PAC1-expressing structures of neural retina alter their PAC1 isoform splicing during postnatal development

Pituitary adenylate cyclase-activating polypeptide (PACAP), a member of the secretin/glucagon/vasoactive intestinal peptide family, exerts various effects on neuronal development as mediated by the differential expression of PAC1 receptor (PAC1-R) isoforms. The expression changes of PAC1-R isoforms (Hip, Hop1) reported in correlation with retinal development suggest an isoform switch during the second postnatal week. Our aim is to determine the exact period of the isoform shift and to describe the PAC1-R-immunoreactive structures appearing from postnatal day 5 (P5) to P10 in the rat retina. The ratio of Hip and Hop1 receptors was assessed and changes in their expression were followed by Taqman and SybrGreen-based quantitative polymerase chain reaction. For the detection of PAC1-R-expressing retinal structures, anti-PAC1-R, anti-calbindin, anti-protein kinase C, anti-glutamine synthetase, anti-HPC1 and anti-Brn3a antibodies were utilized. At the transcript level, a marked decrease to an undetectable level was measured in Hip mRNA expression from P6 to P9. Hop1 expression appeared to be unchanged from P6 to P9, followed by a significant elevation at P10. A Hip/Hop1 isoform shift occurred between P6 and P7. Immunostaining showed strong PAC1-R labeling from P5 to P10 in ganglion, amacrine, horizontal and rod bipolar neurons and in glial Muller cell processes. The Hop1 isoform was predominantly expressed in various types of retinal cell beginning at P7, because of a dramatic reduction in Hip mRNA level. As the Hop1 receptor is coupled to different signaling cascades, this isoform shift might alter the physiological role of PACAP during this particular period.