Abnormal Expression of Calcium/Calmodulin-Stimulated Adenylyl Cyclase in Photoreceptors of Dopamine D4 Receptor-Deficient Mice

Abstract

Abstract: : Purpose: cAMP levels in photoreceptor cells are high in darkness and reduced by light exposure. Photoreceptor cells express the D4 subtype of dopamine receptor, activation of which inhibits cAMP formation. In retinas of mice with targeted deletion of the dopamine D4 receptor (D4KO mice), cAMP levels are low relative to those in wildtype (WT) mice and photoreceptor cells are insensitive to the cAMP-lowering effect of light exposure. The aim of the present study was to test the hypothesis that altered expression of Ca²⁺/calmodulin-stimulated adenylyl cyclase isozymes underlies the dysfunctional regulation of cAMP formation in photoreceptors of D4KO mice. Methods: WT and D4KO mice were housed on a 12 h light : 12 h dark cycle. The genotype of the mice was confirmed by PCR using tail DNA. Total RNA was extracted from retinas and microdissected photoreceptor inner segments of WT and D4KO mice and reverse transcribed. Transcript levels of the type 1 and type 8 adenylyl cyclases, AC1 and AC8, were quantified by real-time PCR. Inner segments were dissected from retinal frozen sections by laser capture microdissection (LCM). Results: AC1 and AC8 mRNA levels in retinas of WT mice were relatively low during the night and increased during the daytime, peaking approximately 8 h after light onset of the imposed daily light-dark cycle. When examined at this time of day, AC1 and AC8 transcript levels were significantly lower in retinas of D4KO mice than in those of WT controls. The AC1 mRNA level of the D4KO mice was also significantly reduced in LCM-dissected inner segments. Conclusions: Expression of mRNAs encoding the Ca²⁺/calmodulin-stimulated adenylyl cyclases is abnormally low in photoreceptors of dopamine D4 receptor-deficient mice. These results suggest that dopamine, acting through the D4 receptor subtype, may normally regulate the expression of these cAMP-synthesizing enzymes in photoreceptor cells.

Keywords: dopamine • photoreceptors • gene/expression