Expression and role of phosphodiesterase 6 in the chicken pineal gland

The chicken pineal gland is directly photosensitive, with light causing an inhibition of melatonin synthesis. A possible role of phosphodiesterase 6 ( PDE6, the primary effector of retinal phototransduction) in mediating this response was investigated. RT-PCR, DNA sequencing and northern blots reveal ed the presence of RNA encoding both catalytic and regulatory subunits of P DE6 in the chicken pineal gland. Both rod and cone forms of PDE6 subunits m RNA were detected. The concentration of the transcripts encoding PDE6 catal ytic subunits peaked at night. Western blot analysis of chicken pineal prot eins with an antibody directed against the catalytic subunits of bovine rod PDE6 identified a single immunoreactive protein of 97 kDa. Anion exchange chromatography of chicken pineal soluble proteins revealed a peak of PDE6 a ctivity that accounted for about 30% of cyclic GMP-hydrolysis. In cultured chick pineal glands, arylalkylamine N-acetyltransferase (AA-NAT), the rate- limiting enzyme of melatonin synthesis. was protected from inhibition by li ght when selective PDE5/6 inhibitors (zaprinast, DMPPO) were added to the c ulture medium. PDE5/6 inhibitors did not affect AA-NAT activity in the dark . In contrast, a general PDE inhibitor (IBMX) increased AA-NAT in a light-i ndependent manner. Together, the data indicate that rod and cone forms of P DE6 are expressed in chick pineal cells and that this enzyme plays a role i n the inhibition of melatonin synthesis by light.