development.