Although vasoactive intestinal peptide (VIP) is a well characterized physiological PRL-releasing factor in avian species, its regulated expression is not fully understood. We cloned complementary DNAs encoding the prepro-turkey VIP (prepro-tVIP) molecule from an adult turkey hypothalamic complementary DNA library. When the amino acid sequence of the prepro-tVIP was compared to chicken and mammalian sequence, it was found that the isolated tVIP molecules lacked the 27-amino acid peptide histidine isoleucine (PHI) portion of the precursor protein. Several tissues showed an alternatively spliced tVIP transcript that lacked the PHI sequence. Only in the hypothalamus did tVIP-specific primer pairs and reverse transcription-polymerase chain reaction produce two alternatively spliced fragments. The larger hypothalamus-specific fragment was subjected to nucleotide sequence analysis and identified as containing the alternatively spliced PHI-containing exon, which encoded a 27-amino acid PHI peptide in addition to the 8 amino acids that flanked the peptide. Hypothalamic tVIP expression was shown to be up-regulated during the incubation phase of the reproductive cycle. The increased steady state level of tVIP messenger RNA appears to be regulated by nesting behavior, because nest deprivation dramatically suppressed its expression. Levels of the minor tVIP transcript containing both the PHI- and VIP-encoding exons did not significantly change between reproductive stages and were maintained at approximately 4-6% of the total tVIP transcript level. Our findings provide further evidence that VIP is the most important PRL-releasing factor in birds. Our study should serve as a useful model for determining whether PHI contributes in any way to the physiological role of PRL regulation. Revealing the tissue distribution of VIP and PHI gene expression and tissue-specific alternative splicing could contribute to an understanding of the physiological functions of the two peptides as well as their relative roles in PRL regulation.
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